Power Engineering Training Systems Courseware

PART A2

PPOWEROWER E ENGINEERINGNGINEERING FOOURTHURTH CLLASSASS

EDDITIONITION 22.5.5

PRINTED - JUNE 2010 Fourth Class Part A2

Edition 2.5 June 2010

Published by PanGlobal Training Systems Ltd. Publisher of Power Engineering Training Systems Courseware

The material in this series is aligned with Fourth Class Syllabus, dated July 31, 2002. For more info visit http://www.sopeec.org/Syllabus/SyllabusFourthClass.pdf This material copyright © Power Engineering Training Systems, a division of PanGlobal Training Systems 2006, 2008. All rights are reserved.

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We would like to acknowledge the contribut ons of Fred Ohlmann, Gerry Parker, & Wilmer Doerksen for their authoring, Lorne Shewfelt for his technical edit ng and the Power Engineering instructors of NAIT & BCIT for their reviews.

We also acknowledge the guidance provided by the joint IPECC/SOPEEC 4th Class review commit ee.

Power Engineering 4th Class Project Team: Andy Shorthouse, Dan Violini, Jennifer Landree, Kyla Brassard, Deb Ross, Terry Lazenby & Scot MacNaughton

This curriculum is endorsed by the Canadian Inst tute of Power Engineers (IPE).

Cover image courtesy of EnCana Corporat on. The image is a SAGD facility at EnCana’s Christ na Lake Operat on, located in Northeast Alberta.

LEGAL NOTICE The informat on, fi gures and procedures presented in this book are provided as general training and background informat on, and should not be taken as specifi c operat ng pract ce for any specifi c piece of equipment or in any individual plant implementat on. Training for specifi c pieces of equipment, and their implementat ons should be provided by the employer and the equipment manufacturer.

The informat on and illustrat ons contained in this book are provided “As Is”, without warranty or any kind, express or implied, including without limitat on any warranty concerning the accuracy, adequacy, or completeness of such informat on. Neither the publisher, the authors, nor the copyright holders shall be held responsible for any claims at ributable to error, omissions, or inac- curacies contained in this book. Nor shall they be liable for direct, indirect, special, incidental or consequent al damages arising out of the use of such informat on or materials.

Printed by Data Group in Calgary, Canada Fourth Class - Part A2 Edit on 2.5, June 2010 ISBN13: 978-1-897461-28-0

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1301 16th Ave. NW Calgary, Alberta T2M 0L4 What is new in the Fourth Class, Edition 2.5?

Edition 2.5 of the Fourth Class incorporates a number of new additions. It incorporates all corrections reported to PanGlobal Training Systems (the Pub- lisher of the Power Engineering series books) and validated as appropriate changes by both internal and external subject matter experts. This edition represents a minor revision and does not reflect any changes to the SOPEEC syllabi. It does, however address a number of educational quality issues which have arisen over the past years. Although the general book organization continues in support of Part A and Part B segmentation, a large number of content elements which had previously been repeated in several areas of the text have been replaced with appropriate references. This has facilitated the combining of several of the chapters and an overall reduction from 149 to 144 chapters. The Learning Objectives in each chapter have been separated into distinct units and this has necessitated some adjustment in the wording of some Objectives to reflect this reorganization. This new format has resulted in a new pagination of each book differing from edition 2.0. Therefore, any instructional references made to the Second Edition will have to be adjusted for use with Edition 2.5. The Second Edition books remain valid and may be used in conjunction with cor- rection sheets posted to http://www.powerengineering.org/index.php/corrections-a-updates#pe4 to provide the same learning resources that are available in this edition. Get the most out of your learning materials; visit the “Learner Support” section of our website at www.powerengineering.org

The “Learner Support” section of PanGlobal’s website provides students with a variety of learning materials Certification Assistance Certification of Power Engineers exists countrywide under the auspices of the Association of Chief Inspectors (ACI), which maintain a interprovincial exam Syllabus for each Power Engineering Class. Examinations are administered by Standardization of Power Engineering Examination Committee (SOPEEC), a sub-committee of ACI. Contact your instructor or local jurisdiction to obtain instructions for certification. A link to SOPEEC’s website (http://www.sopeec.org), is located on our site and we also have links to the Power Engineering learning institutes across Canada. Companion Website (online learning site) You have received a user ID and Password (on the Notice to Reader page). By accessing the online resources page (http://mypower-net.com) and logging in, you can access additional educational self-study resources to support your learning. Your school may also use these resources to guide and manage your learning. Please check with your instructor. Supplemental Media In addition to the images in your textbooks, our companion web contains hundreds of media elements, identified by the Media Element (above) symbol in your texts. Self Assessments Your online account contains self assessment tests for each chapter. These assessments test your compre- hension of key concepts and provide you with detailed feedback which will tell you where you are weak. They provide you with a realtime testing environment that many educators have recognized as providing valuable preparation skills. Academic Supplement Your newly purchased package of learning materials contains an Academic Supplement booklet which may be taken into your national examination if unmarked. Individual sections (Handbook of Formulas, Steam Tables and Refrigerant Properties) of the Academic Supplement are available for download. Website Resources A number of additional resources are available on the Learner Support section including: A Glossary of Power engineering specific terms, a series of Industry links and a growing list of educational supports files. Corrections, Updates and Product Suggestions PanGlobal and its educational partners attempt to ensure that all errors and ambiguity are removed from our products before you purchase them. However, those that do get through our quality review process are fixed as soon as possible We post the corrections on the Learning Products section of our website. You are able to add your own valuable input to this process by filling out our online form in this same area. Comments on all aspects of our products and the service you have been provided are welcome. Committed to Student Success PanGlobal’s Power Engineering Training Systems Learner Resources provide an easy to navigate, dynamic tool to assist students as they progress through the textbooks. We are dedicated to the Power Engineering profession and are providing the resources and tools to help students get the most out of their courses and maximize their potential for success. Fourth Class Part A2 Edition 2.5

Table of Contents

Unit Nine: Environment 1 Chapter 40 Environmental Introduction ...... 3 Chapter 41 Gaseous & Noise Pollutants ...... 13 Chapter 42 Solid & Liquid Thermal Pollutants ...... 29 Chapter 43 Potential Environmental Impact of Liquids ...... 43 Chapter 44 Potential Environmental Impact of Gases ...... 55 Chapter 45 Potential Environmental Impact of Operating Facilities ...... 69

Unit Ten: Materials & Welding 79 Chapter 46 Engineering Materials ...... 81 Chapter 47 Welding Methods & Inspection ...... 93 Chapter 48 Welding Terms, Forge & Fusion Welding Processes ...... 107

Unit Eleven: Piping 123 Chapter 49 Introduction to Piping & Pipe Fittings ...... 125 Chapter 50 Introduction to Valves ...... 151

Unit Twelve: High Pressure Boiler Design 171 Chapter 51 Introduct on to Boilers ...... 173 Chapter 52 Firetube Boilers ...... 185 Chapter 53 Watertube Boilers ...... 197 Chapter 54 Electric Boilers ...... 213 Chapter 55 Basic Boiler Construct on...... 223 Table of Contents (continued...)

Unit Thirteen: Draft, Combustion & High Pressure Boiler Fittings 251 Chapter 56 Boiler Draf Equipment ...... 253 Chapter 57 Introduct on to Boiler Combust on ...... 273 Chapter 58 Fluidized Bed Combustion ...... 301 Chapter 59 Safety & Relief Valves ...... 313 Chapter 60 Water Columns & Gauge Glasses ...... 329 Chapter 61 Drum Internals ...... 347

Unit Fourteen: High Pressure Boiler Operation 359 Chapter 62 Sootblowers...... 361 Chapter 63 Continuous & Intermittent Blowdown ...... 371 Chapter 64 Boiler Preparation, Start-up & Shutdown ...... 381 Chapter 65 Routine & Emergency Boiler Operation ...... 393

Unit Fifteen: Feedwater Treatment 407 Chapter 66 External Feedwater Treatment ...... 409 Chapter 67 Internal Feedwater Treatment & Testing Methods ...... 429 4th Class • Part A2 U N I T 9

ENVIRONMENT

Chapter 40 Environmental Introduction 3

Chapter 41 Gaseous & Noise Pollutants 13

Chapter 42 Solid & Liquid Thermal Pollutants 29

Chapter 43 Potential Environmental Impact of Liquids 43

Chapter 44 Potential Environmental Impact of Gases 55

Chapter 45 Potential Environmental Impact of Operating Facilities 69

1 2 4th Class • Part A2 C HAPTER 40

Environmental Introduction

LEARNING OUTCOME

When you complete this chapter you should be able to: Describe the interaction and interdependency between the various elements of the environment.

LEARNING OBJECTIVES

Here is what you should be able to do when you complete each objective:

1. Describe the cycles that make up our environment.

2. Describe the interdependency of the “ecosystem”.

3 Unit 9 • Chapter 40 • Environmental Introduction 4

INTRODUCTION

The environment is all around us. It is our surroundings. It is the air we breathe, the water we drink and the source of our food. According to the dict onary, it is all the condit ons that aff ect the development of an organism. The environment consists of both natural and man-made elements. The natural environment is designed to be independent and contains delicate interrelat onships between living and non-living things. A man-made environment contains synthet c or art fi cial elements that couldn’t exist without human help. Human act vit es can be either benefi cial or harmful to the environment. For example, habitat enhancement programs provide food and shelter for wildlife, while improperly run plants contribute to air pollut on. The natural environment moves in cont nuous cycles and nature completely converts its wastes and resources to be reused. This conversion is evident in three cycles that occur on earth: air, water and soil cycles.

4th Class • Part A2 Unit 9 • Chapter 40 • Environmental Introduction 5

OBJECTIVE 1

Describe the cycles that make up our environment.

AIR CYCLE

Living beings can live without food and water for a few days, but only minutes without air. Air is invisible, odorless and tasteless. The air we breathe is a mixture of gases, as shown in Table 1. Water vapour, also known as humidity, is present depending on the geographical locat on.

Table 1 Composition of Air

Gas %

Nitrogen (N2)78

Oxygen (O2)21

Argon (Ar) 0.9

Carbon dioxide (CO2) 0.03

Traces of other gases 0.07 People and animals exhale carbon dioxide which is consumed by plants along with water and the sun’s energy to make food during photosynthesis. Oxygen is released during this process and inhaled by animals and humans.

WATER (H2O) CYCLE

Water is one of our most important natural resources. At least half the body weight of all living plants and animals is composed of water and about 70% of a human body weight is water. The sun’s energy moves the earth’s water through its cycle, called a hydrological cycle. The sun’s energy heats the rivers and oceans and evaporates the water. The water vapour condenses to form clouds and when the temperature cools, the moisture falls as precipitat on (rain or snow) from the clouds back to the rivers and oceans via surface water runoff . Water is also purifi ed in this cycle through evapora- t on, biological processes and fi ltering through the soil. In this cycle, water is purifi ed over and over again. Much of the fresh water consumed comes from groundwater sources; therefore, it is a vital resource. It has a role in the format on of landscape and in the migrat on and accumulat on of minerals. Figure 1 shows the zones of subsurface water. The origin of groundwater and surface water is precipitat on (rain or snow). From the land’s surface, water fi lters through a porous or aerated upper zone to the water saturated zone below. In the upper zone, water shares the spaces between soil part cles with air and provides moisture for plant life. Only a small port on fi lters down to the water saturated zone bounded by the water table. In the saturated zone water occupies all the spaces within the soil (no air space) and is able to move slowly. Direct on of fl ow is generally from upland areas to lower areas such as river valleys or lakes. However, the direct on of fl ow is somet mes more complex. It depends on the water table confi gurat on and geology.

4th Class • Part A2 Unit 9 • Chapter 40 • Environmental Introduction 6

Figure 1 Divisions of Subsurface Water

Figure 2 shows an example of the direct on of groundwater ﬂ ow. At some places, called discharge points, the water leaves the groundwater system to become surface water. Common discharge points are: R ivers• Rivers• Lakes• Lakes• Springs• Springs• Sloughs• Sloughs• Somet mes rivers, lakes, springs and sloughs feed into the groundwater system and serve to recharge the groundwater.

Figure 2 The Hydrological Cycle

4th Class • Part A2 Unit 9 • Chapter 40 • Environmental Introduction 7

SOIL CYCLE

Soil is naturally occurring loose mineral or organic material at the earth’s surface which is capable of supporting plant growth thus, it is essent al in the product on of food. The process of soil format on begins with a parent material which may be mineral or organic mat er deposited on the landscape in a variety of ways. Mineral mat er is of en deposited by wind, water, glaciers or gravity; organic mat er is deposited primarily by vegetat on. Soil development depends on the: • type of parent material. • topography of the area. climate.• climate.• • types of organisms present above and below the surface. The soil cycle is somewhat like a bank account. Deposits of nutrients and organic mat er are made by plants and animals in the form of their dead t ssues and wastes. Organisms within the soil break down and redis- tribute these deposits. Withdrawals are made as plants use soil nutrients for growth; microscopic organisms in the soil (microbes) may withdraw nutrients from it as well. When plants and microbes die, deposits are made once again. The process of soil development began in most of northern North America on parent material deposited by the glaciers which retreated about 10,000 years ago. Soils in other parts of the world are much older. Figure 3 shows a typical soil proﬁ le where the stage of the weathering varies with distance from the surface.

Figure 3 A Soil Proﬁ le

4th Class • Part A2 Unit 9 • Chapter 40 • Environmental Introduction 8

OBJECTIVE 2

Describe the interdependency of the “ecosystem”.

ECOLOGY

Many areas of study contribute to our understanding of the environment. Ecology, the study of plants and animals in relat on to their environment, brings these many areas together. Ecosystems, plants and animals living in balance with their environment, can be complex, but most are balanced. Figure 4 shows a schemat c diagram of an ecosystem. The three major components in the system are producers, consumers and inact ve organic mat er which includes the soil, nutrients in water and sediments. The dashed line represents the boundary of the system while the solid arrows indicate interact ons within the system.

The environment inputs CO2, H2O, O2, nutrients and radiant energy to the ecosystem and the ecosystem puts CO2, O2, H2O, some nutrients and heat of respirat on back into the environment. Such chemicals as nitrogen, carbon and oxygen cycle cont nuously between plants, animals and their habitat. The sun’s energy is the driving force of the ecosystem. Green plants (producers) collect energy from the sun by photosynthesis which both creates sugar from CO2 (ﬁ xing ) and releases O2. Consumers are herbivores and carnivores since they feed on plants or other animals to gain energy. Decomposers, like bacteria, obtain their energy by decomposit on of dead plants or animals. Each group of organisms, producers, consumers and decomposers, uses energy to survive and releases heat energy back into the atmosphere. Since energy is lost cont nuously, it must be gained in a cont nuous manner, emphasizing the importance of the sun’s energy to the survival of an ecosystem.

Figure 4 Schematic Diagram of an Ecosystem

4th Class • Part A2 Unit 9 • Chapter 40 • Environmental Introduction 9

FOOD WEBS

Food webs are the pat ern of feeding relat onships in an ecosystem. Figure 5 shows a food web within a prairie grassland community. The arrows ﬂ ow from the eaten to the consumer. The food energy stored by plants is passed through the ecosystem in a series of steps. In Figure 5, for example, grass plants are consumed by grasshoppers, grasshoppers become food for clay-colored sparrows and these sparrows are preyed on by marsh hawks.

Figure 5 Prairie Grassland

The complexity of food webs varies depending on the ecosystem. Many become intricate as shown, while others are not as complex.

CONCLUSION

All of the components of an ecosystem are connected; it is impossible to alter one without aff ect ng another. When man ignores these connect ons, an ecosystem can become unbalanced and permanent changes in the environment can occur. Somet mes the eff ects don’t occur immediately and of en it is hard to know what they may be, but a more comprehensive approach to development and conservat on is emerging. Too of en, air and water resources are presumed to belong to everyone, but no one takes responsibility for their conservat on. Therefore, they are undervalued and assumed to be available for the benefi t of any who take advantage of them with a “what can’t be seen, can’t be hurt” at tude. All ecosystems are essent al. Wise decision-making and long term planning are necessary for them to be maintained.

4th Class • Part A2 Unit 9 • Chapter 40 • Environmental Introduction 10

4th Class • Part A2 Unit 9 • Chapter 40 • Environmental Introduction 11

CHAPTER 40 - QUESTIONS ENVIRONMENTAL INTRODUCTION

1. Air is predominantly composed of what two gases? a) nitrogen and carbon dioxide b) nitrogen and oxygen c) argon and carbon dioxide d) oxygen and argon

2. Ground water and surface water originate from a) lakes. b) oceans. c) precipitat on. d) underground streams.

3. Organic mat er is primarily deposited on the landscape by a) wind. b) water. c) glaciers. d) vegetat on.

4. Parent material for soil format on a) is only mineral material deposited by glaciers, wind, and water. b) is only organic material. c) has been mostly deposited by glaciers in northern North America. d) is the same throughout the world.

5. Components of an ecosystem are a) independent of each other. b) interconnected and interdependent. c) vert cally ordered. d) unchangeable.

6. The gas that is released during the process of photosynthesis is a) oxygen. b) carbon dioxide. c) nitrogen. d) argon.

Fourth Class • Part A2 Unit 9 • Chapter 40 • Environmental Introduction 12

CHAPTER 40 - ANSWERS ENVIRONMENTAL INTRODUCTION

1. (b)

2. (c)

3. (d)

4. (c)

5. (b)

6. (a)

Fourth Class • Part A2 4th Class • Part A2 C HAPTER 41

Gaseous & Noise Pollutants

LEARNING OUTCOME

When you complete this chapter you should be able to: Name gaseous pollutants related to power plants, describe their effect upon the environment, and discuss some methods used for their control as well as describe noise pollution related to power plants.

LEARNING OBJECTIVES

Here is what you should be able to do when you complete each objective:

1. Describe the adverse effects of, and the associated control systems for, various gaseous pollutants.

2. Describe how noise pollution is measured and controlled.

3. Describe typical devices and systems for monitoring gaseous and noise pollutants.

13 Unit 9 • Chapter 41 • Gaseous & Noise Pollutants 14

4th Class • Part A2 Unit 9 • Chapter 41 • Gaseous & Noise Pollutants 15

OBJECTIVE 1

Describe the adverse effects of and the associated control systems for various gaseous pollutants.

GASEOUS POLLUTANTS

Gaseous pollutants from a power plant result from the burning of common fuels. The gaseous pollutants of which a Power Engineer has to be aware are carbon monoxide (CO), carbon dioxide (CO2), sulphur oxides (SO2, SO3, SOX) and nitrogen oxides (NO, NO2, NOX). The product on of these gaseous pollutants is detrimental to the operat on and life of a power plant, as well as to the environment. For these reasons, a Power Engineer must be fully aware of the eff ects of these pollutants, how to detect them and, most of all, how to control their emission into the atmosphere. Carbon Monoxide (CO) Carbon monoxide is a product of incomplete combust on in the boiler furnace. When fuel is burned, carbon reacts with oxygen. If there is enough oxygen, the carbon and the oxygen form carbon dioxide (CO2). If there is a shortage of oxygen, the carbon combines with the oxygen to form carbon monoxide (CO). Carbon monoxide is a serious pollutant as it is a deadly, toxic gas. Its presence in the stack emissions also represents a loss for the operat on. Heat energy is lost when the carbon element of fuel burns to carbon monoxide rather than carbon dioxide because the oxidat on process that releases heat is not completed. As well, carbon monoxide is very explosive. Should it collect somewhere in the boiler passes and form a stagnant pocket, then it would only take a spark or some source of ignit on to cause a furnace explosion. Diff erent condit ons in a furnace can create a shortage of oxygen. The most obvious case is that there is not enough air being supplied to the furnace. Another reason is, due to insuffi cient turbulence, the air and the fuel do not mix well enough for the carbon from the fuel to react completely with the oxygen in the air. Lo- calized shortages of oxygen result in the product on of carbon monoxide. Also, overloading of the boiler can cause quick combust on, depriving the fuel of the t me needed to be completely burned in the hot enclosure of the furnace. The three causes of incomplete combust on ment oned above can all be controlled by the Power Engineer. Most plants are equipped with automat c fl ue gas analyzers which cont nuously monitor the composit on of the fl ue gas leaving the furnace. Even where there are no fl ue gas analyzers, a Power Engineer should, by experience, know how to determine proper combust on from the appearance of the fl ame, the temperature of the gas at various points in its path and the appearance of the stack emissions. If necessary, the Power Engineer should be able to perform an analysis of the fl ue gas.

Carbon Dioxide (CO2)

Carbon Dioxide (CO2), unlike carbon monoxide, has not historically been classifi ed as a gaseous pollutant. Growing concern over the possible or potent al eff ects of increased levels of atmospheric carbon dioxide has prompted new interest in removing the gas from the smokestacks of such large-scale sources as coal-fi red electric power plants. A number of jurisdict ons across North America have been act ve in legislat ng limits or controls of product on of CO2 from industrial sources, and CO2 is now also a common component of stack gases which is measured.

The economic key to reducing or removing CO2 emissions from stacks is ident fying and using a cost-eff ect ve way to capture large quant t es of carbon dioxide from coal and other fossil fuel burning facilit es and then binding it up or sequestering it into another form. Exist ng CO2 capture techniques involve the use of solid or liquid adsorbents that are expensive and require signifi cant amounts of energy to minimize their economic impact. In power generat ng facilit es, the cost of adding such controls must be minimized so they don’t raise the price of electricity signifi cantly. Once removed from the stack gases, the CO2 might be sequestered in the deep ocean, in mined-out coal seams or in depleted reservoirs.

4th Class • Part A2 Unit 9 • Chapter 41 • Gaseous & Noise Pollutants 16

Sulphur Oxides (SO2, SO3 and SOx) Sulphur oxides are produced by the burning of sulphur, an element contained in almost all industrial coals and most fuel oils. During the combust on process in a furnace, the sulphur in the fuel is oxidized into sulphur

dioxide (SO2) and sulphur trioxide (SO3), with the sulphur dioxide usually equal to at least 95% of the total volume of sulphur oxides. These products are not as toxic as carbon monoxide, but can be injurious to humans and animals as well as vegetat on. In the presence of moisture, they form a weak sulphurous or sulphuric acid which irritates skin, corrodes most metals and disﬁ gures the exterior appearance of most painted surfaces. The acid will become entrained in raindrops, producing acid rain responsible for poisoning of lakes and destruct on of trees. As part of industrial smog, acids cause itchy skin, watering eyes, coughing and fat gue.

Since most of the oxides are SO2, processes concentrate on its removal from the fl ue gas. Studies have shown that systems designed to reduce the concentrat on of SO2 usually remove around 50% of it. Many economical systems designed to remove sulphur oxides from fl ue gas omissions do not produce a useful product. Sulphur dioxide can be eff ect vely eliminated from the stack emissions by wet lime or limestone scrubbing, double alkali or dilute sulphuric acid systems. Scrubbers produce a gypsum byproduct which may be marketable for wall board manufacture. Regenerable systems are not used just for cleaning up fl ue gas to meet emission standards. The amount of sulphur in the fl ue gas is usually too small for economical recovery because the regenerable processes are complex and expensive to build and operate. Another major disadvantage is that all regenerable systems use large volumes of toxic or gases such as: hydrogen sulphide, carbon monoxide and hydrogen. They are usually only used for fl ue gas clean-up if there is a waste disposal problem. Some of these processes are magnesium oxide, catalyt c oxidat on and aqueous carbonat on. Because the regenerable systems are not very common and relat vely complex, they will not be discussed any further at this level. The following three nonregenerable processes ment oned will be described in more detail: • Lime/limestone scrubbing • Double alkali systems • Dilute sulphuric acid Lime/Limestone Scrubbing In the past, lime/limestone systems have been used to remove sulphur at the combust on stage in the furnace or from the fl ue gas with a wet scrubber. In the late sixt es and early sevent es, some systems used the method at the combust on stage where pulverized limestone was injected into the furnace with the pulverized coal to react with the sulphur in the combust on chamber. Because of problems with scaling and plugging of boiler convect on surfaces, most of these combust on stage type systems are no longer built. They have been changed to some sort of wet fl ue gas scrubbing system.

The wet scrubbing process has several advantages over the dry process, including higher effi ciency of SO2 removal, less boiler operat on interference and usually a lower operat ng cost for large power plants. The wet process also requires less limestone for a much higher percentage of SO2 removal. Because less limestone is required, less material handling facilit es and less solid waste disposal is required. Figure 1 shows a fl ow diagram for a lime/limestone wet scrubbing process. The fl ue gas enters the scrubber where it comes in contact with limestone slurry. The slurry absorbs the sulphur dioxide which reacts with the limestone and precipitates out as calcium salts in the effl uent hold tank. Fresh limestone is added to replace the spent absorbent and the slurry is recirculated through the scrubber. The absorbent slurry usually contains about 15% dissolved solids. To control the build up of solids, there is a bleed line from the scrubber hold tank which feeds a clarifi cat on tank.

4th Class • Part A2 Unit 9 • Chapter 41 • Gaseous & Noise Pollutants 17

Figure 1 Lime/Limestone Scrubbing Process

(Courtesy of Combustion Engineering) Double Alkali Systems Double alkali systems are similar to the lime/limestone systems in that limestone is consumed and a waste product of calcium sulphite or calcium sulphate is produced. Figure 2 shows a process diagram for a double alkali system. In this method, an alkali solut on such as sodium carbonate is circulated through the scrubber to absorb the SO2 where it forms sodium sulphate and goes to the reactor. It is combined with a lime or limestone solut on. The react on with the lime produces a precipitate of calcium solids. The calcium solids are separated from the liquid in convent onal separat on equipment such as the thickener. The solids are disposed of and the clear liquid is replenished with sodium and recirculated through the system.

Figure 2 Double Alkali System Process

(Courtesy of Combustion Engineering) This system has a major advantage over the lime/limestone method because the absorbent sodium solut on contains no suspended solids which tend to create scaling and plugging of interior scrubber parts.

4th Class • Part A2 Unit 9 • Chapter 41 • Gaseous & Noise Pollutants 18

Dilute Sulphuric Acid

The dilute sulphuric acid method uses a dilute solut on of sulphuric acid in water to absorb the SO2. The absorbed SO2 is then oxidized to sulphuric acid using an iron oxidizat on catalyst. Some of the sulphuric acid is recirculated through the absorber and some of it goes to a crystallizer where limestone is added and gypsum is formed. Figure 3 shows a dilute sulphuric acid process diagram. The disadvantages of this system are a high init al cost and corrosion of system parts due to the acidity of the absorbent solut on.

Figure 3 Dilute Sulphuric Acid Process

(Courtesy of Combustion Engineering)

Nitrogen Oxides (NO, NO2 and NOx) In recent years, nitrogen monoxide and nitrogen dioxide have been shown to react with sunlight in a complicated fashion to form a photochemical smog. As well, in the presence of other hydrocarbons, they may form cyanides - lethal poisons. Their presence in industrial emissions is now closely regulated and power plants must ensure that emissions do not exceed the permit ed level. The nitrogen from these oxides originates from both atmospheric nitrogen and that contained in all fossil fuels. The react ons that create nitrogen oxides (NOX) during the burning process are hard to prevent. There are two pract cal means of controlling emissions of nitrogen oxides from power plants. Control may be accomplished either by minimizing their format on, which is mostly dependent on high temperature and the availability of oxygen or removing the oxides from the ﬂ ue gas.

It is diffi cult to control NOX product on in power plants because the condit ons favourable for high NOX product on are created by combust on pract ces developed to increase power plant operat ng effi ciency and control other air pollutants. Boilers are ideally designed to have high fl ame temperature and use excess air to ensure complete combust on. This design controls the amount of the undesirable by-products such as smoke and carbon monoxide, but the high temperature and increased oxygen encourage high NOX format on.

4th Class • Part A2 Unit 9 • Chapter 41 • Gaseous & Noise Pollutants 19

Large coal-fi red power plants can use fi ring system modifi cat ons to reduce the init al excess air in the combust on process. The reduct on in air decreases the amount of oxygen available to form NOx. These modifi ca- t ons include: • Reduced excess air percentages • Staged combust on • The use of overfi re air • Tangent al fi ring • Gas recirculat on

If format on of NOX is not suffi ciently controlled during fi ring, some form of fl ue gas scrubbing must be used to bring its concentrat on down to an acceptable level. Dry processes, using a reduct on react on with ammonia, are most common. However, at the present t me, modifi cat ons in combust on techniques and furnace design appear to be the most eff ect ve means for reduct on of nitrogen oxide emissions from power plant operat ons. Other Pollutants Commonly Monitored A number of other gaseous pollutants are also commonly monitored either through regulatory or plant effi ciency requirements. Compounds can include: Arsenic• Arsenic• Chlorine• Chlorine• • Hydrogen chloride • Mercury Selenium• Selenium• Mercury has been a part cularly regulated effl uent for a number of years. Both Canada and the US federally regulate emissions of mercury due to its potent al for permanent neurological damage at low levels. Part cular controls for the effl uent include: • Coal washing to remove sulphur and ash before combust on can reduce mercury levels by up to 60% • Electrostat c precipitators capture part culates and have been shown to reduce mercury by up to 25% • Fabric fi lters, have historically been used and reduce part culate mercury levels by over 50% • New technologies ut lizing custom absorbents may see removal levels up to 90%

4th Class • Part A2 Unit 9 • Chapter 41 • Gaseous & Noise Pollutants 20

OBJECTIVE 2

Describe how noise pollution is measured and controlled.

NOISE POLLUTION

The eff ects of industrial noise on workers and the surrounding community have been the subject of much research over the past twenty years. This research has shown that excessive noise can cause irreversible hearing loss, and some studies even suggest that noise may also be the cause of other health problems. Research is cont nuing in this area. The eff ects of noise on the communit es near the source are mostly sleep interference and annoyance, depending on the frequency, intensity and durat on of the sound. For example, high frequency tones are per- ceived as louder than lower frequency tones at the same volume. Intermit ent or impulse noises are of en more annoying than a constant noise. Public awareness is increasing and the requirements for noise limitat on by commercial operat ons have also become greater. Power plants can be very noisy. In the past, some hearing loss suff ered by plant workers was mostly taken for granted. Now employers can be liable for compensat on for hearing loss and occupat onal health regulat ons on noise have become more specifi c and demanding. Sound Waves Sound travels as waves through the air in the same way that waves travel through water. The waves are formed by vibrat ng bodies or air turbulence which causes a variat on in air pressure. This variat on is passed via the air molecules through the air and represents an energy transfer or fl ow through the medium (air). Because it is a form of energy, sound can be expressed as having specifi c power and intensity levels. Sound waves have varying frequencies and wavelengths; they can be refl ected, defl ected and absorbed. Absorpt on of sound occurs when a sound wave strikes a nonrigid barrier. The energy of the wave moves the fi bres on the barrier’s surface. Internal frict on in the barrier opposes this movement and the energy of the air is converted to heat energy within the barrier. Refl ect on and defl ect on of waves occur when the sound strikes a rigid barrier. Very lit le of the energy of the wave is absorbed by the barrier; only the direct on of the wave is changed. Sound Measurement Variat ons in sound are caused by the diff erent pressures and frequencies of the sound waves. A human ear is usually capable of hearing sounds which represent pressures from 2 x 10-5 Pa to 200 Pa. Because of this large range, the concept of a decibel was created to compress this range of sound levels into a more meaningful scale. A decibel (dB), in mathemat cal terms, is:

Decibel = 10 log(A/AO)

Where A is the measured quant ty and AO is a ﬁ xed reference quant ty. A decibel can represent a sound power level, a sound intensity level or a sound pressure level. For example, the sound power level, (Lw) can be expressed as:

Lw = 10 log(W/WO)

Where W is the sound power, in wat s (W), of the measured sound and WO is a reference sound power, generally 10 -12 W. Table 1 gives sound pressure levels (SPL) in decibels for some common noises and their corresponding sound pressure. The most important thing to realize from these deﬁ nit ons is that because decibels represent a logarithmic scale, sound levels given in decibels do not add directly together.

4th Class • Part A2 Unit 9 • Chapter 41 • Gaseous & Noise Pollutants 21

Table 1 Sound Pressures and Sound Pressure Levels Sound SPL Source Distance Pressure (Pa) (dB ref 0.00002 Pa) (long time average) (m) 200 140 threshold of pain 130 pneumatic clipper 2 20 120 threshold of discomfort 110 automobile horn 6 2 100 New York subway train inside 90 motor bus inside 0.2 60 typical business ofﬁ ce inside 50 quiet residence inside 0.002 40 library inside 30 0.0002 20 whisper 1.5 10 0.00002 0 threshold of hearing

Example 1:

A man stands at a point and hears noise from two dist nct sources, S1 and S2. The level of sound power from each source LS, equals 80 decibels. This does not mean that the total level of sound power, LT, received by the man is 160 decibels. In fact the sound power level at his point is 83 decibels, which can be shown by the following calculat ons: -12 Let the reference level, Wo equal 10 wat s (W).

LS = 10 log (Ws/Wo) -12 Then: 80 dB = 10 log (Ws/10 ) -12 8 = log (Ws/10 ) -1 -12 log (8) = Ws/10 8 -12 10 = Ws/10

Solving for WS: 8 -12 WS = 10 x 10 = 10-4 W (watts)

Because there are two sources, WS, the level of the power intensity at the point is 2Ws then: -4 -12 LS = 10 log (2 x 10 /10 ) = 10 log (2 x 108) = 10 x 8.3 = 83 dB From this calculation, it is easily seen that doubling the sound intensity at point A is represented as only a shift of 3 decibels. Therefore, it is important to remember that decibel levels from multiple source cannot be simply added together. Decibels from two or more sources require a conversion of each level back to its antilogarithm before addition can occur. Sources of Noise There are numerous sources of noise in a plant. These sources include: • machinery furnaces• furnaces• • air movement (fans and compressors) • structural vibrat ons transferred from moving parts

4th Class • Part A2 Unit 9 • Chapter 41 • Gaseous & Noise Pollutants 22

Figure 4 shows some common sources of noise, their levels and their eﬀ ects on people. Furnaces, fans, reciprocat ng compressors and coal pulverizers generally produce low frequency noise while gas or steam passing through vents and valves produce high frequency noise.

Figure 4 Noise Pollution Graph

Exposure Limits Noise levels in industrial environments are both variable and cont nuous and so criteria have been established to determine exposure limits based on both SPL over a speciﬁ c t me as well as maximum t me at a set SPL. Table 2 provides maximum exposure limits over an eight hour cont nuous t meframe for various jurisdict ons in Canada.

Table 2 2007 Canadian Exposure Limits over 8 hours Maximum Permitted Exposure Level Jurisdiction for 8 hours continuous (dB) Canada (Federal) 87 British Columbia, Alberta, Saskatchewan, Manitoba, Ontario, New Brunswick, Nova Scotia, Prince Edward Island, 85 Newfoundland, Northwest Territories, Nunavut, Yukon Territories Québec 90

4th Class • Part A2 Unit 9 • Chapter 41 • Gaseous & Noise Pollutants 23

These maximum exposure levels, of en called criterion and abbreviated Lc are used as the basis to determine maximum daily exposure limits in varying industrial environments. For example, a criterion level of 85 dB such as is seen in Saskatchewan allows only a two hour exposure to 91 dB each day. Table 3 provides diﬀ erent maximum durat ons for the two typical criterion levels seen in Canada as jurisdict ons (non-federal).

Table 3 Noise Exposure Limits with Different Criterion Noise exposure limits when Lc equals 85 dB Noise exposure limits when Lc equals 90 dB Allowable Level decibel Maximum permitted Allowable Level decibel Maximum permitted (3 dB Exchange Rate) daily duration (hours) (5 dB Exchange Rate) daily duration (hours) 85 8 90 8 88 4 95 4 91 2 100 2 94 1 105 1 97 0.5 110 0.5 100 0.25 115 0.25 (Canadian Center for Occupat onal Health and Safety - ht p://www.ccohs.ca/oshanswers/phys_agents/exposure_can.html#anchor2) Noise Control Eff orts to control noise are generally aimed at lowering the sound intensity at a given locat on. The minimum not ceable noise reduct on is approximately ten percent on the decibel scale which means a reduct on in noise of 5 to 10 fold is necessary to be worthwhile and not ceable. There are three ways to reduce noise at a given locat on: 1. reduce the sound at the source. 2. modify the sound wave path to the locat on. 3. protect hearing with a barrier device, such as: ear plugs or ear muff -type protect ons (most eff ect ve and preferred method). Noises are of en controlled by a combinat on of these methods. Reducing the sound at the source can be achieved by installing vibrat on isolat on mounts and damping equipment for machinery as well as by reducing speed of operat on and blade t p velocit es in fans. To be cost eff ect ve, most of these modifi cat ons must be considered and included at the design phase. Af er the design stage, modifi cat on of the sound path to the receiver is of en used. Enclosures are a form of sound path modifi cat on. They are designed using special materials which refl ect sound back to the source, and in some cases absorb it as well. All materials have some sound absorbing propert es. The energy of a sound wave striking a material surface is either refl ected, absorbed or transmit ed. The relat ve amounts will depend on the sound absorbing quality of the material. Soundproof materials absorb sound waves by trapping them in the material; as a result, only very small sound waves are refl ected back or transmit ed through the materials. The best materials for sound absorpt on are generally porous and are relat vely easy to be found. Lead, loose sand, brick and almost all heat insulators (fi berglass, mineral wool, polyurethane and cork) are good sound absorbers.

4th Class • Part A2 Unit 9 • Chapter 41 • Gaseous & Noise Pollutants 24

Figure 5 illustrates how sound wave paths are changed by a mainly reﬂ ect ve barrier and an absorbent barrier.

Figure 5 Sound Reﬂ ectors and Absorbers

Figure 6 shows two types of noise enclosures that absorb and reﬂ ect sound: muﬄ ers and lined ducts. Inlet silencers on forced draf fans are also a form of enclosure.

Figure 6 Sound Path Modiﬁ ers

Somet mes, it is necessary to enclose an ent re piece of equipment. If such an enclosure is necessary, it must be completely free of leaks. A hole as small as 1/1000 of the total wall area of the enclosure would leak enough sound to make the enclosure non-eff ect ve. For high frequency noises, the best results are obtained using a double structure with a sound absorbing material between the two structures. Another more obvious form of sound path modifi cat on is creat ng a buff er zone around the source. By doubling the receiver’s distance from the source, the sound pressure can be halved. In many situat ons in a plant, it is very diffi cult to use sound absorpt on or path modifi cat on techniques. If they cannot be employed, personal protect on, such as earplugs or muff s, must be used to reduce noise to the receiver and prevent damage to hearing.

4th Class • Part A2 Unit 9 • Chapter 41 • Gaseous & Noise Pollutants 25

OBJECTIVE 3

Describe typical devices and systems for monitoring gaseous and noise pollutants.

POLLUTION MONITORING

In this chapter, pollutants from power plants and their adverse eff ects are discussed. As the government and public demand stricter measures on pollut on, the challenge of environmental control will cont nue to grow. Many pollutants cannot be totally eliminated; only their concentrat on can be controlled. In order to eff ec- t vely fi ght environmental pollut on in any form, plant outputs must be cont nually monitored. This monitoring also serves as an indicator of how well the exist ng equipment is funct oning. Gaseous Emission Monitoring For the power plant, the boiler stack is usually the main source of atmospheric pollut on. Vent lat on and exhaust stacks also contribute. Whatever the case, a clear picture of what is going out from the stacks is of vital importance for eff ect ve pollut on control. Cont nuous emission monitoring systems (CEMS) involve the installat on of equipment which sample, ana- lyze and report data at predetermined t mes in a stack or duct. Regulatory agencies may require the inclusion of a CEMS to ensure compliance with emission standards. The following example is one which may be seen for a boiler stack. The sampling train, shown in Figure 7, may, through a selector switch, sample the fl ue gas in the stack alternat vely with the ambient air at a remote locat on. The pump operates cont nuously to fi ll the storing box with the gas being tested and move the sample to be tested through the sampling train. The posit on in Figure 7 shows the fl ue gas being tested. Valves A and D are open while B and C are closed. When the storing box is full, valves A and D close. The sample is cooled by circulat ng cooling water. When the temperature is correct, valves B and C open and the sample is pumped through the series of analyzers: #1, #2, #3, #4, #5 and #6.

Figure 7 Automatic Sampling and Analyzing Train

Each analyzer contains a reagent or sensor designed to absorb or react with a dist nct pollutant thereby allowing the amount of that gas present to be determined. Results from the analyzers (electric or pneumat c) are sent to recorders and indicators in the control room. Alarms may be installed and set to go oﬀ if a preset allowable limit of any gas is exceeded. The cont nuous monitoring of the plant emissions is vital. As well as ensuring that they are kept at an acceptable level, the results from the monitoring can be used to see how well exist ng equipment is funct oning. Cont nual monitoring also provides records of plant emissions.

4th Class • Part A2 Unit 9 • Chapter 41 • Gaseous & Noise Pollutants 26

Noise Monitoring As discussed previously, sound levels are measured in decibels without dist nguishing between frequencies. Sound measurement instruments (sound meters) contain weight ng networks which take into account frequency and adjust the readout of the meter to correspond to how loud a person would perceive the sound to be. There are three weight ng networks: A, B and C. When noise is expressed in decibels, there may be an extra let er af er the dB, such as dBA. This let er, A, B or C indicates which, if any, weight ng network was used to measure the noise. Figure 8 shows a hand-held level meter which is rated as Type 1, 2, 3 or S in accordance with noise legislat on. Noise laws or regulat ons will specify which of these types of sound meters and which weight ng network must be used to invest gate noise levels for conformance to the regulat ons.

Figure 8 Sound Level Meter

(Courtesy of B & K Precision Corp.)

4th Class • Part A2 Unit 9 • Chapter 41 • Gaseous & Noise Pollutants 27

CHAPTER 41 - QUESTIONS GASEOUS NOISE POLLUTANTS

1. Carbon monoxide produced in a furnace is caused by what primary factor? a) boiler ﬁ ring rates too high b) a shortage of oxygen in the furnace c) unclean furnace walls d) incorrect fuel composit on

2. Nitrogen oxides in ﬂ ue gas may be reduced by a) raising the furnace temperature. b) decreasing the size of the furnace. c) scrubbing the ﬂ ue gas with lime. d) reducing the percentage of excess air.

3. A sampling and analyzing train a) measures the frequency composit on of noise. b) monitors ﬂ ue gas for one type of emission. c) cannot be operated automat cally. d) measures the amount of several types of emissions.

4. Decibels are represented on what type of scale? a) sound pressure b) exponent al c) logarithmic d) 0 – 100 kPa

5. Damaging sound pressure levels aﬀ ect ng plant personnel could not be reduced by a) enclosing the noise source. b) modifying the equipment operat ng condit ons. c) wearing protect ve equipment. d) changing the frequency of the sound.

6. Noise monitors (sound meters) measure the decibel rat ng and contain ______net- works. a) weight ng b) frequency c) analyzing d) boiler ﬁ ring control

Fourth Class • Part A2 Unit 9 • Chapter 41 • Gaseous & Noise Pollutants 28

CHAPTER 41 - ANSWERS GASEOUS NOISE POLLUTANTS

1. (b)

2. (d)

3. (d)

4. (c)

5. (d)

6. (a)

Fourth Class • Part A2 4th Class • Part A2 C HAPTER 42

Solid & Liquid Thermal Pollutants

LEARNING OUTCOME

When you complete this chapter you should be able to: Discuss methods of handling solid pollutants produced by power plants and the problems and solutions in regard to liquid thermal pollutants.

LEARNING OBJECTIVES

Here is what you should be able to do when you complete each objective:

1. Describe the construction and operation of various types of mechanical collectors.

2. Describe the construction and operation of electrostatic precipitators.

3. Describe how ﬂ yash is removed from a steam generator and the operation of a cooling pond.

29 Unit 9 • Chapter 42 • Solid & Liquid Thermal Pollutants 30

INTRODUCTION

The combust on of solid fuel, especially coal, produces varying amounts of a solid residue called ash. Thermal coal produces varying amounts of ash, dependent upon the quality of coal used, the grinding equipment, the furnace and the combust on process itself. Ash exists in several forms called fl yash and bot omash. Bot- tomash is lef as a residue in the bot om of the furnace and is typically used as base material for pavement and landfi ll drainage layer construct on. A large percentage of the ash carried through the fl u gas stream is a very fi ne solid material. If it is not collected it escapes the plant through the stack result ng in an environmentally unacceptable emission. This fl yash is a fi ne powder, which can vary in size from <1 μm to over 150 μm. The major port on of the fl yash is less than 30 μm in diameter.

4th Class • Part A2 Unit 9 • Chapter 42 • Solid & Liquid Thermal Pollutants 31

OBJECTIVE 1

Describe the construction and operation of various types of mechanical collectors.

MECHANICAL COLLECTORS

Baffl e-type fl yash collectors (Fig. 1) separate the fl yash from the fl ue gases by project ng the part cles out of the gas stream when the gases make an abrupt change in direct on. Usually, the baffl es are arranged in rows so that the gas stream is divided into a series of narrow ribbons. This type of collector can be designed to operate on natural draf installat ons or induced draf (I.D.) fans when the available draf is limited. The collect on effi ciency is best with the larger, heavier part cles. In Figure 1, two confi gurat ons are shown in which the change in direct on is caused either by an abrupt change in the fl ow path (a) or an obstacle in the fl ow path (b). Locat on A, B, and C show the relat ve part cle numbers before, during and af er the change in direct on respect vely.

Figure 1 Combination Flyash Collector and Induced Draft Fan

Mechanical Centrifugal Collectors These devices achieve part culate removal by centrifugal, inert al, and gravitat onal forces developed in a vortex separator. The dust-laden gas is admit ed either tangent ally or axially over whirl vanes (Fig. 2) to create a high velocity in the cylindrical port on of the device. The dust-laden gas circulates around the inner wall of the vortex separator and the dust is sucked into the hopper by the act on of the I.D. fan. The dust “centrifuges” onto the walls of the hopper and falls out for collect on. The ﬂ ue gas cont nues into the stack.

4th Class • Part A2 Unit 9 • Chapter 42 • Solid & Liquid Thermal Pollutants 32

Figure 2 Cyclone Separator Operation

Part cles are subjected to an outward centrifugal force and an oppositely directed viscous drag. The balance between these two forces determines whether a part cle will move to the wall or be carried into the vortex and be passed on to the clean-gas outlet tube and I.D. Fan. The collect ng force developed is more eff ect ve with larger part cles. The centrifugal collector is relat vely simple in design, construct on, and operat on, having no moving parts; it is, therefore, a relat vely inexpensive collector both in init al cost and operat ng cost. This mechanical collector is not suitable for high effi ciency collect ons of fi ne part culates such as those found in a coal-fi red ut lity boiler. It has the added disadvantage of a relat vely high pressure drop which results in excessive power consumpt on by the induced draf fan motor.

4th Class • Part A2 Unit 9 • Chapter 42 • Solid & Liquid Thermal Pollutants 33

Figure 3 illustrates a centrifugal collector arrangement.

Figure 3 Centrifugal Collectors

(Courtesy of Western Precipitation Corp.) Fabric Filters Fabric fi lters operate by trapping or impinging the dust on fi ne cloth fi lters or bags, usually tubular in shape; a number of bags are enclosed in a large chamber. As the collect on of fi ne fl yash and dust cont nues, the dust part cles adhere to the fabric surface. The fabric fi lter obtains its maximum effi ciency during this period of dust buildup. Af er a fi xed operat ng period, the bags are cleaned by a rapper system or by pulses of compressed air which simply shake the bags; the dust or fl yash falls into a hopper. Figure 4 shows the air fl ow through a bag house fi lter system

Figure 4 Bag House Filters (Fabric)

4th Class • Part A2 Unit 9 • Chapter 42 • Solid & Liquid Thermal Pollutants 34

Figure 5 illustrates the principle of operat on of a bag fi lter. Immediately af er cleaning, the fi ltering effi ciency is reduced unt l the buildup of collected ash takes place. The fabric fi lter (baghouse) can be applied in any process area where dry collect on is desired and where the temperature and humidity of the gases to be handled will not damage the cloth. For part culate mat er, effi ciencies above 99% can be achieved with fabric fi lters.

Figure 5 Fabric Filter

(Courtesy of Flakt Canada Ltd.) Wet Collectors or Scrubbers Wet scrubbers remove dust from a gas stream by collect ng it with a suitable liquid. Figure 6 illustrates a spray type wet scrubber.

Figure 6 Spray-Type Wet Scrubber

Wet scrubbers operate by passing the gas stream through a sprayed mist. Deﬂ ectors may be added to provide an impinging surface and the part culate mat er is removed by the liquid stream. Figure 7 illustrates the principle of operat on of a wet cyclonic scrubber.

4th Class • Part A2 Unit 9 • Chapter 42 • Solid & Liquid Thermal Pollutants 35

Figure 7 Cyclonic Scrubber Operation

Unlike other mechanical part culate collectors, wet scrubbers simultaneously remove dust and some gaseous pollutants. They are eﬃ cient on part cles 1 to 5 micrometers (μm) in diameter. However, they have a serious drawback in cases where the combust on gases contain sulphur dioxide (SO2). Sulphur dioxide, in water, forms a weak acid which is corrosive to steel, an irritant to human skin and a soil pollutant. Since, in this case, the discharge water from the spray type scrubber is acidic, it requires a safe disposal site. The scrubbing water may be treated with lime or dolomite to control water pH, result ng in higher operat ng and maintenance costs. Figure 8 illustrates a venturi type wet scrubber.

Figure 8 Venturi Scrubber

(Courtesy of Babcock & Wilcox)

4th Class • Part A2 Unit 9 • Chapter 42 • Solid & Liquid Thermal Pollutants 36

OBJECTIVE 2

Describe the construction and operation of electrostatic precipitators.

ELECTROSTATIC PRECIPITATORS

Electrostat c precipitators (Fig. 9) produce an electric charge on the part cles to be collected and then propel the charged part cles by electrostat c forces to the collect ng electrodes. This supplied voltage is between 10 000 and 100 000 volts DC.

Figure 9 Electrostatic Precipitator

4th Class • Part A2 Unit 9 • Chapter 42 • Solid & Liquid Thermal Pollutants 37

The precipitator operat on involves four basic steps: (Fig.10) 1. An intense electrostat c fi eld is maintained between the high voltage discharge electrode and the collect ng electrodes. 2. Part cles, entrained in a gas, become electrically charged when subjected to a strong electrostat c fi eld. 3. The negat vely charged part cles, st ll in the presence of an electrostat c fi eld, are at racted to the posit vely charged (grounded) collect ng electrodes. 4. The collected dust is knocked off the electrodes, by rapping or vibrat ng, into storage hoppers.

Figure 10 Principle of Electrostatic Precipitator Operation

4th Class • Part A2 Unit 9 • Chapter 42 • Solid & Liquid Thermal Pollutants 38

Figure 11 illustrates the details of an electrostat c precipitator’s basic components. The discharge electrodes are usually rows of wires suspended vert cally in the gas stream, held in posit on by weights at ached to their bot om ends. Collector electrodes are usually steel plates; and the two types of electrodes are arranged in alternat ng rows parallel to the gas stream.

Figure 11 Electrostatic Precipitator

Precipitators operate at 80–99% effi ciency, result ng in a high percentage of part culate removal from the fl ue gases from solid fuel-fi red boilers.

4th Class • Part A2 Unit 9 • Chapter 42 • Solid & Liquid Thermal Pollutants 39

OBJECTIVE 3

Describe how ﬂ yash is removed from a steam generator and the operation of a cooling pond.

FLYASH REMOVAL

In coal-ﬁ red power plants, ﬂ yash removal is typically accomplished by use of either mechanical or electrical collectors, or a combinat on of both, to ensure that no atmospheric pollutants escape. The ash removal system is either hydraulic or pneumat c. A pneumat c transfer disposal system is shown in Figure 12.

Figure 12 Pneumatic Flyash Removal System

In the pneumat c system, fl yash from the dust collectors drops into hoppers, and a high velocity air stream picks up the ash and carries it to an ash storage bin for disposal. The most eff ect ve method of disposal is to bury the fl yash under a suitable landfi ll. At some plants, fl yash is a saleable product to the concrete industry; therefore, the equipment becomes more than just a pollut on control device.

4th Class • Part A2 Unit 9 • Chapter 42 • Solid & Liquid Thermal Pollutants 40

LIQUID THERMAL POLLUTANTS

One of the main factors to be taken into considerat on when choosing the locat on of a power plant is the availability of a suffi cient and suitable supply of cooling water. The condensing steam turbine requires considerable quant t es of cooling water for use in the main condensers. Af er the water travels through the condensers and other related equipment, it is discharged back to its source at an elevated temperature. This process causes thermal pollut on. As water warms, its ability to dissolve oxygen decreases, (the “deaerat on principal”). Reduct ons in the percentage of dissolved oxygen can have lethal eff ects for many kinds of water life. These eff ects have been proven both by experiment and actual observat ons in the vicinity of power plant effl uent discharge. While certain tropical forms of water life may survive in a temperature as high as 35°C (Persian Gulf in August), water life on the North American cont nent can not adapt to such a thermal environment. Especially in a narrow river, the power plant effl uent in moderate quant t es may represent a deadly asphyxiat on snare to the fi sh passing through it. Cooling Ponds Cooling ponds are an eff ect ve means to combat thermal pollut on. If cooling water can be supplied in unlimited quant t es and there is also suffi cient fl at space around the power plant, a cooling pond system might be the answer.

Figure 13 Cooling Pond System

The arrangement shown in Figure 13 employs two ponds in series, although only one might be used at a t me. When both ponds are used in series, then valves A, C, F and G are open, while B, D and E are closed. High temperature water enters through A and its temperature is recorded at thermometer “a”. The water then goes through the sprays of the main pond where it is discharged in a ﬁ ne spray above the surface. The droplets coming into contact with the atmospheric air not only cool down, but also have a chance to enrich themselves in oxygen. A barrier, as shown, retains the water in the pond for a maximum length of t me. The water now exits through valve C, its temperature is recorded at “b” and it enters the suct on of the auxiliary pump. Now, the spraying process is repeated in the auxiliary pond and the temperature is recorded at “c”. The cooled water is discharged into the river or lake.

4th Class • Part A2 Unit 9 • Chapter 42 • Solid & Liquid Thermal Pollutants 41

CHAPTER 42 - QUESTIONS SOLID LIQUID THERMAL POLLUTANTS

1. Wet scrubbers remove ______and ______from a stack. a) solid part cles, airborne liquids b) gases, dust c) carbon monoxide, sulphur dioxide d) unburned gases, large pieces of ﬂ yash

2. The voltage supplied to an electrostat c precipitator is between ______volts DC and ______volts DC. a) 25 000, 50 000 b) 5000, 15 000 c) 10 000, 100 000 d) 1000, 5000

3. Flyash is removed from bag ﬁ lters by a system of ______or compressed air. a) vacuum pumps b) vibrators c) high pressure water d) rappers

4. Flyash is normally less than ______in diameter. a) 1 mm b) 0.3 mm c) 30 microns d) 1000 microns

5. One of the main factors in considering a locat on for a power plant is the availability of a reliable supply of a) natural gas. b) steam. c) manpower. d) cooling water.

Fourth Class • Part A2 Unit 9 • Chapter 42 • Solid & Liquid Thermal Pollutants 42

CHAPTER 42 - ANSWERS SOLID LIQUID THERMAL POLLUTANTS

1. (b)

2. (c)

3. (d)

4. (c)

5. (d)

Fourth Class • Part A2 4th Class • Part A2 C HAPTER 43

Potential Environmental Impact of Liquids

LEARNING OUTCOME

When you complete this chapter you should be able to: Explain the impact of liquid waste on the environment.

LEARNING OBJECTIVES

Here is what you should be able to do when you complete each objective:

1. List the common sources and effects of liquid waste and thermal pollution.

2. Describe the preventive measures that can be taken to prevent liquid pollution.

3. Describe current and alternate methods of liquid waste disposal.

43 Unit 9 • Chapter 43 • Potential Environmental Impact of Liquids 44

4th Class • Part A2 Unit 9 • Chapter 43 • Potential Environmental Impact of Liquids 45

OBJECTIVE 1

List the common sources and effects of liquid waste and thermal pollution.

LIQUID WASTES

Liquids are forms of mat er that fl ow when subjected to a diff erence in pressure or other external force. They are neither solid nor gas, but may become either through a change of state usually brought about by a change in temperature, pressure or react ons with other chemicals. Some pollut ng substances may not be true liquids by exact defi nit on. Instead, they may be carried within a harmless liquid. For the purpose of this chapter, carrier liquids will be considered a hazard unt l the hazardous materials are removed from them. Man has produced some extremely hazardous materials which are not a problem as long as they are properly controlled. Diffi culty arises when these materials are out of control. Loss of control is experienced by a: spill• spill• leak• leak• • uncontrolled release • process excursion Materials become pollutants when they begin to have adverse eff ects on other occupants of the ecosystem. The eff ects of pollutants vary considerably depending on the following: 1. what the pollutant is and how much of it is there 2. what it is aff ect ng 3. what other condit ons complicate or at enuate the eff ects of the pollutant When spills occur, liquids present problems in some of the following ways: • fi re or explosion hazard • toxicity to humans, animals or plant life • fumes, vapours or clouds carried from the spill • react vity problems where hazardous byproducts can be formed The most common way of fi nding a pollut on problem is to witness some adverse condit on becoming evident. In an at empt to fi nd causes for these condit ons, a process of invest gat on is undertaken to eliminate possible causes. Somet mes elaborate test ng and simulat ng is necessary to verify a certain pollutant or combinat on of pollutants is really the cause of the adverse condit on. Evidence of pollut on may show up as a decrease in populat on of one or more given species, or as a sickness, deformity, abnormal behaviors or sudden populat on increase. Thousands of dead fi sh found along the banks of a river are a good indicator that some adverse condit on has occurred. In an at empt to fi nd pollutants before they become a severe problem, many test ng methods have been developed to detect abnormal concentrat ons of materials. There are naturally occurring condit ons that are no fault of man. These condit ons have usually stabilized to the point that species have adapted to the condit on, or rapidly recover from short-term deviat ons. Researchers try to ident fy these natural occurrences and quant fy them as standard background condit ons. On occasion, man-made pollutants may interact with background materials to produce adverse eff ects.

4th Class • Part A2 Unit 9 • Chapter 43 • Potential Environmental Impact of Liquids 46

The keys to limit ng the eff ects of an uncontrolled release of liquid are to contain or immobilize the material and neutralize or lower its hazard potent al. Eff ect ve isolat on may require a specifi c process to be housed in its own room or building with dedicated water supply, vent lat on and effl uent treatment. Containment dikes or catch basins are used to collect and hold any excursion from the process area unt l it is neutralized. Increased training and vigilance on the part of operators is also a most eff ect ve method of pollut on control. Improved process control st ll has a factor of human error or natural catastrophe beyond its control. In many cases, new thought has developed advanced processes using smaller quant t es or less hazardous materials to accomplish the same job, of en more effi ciently. This pract ce may eliminate the problem before it can happen. Some liquids may cause fumes, mists or vapours to be liberated from their surfaces just by exposing them to air or because of heat ng or spraying processes. Of en, these fumes diff use in the air and again become a liquid problem when they mix with precipitat on. Some fumes may have large enough droplets to set le out in close proximity to the source, or, by combinat on with moisture in the air, drop to the ground surface. High vent stacks at empt to scat er the problem over a wider area to reduce localized concentrat ons of pollutants. If these pollutants do not degrade to less harmful materials quickly when they accumulate on the surface, they can be picked up by precipitat on runoff and they become a liquid pollutant. Industrial dusts may become liquid pollutants in a similar way. The most prevalent liquid on earth is water. Nearly three quarters of the earth’s surface is covered with water, most of which is either frozen or salty. Only a small amount (about 0.01%) is fresh water as is observed in lakes or streams, known as surface water. Some water exists in soil and rock format ons; it is known as groundwater. Civilizat on demands a supply of water; these demands are met by the use of surface water and groundwater drawn from wells or springs. The list of uses to which water is applied is very long and diverse. However, the uses can be generally categorized as follows: • Irrigation • Consumption • Washing • Cooling

• Processing Water not used up fi nds its way back to natural water sources in one way or another. The most common way is as effl uent. Effl uent means “out fl ow” and the use of the word usually carries connotat ons of something undesirable. Water alone is not usually considered to be a contaminant, but many of the thousands of materials suspended or dissolved in it are. When water is used for any purpose, it is usually discarded in a state of lower quality than when it was taken in. Potent ally hazardous materials get into water either by direct dumping or by various processes where water is a carrier, a cleaner or an unintent onal solvent of the contaminant. Once undesirable material is dissolved or suspended in water, it becomes a liquid pollutant. Not many liquids on their own become a wide ranging pollut on problem; it is usually when they come in contact with surface or groundwater that pollutants are spread or transported. As a result, containment of a spill is vital before it reaches a watercourse. The oil-soaked bodies of shore birds are immediate evidence of the problems created by an oil spill. Other pollut on problems may be far slower and more subtle in their work of destruct on. For example, one may wonder how a small leak in the air condit oning system in a car (Few modern A/C systems use CFC based refrigerants today) could contribute to increased skin cancer on a beach half a world away. Every kilogram of CFC refrigerant that is released has an impact on the ozone layer. Whether released by industrial plants or individuals, the cumulat ve eff ect is the same. Following are some of the common tests applied to water to determine contaminat on. The biochemical oxygen demand (BOD) test is used to determine the level of organic contaminat on such as from domest c sewage effl uent. The test determines how much the natural oxygen content of water is taxed by the incoming pollut on. Chemical oxygen demand (COD) also determines how much natural oxygen is consumed by nonorganic pollut on. pH test ng of effl uent can determine whether the fl ow contains excessive amounts of alkaline or acidic materials. Turbidity test ng determines how much suspended material is being carried in water. Colour test ng indicates the presence of certain materials dissolved in water. Some turbidity and colour is natural and seasonally occurring, such as the mud in water during high fl ows or the brown colour in spring runoff produced from de- composing leaves and grass. Sophist cated equipment and procedures are necessary to detect even minute traces of hazardous materials, such as: pest cides, metals and carcinogens. Some hazardous materials can be detected in parts per trillion (ppt). Although an extremely small amount, concentrat on through the food web, from simple to more complex life forms, places humans in a high risk group.

4th Class • Part A2 Unit 9 • Chapter 43 • Potential Environmental Impact of Liquids 47

Excessive biological act vity in a water source can reduce its quality. Several factors determine the growth rate of plants. Sunlight, favourable temperature and nutrients are necessary to have sustained growth. Do- mest c sewage carries the two main growth nutrients required by plants: nitrates and phosphates. Nitrates are a natural byproduct of sewage digest on. Phosphates are derived from phosphate based detergents, fert lizers and other industrial and agricultural sources.

THERMAL POLLUTION

When water temperatures are warm enough with a good supply of sunlight, plants ut lize the enriched supply of nutrients to cause rapid growth of biological contaminat ons. The rapid growth can cause the receiving water to be strangled with algae and weeds. The use of reduced phosphate detergents has helped to slow some of the stream fouling, but phosphate extract on is being introduced to further reduce concentrat ons from domest c sewage treatment plants. Elevated temperatures are common below sewer discharge points. Some stretches of rivers or lakes now remain open year round and the normal migrat on pat erns of waterfowl are being interrupted, apparently because of the accessibility of open water. Other contributors to warm water discharge are cooling towers for thermal generat ng stat ons and industries with high cooling loads, such as: steel mills, pulp mills, chemical processing plants and refrigerat on or air condit oning cooling systems. A side eff ect to increased temperature is the inability of water to maintain enough dissolved oxygen to support the animal life it contains, a problem known as deaerat on. For this reason, temperatures of discharging water must be reduced suffi ciently before contact ng the receiving water so that no threat is posed to fi sh or other organisms living in the lake or stream. The amount of temperature dilut on that occurs when the fl ows meet is governed by the diff erence in temperature of the two streams and the volume of the receiving stream compared to that of the effl uent. In t me, the stream will approach the average ambient temperature (i.e. relat ve humidity) of the surroundings. For streams fed by mountain runoff , this equalizat on means a further increase in temperature during summer months, but in winter t me or for slow meandering streams, it usually means the stream will cool off again. Thermal strat fi cat on can occur in lakes where wind and currents are not suffi cient to cause mixing of incoming warm water; strat fi cat on may trap other pollutants in layers at varying depths in a lake. These thermally separated layers are called thermoclines; this condit on can lead to lower regions of lakes being starved for oxygen. The simplest solut on to thermal pollut on is the use of retent on ponds which bring effl uent closer to ambient temperature before release into the receiving waterway. However, space restrict ons may call for spray cooling or cooling towers if there is not enough room for a large cooling pond.

4th Class • Part A2 Unit 9 • Chapter 43 • Potential Environmental Impact of Liquids 48

OBJECTIVE 2

Describe the preventive measures that can be taken to prevent liquid pollution.

PREVENTIVE MEASURES

The following are prevent ve measures that can be taken to combat liquid pollut on: • pH control • Set ling ponds • Vacuum fi lters • Grease traps pH Control Effl uents dumped into an exist ng stream should have a similar pH value to the receiving water. A deviat on from this exist ng pH can have detrimental eff ects on stream life. Industries generat ng strongly alkaline or acidic effl uents must neutralize the material before it is released. Even within cit es or municipalit es with common sewer collect on systems, companies are required to maintain their waste water discharge between given pH values. Sewer line sampling monitors can detect sources of deviat on and penalt es are assessed against the of- fenders. Responsible companies monitor their effl uent, (where their business has a potent al to cause deviat ons) and neutralize any excursion before it enters the sewer system or waterway. With increased focus on environmental issues, companies that require large quant t es of water of en have their own dedicated supply of water. Permits are issued to the company to use the water; strict effl uent guidelines are established. The company must do effl uent monitoring to prove that the system has not exceeded the guidelines during operat on. One common method of control uses a dilut on tank or pond. If a surge of acid is detected by the monitors, a chemical pump is act vated to add enough alkaline material to bring the fl ow back to the desired pH value. If the system shows an alkaline deviat on, an acid pump is used to bring the pH under control. When the fl ow is neutralized, it can be discharged. Settling Ponds Some industries discharge part culate laden water from the process. In years past, it was common pract ce to dump the discharge into the nearest water course and forget about it. The downstream eff ects did not concern management. But through responsible management and environmental awareness, this pract ce is rapidly being remedied. One method used to improve the problem is the use of set ling ponds or tanks. Effl uent is allowed to fl ow slowly through a quiet pond where part culate mat er set les to the bot om and the clean effl uent is dumped into the water course. Very fi ne part cles, colloidal part cles, are so small that they will not set le out when water is allowed to stand quietly. Industries generat ng large quant t es of colloidal material have to use coagulant aids to help cluster these small part cles together so that they will set le. This process is also used in some water supplies to cause accelerated set ling of fi ne material. Occasionally, effl uent can be cleaned up to the point that part of it is cycled back to the water intake rather than dumped. This process also reduces the amount of water required from the source. Some plants, notably in the pulp and paper industry, are now being designed so that most or all of their waste water can be re-used, producing “zero effl uent.” A side eff ect of cleaner effl uent is that some of the set led material represent ng previous losses may be returned to the process where it is used rather than lost. This occurrence could represent increased profi ts because of improved effi ciency.

4th Class • Part A2 Unit 9 • Chapter 43 • Potential Environmental Impact of Liquids 49

Vacuum Filters Another method of part cle capture is the vacuum fi lter. Water fl ows inward through the very fi ne mesh of a fi lter medium formed into the shape of a horizontal cylinder. The cylinder is part ally submerged in an effl uent tank; it slowly rotates and material gathers on the outside of the fi lter. A vacuum is applied to the inside of the cylinder causing dewatering of the fi ltered part cles as they rotate above the water line. The material, caked on the fi lter, is then scraped or blown off by an air jet and collected. The cleaned fi lter then rotates down into the effl uent tank again to fi lter more water. The fi ltered water on the inside of the cylinder is pumped away for recycling or it may be dumped if no further treatment is required. Grease Traps Some plant effl uent may contain materials that fl oat on the surface of the water, such as oils and greases. When the density of the materials varies considerably from that of water, the material fl oats readily and can be separated by a simple skimming process which may be accomplished using the grease trap principle. In a grease trap, water carrying oil or grease enters the chamber where a calm area allows separat on to occur by gravity. The grease fl oats on the top and clean water is conducted away through the bot om outlet pipe. The grease or oil is then removed as it accumulates on top of the water. Large units may have several chambers in series to assure bet er capture. A surface skimming mechanism may also be used for cont nuous oil extract on. When the density diff erence between the water and the material is minimal or when the material is emulsifi ed with the water, a long calm period may not be enough to cause separat on. In some cases, addit ves may be used to help break the emulsion so that separat on can occur by gravity. Another method is to use centrifugal separat on. Centrifuges spin material at high speed and have the eff ect of mult plying gravity hundreds or even thousands of t mes. This process is similar to the way that cream is separated from milk in a separator. Some chemicals, such as agricultural chemicals and soluble industrial residue, fi nd their way into the water course. Set ling, skimming, fi ltrat on and addit ves have no eff ect on these pollutants. Of en, these chemicals are in very small amounts, but have a cumulat ve eff ect on stream life.

4th Class • Part A2 Unit 9 • Chapter 43 • Potential Environmental Impact of Liquids 50

OBJECTIVE 3

Describe current and alternate methods of liquid waste disposal.

LIQUID WASTE DISPOSAL

Several methods of disposal have been used through the years. Landfi ll was the quick and easy way of hiding thousands of drums of hazardous material. The risk was not well assessed as numerous old landfi ll sites are now leaking dangerous materials into waterways. Encasement and burial at sea has been used, but the hazard is not gone, just moved elsewhere. Deep well inject on has been used to dispense hazardous liquids into subsurface rock format ons. This pract ce is a vast improvement over landfi lls, but again the hazard has only changed locat on. The only certain way to destroy many materials is through total burning which can be accomplished in high temperature incinerators. Even in this process, close control and vigilance is necessary to assure complete destruct on. Crude oil and all its derivat ves, gasoline, LPG, solvents, fuel oils, lubricat ng oils, heat transfer oils and hundreds of liquid products each present a hazard if a spill or leak occurs. Spills in lakes, streams, oceans or environmentally fragile areas cause a news media frenzy even if relat vely small amounts are involved. The fi re or explosion hazard is more visible when a transport system, such as a truck, rail car, pipeline or fl oat ng tanker, encounters diffi culty in a populated area. Evacuat on of residents to a safe distance is necessary because of toxicity or the possibility of an explosion. Hast ly constructed earth berms or sandbag dikes or booms can limit the spread of the problem. Sewer systems involved with a spill must be blocked to stop the spread of the material. For example, a gasoline tank truck, rolled over in an urban intersect on, could spill fuel into storm and sanitary sewer systems. Unchecked, explosive vapour concentrat ons could travel several kilometres underground in a relat vely short t me. A spark from any source could create a massive explosion and fi re. The thrill-seeker cannot be ruled out as a source of ignit on in these situat ons. Pipelines carrying other than petroleum products are usually of minimal length and present a spill hazard only in the plant site or immediate surrounding area. Liquid spills, other than pipelines, are then restricted to process storage and transportat on leaks. Pressurized tanks holding liquid chlorine, liquid anhydrous ammonia, various refrigerants and other liquefi ed gases pose only a short term problem as a liquid because they quickly turn to vapour when released

from the tank. Liquefi ed N2, O2 and other liquefi ed gases create short term cryogenic hazards if they are depressurized due to the extremely cold temperature at which they are stored. Acid spills are more common occurrences. The type, quant ty and locat on of the spill are factors that govern the severity of the incident. Some acids, such as sulphuric or hydrochloric (somet mes called muriat c acid), can become neutralized quickly by high calcium or magnesium soil with not much impact on the surroundings. In the mid-1980s, Denver, Colorado experienced a train wreck which involved a fume producing acid (concentrated nitric acid). A large evacuat on was necessary because of the hazardous cloud which developed. Emergency workers had diffi culty approaching the area and fi nally eff ected neutralizat on by using rotary snow blowers to throw soda ash onto the spill from a safe distance away. Other acids and liquids may present a hazard because of the toxic cloud produced. Each substance has its own peculiarity and must be handled with the best cleanup data available for that material. For example, if picric acid solut on spills on concrete or calcium soil and goes un-neutralized, calcium picrate is formed; when the spill dries up, calcium picrate crystals present a severe explosion hazard. Just the foot all of a person can detonate this extremely shock sensit ve material. For this reason, we should not assume that just because a spill seems to have dried up, the problem is gone. A spill of metallic mercury on the ground may appear to be minimal unt l probing deeper. The high relat ve density of mercury causes it to penetrate deeply and even fl oat soil and rock on its surface. Mercury does not “wet” its environment, so capillary act on does not draw it into the ground. Other heavy liquids with wet ng act on can rapidly move through the soil, displacing water and penetrat ng down unt l they reach bedrock or an impervious layer in the soil. The viscosity of the liquid also governs the rate of progress from a spill locat on.

4th Class • Part A2 Unit 9 • Chapter 43 • Potential Environmental Impact of Liquids 51

The cumulat ve total of creosote spilled over many years at a wood treatment plant in Calgary recently became evident when a layer of the heavy liquid was found lying at the bot om of deep pools in the Bow River. The material had penetrated to an impervious layer, then slowly migrated horizontally unt l it oozed into the river bed and ﬂ owed into the deep pools. Each liquid must be handled according to informat on on current material safety data sheets. Neutralizing cleanup and disposal procedures should be followed. Appropriate clothing, tools and containers must be employed and personnel working on the cleanup must be thoroughly decontaminated and monitored for eﬀ ects—even af er the cleanup is complete.

4th Class • Part A2 Unit 9 • Chapter 43 • Potential Environmental Impact of Liquids 52

4th Class • Part A2 Unit 9 • Chapter 43 • Potential Environmental Impact of Liquids 53

CHAPTER 43 - QUESTIONS POTENTIAL ENVIRONMENTAL IMPACT OF LIQUIDS

1. One of the main growth nutrients carried by domest c sewage is a) calcium. b) granite. c) phosphates. d) mercury.

2. Thermally separated layers in lakes are called a) thermopiles. b) thermistors. c) thermocouples. d) thermoclines.

3. Before being released, strongly alkaline or acidic eﬄ uents from an industry must be a) recorded in a log book. b) quant ﬁ ed. c) neutralized. d) blended.

4. Chemical emulsions can be removed by a) set ling. b) addit ves. c) centrifugal act on. d) all of the above.

5. Acids and other hazardous liquid spills may present a clean-up hazard because of a) the toxic cloud produced. b) the sheer volumes produced. c) unknown types produced. d) limited manpower.

Fourth Class • Part A2 Unit 9 • Chapter 43 • Potential Environmental Impact of Liquids 54

CHAPTER 43 - ANSWERS POTENTIAL ENVIRONMENTAL IMPACT OF LIQUIDS

1. (c)

2. (d)

3. (c)

4. (d)

5. (a)

Fourth Class • Part A2 4th Class • Part A2 C HAPTER 44

Potential Environmental Impact of Gases

LEARNING OUTCOME

When you complete this chapter you should be able to: Explain the impact of gases and vapours on the environment.

LEARNING OBJECTIVES

Here is what you should be able to do when you complete each objective:

1. List and identify the sources of the common domestic, industrial, and naturally occurring gases and vapours that have environmental impact.

2. Identify the various gases that create the formation of acid rain.

3. Discuss the makeup and effect of greenhouse gases.

4. Describe current and alternative methods of reducing gas and vapour pollution.

55 Unit 9 • Chapter 44 • Potential Environmental Impact of Gases 56

4th Class • Part A2 Unit 9 • Chapter 44 • Potential Environmental Impact of Gases 57

OBJECTIVE 1

List and identify the sources of the common domestic, industrial, and naturally occurring gases and vapours that have environmental impact.

GASEOUS POLLUTANTS

The gases and vapours responsible for environmental pollut on are numerous; the full impact of individual gases in combinat on with other gases and elements is not thoroughly understood. This chapter will emphasize gases responsible for acid rain and the so called “greenhouse gases”. Common gaseous pollutants are listed in Table 1.

Table 1 Common Gaseous Pollutants

Pollutant Problem Produced

Sulphur Oxides Responsible for about 70% of acid rain

Responsible for about 30% of acid rain, produce smog and trap Nitrogen Oxides heat that increase greenhouse effect

The main heat trapping gas responsible for the Carbon Dioxide greenhouse effect

Carbon Monoxide A very poisonous gas

Methane A greenhouse gas

Chloroﬂ uorocarbons Destroy ozone after rising to the stratosphere

Hazardous to health and corrosive to many materials Ozone in the lower atmosphere

HOW POLLUTANT GASES ARE PRODUCED

Sulphur Oxides

Sulphur oxides [sulphur dioxide (SO2) and sulphur trioxide (SO3)] are produced whenever a fuel containing sulphur is burned. Sulphur is contained in most coals and fuel oils used by industry. Low sulphur fuels are much more expensive than high sulphur fuels; hence, the use of high sulphur fuel has a great advantage from an economic standpoint. Some mining and smelt ng operat ons discharge great quant t es of sulphur oxides to the atmosphere. Pulp mills are also a source. Sulphur oxides also enter the atmosphere from natural sources. Some hot springs emit them and, during a volcanic erupt on, millions of tons of sulphur oxides may be introduced into the atmosphere.

4th Class • Part A2 Unit 9 • Chapter 44 • Potential Environmental Impact of Gases 58

Nitrogen Oxides (NOx) Oxygen and nitrogen react to form the following compounds: • Nitric oxide (NO)

• Nitrous oxide (N2O)

• Nitrogen dioxide (NO2)

• Nitrogen trioxide (N2O3)

• Nitrogen pentoxide (N2O5)

These gases have varying degrees of stability and are referred to collect vely as NOx. In many cases, more than one of these gases is given oﬀ by a single source.

Nitric oxide (NO) and nitrogen dioxide (NO) are the major NOx gases formed in the combust on process (e.g. in boilers and internal combust on engines.) NO2 format on is favoured at temperatures below 1000 K, while NO format on is favoured above that temperature. The total NOx formed during the combust on process increases with an increase in temperature and excess O2 in the burning zone. Usually, more than 90% of the NOx formed in the combust on zone is NO. Some of this NO then combines with the excess O2 to form NO2. This lat er react on occurs as the temperature of the exhaust gases drops below 1000K. The rate of react on slows unt l it ceases as temperatures approach ambient and some of the NO will decompose back to O2 and N2 before being discharged from the exhaust system. Some decomposit on of NO does occur slowly in the atmosphere.

NO and NO2 convert back and forth as components in the photochemical smog problem. When water is added, NO2 is a contributor to the format on of nitric acid (HNO3) which accounts for roughly 30% of the acidity in most acid rain.

Carbon Dioxide (CO2)

Any t me a fossil fuel is burned, CO2 is produced. It is one of the most important gases on our planet because all plant life depends on it. It’s concentrat on in the atmosphere may be connected to variabil- ity of weather pat erns. All animal life produces it and forest fi res are also large contributors of this gas. CO2 cannot be eliminated from the combust on process as long as the fuel contains carbon. Carbon Monoxide (CO) Carbon monoxide, an extremely poisonous gas, is produced in any combust on process that does not have suffi cient air. One of the greatest producers of CO is the internal combust on engine because there is insuffi cient t me for complete combust on to take place. Overloaded boilers, forest fi res and burning refuse can also produce CO.

Methane (CH4) Methane, the main component of natural gas, is a product of decomposed organic mat er such as in swamps and sewage lagoons. It is also formed in the digest on processes of animals, especially ruminants

Ozone (O3)

Ozone is a form of oxygen which contains three oxygen atoms (O3). In the upper atmosphere, O3 is formed by the act on of high intensity sunlight on O2. Only a small percentage of the result ng ozone works down to the lower atmosphere and ground level. The upper level ozone is of crit cal importance as it shields the earth’s surface from hazardous ultraviolet and other radiat on. CFCs not only produce a greenhouse eﬀ ect, but they also destroy this upper level ozone layer. Ozone is also produced in the lower atmosphere by electrical arcs such as brushes in electric motors, arc

welding and lightning. The act on of sunlight on a mixture of NOx and volat le organic compounds (frequently referred to as photochemical smog) also produces ozone in this zone.

4th Class • Part A2 Unit 9 • Chapter 44 • Potential Environmental Impact of Gases 59

OBJECTIVE 2

Identify the various gases that create the formation of acid rain.

ACID RAIN

Acid rain is formed when NOx and SO2 in the atmosphere undergo complex and lit le understood changes in a medium of water vapour and sunlight. The result ng interact on produces sulphuric and nitric acids which precipitate with rain, snow, hail, sleet, fog and, in some cases, deposit on the ground in dry form. Since the acid can come down in either a wet or dry form, the condit on is referred to as acid deposit on. The contaminants which form acid rain can be carried great distances from their source. The smogs of California deposit their pollut on in the snows of Colorado. The snows melt and the acid runoff leaches hazardous minerals from the mountains. Some of the meltwater returns in the rivers as a water source for California. Effects of Acid Rain on Lakes and Rivers When the acid level increases in a lake, microscopic life dies and interference with the reproduct ve cycle of aquat c animals becomes evident. Metals leached from the soil, in higher concentrat ons than normal, restrict the ability of some animals to breathe properly. For example, fi sh gills become fouled and the fi sh suff ocate. The fi rst melt of spring releases a surge of acid from the snow. In this toxic meltwater, many eggs and hatchlings cannot survive. In lakes and streams, clams, snails and crayfi sh are fi rst to die, followed by insects such as the mayfl y, dragonfl y, damsel fl y and other larvae. Then, by acid at ack or starvat on, the amphibians and fi sh die off . In Ontario, Quebec, and the Northeastern United States about 2500 lakes per year are dying. In other parts of the world, acid rain is also a problem; for instance, in Sweden, acid rain is blamed for as many as 20 000 dead lakes. Effects of Acid Rain on Plant Life Forests are being killed by acid rain. Some problems are directly created by acid deposit on on the leaves; however, the greater problem appears to be the eff ects of acid deposit on on the soil. Nutrients normally dissolved and used by tree roots have either been leached away or lost, or toxic quant t es of other materials have been taken into solut on by the acidic groundwater. These materials interfere with the plants ability to acquire proper nourishment. The result is that plants weaken and die. We must realize that these eff ects impact on all plant life, not just the forests.

4th Class • Part A2 Unit 9 • Chapter 44 • Potential Environmental Impact of Gases 60

Effects of Acid Rain on People Acid rain causes the following economic impacts: • The eff ect on maple trees in Eastern Canada and the Northeastern United States is result ng in a diminishing maple sugar industry • Logging operat ons are being curtailed and reforestat on eff orts are fut le in heavily impacted areas • Fruit trees produce substandard fruit, or die off • Vegetables become discoloured and unhealthy, making them diffi cult to market • Fish from certain waters may be banned for human consumpt on due to the impact of acid rain on the watercourse The full extent of the economic impact of acid rain is diffi cult to assess, but the above examples provide some indicator of how widespread the impact may be. The eff ects of acid rain on human health are varied. Acid water can leach toxic heavy metals from the soil, as well as copper and lead from plumbing systems causing a toxic hazard to humans. Studies suggest that sulphur dioxide in the air causes bronchit s, emphysema and a strain on the heart and circulatory system. Some recent reports suggest a possible relat onship between acid haze and an increase in some types of cancer, probably due to the blocking of sunlight at the wavelength needed by humans to produce Vitamin D in the body.

4th Class • Part A2 Unit 9 • Chapter 44 • Potential Environmental Impact of Gases 61

OBJECTIVE 3

Discuss the makeup and effect of greenhouse gases.

GREENHOUSE GASES

Water Vapour The water vapour content of the air depends on its temperature. The warmer the air, the more moisture it can hold and water vapour can hold a tremendous amount of heat. By itself, it cannot produce a “greenhouse eff ect”, but water vapour amplifi es the eff ect of other gases. Carbon Dioxide Carbon dioxide is the principal greenhouse gas account ng for about 50% of the total problem. The amount of CO2 in the atmosphere is est mated to be increasing by about 0.5% per year. That may not sound like much, but it means an addit on of 4.5 x 109 tonnes of carbon/year. About one-half of that carbon is used by plants or absorbed by the oceans; the rest stays in the atmosphere. Methane Methane is believed to comprise about 20% of the greenhouse gas problem. The carbon in methane makes it a heat trapping gas like carbon dioxide and, in fact, it has about thirty t mes the act vity of CO2. Although the greenhouse eff ect of methane is much higher than that of CO2, methane has an est mated half-life of about 7–10 years compared to 500 years for CO2. Some scient sts are concerned that methane is responsible for prevent ng the atmosphere from ridding itself of CFCs. Chlorofluorocarbons CFCs contribute about 15% of the greenhouse gas problem. They are very stable man-made compounds that help destroy ozone af er they reach the stratosphere. Ozone shields the planet from harmful cosmic radia- t on that can cause cancer. In the lower atmosphere, CFCs are able to absorb infrared rays about 10 000 t mes as eff ect vely as carbon dioxide making them powerful greenhouse gases. Nitrous Oxide

Nitrous oxide (N2O) (laughing gas) is very stable, last ng an est mated 150 years or longer in the atmosphere. It is one of the gases contribut ng to the greenhouse eff ect, account ng for about 10% of the problem. When it rises to the stratosphere, it is responsible for destroying ozone. Nitrous oxide levels in the atmosphere are est mated to be rising at a rate of 0.25% per year. Ozone Ozone in the lower atmosphere is a pollutant responsible for about 5% of the greenhouse eff ect. This gas is an irritant to the eyes and respiratory system. In the stratosphere, ozone is very benefi cial because it blocks out cancer-causing ultra violet radiat on. In the lower atmosphere, it is a hazard in that it is dangerous to health and corrosive to many products including rubber.

4th Class • Part A2 Unit 9 • Chapter 44 • Potential Environmental Impact of Gases 62

THE GREENHOUSE EFFECT

The earth has existed for a considerable period of t me in an equilibrium state, where the amount of energy received from the sun is balanced with an equal amount of energy radiated from the earth back into space. Some high energy radiat on incoming from the sun (such as X-rays and high energy ultraviolet light) encounters upper atmosphere ozone and is absorbed or reﬂ ected back into space. Slightly lower energy radiat on (such as visible light and infrared radiat on) passes through the atmosphere to ground level. At ground level, it is largely absorbed and eventually radiated back towards space at a lower energy level. The greenhouse gases allow high energy radiat on to pass through, but restrict the transmission of the lower energy radiat on that would normally move towards space.

Figure 1 Greenhouse Effect

The relat vely stable concentrat on of greenhouse gases accounts for the earth’s overall average temperature being reasonably stable for many centuries. With an increase in greenhouse gases, more outgoing radiat on is trapped by the atmosphere causing an increase in average temperatures. The increasing average temperatures can cause: • drought condit ons in presently fert le growing regions. • polar ice masses to be reduced, causing rising ocean levels which would impact severely on man’s act vit es. There is considerable controversy regarding the overall eﬀ ect of changes in global temperatures and how ult mately this will aﬀ ect the world’s environment.

4th Class • Part A2 Unit 9 • Chapter 44 • Potential Environmental Impact of Gases 63

OBJECTIVE 4

Describe current and alternative methods of reducing gas and vapour pollution.

CURRENTLY USED METHODS

Sulphur Dioxide Removal Many plants have met environmental regulat ons by ut lizing naturally occurring low sulphur, western source coal. Others have achieved the required emission levels by installing fl ue gas desulphurizat on (FGD) systems. The FGD systems are categorized as nonregenerable or regenerable. Nonregenerable systems are the best developed and most widely applied. Most of the processes involve wet scrubbing of the combust on fl ue gas in a gas-liquid contactor using the following: • Lime or limestone • Alkaline fl y ash with supplemental lime or limestone • Sodium carbonate and dilute sulphuric acid A typical limestone system is shown in Figure 2. Effi ciency for these systems can be as high as 90 to 95% with combust on gases containing up to 5000 ppm SO2.

Figure 2 Limestone Wet Scrubbing System

(Courtesy of Babcock & Wilcox)

4th Class • Part A2 Unit 9 • Chapter 44 • Potential Environmental Impact of Gases 64

A regenerable FGD system is shown in Figure 3; its advantage is that the SO2 recovered is converted into a marketable by-product such as sulphur, sulphuric acid or liquid SO2. These units have a higher init al cost, some of which may be recovered by the sale of the by-product. Some of these processes have been installed in systems over 100 MW capacity. Wet scrubbing units use a solut on of magnesium oxide to remove the SO2.

Figure 3 Regenerable Wet Scrubbing System

(Courtesy of Babcock & Wilcox) Oxides of Nitrogen

Unlike sulphur oxides, which are formed only from the sulphur contained in the fuel, nitrogen oxides (NOx) are formed from both fuel-bound nitrogen and that contained in the combust on air introduced into the furnace. The NOx produced from nitrogen in the fuel is dependent on the: • Percent of nitrogen in the fuel • React vity of nitrogen compounds contained in the fuel • Oxygen availability in the combust on zone

The conversion of nitrogen in the air to NOx is highly dependent on temperature. The format on of NOx proceeds rapidly at combust on zone temperatures in excess of 1650°C. By maintaining the combust on temperature below 1650°C, the NOx emission can be reduced.

The following two words are methods used to reduce the format on of NOx: • Two stage combust on • Low excess air operat on • Gas recirculat on • Dual register burner Two Stage Combustion

This is the most eﬀ ect ve method of reducing the format on of NOx. Init al combust on takes place in a fuel rich environment. The remaining air for combust on, and excess air, is introduced in an area away from the init al combust on zone to complete the combust on process. By reducing the availability of oxygen in the combust on zone, the fuel-bound nitrogen is less likely to be converted to NOx. This process also prolongs combust on, thus reducing the ﬂ ame temperature which reduces the conversion of atmospheric nitrogen to NOx.

4th Class • Part A2 Unit 9 • Chapter 44 • Potential Environmental Impact of Gases 65

Low Excess Air Operation By fi ring with the least amount of excess air possible, while st ll maintaining carbon losses to a minimum, the O2 available is reduced. This means that the amount of fuel-bound nitrogen converted to NOx is reduced. The amount of NOx formed due to nitrogen in the air is also reduced due to the lower availability of O2 and reduced quant t es of nitrogen entering the combust on zone. Gas Recirculation On gas and oil-fi red units, gas recirculat on into the combust on zone provides dilut on of the combust on air, thus prolonging combust on and reducing the fl ame temperature. Since all the NOx from gas-fi ring and 50% of the NOx from oil-fi ring are produced by thermally converted nitrogen, gas recirculat on to the burners appears to be eff ect ve on overall NOx reduct on. With coal fi ring, the eff ect of this method of NOx reduct on is minimal. Dual Register Burner Figure 4 shows a dual register burner developed by Babcock and Wilcox. It limits turbulence, thus reducing the peak fl ame temperature at the burner. The reduced turbulence also delays combust on, producing a slower burning fl ame in the combust on zone. While maintaining stable effi cient operat on, the burner has demonstrated a 50% reduct on in NOx over previous high turbulence burner designs. Ut lizing this type of burner in conjunct on with low excess air operat on has produced an acceptable reduct on of NOx.

Figure 4 Dual Register Burner

(Courtesy of Babcock & Wilcox) Diluting Pollution To reduce pollut on in the immediate area of stack emissions, super stacks have been built. The local pollu- t on has been reduced because the emissions are discharged higher into the atmosphere. This “solut on” to the problem has only spread the problem over a greater area; it has not reduced the amount of gases that have adverse environmental impacts.

4th Class • Part A2 Unit 9 • Chapter 44 • Potential Environmental Impact of Gases 66

ALTERNATIVE METHODS

Solar energy is a nonpollut ng source of energy. The energy is free, but harnessing it is expensive. It was reported in 1989 in the New York Times that “the conversion of solar energy to electrical power could become comparable in eﬃ ciency to convent onal power generat on”. Tremendous gains in the development of solar cells have taken place in the last few years and, if fossil fuel prices dictate, more research will take place. It is possible to reduce SO2 and NOx emissions to an acceptable level, if laws are made strict enough. But, as long as we burn fossil fuels, the amount of CO2 in the atmosphere will cont nue to increase. Hydrogen Fuel Using hydrogen as a fuel is not a new idea, but the methods of product on, storage and handling must be reﬁ ned before it receives wide acceptance. Hydrogen is an ideal fuel in that the only product of combust on is water vapour. A kilogram of hydrogen delivers about 4.25 t mes more energy than a kilogram of carbon when burned completely. Hydrogen is of en stored in cryogenic containers to keep the storage pressures from being unreasonably high. Even a small hydrogen leak poses an explosion hazard due to the wide range in the explosive limits. Hydrogen rich fuels, such as methane and propane, contribute less CO2 pollut on to the air than regular gasoline or diesel fuels. A greater usage of these fuels would reduce equipment costs, but the higher demand would likely result in a price increase. Carbon Dioxide Removal from the Atmosphere

Many scient sts are urging mass reforestat on of the earth. Trees require CO2; it is est mated that 10 000 000 acres of new forest would use up all the CO2 that would be emit ed by power plants in the next 10 years.

There are solvents that can remove CO2 from power plant emissions, but the process is cost prohibit ve at the present t me. The CO2 removed using solvents could be injected into oil ﬁ elds to maintain format on pressure. CFCs These man made substances are being phased out and alternate refrigerants are used. Possibly, refrigerat on systems like the ammonia and lithium bromide absorpt on systems will be researched and developed more extensively. These systems produce no pollut on and can be energy savers by making use of low level waste otherwise dumped into the atmosphere.

4th Class • Part A2 Unit 9 • Chapter 44 Potential Environmental Impact of Gases 67

CHAPTER 44 - QUESTIONS POTENTIAL ENVIRONMENTAL IMPACT OF GASES

1. The upper level gas that shields the earth from ultraviolet radiat on is a) oxygen. b) ozone. c) carbon dioxide. d) nitrogen.

2. A gaseous pollutant that contributes to acid rain is a) carbon dioxide. b) ozone. c) CFCs. d) sulphur dioxide.

3. In order for acid rain to be formed a) atmospheric temperatures must be correct. b) water vapour must be present. c) sunlight must be present. d) water vapour and sunlight must be present.

4. Chloroﬂ uorocarbons a) contribute to the acid rain problem. b) are the main heat trapping gases responsible for the greenhouse eﬀ ect. c) destroy ozone af er they reach the stratosphere. d) combine with ozone to produce toxins.

5. The conversion of nitrogen to nitrogen oxides occurs at temperatures above a) 1000°C. b) 1225°C. c) 1450°C. d) 1650°C.

6. Greenhouse gases allow the passage of ______energy radiat on. a) high b) low c) potent al d) heat

Fourth Class • Part A2 Unit 9 • Chapter 44 • Potential Environmental Impact of Gases 68

CHAPTER 44 - ANSWERS POTENTIAL ENVIRONMENTAL IMPACT OF GASES

1. (b)

2. (d)

3. (d)

4. (c)

5. (d)

6. (a)

Fourth Class • Part A2 4th Class • Part A2 C HAPTER 45

Potential Environmental Impact of Operating Facilities

LEARNING OUTCOME

When you complete this chapter you should be able to: Explain the environmental impacts of industrial operating facilities.

LEARNING OBJECTIVES

Here is what you should be able to do when you complete each objective:

1. List the types of impacts that operating facilities can have on the environment.

2. Discuss how plant personnel are alerted to the environmental problems of operating facilities.

3. Discuss the overall noise implications of operating facilities.

4. Discuss and give examples of the importance of “attitude” in limiting the environmental impact of operating facilities.

5. Describe the long term environmental impact after the decommissioning and abandonment of operating facilities.

69 Unit 9 • Chapter 45 • Potential Environmental Impact of Operating Facilities 70

4th Class • Part A2 Unit 9 • Chapter 45 • Potential Environmental Impact of Operating Facilities 71

OBJECTIVE 1

List the types of impacts that operating facilities can have on the environment.

An operat ng facility may adversely impact the environment as a result of several diff erent operat onal characterist cs, such as: • incomplete combust on. • planned or unplanned vapour release. • hazardous or contaminated liquid release. • airborne part culate mat er. • solid waste. • noise generat on. odour.• odour.• • surface disturbance. These excursions have immediate and long-term eff ects on the environment. Incomplete combust on, for example, discharges an excessive amount of CO which aff ects the atmosphere. Also, unless steps are taken to prevent it, a form of NOX is produced during all combust on processes. Under certain condit ons, this NOX will produce ozone, visible as a brown haze in the atmosphere. When combined with rain, CO and NOX form acids which may be harmful to the soil and waters upon which they fall. Noise pollut on, on the other hand, will have a more immediate eff ect. Not only is it an annoyance, but, over a period of t me, personnel without proper protect on will experience hearing loss also. Pollut on in the liquid form has the potent al for long-term soil as well as groundwater contaminat on. Sus- tained pollut on over a long period of t me may render the soil hazardous and unproduct ve. Contaminat on of groundwater could introduce excessive levels of undesirable components into drinking water or irrigat on streams. During the normal course of operat on, or a plant upset which releases material to the atmosphere, a very dist nct and far-reaching odour may result. For example, the odours of hydrogen sulphide, mercaptans or C5+ streams are very not ceable. These products may or may not be harmful, depending on the concentrat on level; however, the presence of any of these products in the atmosphere raises a concern for the public exposed to the odours. Care must be taken in handling any material which may produce odours. For example, when inject ng mercaptan into a load of propane, or offl oading mercaptan, ensure adequate measures are taken to minimize the chance of a release.

4th Class • Part A2 Unit 9 • Chapter 45 • Potential Environmental Impact of Operating Facilities 72

OBJECTIVE 2

Discuss how plant personnel are alerted to environmental problems of operating facilities.

RECOGNITION

Diff erent methods may be employed to alert operators to the existence of a problem. For example, instrumentat on may be used to indicate a change in: • level. • fl ow. pressure.• pressure.• • incomplete combust on. Other equipment may be used to monitor for seal leaks, the integrity of crit cal equipment or contaminated sewers. The operat ng and maintenance personnel provide the most eff ect ve means of recognit on of problems. Through frequent and comprehensive checks, early recognit on of spills, leaks or any unplanned condit on can minimize the impact. It is important that all employees know the characterist cs of the materials with which they are dealing in order to spot excursions and protect themselves and the environment. As a person becomes int mately familiar with the facility in which he/she works, various sights, sounds and smells become standard or normal. One way to detect possible trouble is to recognize condit ons diff erent from normal. Does your area have an unusual sound? Should that vapour plume be there? What is the source of the diff erent odour? If any of these quest ons surface during the execut on of dut es, make sure the cause is followed to the source. The Workplace Hazardous Material Informat on System (WHMIS) is a valuable resource in this monitoring process. Familiarity with the facility which one operates and maintains makes it possible to ident fy design defi ciencies and seek alternate methods of doing the job to ensure that current methods address the problem of environmental impact.

Example 1: Some facilit es use gaskets or valve packing with asbestos fi bres. In the new, undamaged state, these are not hazardous; however, as they wear, some asbestos fi bres may be exposed. During the changeout of this material, ensure that this hazard is recognized. Establish procedures for the removal and disposal of this material to ensure that asbestos fi bres are not released into the atmosphere.

4th Class • Part A2 Unit 9 • Chapter 45 • Potential Environmental Impact of Operating Facilities 73

OBJECTIVE 3

Discuss the overall noise implications of operating facilities.

NOISE

Applicat on for new permanent facilit es or for modifi cat ons to exist ng permanent facilit es will, where there is a reasonable expectat on of cont nuous noise generat on, require a noise impact statement. Facilit es such as: compressor and pumping stat ons, gas processing plants and gas pipeline compressors are examples of installat ons for which a noise impact statement would be required. The noise impact statement should specify the design sound level at the nearest or most impacted permanently or seasonally occupied dwelling. It is intended to encourage applicants to consider possible noise impacts before a facility is constructed or in operat on. Since the cost to retrofi t may be signifi cantly more than if noise mit gat on measures are incorporated into the design of a facility, designers should discuss noise mat ers with residents during the design and construct on phases of an energy facility. While residents, part cularly in rural areas, would generally prefer no increase in sound levels result ng from energy-related developments, it is somet mes not possible to completely eliminate these increases. How- ever, if proper sound control measures are incorporated into facility design, increases in sound levels can be kept to acceptable minimums. Sound levels are generally considered to be acceptable when overall quality of life or indoor sound levels for residents are not adversely aff ected. Even for facilit es with no dwellings within 3 km, uncontrolled sound generat on will not be allowed. There- fore, reasonable measures should be taken to moderate potent ally bothersome sound generat on even though no sound level criteria are specifi ed. Retrofi t may be required if a residence is built so close to a facility that it can no longer be considered remote. While noise impact from energy facility-related heavy truck traffi c is not always specifi cally ment oned, it should be noted that its impact is not excluded from noise regulat ons. It is expected that every reasonable measure will be taken by industry to avoid, or at least minimize, the impact of heavy truck traffi c in any given area.

4th Class • Part A2 Unit 9 • Chapter 45 • Potential Environmental Impact of Operating Facilities 74

OBJECTIVE 4

Discuss and give examples of the importance of “attitude” in limiting the environmental impact of operating facilities.

ATTITUDE

The act on taken by the operat ons’ personnel is crit cal in minimizing the impact of an unplanned excursion. An act on may be as simple as the adjustment of air fl ow to ensure complete combust on. Other more complex situat ons may require the isolat on of equipment and containment of a release. For example, suppose a pump seal fails and releases liquid hydrocarbon. Prompt isolat on of the pump will reduce the amount of material spilled. Containment to prevent the release from entering storm sewers, etc. will aid in minimizing the damage caused. Perhaps the most benefi cial and eff ect ve act on in the reduct on of environmental impacts is at tude. No longer can act ons be taken just because “They were always done like that!” One must be conscious of the impact of any act ons taken. Quest ons, such as, “Is there a bet er way?” should be asked to raise awareness and thus potent ally reduce the environmental eff ects.

Example 2: Can the transmit er that has always been blown down to the ground be routed to a collect on system, e.g., to a fl are or contaminated sewer? How about the fl ange or packing leaks which have been there for a while and don’t seem to be get ng worse? These fi t ngs should be t ghtened, if possible, or placed on a scheduled maintenance list. If they are chronic problems, perhaps a prevent ve maintenance schedule is necessary. When isolat ng and preparing equipment for maintenance or other act vit es, care should be taken to minimize the release of material. Where possible, liquids should be retained in the system and vapours purged to a contained system. Plans must be made to deal with the residual material, such as trapped liquids or sludge deposits. Procedures for the containment and disposal of materials and regulat ons governing the use of correct personal protect ve equipment are necessary. When an act on is undertaken, whether it be in normal rout ne dut es or in response to some unplanned event, other problems that may arise as a result of that act on should be considered. Of en, the most probable scenarios are ident fi ed and procedures, whether normal operat ng or emergency, are writ en to address these situat ons. However, during the execut on of these act vit es, a thought process should be inst lled which prompts the employee to ask why each step is being carried out and what the impact of the procedure is likely to be. As a facility ages, certain aspects of the original design may no longer address the operat ng parameters. Changes will occur that require the addit on of equipment or the retrofi t ng of exist ng equipment to deal with environmental concerns. It is the responsibility of the company and of each worker within a facility to ident fy and act on these concerns as they become apparent. Each step in a change of design or stage of operat ng methods should be evaluated to assess the consequence of the change. Normally, a hazard analysis and risk assessment is included in the design stage to ident fy, minimize and control any hazardous situat on. Prior to commissioning or operat ng a design change, an addit onal hazard and operability study should be conducted to address the potent al eff ects of any modifi cat ons. The following three types of act ons can be taken when a problem area is ident fi ed: • Permanent Interim • Interim • Adapt ve

4th Class • Part A2 Unit 9 • Chapter 45 • Potential Environmental Impact of Operating Facilities 75

Permanent Action Permanent act on is the most desirable since it deals with and eliminates the root cause.

Example 3: A chemical tank is purged through an atmospheric vent on a frequent basis. A permanent act on would be to vent the vessel into a closed fl are system. Interim Action An interim act on allows an individual to live with the eff ects of a problem while making defi nite plans to address the cause at the earliest opportunity. To ensure that this act on is eff ect ve, it is crit cal to assign responsibility and establish a t me frame for follow up.

Example 4: A sect on of pipeline ruptures and spills oil. An interim act on would be to repair the rupture. Responsibility would then be assigned to test and replace any suspect areas by a certain date. Adaptive Action Adapt ve act ons allow the plant to operate with the condit on indeﬁ nitely.

Example 5: A transmit er has been blown down to the ground twice a day since the facility started up. It is impract cal to route the drains to a collect on system. In this case, the frequency of blowdown can be addressed and the transmit er may only be blown down on an as-required basis. By recognizing and analyzing the operat on, the blowdown frequency may be reduced. The ability to deal with abnormal condit ons in a t mely and effi cient manner will signifi cantly reduce adverse environmental impacts. Quest oning and developing cont ngency plans for the occasions when something goes wrong will assist in solving environmental problems. Companies will generally have ident fi ed the most likely cases and have established emergency plans. The following answers to, “If something goes wrong, how am I going to handle it?,” help to establish emergency procedures: • Know the material you are dealing with. • Know the isolat on points in the process. • Ident fy what is required to contain a release and protect oneself. • Ident fy what other problems to the environment or process an upset may cause. • Inform other people in your area of what is happening so they may assist, if needed. • Discuss the “what ifs” with others to gain their experience in dealing with unusual occurrences.

4th Class • Part A2 Unit 9 • Chapter 45 • Potential Environmental Impact of Operating Facilities 76

OBJECTIVE 5

Describe the long term environmental impact after the decommissioning and abandonment of operating facilities.

DECOMMISSIONING ABANDONMENT

When an exist ng facility is shut down and decommissioned, the site must be lef in a condit on suitable for alternate uses. It must pose no future threat to public health or the environment, nor create any future liability for the company. A condit on of approval for a new facility requires that an acceptable development and reclamat on plan be prepared prior to construct on. It will be necessary to remove from the soil all hydrocarbon contaminat on result ng from on-site waste management and the inevitable spills and leaks that occur in operat ng a facility. This process may involve actual removal or cleansing of the soil, depending on the levels and types of contaminants present. The highest pract cal reclamat on standards must be established and met. The public must be provided with a full disclosure of all reports and data generated during the reclamat on project. Before the closure is announced, ant cipated environmental impacts must be ident fi ed and appropriate remedial plans formu- lated. The plans should address such details as: closure, dismantling, site evaluat on and reclamat on procedures. Even before shutdown, the process of determining the history and locat on of potent al sources of past contaminat on, such as tank leaks, process spills and landfi lls, can proceed through a history and records search and discussion with long term employees. As areas are abandoned periodically over the life of the facility, the site and equipment should be dismantled and cleaned. It must be recognized that while these steps are helpful in init at ng the cleanup, the full scale of the project may expand as the shutdown phases progress. The cost of reclamat on can be greatly infl uenced by the act vit es of the operat ons personnel. If people operate a facility in an environmentally conscious manner, there will be far less work to do in returning the site to a condit on suitable for alternate uses.

LONG-TERM ENVIRONMENTAL IMPACT

Long term environmental impacts are greatly infl uenced by the manner in which companies and facilit es manage the hazardous waste generated. The best principle of hazardous waste management is to avoid the generat on of the waste in the fi rst place. Recovering and reducing the amount of hazardous material is preferred over the necessity of its treatment or destruct on. If the hazardous material cannot be eliminated, it must be securely contained and monitored to protect people and the environment. Long-term storage of hazardous material should be avoided where commercial treatment or disposal facilit es exist. When an outside contractor is used for hazardous waste disposal, the following key points must be checked: 1. Ensure that the transportat on of hazardous waste conforms with the Transportat on of Dangerous Goods (TDG) legislat on. 2. Conduct periodic assessments of the disposal company to ensure the methods used comply with all required legislat on. Hazardous materials that may migrate, leak or accumulate from storage or operat ng facilit es must be dealt with. The most common approach is to deal with these as they occur; however, some residual material will accumulate during the life of the facility. When a contaminated area is ident fi ed, the boundaries must be defi ned. This will involve core sampling of the soil to determine the circumference and depth of the area aff ected. Once these are established, the contaminated soil is removed, placed in sealed containers and taken for disposal/destruct on. The void created by the contaminated material removal is then replaced by fresh fi ll. Another area that may be cause for concern is the disposal of samples that are taken within a facility. If the samples contain hazardous material, they should be disposed of in a careful, responsible manner. This process may mean collect ng in appropriate containers and removing to in-plant contaminated sewer facilit es or off -site disposal facilit es. The samples should never be poured or released to a sanitary or storm sewer system where water or soil contaminat on may occur.

4th Class • Part A2 Unit 9 • Chapter 45 • Potential Environmental Impact of Operating Facilities 77

CHAPTER 45 - QUESTIONS POTENTIAL ENVIRONMENTAL IMPACT OF OPERATING FACILITIES

1. A valuable tool in recognit on of an environmental problem is the use of the ______program. a) Plant Health & Safety Commit ee b) WHMIS c) Transportat on of Dangerous Goods d) Provincial Environmental Guidelines

2. As a plant ages, changes in operat ng condit ons that aﬀ ect the environment are the responsibility of the a) company and workers operat ng the plant. b) OH&S. c) residents within 3km of the plant. d) Environmental Review Board.

3. “Permanent act on” is most desirable because it a) allows the plant to operate with the condit on indefi nitely. b) allows an individual to live with the eff ects of the problem while making plans to address the cause at the earliest opportunity. c) sat sfi es upper management. d) eliminates the root cause.

4. The best method of dealing with environmental impacts due to wastes is to a) avoid the generat on of hazardous waste. b) ensure that disposal of material waste conforms with required legislat on. c) conduct periodic inspect ons of the company used to dispose of hazardous waste. d) use long term storage.

5. One of the most beneﬁ cial and eﬀ ect ve factors in the reduct on of environmental impacts is a) working safely. b) good on the job training. c) at tude. d) knowledgeable Supervisor.

Fourth Class • Part A2 Unit 9 • Chapter 45 • Potential Environmental Impact of Operating Facilities 78

CHAPTER 45 - ANSWERS POTENTIAL ENVIRONMENTAL IMPACT OF OPERATING FACILITIES

1. (b)

2. (a)

3. (d)

4. (a)

5. (c)

6. (a)

Fourth Class • Part A2 4th Class • Part A2 UNIT 10

MATERIALS WELDING

Chapter 46 Engineering Materials 81

Chapter 47 Welding Methods & Inspection 93

Chapter 48 Welding Terms, Forge & Fusion Welding Processes 107

79 80 4th Class • Part A2 C HAPTER 46

Engineering Materials

LEARNING OUTCOME

When you complete this chapter you should be able to: Describe the mechanical properties of engineering materials and the ability of alloying elements to change the mechanical properties of materials, and identify nonferrous materials as used in engineering.

LEARNING OBJECTIVES

Here is what you should be able to do when you complete each objective:

1. Discuss the mechanical properties of materials.

2. Describe the various types of ferrous materials with respect to their mechanical properties.

3. Describe the various types of non-ferrous materials with respect to their mechanical properties.

81 Unit 10 • Chapter 46 • Engineering Materials 82

INTRODUCTION

Propert es of a metal are the characterist cs by which it can be accurately ident ﬁ ed, or by which its range of usefulness can be determined. Power Engineers are concerned with the applicat ons and propert es of two main groups of metals: Ferrous• Ferrous• • Nonferrous The word ferrous is derived from “ferrum”, the Lat n word for iron. Ferrous metals include pure iron and alloys of iron. Nonferrous metals contain no iron, or possibly only a trace. Some nonferrous metals are copper, lead, aluminum, zinc, nickel, t n and magnesium. Serviceability and safety are the ult mate criteria in choosing metals. Knowledge of types, performance and preservat on are absolutely essent al. By knowing to what extent each property exists in a metal, the metal can be used with the assurance that it will meet requirements for a speciﬁ c applicat on.

4th Class • Part A2 Unit 10 • Chapter 46 • Engineering Materials 83

OBJECTIVE 1

Discuss the mechanical properties of materials.

MECHANICAL PROPERTIES OF METALS

Hardness Hardness is the ability to resist wear, abrasion, cut ng and indentat on. It may be a surface condit on of a metal such as when it is subjected to case hardening, or it may be uniform throughout the metal. Resistance to indentat on is the basis for a number of hardness tests. A ball indenter, or a cone or pyramid, made of hardened steel or diamond is loaded so that it produces some indentat on. The indenter may be dropped from some height and its rebound af er striking the metal is a measure of the surface hardness of the metal. The most commonly used tests are: The Brinell Tester Figure 1 shows a Brinell hardness test ng machine. The test specimen is placed on top of the jack screw and raised unt l it touches the tungsten carbide ball. By means of a pump, oil is forced above the plunger to press the ball into the test specimen. The pressure gauge gives an approximate indicat on of the load. To assure no overload, the masses act as a pressure relief system by absorbing pressure. The ball, 10 mm in diameter, is forced into the piece being tested by a standard 3000 kg load, if the material is steel. For nonferrous metals the standard load is 500 kg. The ball will make a circular mark. The diameter may be measured by a low- powered microscope, the surface area of the mark is determined and the hardness calculated:

Load (kg) Brinell Hardness Number (BHN) = ______Area of mark (mm2)

Normally, the area of the mark is not calculated. Instead, the diameter of the depression is measured in mm and a chart consulted from which the hardness number can be read directly.

Figure 1 Brinell Hardness Tester

4th Class • Part A2 Unit 10 • Chapter 46 • Engineering Materials 84

Rockwell Tester The Rockwell tester uses a somewhat diﬀ erent principle. A 10 kg load is used to hold a 1.6 mm ball on the piece being tested. Another 90 kg load (100kg in all) is then applied to make the impression. The 90 kg is removed and the 10 kg lef on. The dial of the instrument measures the depth of the mark to within 0.002 mm and converts the depth to a hardness reading simply read oﬀ the dial. For hard materials, a diamond cone is used and the total load is 150 kg. To avoid confusion, readings made with the ball are called Rockwell B readings, while those with the cone are called Rockwell C readings. Brittleness Brit leness is that property of a metal which permits no permanent deformat on before breaking. Brit le materials generally break instantly, without any intermediate stage of bending. An example of a brit le material is cast iron. Ductility Duct lity is the property of a material that enables it to be drawn out to a considerable extent before rupture and, at the same t me, to sustain appreciable load. It is somet mes considered to be the ability of a material to be permanently deformed without breaking. Mild steel is a duct le material. Duct le metal may be cold- drawn into wires, as in annealed copper. Plasticity A material is said to exhibit plast city, or to be plast c, if it is very sof and easily deformed. Examples of plast c materials include wax, lead and babbit . Plast c materials have very lit le elast city; that is, they do not return to their original shape af er the deforming force has been removed. Plast city is the opposite of brit leness. Elasticity Elast city is the ability of a material to return to its original shape af er any force act ng on it has been removed. It is one of the most important propert es, from the engineering point of view, as it helps to determine the behavior of the material under a load. Materials that are tough and duct le, such as wrought iron, possess a certain amount of elast city. Hard and brit le materials, such as cast iron, have very lit le elast city. Malleability Malleability is that property which allows a material to be hammered or rolled into other sizes and shapes. Copper, a duct le material, is also malleable. The malleability of most materials will increase when the material is heated, as when iron or steel are heated before forging. Toughness Toughness is the property that determines whether or not a material will break under a sudden impact or hard blow. This property is also referred to as “impact strength”; impact tests are used to determine the toughness of a material. Two commonly used tests are the Izod and the Charpy In the Izod test (Figure 2), one end of a material specimen is held in a vice; the free end is struck with a hammer. The energy required to break the specimen is measured and indicates the toughness of the material.

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Figure 2 Principle of Izod Impact Test

The Charpy test is similar, except that the specimen to be tested is supported at each end and struck in the center with the hammer. In the Charpy Test (Figure 3), a round or square specimen is notched and then broken by a blow from a pendulum. The specimen, struck when the pendulum is at the bot om of its swing, is struck in such a way that the notch tends to be opened up. The amount of energy used in breaking the specimen is measured simply by not ng the height to which the pendulum rises af er the break. The results are given as the number of joules of energy absorbed.

Figure 3 Principle of Charpy Impact Test

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OBJECTIVE 2

Describe the various types of ferrous materials with respect to their mechanical properties.

TYPES OF FERROUS MATERIALS

The following are types of ferrous materials that are used for the engineering of many components: Iron• Iron• Cast iron• Cast iron• • Wrought iron Steel• Steel• Iron Iron is produced in a blast furnace from iron ore. The iron ore is added to the furnace together with coke (for fuel) and limestone which combines with the impurit es to form slag. The slag fl oats on top of the molten iron, is drawn off and discarded. The molten iron is drawn off into ladles and cast into molds to form pig iron. Pig iron, by itself, is of lit le or no use. It is hard, brit le and almost impossible to machine. It cannot be worked whether hot or cold. To make the pig iron useful, it must be refi ned further to give one of two broad classes of materials: cast iron or steel. Cast Iron Cast iron is produced by melt ng pig iron together with some scrap iron in a cupola furnace, similar to a small blast furnace. The result ng molten iron contains 2%–4% carbon. If most of the carbon is combined chemically with the iron, the material is called white cast iron. It can be produced by cooling the molten material rapidly in the mold thus keeping the carbon combined with the iron. If most of the carbon is mechanically mixed with the iron in the form of graphite, the material is known as grey cast iron. It can be produced by cooling the molten material slowly which allows the carbon to disassoci- ate and form graphite within the iron. White cast iron is very hard and brit le. It is used for machinery parts that are subjected to excessive wear, such as crusher jaws and grinding mill balls and liners. Grey cast iron is sof er than white cast iron and is easily machined. It has good compressive strength and is widely used for machinery bases and supports. Another type is malleable cast iron which is produced by annealing (heat ng and cooling at a controlled rate) white cast iron. The result ng product has increased toughness and duct lity; it is used as material for some farm implements, automobile parts, pipe fi t ngs and tools. Wrought Iron Wrought iron is produced by a process known as a puddling. Pig iron is melted in a furnace and, as it melts, the carbon and other impurit es oxidize and leave the iron. As the impurit es pass off , the iron forms a plast c mass which is formed into a ball by the manipulat on of a puddling bar. The ball is then removed from the furnace and squeezed and rolled to remove most of the slag. The result is wrought iron. The important propert es of wrought iron are its duct lity and resistance to corrosion. It was formerly used extensively for boiler tubes, piping and bolts but has been largely replaced by steel.

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Steel Steels are alloys of iron and carbon containing less than 2% carbon. If the carbon content is greater than 2%, the alloy is cast iron. Steels may be divided further into plain carbon and alloy steels. Plain carbon steels are alloys of iron and carbon only. The majority of steel manufactured in the world is produced using the basic oxygen furnace. Modern furnaces will take a charge of iron of up to 350 tonnes and convert it into steel in less than 40 minutes. The basic oxygen furnace is the place within a steel mill where scrap iron, pig iron from the blast furnace and limestone are changed into liquid steel. The process is known as “basic” due to the high pH of the refractories - calcium oxide and magnesium oxide - that line the vessel. Oxygen is blown into the furnace through a water- cooled oxygen lance, which results in an oxidizat on of the carbon and other unwanted elements in the hot metal. As the charge in the furnace is heated and melted, the carbon content is reduced to the proper point by oxidat on; in this way carbon steel is produced. If alloy steel is desired, the required alloying materials are added to the molten carbon steel. Carbon is oxidized to carbon monoxide gas which passes from the furnace to a cleaning plant. Af er cleaning, it can be re-used as a fuel gas. The rest of the elements in the metal are converted to acidic oxides. They combine with the lime and other fl uxes that are added during the init al furnace charging, mainly to neutralize the acidic oxides and prevent excessive wear of the furnace lining. This process produces a slag that fl oats on the surface of the metal. When it is at the correct temperature and composit on, the steel is tapped from the furnace. The furnace is t lted and the molten metal runs out via the taphole into a ladle. Once the steel has been removed, the furnace is turned upside down and the slag remaining inside runs into another ladle. The solidifi ed slag can be used in the product on of cement and as an aggregate in road building. Special high alloy steel is frequently produced in electric furnaces where the heat is furnished by electrical arcs. Plain Carbon Steels Carbon steels are grouped according to their carbon content. Low Carbon Steels Low carbon steels have carbon content between 0.05% and 0.3%, and are commonly referred to as mild steel. Medium Carbon Steels These steels have a carbon content varying between 0.3% and 0.45%. They are strong and hard, but not easily welded. Whenever the carbon content exceeds 0.35%, the steel becomes increasingly diffi cult to weld due to a greater tendency toward brit leness. High and Very High Carbon Steels The carbon content ranges from 0.45% to 0.75% and from 0.75% to 1.5%, respect vely, for high and very high carbon steels. They are very strong and hard. Hardness and strength increase with an increased carbon content. Impurit es, such as phosphorus or sulphur, will lower the duct lity, malleability, and welding qualit es of steel. High and very high carbon steels respond well to heat treatments. Most of these materials may, in the annealed state, be readily machined. Alloy Steels Alloy steels are carbon steels to which certain elements have been added. Each of these elements gives certain qualit es to the steel. Some of the alloying elements combine with the carbon to form compounds; other elements do not form compounds, but remain in solut on in the ferrite. “In solut on” means the elements do not combine with other elements, but are held suspended as crystals in the basic ferrite. The main advantages of alloy steels are: • ability to respond to heat treatment. • improved corrosion resistance. • improved propert es at high and low temperatures. • combinat on of high strength with good duct lity.

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Most alloy steels may be welded, provided the carbon content is within welding range. Generally, these steels require heat ng before, during and af er welding in order to avoid residual stresses. An example of an alloy steel is Specifi cat on SA-335-P22, chrome molybdenum steel used for high temperature steam piping. This material is suitable for severe service because of its high creep strength and resistance to oxidat on and corrosion at high temperatures (above 500°C). Creep is slow, permanent stretching of a material under stress at high temperatures. The following are some of the most important elements added to steel to produce alloy steel and their eff ect on the propert es of the steel. Nickel Nickel is a tough, silvery element of about the same density as copper. It has excellent resistance to corrosion and oxidat on even at high temperatures. It improves toughness and prevents brit leness at low temperatures. Nickel steels are especially suitable for the case hardening process for such applicat ons as roller bearings and gears. These steels provide strong, tough cases resistant to wear and fat gue. Chromium Chromium resists oxidat on caused by hot gases, maintains high strength at elevated temperatures and increases hardness and abrasion resistance. When chromium is present in amounts in excess of 4.0%, corrosion resistance is greatly promoted. With a minimum of 12% chromium, the steel is called stainless steel. Molybdenum Molybdenum increases hardness and endurance limits of steel and decreases the tendency towards creep. It also increases the steel’s resistance to corrosion. Vanadium Vanadium produces a fi ne grain structure during heat treatment, promotes hardening ability and increases duct lity. Copper Copper readily combines with many other elements and improves the atmospheric corrosion resistance qualit es of the steel. Lead Lead improves machinability. Manganese Manganese increases strength and hardness, promotes high impact strength, and off ers excellent resistance to wear by abrasion. Tungsten Tungsten produces a fi ne grain structure. The alloy retains hardness and strength at high temperatures.

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OBJECTIVE 3

Describe the various types of non-ferrous materials with respect to their mechanical properties.

NONFERROUS METALS

The following are types of nonferrous materials that are used for the engineering of many components. Copper Copper is obtained from copper ore which is smelted and then further refi ned by electrolysis. It is then made into cast ngs, wire, bars, sheets, plates and tubes. These propert es make copper desirable as an engineering material: • High electrical conduct vity • High heat conduct vity • High corrosion resistance • Duct lity • Toughness In a power plant, copper is used primarily for electrical equipment and as an alloy in the materials used for heat exchanger tubes, valves and fi t ngs. Copper Alloys If copper is mixed with other metals, the result ng copper alloy has superior propert es to pure copper. For example, copper alloys are stronger, easier to machine, and have bet er corrosion resistance. The most commonly used copper alloys are various brasses and bronzes, which fi nd use as condenser tubes, piping, valves, fi t ngs and bearing shells. Brass Brasses are primarily mixtures of copper and zinc (up to 40%). Frequently, small amounts of other metals such as lead, t n, nickel, aluminum and manganese are also included in the mixture. Bronze Bronze, an alloy of copper and t n, may also contain zinc to ensure non-porous cast ngs and lead to improve machining qualit es. Addit ons of up to 1% phosphorus produce bearing bronzes which are hard but not abrasive. Bronze has a resistance to corrosion approximately equal to that of copper. Aluminum Aluminum is produced by electrolysis from bauxite ore. One of its most important propert es is its low density as it is only one third as heavy as iron or steel. Aluminum is: • very malleable and duct le. • a good conductor of electricity. • an excellent conductor of heat. • highly resistant to corrosion. A disadvantage is that, in its pure form, aluminum has a low tensile strength. Aluminum is usually alloyed with other materials such as copper, silicon, manganese, zinc, nickel, magnesium and chromium in order to improve its propert es. For example, an aluminum alloy may contain 4% copper and 0.5% each of manganese and magnesium. Aluminum alloys are used for internal combust on engine parts, aircraf parts, tubing and water jackets.

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White Metal White metal (also known as babbit ) is the name given to alloys made primarily of lead and t n which in some cases have small amounts of other elements added such as ant mony, copper or arsenic. They are chiefl y used for bearing materials because they are easily melted and cast in the bearing shell, and also have suffi cient strength and duct lity not to crack or squeeze out under heavy loads. In addit on, they are sof enough to wear suffi ciently to conform to the shape of the shaf and they have good thermal conduct vity to carry heat away from the bearing surface. The composit on used, changes for diff erent applicat ons, such as turbines, internal combust on engines, marine use, and so on. The material needs to be able to adsorb and hold a lubricant fi lm on the surface, and carry away some of the foreign mat er that does not get embedded into the bearing material. Tin based white metal, typically composed of 85% t n (Sn), 7% ant mony (Sb), 7% copper (Cu), and 0.35% (max) lead (Pb), is of en used for a turbine bearing. Tin based white metal is bet er for higher speeds (730 m/ min, 2400 f /min at shaf ) and higher loads, (900 kg (2000 lbs)) than lead based white metal. Lead-based white metal is typically composed of 5% t n 14.5% ant mony, 0.3-0.6% arsenic (As) and the balance, approximately 80% Lead. The Lead based Babbit is acceptable for lower speeds, 304 m/min (1000 f / min) and lower loads, 225 kg (500 lbs). The main advantage to lead based babbit is its cost, lead is relat vely cheap compared to t n.

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CHAPTER 46 - QUESTIONS ENGINEERING MATERIALS

1. Malleability of a metal usually increases with the addit on of a) aluminum. b) heat. c) molybdenum. d) copper.

2. Materials, which will break rather than bend when subjected to an outside force are said to be a) tough. b) elast c. c) brit le. d) plast c.

3. What results from removing nearly all the impurit es and carbon from pig iron? a) mild steel b) cast iron c) stainless steel d) wrought iron

4. When nickel is added to alloy steel, it produces improved a) breakage resistance. b) plast city. c) corrosion resistance. d) duct lity.

5. Copper will become sof when it is a) mixed with wrought iron. b) stretched. c) hammered. d) heated and cooled quickly in water.

6. White metal is an alloy of ______and ______. a) copper and zinc b) lead and t n c) copper and vanadium d) copper and tungsten d) analyzing and then disposing of the sample in an approved manner

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CHAPTER 48 - ANSWERS WELDING TERMS, FORGE FUSION WELDING PROCESSES

1. (b)

2. (c)

3. (d)

4. (c)

5. (d)

6. (b)

Fourth Class • Part A2 4th Class • Part A2 C HAPTER 47

Welding Methods & Inspection

LEARNING OUTCOME

When you complete this chapter you should be able to: Describe electric arc welding processes and weld inspection and testing methods.

LEARNING OBJECTIVES

Here is what you should be able to do when you complete each objective:

1. Describe metal arc and brazing welding processes.

2. Discuss commonly used methods of weld inspection and testing.

93 Unit 10 • Chapter 47 • Welding Methods & Inspection 94

INTRODUCTION

Welding consists of joining two or more pieces of metal by the applicat on of heat and somet mes pressure. In arc welding, the heat comes from an electric arc and no pressure is employed to fuse the metal parts. Of en the heat from the arc is used to melt and fuse the parts together without adding extra metal. In most applicat ons of arc welding, however, molten metal is added to the joint and usually this joint is specially prepared to receive such metal. Since welding is a joining process, the student should ﬁ rst have knowledge of joints themselves - what they look like, what they are called, how they are prepared and what their uses are - and be familiar with the language used in the welding industry.

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OBJECTIVE 1

Describe metal arc and brazing welding processes.

METAL ARC WELDING PROCESS

Arc welding is a fusion welding process wherein the pieces of metal are joined together by means of an electric arc. The heat is obtained from an electric arc formed between the base metal and an electrode. The temperature produced by the arc ranges from 3000°C to 8300°C, result ng in a molten pool forming on the work at the arc locat on. By manipulat on of the electrode, the molten pool is made to travel along the joint as desired. Shielded Metal Arc Welding The covered metal electrode (ﬁ ller rod) melts and carries metal across the arc from the electrode to the work. Very small globules are deposited in the molten pool on the work. The electrode is consumed and the metal is deposited at a rate dependent on the value of the current. In metal arc welding, the weld is shielded from the atmosphere during welding by the decomposit on of the electrode covering. A shielded metal arc weld being produced is illustrated in Figure 1.

Figure 1 Shielded Metal Arc Welding

(a) Basic Shield Metal Arc Welding Circuit (b) Shielded Metal Arc Welding Process This method of welding has a wide applicat on for high-speed welding on thin sect ons. It is used in the fabricat on of steel barrels and tanks, and also for welding pipe and pressure vessels. The procedure is adaptable for welding of sect ons in fl at, vert cal, or overhead posit ons. Arc Welding Equipment Either alternat ng or direct current may be used for arc welding, depending on the type of electrodes. The most sat sfactory source of direct power is a portable motor-generator set. The generator is of the variable voltage type, which adjusts its voltage to the arc demands. Alternat ng current is supplied from single-phase transformers, either with variable voltage taps or with a variable inductance in each arc circuit to obtain desired current values. Another source of power is the rect fi er, which can convert AC into DC. Figure 2 shows a simplifi ed diagram of an arc welding circuit. The electric current passes through the metal being welded and then returns to the welding power source through another cable, thus complet ng the circuit.

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Figure 2 Arc Welding Circuit

Electrodes Carbon electrodes, bare wire, and covered electrodes with a covering principally of combust ble materials, generally require direct current. Covered electrodes with a covering principally of minerals are, in general, sat sfactory for direct or alternat ng current. The combust ble electrode type gives protect on to the arc largely by producing a gaseous envelope around the arc which excludes oxygen and nitrogen. The mineral-coated types give protect on by forming a slag coat on the individual globules of metal as they are transferred through the arc, thus providing a heavy slag layer over the weld metal pool. In other words, the slag acts as insulat on against the air while cooling takes place. The coverings of electrodes also aid in stabilizing the arc, and to a very large degree, determine the welding characterist cs of the electrode. The American Welding Society has developed speciﬁ cat ons and ident ﬁ cat on numbers for shielded metal arc hand held welding rods as shown in Figure 3.

Figure 3 Covered Electrode Identiﬁ cation

Examinat on of an electrode will reveal a four fi gure number preceded by the let er E, painted on the covering. The E indicates that the rod is suitable for electric welding. The fi rst two digits give the minimum tensile strength of the weld bead if the weld is soundly performed. Mult ply these two digits by 1000 to get this fi gure in pounds per square inch. Note that 1 psi = 6.895 kPa. The third digit gives the weld posit on or posit ons the rod may be used in. 1 = All posit ons 2 = Flat horizontal fi llet 3 = Flat only 4 = Down hand only

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The fourth or last digit may be numbered from 0 to 8, and is a guide to the rod’s characterist cs. 0 = Reverse polarity DC. Deep penetrat on possible. 1 = Reverse polarity DC or AC. Deep penetrat on. 2 = Any polarity DC or AC. Medium penetrat on possible. 3 = Straight polarity DC or AC. Shallow penetrat on. 4 = Any polarity DC or AC. Medium penetrat on. 5 = Reverse polarity DC. Medium penetrat on. 8 = Reverse polarity DC or AC. Shallow penetrat on. The above are the “last digits” in common use today. For example, consider an electrode having the ident fying number E-7014. This is a general purpose rod that is suitable for thin metal welding. It must NEVER be used for dynamically loaded structures or pressure vessel construct on. This is the rod most handymen or learners fi nd easy to handle and is of en nicknamed a “High Heat—Low Talent” rod. This rod may be used with AC or any polarity of DC power. Electric Arc Welding Machines Most electric welding machines used for vessel or pipeline construct on are direct current generators, powered by an AC electric motor or a gasoline engine. The current from a DC machine fl ows in one direct on only and results in good control of the welding process. For this reason, DC machines are usually preferred to AC machines for quality work. Convent onal current fl ow states that the current fl ows from posit ve to negat ve. Assuming this, if the electrode holder is connected to the negat ve lead, a situat on described as straight polarity is created. This is shown in Figure 4.

Figure 4 Straight Polarity

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If the electrode holder is connected to the posit ve lead and the ground lead is on the negat ve side of the machine, then reverse polarity is created. This is shown in Figure 5.

Figure 5 Reverse Polarity

Low voltage and high amperage are required for welding. The choice of straight or reverse polarity depends upon the type of welding job being done and the electrode being used. With straight polarity, most of the heat energy (about 2/3) is concentrated in the electrode, which causes the electrode to melt faster and produce faster welding speeds. This is favoured where shallow, wide weld deposits are preferred. On the other hand, with reverse polarity, most of the heat energy (again, about 2/3) is the work (base metal) and less heat at the electrode. The electrode melts more slowly in this case, result ng in slower welding speeds. However, penetrat on tends to be deeper and the weld widh is narrower. This would be preferred when doing overhead welding, where the fi ller metal from the electrode must be quickly solidfi ed to prevent it from falling away from the work. Submerged Arc Welding Process This is a method of welding electrically beneath a protect ve layer of granular and molten mineral material. Submerged arc welding produces seams which are neat and uniform in appearance otherwise impossible to imitate by hand electric welding. Submerged arc welding is a machine electric welding process, where the welding zone is shielded by a blanket of granular, fusible material on the work. The fusible shielding material is known as the melt, fl ux, or welding composit on. The term fl ux is the most common one used, although the fusible material serves other purposes. During submerged arc welding, the arc is not visible to the welder, since it takes place below the fl ux. The electrode is not in actual contact with the work piece, the current being carried across the gap through the fl ux. Very lit le gas or fumes rise from the weld. This type of welding can be performed manually, but the process is normally automat c. The fi nely crushed mineral composit on fl ux is the secret of good submerged arc welding. Start ng the arc may be accomplished in several ways. One way is to put a piece of steel wool between the electrode and the work-piece before switching on the electrical power. The submerged arc welding process is shown schemat cally in Figure 6. As seen in the sketch, granular fl ux is deposited on the unwelded seam ahead of the consumable solid electrode. The arc is struck underneath the fl ux which, although a nonconductor when cold, becomes highly conduct ve when molten at about 1310°C. This forms a path for the current and the generated heat keeps the fl ux molten. The welding operat on takes place beneath the fl ux without sparks, spat er, smoke, or fl ash; thus protect ve shields or helmets are not needed.

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Figure 6 Submerged Arc Welding Process (Granular Flux Shielding)

GMAW Welding Process Gas metal arc welding (GMAW), somet mes referred to by its subtype, metal inert gas (MIG) welding, is a semi-automat c or automat c arc welding process in which a cont nuous and consumable wire electrode and a shielding gas are fed through a welding gun. A constant voltage, direct current power source is most commonly used with GMAW, but constant current systems, as well as alternat ng current, can be used. There are four primary methods of metal transfer in GMAW, called globular, short-circuit ng, spray, and pulsed-spray, each of which has dist nct propert es and corresponding advantages and limitat ons. Originally developed for welding aluminum and other non-ferrous materials in the 1940s, GMAW was soon applied to steels because it allowed for lower welding t me compared to other welding processes. The cost of inert gas limited its use in steels unt l several years later, when the use of semi-inert gases such as carbon dioxide became common. GMAW is used in industries such as the automobile industry, where it is preferred for its versat lity and speed. Unlike welding processes that do not employ a shielding gas, such as shielded metal arc welding, it is rarely used outdoors or in other areas of air volat lity. GTAW Welding Process Gas tungsten arc welding (GTAW), also known as tungsten inert gas (TIG) welding, is an arc welding process that uses a non-consumable tungsten electrode to produce the weld. The weld area is protected from atmospheric contaminat on by a shielding gas, usually an inert gas such as argon. A fi ller metal is normally used, though some welds, known as autogenous welds, do not require it. A constant-current welding power supply produces energy which is conducted across the arc through a column of highly ionized gas and metal vapors known as plasma. GTAW is most of en used to weld thin sect ons of stainless steel and light metals such as aluminum, magnesium, and copper alloys. This process grants the operator greater control over the weld than compet ng procedures such as shielded metal and gas metal arc welding, thus allowing for stronger and higher quality welds. However, GTAW is comparat vely more complex and diffi cult to master, and it is signifi cantly slower than most other welding techniques. Gas tungsten arc welding is shown in Figure 8. The molten fl ux, which is lighter than the weld metal, rises to the surface of the weld and solidifi es as a glass-like covering for the weld bead. It protects the weld from oxidat on, slowing its rate of cooling, and producing a smooth, well shaped bead. The cold fl ux is readily removable, somet mes popping off the bead spontaneously. Excess unmelted fl ux can be salvaged and reused af er proper processing. Both DC and AC are used and the machines may be of either the convent onal drooping voltage characterist cs or constant potent al type. There are advantages to the use of each of these types of current supply, dependent upon the applicat on.

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With constant potent al, the arc length is self-adjust ng. With a machine with drooping characterist cs, a voltage sensit ve relay adjusts the wire feed to maintain the desired arc voltage. Two electrodes may be used in series or parallel and two or more heads with separate power supplies for each. Currents as high as 4000 amperes may be used, although normally they will not exceed 2000 amperes.

BRAZE WELDING

In the previously described metal arc processes, the metal pieces to be welded (the base metal) were brought to a molten state and fused together with metal from the fi ller rod or electrode. Both these methods are forms of fusion welding. With braze welding (Fig. 9), which is not a form of fusion welding, the base metal is not fused but is heated, by means of an oxyacetylene torch, to a t nning temperature (dull red color) above 450°C, and a bronze fi ller rod is then applied. The bronze melts and fl ows smoothly and evenly over the ent re weld area. At its liquid temperature, the molten fi ller metal and fl ux interact with a thin layer of the base metal, cooling to form an except onally strong, sealed joint due to grain structure interact on. The brazed joint becomes a sandwich of diff erent layers, each metallurgically linked to the adjacent layers. Common brazements are about 1/3 as strong as the materials they join because the metals part ally dissolve each other at the interface and usually the grain structure and joint alloy is uncontrolled. To create high-strength brazes, somet mes a brazement can be annealed, or cooled at a controlled rate, so that the joint’s grain structure and alloying is controlled. Braze welding is used extensively in the repair of cast iron and malleable iron parts.

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OBJECTIVE 2

Discuss commonly used methods of weld inspection and testing.

WELD FAULTS

Faults in welding may range from faulty metallurgical characterist cs to such physical imperfect ons as: Cracks• Cracks• Porosity• Porosity• • Slag inclusions • Lack of fusion • Undercut ng • Lack of penetrat on • Dimensional defects The importance of weld defects, however, both as to type and quality, is relat ve to the type of weld and the service required. An imperfect on, harmful in one joint, need not be so in a diff erent joint which may have diff erent loading or stress. The service condit ons must be known before a suitable welding inspect on method can be chosen. For example, in nuclear power plant pressure vessel construct on, the maximum in weld quality is essent al. The Power Engineer will meet welding inspectors on power plant or fi eld construct on sites. The welding inspector may be a representat ve of the manufacturer, purchaser, or a government agency such as the provincial Boilers Branch.

NON-DESTRUCTIVE METHODS FOR TESTING WELDS

Various methods of non-destruct ve test ng for welds are employed. These types of inspect on include the following: Visual• Visual• • Liquid dye penetrant • Radiographic • Ultrasonic Visual Inspection Visual inspect on is of great importance because it const tutes the principal basis of acceptance for many types of weldments. It is the most extensively used method of inspect on because it is easy to apply, is quick, is relat vely inexpensive, and gives very important informat on with regard to the welds and general confor- mity of the weldment to specifi cat on requirements. This type of inspect on usually begins prior to fabricat on and includes drawings, specifi cat ons, welding procedures, and materials. Such defects as laminat ons, scabs, seams, scale, or other harmful surface condit ons may be detected prior to use. Af er the parts are assembled for welding, the inspector can note incorrect root openings, improper edge preparat on and alignment, and other features of joint preparat on that may aff ect the quality of the fi nished welded joint. Visual inspect on is also used to check details of the work while welding is in progress, and also items involved in the welding procedure. To complete the visual inspect on cycle, inspect on before and during welding must be followed by inspect on af er welding. It can be readily applied at all stages of product on and has no equal in avoiding errors and detect ng faults while they are st ll easily rect fi ed.

4th Class • Part A2 Unit 10 • Chapter 47 • Welding Methods & Inspection 102

Liquid Dye Penetrant Inspection Liquid dye penetrant inspect on consists of the following methods: • Fluorescent D ye• Dye• Fluorescent Penetrant The fl uorescent penetrant principle of determining weld defects that extend to the surface is of en used to examine nonmagnet c and magnet c materials. The process employs a penetrat ng fl uid which is fl uorescent under “black” or near ultraviolet light. This fl uid is applied to the surface being checked and, af er allowing a short t me for absorpt on into the fl aw, the surplus fl uid is wiped or washed off . “Black” light is then beamed on the treated area, which reveals the defect by means of the fl uorescent material absorbed in the fl aw. Detect on of leaks by fl uorescent penetrant through welded joints in tanks and containers is very eff ect ve and widely used. The operator applies the penetrant on one side of the weld and, af er a suitable t me, the penetrant travels through any leak passages which are revealed by scanning the opposite side under black light. In addit on to the normal detect on of cracks and leaks, this method is part cularly suited to revealing micro cracks in welds and has also been used in lieu of linear t ghtness tests and in the inspect on of welds made in at aching corrosion resistant linings to vessel walls. Dye Penetrant Dye penetrant inspect on is a simplifi ed version of the fl uorescent penetrant method. It too is a mechanical detect on method for locat ng defects that extend to the surface. It indicates surface discont nuit es or openings on metal parts as bright red lines or dots on a white background. It requires an absolute minimum of equipment: • Three bot les or cans of non-corrosive liquids (the dye penetrant, the solvent and the developer compound) • Three brushes Briefl y, the process consists of the following: 1. The surface to be inspected is thoroughly cleaned. 2. A low viscosity fl uid containing a red dye is applied to the surface by brushing, spraying, or dipping. An interval of fi ve to fi f een minutes permits the penetrant to soak into surface imperfect ons. The penetrant travels easily by capillary act on. 3. At the end of the penetrat on t me, the excess penetrant is removed from the surface by washing with the solvent, followed immediately by a thorough water rinse. 4. The surface is then dried and a developer compound is applied in an even, fi ne spray with a paint spray gun. 5. The developer will then draw, by capillary at ract on, the red penetrant from any hidden fl aws, revealing such fl aws by the “bleedout” of the red dye against the white background of the developer. In some welded tanks, large pipelines, and other similar objects, Dye Penetrant Test ng can be used for locat ng porosity or other defects and leaks by applying the penetrant on one side and the developer on the other (similar to fl uorescent penetrant leak detect on). If porosity or cracks extend through the weld, the developer will pull the penetrant through such defects to reveal the fl aw. In other words, the red penetrant will show up on the side opposite to that on which it was originally applied. Radiographic Inspection Radiography is a nondestruct ve test method that shows defects in the interiors of welds that would not be visible to the eye. This inspect on method makes use of the ability of short wavelength radiat ons, such as x-rays or gamma rays, to penetrate the weld. The rays penetrate objects opaque to ordinary light, and like ordinary light, can aff ect sensit zed photographic paper. Therefore, if a source of such radiat on is placed on one side of the welded joint, and a sensit zed photographic paper is placed on the other side, it will be aff ected by the radiat on coming through the plate and the weld. A certain amount of the radiat on is absorbed by the metal and this amount depends on the thickness and the type of metal. If there are any cracks, porosity, or light slag

4th Class • Part A2 Unit 10 • Chapter 47 • Welding Methods & Inspection 103 inclusion in the weld, more light will come through at that point and the fi lm will be darker as a result of this extra light. Thus, an image or shadow of the defect is revealed on the fi lm. When using gamma rays for radiography, a radioact ve material such as radium or cobalt is used. The gamma rays emit ed by this material pass through the weld metal and register on a fi lm located on the opposite side of the weld and any defects in the weld will be evident. To control the quality and reliability of the test, strips of metal called penetrameters are used during the radiographic inspect on of a weld. These are made of the same materials as the welded parts. Usually, several small holes are drilled in the penetrameter and it is placed adjacent to the weld. When the radiographic “picture” of the weld is taken, the outline of the penetrameter will be visible on the fi lm and this will give an indicat on of the sensit vity and clarity of the radiograph. Ultrasonic Inspection Ultrasonic inspect on is a nondestruct ve method of detect ng defects in welds. High frequency vibrat ons are more sensit ve to fi ne cracks and defects than the other common inspect on methods such as x-rays. The frequencies cannot be heard by the human ear, hence the word, ultrasonic. The sound waves or vibrat ons are directed into the metal to be tested, and are refl ected back and measured. The wave signal is visible on a small screen to the operator guiding the hand held instrument over the weld. Ultrasonic inspect on may be used to detect fl aws in all types of welded joints such as nozzles, manholes, and tubeplates to shell. Ul- trasonic inspect ons may be carried out on boilers, pressure vessels, and piping systems which are in service during the inspect on. Rules for the uses of radiographic and ultrasonic inspect ons are covered in the ASME Code, Sect on 1, Power Boilers. Figure 7 illustrates weld defects detected by the various non-destruct ve inspect on methods.

Figure 7 Weld Faults

4th Class • Part A2 Unit 10 • Chapter 47 • Welding Methods & Inspection 104

CHAPTER 47 - QUESTIONS WELDING METHODS INSPECTION

1. If a welding electrode has the following ident fying number “E7015”, what does the “70” represent? a) 70 mm b) 70 000 psi c) 70 000 kPa d) best angle to be used when welding

2. With straight polarity the welding electrode is connected to a) the posit ve terminal. b) either terminal. c) the negat ve terminal. d) a good ground.

3. Submerged arc welding is a) an oxyacetylene welding process. b) an AC electric welding process. c) a DC welding process. d) an AC or DC electric welding process.

4. Braze welding is used extensively in the repair of ______and ______parts. a) wrought iron, cast iron b) stainless steel, carbon steel c) aluminum, white metal d) cast iron, malleable iron

5. A method of test ng a weld which is quick, easy, and inexpensive is the a) ﬂ uorescent penetrant method. b) visual method. c) ultrasonic method. d) dye penetrant method.

Fourth Class • Part A2 Unit 10 • Chapter 47 • Welding Methods & Inspection 105

6. When using radiographic inspect ons, defects such as cracks and porosity show up as ______on the photographic paper. a) hairline cracks b) pinholes c) slag inclusion d) darker shadows

7. The method of detect on that is eﬀ ect ve and widely used to detect leaks in the welded joints of tanks and containers is a) hydrostat c test. b) ultrasonic test. c) ﬂ uorescent penetrant. d) magpart cle.

Fourth Class • Part A2 Unit 10 • Chapter 47 • Welding Methods & Inspection 106

CHAPTER 47 - ANSWERS WELDING METHODS INSPECTION

1. (b)

2. (c)

3. (d)

4. (d)

5. (b)

6. (d)

7. (c)

Fourth Class • Part A2 4th Class • Part A2 C HAPTER 48

Welding Terms, Forge & Fusion Welding Processes

LEARNING OUTCOME

When you complete this chapter you should be able to: Describe welding terms, forge and oxyacetylene welding.

LEARNING OBJECTIVES

Here is what you should be able to do when you complete each objective:

1. Describe the common terms used in welding.

2. Describe forge and oxyacetylene fusion welding processes.

107 Unit 10 • Chapter 48 • Welding Terms, Forge & Fusion Welding Processes 108

INTRODUCTION

Since welding is a joining process, the student should ﬁ rst have knowledge of the joints themselves, what they look like, what they are called, how they are prepared and what their uses are, and be familiar with the language used in the welding industry.

4th Class • Part A2 Unit 10 • Chapter 48 • Welding Terms, Forge & Fusion Welding Processes 109

OBJECTIVE 1

Deﬁ ne the common terms used in welding.

WELDING TERMS

Arc Welding Arc welding is the process where fusion (or coalescence) is obtained by heat ng with an electric arc or arcs, without using a fi ller metal. Axis of Weld The axis of a weld is an imaginary line through the length of a weld, perpendicular to the cross sect on, at its centre of gravity. Backfire Backfi re is the retreat of the fl ame into the torch t p, followed by its immediate reappearance and is accom- panied by noise and possible fl ameout. Backing Pass Backing pass is the pass made to deposit a backing weld behind the root pass. Backing Ring Backing rings are strips in the form of a ring placed inside piping or vessels to facilitate obtaining a sound, nonporous weld at the root. Backing Strip Backing strips are in the form a strip of fl at bar, placed behind the root before welding. They are used to increase the thermal capacity of the joint to aid in the prevent on of excessive warping of the base metal. Backing Weld A backing weld (Fig. 1) is a weld bead applied to the root of a single groove joint to assure complete root penetrat on.

Figure 1 Backing Weld

4th Class • Part A2 Unit 10 • Chapter 48 • Welding Terms, Forge & Fusion Welding Processes 110

Bare Electrode Bare electrodes are a ﬁ ller metal electrode with no coat ng. Base Metal The base metal is the metal to be welded or cut. Bevel Angle Bevel angle is the angle between the prepared edge and a plane, perpendicular to the surface of the metal. Blowhole A blowhole is a gas pocket or weld cavity caused by gas or moisture trapped in the weld. Boxing Boxing (Fig. 2) is the operat on of cont nuing a ﬁ llet weld around a corner of a member as an extension of the principal weld.

Figure 2 Boxing

Braze Welding Braze welding is the process of soldering with a nonferrous alloy that melts at a lower temperature than that of the metals being joined. This process is used extensively in repair of cast and malleable iron parts. Butt Joint A but joint is the joint between two plates or parts that lie approximately in the same plane. See Figure 3.

Figure 3 Typical Butt Joints

4th Class • Part A2 Unit 10 • Chapter 48 • Welding Terms, Forge & Fusion Welding Processes 111

Carbon Arc Cutting Carbon arc cut ng is a process where a metal is cut with the heat of an arc between a carbon electrode and the base metal. Coalescence Coalescence is the union or fusing of two pieces of metal into one. Coated Electrode Coated electrodes are electrodes having a fl ux that, upon burning, produces a gas which envelops the arc to protect the molten metal from the atmosphere. Crater A crater is a depression or hole at the end of an arc welding bead. Downhand Downhand is when welding is performed from the upper side of the joint. The weld face is approximately horizontal. Filler Metal Filler metal is the material to be added in making a weld; for example, electrodes or rod. Flux Flux is a granular substance deposited with the weld metal during the welding process which helps deoxidize and cleanse the molten weld metal, then rises to the top of the weld and forms a protect ve slag over the surface of the new weld. Forehand Welding Forehand welding is a gas welding technique using the fl ame directed in the direct on of weld progress. Fusion Fusion is a complete melt ng together of the fi ller rod and base metal. Lap Joint Lap joint (Fig. 4) is the joint between two overlapping members.

Figure 4 Lap Joint

4th Class • Part A2 Unit 10 • Chapter 48 • Welding Terms, Forge & Fusion Welding Processes 112

Lead Angle Lead angle is the angle that the electrode makes in advance of a line perpendicular to the weld axis. Machine Welding This is welding with equipment that performs the operat on, under the observat on and control of a trained operator. Manual Welding Manual welding is when the ent re operat on is performed by hand. Neutral Flame Neutral fl ame is a gas fl ame which is neither oxidizing nor reducing; that is, it is neither rich in oxygen nor acetylene. Oxyacetylene Cutting Oxyacetylene cut ng is a process in which the required cut ng temperature is maintained by the chemical react on of oxygen with the base metal. The necessary heat is provided by burning acetylene with oxygen. Plug Weld A weld is made through a hole in one plate of a lap joint, joining that plate or member to the other. The walls of the hole may or may not be parallel, and the hole may be part ally or completely fi lled with the weldment. See Figure 5.

Figure 5 Plug Weld

Porosity Porosity is the presence of gas pockets or voids in the metal or welds. Postheating Postheat ng occurs by adding heat to a welded part immediately af er welding to prevent cracking by slowing the cooling rate of the weld and heat-aff ected zone. Preheating Preheat ng is heat ng of the base metal or plates immediately before welding to slow the cooling rate of the weld and prevent cracking. Procedure Qualifications Procedure qualifi cat ons is the approval of a welding procedure by the governing authority to be used for a specifi c applicat on. Approval is granted when it is sat sfactorily demonstrated by test ng that welds made using a proposed procedure will meet the required standards.

4th Class • Part A2 Unit 10 • Chapter 48 • Welding Terms, Forge & Fusion Welding Processes 113

Reaction Stress React on stress is the residual stress created by welding restrained parts which are not free to shrink during cooling af er welding. Reducing Flame Carbonizing ﬂ ame is an oxyacetylene ﬂ ame in where is an excess of acetylene. Residual Stress Residual stress is the stress remaining in a structure as a result of welding. Reverse Polarity Reverse polarity is the arrangement of direct current arc welding leads in which the work is the negat ve pole and the electrode is the posit ve pole of the welding arc. Root Crack Root crack is a crack originat ng in a weld root. See Figure 6.

Figure 6 Root Crack

Slag Inclusion Slag inclusion is solid non-metallic material entrapped in the weld metal or between the weld and the base metal. Slugging Slugging is adding of unspeciﬁ ed pieces of rod or metal in a joint before or during welding, result ng in an unacceptable joint that does not comply with design drawing or speciﬁ cat on requirements. See Figure 7.

Figure 7 Slugging

4th Class • Part A2 Unit 10 • Chapter 48 • Welding Terms, Forge & Fusion Welding Processes 114

Spatter Spat er occurs when metal part cles (which do not form part of the weld) spray out during arc or gas welding. Spot Welding Spot welding is the joining of two thin plates at spots by the heat obtained from the resistance to the ﬂ ow of current through the work parts, which are held together under pressure by electrodes. The size and shape of the individually formed welds are limited by the size and contour of the electrodes. Tack Weld Tack welds are welds made to hold parts together in the correct alignment unt l the ﬁ nal welds can be made. Toe Crack Toe crack is a crack in the plate at the edge of a weld. See Figure 8.

Figure 8 Toe and Underbead Cracks

Underbead Crack Underbead crack is a crack in the heat-aﬀ ected zone, generally not extending to the surface of the weld or base metal. See Figure 8. Undercut Undercut is a groove melted into the base metal adjacent to the toe or root of a weld which is lef unﬁ lled by weldment. See Figure 9.

Figure 9 Undercut

4th Class • Part A2 Unit 10 • Chapter 48 • Welding Terms, Forge & Fusion Welding Processes 115

Weave Bead Weave bead is a weld bead made with transverse oscillat on of the rod. See Figure 10.

Figure 10 Weave Bead

Weld Bead Weld bead is a weld deposit result ng from a pass with a welding rod. Welder Certification This is a cert ﬁ cate in writ ng that a welder has produced welds meet ng prescribed standards. Welding Transformer Welding transformer is the electrical transformer used for supplying current of desired voltage for welding.

4th Class • Part A2 Unit 10 • Chapter 48 • Welding Terms, Forge & Fusion Welding Processes 116

OBJECTIVE 2

Describe forge and the oxyacetylene fusion welding processes.

FORGE WELDING

Forge welding involves methods in which the joint surfaces are heated to the temperature range where the metal becomes suﬃ ciently plast c, and pressure is then applied. Forge welding is the oldest form of welding. The parts to be joined are heated in a forge or furnace to the proper temperature range and then subjected to a pressure by hammering or rolling by hand or machine. This results in the parts being united. This process was used for a wide variety of applicat ons, from the miscellaneous work of the blacksmith to the joining of steel plates in the fabricat on of boiler drums and other pressure vessels. However, forge welding has been replaced by the fusion welding processes.

FUSION WELDING USING THE OXYACETYLENE PROCESS

In the fusion welding process, the joint surfaces are heated above the melt ng temperature of the base metal and fusion is obtained without the applicat on of pressure. Oxyacetylene Welding Process In this method, which is also referred to as gas welding, acetylene gas is burned with oxygen in a special torch to produce an intensely hot fl ame. This fl ame is used to melt the edges of the pieces of metal to be welded (the base metal). A fi ller metal, in the form of a welding rod, is melted into the molten pool of base metal at the same t me. The welding rod used is the same material as the base metal and they all join together to form a single piece. This process is of part cular advantage in making welds of any shape, or in any posit on, on comparat vely thin sect ons, especially at sharp edges and corners. It is not, however, limited to thin sect ons, but may be sat sfactorily used to produce welds in thick metal parts. For joining thin parts, a fi ller metal may or may not be required, but for thicker sect ons, a fi ller rod of suitable composit on must be used. Oxyacetylene Welding Equipment The welding torch consists essent ally of a mixing chamber with oxygen and acetylene connect ons at one end. Each connect on is fi t ed with a needle valve to control the fl ow of the gases. At the other end of the mixing chamber, a welding t p is at ached. Various sizes of t ps may be used to weld diff erent metal thicknesses. A typical welding torch is illustrated in Figure 11.

Figure 11 Welding Torch (Airco)

4th Class • Part A2 Unit 10 • Chapter 48 • Welding Terms, Forge & Fusion Welding Processes 117

The type of torch in general use is the Equal Pressure Torch. Torch ident fi cat on numbers vary around the world, but in North America, the system adopted is a simple one of numbers that indicate the pressures to be used. A number two t p requires 14 kPa of oxygen and acetylene pressure. Likewise, a slightly larger t p showing a fi ve stamped on it requires about 35 kPa of oxygen and acetylene pressure. The other equipment includes: • regulators fi t ed to the acetylene and oxygen cylinders. • a torch handle with needle valves for control of the fl ame. • cut ng t ps for cut ng metal with the fl ame. • a cut ng at achment. • welding t ps, with gas mixers, to suit the size of t p. • “Dualine” hose, in two colors: green for oxygen and red for acetylene. • a t p cleaner. • dark glass goggles. • a sparklighter. • fl ux coated brazing rods. • low carbon steel fi ller rods. The oxygen for the process is supplied in steel cylinders at a pressure of about 15 400 kPa. The acetylene is also supplied in cylinders, but as it is unstable in its free state, it is carried in an absorbed state in acetone, which is held in the cylinder by a porous fi ller. In this way, it can be stored under a pressure of 1720 kPa. Both the oxygen and acetylene cylinders must be fi t ed with regulators to reduce the pressure of each gas to a suitable amount for the welding process. For example, the oxygen pressure necessary for welding a certain thickness of metal might be 28 kPa. Therefore, the regulator would have to reduce the cylinder pressure from 15 400 kPa to 28 kPa. Similarly, the acetylene pressure would have to be reduced from 1720 kPa to about 28 kPa. The regulators are actually pressure reducing valves and are fi t ed with two pressure gauges. One gauge shows the cylinder pressure while the other shows the operat ng pressure.

4th Class • Part A2 Unit 10 • Chapter 48 • Welding Terms, Forge & Fusion Welding Processes 118

Figure 12 illustrates a complete oxyacetylene out it and necessary equipment for operat on.

Figure 12 Oxyacetylene Outﬁ t (Airco)

Oxyacetylene Cutting When iron or steel is heated to a temperature of 870°C, it will burn when brought into contact with oxygen. If a blast of pure oxygen in the form of a thin jet is directed on the hot iron or steel, the metal will be quickly burned away in the form of a narrow cut. A common misconcept on is that the steel melts away due to the heat, but this is not the case. The steel actually burns, combining chemically with oxygen. To cut steel plate, normal pressure would be about 20 kPa for acetylene and about 280 kPa for oxygen. The oxygen pressure would be increased or decreased to suit the thickness of the plate.

4th Class • Part A2 Unit 10 • Chapter 48 • Welding Terms, Forge & Fusion Welding Processes 119

An oxyacetylene welding out it with a cut ng torch at achment is illustrated in Figure 13.

Figure 13 Oxyacetylene Welding Outﬁ t

Cast iron cannot be cut successfully and neatly by this method since it does not burn progressively. To cut stainless steel neatly with an oxyacetylene torch, a waster plate of ordinary low carbon steel must be clamped to the stainless steel plate and the two burned together. Nonferrous metals such as copper, brass, aluminum, and their alloys do not burn evenly and progressively and are not normally cut with standard oxyacetylene equipment.

4th Class • Part A2 Unit 10 • Chapter 48 • Welding Terms, Forge & Fusion Welding Processes 120

4th Class • Part A2 Unit 10 • Chapter 48 • Welding Terms, Forge & Fusion Welding Processes 121

CHAPTER 48 - QUESTIONS WELDING TERMS, FORGE FUSION WELDING PROCESSES

1. A gas pocket or weld cavity caused by gas or moisture trapped in a weld is called a a) blowhole. b) crack. c) crater. d) root crack.

2. A “reducing ﬂ ame” is one which is rich in a) oxygen. b) acetylene. c) nitrogen. d) argon.

3. Downhand is welding performed from the a) upper side of the joint and the weld face is horizontal. b) lower side of the joint and the weld face is horizontal. c) upper side of the joint and the weld face is vert cal. d) lower side of the joint and the weld face is vert cal.

4. To cut stainless steel neatly with an oxyacetylene torch, what must be used? a) an oxygen rich ﬂ ame b) a plasma torch c) a waster plate of ordinary low carbon steel d) excess heat

5. The pressure in a fully charged oxygen cylinder is about ______kPa. a) 15 400 b) 22 500 c) 6500 d) 3300

6. The normal acetylene and oxygen pressure for cut ng steel plate is ______and ______kPa, respect vely. a) 20 280 b) 10 150 c) 8100 d) 35 425

Fourth Class • Part A2 Unit 10 • Chapter 48 • Welding Terms, Forge & Fusion Welding Processes 122

CHAPTER 48 - ANSWERS WELDING TERMS, FORGE FUSION WELDING PROCESSES

1. (a)

2. (b)

3. (a)

4. (c)

5. (a)

6. (a)

Fourth Class • Part A2 4th Class • Part A2 UNIT 11

PIPING

Chapter 49 Introduction to Piping & Pipe Fittings 125

Chapter 50 Introduction to Valves 151

123 124 4th Class • Part A2 C HAPTER 49

Introduction to Piping & Pipe Fittings

LEARNING OUTCOME

When you complete this chapter you should be able to: Discuss the basic types of piping, piping connections, supports and drainage devices used in industry.

LEARNING OBJECTIVES

Here is what you should be able to do when you complete each objective:

1. State the applications for the most common materials and identify the sizes of commercial pipe.

2. Describe methods of connection for screwed, ﬂ anged and welded pipe and identify ﬁ ttings and their markings.

3. Describe methods and devices used to allow for pipe expansion and support.

4. Explain the methods used to promote good drainage of steam piping, including the installation and maintenance of steam traps. Explain water hammer.

5. Explain the need for piping insulation and describe materials and methods of insulation.

125 Unit 11 • Chapter 49 • Introduction to Piping & Pipe Fittings 126

INTRODUCTION

Piping is used in power plants to convey fl uids to and from their place of usage and forms an essent al part of the operat on of the plant. It is advantageous for the student to understand the diff erences between piping requirements for the various fl uids concerned, the best materials to use, commercial pipe sizes, methods of connect on and fi t ngs.

4th Class • Part A2 Unit 11 • Chapter 49 • Introduction to Piping & Pipe Fittings 127

OBJECTIVE 1

State the applications for the most common materials and identify the sizes of commercial pipe.

PIPING MATERIALS

The material to be used for pipe manufacture must be chosen to suit the operat ng condit ons of the piping system. Guidance in select ng the correct material can be obtained from standard piping codes. As an example, the ASME Codes (B31.1, B31.3, etc.) contain sect ons on: • Power Piping • Industrial Gas and Air Piping • Refi nery and Oil Piping • Refrigerat on Piping Systems The object is to ensure that the material used is ent rely safe under the operat ng condit ons of pressure, temperature, corrosion, and erosion expected. Some of the materials most commonly used for power plant piping are: Steel• Steel• Cast iron• Cast iron• • Brass and copper Steel Steel is the most frequently used material for piping. Forged steel is extensively used for fi t ngs while cast steel is primarily used for special applicat ons. Steel pipe is manufactured in two main categories—seamless and welded. Cast Iron Cast iron has a high resistance to corrosion and abrasion and is used for ash handling systems, sewage lines and underground water lines. However, it is very brit le and is not suitable for most power plant services. It is made in diff erent grades such as gray cast iron, malleable cast iron and duct le cast iron. Brass and Copper Non-ferrous materials such as copper and copper alloys are used in power plants in instrumentat on and water services where temperature is not a prime factor.

COMMERCIAL PIPE SIZES

Commercial pipe is made in standard, nominal sizes, each having several diff erent wall thicknesses or weights. Up to and including 304.8 mm pipe, the size is expressed as nominal (approximate) inside diameter. Above 304.8 mm, the size is expressed as the actual outside diameter. All classes (ie. schedules) of pipe with the same nominal size have the same outside diameter, but different inside diameters, since wall thickness is diff erent for diff erent schedules. For example, referring to Table 1, if a pipe is designated as 152.4 mm size, it would have a nominal or approximate inside diameter of 152.4 mm. The outside diameter is 168.28 mm, which is a constant value no mat er what the pipe schedule is. The actual inside diameter of each schedule of pipe will depend upon the wall thickness. For example, for a standard wall thickness, the actual inside diameter of 152.4 mm pipe is 154.06 mm, but for an extra strong wall thickness, the actual inside diameter is 146.34 mm.

4th Class • Part A2 Unit 11 • Chapter 49 • Introduction to Piping & Pipe Fittings 128

To calculate the actual inside pipe diameter (d), the following equat on is used: d = D – 2t where D = outside diameter t = wall thickness There are two systems used to designate the various wall thicknesses of diﬀ erent sizes of pipe. The older method lists pipe as standard (S), extra strong (XS) and double extra strong (XXS). The newer method, which is superseding the older method, uses schedule numbers to designate wall thickness. These numbers are 10, 20, 30, 40, 60, 80, 100, 120, 140 and 160. In most sizes of pipe, schedule 40 corresponds to standard and schedule 80 corresponds to extra strong. Table 1 lists the dimensions and the mass per metre in kilograms of diﬀ erent sizes of steel pipe with varying wall thicknesses.

Table 1 Dimensions and Masses of Steel Pipe SCHEDULE

Double Std. Extra 10 20 30 40 60 80 100 120 140 160 Extra wall Strong

nominal pipe size Imperial units equivalent nominal pipe size mm outside diameter Strong

1/2 12.70 21.34 ------2.77 2.77 ------3.73 3.73 ------4.75 7.47 ------1.26 1.26 ------1.61 1.61 ------1.92 2.53

3/4 19.05 26.67 ------2.87 2.87 ------3.91 3.91 ------5.54 7.82 ------1.67 1.67 ------2.17 2.17 ------2.87 3.61

1 25.40 33.40 ------3.38 3.38 ------4.55 4.55 ------6.35 9.09 ------2.48 2.48 ------3.21 3.21 ------4.20 5.41

1 1/4 31.75 42.16 ------3.56 3.56 ------4.85 4.85 ------6.35 9.70 ------3.36 3.36 ------4.43 4.43 ------5.57 7.70

1 1/2 38.10 48.26 ------3.68 3.68 ------5.08 5.08 ------7.14 10.16 ------4.02 4.02 ------5.37 5.37 ------7.18 9.47

2 50.80 60.33 ------3.91 3.91 ------5.54 5.54 ------8.71 11.07 ------5.39 5.39 ------7.42 7.42 ------11.00 13.35

2 1/2 63.50 73.03 ------5.16 5.16 ------7.01 7.01 ------9.53 14.02 ------8.56 8.56 ------11.32 11.32 ------14.79 20.25

3 76.20 88.90 ------5.49 5.49 ------7.62 7.62 ------11.13 15.24 ------11.20 11.20 ------15.15 15.15 ------21.16 27.46

3 1/2 88.90 101.60 ------5.74 5.74 ------8.08 8.08 ------16.15 ------13.46 13.46 ------18.49 18.49 ------33.77

4 101.60 114.30 ------6.02 6.02 ------8.56 8.56 ------11.13 ------13.49 17.12 ------15.95 15.95 ------22.14 22.14 ------28.10 ------33.27 40.70

5 127.00 141.30 ------6.55 6.55 ------9.53 9.53 ------12.70 ------15.88 19.05 ------21.61 21.61 ------30.71 30.71 ------39.97 ------48.71 56.98

6 152.40 168.28 ------7.11 7.11 ------10.97 10.97 ------14.27 ------18.24 21.95 ------28.04 28.04 ------42.23 42.23 ------53.78 ------66.95 78.57

8 203.20 219.08 ------6.35 7.04 8.18 8.18 10.31 12.70 12.70 15.06 18.24 20.62 23.01 22.23 ------33.05 36.51 42.20 42.20 52.68 64.13 64.13 75.18 89.61 100.15 110.39 111.47

10 250.40 273.05 ------6.35 7.80 9.27 9.27 12.70 12.70 15.06 18.24 21.41 25.40 28.58 ------41.44 50.61 59.83 59.83 80.91 80.91 95.08 113.17 131.84 153.90 170.93 ------

12 304.80 323.85 ------6.35 8.38 9.53 10.31 14.27 12.70 17.45 21.41 25.40 28.58 33.32 ------49.34 64.69 73.25 79.16 108.13 96.69 130.82 158.44 185.47 206.45 236.88 ------

14 355.60 355.60 6.35 7.92 9.53 9.53 11.13 15.06 12.70 19.05 23.80 27.76 31.75 35.71 ------54.26 67.52 80.65 80.65 93.66 125.50 106.55 156.86 193.22 222.69 251.59 279.52 ------

16 406.40 406.40 6.35 7.92 9.53 9.53 12.70 16.66 12.70 21.41 26.19 30.94 36.53 40.46 ------62.15 77.39 92.49 92.49 122.33 158.89 122.33 201.69 243.62 284.20 330.33 362.27 ------

18 457.20 457.20 6.35 7.92 11.13 9.53 14.27 19.05 12.70 23.80 29.36 34.93 39.67 45.24 ------70.04 87.25 121.28 104.33 154.82 204.22 138.12 252.37 307.36 360.84 405.31 455.98 ------

20 508.00 508.00 6.35 9.53 12.70 9.53 15.06 20.62 12.70 26.19 32.54 38.10 44.45 49.99 ------77.93 116.17 153.90 116.17 181.66 245.94 153.90 308.71 378.52 438.03 504.15 560.18 ------

24 609.60 609.60 6.35 9.53 14.27 9.53 17.45 24.59 12.70 30.94 38.89 46.02 52.37 59.51 ------93.72 139.85 208.10 139.85 252.99 341.33 185.74 438.02 543.02 634.64 714.07 800.99 ------

30 762.00 762.00 8.03 12.70 15.88 9.53 ------12.70 ------117.40 232.83 289.81 175.36 ------232.83 ------

Note: Upper ﬁ gures are wall thickness in millimetres. Lower ﬁ gures are mass per metre in kilograms.

4th Class • Part A2 Unit 11 • Chapter 49 • Introduction to Piping & Pipe Fittings 129

OBJECTIVE 2

Describe methods of connection for screwed, ﬂ anged and welded pipe and identify ﬁ ttings and their markings.

METHODS OF CONNECTING PIPE

There are three general methods used to join or connect lengths of pressure piping. These include: • using threaded pipe and screwed connect ons. • using fl anges. • using welded joints. Screwed Connections With this method, threads are cut on each end of the pipe and screwed fi t ngs such as unions, couplings, and elbows are used to join the lengths. This method is generally used for pipe sizes less than 101.6 mm for low and moderate pressures. It has the advantage that the piping can be easily disassembled or assembled. However, the threaded connect ons are subject to leakage and the strength of the pipe is reduced when threads are cut in the pipe wall. Figure 1 illustrates various screwed fi t ngs that may be used when fabricat ng a pipe system.

Figure 1 Threaded Pipe Fittings

The purposes of the ﬁ t ngs illustrated in Figure 1 may be generally stated as follows:

Elbows - for making angle turns in piping. Nipples - for making close connections. They are threaded on both ends with the close nipple threaded for almost its entire length. Couplings - for connecting two pieces of pipe of the same size in a straight line.

4th Class • Part A2 Unit 11 • Chapter 49 • Introduction to Piping & Pipe Fittings 130

Unions - for providing an easy method for dismantling piping. A union is made in two pieces that are joined together by a large nut, which is hexagonal to enable easy application of a wrench. Tees and Crosses - for making branch line connections at 90°. Y-bends - for making branch line connections at 45°. Return Bends - for reversing direction of a pipe run. Plugs and Caps – for closing off open pipe ends or fittings. Plugs have male (external) threads and caps have female (internal) threads. Bushings - for connecting pipes of different sizes. The male end fits into a coupling and the smaller pipe is then screwed into the female end. The smaller connection may be tapped eccentrically to permit free drainage of water. Reducers - for reducing pipe size. Reducers have two female connections into which the different sized pipes fit. They may also be made with one connection eccentric for free drainage of water. Pipe Cutting and Threading When making up a piping system with screwed connect ons, it is necessary to cut the pipe into the required lengths and then thread the ends onto which the ﬁ t ngs will be screwed. The pipe is supplied from the manufacturer in standard lengths and may be cut to the required length by means of a pipe cut er. The type of cut er usually employed for small piping sizes consists of a cut ng wheel and adjustable guiding rollers, as illustrated in Figure 2.

Figure 2 Pipe Cutter

(Courtesy of Ridge Tool Co.) When pipe is cut with a wheel and roller cut er, a burr is lef on the inside of the pipe and a shoulder is formed on the outside of the pipe. The external shoulder may be removed by ﬁ ling and the internal burr is removed with a special tool known as a pipe reamer, which is illustrated in Figure 3.

Figure 3 Pipe Reamer

(Courtesy of Ridge Tool Co.) It is extremely important that the internal burr be removed completely because it will tend to catch foreign material passing through the pipe and an obstruct on will be formed.

4th Class • Part A2 Unit 11 • Chapter 49 • Introduction to Piping & Pipe Fittings 131

Af er the pipe has been cut to the proper length, reamed, and the external shoulder removed, the threads are cut on the pipe ends. Male threads are cut by means of a set of cut ers known as dies, which are held in a frame called a stock. These may be moved around the pipe by means of a hand driven ratchet lever or else a power driven machine is used to turn the pipe while the dies are held stat onary. Female threads are cut by a cut er called a tap. The ratchet type dies are shown in Figure 4 and the power driven threading machine in Figure 5.

Figure 4 Power Driven Threading Machine

(Courtesy of Ridge Tool Co.)

Figure 5 Ratchet Pipe Dies

(Courtesy of Ridge Tool Co.) Flanged Connections This method uses fl anges which are bolted together face to face, usually with a gasket between the two faces. Flanged connect ons have the advantage over welded connect ons of permit ng disassembly and are more convenient to assemble and disassemble than the screwed connect ons. There are three general types of pipe fl anges used and these are classifi ed according to the method of at aching to the pipe end. These types of fl anges, illustrated in Figure 6, are: • Screwed Welded• Welded• • Loose or lapped Van Stone fl ange Note that two types of welded fl anges are shown: the slip-on weld and the weld-neck. In the weld-neck type, the hub of the fl ange is but welded to the pipe whereas the slip-on fl ange is at ached by two fi llet welds. Van Stone fl anges are used in piping systems constructed from stainless steel or other costly materials. The fl anges in the system are usually of carbon steel. In the Van Stone type, the fl ange fi ts loosely on the pipe and the pipe end is lapped over and faced off as shown in Figure 6. The pipe to be connected is similarly lapped and has a loose companion fl ange. A gasket is used between the two lapped faces of the pipes.

4th Class • Part A2 Unit 11 • Chapter 49 • Introduction to Piping & Pipe Fittings 132

Figure 6 Pipe Flange Types

To prevent leakage at fl anged connect ons, the fl ange faces, which but together, need to be absolutely fl at and smooth. While it is theoret cally possible to grind the faces to this condit on, it is a t me consuming and expensive proposit on. Therefore, gaskets are usually used between fl ange faces. Gaskets are made of a comparat vely sof material which, when the fl anged connect on is t ghtened, will fi ll in any small depressions in the fl ange faces and thus prevent leakage. Gaskets are made in three general designs. The full-face gasket covers the ent re face of the fl ange and has holes provided for the fl ange bolts. This is primarily a low-pressure type of joint and is used for joining cast iron to steel fl anges. The fl at ring gasket covers the fl ange face up to the bolt holes in the fl ange. The ring joint gasket is an oval ring, which fi ts in the grooves that are machined in ring joint fl anges. Ring joint gaskets are generally used for high pressure and special services. When making up a fl anged joint, it is important that fl ange and gasket surfaces be clean and free from any foreign part cles. The fl anges must be carefully aligned so that the fl ange faces fi t together properly. The bolt threads should be lubricated and the nuts screwed on by hand as far as possible. When pulling up with a wrench, the bolts should not be t ghtened in rotat on, but the cross-over method should be used where opposite bolts around the fl ange are t ghtened gradually and evenly. A suitable sequence for bolt t ghtening is shown in Figure 7.

Figure 7 Bolt Tightening Sequence

4th Class • Part A2 Unit 11 • Chapter 49 • Introduction to Piping & Pipe Fittings 133

Welded Connections In this method, the pipe lengths are either welded directly to one another or to any valves or fi t ngs that may be required. The use of these welded joints for piping has several advantages over screwed or fl anged connect ons: 1. The possibility of leakage is removed with the eliminat on of screwed or fl anged joints. 2. The weight of the piping system is reduced due to the eliminat on of connect ng fl anges or fi t ngs. 3. The cost of material and the need for maintenance are reduced with the eliminat on of fl anges and fi t ngs. 4. The piping looks neater and is easier to insulate with the eliminat on of bulky fl anges and fi t ngs. 5. Welded joints give more fl exibility to the piping design as the pipes may be joined at pract cally any angle to each other. The main disadvantage of using welded joints for piping is the necessity of obtaining a skilled welder whenever a connect on is to be made. Piping of 50.8 mm (2 in.) size and smaller, when welded, is usually socket welded. The couplings, valves and other fi t ngs have a recessed port on into which the pipe fi ts and the weld is made around the socket edge. This method is illustrated in Figure 8.

Figure 8 Socket Welding Elbows

For larger sizes of pipe, the pipe ends are either but welded together or to valves or fi t ngs. When this method is used, the edges of the pipes or fi t ngs are beveled to form a groove for deposit ng the weld metal. Backing or back up rings, which fi t inside the pipe at the weld, are used for lining up the pipe and also to prevent weld metal from protruding inside the pipe. This method is shown in Figure 9.

Figure 9 Butt Weld Groove with Backing Ring

4th Class • Part A2 Unit 11 • Chapter 49 • Introduction to Piping & Pipe Fittings 134

IDENTIFICATION OF FITTINGS

To ensure that valves, fi t ngs, fl anges, and unions are of the proper strength and material for the part cular service for which they are used, it is necessary they be clearly marked or ident fi ed. ALL FITTINGS NOT PROPERLY OR CLEARLY IDENTIFIED SHOULD BE REJECTED. All markings, which shall be legible, must indicate the following minimum requirements: 1. Manufacturer’s name or trade mark. 2. Service designat on, such as pressure-temperature rat ng for which the fi t ng is designated. 3. Material designat on, steel, cast, malleable or duct le iron, and ASTM No. (Assigned by the American Society for Test ng and Materials.) The above markings are listed according to the degree of importance, however, for cast and duct le iron fi t ngs, (2) and (3) will be reversed in order since the material ident fi cat on is more important than the service designat on.

Table 2 Service Symbols

Symbol Reference Symbol Reference A Air O Oil G Gas S Steam L Liquid W Water

Table 3 Material Markings Symbols or Ident ﬁ cat on System

Material Marking Abbreviated Symbol or Identiﬁ cation System

Malleable Iron MI Cast Iron not required for gray cast iron Ductile (Nodular) Cast Iron Ductile or DI Carbon Steel Steel or ASTM Speciﬁ cation No. and Grade Alloy Steel Grade Identiﬁ cation Symbol and Steel ASTM No.

4th Class • Part A2 Unit 11 • Chapter 49 • Introduction to Piping & Pipe Fittings 135

OBJECTIVE 3

Describe methods and devices used to allow for pipe expansion and support.

PIPING EXPANSION

As piping is heated, it will expand in length. Unless this is allowed for in the design of a piping system, it will cause addit onal stress in the piping. The following are methods used to provide expansion in pipelines: • Expansion bends • Expansion joint Expansion Bends Expansion bends make use of pipe fabricated with special bends. The increase in the length of pipe due to expansion is taken up by ﬂ exing or springing of these bends. Figure 10 illustrates some typical shapes of expansion bends. The use of expansion bends is usually preferred for high-pressure work as there is no maintenance involved and lit le likelihood of leaks developing.

Figure 10 Expansion Bends

Expansion Joints Two types of expansion joints in general use are the: Slip• Slip• • Corrugated

4th Class • Part A2 Unit 11 • Chapter 49 • Introduction to Piping & Pipe Fittings 136

Slip Expansion Joint This type, which is illustrated in Figure 11, features a slip pipe with a fl ange bolted to an adjoining pipe. The slip pipe fi ts into the main body of the joint, which is fastened to the end of the other adjoining pipe. When the pipe line expands, the slip pipe moves within the joint body. To prevent leakage between the slip pipe and the joint body, semi-plast c packing is used around the outside of the slip pipe and the slip pipe moves within the packing. Addit onal plast c packing may be added while the joint is in service by means of a packing ram. Grease fi t ngs are used to provide lubricat on.

Figure 11 Slip Expansion Joint

Corrugated Expansion Joint A corrugated expansion joint consists of a ﬂ exible corrugated sect on which is able to absorb a certain amount of endwise movement of the pipe. The bellows type corrugated expansion joint shown in Figure 12 is suitable for pressures up to 2000 kPa. This type may be supplied with or without anchor bases.

Figure 12 Bellows Type Corrugated Expansion Joint

(Courtesy of ADSCO Manufacturing Corporation)

4th Class • Part A2 Unit 11 • Chapter 49 • Introduction to Piping & Pipe Fittings 137

Piping Supports Piping must be supported to prevent its weight from being carried by the equipment to which it is at ached. The supports used must prevent excessive sagging of the pipe and, at the same t me, must allow free movement of the pipe due to expansion or contract on. The support ng arrangement must be designed to carry the weight of the pipe, valves, ﬁ t ngs and insulat on plus the weight of the ﬂ uid contained within the pipe. Figure 13 illustrates various types of pipe supports and hangers.

Figure 13 Pipe Supports and Hangers

(Courtesy of Crane Limited) A pipe hanger or support at 3 metre intervals is considered good rule-of-thumb pract ce for ordinary installa- t ons. Hangers and supports should be placed close to valves and other heavy equipment.

4th Class • Part A2 Unit 11 • Chapter 49 • Introduction to Piping & Pipe Fittings 138

OBJECTIVE 4

Explain the methods used to promote good drainage of steam piping, including the installation and maintenance of steam traps. Explain water hammer.

PIPING DRAINAGE

In the case of steam piping, it is necessary to constantly drain any condensate from the lines. If this is not done, the condensate will be carried along with the steam and may produce water hammer and possibly rupture pipes or fi t ngs. In addit on, the admission of moisture-carrying steam to turbines or engines is most undesirable. The following devices are used to remove this condensate and moisture from the lines: • Steam separators • Steam traps Steam Separators Steam separators, somet mes called steam purifi ers, are devices which, when installed in the steam line, will remove moisture droplets and other suspended impurit es from the steam. To accomplish this, the separator either causes the steam to suddenly change its direct on of fl ow or it imparts a whirling mot on to the steam. Both of these cause the moisture and other part cles to be thrown out of the steam stream. The separators shown in Figure 14 use baffl es to cause the steam fl ow to suddenly change direct on. The moisture part cles thus removed collect at the bot om and pass out through a drain opening.

Figure 14 Bafﬂ e Type Steam Separators

Steam Traps The purpose of the steam trap is to discharge the condensed water from steam lines, separators and other equipment, without allowing steam to escape. Also, most traps are designed to discharge air from the lines or equipment. Steam traps should be installed where condensate must be drained as rapidly as it accumulates, and where condensate must be recovered for heat ng, hot water needs, or for return to boilers. They are a “must” for steam piping, separators, and all steam heated or steam operated equipment. There are numerous trap designs, which can be classiﬁ ed into three types, according to their principles of operat on. These classes are: • Mechanical • Thermostat c • Thermodynamic

4th Class • Part A2 Unit 11 • Chapter 49 • Introduction to Piping & Pipe Fittings 139

Mechanical Traps In the mechanical trap design, use is made of either a ball or bucket fl oat to open and close the trap outlet valve, depending upon whether any condensate is present within the trap body. The ball fl oat trap, illustrated in Figure 15, has a hollow fl oat. As the condensate enters, the fl oat rises and opens the outlet valve. Then, as the condensate is discharged, the fl oat sinks and closes the outlet valve. This type also features a bellows controlled air vent located near the top of the trap. If steam surrounds the bellows, the bellows expands and closes the vent outlet. However, if air (which is cooler than steam) surrounds the bellows, the bellows contracts and opens the vent, allowing air to escape.

Figure 15 Float Trap with Thermostatic Air Vent

(Courtesy of Spirax Sarco Limited) Figure 16 illustrates the operat on of a similar type of ﬂ oat trap.

Figure 16 Float Trap Operation

(Courtesy of Armstrong International, Inc.) The fl oat trap works equally well whether the condensate load is light or heavy as its operat on is not aff ected by changes in steam pressure. It does not become air-locked upon start-up when there is a large amount of air present, as it will discharge this air immediately and automat cally. However, the ball fl oat trap has the disadvantage of being vulnerable to damage from water hammer. In addit on, this type of trap is not suited for outdoor use because it will freeze in cold weather.

4th Class • Part A2 Unit 11 • Chapter 49 • Introduction to Piping & Pipe Fittings 140

Another common mechanical trap is the inverted bucket trap, also called an inverted open-ﬂ oat trap. Two styles are shown in Figure 17.

Figure 17 Inverted Bucket Trap

(Courtesy of Spirax Sarco Limited) The operat ng principle is the same for both styles. The bucket init ally hangs down and holds the discharge valve open. Condensate enters the trap and fl ows under the bot om edge of the bucket to fi ll the trap body. Then the condensate will fl ow out through the open discharge valve at the outlet. Any steam that enters the trap will collect at the top of the inverted bucket giving it buoyancy and causing it to rise, thus closing the discharge valve. Air and CO2 gas also collects at the top of the inverted bucket and passes through the vent at the top of the bucket to the upper part of the trap body. Figure 18 illustrates the operat on of an inverted bucket steam trap.

Figure 18 Operation of an Inverted Bucket Steam Trap

(Courtesy of Spirax Sarco Limited) Thermostatic Traps The operat on of thermostat c traps depends upon the diff erence in temperature between the steam and the condensate. They are commonly used on radiators in steam heat ng systems, hence they are also known as radiator traps. Figure 19 shows the construct on of a bellows type (radiator) steam trap. As the steam in the radiator gives up its heat to the room, it condenses to water. This water must be removed from the radiator as fast as it is formed. The radiator trap is a device that allows the condensed steam or water to be discharged from the radiator, but prevents any steam from discharging. Essent ally, the trap consists of a corrugated bellows or fl at hollow disc to which is at ached a valve-shaped plunger. The bellows contains a volat le fl uid, which boils by the heat from the steam that surrounds the bellows.

4th Class • Part A2 Unit 11 • Chapter 49 • Introduction to Piping & Pipe Fittings 141

Figure 19 Thermostatic Steam Trap

Figure 20 illustrates the operat on sequence of a bellows type trap. In Figure 20(a), only condensate is being discharged from the heat ng unit. The trap is wide open, so condensate ﬂ ows freely. As steam enters the trap, Figure 20(b), pressure within the bellows increases and causes it to expand. In Figure 20(c), most of the condensate is drained from the heat ng unit. When steam completely surrounds the bellows, Figure 20(d), pressure forces the plunger to seat securely onto the discharge port.

Figure 20 Operation of a Radiator Trap

Another type of thermostat c trap is the bimetal steam trap shown in Figure 21. This design consists of bimetal strips (dissimilar metals welded together), which defl ect when heated. As the condensate passes through the trap, its temperature will increase, defl ect ng the bimetal strip, which allows the valve to open and the condensate to fl ow through the trap. However, when the trap fi lls with steam, the bimetal strip will defl ect downwards enough to fully close the valve.

Figure 21 Bimetal Steam Trap in the Open Position

(Courtesy of Spirax Sarco Limited)

4th Class • Part A2 Unit 11 • Chapter 49 • Introduction to Piping & Pipe Fittings 142

Thermodynamic Traps This type of trap employs the heat energy in the steam and condensate to control its operat on. One design of thermodynamic trap is the impulse trap, shown in Figure 22.

Figure 22 Impulse Trap

This design consists of a piston type valve working within a control cylinder. When cool condensate enters the trap, the pressure of the condensate act ng upon the piston disk will lif the valve to the open posit on, thus allowing the condensate to escape through the outlet orifi ce. However, a port on of the condensate, instead of escaping through the outlet orifi ce, passes up past the piston disk into the upper part of the control cylinder and then down through a small hole drilled through the centre of the piston valve to the outlet. If the condensate entering the trap is at steam temperature, then the part entering the upper sect on of the control cylinder will fl ash into steam as the sect on is at a lower pressure (outlet pressure). The large volume of steam result ng will plug or choke the small hole through the centre of the valve and pressure will build up above the piston disk, thus forcing the valve into the shut posit on. Figure 23 is a cutaway view of the impulse trap.

Figure 23 Impulse Trap Cutaway View

(Courtesy of Yarnall Waring Co.) Another design of thermodynamic trap is the disk type, shown in Figure 24. In this type, the condensate entering the trap raises the disk and ﬂ ows under it to the discharge. When steam enters the trap, the steam expands and travels at high velocity across the underside of the disk. Some of the steam also passes up to the small space above the disk. The steam, passing at high velocity under the disk, causes a pressure reduct on in that area and the steam above the disk, being at full pressure, forces the disk down, thus closing oﬀ the outlet of the trap.

4th Class • Part A2 Unit 11 • Chapter 49 • Introduction to Piping & Pipe Fittings 143

Details of the thermodynamic disk trap are shown in Figure 25.

Figure 24 Disk Trap Figure 25 Disk Trap Details

(Courtesy of Spirax Sarco Limited)

Trap Installation New lines must be blown clear of all foreign mat er before installing the trap. The trap is installed below the lowest point in the system with the line leading to the trap pitched downward to assure good drainage. A sediment separator is always installed just ahead of the trap to prevent the entry of foreign mat er. Inlet and outlet gate valves permit isolat ng the trap for inspect on and cleaning. A bypass line with a globe valve makes it possible to rid the system of condensate by hand throt ling while the trap is out of service. Figure 26 shows the piping arrangement. To put the trap into operat on, close the valve in the outlet line, open the inlet valve, and allow condensate to ﬁ ll the trap. When the trap is ﬁ lled, open the outlet valve.

Figure 26 Typical Piping Arrangement for Inverted Open Float Steam Trap

(Courtesy of Crane Limited) Trap Inspection and Servicing Frequency of inspect on depends on condit on of the line. To isolate the trap for inspect on, close the inlet and outlet gate valves and open the sediment blowoﬀ or the test valve to relieve the pressure. Inspect on or repairs can be made while the trap is in the line or the trap can be removed from the line easily by loosening the unions.

4th Class • Part A2 Unit 11 • Chapter 49 • Introduction to Piping & Pipe Fittings 144

WATER HAMMER

Water hammer is the term used to describe a series of shocks produced by a sudden change in velocity of water fl owing within a pipeline. When steam is introduced into a cold pipe, or when fl ow in a steam pipe is very slow and normal cooling occurs, the steam revert ng to water forms condensate. If the condensate can be removed from the pipe as fast as it is being formed, there will not be any problem with water hammer occurring. However, there are not always drains located at all the points where condensate may form. Tests have been conducted in transparent piping, and observat ons indicate that water hammer occurs when a bubble of steam has become enclosed by cooler condensate. Steam in the bubble transfers heat to the surrounding water and then reverts to condensate. This rapid condensat on leaves a low pressure void and condensate rushes in to fi ll the void. In other words, the steam bubble implodes with the result that the inrushing water from one side of the bubble is met by inrushing water from the opposite side of the bubble. This causes a bang, or shock wave, generated by a collision of the masses in mot on. Figure 27 illustrates the progressive collapse of a steam bubble.

Figure 27 Collapsing Steam Bubble

(Courtesy of Crane Limited) Another situat on that produces water hammer is the sudden stopping of a motor driven centrifugal pump due to a power interrupt on or “trip out”. When this happens, the water in the pump discharge line will stop and then reverse direct on. Subsequent rapid closing of the check valve at the pump will cause water hammer. Water hammer will also be produced if steam is admit ed to a pipe containing water or condensate. The steam, on passing above the surface of the water, will raise up behind it a mass of water and thus a pocket of steam will be formed. This steam will rapidly condense due to contact with the water and a vacuum will be formed in the pocket. The water rushing into this vacuum will produce water hammer, which can rupture piping or fi t ngs. Higher steam pressures applied to the system serve to increase the pressure diff erent al, driving the water into the collapsing void with greater ferocity. Shat ered pipe fi t ngs, broken mains or damaged equipment are visible evidence of the violent results of uncontrolled water hammer.

4th Class • Part A2 Unit 11 • Chapter 49 • Introduction to Piping & Pipe Fittings 145

Figure 28 shows how a steam trap failure, or trying to bring on a steam main faster than the trap can remove the condensate, could cause water hammer to occur.

Figure 28 Condensate, form Collapsing Bubbles

Courtesy of Crane Limited As shown in Figure 28, steam entering from the lef overruns the cold condensate. The steam can displace any air exist ng above the condensate so that if a bubble is formed, there is no air in the collapsing bubble to cushion the impact. As the velocity of the steam increases, it causes ripples to form on top of the water. These ripples come in contact with the top of the pipe and steam chambers are formed. For this reason, mult ple shock waves are produced. In small piping, such as in a steam heat ng radiator, repeated banging is common. When the thermostat calls for steam to be admit ed to the radiator, intense banging may begin. As steam progresses through the radiator, residual condensate is warmed and the banging becomes less violent. Therefore, it must be stressed that before admit ng steam to any piping system, all water or condensate must be posit vely removed from all parts of the system. Traps which are ﬁ t ed to main and branch lines, and separators for drainage purposes, must be installed with bypass lines around them which may be opened to ensure posit ve drainage, as shown in Figure 26.

4th Class • Part A2 Unit 11 • Chapter 49 • Introduction to Piping & Pipe Fittings 146

OBJECTIVE 5

Explain the need for piping insulation and describe materials and methods of insulation.

PIPING INSULATION

Most power plant piping systems are used to convey substances that are at temperatures much higher than that of the surrounding air. Examples include the main steam and feedwater piping. To reduce the amount of heat lost to the surrounding air from the hot substance, the piping is covered with insulat on. The insulat on not only retains the heat in the hot lines but also prevents the temperature inside the power plant building from becoming uncomfortably high. In addit on, insulat on of hot pipe lines will prevent injury to personnel due to contact with the bare surfaces of the pipe. In the case of piping that carries substances at a lower temperature than that of the surrounding air, insulating the piping will prevent sweat ng of the pipe and consequent dripping and corrosion. A material that is suitable for use as insulat on should have the following characterist cs: • High insulat ng value Long life• Long life• • Vermin proof • Non corrosive • Ability to retain its shape and insulat ng value when wet • Ease of applicat on and installat on An insulat ng material may be defi ned as one that transmits heat poorly. It has been found that substances having a large number of microscopic air pockets dispersed throughout the material make the most effi cient insulators. This is because the extremely small air spaces restrict the format on of convect on currents as air is a poor conductor of heat. Piping Insulation Materials Some of the more common materials used for piping insulat ng are: • Calcium silicate • Glass fi bre • Mineral fi bre • Expanded silica • Elastomeric • Foamed plast c • Refractory fi bre • Insulat ng cement • Magnesia (85%) • Refl ect ve metal insulat on Calcium Silicate Calcium silicate is a granular insulat on made of lime and silica, reinforced with organic and inorganic fi bres and molded into rigid forms. Service temperature range is 38°C to 650°C. Its fl exibility is good. Calcium silicate is water absorbent, but it can be dried out without deteriorat on. The material is non-combust ble and is used primarily on hot piping and surfaces.

4th Class • Part A2 Unit 11 • Chapter 49 • Introduction to Piping & Pipe Fittings 147

Glass Fibre This is glass that has been processed into fi bres and then formed into pipe covering sect ons, which are suitable for temperatures up to 454°C. It is non-combust ble but is water absorbent. Mineral Fibre (Rock and Slag Wool) Rock and/or slag fi bres are bonded together with a heat resistant binder to produce mineral fi bre or wool available in loose blanket, board, pipe insulat on, and molded shapes. Upper temperature limit can reach 1040°C. The material has a pract cally neutral pH, is non-combust ble, and has good sound control quali- t es. Expanded Silica, or Perlite Perlite is made from an inert siliceous volcanic rock, combined with water. Heat ng, causing the water to vaporize and its volume to expand, expands the rock. This creates a cellular structure of minute air cells surrounded by vitrifi ed product. Added binders resist moisture penetrat on and inorganic fi bres reinforce the structure. The material has low shrinkage and high resistance to corrosion. Perlite is non-combust ble and operates in the intermediate and high temperature ranges. The product is available in rigid pre-formed shapes and blocks. Elastomeric Foamed resins, combined with elastomers, produce a fl exible cellular material. It is available in pre-formed shapes and sheets. Elastomeric insulat ons possess good cut ng characterist cs and low water and vapour permeability. The upper temperature limit is 104°C. Elastomeric insulat on is cost effi cient for low temperature applicat ons with no jacket ng necessary. Resiliency is high. Considerat on should be made for fi re retardancy of the material. Foamed Plastic Insulat on produced from foaming plast c resins creates predominately closed-cellular rigid materials. Foamed plast cs are light weight with excellent moisture resistance and cut ng characterist cs. The chemical content varies with each manufacturer. It is available in pre-formed shapes and boards. Foamed plast cs are generally used in the low and lower intermediate service temperature range, 150°C to 183°C. Considerat on should be made for fi re retardancy of the material. Refractory Fibre Refractory fi bre insulat ons are mineral or ceramic fi bres, including alumina and silica, bound with extremely high temperature binders. The material is manufactured in blanket or rigid form. Thermal shock resistance is high. Temperature limits reach 1650°C. The material is non-combust ble. Insulating Cement Insulat ng and fi nishing cements are a mixture of various insulat ng fi bres and binders with water and cement, to form a sof plast c mass for applicat on on irregular surfaces. Insulat on values are moderate. Cements may be applied to high temperature surfaces. Temperature limits reach 1038°C. Finishing cements, or one-coat cements, are used in the lower intermediate range and as a fi nish to other insulat on applicat ons. Magnesia (85%) This material is composed of magnesium carbonate with asbestos fi bre. It is available in molded form for pipe covering and is also supplied in powdered form to be mixed with water to form an insulat ng cement, which is used to cover pipe fi t ngs. Magnesia pipe covering is suitable for service up to 320°C. Reflective Metal Insulation This is a fairly new type of insulat on constructed of metal refl ect ve sheets of stainless steel, spaced and baffl ed to form isolated air chambers around the piping. The highly polished refl ect ve sheets refl ect the heat and prevent loss due to radiat on, yet absorb lit le heat by conduct on. This is used for temperatures above 1040°C. Pipe Insulation Types Piping insulat on is normally fabricated in half-cylindrical sect ons for fi t ng over the pipe. It is held together by metal wire or bands, and then covered with canvas, sheet metal, aluminium or galvanized steel. Some typical examples of pipe insulat ng sect ons are shown in Figure 29. Figure 29(a) shows insulat on for dmall diameter pipe. It is split along its length and opens up to fi t over the pipe. Figure 29(b) and 29(c) show the half-cylindrical sect ons of insulat on for various larger size pipes.

4th Class • Part A2 Unit 11 • Chapter 49 • Introduction to Piping & Pipe Fittings 148

Figure 29 Molded Pipe Insulation

(a) (b) (c)

Figures 30, and 31 illustrate the molded form used for piping and piping ﬁ t ngs.

Figure 30 Insulated Piping Systems

(Courtesy of Owens Corning)

Figure 31 Pipe Elbow Insulation

4th Class • Part A2 Unit 11 • Chapter 49 • Introduction to Piping & Pipe Fittings 149

CHAPTER 49 - QUESTIONS INTRODUCTION TO PIPING PIPE FITTINGS

1. For power plants, materials used for the manufacturing of pipes must be a) cast iron. b) seamless stainless steel. c) suited to the operat ng condit ons of the piping system. d) case hardened.

2. Copper and copper alloy piping and tubing is not used in power plants when ______is a prime factor. a) pressure b) metallurgy c) temperature d) ﬂ uid commodit es

3. Pipe ﬁ t ngs which are not clearly ident ﬁ ed should be a) marked by the local Boiler Inspector. b) hydrostat cally tested. c) used in low pressure service. d) rejected.

4. The type of pipe ﬂ ange gasket not in normal use today is the a) full face b) ring joint c) ﬂ at ring d) graphite

5. Gaskets are used in pipe fl ange joints to a) prevent leakage because of small imperfect ons in the fl ange face. b) allow for pipe expansion between each fl anged joint. c) make up for the misalignment of the pipe. d) provide cushioning in the event of water hammer.

6. A good rule-of-thumb to use for the spacing of pipe supports in a piping system is every ______metres. a) 10 b) 7.5 c) 5 d) 3

Fourth Class • Part A2 Unit 11 • Chapter 49 • Introduction to Piping & Pipe Fittings 150

CHAPTER 49 - ANSWERS INTRODUCTION TO PIPING PIPE FITTINGS

1. (c)

2. (c)

3. (d)

4. (d)

5. (a)

6. (d)

Fourth Class • Part A2 4th Class • Part A2 C HAPTER 50

Introduction to Valves

LEARNING OUTCOME

When you complete this chapter you should be able to: Discuss the design and uses of the valve designs most commonly used in industry and on boilers.

LEARNING OBJECTIVES

Here is what you should be able to do when you complete each objective:

1. Describe standard valve designs.

2. Describe design and operation of specialized boiler valves.

3. Describe piping arrangements and the design and operation of steam system pressure- reducing valves.

4. Discuss valve details, including materials of construction and identiﬁ cation markings.

5. Describe typical valve maintenance requirements.

151 Unit 11 • Chapter 50 • Introduction to Valves 152

INTRODUCTION

Various types of valves are required in any piping system to regulate the fl uid fl ow within that system. Valves can be manually operated or have an actuator to change and control the valve opening. The actuator may be pneumat cally, hydraulically or electrically operated. The valves represent a considerable percentage of the overall cost of the system; therefore, they must be carefully selected. Considerat on must be given to the following details: • Working pressure and temperature • Type of fl uid (corrosive or erosive) • Rate of fl ow • Valve characterist cs desired (percentage valve travel to rate of fl ow) • Valve being used for isolat on purposes only (wide open or closed) • Cost of installat on and maintenance

Fourth Class • Part A2 Unit 11 • Chapter 50 • Introduction to Valves 153

OBJECTIVE 1

Describe standard valve designs.

VALVE DESIGN

There are a number of basic designs of valves, including: Gate• Gate• Globe• Globe• Needle• Needle• • But erfl y Ball• Ball• Plug • Plug Check• Check• Gate Valve The gate valve, illustrated in Figure 1, consists of a gate-like disk actuated by a screwed stem and handwheel which moves up and down at right angles to the path of fl ow. In the closed posit on, the disk seats against two faces to shut off the fl ow. Gate valves are not suitable for throt ling service because excessive wear due to wire drawing (erosion) occurs on the gate and gate seats. However they are suitable as stop (or isolat ng) valves, where condit ons require either full or no fl ow. They have the advantage that, when fully opened, the resistance to fl ow is low with a minimum of pressure drop as the fl uid fl ow moves in a straight line.

Figure 1 Gate Valve

Fourth Class • Part A2 Unit 11 • Chapter 50 • Introduction to Valves 154

Globe Valve The globe valve, shown in Figure 2, is constructed to cause the fl ow of the fl uid passing through it to change direct on twice. The disk and seat are parallel to the main fl ow path; the disk is moved toward, or away from, the seat by means of a threaded stem. Due to its construct on, the globe valve is ideal for throt ling or regulat ng fl ow with a minimum of wire drawing and seat erosion. Another advantage it has compared to the gate valve is that it is cheaper to manufacture. On the other hand, the globe valve off ers much more resistance to fl ow than does the gate valve. The unbalanced single disk type of globe valve, shown in Figure 2, is seldom used in sizes larger than 305 mm due to diffi culty in opening and closing against fl uid pressure.

Figure 2 Globe Valve

Needle Valve Needle valves are designed to allow precise fl ow control. Their name is derived from the sharp pointed disk and matching seat. They are extensively used for cont nuous blowoff or chemical feed control services. The stem threads are fi ner than usual so that considerable movement of the hand wheel is required to increase or decrease the opening through the seat. Usually, these valves have a reduced seat diameter in relat on to the pipe size. See Figure 3.

Fourth Class • Part A2 Unit 11 • Chapter 50 • Introduction to Valves 155

Figure 3 Needle Valve

Butterfly Valve The but erfl y valve is of en used as a fi nal control element in air or large water piping systems. It comes in sizes from 25 to 3800 mm in diameter and is designed for pressures and temperatures up to 13 800 kPa and 1100°C, respect vely. But erfl y valves are of en lined with a resilient material so the rotat ng disk seats t ght when closed. They provide a bubble t ght seal with low operat ng torque and operate by the wing-like act on of the disk; when open, the disk is parallel to the fl ow. The fl at disk can be rotated through 90° from wide open to the fully closed posit on. But erfl y valves fi t into the piping in two ways: the two-fl ange (or double ported type) and the wafer type. Referring to Figure 4(a), the double-ported type has a fl anged body and the liner terminates within the body. The small port works independently and can increase the control range considerably at low fl ows. The body is frequently a solid ring type mounted between pipe fl anges. The disk is generally cast in one piece. Correct alignment of this valve is required to prevent binding of the swing-through disk. The thickness of the disk is determined by the pressure drop across the valve (throt ling or closed posit on). The wafer body type, shown in Figure 4(b), does not have fl anges. It is installed by sliding it between two fl anges in the piping. It has a sealing surface which matches up with the sealing surfaces of the piping fl anges. The wafer valve of en has a molded in-seat for extra life and a bet er seal. The valves, shown in Figure 4, are fi t ed with levers for manual operat on. A power actuator is required to posit on the disk for bigger sizes because large pressure diff erent als can exist across the disk.

Fourth Class • Part A2 Unit 11 • Chapter 50 • Introduction to Valves 156

Figure 4 Butterﬂ y Valves

(Courtesy of DeZurik) The valve shown in Figure 5 can be manually or electrically operated.

Figure 5 Power Operated Butterﬂ y Valve

(Courtesy of Rockwell Manufacturing) But erﬂ y valves are used in the following: • Thermal and hydroelectric power stat ons • Oil and gas processing industries • Oil and gas transmission • Water and sewage plants They have the following advantages: • Relat vely light weight • Ease of operat on self cleaning • Negligible pressure drop across the valve when it is fully open

Fourth Class • Part A2 Unit 11 • Chapter 50 • Introduction to Valves 157

Ball Valve The ball valve, illustrated in Figure 6, contains a spherical plug with a passage bored through it which controls the ﬂ uid ﬂ ow through the valve body. The basic type of ball valve requires a quarter turn to move from fully open to the fully closed posit on. The valve can be operated by means of a lever, which also serves as an open or shut indicator, or by using an automat c actuator.

Figure 6 Ball Valve

The spherical plug not only allows precise control of the ﬂ ow through the valve, but also gives a t ght shut- oﬀ when in the closed posit on. The valves are designed so that internal lubricat on is not required and the torque to rotate the ball is negligible. The ball and stem are generally machined from one piece. For larger sizes and higher pressure rat ngs, the ball is constructed with a double stem and is supported by bearings. This construct on requires a seal for one end and packing box for the opposite end. Figure 7 shows the arrangement of a “V” ball valve.

Figure 7 “V” Ball Valve

Figure 8 illustrates the alternat ve manual-automat c operat on for this part cular valve. Ball valves are manufactured in sizes from 3 to 1000 mm and for pressures up to 69 000 kPa with service temperatures from -185°C to 550°C. Ball valves are suitable for handling slurries and ﬂ uids with a high solid content and, for this reason, have found wide applicat ons in the paper industry, chemical plants and sewage treatment plants.

Fourth Class • Part A2 Unit 11 • Chapter 50 • Introduction to Valves 158

Figure 8 Manual-Automatic Positioning

(Courtesy of Fisher Controls) Plug Valve The plug valve is a quarter turn valve, as are but erﬂ y and ball valves. It consists of a tapered or straight cylinder containing a hole inserted into the cavity of the valve body. The hole in the plug lines up with the axis and opening in the valve body. The valve illustrated in Figure 9 has a tapered plug secured in the valve body by the valve cover. A packing box is recessed in this cover, with packing held in place by the gland, thus prevent ng leakage along the valve stem. The tapered plug has a tendency to jam in the tapered seat and can cause scoring if forced to turn. Most plug valves are lubricated to eliminate this problem. The lubricant is supplied through the center of the stem and is distributed through channels to the seat ng surfaces. Other valves are equipped with a ﬂ exible, smooth liner which eliminates the need for lubricat on. The pressure drop across this valve, when it is in the wide open posit on, is very low. This valve is also self-cleaning. Plug valves are used as quick opening valves in the following: • Gas supply lines • Low pressure steam lines • Water treatment plants • Pulp and paper and chemical industries

Fourth Class • Part A2 Unit 11 • Chapter 50 • Introduction to Valves 159

Figure 9 Plug Valve

(Courtesy of Flowserve) Check Valve The check valve prevents reversal of fl ow in piping. The fl ow of fl uid keeps the check valve open while gravity and reversal of fl ow cause the valve to close. The two basic types of check valves are the swing check (Fig. 10(a)) and the lif check (Fig. 10(b)).

Figure 10(a) Swing Check Valve Figure 10(b) Lift Check Valve

Referring to Figure 10(a), the swing check valve consists of a fl ap or disk of the same diameter as the pipe bore which hangs down in the fl ow path. With fl ow in the forward direct on, the pressure of the fl uid forces the disk to hinge upwards, allowing fl ow through the valve. A reversal in fl ow causes the disk to shut against the seat and stop the fl uid from going back down the pipe. In the absence of fl ow, the weight of the fl ap is responsible for the closure of the valve. Swing check valves produce relat vely high resistance to fl ow in the open posit on, due to the weight of the disk. In addit on, they create turbulence, because the fl ap ‘fl oats’ on the fl uid stream which typically causes a larger pressure drop across a swing check valve than across other types. With abrupt changes in fl ow, the disk can slam against the valve seat which can cause signifi cant wear of the seat and generate water hammer along the pipe system. This problem can be overcome by fi t ng a damping mechanism to the disk and by using metal seats to limit the amount of seat wear.

Fourth Class • Part A2 Unit 11 • Chapter 50 • Introduction to Valves 160

The fl ow through the lif check valve, undergoes two changes in direct on as it passes through a horizontal sect on upon which the disk seats. The disk moves upward to allow the fl uid to pass through and moves downward to close if the fl ow should reverse. A dashpot is used to cushion the act on of the disk in this design. Lif check valves are similar in confi gurat on to globe valves, except that the disk or plug is automat cally operated. The inlet and outlet ports are separated by a cone shaped plug that rests on a seat typically made of metal. In some valves, the plug may be held on its seat using a spring. When the fl ow into the valve is in the forward direct on, the fl uid pressure lif s the cone off its seat, opening the valve. With a reversal in fl ow, the cone returns to its seat and is held in place by the reverse fl ow pressure. If a metal seat is used, the lif check valve is only suitable for applicat ons where a small amount of leakage under reverse fl ow condit ons is acceptable. Since the design of a lif check valve generally limits its use to water applicat ons, it is used to prevent reverse fl ow of condensate in steam traps. The main advantage of the lif check valve is its simplicity. As the cone is the only moving part, the valve is robust and requires lit le maintenance. In addit on, the use of a metal seat limits the amount of seat wear. The valve is designed only for installat on in horizontal pipelines.

Fourth Class • Part A2 Unit 11 • Chapter 50 • Introduction to Valves 161

OBJECTIVE 2

Describe design and operation of specialized boiler valves.

BOILER VALVE TYPES

ASME Code, Sect on I, states that each outlet from a power boiler (except safety valve connect ons) is to be fi t ed with a stop valve located as close as is pract cal to the boiler. The two types of valves commonly used on the steam outlet of a boiler are the gate valve and the globe valve, as illustrated in Figures 11 and 12, respect vely. The connect ons may be threaded, fl anged or welded. The gate valve is more likely to be used since it off ers the least resistance to fl ow and because it will be wide open during operat on, no throt ling is involved. It is recommended to use a stop valve of the outside-screw-and-yoke type with rising spindle when the outlet is 51 mm pipe size and larger. This type has the advantage that the operator can see, even from a distance, whether the valve is open or closed. Also, since the threaded part of the spindle is outside the valve body, it is not exposed to the corrosive act on of steam or water, and the thread can be easily lubricated. The stop valve (gate valve) in Figure 11 features an outside-screw-and-yoke with rising spindle. The handwheel is carried on the yoke and does not rise with the spindle. The stop valve (globe valve) in Figure 12 also features an outside-screw-and-yoke with a rising spindle. How- ever, the handwheel is carried on the spindle and rises with it. Non-Return Stop Valve The non-return stop valve, also referred to as a stop-and-check valve, is installed at the boiler outlet in cases where the boiler is connected to a common main with other boilers. In principle, it is a stop valve which includes a device for prevent ng a reversal of fl ow through the pipe line when the valve is open. The check valve will prevent a reverse fl ow of steam into the boiler from the common main. When the valve is opened, the valve head or piston not being connected to the spindle is lif ed from its seat by the steam pressure at the inlet, and is free to reseat itself independently. In the event of a reversal of fl ow, as illustrated in Figure 12, the pressure in the header is greater than that in the boiler, thus the valve or disk is shown in the closed posit on. This valve is designed for one-way fl ow only. If a boiler tube were to rupture, the pressure in the damaged boiler would drop and the non-return stop valve would automat cally close, thus prevent ng steam from the other boilers connected to the header from entering the damaged boiler. The non-return stop valve may be secured in the closed posit on by turning the spindle down against the valve.

Fourth Class • Part A2 Unit 11 • Chapter 50 • Introduction to Valves 162

Figure 11 Non-Return Stop Valve (Gate)

The stop valve in Figure 12 features an outside-screw-and-yoke with rising spindle.

Figure 12 Non-Return Stop Valve

Fourth Class • Part A2 Unit 11 • Chapter 50 • Introduction to Valves 163

OBJECTIVE 3

Describe piping arrangements and the design and operation of steam system pressure-reducing valves.

PIPING ARRANGEMENTS

Various arrangements of piping from boiler to header are illustrated in Figure 13. Each arrangement features a non-return valve closest to the boiler and a stop valve at the header. Drains are provided in each case to drain the piping between the two valves. (Ref. ASME Code, Sect on I).

Figure 13 Non-Return Stop Valves and Header Valve Arrangements

PRESSURE REDUCING VALVES

Quite of en, the supply pressure of such ut lit es as city water, natural gas and compressed air is considerably higher than the service pressure desired for certain types of equipment. Pressure reducing valves are used to lower the supply pressure to the equipment and maintain it at the required pressure. A descript on of two of the many diﬀ erent types of pressure reducing valves is as follows: • Spring operated reducing valve (internal diaphragm) • Spring-operated reducing valve (external diaphragm)

Fourth Class • Part A2 Unit 11 • Chapter 50 • Introduction to Valves 164

Spring-Operated Reducing Valve (Internal Diaphragm) This valve, illustrated in Figure 14(a), consists of two main parts, the valve housing and the bonnet, separated by a fl exible diaphragm. The bonnet is open to the atmosphere. A compression spring acts downward on the diaphragm forcing the valve to open. The space below the diaphragm is connected to the low pressure side of the valve and the fl uid (liquid or gas) exerts an upward force against the diaphragm counteract ng the force of the spring. At a set outlet pressure, these two forces are balanced and the valve is held open a certain distance allowing a certain level of fl ow through the valve. When the demand for fl uid increases, the pressure on the outlet side drops slightly result ng in a reduced upward force on the diaphragm. The spring force moves the diaphragm downwards which, in turn, forces the valve to open more. More fl uid is allowed to pass through and the outlet pressure is restored. The opposite happens when the demand for fl uid decreases. The outlet pressure can be adjusted by changing the compression of the spring by means of the adjust ng screw through the top of the bonnet. Turning the screw downwards increases the spring compression and raises the outlet pressure; turning the screw upwards results in a reduced outlet pressure. The adjust ng screw is secured by a locknut af er the fi nal adjustment has been made. The valve can either be directly connected to the diaphragm or, as is illustrated in Figure 14, be separate from the diaphragm, in which case a small spring plus the pressure of the fl uid on the high pressure side of the valve force the valve to follow the upward movement of the diaphragm.

Figure 14(a) Spring-Operated PRV Figure 14(b) Spring-Operated PRV (External Diaphragm) (Internal Diaphragm)

Spring-Operated Reducing Valve (External Diaphragm) The reducing valve, illustrated in Figure 14(b), operates on the same principle as a valve with an internal diaphragm. However, it diff ers in basic design in that the diaphragm and spring are mounted outside the valve housing on a yoke and they operate the valve by means of an extended valve stem. A control line connects one side of the diaphragm casing to the low pressure side of the valve. With this arrangement, the diaphragm is not aff ected by the temperature of the fl uid which can have a harmful eff ect on the condit on and life span of the diaphragm. The fl uid in the control line and the diaphragm casing assumes the temperature of the surrounding air. When used in steam lines, the line and casing are fi lled with condensate. All reducing valves should be installed with isolat ng valves on either side and with a by-pass valve and line so that the valve can be removed for repairs without a complete interrupt on of the supply. They should also be equipped with a pressure gauge and safety valve on the low-pressure side. This equipment is needed to ensure safe and cont nuous operat on of the system. Since the isolat ng valves are either wide open or fully closed (not subjected to throt ling), gate valves are preferred since they off er minimum resistance to fl ow. The by-pass valve, which throt les the fl ow when the reducing valve is out of service for maintenance or repair, should be a globe valve.

Fourth Class • Part A2 Unit 11 • Chapter 50 • Introduction to Valves 165

OBJECTIVE 4

Discuss valve details, including materials of construction and identiﬁ cation markings.

VALVE DETAILS

Materials of Construction Materials of construct on for valve bodies will be determined mainly by the pressure, temperature and type of fl uid in the applicat on. For example: • Cast iron is used for low pressure and temperature applicat ons • Bronze is used for moderate pressures and temperatures up to 280°C • Carbon steel is used for services up to 425°C • Alloy steel is used for high pressure applicat ons and temperatures up to 650°C • Special alloy steels are used for temperatures in excess of 650°C • Stainless steel is used for corrosive surfaces The body can have screwed or fl anged ends or be welded into the piping system. Valve trim, consist ng of the disk, seat ring, valve stem and guide bushings, if applicable, are manufactured from bronze, mild steel, alloy or stainless steel. Valve packing material, depending on the service, is made of Tefl on, Tefl on impregnated asbestos, graphi- t zed asbestos or semi-metallic packing. Non-Rising Stem In the case where a gate valve is to be used and head room is limited, the non-rising stem, inside screw design (Fig. 15) is used. With this type, as the stem is turned, the gate climbs up the threaded part of the stem which is inside the valve body.

Figure 15 Non-Rising Stem Valve

Fourth Class • Part A2 Unit 11 • Chapter 50 • Introduction to Valves 166

IDENTIFICATION OF VALVES

It is extremely important that the proper type of valve be used for a part cular service. Accidents have frequently occurred when a valve made of the wrong material has been installed in a pipe line. Therefore, all valves must be properly ident fi ed as to the material of construct on and service condit ons for which they are designed. All valves not properly or clearly ident fi ed should be rejected. All markings shall be legible and must indicate at least the following: • Manufacturer’s name or trademark • Service designat on, i.e., pressure-temperature for which the fi t ng is designated • Material designat on, i.e., steel or cast iron, ASTM Number, etc

Fourth Class • Part A2 Unit 11 • Chapter 50 • Introduction to Valves 167

OBJECTIVE 5

Describe typical valve maintenance requirements.

VALVE MAINTENANCE

Valve Leaks Visually inspect valves frequently and repair small leaks immediately. A leak in a valve of en can be remedied in a simple and fast way. Neglect in doing so can result in a mess and may lead to damage to bonnet and fl ange surfaces which, in turn, may require costly and t me consuming repairs. Valve leaks usually occur in the: • stem • bonnet and fl ange Stem Leaks Stem leaks can normally be fi xed by slightly t ghtening the packing nut or gland. Always t ghten up bolted glands evenly; otherwise, the gland will bind the valve stem. If insuffi cient packing is lef to stop leakage, renew the packing. The procedure for renewal of packing in stuffi ng boxes of valve stems is similar to that of pump shaf s. The only diff erence is that the valve stem packing may be t ghtened enough to stop all leakage since no constant fl ow of liquid is required for lubricat on. Never t ghten more than necessary to stop leakage since over t ghtening causes extra frict on result ng in wear and added eff ort in opening and closing the valve. Wear on stem packing is mainly due to the rising and turning mot on of the valve stem. New packing contains suffi cient lubricant to reduce the frict on to a minimum, but older packing may have exhausted its lubricant. A few drops of oil applied occasionally to the valve stem will reduce frict on and extend the life of the packing. Bonnet and Flange Leaks These leaks can be caused by insuffi cient t ghtening of the bolts or by bolts loosening under service strain. Try t ghtening the bolts fi rst to stop the leak, but do not overstress them. If t ghtening the joint does not stop the leak, the gasket should be replaced as soon as possible. Neglect ng to stop fl ange leaks will result in “wiredrawing” on the faces of the fl anges. Wiredrawing is the forming of deep grooves, running from the center of the fl ange to the outside, caused by the scouring act on of the escaping fl uid. Lubricat ng the external thread of the valve stem will cut down frict on, wear and eff ort in operat ng the valve. Valves equipped with a grease nipple should receive a shot of grease periodically to supply the threaded bushing in the top part of the yoke with lubricant. Internal Inspection Most valves are designed to permit internal inspect on without removing the valve body from the line. Periodic inspect on of the valve disk and seat is the best prevent ve maintenance. The complete bonnet and disk assembly can be removed for cleaning and inspect on. Check the seat ng surfaces in the body at the same t me. If inspect on of the valve disk (or the wedge) and the seat shows damage, such as grooving and wire-drawing, the seat ng surfaces should be refaced. Globe valves equipped with a composit on disk should have the disk replaced as soon as a seat leak is discovered. It is a simple procedure and early replacement will prevent damage to the seat.

Fourth Class • Part A2 Unit 11 • Chapter 50 • Introduction to Valves 168

To repair damaged valve disks and seats of globe valves, the following procedures can be used: 1. Slightly damaged surfaces can be refaced by grinding the valve disk in on the valve seat in combina- t on with a grinding compound. 2. If damage is more extensive, the seat should be refaced with a reseat ng tool. The valve disk should be refaced either by carefully ﬁ ling down the damaged seat ng surface or by taking a slight cut oﬀ on the lathe or grinding machine. Then the disk and seat should be ground in together to obtain matching seat ng surfaces. 3. When the valve is equipped with a renewable seat and the disk and seat are extensively damaged, it will be cheaper to replace both rather than to repair. The seat ng surfaces of gate valves are much harder to reface than those of globe valves. Special equipment is required to do a sat sfactory job.

Fourth Class • Part A2 Unit 11 • Chapter 50 • Introduction to Valves 169

CHAPTER 50 - QUESTIONS INTRODUCTION TO VALVES

1. The kind of valve that has minimum wire drawing and seat erosion is the a) ball valve. b) globe valve. c) check valve. d) gate valve.

2. To ensure safe and cont nuous operat on of a pressure-reducing valve, the valve stat on should have a) a safety valve on the downstream side. b) a by-pass loop, downstream safety valve and a downstream pressure gauge. c) the piping to the stat on made of at least Schedule 80 piping. d) a Y-strainer immediately af er the valve.

3. What is the maximum temperature (°C) for which a non-special alloy steel valve can be used? a) 343 b) 425 c) 925 d) 650

4. Pressure and/or temperature rat ng of a fi t ng is known as the a) material designat on. b) ident fi cat on of the type of construct on material. c) type of appliance that can be at ached to the fi t ng. d) service designat on.

5. The basic type of ball valve requires a ______turn from the fully open to the fully closed. a) half b) quarter c) full d) three quarters

6. But erﬂ y valves have which of the following advantages? a) relat vely light weight b) self cleaning c) negligible pressure drop across the valve when fully open d) all of the above

Fourth Class • Part A2 Unit 11 • Chapter 50 • Introduction to Valves 170

CHAPTER 50 - ANSWERS INTRODUCTION TO VALVES

1. (b)

2. (b)

3. (d)

4. (d)

5. (b)

6. (d)

Fourth Class • Part A2 4th Class • Part A2 U N I T 1 2

HIGH PRESSURE BOILER DESIGN

Chapter 51 Introduction to Boilers 173

Chapter 52 Firetube Boilers 185

Chapter 53 Watertube Boilers 197

Chapter 54 Electric Boilers 213

Chapter 55 Basic Boiler Construction 223

171 172 4th Class • Part A2 C HAPTER 51

Introduction to Boilers

LEARNING OUTCOME

When you complete this chapter you should be able to: By using common terms relating to boilers discuss the historical developments of, and the general requirements for proper boiler design.

LEARNING OBJECTIVES

Here is what you should be able to do when you complete each objective:

1. Apply common terminology used in the description of boilers.

2. Describe early boiler designs and explain developments that improved boiler operation.

3. List the general requirements for proper boiler design.

173 Unit 12 • Chapter 51 • Introduction to Boilers 174

INTRODUCTION

The majority of larger industrial, commercial or inst tut onal buildings are equipped with boilers to supply steam or hot water for processing or to the heat ng systems and/or the heat ng coils in air condit oning systems during the colder seasons to maintain the temperature at a level which is comfortable for the occupants. Even though the modern boiler is automated for easier operat on, it requires regular supervision and maintenance to achieve uninterrupted, effi cient and safe operat on. The sat sfactory operat on of a boiler depends to a great extent on the knowledge that the boiler operator has of the proper operat onal and maintenance procedures. A steam boiler is basically a closed container, part ally fi lled with water which is evaporated into steam under pressure by the applicat on of heat. This heat is usually obtained from the burning of a fuel such as gas, oil or coal in a furnace, although in some cases, electrical elements may be used to provide the heat. A hot water boiler is also a closed container, but it is completely fi lled with water. By applying heat, the temperature and pressure of the water is raised but no steam is generated. Although it is called a boiler, the water in it does not “boil”, since no steam is produced. Provincial legislat on defi nes a boiler as a pressure vessel in which a gas or vapour can be generated under pressure, or in which a liquid can be put under pressure, by the direct applicat on of a heat source.

4th Class • Part A2 Unit 12 • Chapter 51 • Introduction to Boilers 175

OBJECTIVE 1

Apply common terminology used in the description of boilers.

GENERAL BOILER TERMS

Baffle A baffl e is a wall, barrier or panel used to change the direct on of fl ow of a liquid or gas. On the waterside of a boiler, baffl es may be used to direct the fl ow of water inside drums or headers. On the fi reside of the boiler, baffl es may be used to redirect the fl ow of hot combust on gases through banks of tubes. Fireside baffl es are usually made of high temperature refractory material. Blowdown Valves Blowdown valves are located on the line leading from the lowest part of the waterside of the boiler. Power (high-pressure) boilers usually require a fast opening (also called a guard valve) and a slow opening valve. Heat ng (low-pressure) boilers usually only require one blowdown valve. Boiler Failure Boiler failure, a term usually applied to the pressurized parts of the boiler, occurs when a ruptured or cracked tube allows high-pressure steam or water to escape, requiring the boiler to be shut down. Catastrophic boiler failure is observed when a combust on explosion, or rapid pressure release from the water side of the boiler, causes further destruct on of the boiler and may cause damage to surrounding buildings, equipment or personnel. Combustion Chamber The combust on chamber, also called the furnace or fi rebox, is where air and fuel combine in a chain react on to cause sustained burning. Combustion Gases Combust on gases are the hot gases from the fi re. These are also called “fl ue gases”. Combustion Gas Pass The combust on gas pass is the path that the combust on gases travel along the length of the boiler. If the gases are reversed and passed through the boiler again, this is a second pass. For example, in a two pass boiler, combust on gases make two sweeps or passes through the boiler. Condensate When steam from a boiler is used for heat ng, processing, or power generat on, heat energy is given up to the process and the steam condenses. The water formed from condensing steam is called condensate. Condensate is usually returned to the boiler to be converted to steam again. Drum The boiler shell, together with the heads, forms a drum to contain the fl uid being heated. The term shell and drum are of en used interchangeably. Shell may be more common in terms of fi retube boilers, where drum is more of en used for watertube boilers. Externally-Fired Boiler An externally-fi red boiler is a boiler with the combust on chamber outside of the boiler shell. This type of furnace is not surrounded by water but is surrounded by brickwork.

4th Class • Part A2 Unit 12 • Chapter 51 • Introduction to Boilers 176

Feedwater Feedwater is the water that is fed into a steam boiler to replace the water which has been converted to steam and drawn off from the boiler. Feedwater usually includes the condensate. Firetube Boiler A fi retube boiler is a boiler consist ng of a drum containing straight tubes through which the hot combust on gases from the fi re travel. Water in the drum surrounds the tubes and the heat from the hot gases is transferred through the tube walls to the water. Fitting A fi t ng on a boiler is any valve, gauge, regulat ng or controlling device, fl ange, pipe fi t ng or other at achment on the boiler. Flame Scanner A fl ame scanner is a device used to monitor the fl ame in a boiler. If the fl ame is ext nguished for any reason, the scanner may send a signal to close the fuel supply valve to prevent a possible explosion. Flue Gases Flue gases are the hot gaseous products from the fi re. These are of en referred to as “combust on gases”. Forced Draft Fan A forced draf fan supplies air to the furnace for the combust on process. Gauge Glass A gauge glass is a strong tubular glass used to indicate the water level in a steam boiler. Some high pressure boilers use heavy fl at glass sect ons fi rmly clamped to a hollow metal frame, or circular glass “bullseyes” arranged in a vert cal column. Generally Supervised Boiler A generally supervised boiler is one that can operate without cont nual control by an operator. The boiler may operate overnight or on weekends without an operator present. However, through the week, someone must occasionally check the boiler and verify that: • All controls are working • The boiler water is correctly treated • The boiler is working properly This type is also known as an automat c or protected boiler. Handhole A handhole is a small, hand-sized inspect on, cleaning, and maintenance port leading from the outside of the boiler into the pressure area of the boiler. A handhole cover seals the port during operat on. Header A header is a large pipe which supplies to, or collects from, a series of smaller pipes or tubes. It is also called a manifold. Heads Heads are the steel plates which close off the ends of the boiler drum. They are also referred to as end plates. If the shell contains tubes which are held in posit on by the heads, then they are usually called tube sheets. Heating Surface Heat ng surface includes all parts of the boiler through which heat from the burning fuel is transferred to the waterside of the boiler. It includes all parts of the boiler plates and tubes which have water or steam on one side and are swept by fi re or hot combust on gases on the other side.

4th Class • Part A2 Unit 12 • Chapter 51 • Introduction to Boilers 177

High-Pressure Steam Boiler A high-pressure steam boiler is one which operates at pressures above 103 kPa. They are also called power boilers. Horizontal Return Tubular Boiler (HRT) An HRT boiler is a fi retube boiler usually supported in a brick combust on chamber. The hot gases from combust on sweep along the underside of the shell then return through the fi retubes to the chimney connect on. Hot Water Boiler A hot water boiler is a closed vessel completely fi lled with water to which heat is added to raise the temperature of the water. No steam is generated in this boiler. Types of hot water boilers include: • Low-pressure hot water heat ng boilers • Hot water supply boilers (commonly called water heaters or hot water tanks) • High temperature hot water boilers Depending on the regulat ons of specifi c jurisdict ons, hot water supply boilers are not usually considered to be low-pressure heat ng boilers unless one or more of the following is exceeded: • Capacity – 454 L • Temperature – 93°C • Energy input – 58.7 kW Low-pressure heat ng boilers are considered as such unless the temperature and/or pressure exceed 120°C and 1100 kPa, respect vely. Above this range, they are classed as high temperature hot water boilers. Induced Draft Fan An induced draf fan draws combust on gases out of the boiler and discharges them up the chimney. Internally-Fired Boiler An internally-fi red boiler has the combust on chamber located within the shell of the boiler, or a furnace surrounded by watertubes. Limit Controls Limit controls are the controls that have their set point adjusted higher than the boiler operat ng controls set point. If, for any reason, the normal operat ng controls fail to limit the rise in temperature or pressure to a safe value, the limit controls will shut off the boiler fuel supply. Lowest Permissible Water Level The lowest permissible water level is the lowest level at which the boiler can be safely operated without damaging or overheat ng any part of the boiler. For heat ng boilers, this level is specifi ed and marked by the manufacturer. The lowest visible part of the water gauge glass is 25 mm above this lowest permissible water level. For power boilers, the lowest visible part of the gauge glass is 50 mm above this lowest permissible water level. For HRT boilers, the lowest visible part of the gauge glass is set not less than 76 mm above the highest point of the tubes, fl ue or crown sheet. Low-Pressure Steam Boiler A low-pressure steam boiler is a boiler which operates at a pressure not above 103 kPa. They are also called heat ng boilers. Low-Water Fuel Cutoff A low-water fuel cutoff is a safety device which cuts off the fuel supply to the burner if the boiler water level drops below a safe level.

4th Class • Part A2 Unit 12 • Chapter 51 • Introduction to Boilers 178

Manhole A manhole is an opening or hatch through which a person may enter into the shell or drum of a boiler. Operating Controls Operat ng controls are the controls which operate the combust on and feedwater equipment on a boiler. Changes in steam demand from the boiler require a corresponding change in the boiler fi ring rate and feedwater fl ow rate. Packaged Boiler A packaged boiler is a boiler supplied by the manufacturer completely equipped and mounted on its own base. Pressure Gauge A pressure gauge is a fi t ng at ached to a boiler to indicate the internal pressure of the boiler. All steam boilers must be equipped with an accurate pressure gauge. It is somet mes called a steam gauge. Refractory Refractory is the protect ve layer of material that is applied to various parts of the boiler to withstand high temperatures and abrasion. It usually has an appearance similar to cement or bricks. Rupture Disk A rupture disk is a safety device which acts like a safety valve to protect against excessive pressure build-up in a system. However, the disk ruptures when its maximum pressure is reached and must be replaced each t me it act vates. Safety Valve A safety valve is a fi t ng that prevents the pressure within a steam boiler from exceeding safe limits. When the pressure inside the boiler reaches a set point, the valve will pop open and reduce the pressure to another preset point and then close. Safety relief valves are used where liquids are present. This type of valve does not “pop” open but only bleeds off enough liquid to relieve the excess pressure. Setting The set ng is the brickwork used to support a boiler and surround the combust on chamber. This term is part cularly used for such boilers as a horizontal return tubular boiler (HRT). Sootblower A sootblower is a device used to blow accumulated soot off tubes and heat ng surfaces in a boiler. Air or steam issues from nozzles to dislodge the soot or fl yash as the sootblower traverses or is rotated to clear the deposit from a sect on of the boiler. Stack The stack is the hollow duct through which combust on gases are elevated for discharge to the atmosphere. The hot combust on gases, rising through the stack, cause a draf to be created in the boiler. A stack may also be called a smoke stack or chimney. Steam Drum A steam drum is a closed container part ally fi lled with water. The water is evaporated to steam under pressure by the applicat on of heat. The steam is piped away for specifi c purposes. Steam Space The steam space is the space above the water line in a steam boiler where the boiling water and steam can separate from each other. The space also acts as a pressure reservoir to accommodate small load fl uctuat ons. Steam Stop Valve

4th Class • Part A2 Unit 12 • Chapter 51 • Introduction to Boilers 179

The steam stop valve, the main valve on the steam line leaving the boiler, must be able to posit vely halt the fl ow of steam. Uptake The uptake is the duct used to convey the spent combust on gases from the boiler to the stack or chimney. This is also known as the fl ue, vent, or breeching. Vertical Tubular Boiler A vert cal tubular boiler is a smaller sized fi retube boiler with vert cal fi retubes. Some hot water supply boilers are of the vert cal tubular type. Water Column A water column is a chamber at ached to the top and bot om of a boiler’s steam drum. The water level in the column should be the same as in the boiler. A gauge glass is normally at ached to the column to give a visual indicat on of the water level. Water Line The water line is the actual level of water in a boiler. It is the point at which water and steam separate. Water Space The water space is the port on of the boiler which is normally fi lled with water. Waterleg A waterleg is a water-fi lled sect on extending from the shell, which surrounds the fi rebox of some types of fi retube boilers. Most locomot ve boilers are equipped with waterlegs around the fi rebox. Watertube Boiler A watertube boiler is a boiler consist ng of drums and headers that circulate water through tubes which are heated by fi re and the products of combust on. The heat from the fi re is transmit ed through the tubes to the water. Windbox The windbox is the box surrounding the burner damper on a boiler. The forced draf fan blows combust on air into the box and the damper regulates and directs the air into the burners.

4th Class • Part A2 Unit 12 • Chapter 51 • Introduction to Boilers 180

OBJECTIVE 2

Describe early boiler designs and explain developments that improved boiler operation.

BASIC DEVELOPMENT OF BOILERS

Steam boilers have been in use for thousands of years. The ancient Greeks and Romans used them extensively for heat ng buildings and public baths and had even invented a simple steam engine by the ﬁ rst century AD. The earliest steam boiler (Fig. 1) consisted of a single closed container or shell, part ally ﬁ lled with water and enclosed in a brick set ng which formed a furnace under the shell. An example of this early design was the Haycock Boiler from 1720.

Figure 1 Early Boiler Design

The fuel was burned on a grat ng near the front of the boiler. The fl ames and hot gases played against the lower part of the shell as they travelled towards the rear of the boiler on the way to the chimney. The combust on heat was conducted through the lower half of the shell and heated up the water inside the shell. When the water reached its boiling temperature, steam was generated and collected in the upper part of the shell. The part of a boiler through which the heat is transferred from the combust on gases to the boiler water is called the heat ng surface. In this early boiler, the heat ng surfaces consisted only of the lower half of the shell. Since the furnace was located outside the shell, this type of boiler was classifi ed as an externally fi red boiler. This boiler design was very ineffi cient because the heat ng surface was small and the t me the hot gases were in contact with the heat ng surface was short. Most of the heat generated by the combust on of the fuel went up the chimney instead of being absorbed by the boiler water. Also, a large amount of heat was lost through the brick walls of the furnace. Advances Over the years, various improvements were made to the design of the earlier boiler; as a result, the effi ciency improved considerably. Major improvements were made by the following changes. • The addit on of a chimney or stack improved combust on under the boiler • The shell or drum was lengthened to contain higher pressures and increase the heat ng surface • To reduce heat losses through the brick walls the furnace (fi rebox) was placed inside the shell and completely surrounded by water; the boiler was now internally fi red

4th Class • Part A2 Unit 12 • Chapter 51 • Introduction to Boilers 181

• The heat ng surface was further increased by the installat on of tubes mounted horizontally between the heads of the boiler. The hot combust on gases passed through these tubes and gave up their heat to the boiler water surrounding the tubes. Since these tubes conducted hot gases or fi re they were called fi retubes. Today, a boiler pass is the travel of the combust on gases once along the length of the boiler. Some boilers pass the hot gases through the boiler water via fi retubes 4 t mes or more.

Figure 2 A 2 Pass Horizontal Return Tubular (HRT) Boiler

The advent of fi retubes represents an important stage in boiler evolut on. Unt l this point, all designs were very similar, basically ket les on fi res. Firetubes led boiler designers to realize that there were two ways of maximizing the interact on between boiler water and combust on gases. They could pass hot gases through tubes running through the liquid (fi retubes) or they could pass the water through tubes surrounded by the hot combust on gases. These are known as watertubes since the tubes are fi lled with water. Boilers consist ng of a water fi lled shell containing tubes through which the hot gases travel from the fi re to the stack are classifi ed as fi retube boilers. An HRT fi retube boiler is shown in Figure 2. Boilers that feature drums connected by tubes which are fi lled with water are classifi ed as watertube boilers Water circulates from the drum through the tubes and back to the drum. The heat from the fi re and hot gases comes in contact with the outside of the tubes. Figure 3 shows an early style of straight tube watertube boiler.

Figure 3 Watertube Boiler

4th Class • Part A2 Unit 12 • Chapter 51 • Introduction to Boilers 182

OBJECTIVE 3

List the general requirements for proper boiler design.

GENERAL REQUIREMENTS FOR PROPER BOILER DESIGN

The chief aim in modern boiler design is the product on of a boiler that will be safe and effi cient in operat on and economical in fuel consumpt on. To ensure this result, the following points must be observed: 1. A boiler must have a large heat ng surface so that the maximum amount of heat can be absorbed. 2. All parts of the boiler heat ng surface exposed to fi re or hot gases must be covered by water. 3. A boiler must have a thorough circulat on of water through all parts of it to prevent overheat ng of any part of the heat ng surface. 4. A boiler must be properly insulated to minimize heat loss to the surroundings. 5. The steam space must be large and the steam able to rise freely from the surface of the water, so that any water contained in the steam may be separated before the steam is drawn off from the boiler. 6. All parts of the boiler must be readily accessible for inspect on, cleaning, and repairs. 7. A boiler must incorporate proper workmanship, simple construct on, and low maintenance costs. 8. A boiler must have strong enough construct on to withstand the high pressures and temperatures. 9. A boiler must have proper allowance for expansion and contract on of all parts. 10. A boiler must be furnished with the approved fi t ngs such as gauges and safety valves. In general, the boiler should be designed to absorb the maximum amount of heat available from the furnace and, at the same t me, provide maximum safety and reliability in operat on. The design should also feature compactness to keep down building costs but provide adequate access to the parts for maintenance and inspect on. From their beginnings, diff erent types of boilers have evolved to take care of specifi c industrial requirements. High pressure, temperature, and capacity boilers are used for producing electrical power. Thermo fl ooding boilers are used to inject large amounts of heat energy into geological format ons. Some process boilers have been developed which use fl uids other than water as the heat transfer medium. In buildings, fi retube, watertube and cast-iron sect onal boilers are used for heat ng and cooling systems.

4th Class • Part A2 Unit 12 • Chapter 51 • Introduction to Boilers 183

CHAPTER 51 - QUESTIONS INTRODUCTION TO BOILERS

1. The steam space of a boiler consists of a) highest point of the heat ng surface. b) port on of the boiler not ﬁ lled with water. c) inside area of the economizer. d) port on of the boiler below the waterline.

2. The steam space of a boiler must be large enough to a) accommodate all the steam riser tubes. b) separate the water from the steam. c) accommodate a steam separator. d) keep the steam pressure down.

3. All parts of the boiler exposed to ﬁ re or ﬂ ue gases must be covered by a) blanket insulat on. b) metal cladding. c) refractory. d) water.

4. A steam boiler should be compact in design in order to a) allow a larger steam generat ng capacity. b) reduce the size of the feedwater supply line. c) reduce the size of the stack. d) reduce building costs.

5. Power boilers usually require ______blowdown valves. a) 1 b) 2 c) 3 d) 4

6. High-pressure boilers operate at pressures above a) 1000 kPa. b) 500 kPa. c) 2500 kPa. d) 101.3 kPa.

Fourth Class • Part A2 Unit 12 • Chapter 51 • Introduction to Boilers 184

CHAPTER 51 - ANSWERS INTRODUCTION TO BOILERS

1. (b)

2. (b)

3. (d)

4. (d)

5. (b)

6. (d)

Fourth Class • Part A2 4th Class • Part A2 C HAPTER 52

Firetube Boilers

LEARNING OUTCOME

When you complete this chapter you should be able to: Discuss the design, components and characteristics of HRT, locomotive, ﬁ rebox, Scotch and packaged ﬁ retube boilers.

LEARNING OBJECTIVES

Here is what you should be able to do when you complete each objective:

1. Describe horizontal return tubular and locomotive type boilers.

2. Describe ﬁ rebox, scotch and heating boilers.

3. Describe vertical and packaged ﬁ retube boilers.

185 Unit 12 • Chapter 52 • Firetube Boilers 186

INTRODUCTION

Firetube boilers consist of a water ﬁ lled shell containing tubes through which the ﬂ ue gases travel. The water in the shell surrounds the tubes and the heat from the hot gases is transferred to the water. Firetube boilers feature simple, rugged construct on and low cost. Their large water volume makes them somewhat slow in coming up to operat ng pressure and temperature, but the large amount of heat stored in the water makes it possible to meet load changes quickly.

BOILER PASSES

The longer the heat ng surface, the more heat exchange can occur between the boiler water and the ﬂ ue gases. To accomplish this exchange, ﬁ retubes pass repeatedly through the boiler drum (shell), as shown in Figure 1. A boiler pass is the travel of the combust on gases once along the length of the boiler.

Figure 1 Firetube Boiler Passes

While addit onal passes improve the effi ciency of the boiler, they also slow the fl ue gas velocity. To maintain fl ue gas velocity, addit onal passes must have fewer tubes or fans may be added to force the fl ue gases through the boiler. In a four-pass boiler, the fl ue gases fl ow from front to rear through the furnace fl ue marked in black in Figure 2(a), then the rear refractory baffl es direct the fl ue gases from rear to front through the lower fi retubes as shown in Figure 2(b). At the front, the gases are directed upward into the third pass toward the rear again, Figure 2(c). At the rear, another refractory baffl e directs the gases into the top rows of tubes toward the front and into the stack. As the gas temperature decreases during the travel through the boiler passes, the volume of the gases will decrease proport onally. To maintain high gas velocit es throughout all gas passes, the cross-sect onal area of each succeeding pass is decreased. To accomplish this, fewer tubes are used in each successive pass. This way the same size tubes can be used for all passes.

Figure 2 Flue-Gas Flow in a Four Pass Boiler

4th Class • Part A2 Unit 12 • Chapter 52 • Firetube Boilers 187

OBJECTIVE 1

Describe horizontal return tubular and locomotive type boilers.

HORIZONTAL RETURN TUBULAR (HRT) BOILERS

The horizontal return tubular boiler (HRT) is an early fi retube boiler design. It has an externally-fi red brick furnace; the combust on gases make two passes as they travel from the furnace to the chimney. The HRT boiler is less costly to build, is easy to clean and has a relat vely large water capacity. The HRT boiler is labour intensive to construct and ineffi cient to operate. Although these boilers are no longer constructed, there are st ll some in operat on, mostly in small industrial and heat ng plants. A type of HRT boiler is shown in Fig. 3.

Figure 3 HRT Boiler

LOCOMOTIVE TYPE BOILERS

The locomot ve type boiler which has a capacity up to 6800 kg/h at 2400 kPa is characterized by a fi rebox surrounded by waterlegs on four sides. Having the fi rebox built into the boiler (internally-fi red) increases the overall heat ng surface and eliminates the brick set ng. Locomot ve boilers also have an extensive network of stays, structural braces needed to support the surfaces exposed to steam or water pressure. Rugged construct on makes this unit suitable for portable service in heat ng plants, small industrial plants and the oil fi eld industry. Diagonal stays at ached with rivets are used to support the upper sect ons of the end plates from the shell. Radial stays are used to stay the crown sheet which is curved in current pract ce. A locomot ve boiler, with a waterleg that extends only down the sides, is called a “Dry Bot om”, shown in Figure 4.

4th Class • Part A2 Unit 12 • Chapter 52 • Firetube Boilers 188

Figure 4 Dry Bottom Locomotive Boiler

A locomot ve boiler with a waterleg that encloses the bot om of the furnace is referred to as “Wet Bot om,” shown in Figure 5.

Figure 5 Wet Bottom Locomotive Boiler

Locomot ve type boilers are generally inexpensive to construct. Unfortunately, they also required a large amount of staying, have poor water circulat on, are diﬃ cult to maintain and provide poor cleaning and inspect on access.

4th Class • Part A2 Unit 12 • Chapter 52 • Firetube Boilers 189

OBJECTIVE 2

Describe ﬁ rebox, scotch and heating boiler

FIREBOX BOILER

The fi rebox boiler is an externally-fi red, two-pass, horizontal fi retube boiler, similar to the HRT. The shell is made in two sect ons and there are two groups of fi retubes. The combust on gases travel from the fi rebox, through the tubes in the lower shell sect on to the rear of the boiler. Then they reverse through the tubes in the upper shell sect on to the chimney. Usually the fi rebox is surrounded by a brick set ng enclosed in a steel casing, as shown in Figure 6.

Figure 6 Firebox Boiler Showing Gas Passes

Firebox boilers are mostly used as heat ng boilers. They are compact, eﬃ cient and init ally inexpensive. How- ever, they may be diﬃ cult to access for cleaning and inspect on. In higher pressure service, the brick set ng type can have a capacity of 6800 kg/h up to 1720 kPa.

4th Class • Part A2 Unit 12 • Chapter 52 • Firetube Boilers 190

SCOTCH MARINE BOILER

The scotch boiler is a horizontal, internally-ﬁ red, ﬁ retube boiler. If the rear of the boiler is brick-lined, as shown in Figure 7, the boiler is classed as a dryback Scotch boiler, commonly used for heat ng. Models with a waterleg in the back are known as wetback Scotch boilers, designed for use in a marine service applicat on.

Figure 7 Scotch Dryback Boiler

Scotch boilers incorporate a corrugated furnace that allows for diff erent al expansion between the furnace tube and fi re tubes, thus reducing stress on the tubesheets and increasing the strength of the furnace while allowing the wall to be thinner which improves heat transfer. In both types, the large furnace tube is surrounded by water and provides addit onal heat ng surface Scotch boilers are very popular due to their effi ciency, low cost and compactness. Their main drawback is that they can be diffi cult to access for cleaning and inspect on.

HEATING BOILERS

Firetube heat ng boilers are essent ally designed the same as power boilers. The primary factor separat ng heat ng boilers from power boilers is pressure. The pressure in heat ng boilers must not exceed 101.3 kPa. The temperature of the water does not exceed 120°C and the pressure does not exceed 1100 kPa in hot water boilers. Hot water supply boilers (similar to the hot water tank in your home) are not considered to be heat ng boilers unless one of the following is exceeded: 1. capacity 454 litres 2. temperature 93°C 3. energy input 58.6 kW Firetube boilers may be used for steam or hot water operat on. In hot water service, a forced circulat on pump is usually necessary to maintain even heat distribut on inside the boiler.

4th Class • Part A2 Unit 12 • Chapter 52 • Firetube Boilers 191

OBJECTIVE 3

Describe vertical and packaged ﬁ retube boilers.

VERTICAL FIRETUBE BOILERS

The vert cal fi retube boiler is similar to the horizontal fi retube boiler. However, the fi retubes run vert cally from the furnace to the chimney, a useful feature when fl oor space is limited as the boiler occupies only a small area. A common type of vert cal heat ng boiler is the steel plate boiler (Fig. 8), a small capacity boiler commonly used for hot water heat ng systems in residences and small apartments. The lower part is the combust on chamber or furnace; the upper part is the actual boiler fi lled with water. The combust on gases travel upwards through the fi retubes and are collected by a fl ue gas collector box connected to the chimney. In order to help the heat exchange from the gases to the tube walls, baffl es made of thin steel strips are installed inside the tubes which give the gases a swirling mot on. Water returning from the heat ng system is forced into the lower part of the boiler shell by a circulat ng pump. The water then fl ows upward around the tubes, picks up heat from the tube walls, leaves the boiler through the top outlet and goes back to the heat ng system. The gases travel through the boiler in one direct on only; for this reason, this boiler can also be classed as a single-pass boiler.

Figure 8 Oil-Fired Steel Plate Hot Water Boiler

(Courtesy of American Standard)

4th Class • Part A2 Unit 12 • Chapter 52 • Firetube Boilers 192

PACKAGED FIRETUBE BOILER

Early fi retube boilers were always manufactured at the factory, shipped to the site, and then assembled. Re- fractory, insulat on, boiler fi t ngs, controls and fi ring equipment were installed locally by various tradesmen. Today, packaged fi retube boilers represent the majority of those being manufactured. Designs usually range from two-pass to four-pass Scotch boiler type, and can be either dryback or wetback. A packaged fi retube boiler (Fig. 9) is engineered, built and fi re tested before shipment. It comes as a completely equipped unit mounted on its own base, ready for operat on as soon as it is placed in the boiler room and hooked up to the various supply and discharge piping. Packaged fi retube boilers are used as power boilers or for heat ng service. In power boilers, the tube sheets, drum, tubes and all boiler fi t ngs will be of heavier design and construct on to match the condit ons under which they operate.

Figure 9 Three-Pass Packaged Firetube Boiler

Advantages of a Packaged Boiler The main advantage of a packaged boiler is that it can be mass produced which reduces product on costs and makes it lower in fi rst cost than a boiler of the same capacity assembled at the site. Also, bet er workmanship can be expected as the boiler is assembled by more experienced and qualifi ed personnel. Packaged boilers require a much smaller fl oor space, building volume and height. The boiler can be put to use in a shorter length of t me af er shipment as it can be levelled quickly and requires only a reinforced, fl at concrete slab as a foundat on. The connect on of steam, water, fuel piping, stack and electricity is all that is necessary. Most packaged boilers are easily accessible for cleaning, inspect on and repair due to hinged doors at the front and rear. These doors can be opened by taking out a few bolts, thus making inspect on or repair easier and more convenient. Another advantage of the fi retube boiler is in the water treatment area. Watertube and tubular boilers must be chemically controlled to prevent the format on and deposit on of scale or sludge in the tubes. The fl ow area inside a tube decreases with a deposit. As fl ow is restricted, the temperature inside a tube may increase causing the deposit to form more quickly. Tubes can be completely choked off by scale and the temperature of the tube metal will rise to the point where failure could occur. Scale format on on the outside of fi retubes is not desirable either, but the overall heat transfer of the tube cannot be restricted by a deposit at just one point on the tube. Water treatment is important in fi retube boilers but errors in treatment do not have immediate eff ects to the same extent as in watertube boilers. Figure 10 shows a packaged fi retube boiler in which the combust on gases make four passes through the boiler on their way from burner to uptake. A forced draf fan is located at the boiler front to deliver combus- t on air to the burner and to force the combust on gases through the boiler.

4th Class • Part A2 Unit 12 • Chapter 52 • Firetube Boilers 193

Figure 10 Firetube Packaged Boiler - Cutaway View

(Courtesy of Cleaver-Brooks) Figures 11(a) and 11(b) show the hinged front and back doors, which allow access to the tubes and tube sheets.

Figure 11 (a) Hinged Back Door (b) Hinged Front Door

4th Class • Part A2 Unit 12 • Chapter 52 • Firetube Boilers 194

A schemat c of a four-pass design is shown in Figure 12.

Figure 12 Four-Pass Design

4th Class • Part A2 Unit 12 • Chapter 52 • Firetube Boilers 195

CHAPTER 52 - QUESTIONS FIRETUBE BOILERS

1. What is a major disadvantage of a fi rebox boiler? a) fi rst cost is relat vely high b) cleaning and inspect on can be diffi cult c) it is not very compact d) it has a low heat transfer effi ciency

2. For what use was the wetback Scotch boiler primarily designed for? a) very high capacity electric generat on b) marine service c) building heat ng service d) railway service

3. What boiler type is built ready for installat on? a) Locomot ve boiler b) Scotch marine boiler c) HRT boiler d) Packaged boiler

4. Hot water supply boilers are not considered to be heat ng boilers unless the operat ng temperature exceeds a) 50°C. b) 200°C. c) 175°C. d) 93°C.

5. What is increased to augment the heat transfer capability in a ﬁ retube boiler? a) diameter of tubes b) number of tube passes c) thinner walled tubes d) boiler feedwater temperature

Fourth Class • Part A2 Unit 12 • Chapter 52 • Firetube Boilers 196

CHAPTER 52 - ANSWERS FIRETUBE BOILERS

1. (b)

2. (b)

3. (d)

4. (d)

5. (b)

Fourth Class • Part A2 4th Class • Part A2 C HAPTER 53

Watertube Boilers

LEARNING OUTCOME

When you complete this chapter you should be able to: Describe various watertube boiler designs, including large generating units.

LEARNING OBJECTIVES

Here is what you should be able to do when you complete each objective:

1. Describe the operating principle and design of watertube boilers.

2. Explain the design and application of packaged watertube boilers.

3. Describe the design, construction and components of large scale steam generating units.

4. Describe the design of watertube and copper-tubular heating boilers.

197 Unit 12 • Chapter 53 • Watertube Boilers 198

INTRODUCTION

Boilers that feature drums connected by water ﬁ lled tubes are classiﬁ ed as watertube boilers. The combus- t on gases travel over the outside surfaces of these tubes and transfer their heat to the water within.

4th Class • Part A2 Unit 12 • Chapter 53 • Watertube Boilers 199

OBJECTIVE 1

Describe the operating principle and design of watertube boilers.

OPERATING PRINCIPLE

Circulation of Water Hot water rises. This simple principle is crit cal to the operat on of a watertube boiler. Although all of the water in a boiler is hot, diﬀ erences in temperature do exist. Those tubes located in the hot est part of the furnace receive more heat than those sheltered from the direct heat of the furnace. Water circulat on, constantly replacing the hot est water in the boiler with cooler water, protect ng the thin watertubes from the intense heat of the furnace. Any blockage that hinders this circulat on may result in damage to the watertubes. Parts of a Watertube Boiler Watertube boiler designs vary greatly, but they share the same basic parts. Figure 1 shows the general locat on of each of these parts: • Steam drum: the top drum(s) • Mud drum: the bot om drum(s) • Furnace: where the fuel is burned • Watertubes: tubes which contain boiler water • Downcomer: watertube conduct ng cooler water down to the mud drum • Riser: watertube conduct ng hot er water up to the steam drum

Figure 1 Overview of Watertube Boiler Parts

4th Class • Part A2 Unit 12 • Chapter 53 • Watertube Boilers 200

WATERTUBE BOILER DESIGNS

Straight Tube Boilers As its name indicates, this early watertube boiler design has straight, inclined tubes which run between vert cal headers connected to the front and rear of the steam drum. The drum in Figure 2 runs longitudinally in relat on to the tubes. Therefore, this boiler is called a Longitudinal Straight Tube Boiler. In some straight tube boilers, the steam drum runs crossways. These are known as Cross Drum Straight Tube Boilers, shown in Figure 3.

Figure 2 Straight Tube Design (Longitudinal)

Some of these boilers are st ll in service today, but none are being built for industry.

Figure 3 Cross Drum Straight Tube Boiler

4th Class • Part A2 Unit 12 • Chapter 53 • Watertube Boilers 201

Bent Tube Boiler Most watertube boilers manufactured today are of the bent tube design. Bent tube boilers are capable of large steam capacit es, high pressures and versat le arrangements. As well, they allow effi cient use of superheaters, reheaters and other heat recovery components. The furnace is of en lined with watertubes (called a water-cooled furnace) to absorb more radiant heat from the fi re. Bent tube boilers are diff erent ated both by their type and number of drums. The most common type of boilers are A, D and O. A-Type Boiler The A-type, shown in Figure 4, has two small mud drums. The steam drum is larger to permit separat on of water and steam. Bent tubes running from the upper drum to the two mud drums form the furnace enclosure.

Figure 4 A-Type Boiler

D-Type Boiler The D-type boiler (Fig. 5) has two drums. Bent tubes, on one side of the boiler, form a “D” shape which creates a water-cooled furnace. The back wall, which takes the impact of the ﬂ ames, is usually protected with refractory coat ng and some tubes for cooling. Excessive vibrat on is somet mes a problem with The D-type boiler.

Figure 5 D-Type Boiler

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O-Type Boiler The O-type boiler (Fig. 6) is a two drum design. The tubes connect ng these two drums are arranged in an “O” shape to form a water-cooled furnace. While the O-type boiler exposes less tube surface to radiant heat than the A or D types, its compact design makes it a popular choice where space is limited.

Figure 6 O-Type Boiler

In each of the three designs, steam bubbles form in the hot est tubes (risers) and rise to the steam drum, where the steam is separated out of the water and steam mixture. Circulat on is maintained by water returning to the mud drum through the cooler tubes (downcomers.) Four-Drum Bent Tube Boiler (St rling Type) The four-drum boiler shown in Figure 7 was the ﬁ rst bent tube type to be developed. It is known as the St rling type and it features three upper drums and a lower mud drum. This early boiler was arranged for hand ﬁ ring.

Figure 7 Four-Drum Boiler

Three drums are connected by watertubes to the mud drum which is provided with a blowoﬀ connect on. The space above the water level in the three upper drums serves as a steam space and the three drums are interconnected by both steam and water circulat ng pipes. The steam outlet is from the rear upper drum which has a safety valve and feedwater inlet connected. Water circulat on is as illustrated in Figure 7 and combust on gas is directed across the tube banks by means of brick baﬄ es. Steelwork supports the three upper drums while the mud drum is freely suspended from the tubes. The furnace is constructed of brickwork and is not water-cooled.

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OBJECTIVE 2

Explain the design and application of packaged watertube boilers.

PACKAGED WATERTUBE BOILERS

The bent tube, watertube boiler is of en produced as a packaged unit. These units have water-cooled furnaces and the air for combust on is supplied from a forced draf fan. A steel casing which prevents combust on gas leakage into the boiler room covers the outside of the boiler and furnace. A skid-type steel foundat on is included and the packaged boiler is bot om-supported, meaning the mud drum is supported by a steel casing and the steam drum by the tubes. This support requires special concrete foot ngs or piers to eliminate excessive vibrat on in the boiler set ng which can cause failures of the insulat on, casing and supports. The units are available for high as well as low-pressure applicat ons. Advantages of the Packaged Boiler 1. Construct on costs are lower since the ent re boiler is built and assembled in the factory (“shop assembled”). 2. Shipping costs are lower, as the boiler is transported as a single unit, complete with fuel system, draf equipment and controls. 3. Prior to delivery to the site, the boiler is pre-tested and inspected to ensure high quality. 4. On-site installat on is reduced, with only posit oning of the boiler and connect on of external auxiliary systems and piping required. 5. It is compact and versat le in design and can occupy a relat vely small ﬂ oor area. Figure 8 shows a “D” type packaged watertube boiler with the accessories and control panel in posit on.

Figure 8 Packaged Watertube Boiler

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OBJECTIVE 3

Describe the design, construction and components of large scale steam generating units.

STEAM GENERATING UNIT

Any heat going up the stack is a waste of money. Fuel costs are a signifi cant expense to the plant; thus, effi cient plant operat on depends on extract ng as much heat from the fl ue gases as possible. A steam generat ng unit consists of all the elements which contribute to the effi cient product on of steam including: B oiler• Boiler• • Superheater Reheater• Reheater• • Economizer • Air heater • Fuel equipment • Draf fans • Ash removal equipment

STEAM GENERATOR COMPONENTS

The purpose of each component is the same, to extract heat from the ﬂ ue gas and transfer it to the substance within, whether it be water, steam or air. The components of a steam generat ng unit, shown in Figure 9, consist of the following: • Superheater Reheater• Reheater• • Economizer • Air heater Superheater A superheater consists of a bank of tubes located at the furnace outlet, or radiant area of the furnace, which give extra heat to dry saturated steam taken from the steam drum. Superheaters are used when the steam required for power generat on or a process is at a temperature higher than that of saturat on. They minimize the chance of condensat on of the steam in the lat er stages of a steam turbine. Superheated steam also increases the overall plant eﬃ ciency in driving turbines by increasing the amount of energy that can be extracted from each kilogram of steam. Reheater A reheater consists of a bank of tubes located within the furnace outlet, or radiant area of the furnace. It reheats the steam drawn from a medium pressure stage of the turbine to further reduce the chance of condensat on and increase the energy available to the turbine. The reheated steam is returned to the remaining stages of the turbine.

4th Class • Part A2 Unit 12 • Chapter 53 • Watertube Boilers 205

Economizer Economizers are used to remove waste heat from the fl ue gas and transfer it to feedwater fl owing through the economizer tubes. This process maximizes the fuel economy of the system. Air Heater The air heater preheats the combust on air for the burners, which improves combust on effi ciency and assists in the burning of pulverized coal. Air heaters get their heat from fl ue gases leaving the economizer sect on.

Figure 9 Steam Generating Unit

4th Class • Part A2 Unit 12 • Chapter 53 • Watertube Boilers 206

OBJECTIVE 4

Describe the design of watertube and copper-tubular heating boilers.

WATERTUBE HEATING BOILERS

Industrial type watertube boilers are seldom selected for low-pressure heat ng plants except for large building complexes. The main reasons are the high cost of this type of boiler as compared to the cast-iron or steel ﬁ retube boiler and the need for closer supervision, especially with regard to water treatment. Specially designed watertube boilers, usually supplied as packaged units are used for low-pressure heat ng applicat ons. Watertube Heating Boiler with Serpentine-Shaped Tubes A popular type of boiler consists of a lower and upper header connected by copper or steel serpent ne- shaped tubes ,which are at ached with threaded connect ons. A low-pressure watertube boiler is shown in Figure 10.

Figure 10 Low-Pressure Watertube Boiler

4th Class • Part A2 Unit 12 • Chapter 53 • Watertube Boilers 207

The combust on chamber of this boiler is below the tubes, thereby exposing the lower part of the tubes to the radiant heat of the fi re. The combust on gases travel between the tubes upwards to the fl ue. Even though the gas travel is relat vely short, heat transfer is effi cient due to the arrangement of the tubes which causes turbulent and intensive scrubbing of the gases around them. Advantages of this boiler design are the fl exibility of the serpent ne-shaped tubes which eliminate expansion and contract on stresses, and the ease of replacement of a defect ve tube since no welding or tube end expanding is needed. Bent-Tube Watertube Heating Boiler Another type of watertube boiler that has become increasingly popular is illustrated in Figures 11 and 12. This boiler is actually a special version of the industrial “O” type packaged watertube boiler.

Figure 11 Watertube Boiler with Membrane Waterwalls

(Courtesy of Cleaver-Brooks) Figure 10 shows the basic boiler, the pressure part, before being enclosed. It consists of a large upper drum, a small lower drum and a number of bent watertubes which connect the upper and lower drums. The tubes are arranged in such a way as to form the furnace enclosure, thus, the furnace walls are formed by water-fi lled tubes and therefore called “waterwalls”. The tubes in these walls are spaced apart but connected to each other by steel plat ng welded to the tubes so that gast ght walls (membrane waterwalls) are formed. Two such walls are used in each side of the boiler, the inner walls form the furnace enclosure, the outer walls passageways for the fl ue gases. Front and rear of the boiler are closed by single waterwalls. Large amounts of radiant heat from the fi re are absorbed by the furnace walls. This heat is carried off by the rapidly circulat ng water in the tubes. The combust on gases, af er leaving the furnace, travel between the inner and outer membrane walls on their way to the stack, giving up their heat to the walls by convect on. The boiler is enclosed by an insulated steel casing. It is either oil or gas fi red and equipped with a forced draf fan. Figure 12 shows an illustrat on of the boiler complete with casing, fi ring equipment, fi t ngs and controls. This boiler can be used as a steam or hot water boiler for low as well as high pressure service depending on its design pressure rat ng.

4th Class • Part A2 Unit 12 • Chapter 53 • Watertube Boilers 208

Figure 12 Watertube Boiler

(Courtesy of Cleaver-Brooks)

PACKAGED TUBULAR STEAM HEATING BOILER

A tubular boiler is in principle a watertube boiler. However, it is not equipped with drums or headers. Instead of having a large number of tubes, it has one or more cont nuous coils of copper or steel tubing, the number depending on the capacity of the boiler. A pump forces water through the coil which is exposed to the hot products of combust on. The tubular boiler was originally only used in hot water systems. When used as a steam boiler, the hardness forming salts (present in most waters) concentrated in the boiler and formed scale on the inside of the tubing, result ng in restrict on of the waterﬂ ow and overheat ng of the tube material. However, this problem has now been overcome, and the tubular steam boiler is gaining popularity rapidly.

COPPER TUBULAR HEATING BOILER

The copper-tubular heat ng boiler, shown in Figure 13, is popular in resident al and commercial hot water heat ng systems. The heat ng surface consists of one cont nuous, small diameter copper tube. Copper is used because it resists corrosion and it has a bet er heat transfer rate than cast iron or steel. The boiler is either gas or oil ﬁ red, with the burner placed in the lower part of the furnace sect on.

4th Class • Part A2 Unit 12 • Chapter 53 • Watertube Boilers 209

The heat ng surface is divided into three parts: 1. The lower sect on consist ng of a t ghtly wound coil which surrounds the combust on chamber. 2. An intermediate sect on made up of several layers of loosely wound spirals, allowing the hot gases to ﬂ ow freely around the tubing af er they leave the furnace. 3. The upper sect on consist ng of a ﬁ n-and-tube type heat exchanger. Fins are crimped or bonded on the tubes, increasing the heat ng surface so more heat is absorbed from the hot gases rising from the furnace on their way to the chimney. Figure 13 Copper-Tubular Boiler

(Courtesy of A. O. Smith) In smaller heat ng systems a single, copper-tubular boiler is used. In larger heat ng systems, instead of using one single, large capacity boiler, many designers prefer to use several smaller units in parallel. The Packaged Tubular Steam Heating Boiler Figure 14 shows a basic diagram of a tubular boiler used for steam generat on. The boiler is supplied as a packaged unit and is equipped with its own feedwater pump and a steam separator. The units are available for high as well as low pressure applicat ons. The heat ng surface consists of one cont nuous steel tube forming a waterwall of t ghtly wound coils around the furnace in the lower sect on, and several layers of loosely wound spirals in the upper sect on. The tubing in the waterwall is protected by steel sheathing against the erosive act on of the fl ames. Water circulates at high velocity through the boiler tubing from the top downwards while the hot gases travel upwards. In this way, no cold water is pumped into the hot est sect on of the boiler, excessive stresses are prevented, and longer life is assured. The boiler can be fi red by oil or gas. A blower, driven by the same motor driving the feedwater pump, supplies the combust on air. The operat on of the unit, as depicted in Figure 14, is as follows: Make-up water and returned condensate enter the feedwater inlet (A) and fl ow to the feedwater pump headers (B). The water is then pumped through the mixing chamber (C) to the heat ng coil (D). In the single pass heat ng coil (D) the water is heated to equivalent steam temperature by the combust on gases fl owing upward through the coil assembly. The steam-water mixture is then passed into the thermostat tube (E) and counterfl ows back across the combust on chamber (F) to the accumulator (G). Here steam and water are centrifugally separated by the separat ng nozzle (H).

4th Class • Part A2 Unit 12 • Chapter 53 • Watertube Boilers 210

Steam is discharged through the steam discharge valve (M). The water is drawn from the accumulator (G) by the recirculat ng pump heads (J) and pumped to the mixing chamber (C) where it blends with the feedwater. Excess water in the accumulator is returned to the condensate tank through a steam trap (K). A liquid fl ow control (L) is incorporated to prevent burner operat on in case of a low water condit on. Automat c half-fi re modulat on is provided to reduce “on-off ” cycling during periods of light steam demand. Sof ened water is used as feedwater to prevent scale forming. Since any salt carried into the boiler with the feedwater stays behind and would concentrate in the boiler water, an automat c blow-down is provided which drains suffi cient water off the accumulator to keep the concentrat on within safe limits.

Figure 14 Packaged-Type Tubular Steam Boiler

(Courtesy of Clayton Manufacturing Co.) Advantages of the Low-Pressure Watertube and Tubular Type Boilers The advantages of these types of boilers include: • Compact - considerably smaller and lighter than fi retube boilers of equal capacity • No special foundat on required • Very short warm-up period required • Bent or coil tube design avoids thermal stress and distort on • Rapid response to fl uctuat ng loads • Supplied as packaged units completely equipped with fi ring equipment, automat c controls and safety devices • Safer than fi retube boilers with respect to tube failure since the boilers contain very lit le water and no disastrous explosion could occur • Maintenance cost low. Boiler has a minimum of refractory.

The disadvantages of watertube and tubular heat ng boilers compared to ﬁ retube heat ng boilers include: • Higher init al cost • Need for closely monitored water treatment

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CHAPTER 53 - QUESTIONS WATERTUBEBOILERS

1. High pressure watertubes a) are always smaller than ﬁ retubes. b) can be of various straight or bent conﬁ gurat ons. c) are always straight. d) require pumps for circulat on.

2. The bent tube boiler design allows the tubes to be at ached radially on the outside of the a) ﬁ rebox. b) economizer. c) air preheater. d) drum.

3. Since furnace pressure is of en posit ve in a packaged watertube boiler, to prevent ﬂ ue gas leakage a) an induced draf fan is incorporated. b) an economizer is used. c) smoke stacks are made taller. d) a steel casing forms a seal for the furnace.

4. A problem with a D-type watertube packaged boiler is a) ceiling height requirements. b) excessive vibrat on. c) refractory maintenance for the external furnace. d) poor water circulat on.

5. Which of the following is not an advantage of the low-pressure watertube and tubular type boilers? a) very short warm-up period required b) bent or coil tube design avoids thermal stress and distort on c) rapid response to ﬂ uctuat ng loads d) lower init al cost

Fourth Class • Part A2 Unit 12 • Chapter 53 • Watertube Boilers 212

CHAPTER 53 - ANSWERS WATERTUBE BOILERS

1. (b)

2. (d)

3. (d)

4. (b)

5. (d)

Fourth Class • Part A2 4th Class • Part A2 C HAPTER 54

Electric Boilers

LEARNING OUTCOME

When you complete this chapter you should be able to: Describe electric boilers in regard to their use and general design.

LEARNING OBJECTIVES

Here is what you should be able to do when you complete each objective:

1. Describe the construction and operating principle of electric boilers.

213 Unit 12 • Chapter 54 • Electric Boilers 214

INTRODUCTION

The boilers described in other chapters, namely the ﬁ retube and the watertube types, obtained the heat necessary for convert ng water to steam from the burning of fuel within the boiler furnace. The electric boiler uses electricity, instead of burning fuel, to heat the water and convert it to steam. Electric boilers are not usually used in large heat ng systems, but are somet mes found in smaller ones. They are of en used in hospitals, schools and hotels to provide steam for heat ng, sterilizing, laundry equipment and kitchen equipment. Another applicat on is in a system where electricity, produced by a gas turbine driving a generator, is used in an electric boiler to produce steam which supplements steam produced in a waste heat boiler. The waste heat boiler uses the gas turbine exhaust as a heat source. Electric boilers are also used in facilit es where large amounts of electricity are available at low cost. For example, aluminum smelters use large quant t es of electricity for the smelt ng process and hydro-electric facilit es are nearby.

4th Class • Part A2 Unit 12 • Chapter 54 • Electric Boilers 215

OBJECTIVE 1

Describe the construction and operating principle of electric boilers.

ELECTRIC BOILER DESIGN

There are two general designs of electric boilers, namely the: • Electrode type • Immersion heater type Electrode Type The sketch in Figure 1 shows the general arrangement of an electrode boiler.

Figure 1 Electrode Type Electric Boiler

(Courtesy of General Electric)

4th Class • Part A2 Unit 12 • Chapter 54 • Electric Boilers 216

Referring to Figure 1, the shell of the boiler contains a separate sect on called the basket into which the electrodes extend. The water level in the basket is varied by means of the circulat ng pump. It draws water from the bot om of the shell and pumps it into the basket through a control valve. The water then drains back into the lower port on of the shell through the basket drain valve. The water acts as a conductor and its depth surrounding the electrodes in the basket determines the amount of electric current fl owing between the electrodes. This current fl owing through the water heats it and converts it to steam. If too much steam is being produced, the control valve at the pump discharge will be part ally closed, thus lowering the level of water in the basket and reducing the amount of current fl ow. If not enough steam is being produced the control valve will open further; the water level in the basket will rise, allowing more current to fl ow. Feedwater is supplied to the outer shell of the boiler through a feedwater regulator to maintain the water in the shell at a constant level. The boiler type shown in Figure 1 operates at voltages up to 16 000 volts and at power rat ngs up to 30 000 kilowat s. The sketch in Figure 2 shows the general arrangement of another electrode type boiler. The electrodes are located in a central generat ng chamber. In both types, power is supplied to the terminals which are connected to the electrode adapters which pass through insulators to the outside of the shell.

Figure 2 Electrode Type Boiler

Pure water does not conduct electricity, but when the water contains a salt, it becomes a conductor through which current can pass. The boiler water is given the proper salinity by the addit on of salt. When power is switched on, the current will fl ow through the water between the conductors. This current heats the water and converts it to steam. In all electrode boilers, the amount of current passing through the water is directly proport onal to the length of the electrodes submerged in the water. In the boiler shown in Figure 2 water is supplied from the outside regulat ng chamber to the generat ng chamber. When the steam load drops, the pressure in the generat ng chamber will increase slightly; this extra pressure will force some of the water out of the generat ng chamber, thus lowering the level. As a result of the lower level, the current fl ow is reduced since a shorter length of the electrodes is submerged. Thus, less steam is produced. An increased load will do the opposite. Pressure will drop slightly, water level rises, current increases and steam product on increases to meet the larger demand. The boiler is thus completely self-regulat ng. At no-load, the pressure will increase enough to force the water out of the generat ng chamber and stop the current fl ow completely, so a high pressure limit switch is not required. The boiler does not need a low-water cutoff either, since the current fl ow will stop when the level drops below the electrodes. The water supply to the regulat ng chamber is controlled by a fl oat valve which regulates the supply from the feedwater pump. The electrode boiler is also manufactured as a packaged unit. Figure 3 is a sketch showing the components within the cabinet. Note that in this type, the feedwater pump and condensate tank are included in the package as well as all controls.

4th Class • Part A2 Unit 12 • Chapter 54 • Electric Boilers 217

Figure 3 Electrode Packaged Boiler Components

1. Electrode Steam Boiler 9. Access Panels 2. Condensate and Feed Water Tank 10. Power Connect on Terminals 3. Cold Column Tank (opt onal) 11. Ammeter 4. Feed Pump 12. Load Control Adjustment 5. Feed Water Float Valve 13. Pump Start ng Switch 6. Control Compartment 14. Emergency Stop Switch 7. Load Control Solenoid Valve 15. Control Switch 8. Steam Pressure Gauge 16. Bleed Solenoid Valve (opt onal)

(Courtesy of Cam Industries Inc.) Immersion Heater Type Figure 4 shows the external view of an immersion heater electric steam boiler and Figure 5 is a sketch label- ing its various parts. The boiler type shown is manufactured in sizes up to 1500 kilowat s and for pressures up to 2100 kPa. The immersion heater boiler diﬀ ers from the electrode boiler in that no electric current travels through the water. Instead, the electric current ﬂ ows through a heat ng element which is ent rely submerged beneath the boiler water level. This principle is the same as is employed in the ordinary electric element tea ket le.

4th Class • Part A2 Unit 12 • Chapter 54 • Electric Boilers 218

Figure 4 Immersion Heater Boiler

(Courtesy of General Electric)

Figure 5 Immersion Heater Boiler Parts

(Courtesy of General Electric) Control of the immersion heater boiler is accomplished by turning on and oﬀ the power supply to one, two or more elements as required. The packaged boiler concept is also used for some immersion heater types. These packaged units may include controls, feed pumps and condensate tank all within one cabinet.

4th Class • Part A2 Unit 12 • Chapter 54 • Electric Boilers 219

The immersion heaters are arranged in the boiler so that they are easily accessible for maintenance or replacement. Figure 6 shows the arrangement of a number of elements and also illustrates the general design of an element which has been removed from the boiler. Connect ons to the heat ng elements can be rearranged to provide full load operat on on various voltages (which may vary due to plant locat on).

Figure 6 Immersion Heater Elements

(Courtesy of Cam Industries Inc.) Advantages/Disadvantages of Electric Boilers The advantages of electric boilers over fuel fi red boilers are as follows: • Electric boilers are very compact as they do not require furnace space for combust on and, therefore, do not require ductwork or a chimney • No fuel storage space is required as in the case of oil or coal fi red boilers • Electric boilers are quickly and easily installed due to the fact that ductwork, chimneys and fuel lines are not required • A high percentage (98%) of the energy delivered by the electricity is absorbed as heat in the boiler • Electric boilers produce no pollut on, such as smoke, dust or ashes. However, the electricity must be produced at another locat on using thermal, hydro-electric or other systems where some environmental impact will result. • Electric boilers are silent in operat on and are safe because there is no possibility of furnace explosion The disadvantages of the electric boiler are as follows: • Usually high comparat ve cost of the electricity required • Most designs are limited in pressure to about 2100 kPa due to the eff ect of high temperature on the electrodes or elements

4th Class • Part A2 Unit 12 • Chapter 54 • Electric Boilers 220

4th Class • Part A2 Unit 12 • Chapter 54 • Electric Boilers 221

CHAPTER 54- QUESTIONS ELECTRIC BOILERS

1. A main disadvantage of an electric boiler is that a) they are not very compact. b) vibrat on requires special foot ngs. c) input energy cost is high. d) energy transfer eﬃ ciency is low

2. Operat ng voltage for an electrode boiler may be as high as ______volts. a) 25 000 b) 16 000 c) 1000 d) 50 000

3. The amount of steam produced by an electrode type boiler depends on the a) voltage supplied to the elements. b) level of water in the basket. c) number of heat ng elements engaged. d) number of baﬄ es.

4. A ______is not required on an electrode type electric boiler. a) thermostat b) water pump c) basket d) low water cutoﬀ

5. An immersion electric boiler capacity can be as high as a) 1000 kW. b) 1500 kW. c) 5000 kW. d) 500 kW.

6. Control of the immersion heater boiler is done by turning on and oﬀ the a) feedwater pump. b) fuel supply. c) number of heat ng elements. d) power supply.

Fourth Class • Part A2 Unit 12 • Chapter 54 • Electric Boilers 222

CHAPTER 54 - ANSWERS ELECTRIC BOILERS

1. (c)

2. (b)

3. (b)

4. (d)

5. (b)

6. (c)

Fourth Class • Part A2 4th Class • Part A2 C HAPTER 55

Basic Boiler Construction

LEARNING OUTCOME

When you complete this chapter you should be able to: Describe fabrication and general construction features of watertube and ﬁ retube boilers.

LEARNING OBJECTIVES

Here is what you should be able to do when you complete each objective:

1. Describe the design and manufacturing of boiler shells and drums.

2. Describe the standard types of welded joints, heat treatments and welding inspection used in the construction of pressure vessels.

3. Describe the general design of riveted joints.

4. Describe the tools and standard methods used to attach boiler tubes to tubesheets, headers and drums.

5. Describe the need for, and application of, boiler stays.

6. Describe boiler access and inspection openings and drum connections.

7. Identify the different types of internal ﬁ retube furnace designs.

8. Describe boiler foundations and supports.

9. Describe the design and construction of water-cooled furnace walls in watertube boilers.

223 Unit 12 • Chapter 55 • Basic Boiler Construction 224

INTRODUCTION

Boilers are vessels in which heat is transferred from one fl uid to another, such as from hot fl ue gases to water. These fl uids must be kept separate from each other and the outside environment. Since the fl uid pressures can be considerable, the boiler surfaces must be able to contain them. Boiler construct on is thus largely the science of joining metal plates, tubing and piping, in required confi gurat ons, so the vessel can withstand these pressure diff erences and stresses. In this chapter, some of the common methods of construct on, such as welding, staying and tube expanding will be examined.

4th Class • Part A2 Unit 12 • Chapter 55 • Basic Boiler Construction 225

OBJECTIVE 1

Describe the design and manufacturing of boiler shells and drums.

SHELLS DRUMS

The main part of the ﬁ retube boiler is referred to as the shell, within which the boiler tubes are contained. The watertube boiler, on the other hand, features a drum, or drums instead of a shell and the tubes run between the drums, or between drums and headers, rather than being contained within them. Shells and drums are made up of steel plates which are rolled to the correct curvature. The thickness of the plates ranges from 6 mm to 250 mm, depending upon the pressure they have to withstand and the required diameter of the shell or drum. If the plate is thick, it is ﬁ rst heated and shaped into half-cylindrical sect ons as shown in Figure 1. These sect ons are then welded together to form the complete cylinder. If the plate is thin, it is rolled without heat ng to form the cylinder from a single sheet. The adjoining edges are then welded.

Figure 1 Rolling Boiler Plate

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Figure 2 shows the process of welding the adjoining edges or seam of a plate that has been rolled to form a shell.

Figure 2 Welding a Shell Seam

Af er the shell, or drum, is formed and welded, the ends are closed oﬀ by means of heads, or end plates which are also made from steel plates. In the case of a watertube boiler, the heads are made in a dished rather than ﬂ at shape. Figure 3 shows a large watertube boiler drum with dished or rounded heads. In the illustrat on, the weld joining one of the heads to the drum is being x-rayed in order to discover any possible faults within the weld.

Figure 3 X-Ray Examination of Head to Shell Joint

In the case of a fi retube boiler, the heads must be fl at rather than dished in order that the tube ends be securely at ached. Because the fi retube boiler heads hold the tube ends, they are called tubesheets.

4th Class • Part A2 Unit 12 • Chapter 55 • Basic Boiler Construction 227

Figure 4 Tube Holes in Firetube Boiler Head

(Courtesy of Rene J. Bender, Power, Hopkinsons Ltd.) Figure 4 shows the tube holes in the ﬂ at head of a three-pass ﬁ retube boiler. As with the watertube boiler, the heads are fastened to the shell by welding. The tube holes in a watertube boiler are located in the drum rather than in the head. Figure 5 illustrates the assembly of tubes to the drilled holes.

Figure 5 Tube Holes in Watertube Boiler Drums

(Courtesy of Stork Boilers)

4th Class • Part A2 Unit 12 • Chapter 55 • Basic Boiler Construction 228

OBJECTIVE 2

Describe the standard types of welded joints, heat treatment and welding inspections used in the construction of pressure vessels.

WELDED JOINTS

The ﬁ ve basic types of welded joints are sketched in Figure 6. Of these ﬁ ve, the but joint is the one used primarily in boiler fabricat on, although under certain circumstances the lap joint may be used.

Figure 6 Types of Welded Joints

Two basic types of welds are sketched in Figure 7. The groove weld is used in making a but joint while the ﬁ llet weld is used to make the lap, or tee joint. Before the welding of a but joint is carried out, the edges of the plates to be joined are beveled or cut in such a way as to form a groove of the proper shape and size.

Figure 7 Basic Weld Types

Various shapes of the grooves used are sketched in Figure 8. The double grooves shown at the bot om are used for thick plates. With these types, the ﬁ ller metal is applied to both sides of the joint; the weld is referred to as a double-welded but joint. If the ﬁ ller metal is applied from one side only, the weld is referred to as a single-welded but joint.

4th Class • Part A2 Unit 12 • Chapter 55 • Basic Boiler Construction 229

Figure 8 Types of Weld Grooves

Figure 9 is a sketch of a single-welded but joint showing the terminology used for the various weld parts.

Figure 9 Weld Terminology

Figure 10(a) is a sketch of a double v groove which may be used for the longitudinal seam of a boiler shell or drum. Figure 10(b) is a sketch of a method of welding a head to a boiler drum, as viewed from the side of the drum.

Figure 10 Welding Groove for Longitudinal Seam (Double-Welded Butt Joint)

4th Class • Part A2 Unit 12 • Chapter 55 • Basic Boiler Construction 230

A sect on of a typical weld is shown in Figure 11. The numbers within the weld refer to the passes or beads used to produce the weld and the sequence in which they are applied. The numbers outside of the weld refer to the layers with those preﬁ xed by the let er B being the layers applied to the underside of the weld.

Figure 11 Typical Double Butt Weld

HEAT TREATMENT

During the welding process, stresses are set up in the metal. These stresses are due to temperature diff erences exist ng in the weld area; these stresses may be removed or reduced by heat treatment of the metal. One method of heat treatment, called preheat ng, is to heat the parts in a special oven or furnace prior to welding. The preheat temperature ranges from 80°C to about 230°C depending upon the material and thickness. Another method of heat treatment is to heat the parts af er the welding has been completed. This method is referred to as postweld heat treatment or stress-relieving. In the case of a boiler drum, af er welding of the drum joints, the ent re drum can be heated to a specifi ed temperature (590°C or above) in a special furnace for a defi nite period of t me. It is then cooled off slowly at a controlled rate unt l its temperature drops to below 315°C.

WELDING INSPECTION

During the fabricat on of the boiler, an authorized Inspector must make sure of the following: • The welding procedures used by the manufacturer are correct • All welding is carried out by pressure vessel welders who are qualiﬁ ed to perform the required type of welding • The correct applicat on of heat treatment is used • Test plates of the same thickness and material as the drum are welded and then subjected to tension and bending tests to verify that the drum welds will safely hold The drum welds are also radiographed (as shown in Fig. 12) for their ent re length using x-rays or other radioact ve material. In this way, any defects within the weld will be evident in the radiographic picture.

4th Class • Part A2 Unit 12 • Chapter 55 • Basic Boiler Construction 231

Figure 12 Radiographic Examination of Shell Weld

(Courtesy of Stork Boilers)

4th Class • Part A2 Unit 12 • Chapter 55 • Basic Boiler Construction 232

OBJECTIVE 3

Describe the general design of riveted joints.

RIVETED JOINTS

Rivet ng is no longer used in boiler fabricat on, but there are st ll some older riveted boilers in operat on. There are two main types of riveted joint: 1. The lap joint 2. The but joint In the lap joint, the edges of the plates are overlapped and fastened by one or more rows of rivets. If one row of rivets is used, the joint is single riveted; with two rows, it is double riveted, and so on. Figure 13(a) shows a single riveted lap joint, and Figure 13(b), a double riveted lap joint.

Figure 13 Riveted Lap Joints

Figure 14 shows a double riveted but joint with the shell in between an inner and outer strap.

Figure 14 Double Riveted Butt Joint

4th Class • Part A2 Unit 12 • Chapter 55 • Basic Boiler Construction 233

In the but joint construct on, the edges of the shell plates do not overlap but but together. Cover straps are riveted to the plates covering the seam. Figure 14 shows a double riveted but joint with equal straps, while Figure 15 shows a treble riveted but joint with unequal straps.

Figure 15 Treble Riveted Butt Joint

Lap joints are used for circumferent al seams in all riveted boiler shells, but longitudinal seams are nearly always of but joint construct on. But joints are stronger and therefore more suitable for longitudinal seams because the force tending to rupture a boiler at the longitudinal joint is greater than that at the circumferent al joint. With the but joint, it is also possible to bend the plate in the form of a true circle as the ends of the shell plate but or meet, but this cannot be done with the lap joint as the ends must overlap to make the joint.

4th Class • Part A2 Unit 12 • Chapter 55 • Basic Boiler Construction 234

OBJECTIVE 4

Describe the tools and standard methods used to attach boiler tubes to tubesheets, headers and drums.

BOILER TUBES

For moderate pressure service, tubes are made from strips of steel, which are formed into tube shapes. The edges are then welded in an electrical resistance welding machine. In this method, the welding is achieved due to the heat produced when an electric current fl ows through the metal at the tube edges. Tubes used for high pressure service are usually seamless and are made by piercing a solid round billet of heated steel to form a rough tube. During piercing the heated billet is spun between rollers as it is forced against the piercing point, forming a rough tube. In the rolling operat on a plug the size of the fi nished tube inner diameter is mounted on a long rolling rod between rollers, which are grooved to the outer dimension of the tube. The roughly shaped tube from the piercing process is then placed between the rollers, which force the tube over the rolling plug, expanding the inner diameter with the plug while reducing the outer diameter between the rollers. The tube is fi nally fi nished in a reeling operat on where it is spun between smooth rollers while being forced over a smooth reeling plug to achieve fi nal dimensions and smoothness. The most common method used to fasten tubes to drums or tube sheets is to expand the tube ends into the tube holes in the drum or sheet. This expanding (or rolling) is done by means of an expander which consists of three rollers mounted in a cage which fi ts inside the tube end. A tapered mandrel fi ts between the rollers and, when the mandrel is turned, the rollers are rotated and forced out against the tube wall, thus expanding the tube against the tube hole. Figure 16(a) shows an expander used for fi retubes. The expander in Figure 16(b) is used for watertube boilers and has a sect on of one of the rollers set at an angle in order to fl are or bell the tube end. Figure 16(c) shows a tube being expanded into a watertube boiler drum.

Figure 16 Tube Expanders

(a)

(b) (c)

In a watertube boiler, the tubes are expanded and fl ared as shown in Figure 17. In a fi retube boiler, in addi- t on to the expanding and fl aring, the fl ared part is beaded over against the tube sheet. This beading of the tube end prevents it from being overheated and burned when the boiler is in service.

4th Class • Part A2 Unit 12 • Chapter 55 • Basic Boiler Construction 235

Figure 17 Expanded and Flared Tube (Watertube)

Figure 18(a) shows the beaded over end of an expanded ﬁ retube. The tube in Figure 18(b) has been expanded, beaded over and then seal welded. The tube in Figure 18(c) has been expanded into a grooved tube hole and then beaded over. This grooved type of construct on is used for very high pressure service in ﬁ retube and watertube boilers.

Figure 18 Tube Attachments for Firetube Boilers

Rather than using the expansion method of tube at achment, many high pressure watertube boilers use welded at achments. The boiler drums and headers have stubs welded to them in the factory; the tubes are welded to these stubs during the erect on of the boiler in the fi eld. These welded stubs will be discussed further under the heading “Drum Connect ons” later in this chapter. Similarly, some fi retube boilers use welded at achments for tubes. However, in the case of the fi retube boiler, the tubes are expanded both before and af er the welding. Figure 19 shows some welded tube at achments for fi retube boilers.

Figure 19 Welded Tube Attachments (Firetube)

4th Class • Part A2 Unit 12 • Chapter 55 • Basic Boiler Construction 236

OBJECTIVE 5

Describe the need for, and application of, boiler stays.

BOILER STAYS

Any fl at surface exposed to pressure will tend to bulge due to the eff ect of this pressure. To prevent this bulging from occurring, it is necessary to brace or support the fl at surfaces present in fi retube boilers including the tube sheets and waterlegs and the crown sheets on the top of the furnace. These surfaces are supported by various types of stays. In the watertube boiler, stays are not normally required because this type of boiler can be built without fl at surfaces. For example, the boiler heads are dished rather than fl at. The fl at surfaces above and below the tubes in the tube sheets of a fi retube boiler must be supported; Figure 20 illustrates the shape of these surfaces. The shaded area in Figure 20 represents the area that must be supported above the tubes. There may also be a similar area below the tubes. The area containing the tubes is supported by the tubes themselves.

Figure 20 Tubesheet Area to be Stayed

The area above the tubes is usually supported by diagonal stays, as illustrated in Figure 21.

Figure 21 Diagonal Stay

As shown in Figure 21, the diagonal stay is welded to the boiler head, or tubesheet, and to the boiler shell. In this way, bulging of the fl at tubesheet is prevented. Diagonal stays are used above the tubes on a fi retube boiler to allow access to the area for inspect on and cleaning. Modern fi retube boilers of en use through stays which are stronger but allow less access. The area below the tubes is of en supported by longitudinal or through stays: long rods extending from the rear tubesheet to the front tubesheet. These stays are usually fastened to the tubesheets by welding or using nuts and washers as shown in Figure 22. Packaged fi retube boilers usually have tubes extending to the bottom of the shell which support the tubesheets; stays are not required in this area. Due to the need for steam space at the top of the boiler, the upper areas must be stayed.

4th Class • Part A2 Unit 12 • Chapter 55 • Basic Boiler Construction 237

Figure 22 Through Stay Attached with Nuts and Washers

In the case of the rear tubesheet of an HRT boiler, the through stays are fastened by means of angle braces which distribute the stress over a large area as shown in Figure 23.

Figure 23 Angle Brace Attachment

Staybolts are used primarily on steam locomot ve boilers to support the boiler and the ﬁ rebox. The size of bolts and the number used are determined in Sect on I of the ASME Code. In working steam locomot ve boilers, loose staybolts are one of the main causes of boiler leakage. Figure 24 shows detail of staybolt design on the waterleg of a locomot ve type boiler. To see how they are at ached, see Figure 6 in the chapter t tled Firetube Boilers. (Note: P in Fig. 24 refers to Pitch or the number of staybolts used).

Figure 24 Waterleg Staybolt

4th Class • Part A2 Unit 12 • Chapter 55 • Basic Boiler Construction 238

OBJECTIVE 6

Describe boiler access and inspection openings and drum connections.

ACCESS OPENINGS

In order to carry out inspect on, cleaning and repair, it is necessary to provide access to various parts of the boiler. Manholes, or manways, are provided to allow entry into drums and handholes give access to smaller parts, such as headers and waterlegs. Manhole and handhole openings are usually ellipt cal in shape, but may be circular. The size of an ellipt cal manhole must not be less than 280 mm by 380 mm or, alternately in diﬀ erent shapes, 250 mm by 405 mm. The diameter of a circular manhole must not be less than 380 mm. Handholes must not be less than 70 mm by 89 mm and it is recommended that larger sizes be used. When the boiler is in service, the manhole and handhole openings are closed by means of doors or cover plates. These doors, or covers, ﬁ t on the inside of the drum or header and are held in place by means of bolts and yoke pieces. When the boiler is in operat on, the pressure within it helps to hold the door in place. Figure 25 shows the locat on of the manholes in a two drum packaged watertube boiler.

Figure 25 Packaged Boiler Showing Manhole Covers

4th Class • Part A2 Unit 12 • Chapter 55 • Basic Boiler Construction 239

Figure 26 shows the general construct on of a manhole cover.

Figure 26 Manhole Cover

DRUM CONNECTIONS

Drum connect ons consist of nozzles or stubs, at ached to the drum. They are used for connect ng various ﬁ t ngs and at achments such as the: • Safety valve • Main steam outlet • Water column • Feedwater inlet • Pressure gauge These connect on nozzles, or stubs, are usually at ached to the drum by welding, although on older boilers, they may be riveted. In some cases, threaded connect ons are screwed into the drum wall via a threaded hole, but these are restricted depending on size and pressure. As ment oned previously, tubes are frequently at ached to drums by expanding but, for higher pressure, welded tube stubs are used. Figure 27 shows the types of welds used for at achment of nozzles that would hold safety valves and steam outlet connect ons.

Figure 27 Welded Nozzle Attachments

4th Class • Part A2 Unit 12 • Chapter 55 • Basic Boiler Construction 240

Figure 28 illustrates the welding of tube stubs to square and round headers.

Figure 28 Welded Tube Attachments

The round header in Figure 29 shows the arrangement of the welded tube stubs to which the tubes themselves are welded. Note that the handholes are welded into place because the header is designed for high pressures.

Figure 29 Header with Welded Tube Stubs

4th Class • Part A2 Unit 12 • Chapter 55 • Basic Boiler Construction 241

OBJECTIVE 7

Identify the different types of internal ﬁ retube furnace designs.

INTERNAL FURNACES DESIGNS in the case of a ﬁ retube boiler having an internal furnace contained within the shell, the pressure within the boiler acts upon the outside of the furnace, tending to collapse it. In order to maintain Code required strength with the use of thinner metal, it is usually made in a corrugated form such as is shown in Figures 30 and 31.

Figure 30 Internal Furnace Firetube Boiler

Figure 31 Packaged Firetube Boiler with Corrugated Furnace

4th Class • Part A2 Unit 12 • Chapter 55 • Basic Boiler Construction 242

Figure 32 shows various cross sect on designs of corrugated and ring reinforced furnaces.

Figure 32 Furnace Designs

Figure 33 shows methods of at aching furnaces to the boiler head by welding.

Figure 33 Furnace Welded Attachments

4th Class • Part A2 Unit 12 • Chapter 55 • Basic Boiler Construction 243

OBJECTIVE 8

Describe boiler foundations and supports.

FOUNDATIONS SUPPORTS

Adequate foundat ons and supports are required for boilers in order to avoid any movement which would put extra stress on the boiler and its connect ng pipework. Also, if the boiler set les at one end, the gauge glass will not give an accurate indicat on of the water level in the boiler. In the case of a packaged fi retube boiler, the boiler room fl oor must be strong enough to act as a foundat on because the boiler is usually mounted on a skid type steel base. The whole assembly can then be placed directly on the fl oor by an overhead crane as shown in Figure 34.

Figure 34 Floor Mounted Packaged Scotch Marine Firetube Boiler

(Courtesy of Cleaver Brooks) Packaged watertube boilers are also frequently ﬂ oor mounted as shown in Figure 35.

Figure 35 Watertube Boiler

4th Class • Part A2 Unit 12 • Chapter 55 • Basic Boiler Construction 244

For large boilers, other than the packaged type and those that are built on-site, it is usually necessary to provide special concrete foundat ons. These large boilers may be either top-supported or bot om-supported. A top-supported boiler is shown in Figure 36.

Figure 36 Top Supported Boiler

There are several advantages to large watertube boilers that are top supported: • Since these boilers can be 30 metres or more in height and the steam drum is the heaviest part, they tend to be top heavy and unstable if bot om supported. • Top support allows expansion downward as the boiler is heated, reducing stress. • The structural steel used to support the boiler can also be used as the boiler house and support for other boiler auxiliaries. • Since these large boilers expand by several cent metres when heated, top support ng is easier for at achments. Almost all of the connect ons on the boiler are at the steam drum when top supported, this steam drum is held in a ﬁ xed posit on. If bot om supported, the steam drum moves up and down with heat ng and cooling. For very large boilers this movement would make at achments diﬃ cult.

4th Class • Part A2 Unit 12 • Chapter 55 • Basic Boiler Construction 245

The bot om-supported boiler in Figure 37 has concrete piers which support the bot om drum and headers. The boiler tubes themselves are used to support the top drum.

Figure 37 Bottom Supported Boiler

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OBJECTIVE 9

Describe the design and construction of water-cooled furnace walls in watertube boilers.

WATER-COOLED FURNACE WALLS

The furnace walls of early boilers, such as the straight tube watertube boiler, were made of brick. This brickwork has been eliminated in the modern watertube boiler by using the tubes to form the furnace walls. This type of furnace wall is called a watercooled wall or a waterwall. Figure 38 illustrates a modern design of watercooled furnace wall. Adjacent tubes are welded to metal ﬁ ns to produce a solid panel, which is exposed to the furnace heat. The welded tubes and ﬁ ns are backed by a layer of insulat on and then an outer metal casing to protect the insulat on, as shown in Figure 38(a). Figure 38(b) shows details of the welded tubes.

Figure 38 Welded Fin Tube Waterwall

(Courtesy of Combustion Engineering )

4th Class • Part A2 Unit 12 • Chapter 55 • Basic Boiler Construction 247

Other designs of watercooled furnace walls are seen in Figure 39 and Figure 40. In the tangent-tube wall (Fig. 39), the inside of the furnace wall is made up of tubes which are side by side and touching each other to form a cont nuous surface. They are backed by a layer of plast c insulat on and then a steel casing. Block insulat on is then placed against the steel casing and another steel casing forms the outside surface of the wall.

Figure 39 Tangent-Tube Wall

In the fl at stud wall (Fig. 40), the tubes have fl at studs welded to them on each side; these studs fi ll the space between adjacent tubes. The tubes and studs are backed up by refractory and insulat on as shown in Figure 40.

Figure 40 Flat Stud Tube Wall

(Courtesy of Babcock and Wilcox)

4th Class • Part A2 Unit 12 • Chapter 55 • Basic Boiler Construction 248

4th Class • Part A2 Unit 12 • Chapter 55 • Basic Boiler Construction 249

CHAPTER 55- QUESTIONS BASIC BOILER CONSTRUCTION

1. Common shell and drum thickness range is ______to ______mm? a) 4 - 200 b) 6 - 250 c) 12 - 500 d) 9 - 400

2. Single-welded but joints a) are used on very thick boiler plates. b) have the ﬁ ller metal applied from one side only. c) have the ﬁ ller metal applied to both sides of the joint. d) are never used for boiler fabricat on.

3. Tubes for high pressure service are usually a) resistance welded construct on. b) pierce and roll construct on. c) lap welded construct on. d) expanded into the drum or tubesheet.

4. Another name for the parts of the tube ,welded to boilers and headers, which are used for the at achment of tubes is a) longitudinal tubes b) vert cal tubes c) expansion nozzles d) tube stubs

5. Stays are not normally required in water tube boilers? a) The water and steam pressures are counterbalanced. b) Watertube boilers have no ﬂ at surfaces to stay. c) The top suspension negates any forces. d) The surfaces are built to withstand the internal pressures.

6. Handhole and manhole openings a) permit inspect on, cleaning and repairs. b) require a cast iron reinforcing ring. c) have their long axis lengthwise along the shell. d) are not required on low pressure hot water boilers.

Fourth Class • Part A2 Unit 12 • Chapter 55 • Basic Boiler Construction 250

CHAPTER 55 - ANSWERS BASIC BOILER CONSTRUCTION

1. (b)

2. (b)

3. (b)

4. (d)

5. (b)

6. (a)

Fourth Class • Part A2 4th Class • Part A2 U N I T 1 3

DRAFT, COMBUSTION HIGH PRESSURE BOILER FITTINGS

Chapter 56 Boiler Draft Equipment 253

Chapter 57 Introduction to Boiler Combustion 273

Chapter 58 Fluidized Bed Combustion 301

Chapter 59 Safety & Relief Valves 313

Chapter 60 Water Columns & Gauge Glasses 329

Chapter 61 Drum Internals 347

251 252 4th Class • Part A2 C HAPTER 56

Boiler Draft Equipment

LEARNING OUTCOME

When you complete this chapter you should be able to: Discuss draft and describe the basic equipment used to supply combustion air to a boiler furnace.

LEARNING OBJECTIVES

Here is what you should be able to do when you complete each objective:

1. Describe the forced, induced and balanced methods of mechanical draft.

2. Discuss the common methods of controlling combustion air ﬂ ow.

3. Discuss the common methods of measuring furnace pressures.

253 Unit 13 • Chapter 56 • Boiler Draft Equipment 254

INTRODUCTION

The term, “draf ” refers to the process of supplying combust on air to the boiler furnace. It can be defi ned as “a diff erent al in pressure which will produce a fl ow of air”. This cont nuous air fl ow is necessary to provide the oxygen required for the combust on of the boiler fuel. The movement of air into the furnace, and the subsequent fl ow of combust on fl ue gases through the boiler, conducts heat to the convect on sect ons of the boiler and conveys exhaust gases out of the stack. Air is supplied to the burner so that its oxygen can combine with fuel. Burners are, therefore, essent ally mixing devices. The air going to the burner is of en separated into diff erent streams to facilitate mixing, especially on oil and solid fuel fi red systems. These diff erent air streams are referred to as primary air and secondary air. Primary air refers to the air which may be pre-mixed with the fuel before being admit ed to the furnace, or to the air admit ed close to the burner t p or fuel bed. Secondary air is the remaining air necessary for complete combust on, supplied close to the burner in the combust on zone. In order to promote thorough mixing of fuel and air, the primary air is of en given a clockwise rotat on and the secondary air a counter-clockwise rotat on or vice-versa. In some cases an addit onal layer of air is introduced at the end of the combust on zone to ensure that the fuel is burned completely. This is of en referred to as tert ary air. With solid fuel fi red systems, where the fuel is burned in a bed on grates, air that is introduced under the grates may be called undergrate air or underfi re air and air admit ed above the bed is called overfi re air.

4th Class • Part A2 Unit 13 • Chapter 56 • Boiler Draft Equipment 255

OBJECTIVE 1

Describe the forced, induced and balanced methods of mechanical draft.

DRAFT EQUIPMENT

Equipment used to supply combust on air to the furnace can be classiﬁ ed into two groups: • Natural draf equipment • Mechanical draf equipment Natural Draft Equipment When a gas is heated, it tends to expand so that each cubic metre has less mass than it did before heat ng. Referring to Figure 1, the hot gases in the stack, or chimney, will have less mass than a column of equal height of the outside colder air. The pressure due to the column of colder air entering the furnace will be greater than the pressure due to the mass of the column of lighter, heated gases at the base of the chimney. The heavier, colder air displaces the lighter gas and, as more is heated in the furnace, the process becomes cont nuous.

Figure 1 Chimney Producing Natural Draft

In order to increase the pressure diﬀ erent al between the pressure at the furnace inlet and the pressure at the furnace exit, there are three alternat ves: 1. Chill the outside air 2. Increase the temperature of the gases leaving the furnace 3. Increase the height of the chimney

4th Class • Part A2 Unit 13 • Chapter 56 • Boiler Draft Equipment 256

The fi rst method is obviously next to impossible to at ain. However, during the cold weather, bet er draf condit ons can be expected compared to those during the summer months. The second method, increasing the stack (chimney) temperature, is easily accomplished, but it should be kept in mind that the higher the stack temperature, the greater the heat loss through the chimney. The third method seems to be the only sensible solut on; however, economically there are limitat ons to this solut on. In addit on, increasing the height of the stack produces a proport onal increase in frict onal resistance when the gases travel through the stack. Another funct on of the stack, besides creat ng draf , is to disperse dust part cles, and occasionally smoke, over a large area. In the interest of public health, various government bodies, in order to reduce air pollu- t on to a minimum, have established rules pertaining to the quant ty and size of dust part cles emit ed from a stack. A taller stack will disperse the pollutants over a larger area and thus reduce the pollut on problem. For coal and other solid fuel-fi red furnaces, dust collectors and precipitators are installed in the fl ue gas ductwork before the fl ue gas enters the stack. Effi cient dust collectors may remove as much as 98 % of the dust from the fl ue gases emit ed from the furnace. The chimney, or stack, is the only means of providing draf in natural draf systems; therefore, its design and construct on are important considerat ons. Chimneys are commonly constructed of reinforced concrete, brick or steel. The reinforced concrete chimney is the most durable type; Figure 2 shows the arrangement and details of this design. The brick chimney gives a nice fi nished appearance as shown in Figure 3. Common brick is not heat resistant and the chimney, for this reason, requires a fi rebrick or clay lining. The mortar tends to disintegrate with t me and is subjected to air infi ltrat on which reduces the available draf .

Figure 2 Concrete Chimney Details

4th Class • Part A2 Unit 13 • Chapter 56 • Boiler Draft Equipment 257

Figure 3 Masonry Chimney Details

Steel chimneys, shown in Figure 4, are the most popular type. They are low in ﬁ rst cost, of light construct on and may be erected on the same structure that carries the weight of the boiler. The disadvantages of the steel chimney are that it requires considerable maintenance and a refractory lining. In order to reduce cost and weight, the lining may extend to only one third of the total length.

4th Class • Part A2 Unit 13 • Chapter 56 • Boiler Draft Equipment 258

Figure 4 Self-Supporting Steel Chimney

Mechanical Draft Equipment The amount of draf that can be created by chimney act on alone is limited, thus the amount of fuel burnt per unit of t me is also limited. For this reason, some addit onal means must be provided to increase the amount of draf to that necessary for high output boilers. There are three methods available to solve this problem: • Induced draf • Forced draf • Balanced draf

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Induced Draft Induced draf , where the velocity of the gases in the chimney is increased, creates a part al vacuum in the furnace, thereby increasing the air fl ow into the furnace. This increase may be accomplished by: • Steam jet • Fan Steam Jet A part al vacuum in the furnace is created by a steam jet. This method was used on steam driven locomot ves where the exhaust steam from the engine was ut lized for supplying steam to a venturi tube placed in the chimney. The high velocity steam escaping from the venturi imparted velocity to the surrounding fl ue gases. Consequently, the increased gas fl ow through the chimney induced a greater draf across the furnace. Fan A fan, consist ng of a rotat ng wheel or impeller, sets up a centrifugal mot on on the air or gas to be moved. The fan casing, usually a spiral type, converts a part of the velocity created by the impeller into pressure energy. Figure 5 shows a double suct on induced draf fan. The gases from the boiler are drawn into the inner core, or eye, of the impeller from both sides. They are then accelerated by the rapidly rotat ng wheel, leave the t p of the impeller blades at a high velocity and are discharged into the surrounding casing. A part of the casing has been removed to give access for cleaning, inspect on or repair. The chimney connect on is at the upper sect on of the casing. The impeller shaf is supported by two end bearings. When used as an induced draf fan, this unit is located at or near the furnace outlet and may be motor or steam turbine driven. It draws the gases through the furnace and forces them up into the stack.

Figure 5 Induced Draft Fan

(Courtesy of Clarage Fan Company)

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Figure 6 shows the posit on of the induced draf fan at the lower sect on of the stack. For this part cular arrangement, the fan and drive are supported by overhead steel work.

Figure 6 Induced Draft Arrangement

Forced Draft Forced draf means forcing air into the furnace. The fan in this case delivers air to the furnace by means of ductwork. The quant ty of air admit ed is usually regulated by a damper. Figure 7 shows a typical forced draf fan suitable for a large industrial boiler. The atmospheric air is drawn into the side of the casing via a set of dampers which control the amount of air handled by the fan. When using a forced draf system, the ent re furnace casing is under posit ve pressure. To prevent the escape of gases, the furnace and all furnace openings must be carefully sealed against outward leakage and the casing must be strong enough to withstand the internal pressure. This type of furnace construct on is called a pressurized furnace.

Figure 7 Forced Draft Fan

(Courtesy of Clarage Fan Company)

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Figure 8 illustrates a reinforced furnace casing suitable for a pressurized furnace.

Figure 8 Reinforced Outer Casing

(Courtesy of Babcock and Wilcox) Figure 9 shows a cross sect on of a water-cooled pressure sealed access door.

Figure 9 Water-Cooled Access Door

Figure 10 shows a combinat on gas-oil burner. The gas burner is at ached to the boiler front and a packing box maintains a seal between the windbox and gas burner barrel. The gas burner is of double walled construct on. The gas ﬂ ows through the annular ring formed by the two tubes and issues from the gas burner ports.

Figure 10 Gas-Oil Firing Equipment with Aspirating Air

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The oil burner is inserted in the inner gas burner tube and can be removed when gas fi ring or oil burner servicing is desired. Sealing air (aspirat ng air) is supplied from the forced draf fan, or other source, at a suffi ciently high pressure to allow for safe removal of the oil burner or automat c gas lighter while the boiler is in service. A duct system is shown in Figure 11. The forced draf fan is connected to the lower duct which conveys the air through an air heater to preheat the air before it is admit ed to the burners. The duct passes underneath the boiler fl oor and is connected at the front to a wind box which distributes the air to a number of burners. The upper duct discharges the fl ue gas from the boiler via the air heater into the stack. It is common to have a set of adjustable louvres (dampers) in the windbox at the point where the air is admit ed to the burner.

Figure 11 Duct System – Forced Air

These louvres, as illustrated in Figure 12, perform the following funct ons: • Give the air a swirling mot on in order to obtain bet er mixing of fuel and air • Shut oﬀ the air supply when the burner has been taken out of service, and in this way prevent the other burners from being starved for air When the boiler is taken oﬀ line, all the louvres are shut, allowing the boiler to cool at a slow controlled rate and thereby avoiding undue stress on the tube joints and other parts.

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Figure 12 Boiler Front – Windbox and Louvres

Figure 13 shows a forced draf fan including all the necessary ﬁ ring equipment, mounted directly on the boiler front of a packaged boiler.

Figure 13 Three-Drum “A” Type Packaged Boiler with Forced Draft Fan

Figure 14 shows a fan suitable for smaller sized boilers. The air ﬂ ows parallel to the motor and the propeller type fan forces it into the boiler furnace.

Figure 14 Axial Flow Fan

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Balanced Draft A balanced draf system forces air into the furnace where the combust on gases are removed by natural or mechanical means. These systems are regulated so that the forced draf fan supplies a suffi cient amount of air necessary for complete combust on of the fuel and the induced draf fan removes the fl ue gases, keeping the furnace pressure slightly below atmospheric. This system is commonly employed for large steam boilers where the gases travel a long distance through a number of boiler passes with a proport onal increase in resistance to fl ow. The induced draf fan works in conjunct on with the forced draf fan allowing the furnace pressure to be maintained slightly below atmospheric. Figure 15 shows a balanced draf system. The forced draf fan forces air into the windbox while the induced draf fan creates a part al vacuum at the fan suct on, inducing the gas to fl ow through the various boiler passes.

Figure 15 Balanced Draft System

When burning solid fuels, openings are normally provided for the fuel to be introduced into the furnace and ash to be removed from the ash pit, grates and fl ue gas passages. For these reasons, a negat ve furnace pressure is required to keep hot gases from being expelled through openings. Therefore, when burning solid fuels an induced draf fan is normally used. Since having only an induced draf fan would cause a very low furnace pressure, result ng in a lot of air being drawn in through these openings, they are usually balanced draf to increase effi ciency. If using waste fuel where combust on effi ciency is less crit cal, an induced draf fan alone may be used. Generally speaking, when burning natural gas or liquid fuels the boiler furnace can be made air t ght and only a forced draf fan is required. Since induced draf fans are larger and handle hot gases they are more expensive and have higher maintenance requirements. Figure 16 shows a typical balanced draf system used on a large boiler (steam generator). Both fans are posit oned for easy access at ground level.

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Figure 16 Gas or Oil-Fired Steam Generator

Advantages of Mechanical Draft 1. It is independent of atmospheric air temperatures. 2. It is independent of chimney temperature; whereas for natural draf , this is an important factor. 3. Mechanical draf can be regulated more accurately than natural draf ; hence, steam product on can be varied to suit a fl uctuat ng demand. 4. Bet er draf regulat on increases the effi ciency of combust on. 5. The chimney needs only to be suffi ciently tall to adequately disperse the fl ue gases. Comparison of Forced Draft and Induced Draft The forced draf fan handles clean cool air which means longer fan life as well as a smaller size fan and fan drive requirement. An advantage of induced draf is that it maintains the furnace pressure below atmospheric prevent ng any toxic gases from entering the boiler room. Also, furnace inspect on and burner openings do not require pressure sealing as is the case with a pressurized furnace. However, the induced draf fan impeller may be subjected to corrosive act on of the fl ue gases and erosion due to fl yash when coal or other solid fuels are used.

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OBJECTIVE 2

Discuss the common methods of controlling combustion air ﬂ ow.

DRAFT CONTROL

The combust on process in the boiler furnace must be regulated in accordance with the demand of steam from the boiler. This regulat on is at ained by controlling fuel and air fl ow into the furnace. In addit on to making sure that suffi cient fuel and air are admit ed, the control system must maintain the proper rat o of air to fuel in order to achieve safe and effi cient combust on. When balanced draf is employed, the control system must also ensure the correct fl ue gas fl ow from the furnace in order to maintain the correct furnace pressure. Control of Air and Flue Gas Flow Controlling the combust on air to the furnace and the fl ue gas fl ow from the furnace is accomplished by: • Register or damper control • Fan speed control • Outlet damper control • Inlet damper control • Variable speed coupling Register or Damper Control When draf is produced by natural means, the fl ow is controlled by means of an air register (Fig. 17) at the point where the air enters the furnace or by a damper at the chimney inlet. Usually, the chimney damper is perforated so that when the damper is in the closed posit on and the boiler is shut down, a slight fl ow of air is maintained through the furnace, thereby prevent ng the possibility of an explosive gas mixture build up.

Figure 17 Natural Draft Control

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Fan Speed Control The most effi cient operat on of a fan demands that the speed be only suffi cient to supply the required amount of air. This control can easily be accomplished when a steam turbine drives the fan and an automat c control system is used to regulate the steam fl ow to the turbine by means of a throt le valve. However, with constant speed electric motors, it is necessary to fi nd other methods to control the fl ow of gases. A simple method is to place a set of dampers at the fan outlet or inlet. Outlet Damper Control A damper acts as a throt le valve and restricts the fan output to any desired quant ty. For fan outlet damper control, the damper is placed in the fan outlet duct. The fan output can, in this case, be easily controlled from minimum to maximum output. Figure 18 shows a schemat c of an outlet damper control while Figure 19 shows the damper control linkage.

Figure 18 Outlet Damper Location

Inlet Damper Control Figure 19 illustrates an induced draf fan with dual fan inlet damper control. This ﬁ gure also shows the control linkage used for outlet or inlet dampers. The damper for controlling the combust on air ﬂ ow may be located directly at the forced draf fan inlet. Figure 20 shows this arrangement. This type of damper is of somewhat more complicated design. However, by employing it, less power is required to drive the fan at reduced loads.

Figure 19 I.D. Fan Inlet Damper Control

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Figure 20 Forced Draft Fan with Inlet Damper Control

Variable Speed Coupling The speed of a fan driven by a constant speed motor can be varied by means of a variable speed coupling. Figure 21 shows one of these units. The init al cost for this system is high, but it gives excellent fan speed control; as well, it requires the least amount of energy to drive the fan at varying speeds compared to any other type of control.

Figure 21 Variable Speed Coupling

Regardless of the method used to control the air fl ow into the furnace, this is a funct on of the combust on control system. The combust on control system dictates the amount of steam produced. This is normally done by sensing the steam pressure in the boiler or main steam header. If the pressure is falling, more steam is required so the combust on control system must increase the fuel to the furnace. But fi rst, the air fl ow must be increased to ensure that the fuel is burned completely. If the steam pressure is rising above setpoint, the fuel and air are cut back by the combust on control system. In a balanced draf system, the control for the induced draf fan is furnace pressure. If the air and fuel are being increased by the combust on control system, the furnace pressure will increase and the furnace pressure controller will increase output of the induced draf fan to maintain the correct furnace pressure.

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OBJECTIVE 3

Discuss the common methods of measuring furnace pressures.

DRAFT MEASURING INSTRUMENTS

The pressures experienced in combust on air systems are relat vely low and, for this reason, the instruments used should be sensit ve to small pressure changes. The simplest device is the “U” tube manometer shown in Figure 22. The tube is made of glass, or any other transparent material, bent in the form of a “U”. With no pressure connect on and both legs open to the atmosphere, the tube is ﬁ lled about half full, usually with water. The surface of the liquid in one leg will then be at the same height as the surface in the other; the scale may be raised or lowered so that the zero line coincides with the liquid surfaces. If pressure is applied to one leg of the manometer, the liquid in that leg will be forced down and the liquid level in the other will rise. When the liquid comes to rest, the pressures in the two legs, measured above some common datum point, will be equal. That is, the force caused by the mass of the liquid in the one column will just balance the total force caused by the applied pressure and the mass of liquid in the other.

Figure 22 “U” Tube Draft Gauge (Manometer)

The inclined tube manometer operates on the same principle as the “U” tube as shown in Figure 23. The pressure to be measured is connected to the vert cal leg while the read out is taken from the slanted or inclined leg. Any given pressure diﬀ erent al between the applied and atmospheric will always create the same diﬀ erent al in vert cal level for a given manometer liquid. By using an inclined scale, the liquid will have a greater movement over the scale for the same vert cal travel of the liquid. This type facilitates reading as there is only one scale to be read. Also it gives a more accurate reading for small pressure changes.

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Figure 23 Inclined “U” Tube Manometer

Another type of sensing element used for pressure measurements is the diaphragm gauge. Referring to Figure 24, the diaphragm is made of ﬂ exible material such as rubber and the pressure to be measured is applied to the housing below the diaphragm. This causes the diaphragm to be moved upward as the pressure increases. This movement is transferred through a push rod to the cant lever range spring linked to a pointer. The movement is used to indicate the applied pressure or to vary a control signal in a pressure transmit er. The gauge can be calibrated using the zero adjustment screw. Like the water ﬁ lled manometer, the diaphragm gauge indicates boiler draf in millimetres of water.

Figure 24 Diaphragm Draft Gauge

(Courtesy of Bailey Meter Co. Ltd.)

SIZE OF AIR OPENINGS

The student should be aware of the importance of having adequate openings for the admission of combust on air in buildings containing boilers. For an enclosed boiler room, it is important that adequate openings for combust on air be provided into the boiler room from the outside. These must be in addit on to the boiler room doors and windows which are usually kept shut in cold weather. The CSA Code B-51 speciﬁ es the size of openings required when burning a given number of heat units in a given t me. If there are not adequate openings in the boiler room and the fan is drawing air from the room and discharging it up the stack, the pressure in the boiler room will be reduced and there may not be adequate air supplied to the furnace for complete combust on. This can reduce boiler eﬃ ciency and may result in a furnace explosion. In addit on, the low pressure in the building can result in a building collapse. Remember that the force on the outside of a building is the atmospheric pressure mult plied by the area, and a building has a large surface area, so that if the pressure inside is below atmospheric, the force on the outside of the building can be extreme.

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CHAPTER 56 - QUESTIONS BOILER DRAFT EQUIPMENT

1. The most eﬃ cient operat on of a fan demands that a) the speed is only suﬃ cient enough to supply the required amount of air. b) inlet damper controls are used. c) outlet damper controls are used. d) constant speed electric motors are used.

2. Balanced draf refers to a) higher fl ow of induced than forced due to the air temperature diff erent al. b) combinat on of forced and induced draf . c) equal mixture of ambient and preheated air supply to the burners. d) induced draf fan being used to induce a part al fl ow of makeup air to the burners and a natural draf system supplying the bulk of the air intake.

3. A boiler with only an induced draf fan a) will have a negat ve furnace pressure. b) needs an airt ght furnace. c) needs a very long stack. d) cannot maintain a stable ﬁ re.

4. When the least amount of energy is desired to drive a fan with variable output control, the type of control used would most likely be a) register or damper. b) fan speed. c) outlet damper. d) variable speed coupling.

5. A diaphragm draf gauge on a furnace typically indicates the draf in a) kilopascals. b) millimetres of water column. c) cent metres of mercury. d) millimetres of mercury.

6. The Code that speciﬁ es the size of openings required into a boiler room when burning a given number of heat units in a given t me is a) CSA B-52. b) ASME Sect on 1. c) ASME Sect on 4. d) CSA B-51.

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CHAPTER 56 - ANSWERS BOILER DRAFT EQUIPMENT

1. (a)

2. (b)

3. (a)

4. (d)

5. (b)

6. (d)

Fourth Class • Part A2 4th Class • Part A2 C HAPTER 57

Introduction to Boiler Combustion

LEARNING OUTCOME

When you complete this chapter you should be able to: Discuss the basic theory of combustion in a boiler, and the equipment used to provide proper combustion conditions.

LEARNING OBJECTIVES

Here is what you should be able to do when you complete each objective:

1. Describe the principles of combustion, combustion equations and the relationships between theoretical and excess air.

2. Describe the three general classes of boiler fuels.

3. Describe the ﬁ ring methods used in the combustion of various fuels, the effects of combustion on refractory and how the ﬂ ow of fuel is controlled.

4. Describe ﬂ ue gas analysis and its relationship to boiler efﬁ ciency.

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INTRODUCTION

The term “combust on” refers to the burning of a fuel in a boiler furnace to produce heat. It is important that the Power Engineer understands this process because, if the combust on is not carried out in the proper manner, then the fuel will not be completely burned and less heat per kilogram of fuel will be produced. Therefore, more fuel than necessary will have to be fed to the furnace with a result ng higher fuel bill. In addit on, if the combust on is not carried out in a proper manner, there will be a danger of explosions occurring in the furnace. In order to understand the basic process of combust on, it is necessary to know what components the various fuels contain and what part they play in the combust on process.

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OBJECTIVE 1

Describe the principles of combustion, combustion equations and the relationships between theoretical and excess air.

COMBUSTION

All fuels, whether in solid, liquid or gaseous state, are composed of combust ble and non-combust ble elements. The combust ble elements include: • Hydrogen Carbon• Carbon• Sulphur• Sulphur• The non-combust ble elements include, in variable quant t es: Moisture• Moisture• Ash• Ash• • Carbon dioxide • Other trace elements Some of the combust ble elements may be chemically united, such as hydrogen and carbon, called volat le mat er or hydrocarbons. Occasionally, in gaseous fuels, the carbon is combined with oxygen and denoted as carbon monoxide (CO), a combust ble gas which can also be produced during an incomplete combust on process.

Moisture (H2O) is another example of a combust ble element combined with oxygen; two atoms of hydrogen combine with one atom of oxygen forming one molecule of water. The presence of moisture in a fuel, such as coal, may be due to the absorpt on of moisture from the atmosphere or from hydrogen and oxygen in the fuel being chemically combined prior to combust on. Theory of Combustion In the process of combust on, the main combust ble elements of the fuel, carbon, hydrogen and sulphur, combine chemically with oxygen from the air. As a result of this combinat on, heat is produced. Thus, combust on is the rapid oxidat on of a fuel whereby large quant t es of heat are produced. To ensure complete combust on of the fuel in the furnace, the following condit ons must be fulfi lled: 1. Enough air must be supplied to the furnace to provide suffi cient oxygen to combine with all the combust ble elements of the fuel. 2. The air and fuel must be thoroughly mixed together (known as turbulence) so that each part cle of fuel can come in contact with the necessary oxygen. 3. The temperature in the furnace must be high enough to ignite the fuel as it enters. 4. The furnace must be large enough to allow suffi cient t me for the combust on to be completed before the gases strike the cooler areas of the heat ng surfaces. The above condit ons may be summed up as enough air plus enough t me, temperature and turbulence. The last three terms are of en referred to as the three T’s of combust on. In other words, the requirements for proper combust on are the three T’s plus enough air. If poor combust on is taking place in a furnace, then one or more of the four condit ons is not being met.

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Products of Complete Combustion Of the combust ble elements in a fuel, carbon usually forms the greatest percentage. If the combust on is complete, each atom of carbon combines with two atoms of oxygen to produce gaseous carbon dioxide (CO2), plus heat.

The hydrogen in a fuel combines with the oxygen to produce water vapour (H2O) plus heat.

The sulphur in the fuel combines with oxygen to produce gaseous sulphur dioxide (SO2), plus heat. The combust on process can be expressed in simple chemical equat ons, which represent the combinat on of these combust ble elements with oxygen during complete combust on (Equat ons 1–3): 1. Carbon + Oxygen → Carbon Dioxide

C + O2 → CO2 2. Hydrogen + Oxygen → Water Vapour

2H2 + O2 → 2H2O 3. Sulphur + Oxygen → Sulphur Dioxide

S + O2 → SO2 The non-combust ble elements of fuel will not combine with oxygen, but will form ash or pass through the furnace unchanged. Products of Incomplete Combustion If any of the requirements for complete combust on are missing, then the combust ble elements will not combine completely with oxygen. Equat ons 4–7 represent the incomplete combining of oxygen and combust bles. 4. Carbon + Insuﬃ cient oxygen → Carbon monoxide

2C + O2 → 2CO Equat on 4 shows the format on of carbon monoxide instead of carbon dioxide. This react on is undesirable because carbon monoxide is a combust ble compound and, in passing out of the furnace without burning, it represents a loss of heat ng value and a waste of fuel, as well as a contribut ng factor to pollut on. However, if the carbon monoxide combines with more oxygen before leaving the furnace, then its combust on will be complete and carbon dioxide will be formed (Equat on 5). 5. Carbon monoxide + Oxygen → Carbon dioxide

2CO + O2 → 2CO2 Equat on 6 shows the format on of free hydrogen. This process is undesirable because hydrogen is a combus- t ble element, which, if not burned, will represent a waste of fuel and a loss of heat ng value. 6. Hydrogen + Insuﬃ cient oxygen → Water Vapour + Free hydrogen

3H2 + O2 → 2H2O + H2 Similarly, the format on of free sulphur (Equat on 7) is undesirable because, being combust ble; it represents a waste of fuel. In actual pract ce, the sulphur in most fuels is considered to be an impurity because, although it is a combust ble element, it tends to produce corrosive acids in the presence of water. The sulphur dioxide discharged into atmosphere also contributes to air pollut on. 7. Sulphur + Insuﬃ cient oxygen → Sulphur dioxide + Free sulphur

2S + O2 → SO2 + S Hydrogen is extremely react ve and will normally burn completely and the amount of sulfur in most fuels is very small. Therefore, the main concern with incomplete combust on is the format on of carbon monoxide. However, if the combust on react on is very poor, free carbon can be produced. This occurs when carbon in the fuel is vaporized but some does not combine with any oxygen; as it leaves the combust on zone and cools, it turns back to solid carbon part cles, result ng in black smoke. Note that the format on of carbon monoxide does not create visible smoke because it is a colourless gas.

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Combustion Air and Excess Air Air is basically composed of a mixture of oxygen and nitrogen. The approximate proport ons are 21 percent oxygen and 79 percent nitrogen, by volume. The other components are low in concentrat on and will be assumed to be zero. The oxygen required for complete combust on must be obtained from the air supplied to the furnace. The amount of air required to supply just enough oxygen for complete combust on is called the theoret cal air. However, in actual pract ce, it is necessary to supply more than this theoret cal amount of air in order to make sure that all part cles of fuel come in contact with oxygen. The amount of air in excess of the theoret cal air is called excess air. It is usually expressed as a percentage of the theoret cal air.

Example 1: If the theoret cal amount of air required for the complete combust on of 1 kg of coal is 12 kg and if the actual amount of air used in the furnace is 18 kg for every kilogram of coal, what is the excess air?

Solut on: The amount of air in excess of 12 kg is 6 kg (18 - 12 = 6). Expressed as a percentage of the theoret cal air (12 kg), this would be: ___ 6 x 100% = 50% (Ans.) 12 The percentage of excess air required for proper combust on of a fuel may vary from 10% to 60% or even higher. The amount required will depend upon: 1. The t me available for the fuel to mix with the air before it comes in contact with the relat vely cool heat ng surfaces and is cooled below ignit on temperature. (Time) 2. How well the fuel and air can be mixed together. (Turbulence) 3. The temperature exist ng within the furnace. (Temperature) Therefore, it can be seen that a gaseous fuel, such as natural gas, that can easily be mixed with the combus- t on air will require less excess air than a solid fuel such as coal. It is desirable to reduce the amount of excess air supplied to the furnace as much as possible since the air is heated to a high temperature in the furnace, and therefore carries a large amount of heat out through the stack. In addit on, the power required for forced and induced draf fans will decrease if less air is supplied. On the other hand, if the excess air is reduced too much, then there will be the possibility of incomplete combust on occurring, result ng in the format on of carbon monoxide and free hydrogen, as explained previously. Flue Gases Analyzing the chemical composit on of the burned gases af er they leave the furnace enables the operator to ascertain whether complete combust on has been at ained. A typical composit on of burned gases, denoted from now on as ﬂ ue gases, may consist of CO2, H2O, SO2, O2, N2 and possibly ash in a ﬁ nely divided state. The presence of oxygen is due to supplying more air than theoret cally is required to assure complete combust on.

The nitrogen (N2) is contained in the combust on air. Air by weight is composed of 23% oxygen and 77% nitrogen and, in terms of volume, 21% oxygen and 79% nitrogen. Nitrogen is actually a waste product because it is heated in the furnace and leaves the chimney at a relat vely high temperature. The heat contained in the nitrogen represents a heat loss as it will be lost into the atmosphere. For this reason, the amount of excess air should be reduced to a minimum. The ideal amount required is determined by the following factors: • Composit on and condit on of the fuel ﬁ red. • Method used for burning the fuel. For example, coal ground to a ﬁ ne powder will require less excess air than coal supplied to the furnace in the form of large part cles.

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The approximate amounts of excess air typically required for the various ﬁ ring methods and types of fuels, as shown in Table 1.

Table 1 Characteristics of Fuel Oil

Excess Air O2% Stoker ﬁ ring 30 - 35% 6 - 7% Coal Pulverized 15 - 20% 3 - 4%

Fuel Oil 15% 3%

Gaseous Fuel 10% 2% If an insuffi cient amount of air is supplied, or any other requirement for complete combust on is not met, the result will be incomplete combust on indicated by CO or, under the most adverse condit ons, soot (carbon) appearing in the fl ue gases. This residue represents a serious waste of fuel and, in addit on, the possibility of a furnace explosion exists due to pockets of CO being formed in parts of the furnace. Air Supply to Boiler or Furnace Room It is extremely important that adequate openings be provided to allow suffi cient outside air to enter the boiler room for the sat sfactory combust on of the fuel. These openings must be in addit on to doors and windows which are usually kept shut in cold weather. A shortage of combust on air results in incomplete combust on of the fuel, which, in turn, can cause: • Soot ng of heat ng surfaces • Plugging of gas passages • Poor heat transfer • Increased fuel consumpt on • The possibility of a disastrous explosion, when fi ring oil or gas • Carbon monoxide poisoning to anyone entering the room CGA Bl49.1, the Installat on Code for Natural Gas Burning Appliances and Equipment, specifi es standards for the type, locat on and size of outside air supply inlets with regard to gas burning equipment of various capacit es with natural or mechanical draf . Strict adherence to this code is required.

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OBJECTIVE 2

Describe the three general classes of boiler fuels.

FUELS

Fuels can be classifi ed into three groups: Solid• Solid• Liquid• Liquid• Gaseous• Gaseous• Solid Fuels Solid fuels used as fuel in boilers include: Coal• Coal• • Wood and Other Biomass Refuse• Refuse• Coal Coal is composed of carbon, hydrogen, oxygen, sulphur, nitrogen, moisture and ash. Of these components, only carbon, hydrogen and sulphur are combust ble. The hydrogen is combined with some of the carbon to form hydrocarbons, called volat le materials since they pass off as gas when the coal is heated. The remaining carbon, that not combined with hydrogen, is referred to as fi xed carbon. The sulphur represents a very small percentage of a coal’s composit on and, although it is combust ble, it is considered an impurity since its combust on product, sulphur dioxide, tends to produce corrosion in the boiler and chimney and also contributes to air pollut on. Wood and Other Biomass Historically, wood and other natural organic materials were an important source of boiler fuel. The growth in pulp and paper and wood manufacturing industries, combined with the development of coal, oil and gas (which are more concentrated and transportable sources of chemical energy) led to the decline of wood as a fuel. Nevertheless, the byproducts of wood processing, such as bark, chips and sawdust, cont nue to be used as fuel in favorable situat ons, such as pulp and paper product on. These materials are of en referred to collect vely as hog fuel. The use of residual organic materials like sugar cane stalks, animal waste, and shells from nuts as fuels is also common to the food industries and community waste processing plants. Recently, the term biomass has been applied collect vely to this type of fuel. Refuse The burning of household garbage is common in many areas of North America and Europe due to the problems of disposing of this waste using landfi lls. Specially designed refuse boilers can eff ect vely burn the waste material and modern fl ue gas treatment systems can reduce harmful emissions to acceptable levels. These operat ons have become commercially viable since they can charge the municipalit es to dispose of the garbage and use the steam generated to drive turbines and produce electrical power that can also be sold. In addit on, many refuse plants sell steam to adjacent industrial facilit es. The refuse may be processed before combust on to remove recyclable materials or simply burned and only the ferrous metals recovered from the ash using magnet c separators. Liquid Fuel Liquid fuels used for boiler fi ring include fuel oil.

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Fuel Oil Fuel oils are derived from petroleum. Petroleum, called crude oil in its unrefi ned state, is a mixture of a large variety of hydrocarbon compounds varying from very light hydrocarbons in the gaseous state through a range of progressively heavier hydrocarbons in the liquid state to very heavy hydrocarbons in the semisolid state. In the refi nery, the fract onal dist llat on process is used to separate crude oil into a number of dist nguish- able groups according to specifi c characterist cs: boiling point, specifi c gravity and viscosity. These groups include: • Gases such as methane and ethane G asoline• Gasoline• Jet fuel• Jet fuel• • Kerosene • Diesel fuel • Light fuel oils • Lubricat ng oils • Heavy oils Residue• Residue• The dist nct on between these groups is not always sharply defi ned; they may overlap. For example, the diesel fuel range overlaps the light fuel oil range. This overlapping means that some diesel fuels are quite similar in composit on and characterist cs to light fuel oils. In fact, No. 2 diesel fuel is nearly ident cal to No. 2 fuel oil, the most widely used oil for packaged fi retube boilers. The main components of fuel oils are carbon and hydrogen combined as hydrocarbons. They also contain small amounts of oxygen, sulphur, nitrogen and traces of ash. Fuel oils are classifi ed into grade numbers according to such characterist cs as: 1. Relat ve density (specifi c gravity) - the rat o of the mass of a certain volume of oil to the mass of an equal volume of water. 2. Viscosity - a measure of the internal resistance of the oil to fl ow. 3. Flash point - the lowest temperature at which the fuel oil gives off suffi cient vapour to ignite when exposed to an open fl ame. The fl ash point is a good indicat on of the fi re hazard involved in the storage and pumping of the oil. Since the fl ash point of fuel oils is well above ambient temperatures, they are relat vely safe fuels to store, even inside a building. Table 2 shows the various classes of fuel oil, their applicat on and the average values of the characterist cs discussed above.

Table 2 Characteristics of Fuel Oil

Commercial Relative Viscosity Minimum Grade Density SSU* Flash Point Application No. at 15.5°C at 38°C °C °F 1 0.815 31 37.8 100 Light Domestic

2 0.86 32 - 39 37.8 100 Medium Domestic

3 0.92 45 - 120 54 130 Light Industrial

4 0.95 140 - 700 54 130 Medium Industrial

5 0.98 900 and up 66 150 Heavy Industrial *SSU stands for Saybolt Seconds Universal. See chapter ent tled Lubricat on Principles for explanat on of this unit.

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Grades 1 and 2, of en called furnace oils, have a relat vely low viscosity and relat ve density. They do not require heat ng before being fi red in the boiler furnace. Grade 2 oil is the most popular fuel oil for domest c and small commercial or industrial furnaces and boilers. Grades 4, 5 and 6 are heavier oils with higher viscosit es. They require heat ng during storage and pumping, and addit onal heat ng, usually to about 95°C, before they can be sat sfactorily burned in a furnace. Fuel oils have certain advantages over coal as a boiler fuel including: • Less storage space is required. • The amount fed to the furnace is more easily controlled. • Less handling equipment and labour is required. • They burn more cleanly (less residuals) and are effi cient to use. Two major disadvantages are that oil is more expensive and less abundant than coal. Gaseous Fuels Gaseous fuels used for combust on in boilers include: • Natural gas • Liquefi ed petroleum gases Natural Gas Natural gas is obtained from gas wells drilled in gas-bearing rock format ons or as a by-product from oil wells. Processing is usually required to remove any undesirable components before it is piped to the markets. The main components of natural gas supplied to the consumer are the hydrocarbons, methane (80–90%) and ethane (10–20%). It also may contain traces of propane, butane, nitrogen, oxygen, carbon dioxide and hydrogen sulphide. Although natural gas may be more expensive per kilojoule than oil in some areas, the cost of heat ng, compressing, and addit onal maintenance required makes natural gas the best choice where it is available. However in most cases, a secondary fuel such as oil is normally required because when the resident al demand for natural gas is high, the gas company may curtail industrial users. The advantages of natural gas as a boiler fuel are: • No ash is produced when gas is burned. • Lit le handling equipment is required. • The amount fed to the furnace is easily controlled. • It is easily mixed with air. • It is very clean - no spills, no mess, no residue. • No storage space is required.

However, natural gas is more expensive than coal and (in some locales) oil and requires long pipelines for transmission from its source to the heat ng plant. The relat ve economics of cost and supply for natural gas and oil will be dependent on a number of factors and will be specifi c to a given locat on. Liquefied Petroleum Gases (LPG) Liquefi ed petroleum gases are petroleum products consist ng of light hydrocarbons (butane, propane or a mixture of the two) which exist in the gaseous state at atmospheric pressure, but can be condensed to form a liquid by the applicat on of moderate pressure. In other words, they are petroleum gases that can be easily liquefi ed. As a liquid, the product takes as lit le as 1/120th of the space it needs as a gas, making it easily stored and transported and then burned as a gas. The liquid is converted to gas by reduct on of the pressure and absorpt on from the surrounding area of the latent heat required for evaporat on. The boiling point of butane at atmospheric pressure is 0°C, and that of propane is -42°C. These gases burn completely and cleanly with a bright fl ame. These fuels are commonly used for pilot fl ames on oil fi red boilers where natural gas is not available.

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Fuel Heating Value When a unit amount of fuel is burned completely, the heat produced by this combust on is called the heat ng value of the fuel. The unit amount can be a mass unit (kilogram) or a volume unit (cubic metre) depending on the type of fuel. The heat ng value of a fuel depends primarily on the amount of carbon, hydrogen and sulphur in the fuel. Typical average heat ng values for various fuels are listed below in Table 3.

Table 3 Average Fuel Heating Valves Heating Values Heating Values Fuel (SI) (USCS) Coal (bituminous) 25 600 kJ/kg 11 000 Btu/lb Fuel Oil (light) 45 360 kJ/kg 19 500 Btu/lb Natural Gas 37 260 kJ/m3 1000 Btu/f 3 Propane 93 150 kJ/m3 2500 Btu/f 3 Butane 122 200 kJ/m3 3200 Btu/f 3

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OBJECTIVE 3

Describe the ﬁ ring methods used in the combustion of various fuels, the effects of combustion on refractory and how the ﬂ ow of fuel is controlled.

BOILER FIRING

Boiler fi ring methods vary according to the type of fuel being used, namely solid, liquid or gaseous. The methods discussed will be for the most commonly used fuels: Coal• Coal• Fuel oil• Fuel oil• • Natural gas Coal Burning Apparatus The type of coal burning equipment used depends upon whether the coal is in large part cles or powdered form. Large coal part cles are used in mechanical stokers in which the coal is moved by mechanical means. Powdered coal is used in pulverizers in which the coal is carried by air. Mechanical Stokers Mechanical stokers may be classifi ed into three main types: underfeed, crossfeed and overfeed. An underfeed stoker is shown in Figure 1. In this type, the coal is fed up through the fi rebed by means of a ram and pusher blocks. The ram is operated by steam, compressed air or oil pressure. The coal fed at the bottom is gradually forced upward to the incandescent layer at the top where it ignites and spills over onto the side grates. The air for combust on is supplied below the grates and passes through openings called tuyeres, which run the length of the fi rebed on either side. As the coal burns, the ashes collect on the grates at each side from where they can be dumped into an ashpit below. The combust on air is supplied by means of draf fans. This stoker is part cularly adapted to burning coal which is high in volat le content.

Figure 1 Underfeed Stoker

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Figure 2 shows the arrangement of a crossfeed stoker. This type is also referred to as a chain grate or a bar grate stoker, depending upon the construct on of the grate surface. The grate surface consists of a moving endless belt, which extends into the furnace from the boiler front. The coal is fed by gravity onto the front port on of the moving grate. The thickness of the fi re and the speed of the grate can be regulated so that the fuel is completely burned by the t me it reaches the back of the grate and is dumped into the ashpit. The combust on air is supplied by a draf fan and passes up through the grate to the fuel bed. Addit onal air is admit ed through overfi re air jets above the bed at the side where the coal enters the furnace. These air jets assist in burning the volat le mat er driven off as the coal is heated.

Figure 2 Crossfeed Stoker Arrangement

Figure 3 shows the overfeed stoker, also known as the spreader stoker. In this type, the coal is spread over the surface of the grate by means of a revolving rotor having blades or paddles which strike the coal pieces and bat them into the furnace. The grates shown in Figure 3 are the power operated dumping types which are arranged so that they can be t lted in order to dump the ash into a pit underneath. Combust on air is supplied under the grates by a draf fan and passes up through openings in the grates to the fuel.

Figure 3 Spreader Stoker with Dumping Grates

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Pulverized Coal Mills and Burners Pulverized coal has been ground to a fi ne powder or dust. It is carried by means of air fl ow through pipes from the grinding mills, or pulverizers, to the burners in the boiler furnace. Due to the powdered form of the coal, it mixes well with the combust on air. In addit on to this advantage, pulverizing the coal allows cheaper grades to be burned sat sfactorily. As well, bet er combust on control is possible with the coal in a powdered rather than a solid form. Figure 4 shows a commonly used pulverizer design. In this type, the raw coal is fed into a rotat ng bowl. Spring loaded rolls are held in place inside the bowl where the coal is ground to a fi ne powder between the rolls and the bowl sides. Hot air enters at the side of the mill and fl ows upward and across the top of the bowl, picking up the powdered coal. This coal-air mixture is drawn by an exhauster fan from the mill and travels to the burners through piping.

Figure 4 Raymond Bowl Mill

(Courtesy of Combustion Engineering)

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The pulverized coal burner is illustrated in Figure 5. The coal-air mixture from the pulverizer enters and ﬂ ows through a large central nozzle. The addit onal air required for combust on, known as secondary air, passes through a housing surrounding the central nozzle and the combust on takes place at the burner throat.

Figure 5 Pulverized Coal Burner

Hog fuel and biomass fuels can be burned either on their own, or in conjunct on with coal, oil or gas ﬁ ring systems. A variety of arrangements have been developed to handle the solid materials, including refractory lined chambers and stat onary, traveling and vibrat ng grates. Hog fuel may be burned separately or with coal. When burned with oil or gas, these fuels have their own burners separate from the hog fuel. Fuel Oil Burners Before oil can be burned properly it must be broken up into a ﬁ ne spray or vapour. This breaking up of the oil is known as atomizat on; it is necessary for the combust on air to mix well with the oil. Heavy oils must be heated to approximately 90°C before they can be atomized while light oils do not require heat ng. The air atomizing oil burner, shown in Figure 6, is commonly used in packaged boilers. It uses compressed air at about 103 kPa as atomizing air (also called primary air). This primary air mixes with the oil near the burner t p and causes atomizat on to occur. The secondary air necessary for combust on then mixes with the atomized oil as it leaves the burner.

Figure 6 Air Atomizing Oil Burner

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The steam atomizing oil burner, shown in Figure 7, is frequently used in boilers larger than the packaged type. Its principle of operat on is much the same as the air atomizing burner except that steam is used to produce atomizat on by coming in contact with the oil before it leaves the burner t p. The combust on air then mixes with the atomized oil as it sprays from the burner.

Figure 7 Atomizing Oil Burner

A mechanical atomizing oil burner is shown in Figure 8. In this type, the oil is pumped under high pressure through slots in a sprayer plate producing atomizat on of the oil. This type of burner is used in both packaged and larger boilers.

Figure 8 Mechanical Atomizing Oil Burner

The rotary cup oil burner, shown in Figure 9, is of en used for larger sized packaged boilers. In this type, the oil is pumped to the inside of a cup which is being rotated at about 3500 r/min by an electric motor. Cen- trifugal force causes the oil to be thrown oﬀ the cup’s rim in a ﬁ ne spray. Primary air is forced in a whirling mot on by a fan into the path of the oil spray. Addit onal secondary air for combust on is supplied to the oil spray as it leaves the burner.

Figure 9 Rotary Cup Oil Burner

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Oil Burner Maintenance and Operation If the installat on is new, the fuel lines should be thoroughly cleaned by blowing out with compressed air or steam before put ng burners into operat on. This cleaning will prevent dirt and other foreign materials from plugging the small holes and passageways in the burner. When oil burners become dirty, they should be replaced promptly with clean spare burners. The dirty burners should then be cleaned. When cleaning burners, use a clean work area and take care not to damage the delicate parts of the burner. Kerosene or special solvents can be used along with compressed air to blow out the burners. Be sure to wear goggles when blowing out with compressed air. Af er cleaning burners, hang them vert cally with the t ps immersed in kerosene. When shut ng off air or steam atomizing burners, the oil should be shut off fi rst and then the burner blown out with steam or air. The freedom of movement of louvres or registers should be checked regularly and the linkage lubricated. When the burner is in operat on, the registers must be adjusted to give the proper rat o of air to oil. Too lit le air will produce a dark, heavy smoke while too much air will produce a gray-coloured smoke. When the boiler is operat ng at low rat ngs, the number of burners in service should be reduced to maintain adequate oil pressure to produce a stable fl ame at the burners st ll in service. When burners are not in use, they should be removed from the furnace. Otherwise, the heat may cause carbon to form within them. Natural Gas Burners Since gas is already in an atomized condit on, further atomizat on at the burner is not necessary. Therefore, the main funct on of the gas burner is to provide complete and turbulent mixing of the gas and combust on air. A type of gas burner of en used in packaged fi retube boilers is sketched in Figure 10. Essent ally, it consists of a tube contained within another tube. Air, blown by a fan, travels through the central tube while the gas is admit ed to the annular space between the two tubes. The gas issues from small holes at the end of the annular space while the air passes out through louvres at the end of the central tube. The two mix together; the mixtures are given a whirling mot on by the velocity of the air. This type of burner is known as an af er-mix or outside mixing type as the gas and air mix together af er they leave the burner.

Figure 10 Packaged Boiler Gas Burner

The burner sketched in Figure 11 is known as a pre-mix type as the gas and some air (primary air) mix together inside the burner itself. The amount of gas entering the burner is controlled by a needle valve; the primary air enters through a controlling shut er at the end of the burner. The two mix together as they pass through a venturi tube. Addit onal (secondary) air for combust on is supplied as the mixture leaves the tube. This type is used in small installat ons.

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Figure 11 Pre-Mix Gas Burner

A gas burner suitable for use in a large boiler is shown in Figure 12. In this ring type burner, gas is supplied to a ring manifold containing numerous gas outlet holes. Air is admit ed through the register louvres and mixes with the gas leaving the ring holes. An oil burner is mounted at the centre of the ring manifold.

Figure 12 Ring Type Gas Burner

Gas Burner Maintenance and Operation Gas shut-off cocks should be lubricated periodically with grease and air registers should be checked for freedom of movement and lubricated when necessary. The amount of air admit ed to the burner should be adjusted to give a stable fl ame. The fl ame color should be blue with yellow t ps. Gas burners should be cleaned whenever the boiler is shut down for rout ne maintenance or inspect on. Carbon deposits can usually be removed with a wire brush and solvents used to cut grease deposits. Burner holes, if seriously plugged, can be drilled out with a proper sized drill. The number of burners should be reduced at low loads in order that the remaining burners in service will have adequate gas pressure and a stable fl ame.

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EFFECTS OF COMBUSTION ON REFRACTORY

The term refractory is used for those materials that can withstand very high temperatures. The most common example of a refractory is fi rebrick. Earlier designs of boilers used refractory materials for furnace walls but the use of the watercooled furnace wall in the modern boiler has largely reduced the use of refractory. Since water fi lled tubes absorb heat from the furnace quickly, some refractory is of en used in the furnace area to absorb heat and keep the furnace temperature high enough to ensure complete combust on. However, refractory is st ll in use in certain areas in the modern boiler. These areas include: • Burner throats • Inspect on door openings • Combust on chambers • Baffl es The fuel used in the boiler will have an eff ect on the refractory, some fuels being more destruct ve than others. Stoker fi red units burning coal produce ash slag which will both corrode and erode refractory. Ash clinkers, forming on the brickwork, will cause port ons to break away. In the case of oil-fi red boilers, the refractory may be corroded by the ash from the oil. In addit on, if the fl ame from the burner impinges directly on the brickwork, the surface will tend to break off . If the oil burner is not posit oned correctly, carbon will build up on the burner throat refractory. When natural gas is used as fuel, the refractory problems are usually reduced although, as with the oil burner, impingement of the fl ame directly on the brickwork surface will cause deteriorat on.

FUEL FLOW CONTROL

Just as proper air fl ow is necessary to meet the boiler steam fl ow and pressure requirements, the fuel fl ow must also be controlled. The fuel fl ow rate is generally adjusted to ensure that boiler steam pressure remains constant regardless of steam demand. Any changes to the fuel fl ow rate must be matched to corresponding air fl ow changes to maintain stable and effi cient combust on condit ons. However, the fuel fl ow is changed using the following three general methods: 1. Solid fuel: adjustment of primary air fl ow (for pulverized coal), grate or stoker speed. 2. Fuel oil: solenoid valve (on-off or mult -range control) or control valve (modulat ng control) 3. Natural gas: solenoid valve (on-of or mult -range control) or control valve (modulat ng control)

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OBJECTIVE 4

Describe ﬂ ue gas analysis and its relationship to boiler efﬁ ciency.

FLUE GAS ANALYSIS

Earlier in this chapter, the need for a certain amount of excess air in order to obtain complete combust on of the fuel was discussed. Too much excess air will cause the eﬃ ciency of the boiler to drop since the air will carry a large amount of heat out through the stack. To check the amount of excess air the ﬂ ue gas contains, a sample of the gas is taken and analyzed for CO2 content.

Carbon in the fuel combines with oxygen in the combust on air to form carbon dioxide (CO2), which will be part of the total amount of fl ue gases leaving the boiler. By determining the percentage of CO2 in the fl ue gases, the amount of excess air supplied can be calculated. The amount of CO2 produced per unit amount of fuel burned will be constant, but the percentage of CO2 in the fl ue gases depends on the amount of air supplied. The higher the amount of excess air, the lower will be the percentage of CO2, and vice versa.

The percentage of CO2 found in the ﬂ ue gases also depends on the kind of fuel burned, as various fuels contain diﬀ erent amounts of carbon per unit amount of fuel.

Analyzing the fl ue gas for CO2 content is a fairly simple procedure; it can be performed by service personnel or a boiler operator provided with the proper equipment. A fl ue gas analyzer of moderate cost commonly used for heat ng boilers is the Fyrite analyzer. Electronic Flue Gas Analyzers Flue gas analysis is used both for effi ciency and emissions purposes. Thanks to advances in electronics, it is now cheaper, easier and therefore more common to monitor fl ue gases. Due to t ghtening environmental regulat ons, monitoring may be mandatory. The electronic instruments used for analysis range from inexpensive small hand-held devices that produce reasonable accuracy to larger, permanently installed units that are capable of producing lab quality results on a cont nuous basis. The lat er, of en referred to as Cont nuous Emissions Monitoring Systems (CEMS) are the standard for regulated emissions. The combust on process inputs are fuel and air. Therefore, the components of fl ue gas are primarily made up of compounds of oxygen, nitrogen, sulphur, hydrogen and carbon. The components that are of interest because they aff ect effi ciency are primarily oxygen (O) and carbon (C). In boilers using coal or wood as fuel, the components that are of most interest from an environmental perspect ve are nitrous oxides (NOx), carbon dioxide (CO2), carbon monoxide (CO), and sulphur dioxide (SO2). Of en a cont nuous or periodic mercury analysis is also commonly performed in regulated emissions test ng programs. Testing Procedures Flue Gas Analysis is performed by either insert ng an analyzer probe directly into the fl ue of the furnace, boiler, duct etc., or sampling the fl ue gas and pumping it to a sampling chamber or tube within the external analyzer unit. The lat er is more typical of cont nuous monitoring equipment and for very large systems where it is not easy to reach a spot in the fl ue to insert a probe or locate a portable meter. Depending on the gas being measured, most electronic analyzers use infrared sensors or undergo some sort of electrochemical react on. The environments within external analyzer units vary according to analysis and sensor needs. The sampling chamber may need to be heated in order to keep the product to be measured from condensing out, such as NO2, SO2, and HCl. In other cases, the fl ue gases must be cooled and dried to prevent moisture damage to the probe. Instruments of en use a device called a ‘Pelt er Cooler’ which is an electrochemical device that produces a cool surface that condenses any moisture out of the fl ue gas before it reaches the measuring sensor. Probes and sensors do not last forever; some electrochemical sensors are consumed by the measurement process and others will wear out with use as they are exposed to high temperatures and corrosive gases. IR Sensors are therefore becoming more popular for many applicat ons, but they are more expensive, not always as accurate and are aff ected by dirt, fogged lenses and other applicat on issues with where and how they can be used.

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Measurement values are either provided in part per million (ppm) or percentage (%) depending on the size of the reading. Larger numbers, such as oxygen and carbon dioxide are generally provided in percentage and

small numbers, such as NOx and carbon monoxide are provided in ppm. Chemical Based Analyzers The body of the Fyrite analyzer is moulded of a clear, high-strength plast c. It consists of a top and bot om reservoir connected by a centre tube (Fig. 13).The bot om of the lower reservoir is sealed off by a synthet c rubber diaphragm which rests on a perforated metal plate. The upper reservoir is covered by a moulded plast c cap, containing a double-seated plunger valve. A spring holds this valve against a seat in the top cap providing a perfect seal making the instrument spill proof in any posit on. When the valve is fully depressed, it vents the top reservoir to the atmosphere and seals the centre tube beneath it. With the valve part ally depressed, the ent re instrument is open to the atmosphere. The bot om reservoir is fi lled with an absorbing fl uid extending approximately 6 mm into the bore of the centre tube when the instrument is held upright. The scale, mounted to one side of the centre tube, is movable so that, before each test, it may be conveniently adjusted to locate the zero scale division exactly opposite the top of the fl uid column in the centre tube.

Figure 13 Flue Gas Analyzer

(Courtesy of Bacharach, Inc.) Testing Procedure To perform a test with the analyzer, the metal sampling tube at the end of the rubber hose is inserted into the chimney fl ue. Then the connector plug at the other end of the rubber hose is pressed down on the spring-loaded valve at the top of the analyzer, (Fig. 14(a)). This act opens a passage into the top reservoir and seals off the centre bore at the same t me. By squeezing and releasing the rubber bulb, a sample of the gas is pumped into the top reservoir. It takes at least 18 bulb strokes to ensure that the rubber hose and top reservoir are thoroughly purged and fi lled with the fresh sample. On the last stroke, the connector plug is released which automat cally returns the valve to its upper posit on against the top seat. With the valve in this posit on, 60 ml of the sample are locked in the analyzer and the top is opened to the centre bore (Fig. 14(b)).

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Figure 14 Fyrite Testing Procedure

(Courtesy of Bacharach, Inc.) The analyzer is now turned over, forcing the gas sample to bubble through the absorbing solut on. The solut on will absorb all CO2 in the gas sample. The analyzer is then turned back upright.

Absorpt on of CO2 by the fl uid reduces the volume of gas in the analyzer, causing a slight vacuum to develop. The atmospheric pressure acts on the fl exible diaphragm in the lower reservoir to equalize the pressure. This equalizat on forces an amount of liquid up the centre tube equal to the volume of CO2 absorbed by the liquid (Fig. 14(c)). If the scale was zeroed as described, the reading corresponding to the top of the fl uid column is the percentage of the CO2 in the sample tested, which in Figure 14(c) is 12 percent.

Instead of analyzing the fl ue gases for CO2 content to determine the amount of excess air, an analyzer could be used to measure the O2 content in the gases for the same purpose. Since the O2 in the excess air is not needed for combust on, it will pass through the boiler and form part of the fl ue gases. A high percentage of excess air will result in a high percentage of O2 content in the fl ue gas and, inversely, a low percentage of excess air will show a low percentage of O2 in the fl ue gas. The same type of analyzer can be used, however it should contain a fl uid that will absorb O2 instead of CO2. The absorbing fl uids used in the analyzer are good for several hundred readings, but they eventually must be replaced. Be sure that the fl uids you are using are not at the end of their usable life. Another type of fl ue gas analyzer using a chemical absorber is the Orsat analyzer, which works on a principle similar to that used by the Fyrite.

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Excess Air Calculations

With the CO2 or O2 content of the ﬂ ue gas known, the amount of excess air can be found either by calculat on or from tables or graphs. Using tables or graphs is the simplest way.

Table 4 shows the percentages of CO 2 and O2 that will be found by ﬂ ue gas analysis for the various percentages of excess air supplied when burning coal, oil or natural gas. If the type of fuel oil or coal used in your boiler is not represented in Table 4, you should be able to obtain equivalent ﬁ gures for your part cular type of fuel by consult ng with your local supplier or ut lity.

Table 4 CO2 and O2 Percentages in Flue Gas Combustion Percentages of Excess Air Fuel Products 010203040506080100

Coal (Bituminous) CO2 18.6 16.9 15.5 14.3 13.2 12.2 11.3 10.2 9.2

O2 0.0 2.0 3.5 5.0 6.1 7.1 8.0 9.4 10.6

Fuel Oil (Light) CO2 15.4 13.8 12.6 11.5 10.6 10.0 9.3 8.2 7.4

O2 0.0 2.0 3.7 5.2 5.2 6.3 7.3 8.2 9.6

Natural Gas CO2 12.2 10.9 9.9 9.1 8.4 7.9 7.3 6.4 5.7

O2 0.0 2.1 3.9 5.4 6.4 7.5 8.4 9.8 10.9

Example 2: Af er analyzing a sample of ﬂ ue gas from a gas-ﬁ red boiler, the reading on the scale of the analyzer shows a CO2 content of 9.6%. What is the percentage of excess air being supplied to the boiler?

Solut on

Using Table 4, follow the horizontal CO2 line for natural gas and see if 9.6% is shown on this line. If not, the values above and below 9.6 (9.1 and 9.9) must be used to interpolate (i.e. calculate) the approximate value of excess air for a CO2 percentage of 9.6. Read the percentage of excess air on top of the vert cal column for each of 9.1 and 9.9. In this case:

20% when the percentage of CO2 is 9.9% And

30% when the percentage of CO2 is 9.1%.

These readings show that, in this range, the percentage of CO2 in the ﬂ ue gas drops 0.8% when excess air is increased by 10%. This is equivalent to a drop of 0.1% CO2 for each 1.25% increase in excess air.

Since the reading of 9.6% CO2 is 0.3% below 9.9% CO2, the excess air will be: 3 x 1.25% = 3.75% higher, or Excess air = 20% + 3.75% Excess air = 23.75% (Ans.) The results of the analysis should be used as a guide to regulate the air supply so that no more excess air to obtain complete combust on is supplied than is necessary. The ideal amount of excess air varies for diff erent types of boilers and fi ring equipment. Boiler operators are advised to consult the boiler manufacturer’s service manual for their part cular boiler for the recommended amount of excess air to be supplied to the boiler. Keep in mind that a clean-burning fl ame, together with the correct amount of CO2 in the fl ue gas, is essent al for good combust on.

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Combustion Efficiency (Boiler Efficiency) The combust on effi ciency indicates what percentage of the heat produced by the combust on of the fuel is actually absorbed by the water in the boiler. Consequently, it also indicates how much of the heat is disappearing up the stack. A certain amount of heat loss cannot be avoided since the fl ue gases carry some heat up the stack. The boiler operator can limit this heat loss by limit ng the excess air to the amount required for complete combust on and by keeping the heating surfaces as clean as possible (soot blowing) so the maximum amount of heat can be transferred from the fl ue gases to the water before the gases leave the boiler. A boiler operator or service person can determine the effi ciency of a boiler simply through the use of a combust on effi ciency chart (Fig. 15). To use the charts in Figure 15, two things must be known:

1. The percentage of CO2 in the fl ue gases leaving the boiler (found by fl ue gas analysis). 2. The net stack temperature, i.e. the diff erence between the temperature of the fl ue gases leaving the boiler (thermometer in the boiler stack) and the temperature of the air entering the combust on chamber (thermometer in the wind box). By applying this informat on to the chart pertaining to the fuel used, the effi ciency of the boiler can be found.

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Figure 15 Combustion Efﬁ ciency Charts

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Example 3:

The fl ue gas analysis of an oil-fi red boiler shows a 12% CO2 content while the net stack temperature is 275°C (527°F). Find: a) The percentage of excess air supplied b) The combust on effi ciency of the boiler

Solut on

a) Following the CO2 line for fuel oil in Table 4, at 12.6% CO2, excess air is 20%. At 11.5% CO2 excess air is 30%. Thus, CO2 content drops 1.1% for a 10% increase in excess air. Since CO2 content is 12%, 0.6% below 12.6%, the excess air increase will be: 0.6% / 1.1% × 10% = 5.5% above 20% Thus, the percentage of excess air is 25.5% (Ans.)

b) Using the eﬃ ciency chart for oil in Figure 15, follow the CO2 column for 12% to the intersect on with the net stack temperature column and read the eﬃ ciency. Since the 275°C net stack temperature falls between the 500°F and 550°F temperature columns

given on the chart, the reading is taken at the intersect ons of both columns. At 12% CO2 content and a 500°F net stack temperature, the effi ciency is 81.5%. At 12% CO2 content and a net stack temperature of 550°F, the effi ciency is 79.5%. Since the temperature is 275°C, almost exactly halfway between 500°F and 550°F, the corresponding effi ciency value of 80.5% (halfway between 79.5% and 81.5%) can be used and will be very close to the exact value obtained using calculat ons.

With a 12% CO2 content and a net stack temperature of 275°C, the boiler eﬃ ciency will be approximately 80.5% (Ans.)

The eﬃ ciency charts show us that:

a) an increase in excess air, result ng in a drop in percent CO2 content in the fl ue gases, lowers the effi ciency of the boiler and, vice-versa, a decrease in excess air results in higher boiler effi ciency. b) a rise in net stack temperature at a specifi c fi ring rate results in a drop in boiler effi ciency and a drop in net stack temperature will give higher boiler effi ciency. A rise in net stack temperature can be caused by the heat ng surfaces fouling due to scale forming on the waterside or soot and ash deposits on the fi reside. It can also be caused by an increase in excess air. Because the extra air increases the total volume of fl ue gases that have to pass through the boiler during a specifi c t me, the gases must travel faster through the boiler. This process results in a lower t me of contact between gas and heat ng surfaces, less heat transferred to the water and, hence, lower effi ciency.

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CHAPTER 57 - QUESTIONS INTRODUCTION TO BOILER COMBUSTION

1. The incomplete combust on of carbon results in the format on of a) carbon tetrachloride. b) carbon monoxide. c) carbon dioxide. d) carbon trioxide.

2. The amount of air required to supply just enough oxygen for complete combust on is called a) excess air. b) theoret cal air. c) tert ary air. d) primary air.

3. The approximate amount of excess air for burning fuel oil is______%. a) 50 b) 5 c) 25 d) 15

4. Carbon monoxide is both explosive and a) inert. b) expensive. c) non-compressible. d) toxic.

5. Which one of the following is not an advantage that oil has over coal? a) less storage space is required b) more handling equipment and labour is required c) the amount fed to the furnace is more easily controlled d) it burns more cleanly and is eﬃ cient to use

6. When using a mechanical fuel oil atomizer a) 70 kPa steam pressure is required at the burner. b) 35 to 40 kPa oil pressure is required. c) high pump pressure is required. d) dry saturated steam or superheated steam should be used.

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CHAPTER 57 - ANSWERS INTRODUCTION TO BOILER COMBUSTION

1. (b)

2. (b)

3. (d)

4. (d)

5. (b)

6. (c)

Fourth Class • Part A2 4th Class • Part A2 C HAPTER 58

Fluidized Bed Combustion

LEARNING OUTCOME

When you complete this chapter you should be able to: Discuss the basic theory and design of a ﬂ uidized bed steam generator and describe the special operational and control aspects of ﬂ uidized bed combustion.

LEARNING OBJECTIVES

Here is what you should be able to do when you complete each objective:

1. Defi ne and discuss the history and benefi ts of “fl uidized bed combustion.”

2. Explain the types and operation of ﬂ uidized bed combustion units.

3. Discuss the advantages and disadvantages of ﬂ uidized bed combustion.

4. Discuss two start-up strategies and explain bed expansion.

301 Unit 13 • Chapter 58 • Fluidized Bed Combustion 302

INTRODUCTION

This chapter will serve as a brief introduct on to the process known as “ﬂ uidized bed combust on” or FBC. This is a relat vely new technology in the steam industry, having been very thoroughly researched over the past few decades. Several large systems are now using FBC and it shows every sign of becoming a convent onal technology for a number of furnace and combust on applicat ons in the power and petrochemical industries.

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OBJECTIVE 1

Defi ne and discuss the history and benefi ts of “fl uidized bed combustion.”

FLUIDIZED BED COMBUSTION

Fluidized bed combust on (FBC) covers a range of systems. There is no unique system. In a ut lity steam generator, it is a method of burning crushed coal in a bed of limestone part cles and ash. The combust on air blows up from the bot om, thus keeping the bed in “fl uidized” mot on. The force of the air velocity on the part cles must be suffi cient to counteract gravitat onal forces, but not so great as to transport the ent re bed of part cles along with the air stream. The bed materials are kept suspended (fl uidized) by the same air used for combust on, so the int mate mixing of the burning coal and heated limestone ensures that combinat ons of sulphur, lime and oxygen become relat vely inert compounds such as calcium sulphate. Figure 1 shows a typical FBC unit. The coal and limestone are fed to the furnace and suspended in an updraf of fl uidizing air from the FD fan. Not ce that watertubes are submerged in the limestone-coal bed. The fl ue gases, exit ng the furnace, are passed through the dust collector (cyclone) and any carryover of limestone part cles is returned to the bed.

Figure 1 Fluidized Bed Furnace

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Benefits of FBC FBC has the potent al to signiﬁ cantly increase power generat on eﬃ ciency while, at the same t me, meet ng the stringent sulphur oxide (SOX) and nitrogen oxide (NOX) emission regulat ons that apply in an increasing number of countries. In the past, coal had always meant pollut on. The sulphur and nitrogen emissions of coal-burning power and heat ng plants, such as SO2 and NO2, dissolved in the water vapour of the atmosphere and came back down as toxic part cles and corrosive acid rain.

Convent onal coal-fi red power stat ons with NOX reduct on, dust separat on and desulphurizat on equipment are complex and expensive. One study est mated that this added cost would increase to 47% of the total capital outlay. At a minimum, the vast majority of power stat ons in the world are equipped with electrostat c precipitators or fabric fi lters for eff ect ve removal of fi ne dust. Flue gas desulphurizat on (FGD) plants deal with sulphur products in the fl ue gas, but they are also expensive to build, troublesome to operate and maintain, and can entail problems with disposal of wastewater and sludge products.

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OBJECTIVE 2

Explain the types and operation of ﬂ uidized bed combustion units.

TYPES OF FLUIDIZED BED COMBUSTION UNITS

There are two types of Fluidized Bed Combust on Units: • Atmospheric ﬂ uidized bed combust on (AFBC) • Pressurized ﬂ uidized bed combust on (PFBC) Atmospheric Fluidized Bed Combustion The AFBC technology has at ained commercial applicat on with large industrial units that produce up to 500 000 kg/h of steam. It is expected that soon the increase in unit size will enable capacit es of 1 000 000 kg/h. Within AFBC types are two major subgroups known as bubbling bed and circulat ng bed. Each of these has various classes.

With the AFBC, the combust on air pressure is typically 25 cm of H2O pressure at the FD fan, 16 cm at the base of the act ve bed and atmospheric pressure at the top of the combust on mass. In this design, the “bubbling” mass is maintained at an approximate depth of 1 metre. Part cles carried over, especially when fuel is added above the bed, are captured with cyclone separators and reinjected into the bed to further improve combust on effi ciency and sulphur reduct on. In AFBC, the fuel content by mass of the turbulent inert material is usually less than two percent. Velocit es through the bed of the fl uidizing air are in the range of 2–3 m/s which makes for more even temperature distribut on, thus enhancing high thermal effi ciency. The watertube heat ng surfaces can be submerged directly into the swirling mass to opt mize heat transfer. One boiler was originally a spreader stoker watertube type that was converted to an FBC design. The steam generat ng system remained as original, but with addit onal steam generat ng tubes immersed in the bed. These immersed tubes now produce about 60% of the total boiler output, with the remaining 40% being produced by the original convect on superheater and economizer sect ons. The combust on can be controlled at a furnace temperature of 850°C instead of 1600°C, because the heat transfer coeffi cient is so high. Since nitrogen oxides are produced at high furnace temperatures, this factor virtually eliminates them from the stack gases, thus reducing a major source of acid rain and haze. A further advantage is that the crushed or pulverized limestone is easily mixed with the crushed coal to act as a desulphuring agent. At 850°C, the limestone is converted to calcium oxide and combines with the sulphur dioxide that is released from coal to form calcium sulphate (gypsum). This gypsum becomes part of the residual ash and can be ut lized as an aggregate for building materials, so that even the furnace waste can be put to good use.

In theory, SO2 emissions could be nearly totally eliminated by this process, but in actual fact this outcome depends on the following: • How much limestone is added • The depth of the bed • Time in the limestone ﬂ ux

For pract cal purposes, about 10% limestone reduces SO2 emissions by 80%. For a coal containing 5% sulphur, close to 1/2 tonne of limestone would be fed for each tonne of coal; this process is st ll more economical than building and operat ng a Flue Gas Desulphurizat on system. The results of one test indicate that by adding three t mes the theoret cal quant ty of lime required for the react ons, it is possible to reduce emissions of sulphur oxides by 80–90%.

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Pressurized Fluidized Bed Combustion The eﬃ ciency of a combined cycle plant based on PFBC is potent ally about 5% higher than a convent onal pulverized coal plant. One type of combined gas/steam cycle uses a PFBC boiler. About 1/3 of the electrical power is provided by the gas turbine and 2/3 by the steam turbine(s). Figure 2 shows a pressurized FBC of this type which has the advantages of FBC and combined steam-gas turbine cycle operat ons. The coal and limestone (dolomite) are treated and fed to the furnace. The combus- t on (pressurizing) air is delivered by the gas turbine compressor. The hot pressurized combust on gases, af er transferring most of their heat to the boiler, are then passed through cyclones and a granulator bed ﬁ lter to drive the gas turbine and its generator. The steam produced in the boiler goes to the steam turbine and condenser in a convent onal power plant cycle.

Figure 2 Boiler Combined Cycle

The potent al advantages of this type of combined cycle include: • Larger total electrical power output compared to a gas turbine alone • Increase in overall effi ciency of power generat on Improved fl uidizat on quality for a PFBC boiler occurs because the higher pressure brings about a reduct on in bubble size that is especially important for beds composed of fi ne part cles. With this design, the hot gases must be expanded through a turbine, so the boiler effi ciency can’t be evaluated by itself. Performance measurement includes the gas turbine operat on. Another approach for a PFBC design uses compressed air as the cooling medium for the combust on chamber. The air is then mixed with the pressurized products of combust on (af er they have been cleaned). The mix then expands through the turbine; any lef over heat energy passes to a waste heat boiler. In this cycle, about 60% of the power comes from the gas turbine and the remainder from the steam turbine. Compared with AFBC systems, the PFBC furnace has the following drawbacks that must be weighed against its advantages when choosing between the two for a specifi c applicat on: • Complexity • Erosion and/or corrosion/fouling of gas turbine blades • Low furnace temperatures limit the steam temperature over the range of heat supply • Feeding of fuel and bed materials into the pressurized system

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OBJECTIVE 3

Discuss the advantages and disadvantages of ﬂ uidized bed combustion.

ADVANTAGES OF FLUIDIZED BED COMBUSTION

The advantages of fl uidized bed combust on include: • Flexibility of fuel choices • Less maintenance • Smaller plant size • Less fuel preparat on • Simplifi ed fuel feed • Pressurized combust on chamber Flexibility of Fuel Choices A wide variety of fuels can be burned using just about anything that has suffi cient heat ng value to sustain steady combust on. FBC units are extremely tolerant of variat ons in fuel characterist cs, so the following fuels can be used: • Low grade fuels with high moisture • High ash content of up to 70% (which will not burn successfully in a convent onal furnace) Wood• Wood• • Heavy oil tar sands • Coal mine tailings • Waste gas refuse • Shredded scrap t res Less Maintenance Slag is eliminated due to the lower operat ng temperature in the furnace, which eliminates the format on of large “clinkers”. Also the dry combust on residue is easier to dispose of than the wet sludge produced by normal furnaces as well, soot-blowers are not required, thus eliminat ng the soot blowing equipment along with its associated maintenance and power requirements. Smaller Plant Size The plant is smaller due to the high heat transfer rate in the furnace and the absence of wet exhaust gas scrubbing equipment. This means the steam generator can be reduced by up to 25% in overall size, a signifi - cant savings in material and construct on costs. Less Fuel Preparation The system, as compared to pulverized coal fi ring, is simplifi ed because there is no need for coal pulverizers.

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Simplified Fuel Feed Coal may be added by a spreader stoker which throws the fuel over the bed in a predetermined pat ern, using either overbed nozzles or feed through tubes located in the furnace bot om. Pressurized Combustion Chamber A pressurized combust on chamber makes cost savings possible because of more compact equipment and the opportunity to use a variety of more eﬃ cient energy cycles.

DISADVANTAGES OF FLUIDIZED BED COMBUSTION

Disadvantages of fl uidized bed combust on include: • High power requirements for combust on air • Carryover • Poor Combust on Control at Low Operat ng Rates High Power Requirements for Combustion Air For a given boiler output, an FD fan on an FBC unit may require as much as three t mes the power of a stoker fi red system. Carryover Gradual loss of the fl uidized bed, including the fuel (carbon) part cles, results in lower effi ciency and higher cost dust collectors. Poor Combustion Control at Low Operating Rates This cannot be avoided without adding expensive control equipment.

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OBJECTIVE 4

Discuss two start-up strategies and explain bed expansion.

OPERATION

Before coal can be burned in an FBC, it is necessary to heat the inert bed material to about 600°C (the ignit on point of coal) using an auxiliary heat ng system. Two common methods of heat ng include: 1. Combust on of auxiliary fuel as a fl ame above the bed fl uidized with air. 2. Passing hot gas through the bed. A typical t me to heat the bed to fuel-ignit on temperature is one hour. One AFBC plant operat ng at a steam pressure of 17 000 kPa and 540°C has a windbox divided into nine independently controlled compartments to allow separate fl uidizat on of the bed for startup and for load reduct on with either underbed or overbed fi ring.

BED EXPANSION

Due to the specialized fuel handling system, furnace heat transfer rates, ash removal and ﬂ ue gas treat ng equipment of an FBC unit compared to a convent onal steam generator, its control systems are quite diﬀ erent. For example, the operator must be aware of bed expansion. This is one of the variables that can be successfully manipulated. The expanded bed height is normally at such a level that any boiler furnace tubes are covered to get maximum heat exchange. During operat on, the working bed height can be adjusted through changes in limestone feed rate and bed drain rate.

CONCLUSION

Simple FBC has been available for decades, but the rising price of premium fuel has generated increased interest in low grade fuels such as high ash/sulphur coal, sawdust, garbage and other industrial wastes. The ability of FBC to handle this wide variety of fuels will ensure its increased use in the future. Full sized commercial and electric ut lity plants are now being built based on pilot plant research. Although it is a relat vely new technology in the generat on of large quant t es of energy, FBC with its many variat ons seems ﬁ rmly entrenched. It is probably the most important advance in furnace design since the development of combined cycle gas turbine-boiler systems.

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CHAPTER 58 - QUESTIONS FLUIDIZED BED COMBUSTION

1. What is added to the coal in a typical FBC unit to ensure that the heated compounds become relat vely inert? a) coke b) limestone c) sulphur d) calcium sulphate

2. NOX emissions are reduced in a ﬂ uidized bed furnace due to the a) removal of nitrogen compounds. b) reduced combust on temperatures. c) levels of excess air. d) increased combust on temperatures.

3. In atmospheric ﬂ uidized bed combust on units, the fuel content by mass of the turbulent inert material is usually less than ______%. a) 10 b) 15 c) 25 d) 2

4. The combust on in a ﬂ uidized bed combust on unit can be controlled at a furnace temperature of

______°C to reduce NOx emissions. a) 500 b) 1000 c) 850 d) 1250

5. Before coal can be burned in a ﬂ uidized bed combust on unit, it is necessary to heat the inert bed material to about ______°C. a) 1000 b) 600 c) 1500 d) 2000

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6. A typical t me to heat the bed in a ﬂ uidized bed combust on unit to fuel-ignit on temperature is ______hour(s). a) 4 b) 5 c) 3 d) 1

7. One major disadvantage of FBC is a) burning of coal. b) plant size is greatly reduced. c) pressurized combust on chamber. d) poor combust on control at low operat ng rates. e) will not work with sulphur in the coal.

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CHAPTER 58 - ANSWERS FLUIDIZED BED COMBUSTION

1. (b)

2. (b)

3. (d)

4. (c)

5. (b)

6. (d)

7. (d)

Fourth Class • Part A2 4th Class • Part A2 C HAPTER 59

Safety & Relief Valves

LEARNING OUTCOME

When you complete this chapter you should be able to: Discuss the design and operation of safety valves for power and heating boilers.

LEARNING OBJECTIVES

Here is what you should be able to do when you complete each objective:

1. Describe the ASME code requirements and the construction and operation of high pressure safety valves.

2. Describe the ASME code requirements and the construction and operation of low pressure heating boiler safety and safety relief valves.

3. Describe the testing and repair of safety valves.

4. Describe the construction and operation of a temperature relief device.

313 Unit 13 • Chapter 59 • Safety & Relief Valves 314

INTRODUCTION

Each boiler is designed to operate below a speciﬁ c maximum pressure. The basic funct on of safety valves is to protect boilers against overpressure. Certain condit ons, such as sudden loss of load or failure of automat c controls, can cause the boiler pressure to rapidly exceed the operat ng pressure. To prevent burst ng of the boiler drum or other pressure parts due to this excessive pressure, at least one pressure operated safety valve must be installed on each boiler. When the pressure in the boiler approaches its maximum allowable value, the safety valve will open and release steam to the atmosphere thus prevent ng any further increase in pressure.

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OBJECTIVE 1

Describe the ASME code requirements and the construction and operation of high pressure safety valves.

ASME CODE REQUIREMENTS

The ASME Code states that each steam boiler shall have at least one safety valve; if the boiler has over 47 m2 (500 f 2) of water heat ng surface, then two or more safety valves shall be installed. All boilers must be fi t ed with an approved type of pop safety valve of suffi cient capacity to discharge all the steam that the boiler can evaporate, without permit ng the pressure to rise more than 6% above the maximum allowable working pressure (MAWP.) (See ASME Sect on I). This chapter contains extensive references to ASME Sect on I, ASME Sect on IV and ASME Sect on VII. The specifi ed sect ons of these codes should be referenced while reading this chapter. The regulat ons governing safety valves are covered in detail in the ASME Code Sect on I.

CONSTRUCTION

A safety valve is held shut by means of a spring which holds the safety valve disc t ghtly against its seat. When the boiler pressure reaches the point at which the valve is set (popping pressure), the disc will be raised slightly from its seat and steam will begin to escape. Figure 1 shows a safety valve of an approved design which has a cast steel body and ﬂ ange connect ons.

Figure 1 Cast Steel Safety Valve

The safety valve shown in Figure 1 has an exposed spring to prevent the high temperature of the steam from destroying the mechanical propert es of the spring. The superheater and reheater safety valves are set to pop open before and to close af er the drum safety valves. In this way overheat ng of the superheater and reheater is avoided, as there will be a folow of steam maintained through the tubes unt l the ﬁ res can be shut down or the pressure reduced. ASME Code Sect on I states the rules governing superheater and reheater safety valves.

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Figure 2 illustrates the construct on of a safety valve. Referring to Figure 2, the safety valve is at ached to the drum at the top of the steam space. The valve disc “D” is held ﬁ rmly on its seat by the pressure of the heavy coil spring “J”. The point at which the valve will lif and relieve the pressure is adjusted by screwing the nut “L” up or down, thereby decreasing or increasing the compression of the spring “J”. The nut “L” is prevented from shif ing af er adjustment by the lock nut “N”. When the valve has been set by adjust ng nut “L” the cap “B” is put in place and the Inspector at aches his seal to a wire passing through the hole “O” thus prevent ng access to the adjust ng nut “L”. The valve can be manually lif ed by the lever which raises the valve spindle connected to the valve disc provided that the valve is subjected to a pressure of at least 75% of boiler operat ng pressure. This ensures that the valve cannot be opened with the try lever if there is no pressure on the boiler. If the safety valve is opened and there is no pressure in the boiler, any scale or debris in the safety valve discharge line could fall into the valve, prevent ng it from closing. (ASME Code Sect on I). In order to ensure consistent operat on, t ghtness and proper seat ng of the disc, the valve disc is equipped with guides at the bot om and top to allow the valve disc to move only in a vert cal direct on, with no horizontal shif ing.

Figure 2 Safety Valve Construction

OPERATION OF SAFETY VALVES

The pop valve is provided with a lip or skirt (E, shown in Fig. 3), which forms a popping chamber that becomes ﬁ lled with steam when the valve starts to open, thereby increasing the eﬀ ect ve area of the disc. As soon as the valve lif s, the pressure of the steam acts on this increased area of the disc, result ng in a greater force applied against the spring, which causes the valve to pop wide open. Once open, the valve will remain so unt l the pressure drops below the popping pressure.

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Figure 3 Pop Type Safety Valve

Figure 4 indicates that, as the valve begins to lif , steam rushes into the pop chamber, acts on an increased area as indicated by the shaded ring and causes the valve to suddenly lif , or “pop”, to its full opening. The lif ing force exerted on the disc by the boiler pressure is dependent on the area of the disc exposed to the pressure and on the freedom with which the steam can escape from under the skirt, or lip. Referring to Figures 3 and 5, the “huddling chamber”, or “pop chamber”, is provided with adjustable outlet ports (F) to allow steam to escape from under the skirt. If the huddling chamber outlet is closed, the pressure under the lip will be greater and the boiler pressure must drop quite low before the spring can close the valve. If the huddling or pop chamber outlet is wide open, the pressure in it, and therefore under the skirt, will be small and the valve will close with very lit le drop of boiler pressure. It was previously explained how the eﬀ ect of the skirt or lip (E) will cause the valve to open wide very soon af er start ng to open. When closing, this act on is reversed. When the boiler pressure drops suﬃ ciently to allow the spring to begin closing the valve, the pressure under the skirt drops and allows the spring to close t

Figure 4 Construction and Operation of a Pop Valve

4th Class • Part A2 Unit 13 • Chapter 59 • Safety & Relief Valves 318

REGULATING THE BLOWDOWN

The diﬀ erence between the pressure at which the valve opens and closes is called the blowdown of the safety valve. The blowdown, according to ASME Sect on I, must be a minimum of 14 kPa, and the maximum blowdown will allow the safety valve to close at a pressure not lower than 96% of the set pressure of the safety valve. A threaded adjustable angular ring (G) may be screwed up or down to vary the amount of port opening (F), as shown in Figure 3. If the angular ring is screwed up toward the port holes, the blowdown will be longer. The raising of the adjust- ing ring decreases the area of the escape ports, direct ng more steam against the lip. The result ng increase in the lif ing force act ng on the disc causes the valve to stay open longer, and close at a lower steam pressure. Conversely, if the ring is screwed downwards, the blowdown will be shorter because the ring increases the area of the ports, causing less steam to contact the lip and closing the valve sooner. The adjustment of the blowdown ring can be made by removing the cap screw, insert ng a screwdriver that will catch in notches or ribs on the outside of the adjust ng ring, and turning the adjust ng ring in the desired direct on. When the adjust ng ring is set in the desired posit on, it is locked in place by the set screw (H). To prevent unauthorized persons from tampering with this adjustment, a cap screw is installed and held in place by a seal installed by the Boiler Inspector to prevent any further adjustments. The cap screw is visible in Figure 5. Refer to ASME Sect on I.

Figure 5 Disc and Seat Details of Pop Safety Valve

ADJUSTMENT OF POPPING PRESSURE

The safety valve springs are designed for a certain opening pressure, or “popping” pressure, which may be increased or decreased by fi ve percent (5%) by changing the compression of the spring, as per ASME Code Sect on I. Before adjust ng the pressure at which the safety valve opens, the lock nut, on the adjust ng nut screwed on the spindle above the spring, must be backed off . To increase the blowoff pressure, screw the adjust ng nut down, thus compressing the spring. To decrease the blowoff pressure, screw the adjust ng nut upwards, releasing some of the pressure on the spring. Note: Adjustment of the safety valve set ng may only be done with the permission of an authorized Boiler Inspector. The regulat ons governing safety valve design, material select on, capacity, test ng, adjustments and sealing are covered in the ASME Code Sect on I. Figure 6 shows a cast-iron safety valve suitable for boilers operat ng at pressures up to 1720 kPa and steam temperatures up to 232°C. Cast-iron valves are not used on superheaters.

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Figure 6 Pop Safety Valve with Adjustable Popping Pressure

SAFETY VALVE DISCHARGE PIPING

The safety valves on the drum, superheater, reheater inlet and outlet headers should be placed in an upright posit on with the spindle vert cal. They must be connected to the boiler independent of any other steam connect on and located as close as possible to the boiler without any unnecessary intervening pipe or ﬁ t ng, as stated in ASME Code Sect on I. It should be provided for safety valve outlets as stated in ASME Code Sect on I. The discharge pipe should be separately supported, leaving ample room for expansion so that no force is exerted on the valve.

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Figure 7 shows the discharge piping arrangement and dimensions for a typical safety valve per ASME Code Sect on I.

Figure 7 Discharge Piping Details

The superheater safety valve shown in Figure 1 has an exposed spring to prevent the high temperature of the superheated steam from destroying the mechanical propert es of the spring. The superheater and reheater safety valves are set to pop open before and to close af er the drum safety valves. In this way overheat ng of the superheater and reheater is avoided, as there will be a fl ow of steam maintained through the tubes unt l the fi res can be shut down or the pressure reduced. ASME Code Sect on I states the rules governing superheater and reheater safety valves. Recommended Rules The ASME Code Sect on VII provides recommendat ons for the test ng, operat on and maintenance of safety valves. Torsion Bar Safety Valve The torsion bar safety valve, shown in Figure 8, is designed for use on boilers operat ng at 8000 kPa or higher. Its operat on is similar to the convent onal types of safety valves except that torsion bars are used to hold the valve closed instead of a heavy spring. Its main advantage is that the torsion bars can be machined to much fi ner specifi cat ons than can a coil type spring.

Figure 8 Torsion Bar Safety Valve

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OBJECTIVE 2

Describe the ASME code requirements, and the construction and operation of low pressure heating boiler safety and safety relief valves.

HEATING BOILER SAFETY SAFETY RELIEF

ASME Code Requirements The ASME Code states that each steam heat ng boiler shall have one or more offi cially rated safety valves of the spring pop type, adjusted and sealed to discharge at a pressure not to exceed 103 kPa. The safety valve must not be smaller than 13 mm or larger than 110 mm seat diameter. The safety valve capacity shall be such that, with the fuel burning equipment installed and operated at maximum capacity, the pressure cannot rise more than 34 kPa above the maximum allowable working pressure when all steam outlets are closed. The safety valve shall be equipped with a body drain connect on below the seat level which shall not be plugged af er installat on of the boiler. This will prevent the collect on of condensate around valve and seat which could result in st cking of the valve due to corrosion. Mounting the Safety Valve ASME Code, Sect on IV, sets the following requirements for mount ng of safety valves on heat ng boilers: 1. The safety valve shall be installed in a vert cal posit on and located in the highest pract cable part of the boiler proper. 2. It shall be connected directly to a tapped or fl anged opening in the boiler, to a fi t ng connected to the boiler by a short nipple, to a Y base, or to a valveless header connect ng steam or water outlets on the same boiler. The opening or connect on between boiler and safety valve shall have at least the same area as the valve inlet and no shut off valve may be installed between safety valve and boiler. 3. The safety valve shall not be connected to an internal pipe in the boiler. 4. The discharge pipe of the safety valve shall be as short and straight as possible to avoid undue stress on the valve. Its internal cross-sect onal area shall not be less than the full area of the valve outlet. No shut-off valve shall be placed in the discharge. The discharge piping shall be properly drained to prevent collect on of water. The discharge shall be so arranged that there will be no danger of scalding the operator. Should a long vert cal discharge pipe be required, then it may be necessary to install a fl exible joint near the safety valve so that the expansion of the pipe will not place any stresses on the safety valve.

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OBJECTIVE 3

Describe the testing and repair of safety valves.

TESTING THE SAFETY VALVE

Safety valves should be kept in good working condit on at all t mes. To assure their proper operat on, the operator can employ one of the following test ng methods: • Try lever • Pop Try Lever Test Pull the try lever on the safety valve to the wide open posit on and allow the steam to escape for 5 to 10 seconds. Release the lever, allowing the spring to snap the disc to the closed posit on. This test determines whether or not the valve is free to operate. However, it does not determine whether or not the valve will open at its set pressure. On low pressure boilers, this test should not be used unless the pressure is up to at least 35 kPa, to ensure that any loose deposits or foreign material will be blown away when the valve opens and will not lodge between the valve and seat. Should the valve simmer af er the test, operate the try lever two or three t mes to allow the disc to seat properly. If the valve cont nues to simmer, it must be replaced or repaired. The ASME Code recommends that this test be performed on a monthly basis and then be recorded in the boiler log. Pop Test By raising the steam pressure in the boiler to the value at which the safety valve is set to open, the operat on of the safety valve and the exact pressure at which it opens can be checked. This procedure is called the “pop test”. Prior to performing this test, the steam discharge and feedwater supply valves should be closed and the accuracy of the pressure gauge should be checked. On automat cally fi red boilers, it will also be necessary to bypass the operat ng control as well as the high limit control, which normally shuts the boiler down well before the popping pressure is reached. The controls are bypassed by placing jumper wires on their electrical terminals. Operators not familiar with or unsure of this procedure are strongly advised to call in a qualifi ed tradesman to assist them. When the boiler is set up for the test, start the burner and raise the steam pressure. The safety valve should open at 103 kPa, but a variat on of plus or minus 14 kPa is acceptable. Just before the valve pops open, a simmering act on will be not ced. When the safety valve opens, shut off the burner. Observe the pressure gauge to note the pressure at which the valve opens and the pressure at which it closes again. Record these pressures in your boiler log book. Should the valve fail to open at 117 kPa, shut the burner off and release the pressure slowly either to atmosphere through the vent valve or into the steam header. When the pressure has fallen below the popping pressure, apply the try lever test a few t mes to ensure that the valve is free to move, then repeat the pop test. If the valve st ll fails to open, it must be repaired or replaced. Note: Never try to free a stuck safety valve by hammering or striking the valve body. It is recommended that the pop test be conducted at least once a year, preferably at the beginning of the heat ng season even if the boiler is used only for space heat ng purposes.

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REPAIR OF A SAFETY VALVE

The ASME Code recommends that the repair of safety and safety relief valves be done by the manufacturer or its authorized repair representat ve. In Canada, provincial regulat ons allow a boiler operator to repair safety valves, but only if fully qualiﬁ ed to do so and authorizat on in writ ng from the Chief Inspector has been obtained. Low pressure steam heat ng boilers are designed to operate at a pressure not higher than 103 kPa. A safety valve especially designed for a low pressure steam heat ng boiler is shown in Figure 9. The valve housing consists of two main parts: the valve body, directly connected to the boiler, and the bonnet, threaded and locked onto the valve body. The bonnet has an outlet opening to atmosphere. A valve disc closes the opening in the upper part of the valve body, and is t ghtly held down upon its seat by a heavy spring. The adjust ng cap in the upper part of the bonnet compresses the spring, and is held in posit on by a locking screw.

Figure 9 Cross-Section of Safety Valve

Safety Relief Valve ASME Sect on IV states that each hot water heat ng boiler shall have at least one offi cially rated pressure relief valve set to relieve at or below the maximum allowable working pressure of the boiler. The valve shall have pop act on when tested by steam. The design of the safety relief valve is basically the same as that of the pop safety valve used for steam boilers, except that it is not fi t ed with a blowdown adjustment ring and bot om guides are not permit ed. The relief valve opens part ally when boiler pressure exceeds the valve set ng slightly; however, the slight fl ow of the escaping water does not have the same lif ing eff ect on the valve lip as steam has, so there is no immediate popping act on. On further pressure rise the valve pops wide open. The pressure relief valve may be installed anywhere in the hot water system reasonably close to the heater, or tank. Frequently, the relief valve is installed in the cold water supply line or in the hot water discharge line. There must not be any valve (shut-off , check or any other type) installed between the relief valve and the heater. The drain, or drip line, from the relief valve should be piped to some point over a fi xture or fl oor drain and kept above the top rim of said fi xture. It must never be connected directly to any drain or vent pipe.

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Frequent opening or spilling of the relief valve may be due to the following causes: 1. Scale has accumulated on the valve seat prevent ng it from closing t ghtly. If the water is hard, then scale will form if the temperature of the heater is carried above 65°C. 2. Pressure in the supply line varies and at t mes exceeds the set ng of the valve. 3. The relief valve is defect ve or designed for the wrong pressure range. 4. The hot water system has become water logged. During operat on a hot water heat ng system is completely fi lled with water except for the expansion tank which holds a pad of air (or nitrogen) above the surface of the water. When the burners come on, the water expands and the air in the expansion tank is compressed slightly. If this pad of air is lost and the expansion tank becomes fi lled with water, whenever the burner comes on, the pressure will increase dramat cally, since water is not compressible. This will cause water to weep out the relief valve. Then, when the burner shuts off and the water cools, makeup water will be added keeping the system water logged. If the relief valve does open frequently, the cause must be found and remedied. Never, under any condit on, at empt to plug the outlet of the relief valve. If the system is water logged, the expansion tank should be isolated from the boiler and drained. Once fi lled with air, the tank is closed up and the valve to the boiler is opened. The water can enter the tank unt l the air in the tank is compressed to operat ng pressure and an air pad is re-established.

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OBJECTIVE 4

Describe the construction and operation of a temperature relief device.

TEMPERATURE RELIEF DEVICE

The temperature relief device protects a domest c hot water system from dangerously high water temperatures. It accomplishes its purpose by opening when the water temperature rises to about 99°C which allows hot water to escape from the system. Cold water from the supply then enters and reduces the temperature. Various types of temperature relief devices are used. One type employs a fusible plug which melts at 99°C and allows the hot water to escape. The water cont nues to ﬂ ow unt l the device is replaced. Another type uses the expansion and contract on of a rod and tube arrangement to open the device at 99°C and close it at 71°C. Figure 10 shows a combinat on pressure and temperature relief valve which will open due to either high pressure or high temperature. With this type, if the pressure in the system rises, it will open the valve at the preset spring pressure. In addit on, if the pressure remains normal but the water temperature rises, the wax ﬁ lled sensor probe will expand and open the valve by means of the piston at approximately 99°C.

Figure 10 Combination Pressure-Temperature Relief Valve

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CHAPTER 59 - QUESTIONS SAFETY RELIEF VALVES

1. The ASME Code states that if a steam boiler has over ______m2 of water heat ng surface, two or more safety valves shall be installed. a) 52.5 b) 47 c) 75.6 d) 65.4

2. If a safety valve opens, the boiler pressure will a) rise 5% above the maximum allowable working pressure (MAWP). b) not rise more than 15% above the MAWP. c) remain at the MAWP. d) not rise more than 6% above the MAWP.

3. The purpose of the spring in a safety valve is to a) help open the test ng lever. b) pull open the valve at blowing pressure. c) adjust the blowdown of the valve. d) hold down the disc on the seat.

4. Each safety valve on a steam heat ng boiler is set to discharge at a pressure not exceeding a) 103 kPa. b) 150 kPa. c) 75 kPa. d) 250 kPa.

5. The safety valves on a boiler meet code requirements and are set to a popping pressure of 1380kPa. What would be the maximum permissible pressure rise in the boiler, if the boiler was ﬁ red at full capacity with the main steam stop valve closed? a) 69 kPa b) 138 kPa c) 82.8 kPa d) 41.4 kPa

6. When performing a safety valve pop test, one of the ﬁ rst things to do is a) not fy the boiler inspector. b) verify the accuracy of the applicable steam pressure gauge. c) not fy the chief engineer. d) check the blowdown of the safety valve.

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CHAPTER 59 - ANSWERS SAFETY RELIEF VALVES

1. (b)

2. (d)

3. (d)

4. (a)

5. (c)

6. (b)

Fourth Class • Part A2 4th Class • Part A2 C HAPTER 60

Water Columns & Gauge Glasses

LEARNING OUTCOME

When you complete this chapter you should be able to: Describe different types of direct and inferential level gauges or indicators.

LEARNING OBJECTIVES

Here is what you should be able to do when you complete each objective:

1. Describe direct type water level indicators. 2. Describe indirect type water level indicators.

329 Unit 13 • Chapter 60 • Water Columns & Gauge Glasses 330

INTRODUCTION

The methods used to sense and display liquid levels in open and closed vessels are quite varied. No single method is universally suitable for the ent re range of applicat ons. Solids, liquids, corrosive and explosive condit ons, extremes of heat and cold and variat on in pressure are some of the condit ons encountered in level measurement and indicat on. Liquid level may be measured and indicated directly or indirectly. Common examples of direct level measurement and indicat on include the: • Dipst ck • Gauge glass • Float gauge

Some indirect level measurement and indicat on may use a pressure sensing device to measure the pressure exerted by the depth of the liquid. The dial (or scale) of the gauge is calibrated to indicate level. Other methods may use electrical sensing devices. Examples of the indirect method include: • Bubbler systems • Diﬀ erent al pressure level meters • Electrical capacitance level gauges • Ultrasonic level gauges

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OBJECTIVE 1

Describe direct type water level indicators.

DIRECT METHODS

One of the simplest techniques for indicat ng liquid level is by means of a transparent chamber through which the level can be seen visually. A common method of providing such visual indicat on is by installing transparent ports in the vessel itself. These are placed at diff erent locat ons to indicate the liquid height. More of en, separately mounted gauge glasses are used to provide a cont nuous indicat on of level over a certain vert cal distance on the vessel. Various types of gauge glasses used to directly indicate the liquid level include: Tubular• Tubular• • Armored-type (fl at) Bicolour• Bicolour• Tubular Gauge Glass The simplest and least costly method of liquid level indicat on is the tubular gauge glass. Two slightly diff erent designs are illustrated in Figures 1 and 2. Both are simply transparent vert cal tubes with their lowest visible point connected to the tank or boiler at the lowest level of interest. The top of the glass may be open to the atmosphere if the tank is open or to the unfi lled part of a closed vessel above or at the highest level permit ed. Isolat ng valves are placed above and below the gauge glass connect ons. Figure 1 shows a gauge glass with slow closing valves. In Figure 2, the valves shown are the quick closing type where a one quarter turn of the valve spindle will change the valve from the fully open to the fully closed posit on. The valve spindles are fi t ed with levers to which chains may be at ached to operate the valves from ground level if the vessel is located at a higher posit on. Drain valves or cocks may also be installed below the gauge glass to remove any solid material that may collect.

Figure 1 Gauge Glass

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Figure 2 Water Gauge with Quick-Closing Valves

(Courtesy of Jerguson Gage and Valve Co.) Since many tanks and pressurized vessels are not under cont nuous supervision, a broken gauge glass may allow a large amount of liquid to escape. To prevent this problem, the lower valve on the gauge glass is of en equipped with a safety shutoff device consist ng of a stainless steel ball which closes off the fl uid passage when the glass breaks. Figure 3 shows one type of installat on. One disadvantage of this type of safety device is that it requires more maintenance.

Figure 3 Safety Shutoff Gauge Valve

(Courtesy of Jerguson Gage and Valve Co.) Under normal condit ons, the steel ball remains in the recess in front of the valve seat. However, when the gauge glass breaks, the sudden rush of fl uid through the valve will force the ball against the valve opening thereby shut ng off the fl ow. The gauge glass is usually surrounded by a number of metal rods or a transparent shield to protect it from breakage and the operator from fl ying part cles in case the gauge glass shat ers. The use of tubular gauge glasses is limited to lower pressures and temperatures, and restricted to non-toxic and non-hazardous material. Tubular gauge glasses should not exceed 750mm in length. If the level range to be observed exceeds this length, then two or more gauge glasses should be installed so that they overlap, as shown in Figure 4. In this system, the gauges are mounted on the vessel. The glass tube is held t ghtly in place at each end by a washer or packing ring and a nut. If the gauge glass leaks, the isolat ng valves should be closed and the drain opened to prevent injuries before any maintenance is done.

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Figure 4 Multiple Gauge Mounting

Boiler Gauge Glass Installations Steam generat ng boilers are equipped with at least one gauge glass. It indicates the level of water in the boiler, so the operator can be assured that the level is within safe limits. The level must be high enough to completely cover all parts of the heat ng surface to prevent overheat ng and low enough so that water is not carried over with the steam. All such boiler installat ons must comply with ASME and CSA regulat ons. The installat on in Figure 5 uses a gauge glass similar to the one in Figure 1. Note the drain valve which permits all the connect ons to be blown through daily to be sure that they are not plugged with sludge or sediment. The valves can be slow or quick opening. This method of at achment is commonly used only on ﬁ retube boilers that provide a straight vert cal surface, unobstructed by reversing chamber or smoke box, such as the cast-iron heat ng boiler or some types of vert cal boilers.

Figure 5 Direct Connected Gauge Glasses

A more usual construct on is to have the gauge glass connected to a water column which, in turn, is connected to the boiler as shown in Figure 6. The water column acts as a reservoir to dampen agitat on in the water. In addit on, the column traps any sludge or sediment and prevents it from collect ng in the glass connect ons. The column also provides a place for installat on of high and low level alarms and controls. Try cocks are installed on the column to provide a means of point level detect on when the gauge glass is being replaced.

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Figure 6 Water Column and Gauge Glass

Note: A.S.M.E. codes require that, for a fi retube boiler, the bot om of the gauge glass must be 76 mm above the top row of fi retubes, fl ue or crownsheet, to prevent overheat ng. This posit oning will ensure that the tubes are covered even when the gauge glass shows zero level. Steam heat ng boilers have a lowest permissible water level marked by the manufacturer. The lowest visible level in the gauge glass should be 25 mm or more above this marker. When a water column houses an alarm, a water level control or a low-water fuel cutoff device, isolat ng valves are not permit ed in the piping between the column and the boiler. Tubular Gauge Glass Replacement Gauge glasses are suscept ble to corrosion caused by alkalinity and silica deplet on at higher temperatures. Alkalinity causes thinning of tubular glasses above the water line; its eff ect increases drast cally as the pH of the water rises. Condensate formed due to cooling of steam in the gauge glass dissolves some of the silica in the glass and weakens it. Both cause eventual failure of the glass. Misalignment of fi t ngs also causes failure of gauge glasses. The following steps should be taken when a gauge glass fails: 1. Shut off the steam and water valves on the gauge. These valves are usually equipped with chains and levers so they can be closed from the operat ng fl oor. 2. Open the drain valve on the gauge. 3. Unscrew the nuts at each end of the glass and remove the washer and broken glass. 4. Crack open the gauge glass valves to blow out any fragments of glass and then close the valves again. A suitable face shield should be worn to avoid injury.

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5. If there is no gauge glass of correct length, cut a new glass to the correct length using a glass cutter. Ensure the new glass is made of the correct material and meets the pressure and temperature specifi cat ons of the boiler. 6. Place the nuts and new washers on the glass. Install the gauge in the gauge fi t ngs. Put ng graphite on the washers to act as a lubricant between the washers and nuts will prevent the glass from turning when t ghtening the nuts. The nuts should be only hand t ghtened. Tighten them alternately by holding one while t ghtening the other. If “O” rings are used instead of washers, t ghtening with a wrench will be required. Note that the top fi t ng of the gauge glass is deeper than the bot om so that the gauge glass must be inserted into the steam fi t ng fi rst and then lowered into the bot om of the water fi t ng before t ghtening the nuts. 7. Heat the glass slowly by cracking open the steam valve and leaving the drain valve open. Then close the drain valve and part ally open the water valve. Open the gauge steam and water valves fully when the water level in the glass stabilizes. The operator should wear a face shield or use a portable shield to prevent injury when opening the valves, especially if the gauge valves are not equipped with chains for remote operat on. If the gauge glass leaks when put into service, do not t ghten the nuts while the glass is under pressure. Always close the steam and water valves on the gauge and open the drain before t ghtening the nuts. During the t me that the gauge glass is out of service, the boiler drum level may be checked by means of try cocks, a second gauge glass or a drum level recorder if so equipped. Armored-Type (Flat) Gauge Glass Round tubular gauge glasses are not recommended for pressures above 2800 kPa. For higher pressures, a fl at type gauge glass is used that consist ng of glass plates bolted in a steel forged housing. A fl at gauge glass is shown at ached to a water column in Figure 7.

Figure 7 Water Column and Flat Glass

(Courtesy of Jerguson Gage and Valve Co.) The following are armored gauge glass types, suitable for temperatures exceeding 250°C and pressures up to 70 000 kPa: • Transparent • Reﬂ ex

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Transparent Gauge Glass The transparent gauge glass, illustrated in Figure 8, consists of a one piece central chamber with cover plates on each side that hold the two glass windows. The chamber and cover plates have machined recesses that keep all the parts aligned and prevent the gaskets and cushions from shif ing. To indicate the level of caust c or acidic fl uids present, the inside surfaces of both glasses are lined with a protect ve coat ng of transparent mica. Af er prolonged exposure to high temperature, chemically-treated water, the mica will become opaque. This discolourat on indicates that the mica has failed and water is now in direct contact with the glass. When this phenomenon is observed, the glass should be changed before it fails. The glass itself is also tempered for resistance to both mechanical and thermal shock. Care must be taken when assembling the unit and t ghtening the bolts to prevent glass failure. It is safest to use the crossover method of t ghtening by start ng at the centre and working outwards. Besides being suitable for caust c and acidic liquids, the fl at glass is also eff ect ve for dirty materials, high pressure steam applicat ons and other service where it is necessary to illuminate the glass from the rear.

Figure 8 Transparent Gauge Glass (view from top)

Reflex Gauge Glass The reﬂ ex gauge, illustrated in Figure 9, is best suited for clean, colourless, non-viscous and non-corrosive ﬂ uids including light and heavy hydrocarbons. This gauge has special opt cal propert es that create a sharp line of demarcat on at the liquid level. A dark area represents the liquid in the glass gauge, contrasted by a light area above the liquid.

Figure 9 Reﬂ ex Gauge Glass

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Armored-Type (Flat) Gauge Glass Replacement The following procedure is recommended when changing the glass: 1. Close the steam and water valves on the gauge glass. Open the gauge glass drain. 2. Remove the bolted covers, glass, gaskets and the mica. At this t me, the threads on the studs should be coated with graphite and the nuts run down to clean the threads. 3. Remove any remaining gasket material, being careful not to create low spots on the surfaces of the joints. Scraping the gasket oﬀ the metal surfaces may form burrs. 4. Clean both ends of the gauge so gasket material will not plug the valves on the gauge. 5. Polish the gauge surfaces perfectly smooth. Check the surfaces to be sure they are perfectly level with no high or low spots. Checking includes the surfaces of the gauge body and the bolted covers. 6. Apply molybdenum disulphide on the contact surfaces of the new glass. This permits the glass to slip into place easily. Never reuse old gauge glasses. Be sure that the glass is suitable for high temperature service. 7. Install a new gasket, new mica and new glass on one side and install the cover. Replace the nuts on the cover. 8. Tighten the nuts on the cover evenly. It is best to start at the centre of the glass and t ghten evenly on both sides of the glass. 9. Repeat steps 6, 7 and 8 on the other side of the glass. 10. If the boiler is in service, allow the new glass to warm up gradually through conduct ng heat. Never open the gauge valves unt l the new glass is heated up. 11. With the drain valve st ll open, crack open the steam valve and permit steam to slowly blow through to heat the glass further. 12. When the glass is at operat ng temperature, close the drain valve and crack open the water valve to allow water into the glass. If everything appears normal and a water level is visible in the glass, open the steam and water valves fully. On high-pressure boilers, many gauge glass failures occur because the new glass is not heated gradually. Bicolour Gauge Glass Figure 10 is an illustrat on of one type of bicolour mult port gauge glass using the point level method of indicat on. Instead of a water column, this gauge is at ached to a circulat ng t e bar that has top and bot om connector blocks with gauge valves, plus a bot om connect on for a drain line. The gauge glass consists of a number of sealed circular glasses (or double bullseye assemblies) with spot lights connected at the back. The steam space is indicated in red while the water space is green.

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Figure 10 Bicolour Multiport Gauge Glass

(Courtesy of Jerguson Gage and Valve Co.) Figure 11 illustrates the method used for indicat ng the drum level. A green and red fi lter or screen is placed between a lamp and each double bullseye (or circular glass). These circular glasses are placed at a slight angle, so the light can be diff racted in the proper direct on. Figure 11(a) shows the passage of light through the water in the gauge glass. Green light is allowed to pass through both glasses and this colour becomes visible to the operator. The red light is bent, or diff racted, so it is not visible. In Figure 11(b), the red light is visible through the steam space while the green light is not allowed to pass through. By proper design, the diff erent refract ve indexes for steam and water can be made to indicate red in the circular glass when steam is present, and green if water is at that level.

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Figure 11 Bicolour Gauge Glass Operation

Figure 12 shows one way this bicoloured gauge glass can be used to indicate the drum level to a person on the operat ng fl oor below. A hooded mirror is connected directly to the front of the gauge glass and adjusted to a proper angle, so it will refl ect the red and green light to another mirror on the operat ng fl oor.

Figure 12 Mirror Arrangement

Gauge Glass Error During normal boiler operat on, the gauge glass generally indicates a water level lower than the actual level in the boiler drum. This indicat on is because the water in the gauge glass and in the water connect on from the gauge glass to the drum is cooler and denser than the water within the drum. This error may be quite substant al. For example, in a boiler operat ng at 13 800 kPa, the level in the gauge glass may be 20 percent lower than the actual level in the drum. The amount of the error depends upon the temperature diﬀ erence between the water in the gauge and its connect on and the water in the drum.

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Error is aff ected by such factors as the: • Ambient temperature • Velocity of the air fl owing past the gauge • Amount of gauge surface radiat ng heat to the atmosphere • Length of the gauge glass • Level of the liquid in the gauge glass Testing a Directly Connected Gauge Glass For a gauge glass to indicate the correct level, all connect ons to the gauge and the gauge glass itself should be free of any obstruct ons. To be sure that this condit on exists, each connect on should be cleared in the following manner: 1. The gauge is assumed to be in service; when the drain valve is shut and the steam and water valves are both open. 2. Close the water valve on the gauge glass and open the drain valve. Blowing of steam through the top connect on will prove this connect on and the gauge glass are clear. Close the drain valve. 3. Close the gauge glass steam valve; open the gauge water valve, then the drain valve. This procedure forces water through the lower connect on and proves that it is clear. 4. Close the drain valve on the gauge and open the steam valve to put the gauge glass back into service. When safety shutoff valves are used on the water gauge, the opening of the drain valve during the test ng procedure would cause a surge of steam or water through the valves and the balls would be forced against their seats. This seat ng would prevent proper fl ow through the respect ve passages. To prevent the balls from seat ng during test ng procedure, each valve is fi t ed with a pin extending through the valve seat. During the test ng procedure, the valves are only part ally opened to keep the ball off its seat which permits the steam or water (depending on which valve is open) to fl ow freely. Testing of Water Column and Gauge Glass during Operation Note: Refer to ASME Sect on I for the requirements concerning the placement of column isolat on valves in the steam and water lines connect ng the column to the boiler. This test ng procedure is for connect ons having the isolat on valves installed. When a gauge glass is connected to a water column, the connect ons to the column from the boiler, in addit on to the gauge connect ons, must be proven clear. The following procedure is recommended for high pressure boilers: 1. Close the gauge steam and water valves to isolate the gauge before checking the column connec- t ons. 2. Close the column water valve, and then open the column drain valve. This act permits steam to blow through the steam connect on and the column to prove them clear. Close the column drain valve. 3. Close the column steam valve, open the column water valve, then the column drain valve. Water fl ow through the water connect on proves that this passage is clear. 4. Close the column drain and open the column steam valve. This process puts the column back in operat on. 5. Open the gauge steam valve, then the gauge drain valve. This procedure permits steam to blow through the top gauge connect on, proving it is clear. Close the gauge drain valve. 6. Close the gauge steam valve, open the gauge water valve, then the gauge drain valve. Water fl ow from the drain proves the lower connect on is clear. 7. Close the drain valve on the gauge and open its steam valve to put the gauge glass back in opera- t on.

4th Class • Part A2 Unit 13 • Chapter 60 • Water Columns & Gauge Glasses 341

All the connect ons to both the column and gauge have now been proven clear. As a ﬁ nal check, the try cocks may be operated in turn to verify the water level. Since shutoﬀ valves are not permit ed in the connect ng piping of a water column on a low-pressure heat ng boiler, the following procedure is used to clear the passages on the column and gauge glass: 1. Close the gauge water valve to prevent steam from bypassing through the gauge glass. 2. Open the column drain to allow steam and water to blow through the connect ons and the column to drain. 3. Close the column drain valve and open the drain on the gauge glass to prove the steam connect on and the glass are clear. 4. Close the gauge steam valve and open the gauge water valve to prove the water passages on the gauge glass are clear. 5. Close the gauge glass drain and open the gauge steam valve to put the gauge glass back in service. The water should rise quickly to its true level indicat ng all the passages are clear. The water column and gauge glass should be blown down every shif to remove any sediment that may collect. This procedure is highly recommended on smaller high-pressure boilers. On large boilers where the gauge glass contains mica, blowing down of the gauge glass would be less frequent. Frequent blowing down will shorten the life of the mica and increase maintenance costs. Gauge glasses should be renewed if they become obscured by internal corrosion or deposits. Every plant should carry a substant al reserve of gauge glasses and washers or packing rings. Gauge glasses should be stored in a safe place where they will not be damaged.

4th Class • Part A2 Unit 13 • Chapter 60 • Water Columns & Gauge Glasses 342

OBJECTIVE 2

Describe indirect type water level indicators.

INDIRECT (REMOTE) LEVEL INDICATORS

A remote water level indicator is one that can be located at the operat ng ﬂ oor. The operat ng element consists of a large sensit ve diaphragm with the top side connected to the steam space of the boiler and the bot om to the water space. One such indicator is the Hopkinsons Remote Water Level Indicator. (Fig.13)

Figure 13 Remote Water Level Indicator

(Courtesy of Hopkinsons Ltd.)

4th Class • Part A2 Unit 13 • Chapter 60 • Water Columns & Gauge Glasses 343

A condenser at the boiler drum is provided to maintain a fi xed head pressure of water on the steam side. The water side is connected at the minimum permissible water level in the drum and is subjected to a varying head pressure as the drum level changes. The diff erence in head pressure on the two sides of the diaphragm is balanced by a spring when the level is at minimum, so the diaphragm moves up and down in accordance with the water level. As the water level in the drum rises, the pressure due to the varying head of water increases causing the diaphragm to rise and move the indicator upwards. As full boiler pressure is exerted equally on both sides of the diaphragm, boiler pressure has no eff ect upon its movement. Coloured screens illuminate the inside of the indicator with blue in the lower port on, represent ng the water level, and red in the upper part to represent steam. These two colours are separated by the refl ect ng shut er which moves the indicat ng pointer. The mot on of the diaphragm is transmit ed to the shut er through a lever mechanism connected to the diaphragm. If the drum level rises, the increased force under the diaphragm causes it to rise and move the shut er upwards so more blue and less red is indicated. This act on is reversed when the drum level drops. The shut er may also be used to energize high and low drum level alarms. A remote level indicator, shown in Figure 14, is widely used on high pressure boilers. The steam condenser maintains a constant head of water act ng at the top of the indicator glass that contains a coloured indicat ng liquid. The right column of water, or varying head, is exposed to the liquid level in the drum and the right side of the U-tube containing the indicat ng liquid (usually blue or green in colour). When the level in the boiler steam drum is at minimum, the pressure diff erent al between the two heads will be the greatest.

Figure 14 “Igema” Remote Water-Level Indicator

The indicat ng liquid will be forced downward to the bot om of the indicator glass. As the level rises in the drum, the indicat ng liquid will also rise in the glass, giving a coloured indicat on of level to the operators on the operat ng ﬂ oor. Dirt traps are installed on both connect ons to the indicat ng glass to prevent contaminat on of the indicat ng liquid. A special liquid whose relat ve density is greater than water is used. Care must be taken when adding this liquid as overﬁ lling will cause the level indicat on to be higher than it should be. A great advantage of this remote indicator is that there are no moving mechanical parts.

4th Class • Part A2 Unit 13 • Chapter 60 • Water Columns & Gauge Glasses 344

4th Class • Part A2 Unit 13 • Chapter 60 • Water Columns & Gauge Glasses 345

CHAPTER 60 - QUESTIONS WATER COLUMNS GAUGE GLASSES

1. If a gauge glass breaks, the ﬁ rst act on by the operator should be a) close the gauge glass isolat on valves. b) call his Supervisor. c) shut down the boiler. d) place the spare gauge glass in service.

2. Tubular gauge glasses should not exceed ______mm in length. a) 250 b) 500 c) 750 d) 1000

3. A.S.M.E. codes require that, for a fi retube boiler, the bot om of the gauge glass must be ______mm above the top row of fi retubes, fl ue or crownsheet, to prevent overheat ng. a) 50 b) 76 c) 100 d) 125

4. Round tubular gauge glasses are not recommended for pressures above ______kPa. a) 1500 b) 10 250 c) 5000 d) 2800

Fourth Class • Part A2 Unit 13 • Chapter 60 • Water Columns & Gauge Glasses 346

CHAPTER 60 - ANSWERS WATER COLUMNS GAUGE GLASSES

1. (a)

2. (c)

3. (b)

4. (d)

Fourth Class • Part A2 4th Class • Part A2 C HAPTER 61

Drum Internals

LEARNING OUTCOME

When you complete this chapter you should be able to: Describe typical internal components of a boiler steam drum.

LEARNING OBJECTIVES

Here is what you should be able to do when you complete each objective:

1. Describe the purposes of and the general principles and equipment used to separate steam and water in a steam drum.

2. Describe steam drum internal feedwater, continuous blowdown and chemical feed pipes.

347 Unit 13 • Chapter 61 • Drum Internals 348

INTRODUCTION

The steam drum in a modern water tube boiler is a complex piece of equipment with several mechanical stages that help separate steam from water during the boiling process. It can consist of primary and secondary separators as well as a sect on for dryers. Steam drums must have internal feed lines for make up feedwater and cont nuous blowdown pipes to maintain good water quality. Also, there has to be provision for chemical feedlines to maintain ongoing water treatment to help prevent corrosion and keep impurit es under control.

4th Class • Part A2 Unit 13 • Chapter 61 • Drum Internals 349

OBJECTIVE 1

Describe the purposes of and the general principles and equipment used to separate steam and water in a steam drum.

PURPOSES OF THE BOILER STEAM DRUM

In a modern boiler, the steam drum can serve several purposes. It provides a place: 1. for the storage of steam, so that load increases can be accomplished quickly and smoothly. 2. in which to install equipment that will separate water out of the steam before the steam goes to superheaters, turbines or other processes. 3. in which equipment can be installed that will remove impurit es from the steam, thus prevent ng these impurit es from deposit ng on other surfaces, such as turbine blades. 4. from which heavy concentrat ons of impurit es in the water can be removed via the blowdown line. 5. for the distribut on of feedwater into the water circuits of the boiler. 6. for the introduct on of water treatment chemicals into the boiler. The term “drum internals” means all the devices installed within the boiler steam drum, including various types of steam separators, chemical feed lines, boiler feedwater lines and cont nuous blowoﬀ lines.

SEPARATION OF STEAM WATER

At low pressures, the lower density steam has a strong natural tendency to rise to, and disengage from, the water surface in the steam drum. The separat on of the water and steam in low pressure systems can be accomplished without the aid of mechanical separators, as shown in Figure 1. As the pressure rises, mechanical equipment becomes necessary. The dry pipe, shown in Figure 2, changes the direct on of ﬂ ow of the steam and water mixture and causes most of the water to drop back into the drum, allowing the relat vely dry steam to leave the drum. The dry pipe is an early form of separator st ll found on some low pressure boilers.

Figure 1 Steam Separation in Boiler Drum without Bafﬂ es

4th Class • Part A2 Unit 13 • Chapter 61 • Drum Internals 350

Figure 2 Dry Pipe

In a modern boiler drum, the separat on of steam from the mixture delivered by the riser port ons of the circuit usually takes place in three steps. The primary and secondary separat ons remove nearly all the water from the mixture, as shown in Figure 3, so that in eﬀ ect no steam is circulated back through the circuit. The third separat on, or steam scrubbing, removes or reduces the amount of contaminants in the steam that leaves the drum.

Figure 3 Simple Circuit Showing Primary Steam Separation

Hd = Enthalpy of Downcomer

Hr = Enthalpy of Riser Primary Separators At higher pressures, the steam becomes denser and its tendency to separate from water is not as strong, thus steam will be carried through the steam drum and back into the downcomers unless means are taken to prevent such act on. The cyclone steam separator was developed to remove the steam from the mixture of steam and water in the steam drum, and thereby provide the downcomers with steam-free water. These mechanical separators are installed in single or double rows in the steam drum, as shown in Figure 4. All of the steam and circulat ng water from the risers is collected behind a manifold baffl e and then discharged into the cyclones. The water in the mixture will have a mass of between two (2) and twenty-fi ve (25) t mes the mass of steam in the mixture. The circulat on rat o depends on the boiler design and the fi ring rate.

4th Class • Part A2 Unit 13 • Chapter 61 • Drum Internals 351

Figure 4 Cyclone Separators and Steam Scrubbers

(Courtesy of Babcock and Wilcox) The steam and water mixture swirls within the cyclones at high velocity, producing a centrifugal force many t mes greater than the gravity separat ng force. This centrifugal act on forces the water toward the periphery of the cyclones. The less dense steam ﬂ ows up the central port on of the cyclones and passes through a small corrugated scrubber at the top of the cyclone cylinder, as illustrated in Figure 5.

Figure 5 Cyclone Steam Separators

(Courtesy of Babcock and Wilcox) The steam leaves the top of the cyclone at a velocity low enough to prevent entrainment of water by the steam, which ensures that the steam quality is not aff ected by large variat ons in the water to steam rat o. Direct onal vanes at the bot om of the cyclone guide the water into the separator drum, ut lizing the velocity energy in the water to overcome the head of water outside the cyclone. This process prevents fl ooding of the cyclones, even when the water level in the steam drum is close to the top. It also permits a reasonably wide variat on in the drum water level without aff ect ng circulat on or steam quality. The cyclone steam separator has no moving parts; it simply transforms a small port on of the circulat ng force into the centrifugal force required to separate the steam and water.

4th Class • Part A2 Unit 13 • Chapter 61 • Drum Internals 352

A second type of separator is the turbo separator. Referring to Figure 6, the steam and water mixture enters near the top of the drum, and is directed by a baffl e down the inside wall of the drum to the turbo separator. The mixture enters the turbo separator at the bot om through a set of angle baffl es called spinner blades. A separat ng force is created when the high velocity steam and water mixture passes through the spinner blades. They impart a spin to the mixture, causing the water to fl ow down the outside of the separator and the steam to pass upwards into the secondary separator. The velocity of the steam is reduced by the t me it reaches the secondary separator, prevent ng water from becoming entrained by the steam. The turbo separator does not have a water seal, as is the case with the cyclone separator. This type of separator has no capacity limit and is not aff ected by water level changes. It is also arranged in rows which run the ent re length of the steam drum.

Figure 6 Turbo Separators

Secondary Separators Once the relat vely slow moving steam leaves the primary separators, it passes into the secondary separators, which are generally rows of closely ﬁ t ed corrugated metal plates located directly above the primary separators. These plates cause the steam to change direct on many t mes: the water deposited on the plates drains from the bot om of the assembly to the water in the drum. The secondary separator also runs the length of the boiler drum. Figure 7(b) shows the secondary separators above the turbo separators and Figure 5 shows the corrugated plates above a cyclone separator. Dryers The dryers, or steam scrubbers, are located at the top of the drum and are the last stage of moisture and contaminant removal before the steam leaves the boiler drum. The dryer or scrubber is a wire mesh, or screen, that will pick up any droplets of water and allow them to drop back to the water in the drum, as shown in Figures 7 and 8. Figures 4 and 9 show corrugated metal plate dryers with the drain pots, or pipes, to return the removed water to the water below. A more complicated type of scrubber is shown in Figure 7(a) where perforated trays, stainless wire mesh and wash water are used for the removal of silica from the steam.

4th Class • Part A2 Unit 13 • Chapter 61 • Drum Internals 353

Figure 7 Drum Internals

(Courtesy of Babcock and Wilcox)

4th Class • Part A2 Unit 13 • Chapter 61 • Drum Internals 354

OBJECTIVE 2

Describe steam drum internal feedwater, continuous blowdown and chemical feed pipes.

INTERNAL FEED PIPE

An internal feedwater pipe is used on most boilers. Figure 8 shows one entering the boiler through the drum head and Figure 4 shows another that extends nearly the ent re length of the steam drum. The internal feedwater pipe is arranged so that it reduces the risk of thermal shock and excessive turbulence by controlling the point at which the feedwater mixes with the water in the drum. On smaller boilers, this control may be accomplished by a short feed pipe which discharges against a baﬄ e. Larger boilers use a feedwater pipe which runs nearly the full length of the steam drum and is perforated over its ent re length. The water is introduced directly above the downcomers. This cooler, denser water enters the downcomers and helps to ensure good boiler circulat on.

Figure 8 Internal Feed Pipe

CONTINUOUS BLOWDOWN PIPE

The cont nuous blowdown, or blowoﬀ , is located several cent metres below the normal water level in the steam drum. This locat on is where the water having the greatest concentrat on of dissolved solids is found. Figure 9 shows the locat on of the cont nuous blowdown pipe. As the name implies, this pipe cont nuously removes a controlled amount of concentrated water from the drum. The amount of blowdown is controlled by a special regulat ng or metering valve equipped with an indicator that shows how much the valve is opened; this depends on the results of periodic boiler water tests.

4th Class • Part A2 Unit 13 • Chapter 61 • Drum Internals 355

Figure 9 Continuous Blowdown Pipe

(Courtesy of Babcock and Wilcox)

CHEMICAL FEED PIPE

Chemicals used for the control of scale, corrosion and sludge within the boiler are fed into the drum by means of an internal pipe, as shown in Figure 10. The perforated chemical feed pipe extends into the drum and is posit oned to ensure rapid mixing of the chemicals with the entering feedwater.

Figure 10 Drum Internals, Including Internal Chemical Feed Pipe

4th Class • Part A2 Unit 13 • Chapter 61 • Drum Internals 356

4th Class • Part A2 Unit 13 • Chapter 61 • Drum internals 357

CHAPTER 61 - QUESTIONS DRUM INTERNALS

1. At lower pressures, steam a) has higher densit es. b) is more diﬃ cult to separate from the water. c) readily rises and separates from the water. d) cannot be separated from water with mechanical devices.

2. Which of the following is not a drum internal? a) the internal chemical feed pipe b) the corrugated scrubber c) the separator d) the intermit ent blow-oﬀ line

3. The cont nuous blowdown valve is usually a ______valve with an indicator. a) regulat ng b) globe c) gate d) non-return

4. Some smaller boilers have a short pipe which discharges against a baﬄ e in the steam drum. This pipe acts as a) a separator. b) a chemical inject on device. c) a scrubber. d) an internal feed pipe.

5. The chemical inject on line is located near the a) cont nuous blowdown pipe. b) internal feed pipe. c) corrugated scrubbers. d) cyclone steam separators.

6. What purpose does the boiler steam drum serve? a) provides a place for storage of steam b) provides a place for the distribut on of feedwater c) provides a place in which equipment can be installed to remove impurit es from the steam d) all of the above

Fourth Class • Part A2 Unit 13 • Chapter 61 • Drum Internals 358

CHAPTER 61 - ANSWERS DRUM INTERNALS

1. (c)

2. (d)

3. (a)

4. (d)

5. (b)

6. (d)

Fourth Class • Part A2 4th Class • Part A2 U N I T 1 4

HIGH PRESSURE BOILER OPERATION

Chapter 62 Sootblowers 361

Chapter 63 Continuous & Intermittent Blowdown 371

Chapter 64 Boiler Preparation, Start-Up & Shutdown 381

Chapter 65 Routine & Emergency Boiler Operation 393

359 360 4th Class • Part A2 C HAPTER 62

Sootblowers

LEARNING OUTCOME

When you complete this chapter you should be able to: Discuss the design and operation of sootblowers.

LEARNING OBJECTIVES

Here is what you should be able to do when you complete each objective:

1. Describe the construction and operation of retractable and stationary sootblowers.

2. Describe an arrangement for shot cleaning.

361 Unit 14 • Chapter 62 • Sootblowers 362

INTRODUCTION

In modern steam generators, rout ne soot blowing is essent al to ensure cont nuing operat on, accuracy of control and reliability of performance. The boiler heat ng surfaces exposed to combust on gases tend to become coated with soot and ash, part cularly in coal and other solid fuel fi red boilers, and to some extent of oil-fi red units as well. The soot and ash act as insulat on which reduces the heat transfer rate through the heat ng surface and, as a result, lowers both the effi ciency and capacity of the boiler. In addit on, these deposits tend to obstruct the passage of the combust on gases through the boiler and so increase the draf power required. The accumulat on of ash in superheater sect ons can restrict fl ow, result ng in overheat ng these areas of the superheater.

4th Class • Part A2 Unit 14 • Chapter 62 • Sootblowers 363

OBJECTIVE 1

Describe the construction and operation of retractable and stationary sootblowers.

TYPES OF SOOTBLOWING SYSTEMS

Figure 1 illustrates a typical sootblower arrangement for a large coal-ﬁ red steam generator, with sootblowers in the convect on and radiant zones. Sootblowers are located in the high temperature zones of the steam generat ng unit, such as furnace walls, superheater, reheater and economizer sect ons, and lower temperature zones such as air heaters.

Figure 1 Sootblower Arrangement

The two cleaning mediums employed are compressed air and steam, with both being equally eﬀ ect ve in deposit removal. In the case of air, large compressors must be installed with an integrated piping system around the boiler. The steam systems are usually supplied from the boiler through a pressure reducing stat on so that af er pressure reduct on, a dry superheated steam is available at the sootblower nozzle. Steam has the advantage of availability whenever the boiler is in service. When using air, the blowing medium will be unavailable when the compressor is out of service. The following types of sootblower systems are employed in the cleaning of solid fuel-ﬁ red boilers: • Retractable • Stat onary

4th Class • Part A2 Unit 14 • Chapter 62 • Sootblowers 364

Retractable Sootblowers A short, single-nozzle retractable blower, called a wall blower, removes the ash deposited on the walls of furnace chambers (see Fig. 2). With some fuels, such as coal, ash contained in the fuel is melted by the high furnace temperature. This ash may then come into contact with the comparat vely cool furnace wall tubes, causing the ash to “freeze” on the tube walls. This buildup of ash can greatly restrict heat transfer and is referred to as slag. The retractable blower is a short-stroke lance which, through special openings, penetrates the furnace wall 25 to 51 mm, depending on furnace design. The jet is slightly angled back toward the furnace wall and uses superheated steam or air to dislodge the slag deposits. The lance rotates through 360° and cleans approximately a 1.5 m radius, the eﬀ ect ve radius depending upon the tenacity of the deposit.

Figure 2 Motor-Driven Retractable Sootblower

The blower shown in Figure 3 is a retractable furnace wall blower illustrat ng its operat on cycle. The wall blower operat on can be controlled remotely and is of en automat c in sequence with other blowers.

Figure 3 Retractable Furnace Wall Sootblower

4th Class • Part A2 Unit 14 • Chapter 62 • Sootblowers 365

Long Retractable Sootblowers These sootblowers are designed to dislodge deposits from the convect on and radiant heat ng surfaces, such as those located in the superheater, reheater and economizer sect ons. The convect on sect ons are cleaned with long, fully retractable lances (Fig. 4) which penetrate the cavit es between major heat absorbing sect ons.

Figure 4 Long Retractable Sootblower

The lance normally has two opposed nozzles at the t p which emit a jet of superheated steam or compressed air perpendicular to the lance, as illustrated in Figure 5. While the lance traverses the boiler, it rotates, forming a helical blowing pat ern which eﬀ ect vely cleans the tubes and spaces between tubes in a superheater, reheater or economizer bank of tubes. During sootblowing periods, the boiler should be operat ng at 30% rat ng or more, to ensure stable combust on. Furnace pressure should be below atmospheric to prevent blowback through inspect on doors and other openings. It is important that sootblowers be adjusted so that they do not impinge directly upon tubes. If they do, erosion of these parts will take place. This erosion would be accelerated if the blowing medium contains any moisture; therefore, if air is used, it must be dry, or if steam is used, it must be dry or preferably superheated.

Figure 5 Retractable Sootblower Cleaning Patterns

4th Class • Part A2 Unit 14 • Chapter 62 • Sootblowers 366

Figure 6 illustrates a typical coal-ﬁ red unit showing furnace wall and long retractable sootblower locat ons.

Figure 6 Coal-Fired Steam Generating Unit

The retractable type of sootblower, when not in service, is withdrawn from the unit. The lance is thereby protected against overheat ng, as it is only cooled by steam while in operat on. Stationary Sootblowers Figure 7 illustrates the blowing pat ern of a mult -nozzle rotary type of sootblower for hand operat on. It is used where there is not enough space for the single-nozzle type and where the ﬂ ue gas temperature is suﬃ ciently low to allow the nozzles or elements to remain permanently in the gases.

Figure 7 Multi-Nozzle Sootblower Pattern

4th Class • Part A2 Unit 14 • Chapter 62 • Sootblowers 367

OBJECTIVE 2

Describe an arrangement for shot cleaning.

SHOT CLEANING

Shot cleaning is a method of en used for removing soot and ash deposits from economizer, air heater and superheater tubes. Iron shot or pellets, usually of 6 mm diameter, fall by gravity onto the surfaces of the tubes and ricochet from one tube to another, thereby dislodging deposits. A hopper at the bot om of the boiler sect on is used to collect the shot which is then returned pneumat cally to a distribut ng chamber at the top of the sect on for recycling. Most of the ash removed by the shot is carried away in the fl ue gas stream. Any large part cles will fall with the shot into the collect ng hopper; they are recycled with the shot unt l they are broken up into fi ne part cles and carried away by the fl ue gas. Advantages of this method of cleaning the surface of boiler tubes include: • A supply of air or steam for the operat on of sootblowers is not required, although air is required for the pneumat c conveying system to return the shot to the top distribut ng chamber. • In small plants, a constant steam supply from the turbine is not aff ected by steam extract on for the sootblowers Figure 8 shows an arrangement for shot cleaning.

Figure 8 Shot Cleaning

4th Class • Part A2 Unit 14 • Chapter 62 • Sootblowers 368

4th Class • Part A2 Unit 13 • Chapter 62 • Sootblowers 369

CHAPTER 62 - QUESTIONS SOOTBLOWERS

1. A furnace wall sootblower sequence of operat on is: a) rotate and blow, extend, retract. b) extend, blow, retract, rotate. c) rotate, extend, blow, retract. d) extend, rotate and blow, retract.

2. Soot blowers are especially required on a boiler ﬁ red with a) natural gas. b) pulverized coal. c) propane gas. d) diesel fuel.

3. During a sootblowing operat on the boiler ﬁ ring rate should be at least a) 30%. b) 50%. c) 10%. d) 60%.

4. Shot cleaning pellets have a diameter of approximately a) 1.0 mm. b) 0.25 mm. c) 6.0 mm. d) 1.25 mm.

5. Soot and ash are undesirable on tube surfaces because they a) increase heat transfer through the heat ng surfaces. b) cause overheat ng of the tubes. c) reduce boiler eﬃ ciency. d) reduce draf for power requirements.

Fourth Class • Part A2 Unit 14 • Chapter 62 • Sootblowers 370

CHAPTER 62 - ANSWERS SOOTBLOWERS

1. (d)

2. (b)

3. (a)

4. (c)

5. (c)

Fourth Class • Part A2 4th Class • Part A2 C HAPTER 63

Continuous & Intermittent Blowdown

LEARNING OUTCOME

When you complete this chapter you should be able to: Describe the purposes, equipment and operation of continuous and intermittent blowdown.

LEARNING OBJECTIVES

Here is what you should be able to do when you complete each objective:

1. Describe the equipment and processes involved in continuous and intermittent blowdown systems.

371 Unit 14 • Chapter 63 • Continuous & Intermittent Blowdown 372

INTRODUCTION

The term blowdown refers to the removal of dissolved and suspended solids from boiler water. As the name suggests, cont nuous blowdown is usually an ongoing process, whereas intermit ent blowdown is performed on an as needed basis. The type of blowdown system used in a plant depends to a large extent on the type of external water treatment system. The choice of water treatment is generally related to the operat ng pressure of the boiler. Boilers that operate above roughly 5000 kPa tend to use demineralized water from a cat on-anion exchange system, reverse osmosis or an evaporator. In these cases, the water will be very pure and the blowdown requirements will be marginal. Large central stat on steam generators may require both cont nuous and intermit ent blowdown only in special circumstances, such as start-up. Ut lity boilers operat ng below 5000 kPa, of en use sodium zeolite and/or hot or cold lime sof ening. In this case, cont nuous blowdown will be in constant operat on and intermit ent blowdown will be performed once per day. The operat on of the blowdown system is an important part of rout ne boiler care; specifi c guidelines are described in Sect on VII of the ASME Boiler and Pressure Vessel Code, the ASME B31.1 Code on Power Piping and the CSA B51 code. These guidelines will be referred to in this module. Terminology The terms blowdown and blow-off are somet mes used interchangeably. The ASME code uses both terms, but in slightly diff erent contexts. Blowdown is used to describe the process of removal and/or the water- sludge solut on itself. For example, reference is made to the cont nuous blowdown line or the removal of the blowdown from the boiler. Blow-off is usually applied to the actual equipment. For example, both the CSA and ASME codes use the terms blow-off valves and blow-off tank, rather than blowdown valves or blowdown tank. However, this is not a hard and fast rule. For example, the ASME B31.1 Code on Power Piping refers to both blowdown piping and blow-off piping in the same sect ons. Unfortunately, the term blowdown also refers to the diff erence between the opening and closing pressures of a safety valve. However, the context of this usage is quite diff erent from the removal of solids from boiler water and should not result in any confusion.

4th Class • Part A2 Unit 14 • Chapter 63 • Continuous & Intermittent Blowdown 373

OBJECTIVE 1

Describe the equipment and processes involved with continuous and intermittent blowdown systems.

CONTINUOUS BLOWDOWN

As steam is produced from the boiler water and drawn off for use, most of the impurit es in the water remain behind. These consist of dissolved and some suspended solids lef over from the external water treatment and from chemicals injected into the boiler as part of internal water treatment. The water in the boiler becomes highly concentrated with these impurit es, and this concentrat on steadily rises as long as steam is being produced and drawn off . If this concentrat on is not reduced by some means, then foaming and carryover of impurit es with the steam will occur, as well as the format on of sludge deposits within the boiler. Cont nuous blowdown (CBD) refers to the removal of the dissolved solid element of these impurit es. Since steam separates from water in the steam drum, it is reasonable to assume this is the region where dissolved solids will concentrate. This is in fact the case; a narrow zone of water just below the normal operat ng level is the region of highest concentrat on. At this locat on, a long collect on tube with inlet holes runs inside the steam drum, usually along its whole length. This tube is connected by external piping to a blow-off tank or pond, so that boiler water with a high level of total dissolved solids (TDS) can be removed. The cont nuous blowdown rate depends on the TDS level in the boiler water. Rout ne water tests must be conducted and the cont nuous blow-off valve adjusted accordingly. This valve usually consists of a needle valve with an external valve posit on indicator, somet mes referred to as a vernier valve. On very high pressure systems, more than one valve may be needed to accurately control the fl ow, as the discharge is at atmospheric pressure. A typical adjustment would be only a port on of a turn. If the valve is not opened suffi ciently, the TDS will rise and foaming condit ons and/or carryover could occur. If the valve is opened too much, the TDS will be low; this is not a problem in itself, but if the boiler is meant to operate with high TDS water, expensive treated water will be wasted with no advantage. On boilers using demineralized water, the CBD line may be almost (or completely) closed under normal operat on. On other boilers, it is typical to have a fl ow rate in the CBD line equal to 1% or more of the overall boiler steam fl ow rate. Heat Recovery The fl ow of CBD water from the boiler causes a slight decrease in overall steam system effi ciency, as this water absorbs heat provided by the furnace. If this heat loss is suffi ciently high, a heat recovery system may be used. Figure 1 shows the basic equipment layout used to accomplish heat recovery. Hot blowdown water fl ows from the boiler through the cont nuous blowdown line to the heat exchanger. Inside the exchanger, this water gives up its heat as it fl ows through tubes. Incoming feedwater from the water treat ng system fl ows over the hot tubes in the heat exchanger, absorbing the heat from the CBD. The cooled CBD then exits to the sewer system by way of the blow-off tank. The feedwater cont nues on its way to the deaerator and boiler, along with the heat recaptured from the CBD.

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Figure 1 Continuous Blowdown with Heat Recovery

INTERMITTENT BLOWDOWN

The purpose of intermit ent blowdown is to remove undissolved (or suspended) solids from the boiler. Although small amounts of these solids are dispersed throughout the boiler, the greatest concentrat on occurs in the bot om points of the water space. On packaged watertube boilers, this concentrat on will be at the bot om of the mud drum(s). On fi retube boilers, it will be at the bot om of the boiler shell. On large steam generators, there may be several such locat ons, such as the bot om of each waterwall sect on. In all cases, the intermit ent blow-off connect on is made at the lowest part of the boiler water space; the term bot om blowdown is somet mes used instead of intermit ent blowdown. The intermit ent blow-off connec- t on also serves as a place where the waterside of the boiler can be drained. As discussed under cont nuous blowdown, the amount and frequency of intermit ent blowdown depends on the quality of the boiler water. On ut lity boilers, it may be a rout ne daily task to open the blow-off line for a short period of t me. On large steam generators, this line is usually opened only during init al start up or when shut ng down and draining the boiler. In part cular, if it is a very high pressure boiler operat ng at close to the pract cal upper limit of natural water circulat on, the blow-off valves may not be opened under normal working condit ons. To do so might cause a temporary loss of water circulat on in that sect on of waterwall tubing and expose the tubes to failure from overheat ng. The overall intermit ent blowdown system consists of several important components, including the: • Blow-off tank (drum or vessel) • Blow-off piping • Blow-off valves Blow-Off Tank Intermit ent blowdown involves hot, high pressure water fl owing for short periods of t me. When this water is reduced to atmospheric pressure, it will part ally vapourize. The combinat on of pressure, vapourizat on and high temperature would result in extensive damage to sewer piping if the blowdown entered the sewer directly from the boiler. To prevent this problem, the blow-off lines are routed through a blow-off tank before entering the sewer piping. Figure 2 shows a typical blowdown tank. The blow-off tank provides a space for fl ash vapours to separate and vent, and for the water to cool. The inlet line from the boilers is posit oned beneath the water level so that incoming hot water is submerged in the cooler tank water. Incoming water also agitates the water in the tank so that sludge is kept dispersed. As new water enters, the tank overfl ows to the sewer, carrying the sludge along with the water.

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CSA Code B51 requires the installat on of a blow-off tank for all boilers operat ng at or above 103 kPa, discharging to a sewer system. The tank is not required if the boiler is low pressure (operat ng below 103 kPa gauge), or if the boiler discharges to a separate pond, such as of en occurs in large, remote thermal sta- t ons. The water temperature at the blow-off tank outlet may not exceed 65°C. Code B51 also specifi es the required: • Vessel and piping thicknesses • Design pressures • Corrosion allowance for thickness • Inspect on and cleaning access

Figure 2 Blow-Off Tank

Blow-Off Piping The piping leading to the blow-off tank from the boiler is subject to short durat on fl ows, which means it will experience rapid changes in temperature when the blowdown occurs. Allowance must be made for this piping to expand and contract during these temperature changes. If this allowance is not made, the piping will become stressed due to restricted thermal expansion. Worse yet, it would at empt to expand against the mud drum or water walls of the boiler, placing heavy stresses on boiler parts already working at high pressure and temperature. Therefore, the piping must be anchored in such a fashion that this type of mot on cannot be transmit ed back to the boiler. Provision must also be made so the piping can be inspected for leakage. ASME B31.1 Code on Power Piping specifi es the materials, pressure rat ngs and required sizes for blowdown piping.

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Blow-Off Valves The boiler is isolated from the blow-off tank by the blow-off valves, which are specially designed to handle the pressure drop from boiler operat ng pressure to atmospheric pressure and to provide a safe means of controlling the fl ow of the hot water and sludge under these condit ons. There are several possible valve types and arrangements, each of which has a specifi c valve opening and closing sequence which must be followed to ensure safe operat on and to protect the piping and valves. ASME B31.1 Code specifi es the types of valves that can be used for part cular boilers. Every high-pressure steam boiler must be equipped with two approved blowoff valves, one of which should be of a slow opening type: one which requires at least fi ve 360° turns of the operat ng mechanism to change from full closed to full open and vice-versa. Figures 3 and 4 show some typical slow opening blow-off valve designs. One of the special features of these valves is that they do not allow sediment to collect under the valve seat.

Figure 3 Slow Opening Y Type Blow-Off Valve

(Courtesy of Everlasting Valve Co.)

Figure 4 Slow Opening Angle Type Blow-Off Valve

(Courtesy of Everlasting Valve Co.) Boilers are of en equipped with one slow opening valve and one quick opening valve, or they may be equipped with two slow opening valves. In the lat er arrangement, these valves may be of the hard seat or seatless variety. The opening and closing sequence of these valves depends on the valve type and arrangement. In general, reference is made to the following common combinat ons of blow-oﬀ valves: • One slow and one quick opening • Two slow opening of the seatless type

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Quick and Slow Opening Valves A quick opening valve is one opened or closed by moving a lever or wrench through a small arc. Figure 5 shows a typical design.

Figure 5 Quick Opening Blow-Off Valve

(Courtesy of Everlasting Valve Co.) If a quick opening valve is used with a slow opening one, then the quick opening one should be installed nearest the boiler. It then acts as a sealing (or guard) valve, and the slow opening valve is the blowing oﬀ valve. Figure 6 shows a typical arrangement.

Figure 6 Quick and Slow Opening Blow-Off Valve

(Courtesy of Everlasting Valve Co.) The sealing or guard valve should be opened fi rst and closed last; the blowing-off valve is opened last and closed fi rst. In this way, the blowing valve will be subjected to wear due to the fl ow of abrasive water through it when start ng to open and just before it closes. On the other hand, the guard valve will not wear as there is no fl ow through it when being opened or closed. This arrangement means that there will always be a t ght valve next to the boiler allowing the blowing valve to be repaired while the boiler is in operat on.

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Figure 7 shows the correct valve sequence for these types of valves.

Figure 7 Opening and Closing Sequence for One Slow and One Quick Valve

(Courtesy of Everlasting Valve Co.) The fact that the blowing valve is slow opening reduces the possibility of water hammer and subsequent damage to pipe and fi t ngs when the blow-off is begun or ended. In addit on, as in the case of opening or closing any valve under pressure, the slow opening valve must be opened and closed slowly to further prevent the possibility of water hammer. Two Seatless Slow Opening Valves This type of valve uses a cylindrical piston as the valve element. This piston has a hard surface, ideally suited to withstand the abrasive eff ect of blowdown water. As the piston descends into the valve body, it displaces water in the valve. If the water in the valve has nowhere to go, the piston will compress it as it closes, which could cause the valve body to crack. The correct closing sequence is designed to prevent this cracking. In this case, the valve furthest from the boiler is opened fi rst and closed last. If it were closed fi rst, water would become trapped between the closest and furthest valves, so that when the closest valve is closed, the compression problem described above could occur. Of course, this system dictates that the valve closest to the boiler is now no longer protected. If this valve begins to leak due to the abrasion of the water, the boiler would have to be shut down to replace it. For this reason, this system of two slow opening valves of en incorporates a third gate valve, located immediately next to the boiler. This valve is usually kept wide open so that it experiences no wear on the valve disk. It is only closed in the event the boiler is being shutdown and isolated, or if the fi rst slow opening valve requires repair or replacement. When blowing down a boiler, one person should watch the gauge glass level while another operates the blow-off valves. The valve operator should remain beside the valves unt l the procedure is complete. When there are several boilers in a bat ery or header arrangement, the valves and boilers should be visibly numbered or ident fi ed, so there is no chance of blowing down the wrong boiler.

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CHAPTER 63 - QUESTIONS CONTINUOUS INTERMITTENT BLOWDOWN

1. A steam boiler is blown down in order to a) clean the soot and carbon out of the ﬁ rebox and tubes. b) lower the operat ng water level in the boiler. c) discharge sediment and scale forming mat er from the boiler. d) test the rated relieving capacity of the safety valves ﬁ t ed to the boiler.

2. A “rule of thumb” rate of cont nuous blowdown is approximately a) 10% of steaming rate. b) 5% of steaming rate. c) 15% of steaming rate. d) 1% of steaming rate.

3. Boiler water having the greatest concentrat on of suspended solids is found a) just below the steam drum water line. b) in the bot om of the mud drum. c) at the bot om of the steam drum. d) in waterwall riser tubes.

4. The CSA B51 Code requires the installat on of a blow-oﬀ tank for all boilers operat ng at or above a) 50 kPa. b) 200 kPa. c) 500 kPa. d) 103 kPa.

5. The water temperature at the blow-oﬀ tank outlet may not exceed a) 65°C. b) 10°C. c) 50°C. d) 100°C.

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CHAPTER 63 - ANSWERS CONTINUOUS INTERMITTENT BLOWDOWN

1. (c)

2. (d)

3. (b)

4. (d)

5. (a)

Fourth Class • Part A2 4th Class • Part A2 C HAPTER 64

Boiler Preparation, Start-Up & Shutdown

LEARNING OUTCOME

When you complete this chapter you should be able to: Describe the basic preparation of a boiler for start-up and shutdown procedures.

LEARNING OBJECTIVES

Here is what you should be able to do when you complete each objective:

1. Describe the steps that must be taken to prepare a boiler for start-up.

2. Describe a typical boiler start-up procedure.

3. Describe the boiler and steam header warm-up procedures.

4. Describe the procedure for shutting down a boiler.

381 Unit 14 • Chapter 64 • Boiler Preparation, Start-Up & Shutdown 382

INTRODUCTION

A boiler must be carefully and properly prepared for start-up. Preparat on is a step by step process and all plants will have their own procedures based on the role the steam system plays in that plant. It is important to note the boiler and steam system are of en prepared and started before the process units in a large plant, because many of the other processes depend on the steam system for start-up or ongoing energy supply. If something has been overlooked in boiler preparat on, it may mean costly downt me later on. Boiler preparat on procedures will also vary depending on the reason that the boiler has been shut down. Obviously, more steps would have to be taken to prepare a newly constructed and commissioned boiler than one which has been shut down overnight to replace a leaking fi t ng. This chapter will look at some general considerat ons that apply to either new boilers, or those having undergone extensive maintenance work. As a point of reference, certain assumpt ons are made: • All safety systems and devices on the boiler, such as low level switches and safety valves, were verifi ed as operat onal during the commissioning period. • The boiler and external piping and fi t ngs have been checked for leaks. • The boiler has a valid inspect on cert fi cate. If not, the Chief Engineer will need to contact the local Boiler Inspector and make appropriate arrangements.

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OBJECTIVE 1

Describe the steps that must be taken to prepare a boiler for start-up.

START-UP PREPARATION

Several checks must be made before a boiler can be started. These checks include the boiler itself, the feedwater system and the fuel and draf systems. As well, old boilers must be dried out and new boilers boiled out. Boiler Before closing up and fi lling the boiler, it must be inspected both internally and externally. The internal inspect on is done to make certain it is free from scale, oil, tools, debris and other foreign material. In part cular, if furnace refractory work has been done, it is important to make sure all debris has been removed from inside the boiler. All internal baffl es should be checked to see they are secure, part cularly if they are steam drum internals. If maintenance has been done on the water side of a watertube boiler, all tools taken into the boiler should be checked off as removed, to ensure that nothing has fallen into a water tube. Check that the burners are clean and ready for fi ring. Check soot blower alignment and clearance of movement. Following these steps, the boiler can be closed up. New handhole and manhole gaskets are to be used and coated with a graphite paste to prevent them from st cking to the metal. This coat ng will make them easier to remove when the boiler is next shut down. Before fi lling the boiler, check that the blow-off valves are closed and a vent valve open, so that air may escape from the boiler as it is fi lling. Feedwater System Verify the proper operat on of the feedwater system if it has been out of service to ensure that: • it is open all the way back to the primary source of water. • the feedwater treatment equipment is funct oning. • the raw and treated water tanks are at their proper levels. • the deaerator is operat onal and full. • the boiler feedwater pumps are in service. Fuel and Draft Systems Verify that all necessary valves in the main and pilot fuel supply systems are open and the oil is at the correct temperature and pressure, or that the gas is at the correct pressure. Ensure the main gas cock and other related valves are in their proper posit ons to init ate the start-up ignit on sequence. Start the draf fan and check for normal operat on. Stroke the air dampers to make sure they are free to move their full range. Ensure there is an adequate supply of air available for the furnace. If it uses inside air, check the room openings and the space around the damper inlet vanes. If an external source of air is used, check the intake screens are clear. This point is especially important in the winter when frost may close them off . Boiler Dry-Out If new refractory has been installed in the furnace, it will have to be slowly dried out before raising the furnace to normal operat ng condit ons. New refractory contains water which will fl ash into steam and damage the material if it does not have a chance to evaporate before reaching normal furnace operat ng temperature. Drying is done by creat ng a very light fi re in the furnace, suffi cient to speed up the evaporat on, but not hot enough to harm the refractory.

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Before start ng the fi re, fi ll the boiler to the normal level with treated, deaerated water. Leave a vent open to prevent pressure build up during the dry out period. Maintain the water level by replacing any water that leaves the drum due to the heat ng process. Next, ensure that the furnace has been adequately purged. Then, according to manufacturer’s recommendat ons, start a light fi re and maintain it for several hours to several days, depending on the amount of refractory replaced. Boiling Out If the boiler is new, it will have to be “boiled out.” The boil-out removes all the grease, welding debris, dirt and oil from construct on. If this step is not taken, the boiler will experience major water level control problems when started up, as the greases and oils contribute to foaming condit ons. In addit on, important sensing lines for instruments and gauge glasses can get plugged. In extreme cases, the accumulat on of debris in one area of the boiler could impede water circulat on enough to cause overheat ng and tube failure. The boiler is cleaned by heat ng the water, adding detergent type chemicals and then periodically blowing down to dislodge dirt and remove sludge. On large units, the process may be repeated several t mes over a period of days. The chemicals used are types of alkaline solut ons, including: Soda ash• Soda ash• • Caust c soda • Trisodium phosphate • Sodium silicate The amount of each chemical to be used varies with the overall cleaning program.

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OBJECTIVE 2

Describe a typical boiler start-up procedure.

START-UP

There are three major areas of at ent on when start ng up a boiler: • Ignit on • Boiler warm-up • Steam header warm-up. Ignition Although boilers have many diff erent specifi c ignit on systems, they all follow the same basic sequence of steps: 1. Pre-purge of the furnace. 2. Reduct on of air fl ow for stable ignit on. 3. Pilot burner ignit on 4. Main burner ignit on. Small boilers of en have completely automat c ignit on systems, such as those described in the chapter, “Ba- sic Boiler Instrumentat on and Control Systems.” Some older, or specialized boilers, may have completely manual systems. Most boilers have a combined system, where some steps, such as 1 and 2 above, are done automat cally, while the remaining steps require the operator to perform specifi c tasks within a set t me period. A typical ignit on procedure for a gas fi red boiler is included in Figure 1 which shows the essent al components of an ignit on system. Not all boilers will have equipment ident cal to what is shown, but it is fairly typical of most packaged boiler systems. To keep things simple, a gas fi red boiler will be discussed. Pilot gas is supplied to the pilot burner through a pressure control valve and solenoid valve. Gas for the main burner is supplied through a pressure control valve, a fl ow control valve, electrically operated automat c shut-off valves and the main gas cock. The automat c shut-off valves are arranged in a “double block and bleed” system that ensures no gas can enter the furnace when the boiler is shut down. When shut down, valves A and B are both closed and vent C is open. When the boiler is operat ng, A and B are both open and C is closed. These valves are designed so that A and B will close and C will open, all automat cally, in the event of a boiler shutdown. During start-up, C will close automat cally, but A and B need to be manually actuated (latched) to open.

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Figure 1 Boiler Ignition Equipment

Figure 1 also displays an outline of the local control panel for the boiler including: A larms• Alarms• • Fan and ignit on switches • Ignit on status indicators (red and green lights) • Steam pressure controller • Fuel and air fl ow controllers • Water level controls (not shown in Fig. 1) To avoid clut er, not all of the normal indicators and recorders are shown, nor are the water controls shown. Before the ignit on sequence can begin, all the boiler shutdown switches must be in their “permissive” states. For example, the water level and fuel pressure must be within their acceptable limits. The assumpt on is made that all the required operat ng condit ons have been sat sfi ed. All valves must be in their proper start-up posit ons. In part cular, the gas cock on the main gas line must be closed; there is usually a posit on switch to check this. If this valve is not closed, the ignit on sequence will not be permit ed to proceed. If the boiler has a superheater, it is common to open the superheater vent at this t me, so that it does not overheat due to lack of steam fl ow.

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Before start ng the fan, the operator must place the steam pressure (also known as the Boiler Master), fuel fl ow and air fl ow controllers in manual mode and set their outputs to the required ignit on posit ons. The ignit on sequence is then init ated by closing the fan switch. The programmer will automat cally put the boiler into a pre-purge mode, with the required amount of air fl ow, for a specifi ed t me period. The minimum air fl ow set ng on the air fl ow controller will be temporarily overridden during the purge period. Following the purge, a light on the panel will indicate “Purge Complete” and the air fl ow dampers will return to the posit on assigned by the air fl ow controller. The operator can now push the pilot igniter push but on, which is held unt l the fl ame scanner senses a stable pilot burner fl ame. Init ally, a red light goes on to indicate the igniter is in operat on and pilot gas is fl owing to the burner. When pilot ignit on occurs, the adjacent green light will go on. The operator may now release the igniter push but on. The operator must now act effi ciently to complete the ignit on procedure, since the programmer will only allow for a specifi c t me period to obtain main burner ignit on. The next step is to open valves A and B by manually reset ng them. Usually this procedure involves reposit oning a latching lever. Vent valve C will close automat cally. Finally, the operator must manually open the main gas cock next to the boiler. Gas will now fl ow to the main burner, where it will ignite from the pilot fl ame. If these steps are done in t me, the ignit on sequence is complete and the fi ring rate can be increased to bring the boiler up to operat ng condit ons. Note: Under today’s gas code, the operat on of valves A & B would be totally automat c.

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OBJECTIVE 3

Describe the boiler and steam header warm-up procedures.

BOILER WARM-UP

The rate at which a boiler can be brought up to normal operat ng status depends on its size and the length of t me it has been shut down. In general, the larger and colder a boiler, the longer the period of t me to pressurize. Care must be taken not to thermally shock the metal and refractory, and to allow thermal expansion to occur. Following ignit on, the fi ring rate is increased on manual control. Care must be taken to ensure the fuel and air fl ow rates are within safe rat o limits. Most boiler control systems will, in fact, have built-in “cross-limit ng” combust on controls to make certain the air fl ow and fuel fl ow rates are properly matched. The water level control system can usually be placed in automat c as soon as the boiler is started. While the boiler is slowly pressurizing, the operator can make a visual check of its condit on. Look for red “hot spots” on the metal surface. These would indicate that refractory has fallen away. Check the furnace draf pressures and compare them to earlier log sheets to make sure that combust on gases are passing normally through the furnace and convect on passages. Check the fan and feedwater pumps for normal operat on. Now is a good t me to verify the proper operat on of the low water level alarm and blow the water column clear of sediment. As boiler pressure approaches its normal setpoint, the steam pressure and combust on control system can be switched over to automat c. From previous log sheets and recording charts, verify the boiler is performing “as expected.” Enter in the daily log a summary of the start-up events.

STEAM HEADER WARM-UP

Steam headers conduct steam from the boiler to other equipment in the plant. They must be properly warmed up to avoid thermal stress and remove condensate as it forms. If this procedure is not done, severe water hammer will result during the overall plant start-up. In some cases, piping damage can occur causing injury. To see how the process of warming up and drying out steam headers is accomplished, Figure 2 shows how they are arranged in a plant. Header arrangements can vary depending on whether one, or more than one, boiler produces into the header. Figure 2 shows the two basic arrangements. In Fig. 2(a), a single boiler produces steam into the header. The block valve at the outlet of the boiler is called the boiler stop valve and is usually of the standard gate valve design. The piping downstream of the valve is referred to as the steam header (or steam main); it supplies all the plant equipment taking steam at that pressure. When there is more than one boiler present, such as Fig. 2(b), each boiler is equipped with a non-return type valve (described in the chapter “Introduct on to Valves”) and a header valve. In addit on to these arrangements, plants can have several diﬀ erent steam headers, ident ﬁ ed by the pressures at which they operate. For example: • 10 250 kPa for high pressure turbine and processes • 2410 kPa for intermediate pressure turbines and processes • 620 kPa for pressure steam for heat ng and auxiliary equipment. Each of these may have its own boilers and steam users. It is common that these headers are connected in sequence; the high pressure header “lets down” to the intermediate pressure header which, in turn, lets down to the low pressure header.

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In a single boiler plant, the easiest way to warm up the piping is to open up the boiler stop valve and header drains as soon as ignit on has been obtained. As the boiler begins to produce steam and slowly pressurize, the header will be warmed from the steam. Condensate, which forms as the steam meets the cold piping, will be removed quickly from the header by the drains. By the t me the boiler is up to its normal operat ng pressure, the headers should be warm and dry. Alternat vely, it is possible to leave the header isolated un- t l the boiler reaches its operat ng pressure. At this point, the stop valve is slowly opened and the header warmed up. As before, it is very important that the header drain valves are opened during this procedure. In a plant with more than one boiler, the procedure is somewhat diff erent. There are two possible situat ons that require diff erent procedures: 1. All the boilers have been shut down. The boiler being started up is the fi rst to go on line. All steam headers are cold. 2. There are other boilers in the plant which are already on line. The main steam headers are already warmed up and pressurized. The fi rst case is treated exactly like the warm up procedure for a single boiler plant. The headers are opened up following ignit on and warmed as the boiler is pressurized. In the second case, the main steam headers are already in service, but the boiler steam line needs to be warmed up. Even though this line is a relat vely short length of piping compared to the overall steam header system, it is extremely hazardous to put it into full service without warming it up and drying it out. The procedure for opening up a boiler into a pressurized steam header is referred to as “cut ng” the boiler into the header and is described in Sect on VII of the ASME code. Figure 2 shows the related equipment.

Figure 2 Non-Return and Header Valve Arrangement

To start the warm up, open the drain on the non-return valve. It is important to know that this drain is actually connected internally to the downstream side of the valve plug. Next, open the header valve a slight amount, allowing steam to fl ow from the steam header into the boiler steam line and out the drain. The boiler steam line will thus be warmed up with steam fl owing back from the steam header. Any condensate present will be forced out the drain valve. The header valve can be slowly opened wide when the pressure has equalized between the steam header and the boiler steam line. During this t me, the boiler has been slowly pressurizing. When the boiler pressure is st ll a few kPa below the header pressure, the non-return valve spindle is backed off (turned in the open direct on) to about the one quarter open posit on. Note that, due to the check valve nature of this valve, it doesn’t actually open at this t me. It will only begin to open when the boiler pressure exceeds the header pressure. Backing off the valve handle, will give the valve some room to open when the pressure is high enough. As boiler pressure cont nues to rise, and fi nally exceeds header pressure, the non-return valve will begin to open unt l it reaches the one quarter limit. The non-return valve can be slowly opened the remainder of its travel, and then the drain may be closed. The boiler is now fully on line.

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OBJECTIVE 4

Describe the procedure for shutting down a boiler.

SHUTTING DOWN A BOILER

When a boiler has to be removed from service for maintenance, inspect on or lay-up, the following procedure should be followed: 1. Before shut ng down the boiler, give it a good blowdown to remove as much sediment as possible. Stop when the drain runs clear. 2. Put the boiler steam pressure control in manual mode and slowly reduce the fi ring rate. Watch the main steam header pressure to make sure the other boilers are taking the load. Do not reduce the fi ring rate below that necessary to maintain a stable fl ame. 3. When the boiler is at the minimum fi ring rate, the fuel can be shut off at the main gas cock. Alterna- t vely, this is a good t me to test the low water level shutdown switch, or some other boiler interlock. If this method is chosen, make sure it is noted in the logbook. 4. Allow the fan to post-purge the furnace with a reduced air fl ow and then shut it down. Be part cularly careful not to let the fan supply large amounts of cold air into the furnace in the winter. 5. Close the boiler header stop valve. 6. Open a steam drum vent valve when the boiler pressure drops to slightly above atmospheric pressure. This act will prevent a vacuum from forming (not opening this valve resulted in a fatality in recent years when maintenance personnel proceeded to open the manhole cover on the water drum of a boiler which had drawn a vacuum). If the boiler is going to be shut down for an extended period of t me, it will need a proper lay-up as indicated by the manufacturer’s instruct ons.

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CHAPTER 64 - QUESTIONS

BOILER PREPARATION, START-UP & SHUTDOWN

1. The external inspect on of a boiler includes making sure the a) valves are in good operat ng condit on. b) furnace refractory is in order. c) tubes are free of scale. d) exterior of the tubes are in good condit on.

2. If a new boiler is not properly boiled out, it may result in a) tubes rupturing. b) a furnace explosion. c) an unwanted high rate of water circulat on. d) a foaming condit on occurring.

3. Superheater vents are usually opened before ﬁ ring a boiler to a) allow the superheater to warm up quicker. b) prevent overheat ng of the superheater. c) ensure drum level does not drop. d) enhance boiler water circulat on.

4. If three boilers each have a pressure of 130 kPa, the pressure in the steam header will be a) 140 kPa. b) 145 kPa. c) 135 kPa. d) 130 kPa.

5. When cooling down a steam boiler that has been taken out of service, a vent valve on the steam drum should be opened to prevent what from occurring? a) an overpressure condit on b) a vacuum c) too much condensing of the steam d) loss of visible level in the sightglass

6. Boiling out of boilers is done with a ______solut on. a) acidic b) neutral pH c) liquid d) alkaline

Fourth Class • Part A2 Unit 14 • Chapter 64 • Boiler Preparation, Start-Up & Shutdown 392

CHAPTER 64 - ANSWERS BOILER PREPARATION, START-UP SHUTDOWN

1. (a)

2. (d)

3. (b)

4. (d)

5. (b)

6. (d)

Fourth Class • Part A2 4th Class • Part A2 C HAPTER 65

Routine & Emergency Boiler Operation

LEARNING OUTCOME

When you complete this chapter you should be able to: Discuss routine and emergency practices for operation of a packaged boiler.

LEARNING OBJECTIVES

Here is what you should be able to do when you complete each objective:

1. Describe the proper routine care and operation of a boiler.

2. Describe emergency conditions in boilers and the required responses.

3. List types and causes of boiler accidents and explosions.

4. Discuss the need for boiler operating and maintenance logs and the type of information that should be recorded.

393 Unit 14 • Chapter 65 • Routine & Emergency Boiler Operation 394

INTRODUCTION

Due to the large variety of boilers, ﬁ ring equipment and boiler controls, it will not be possible in this chapter to give speciﬁ c instruct ons for each part cular type or model of boiler. Out of necessity, the instruct onal material has to be limited and, therefore, will only be presented in a general way. Even though the instruct ons in this chapter may be applicable, without change, to most packaged boilers, they should be considered as general guidelines only. Boiler operators are well advised to study the instruc- t ons supplied by the manufacturer of the boiler in their plant or building. The manufacturer’s instruct ons may vary in certain details from the instruct ons given in this chapter.

4th Class • Part A2 Unit 14 • Chapter 65 • Routine & Emergency Boiler Operation 395

OBJECTIVE 1

Describe the proper routine care and operation of a boiler.

ROUTINE OPERATION

Although most packaged boilers are now fully automat c, it is necessary that they are given regular at en- t on to ensure safe and dependable operat on. The following is a general list of daily rout ne checks to be performed by the operator: • Water level • Steam pressure • Burner operat on • Stack temperature • Heat transfer • Water treatment • Cont nuous blowdown • Inspect on • Other equipment • Housekeeping • Boiler log Water Level THE MOST IMPORTANT RULE IN THE SAFE OPERATION OF STEAM BOILERS IS TO KEEP WATER IN THE BOILER AT THE PROPER LEVEL. NEVER DEPEND ENTIRELY ON WATER LEVEL CONTROLS, EMERGENCY WATER FEEDERS OR AUTOMATIC ALARMS. The gauge glass, water column and connect ng lines should be blown through daily. Af er blowing through, the level should quickly return in the glass as well as the low water level alarm should be checked at the beginning of each shif . On boilers in cont nuous service, a spring loaded temporary boiler shutdown bypass switch must be held closed during the test procedure. Keep the gauge glass clean. A clear and accurate indicat on of the level is vitally important. Appearance of rust in the glass is an indicat on of corrosion that should not be ignored. Check the boiler water to be sure that the water treatment chemicals are at the proper concentrat on. Also check the condensate return line and other parts of the system for evidence of corrosion. Make sure the water level is relat vely steady. A wide fl uctuat on of the water level may indicate the boiler is foaming or priming, which may be caused by the level being carried too high or, especially in low-pressure boilers, a very high rate of steaming. Foaming may also be caused by an excessively high concentrat on of dissolved or suspended solids in the water, or the presence of oil. Mild cases of foaming can be controlled by the use of ant foam chemicals. More severe foaming is usually controlled by blowdown. Lower the level in the boiler 5-7.5 cm and refi ll to the normal level. Repeat this procedure several t mes. In persistent cases, it may be necessary to take the boiler out of service and to cool, drain and wash it out thoroughly. Then refi ll the boiler and put it back in service.

4th Class • Part A2 Unit 14 • Chapter 65 • Routine & Emergency Boiler Operation 396

Steam Pressure Whenever going on duty, check the boiler steam pressure. To maintain the boiler pressure at the desired value, the amount of fuel burned must correspond to the boiler load. That is, if the boiler load (the rate of steam demand of the boiler) increases, then the amount of fuel burned in the boiler furnace must be increased; if the boiler load decreases, the amount of fuel burned must also be decreased. The amount of fuel admit ed to the furnace is normally controlled by an automat c system which senses changes in steam pressure and adjusts the fuel feed accordingly. However, the operator must be able to take over from the control system in case of failure of the automat c arrangement; therefore, he/she should be familiar with the method of changing over from automat c to hand control. Burner Operation Check the condit on of the burner fl ame. When burning natural gas, it should be predominantly blue, but some orange coloring at the fl ame t p is unavoidable and considered acceptable. Check the stack for any signs of dark colored smoke. A white plume is due to the presence of water vapour from the combust on process and is not a problem unless the ground shows signs of accompanying white ash. If smoking occurs, check the air intake to the boiler fi rst. It may have become obstructed by snow and ice build-up or debris. If the intake is clear but smoking persists, the burner nozzle may be dirty (oil fi ring), the fuel temperature may be too low (heavy oil fi ring) or the fuel/air rat o may be incorrect (gas or oil fi ring). Stack Temperature Check and record the stack temperature daily. It is an indicator of the effi ciency of boiler heat transfer. Heat Transfer To allow for the greatest possible amount of the heat produced in the furnace to be transferred to the boiler water, it is necessary to keep the heat ng surfaces free from soot and ashes by means of sootblowers. They should be operated when required, but usually once a shif is suffi cient. The boiler load should be above 30% of maximum when sootblowers are in service, which ensures the burner fl ame is strong enough to be stable, and also that the fi ne dust will be carried rapidly from the furnace before an explosive mixture forms. Gas- fi red boilers will not normally require sootblowing. Another cause of poor heat transfer is the format on of scale on the heat ng surfaces. Scale format on can be prevented by proper treatment of the feedwater and boiler water and regulat on of the amount of bot om and cont nuous blowdowns. For these reasons, the operator must take regular tests of water condit ons as outlined below. Water Treatment Take samples of the boiler water, deaerated feedwater and condensate and perform the required tests at the intervals laid out in the water treatment program. From the results of these tests, determine the amount of chemicals required to give the boilers and the system the greatest protect on. Fill the mixing tanks and pot feeders accordingly and start the feeding of chemicals. Blow down the boilers as needed. If any external water treatment equipment is used, monitor this equipment; backwash and/or regenerate when required. Continuous Blowdown The cont nuous blowdown (CBD) should be opened just enough to keep the concentrat on of dissolved and suspended solids below the maximum allowed. Keep in mind that excessive blowdown is wasteful and will increase operat onal costs. Record the CBD set ng in the daily log. Inspection Make a full inspect on of the boiler. Check for any signs of leakage from the following: • safety valves • manholes • handholes • clean-out plugs • valves and pipe connect ons Determine the cause of any unusual noises or condit ons, init ate work requests, and record these in the daily log.

4th Class • Part A2 Unit 14 • Chapter 65 • Routine & Emergency Boiler Operation 397

Other Equipment Check the operat on of the auxiliary equipment in the boiler room including feedwater, condensate and vacuum pumps. If the boiler is oil-fi red, check the operat on of the fuel pump, clean the fi lter if necessary and check the fuel level in the tank. Housekeeping Keep the boiler room and equipment clean. Wipe up oil spills immediately. Remove all art cles that may present a fi re hazard. Store materials needed in the boiler room in such a way that they cannot cause an accident. Boiler Log Maintain the Boiler Room Log and record the various rout nes and tests performed.

4th Class • Part A2 Unit 14 • Chapter 65 • Routine & Emergency Boiler Operation 398

OBJECTIVE 2

Describe emergency conditions in boilers and the required responses.

EMERGENCY CONDITIONS

The following situat ons are rare occurrences if the boiler has been receiving proper rout ne operat on and care. Nevertheless, when upset condit ons do occur, they can develop rapidly into serious situat ons. The operator must be prepared to act calmly but effi ciently; this requires a prior knowledge of emergency procedures. The following list is again only general in nature. Every operator must become completely familiar, and at ease, with the specifi c procedures in his or her plant. Emergency condit ons include: • low water level • high water level • fan failure • fl ame failure Low Water Level A rapidly falling water level in the boiler may be caused by: • a faulty feedwater level controller. • feedwater or condensate pump failure. • interrupt on of the water supply to the pump. • leakage from the boiler due to ruptured tubes or open blow-off valves. Normally, when the level drops to the low-water cutoff point, the boiler will shut down automat cally. However, should the cutoff fail to shut the boiler down, the water level may drop to a dangerous level. If the operator fi nds the boiler in operat on while unable to see the water level in the gauge glass, the glass is either completely full or empty. This status should be checked quickly by opening the drain on the glass. If the level is found to be below the gauge glass, the boiler must be shut down immediately. CAUTION: Do not feed water into the boiler to raise the level and do not open the safety valve or vent valve to release pressure. A sudden fl ow of cool feedwater will quench any overheated heat ng surface which could cause a catastrophic failure. Let the boiler cool slowly unt l it is at hand-touch temperature. Then drain it and open it to inspect for damage due to overheat ng. If no damage is found, it can be closed up again and fi lled. However, it should not be put back into operat on unt l the causes of the feedwater shortage and the failure of the low-water cutoff s to shut the boiler down are found and corrected. If it appears that damage has been done, the Boiler Inspector must be not fi ed. High Water Level Should the water in the boiler gauge glass show higher than normal, or even climb out of sight, the level should be brought back to normal to prevent carry-over of the water with the steam. This adjustment can be made either by shut ng off the feedwater or condensate pump or, more quickly, by draining the excess water through the blow-off valves. Neither of these methods should be used to maintain normal water level while cont nuing boiler operat on. The fi rst method requires the boiler operator to manually control the operat on of the feedwater pump, while the second method wastes too much water. The cause of the trouble, usually a defect ve level control system, should be repaired as soon as possible. It may be necessary to put the level control system in the manual mode while the fault is being found and repaired.

4th Class • Part A2 Unit 14 • Chapter 65 • Routine & Emergency Boiler Operation 399

Fan Failure Should the fan or blower fail, the supply of combust on air will cease. The fuel supply must be shut off immediately to prevent a furnace explosion. Modern automat cally-fi red packaged boilers are equipped with a low-air cutoff switch that will cut off the power to the solenoid fuel valve as soon as the pressure at the blower discharge drops below the minimum safe value. Flame Failure Some of the reasons why the fl ame may fail during operat on include: • Insuffi cient fuel oil supply due to plugged fi lter • Water in the fuel oil • Excessive air supply • Insuffi cient gas pressure When a fl ame failure occurs, the burner fuel fl ow must be stopped immediately to prevent the furnace from fi lling with unburned fuel which could cause an explosion. Most boilers are now equipped with fl ame detect on devices which will shut off the fuel supply within a few seconds af er the fl ame fails. Regular test ng of the operat on of these devices is a must.

4th Class • Part A2 Unit 14 • Chapter 65 • Routine & Emergency Boiler Operation 400

OBJECTIVE 3

List types and causes of boiler accidents and explosions.

BOILER ACCIDENTS

Accident invest gat ons show that the great majority of accidents including boilers could have been prevented and that the number of such mishaps can be eff ect vely reduced through the proper applicat on of operat ng and maintenance logs and procedures. Some reasons that boiler accidents occur include: • The misconcept on that no supervision is required for automat cally-fi red boilers • The inexperience or lack of training of operat ng personnel • Inadequate maintenance of boilers and controls • Overheat ng of the heat ng surfaces due to a low water condit on caused by failure of operat ng or protect ve controls Although automat cally-fi red packaged boilers are well protected by automat c devices, these devices are only as good as the maintenance they receive. Since these boilers require lit le at ent on during operat on, regular checking of operat ng and protect ve devices is easily forgot en and the controls may become inoperable, unknown to the operator. Such a malfunct on, means trouble when adverse condit ons develop in the boiler operat on.

BOILER EXPLOSIONS

Boiler explosions may be listed under two general classifi cat ons: furnace explosions and pressure explosions. In both cases, the results of an explosion are almost always extensive damage to property and either personal injury or loss of life. The operator must know the basic causes of explosions and refrain from unsafe pract ces that lead to them. Furnace Explosions These are explosions which occur when an accumulat on of combust ble gases ignites and explodes within the furnace or gas passes of the boiler. Their causes include: • Insuffi cient purge of the furnace before light ng. According to insurance company stat st cs, this is st ll the number one cause of furnace explosions. • Admission of the fuel to the main burner before the pilot fl ame or other ignit on source is established • A weak pilot fl ame • Failure of the main fuel valve to close when the main burner fl ame is lost • An insuffi cient amount of combust on air result ng in incomplete combust on • At empt ng to light burners from hot refractory

4th Class • Part A2 Unit 14 • Chapter 65 • Routine & Emergency Boiler Operation 401

Pressure Explosions Pressure explosions occur when a pressure part of the boiler, such as the shell, watertube or fi retube, bursts due to too high steam pressure a structural weakening of the metal. The causes of this weakening include: • The eff ects of corrosion • Overstressing of the material due to heat ng the boiler up too quickly during start-up • Overheat ng of the heat ng surfaces due to a low water condit on (low-water fuel cutoff failure) • Scale and sludge build up • Failure of the boiler operat ng controls combined with an inoperat ve safety valve causing the pressure to rise far above the maximum allowable working pressure Even though most boilers are fully automat c, the boiler operator should pay regular at ent on to the operat on of the boiler and should conscient ously follow the instruct ons regarding test ng and maintenance of fi t ngs and controls in order to prevent an explosion. If an explosion does occur, the person in charge of the boiler or pressure vessel must not fy the proper authorit es. The Boiler Acts of the various jurisdict ons state that, in the event of an explosion, the person in charge must fully report all part culars concerning the explosion, including the exact place or locat on, names of persons killed or injured and the cause of the explosion if known. In addit on, the Acts state that nothing shall be moved or interfered with at the scene unt l an inspector has invest gated the accident and determined the cause of the explosion, unless it is for the purpose of saving life or limb, protect ng property or the removal of the dead.

4th Class • Part A2 Unit 14 • Chapter 65 • Routine & Emergency Boiler Operation 402

OBJECTIVE 4

Discuss the need for boiler operating and maintenance logs and the type of information that should be recorded.

BOILER OPERATING MAINTENANCE LOGS

The best method of determining whether adequate at ent on is being given to the boiler and its control equipment is to provide a boiler log, in which is recorded suffi cient informat on to indicate that the boiler is receiving the necessary at ent on. To be eff ect ve, a boiler log must provide a cont nuous record of boiler operat on, test ng and maintenance. It is common pract ce to use a weekly or monthly log sheet to record rout ne operat onal checks, tests and minor maintenance. The boiler and auxiliary equipment should be checked at regular intervals by a qualifi ed person and the protect ve and operat ng devices should be tested at suffi ciently frequent intervals to determine they are in good operat ng condit on. These checks should be recorded on the log sheet. Major maintenance jobs, test ng, adjustment of controls and safety valves and instruct ons for operators are commonly recorded in a log book. Table 1 is an example of a low-pressure heat ng boiler log sheet. Table 2 was developed by the Hart ord In- surance Company for use with small capacity boilers in a large building complex; these boilers are checked daily. Boiler operators can develop similar log sheets and include all data relevant to their boiler plant. A part al leaf of a boiler room logbook (Table 3) is also included. It shows some of the informat on that should be recorded.

4th Class • Part A2 Unit 14 • Chapter 65 • Routine & Emergency Boiler Operation 403

Table 1 Low-Pressure Heating Boiler Log

LOW-PRESSURE HEATING BOILER LOG COMPANY BUILDING LOCATION BOILER MAKE BOILER NO. WEEK 20

SUNDAY MONDAY TUESDAY WEDNESDAY THURSDAY FRIDAY SATURDAY

TIME AM PM AM PM AM PM AM PM AM PM AM PM AM PM

Steam Pressure Water Level CHECK Feed Pump Pressure AND Feed Water Pressure RECORD Flue Gas Temperature Oil Pressure Low Water Cut-Off TEST Feed Pump Control Gauge Glass & Column Feed Water Pump Cond. Tank Level CHECK Burner Operation Fuel Supply Dissolved Solids

TEST Alkalinity EACH Phosphate SHIFT Sulphite pH Blow-Off (Seconds) Make-Up Water Water Softener Safety Valves TEST or Water Filter CLEAN at Oil Filter least ONCE Oil Burner WEEKLY Ignition

MINOR REMARKS AM Remarks ONLY

USE LOG BOOK for PM Remarks EXTENSIVE DETAILS Operators Initials

4th Class • Part A2 Unit 14 • Chapter 65 • Routine & Emergency Boiler Operation 404

Table 2 Weekly Log Sheet

(Courtesy of Hartford Insurance Company)

Table 3 Sample Boiler Log Book Entries

4th Class • Part A2 Unit 14 • Chapter 65 • Routine & Emergency Boiler Operation 405

CHAPTER 65 - QUESTIONS ROUTINE EMERGENCY BOILER OPERATOR

1. When you fi rst not ce that the water level in the boiler gauge glass can no longer be seen, it is best fi rst to a) ensure that the boiler is not over fi lled. b) start the stand-by feed pump or injector. c) shut off the boiler fuel supply. d) close the main steam valve.

2. The principle cause of furnace explosions is a) safety valve is working. b) too much fuel; not enough air. c) purge cycle before light ng is not long enough or doesn’t occur. d) boiler is not ﬁ lled with water.

3. Pressure explosions occur because a) the gas pressure to the burner is too high. b) feedwater pressure is too high. c) safety valve failed. d) low boiler water level produces weakening of tubes.

4. Which is the best way to prevent an accident on a boiler with automat c controls? a) Control systems are fail-safe. b) Control systems should be checked and maintained regularly. c) Control systems should be computerized. d) PLCs in control systems never need maintenance and do not fail.

5. The best method to provide a cont nuous record for boiler operat on, maintenance, and test ng is to a) rely on the plant superintendent to record notes. b) use a personal log book. c) send emails to the plant superintendent. d) use an oﬃ cial plant log book on a daily basis.

Fourth Class • Part A2 Unit 14 • Chapter 65 • Routine & Emergency Boiler Operation 406

CHAPTER 65 - ANSWERS ROUTINE EMERGENCY BOILER OPERATION

1. (a)

2. (c)

3. (c)

4. (b)

5. (d)

Fourth Class • Part A2 4th Class • Part A2 U N I T 1 5

FEEDWATER TREATMENT

Chapter 66 External Feedwater Treatment 409

Chapter 67 Internal Feedwater Treatment & Testing Methods 429

407 408 4th Class • Part A2 C HAPTER 66

External Feedwater Treatment

LEARNING OUTCOME

When you complete this chapter you should be able to: Discuss the general principles, methods and equipment used in preparing raw feedwater for steam production in a boiler.

LEARNING OBJECTIVES

Here is what you should be able to do when you complete each objective:

1. Discuss the common impurities in raw water and their potential effects on a boiler.

2. Describe the various ways in which water is ﬁ ltered to remove suspended solids.

3. Describe the purpose, processes and equipment used in boiler water softening.

4. Describe the theory, process and equipment used in deaeration.

409 Unit 15 • Chapter 66 • External Feedwater Treatment 410

INTRODUCTION

Boiler feedwater is commonly treated before it enters the boiler. This process is called external treatment, and is achieved through the addit on of chemicals to the feedwater. Boiler water test ng is crit cal to the safe and eﬃ cient operat on of a steam plant. Poorly treated or untreated water can produce corrosion, sludge, pit ng, scaling and quite possibly catastrophic failure. This chapter describes the main impurit es found in raw water sources and explains the methods of external treatment such as ﬁ ltrat on, sof ening, demineralizat on, and deareat on.

4th Class • Part A2 Unit 15 • Chapter 66 • External Feedwater Treatment 411

OBJECTIVE 1

Discuss the common impurities in raw water and their potential effects on a boiler.

IMPURITIES

The water used in a boiler starts out in its raw state, containing many chemicals which are contaminants in the context of steam product on. Therefore, it is necessary to treat boiler feedwater in order to prevent: • sludge from deposit ng on boiler surfaces. • scale from forming on boiler surfaces. • corrosion of boiler metal. • carryover of impurit es with the steam leaving the boiler. The above four items are caused by impurit es in the water being fed to the boiler. These impurit es may be classed into the following main groups: • Suspended mat er • Dissolved solids • Dissolved gases • pH values

SUSPENDED MATTER

Suspended mat er is material not dissolved in the water, but which is fl oat ng on the surface or dispersed throughout the water. It consists of sand, mud, clay and organic material, such as sewage and vegetable mat er. These impurit es are usually quite visible in water. The sand, mud and clay will form sludge-like deposits in the bot om of the boiler shell, mud drums and headers. These deposits will keep the water away from the metal causing it to overheat. To prevent this sludge from forming, these impurit es can be removed by passing the water through fi lters or set ling tanks before it enters the boiler. Also, if any sludge does deposit in the boiler, it can be blown out through the boiler blowoff connect ons.

DISSOLVED SOLIDS

These solids are dissolved in the water and so are not visible. The ones causing the most trouble in the boiler are these compounds of calcium and magnesium:

Calcium bicarbonate Ca (HCO3)2

Magnesium bicarbonate Mg (HCO3)2

Calcium sulphate CaSO4

Magnesium sulphate MgSO4 These cause “hardness” of the water and deposit scale within the boiler.

4th Class • Part A2 Unit 15 • Chapter 66 • External Feedwater Treatment 412

The bicarbonates of calcium and magnesium cause temporary hardness because they can easily be removed from the water by heat ng to below its boiling point. The bicarbonates then drop out of solut on and deposit as a sof scale. If heat ng is carried out before the water enters the boiler, the sof scale will deposit in the feedwater heater. If the bicarbonates are not removed before the water enters the boiler, they will form a sof scale on the boiler surfaces. The sulphates of calcium and magnesium cause permanent hardness since these compounds cannot be removed by heat ng the water. They form a hard, dense scale on the boiler surfaces. Deposits of either the sof bicarbonate or the hard sulphate scale are undesirable as they will keep the water away from the metal of the boiler tubes and plates causing overheat ng of these surfaces. The dissolved solids, being in solut on in the water, cannot be removed by ﬁ lters or set ling tanks. Instead, water sof eners of various types are used to remove the hardness before the water enters the boiler. Also, chemicals can be added directly to the boiler feedwater to prevent format on of scale on the boiler surfaces. This will be discussed in the next chapter.

DISSOLVED GASES

The gaseous impurit es of concern are oxygen (O2) and carbon dioxide (CO2). These are dissolved in the water and can cause corrosion of the boiler metal and pipelines. Oxygen produces a special type of corrosion known as pit ng. These pits may be small or large and are usually covered by blisters of iron oxide. Carbon dioxide corrosion produces grooves in the metal and especially at acks pipe threads. These gases can be removed from the water before it enters the boiler by the use of a deaerator. In the deaerator, the water is heated to the boiling point, causing the gases to be driven oﬀ through a vent in the top. Chemicals added directly to the boiler can also be used to absorb dissolved oxygen and neutralize carbon dioxide. See the chapter on internal water treatment.

pH VALUES

The condit on of the boiler water in regard to its acidity or alkalinity is extremely important. If the water is acidic, corrosion of the boiler metal will occur at a rapid rate. On the other hand, if the water has high alkalinity, foaming within the boiler may occur and there is risk of embrit lement of the boiler metal. In order to indicate the degree of acidity or alkalinity of the boiler water, a scale of numbers, called pH values, from 0 to 14 is used. The pH value of the boiler water is determined by taking a sample and test ng it. • If the water is neutral (neither acidic nor alkaline), then its pH value is 7. • If the water is acidic, its pH value will be less than 7; the more acidic the water, the lower the pH value. • If the water is alkaline, the pH value will be greater than 7; the more alkaline, the greater the pH value. If the boiler water is acidic, then the boiler metal will corrode. Although a solut on with pH 7.0 is said to be neutral, it will st ll at ack steel in an acidic manner unt l the pH is increased to at least 8.5. It should be noted that pH is a logarithmic scale; a reduct on of one (1) in pH value is an increase in acidity of ten (10) t mes. To prevent corrosion, the boiler water should be kept alkaline. Water with a pH value of 10.5 usually provides the necessary alkalinity. Table 1 lists the complete scale of pH values.

4th Class • Part A2 Unit 15 • Chapter 66 • External Feedwater Treatment 413

Table 1 pH Scale

Solution pH Value

Most Acidic 0 1 2 Acidity decreases with increasing pH 3 4 5 Least Acidic 6 Neutral (neither acidic or alkaline) 7 Least Alkaline 8 9 10 Alkalinity increases with increasing pH 11 12 13 Most Alkaline 14

4th Class • Part A2 Unit 15 • Chapter 66 • External Feedwater Treatment 414

OBJECTIVE 2

Describe the various ways in which water is ﬁ ltered to remove suspended solids.

METHODS OF TREATMENT

The methods of treat ng boiler water can be divided into two main groups: • Internal treatment • External treatment Internal Treatment With modern water treatment, chemicals may be added to the water already in the boiler or in the feedwater system before the water enters the boiler. In some cases, chemicals are added to the steam as well. The addit on of these chemicals, regardless of where they are added, is considered to be internal feedwater treatment. Internal treatment of boiler water will be discussed in the following chapter. External Treatment External treatment is, tradit onally, the treatment of water before it enters the boiler. The methods of external water treatment that will be discussed in this chapter: • Filtrat on • Water sof ening • Deaerat ng Filtration Most municipalit es use large set ling basins and fi lters to remove suspended mat er from water before it is pumped into the water supply mains for distribut on to customers. These fi ltrat on systems are usually quite effi cient and there is no need for the customer to provide addit onal fi ltrat on. However, the waterworks in some small cit es and towns only remove the coarse impurit es. The water supply could st ll contain a certain amount of fi ner suspended solids. Thus, it will be necessary to fi lter the water before it can be used as boiler feedwater. Filtrat on also applies to industrial sites, where river or lake water is the prime source of feedwater. A few of the most common types of fi lters on the market include: Pressure• Pressure• • Filter-aid tubular (pressure type) C artridge• Cartridge• Pressure Filter A pressure fi lter, shown in Figure 1, consists of a closed cylindrical vessel. The water fl ows downward through graded layers of sand, anthracite or calcite supported by a gravel bed. Any suspended mat er in the water will be trapped by the porous material. The clarifi ed water is collected at the bot om. Af er a fi lter has been in service for a period of t me, its bed will become plugged with suspended mat er that has been removed from the water. When plugging occurs, the fi lter must be backwashed. Backwashing consists of reversing the direct on of fl ow through the fi lter, thus freeing the trapped material from the bed and washing it to waste.

4th Class • Part A2 Unit 15 • Chapter 66 • External Feedwater Treatment 415

Figure 1 Pressure Filter

(Courtesy of Betz) Filter-Aid Tubular Filter (Pressure Type) This fi lter (Fig. 2) consists of a closed cylindrical housing containing several screen-type cylindrical tubes covered by socks made of dacron or polyethylene fi lter cloth. A slurry of fi lter-aid (usually diatomaceous earth) is fed through the inlet at the beginning of the fi lter cycle. The fi lter-aid coats the tubes and serves as the fi lter medium. The upper part of the fi lter housing of the tubular fi lter is fi lled with air which is compressed by the water during the operat on of the fi lter. When the fi lter starts to clog and requires backwashing, the inlet and outlet valves are closed and the drain valve is opened. The compressed air in the top of the fi lter forces water through the tubes in the reverse direct on at high speed. This blasts the fi lter cake loose from the tubes and it is removed through the drain.

Figure 2 Tubular Filter-Aid Filter

4th Class • Part A2 Unit 15 • Chapter 66 • External Feedwater Treatment 416

Cartridge Filter Cartridge fi lters, shown in Figure 3, are widely used for the removal of suspended solids, especially fi ne solids. The fi lter element consists of one or more renewable cartridges made of various types of fi nely woven fi lter material process.

Figure 3 Cartridge Filter

When the fi ltered-out solids start clogging the fi lter and the pressure drop between the inlet and outlet reaches a specifi ed maximum value, the cartridge must be replaced. Filters are frequently used in conjunct on with set ling tanks. In this process most of the suspended impuri- t es are allowed to set le out in the set ling tank and the water is then passed through the fi lter to remove any remaining impurit es. The fi ne part cles, suspended in water, tend to carry a negat ve surface charge. Since they all have the same charge, they repel each other and therefore stay as small part cles in suspension. To overcome this, a chemical is added that holds a surface charge that is posit ve. This at racts the negat vely charged part cles causing them to collect together to form larger part cles. This chemical added is called a coagulant. This chemical combines with the set led solids and forms a blanket of spongy substance in the bot om sect on of the settling tank or clarifi er called a fl oc. These larger masses will set le out more readily. Flocculat ng agents are also added in some cases to increase the amount of fl oc formed. The most commonly used coagulants include:

• Aluminium sulphate Al2(SO4)3

• Aluminium hydroxide Al(OH)3

• Sodium aluminate Na2Al2O4

4th Class • Part A2 Unit 15 • Chapter 66 • External Feedwater Treatment 417

OBJECTIVE 3

Describe the purpose, processes and equipment used in boiler water softening.

BOILER WATER SOFTENING

Most dissolved solids are more soluble in hot water than cold water. The except on are the salts of calcium and magnesium which do not dissolve in water very well at all, but they dissolve bet er in cold water than hot. Water having dissolved calcium and magnesium compounds is referred to as “hard” water. Since cold water is pumped into the boiler and heated, the hardness salts tend to come out of solut on in the boiler on the heat ng surfaces where they cause scale format on. To minimize the format on of scale, these hardness compounds must be removed from the water before the water enters the boiler. The hardness causing salts can either be removed completely or changed to salts that are more soluble in hot water than in cold water. For example, sodium salts are very soluble in hot water. If the calcium and magnesium are removed and replaced with sodium, the water will no longer be hard and scale will not be formed. The process of removing the calcium and magnesium and replacing them with sodium is called water sof ening. The common types of water sof eners include: • Lime-soda • Sodium zeolite • Demineralizers Lime-Soda Softeners The lime-soda sof ener is a method of removing scale-forming dissolved solids, such as calcium and magnesium compounds, from the feedwater. Calcium hydroxide, Ca (OH)2 (lime) and sodium carbonate, Na2CO3 (soda), are added to the water, causing the scale-forming dissolved solids to precipitate (drop out of solut on) in the sof ener. The sodium from the soda replaces calcium and magnesium dissolved in the water, making the water “sof ”. If, for example, there is calcium carbonate in the water entering the sof ener, it will be changed to sodium carbonate. Lime-soda sof eners are classiﬁ ed into two general types according to whether the water is heated or not during the process. Hot process sof ening refers to the type where the water is heated during the treatment, while Cold process sof ening is the name given to the type involving unheated water. Of the two, the hot process is more commonly used for boiler feedwater treatment. The sketch in Figure 4 represents the general arrangement of the hot process sof ener. The raw water enters at the top where it is heated by exhaust or live steam and mixes with the calcium hydroxide and sodium carbonate, which are also introduced at the top of the sof ener.

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Figure 4 Hot Process Lime-Soda Softener

(Courtesy of Betz) As the chemical react ons take place, precipitates form and set le as sludge at the bot om of the sedimenta- t on tank where they can be removed as required. Part of the sludge forms a blanket through which the water passes on its way to the outlet. Some of this sludge is recirculated to the top of the tank by means of a pump to aid in the format on of precipitates. The sludge is periodically blown down to remove the precipitates and the sof ened water, leaving the sof ener, is usually passed through a pressure ﬁ lter to remove any suspended mat er. An advantage of the hot process sof ener is that large quant t es of water can be treated in a relat vely small unit. The high temperature obtained during the process increases the eﬃ ciency of the chemical treatment. Sodium Zeolite Softening The sodium zeolite sof ener uses the principle of cat on exchange to convert scale-forming calcium and magnesium salts dissolved in the water into soluble, non-scale-forming sodium salts. When water contains certain compounds, such as salts in solut on, the compounds break down (dissociate) to form posit vely charged cat ons and negat vely charged anions. For example, if the water contains the scale-forming salt, calcium sulphate (CaSO4), then it will be in solut on in the form of Ca (calcium) cat ons and SO4 anions.

The same applies to the scale-forming magnesium salts, such as magnesium bicarbonate, Mg (HCO3)2. In solut on, this salt will be in the form of Mg (magnesium) cat ons and HCO3 anions. By removing the calcium and magnesium cat ons from the water and replacing them with sodium (Na) cat ons, the salts are converted into sodium salts, namely sodium bicarbonate (NaHCO3) and sodium sulphate (Na2SO4) which are highly soluble and do not produce scale in the boiler.

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The sodium zeolite sof ener consists of a pressure tank part ally ﬁ lled with a granular exchange material called zeolite, a styrene-based resin. The zeolite is charged with a large amount of sodium cat ons. When water containing calcium and magnesium salts is passed through the zeolite, it removes the Ca and Mg cat ons from the water and exchanges them for Na cat ons. In other words, a cat on exchange takes place result ng in the conversion of the hardness causing Ca and Mg salts into non-hardness causing soluble Na salts. Expressed in chemical formulae, using the let er Z to represent the zeolite material, the sof ening process by cat on exchange is as follows:

Ca(HCO3)2 + Na2Z → CaZ + 2NaHCO3 or MgSO4 + Na2Z → MgZ + Na2SO4 When the zeolite material has given up all its Na cat ons in exchange for the Ca and Mg cat ons, it has to be regenerated before it can resume the sof ening process. The regenerat on is done by removing the zeolite sof ener from service and ﬁ lling it with a strong sodium chloride (NaCl) solut on (brine). The zeolite then absorbs the Na cat ons from the brine and discards the Ca and Mg cat ons to the brine as shown in the following formulae:

CaZ + 2NaCl → Na2Z + CaCl2 or MgZ + 2NaCl → Na2Z + MgCl2 The brine, now containing the Ca and Mg cat ons, is then ﬂ ushed to the sewer and the zeolite bed is rinsed with water. The sof ener is then returned to service unt l regenerat on is again necessary. Figure 5 shows the arrangement of the zeolite sof ener together with a regenerant brine tank. The zeolite or exchange material is supported on a bed of gravel or anthracite and is contained in a steel pressure tank.

Figure 5 Zeolite Softener Arrangement

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The four main steps in the operat on of a zeolite sof ener include: Service• Service• • Backwash • Regenerat on R insing• Rinsing• Service (Softening) Referring to Figure 5, by opening valves A & E and then closing B, C, D and F, the hard water enters the top of the sof ener and travels downward through the bed of zeolite. The Ca and Mg ions of the salts in the water are exchanged for the Na ions held by the zeolite. The sof ened water leaves the sof ener at the bot om. Backwash When the zeolite becomes exhausted, the sof ener is taken out of service and backwashed by manipulat ng the valves so that raw water enters at the bot om and fl ows upward through the bed to the wash water collector and then to waste. The backwashing serves to separate and clean the bed. Regeneration By opening valves D & F and then closing A, B, C & E, untreated or raw water is admit ed to an ejector or eductor. The water fl owing through the ejector produces a vacuum, which draws the brine up from the regenerant or brine tank, and the brine is then forced into the sof ener just above the surface of the zeolite bed. The Na ions of the brine solut on exchange place with the Ca and Mg ions held by the zeolite. The regenerat on is then followed by a slow rinse where the brine is shut off but the water cont nues to fl ow through the eductor. This water fl ow pushes the brine through the sof ener at the same speed unt l all the brine has passes through and the resin has all been regenerated for the same length of t me. The brine, discharged to sewer, contains the calcium (Ca) and the magnesium (Mg) ions. Rinsing Rinsing, also called fast rinse, is done with water passing in through the top of the sof ener and down through the resin bed as when the unit is in service, except the water is discharged to waste. To accomplish this, valves A & D are opened and B, C, E & F are closed. This removes any residual brine and hardness ions remaining in the bed. When hardness tests taken on the water leaving the sof ener show that all salts have been rinsed out, the sof ener is put back into service. Instead of having a number of valves to control the operat on of the sof ener, as shown in Figure 5, many sof eners have a master “mult port” valve. Figure 6 illustrates how the various operat ons of the sof ener are controlled by placing a single lever in certain posit ons.

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Figure 6 Master Valve Positions

SERVICE - Raw water enters BACKWASH - Raw water enters valve, flows from top of tank to valve, flows from bottom of tank to bottom where softened water is top, returns to valve and out to collected and directed through drain. valve to service.

REGENERATION - Raw water RINSING - Raw water enters enters valve, draws in brine, flows valve, flows from top of tank to down through the bed to bottom of bottom where water is directed tank, returns to valve and out to through valve to drain. drain.

Troubleshooting of Zeolite Softeners The capacity of zeolite sof eners depends on such factors as: • quant ty, type and condit on of the exchange material • amount of dissolved minerals in the water • amount and strength of regenerant used • mechanical condit on of the sof ener and regenerat ng equipment

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Problems may be encountered in the operat on of a sof ener. These problems are usually indicated either by a reduct on in capacity or incomplete sof ening of the water. The causes of these problems can be divided into the following groups: 1. A change in water quality - surface waters usually change in hardness from season to season which aff ects sof ener capacity. For example, if a sodium zeolite sof ener is rated to sof en 11 350 L of water when the hardness of the raw or untreated water is 120 ppm (parts per million), and the hardness of the water increases to 160 ppm, the capacity of the sof ener will be reduced to: 120 ____ x 11 350 L = 8513 L 160 On the other hand, a drop in hardness would increase the capacity. For example, if the hardness drops to 100 ppm, the capacity of the above sof ener would increase to: 120 ____ x 11 350 L = 13 620 L 100 Changes in capacity due to varying hardness in the raw water are to be expected and should not worry the operator. 2. Improper fl ow rates - the maximum fl ow rates during the sof ening and backwash cycles should not exceed those recommended by the manufacturer. An excessive fl ow during sof ening results in insuffi cient sof ening. Excess fl ow during backwash will cause zeolite material to be washed out of the sof ener. 3. Improper brine inject on during regenerat on - the amount and strength of the brine injected into the sof ener during regenerat on should be as recommended by the manufacturer. Errat c brine inject on can be caused by an insuffi cient amount of salt for dissolut on in the tank, a worn injector, a defect ve fl oat valve in the tank or a malfunct oning sequence t mer. 4. Fouled exchange material - contaminants in the supply water, such as suspended solids, iron, oil or microbiological growth in the zeolite bed hamper the proper react on in the sof ener result ng in reduced output or improper sof ening. 5. Mechanical defects - broken or plugged baffl es, distributors or collect ng headers cause poor distribut on of water or brine solut on and channelling. The result is again reduced capacity and poor sof ening. Leaking valves cause contaminat on of the sof ened water by the raw water. 6. Loss of exchange material by at rit on - there is a slow wearing down of the surface of the beads. The amounts of fi nes produced and washed out of the sof ener is typically about 3% per year of the total amount of beads, result ng in a corresponding drop in capacity of 3% per year. Demineralizers Demineralizers, used when mineral-free water is required, consist of two ion exchangers, as shown in Figure 7. The fi rst exchanger is a hydrogen cat on exchanger which works on the same principle as the sodium zeolite exchanger described above; however, instead of exchanging the cat ons of the salts in the water for sodium ions, the exchange material supplies hydrogen ions. The exchanger not only removes calcium and magnesium cat ons, but it also removes sodium and all other cat ons as shown in the following formulae:

CaSO4 + H2Z → CaZ + H2SO4

Ca(HCO3)2 + H2Z → CaZ + 2H2O + 2CO2

2NaCl + H2Z → Na2Z + 2HCl As seen in these formulae, the salts dissolved in the water entering the cat on exchanger are converted into acids, such as sulphuric and hydrochloric acid, as well as into carbon dioxide and water.

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Figure 7 Demineralizer Arrangement

The acid containing water now enters the second exchanger which contains anion exchange material charged with hydroxide (OH)- anions. When the water passes through the exchange bed; the anions of the acids (acid radicals) are exchanged for hydroxide anions.

H2SO4 + Z(OH)2 → ZSO4 + 2H2O

2HCl + Z(OH)2 → ZCl2 + 2H2O Summing up the process in a demineralizer: 1. In the cat on exchanger, the cat ons of the salts in the water are exchanged for hydrogen ions, thus

convert ng the salts into acids, CO2 and water. 2. In the anion exchanger, the anions of the acids in the water are exchanged for hydroxide ions, thus convert ng the acids into water. 3. The water leaves the demineralizer pract cally free of minerals. When the exchange materials in the ion exchangers become exhausted, they have to be regenerated in a similar manner as described for the sodium zeolite sof ener. However, instead of using a brine solut on, the cat on exchanger is recharged by passing a diluted solut on of sulphuric acid through the bed. The following formula shows one of the react ons:

CaZ + H2SO4 → H2Z + CaSO4

Thus, the exchange material recharges by reject ng the Ca, Mg and Na cat ons and at ract ng the H2 cat ons from the acid. The salts formed during regenerat on are rinsed out and discharged to waste. The anion exchanger is regenerated by passing a diluted solut on of caust c soda through the bed, and the following chemical formula shows one of the react ons taking place:

ZSO4 + 2NaOH → Z(OH)2 + Na2SO4 This exchanger’s material rejects the acid radicals and at racts the hydroxide anions. The sodium salts formed are rinsed out and discharged to waste. Af er regenerat on of both exchangers, the demineralizer is ready for service.

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OBJECTIVE 4

Describe the theory, process and equipment used in deaeration.

DEAERATION

Dissolved oxygen and carbon dioxide in the feedwater cause corrosion in boilers. The term deaerat on refers to the removal of dissolved gases, such as oxygen and carbon dioxide, from the water by raising its temperature to the boiling point and allowing the released gases to be vented to the atmosphere. The process is assisted by scrubbing the water with a ﬂ ow of steam to sweep away the released gases from the water. The removal of these gases is carried out in a vessel known as a deaerator and, af er deaerat on, the water is pumped from the deaerator to the boiler. An outline sketch of a deaerator is shown in Figure 8 and a cutaway view is shown in Figure 9.

Figure 8 Deaerator (Outline)

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Figure 9 Deaerator (Cutaway)

Referring to Figures 8 and 9, the inlet water enters the heater compartment through spray nozzles and is heated by steam rising from the scrubber compartment. The heated water then ﬂ ows down into the scrubber sect on where it is scrubbed by the steam entering through a central pipe. The steam and the gases scrubbed from the water then rise through the heat ng compartment, where most of the steam is condensed, to the vent condenser chamber. The water spray in this chamber condenses any remaining steam and the gases then pass out through the vent. The steam used for heat ng and scrubbing of the water in most types of deaerators is exhaust steam from some source. If exhaust or waste steam is not available, live steam can be used af er reduct on of pressure. The operat ng pressure is usually about 70 to 100 kPa.

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CHAPTER 66 - QUESTIONS EXTERNAL FEEDWATER TREATMENT

1. Feedwater for a high-pressure boiler should be a) treated with phosphate. b) treated with phenolphthalein. c) treated city water. d) sof ened water.

2. The regenerant used to regenerate a cat on exchanger in a demineralizer is a) hydrochloric acid. b) sodium chloride. c) caust c soda. d) sulphuric acid.

3. A normal operat ng pressure range for a deaerator is ______kPa. a) 70 to 100 b) 30 to 50 c) 100 to 150 d) 10 to 25

4. Which of the following is not a reason to treat boiler water? a) prevent scale from forming on boiler surfaces b) prevent corrosion of boiler metal c) prevent the pressure in the boiler from going too high d) prevent sludge from deposit ng on boiler surfaces

5. Dissolved gases are removed in a deaerator by a) adding chemicals to dissolve the gases. b) raising the water temperature to the boiling point. c) passing the gases through an absorpt on material. d) adding cold water to absorb the gases.

6. Which of the following is not a type of coagulant? a) aluminium sulphate b) aluminium hydroxide c) sodium aluminate d) calcium chloride

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CHAPTER 66 - ANSWERS EXTERNAL FEEDWATER TREATMENT

1. (d)

2. (d)

3. (a)

4. (c)

5. (b)

6. (d)

Fourth Class • Part A2 4th Class • Part A2 C HAPTER 67

Internal Feedwater Treatment & Testing Methods

LEARNING OUTCOME

When you complete this chapter you should be able to: Discuss the general principles, methods and equipment used for the internal treatment of boiler water.

LEARNING OBJECTIVES

Here is what you should be able to do when you complete each objective:

1. Describe the types of problems associated with internal boiler water contamination and their treatment.

2. Describe internal boiler feedwater chemical feed systems.

3. List and describe the standard boiler water tests and what they measure.

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INTRODUCTION

Boiler feedwater can be treated af er it enters the boiler. This process is called internal treatment and is achieved through the addit on of chemicals to the feedwater either in the boiler or before the water enters the boiler, such as in the deaerator. Boiler water test ng is crit cal to the safe and eﬃ cient operat on of a steam plant. Poorly treated or untreated water can produce corrosion, sludge, pit ng, scaling and quite possibly catastrophic failure. Six of the most common boiler water tests will be described in this chapter.

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OBJECTIVE 1

Describe the type of problems associated with internal boiler water contamination and their treatment.

INTERNAL TREATMENT

Internal treatment of boiler feedwater refers to the method whereby the water is sof ened, deaerated, and condit oned by the addit on of chemicals to the water af er it has entered the boiler. For most low capacity, low-pressure heat ng boilers, internal treatment is the only method used to prevent scale, corrosion and sludge format on. The makeup water for larger boilers and high-pressure boilers is usually treated externally. However, this treatment is seldom perfect and small amounts of impurit es may st ll enter the boiler, making it necessary to apply some addit onal internal treatment. When low-pressure steam heat ng boilers are used mainly for the generat on of steam for plant or building heat ng, pract cally all the steam condensate is returned from the heat ng system to the boiler; very lit le addit onal water (makeup) is required af er the init al ﬁ lling of the boiler. The internal water treatment for these boilers is usually quite simple. When part of the steam produced in the boiler is used for purposes other than heat ng, result ng in a reduced amount of condensate returning to the boiler, the makeup requirement will increase considerably and internal water treatment will be more extensive. Low-pressure hot water boilers form part of closed circulat ng heat ng systems. In these systems, the makeup requirements are usually very low and internal treatment requires only proper pH control and corrosion prevent on. High-pressure boilers are more subject to corrosion and overheat ng, and so both external and internal water treatments are likely to be extensive and closely monitored. Chemicals Used for Internal Treatment Some of the chemicals commonly used for internal boiler water treatment are listed in Table 1. Chemicals used for water treatment are usually not sold by their generic names. Most water treatment companies sell the chemicals by trade names and numbers. They are supplied in solid form as balls or briquet es, as a powder, or as a liquid. Of en, several chemicals are combined to serve mult purpose treatment. For example, phosphate briquet es, used to prevent scale forming in the boiler, contain sludge condit oning as well as ant foam chemicals in proper proport ons. For the operator, these combinat ons simplify the feeding of chemicals to the boiler.

Table 1 Internal Treatment Chemicals

Inorganic Organic

Sodium carbonate (soda ash) Lignin sulphonate Sodium chromate Sodium alginate Sodium hydroxide (caustic soda) Starch Sodium nitrite Tannins Sodium phosphate Polymers Sodium sulphite Sodium polyacrylate Sodium nitrilotriacetate

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It must be stressed that any program of water treatment should be undertaken only af er the water has been analyzed and the proper type of treatment has been determined by qualifi ed water treatment consultants. The results of improper water treatment include: • Caust c embrit lement of boiler metal • Foaming of boiler water • Corrosion • Scale format on causing overheat ng of boiler metal • Loosening of old scale which collects in a heap in one locat on • Sludge deposits The use of internal treatment for pH control, scale prevent on, sludge condit oning, chemical deaerat on and prevent on of foaming, caust c embrit lement and return line corrosion is discussed in the following sect ons. pH Control The pH value, a number between 0 and 14, indicates the degree of acidity or alkalinity of the water. A pH value of 7 is neutral, with values below 7 being acid and above 7, alkaline. If the boiler water is acidic or highly alkaline, then corrosion of the boiler metal will occur. To prevent this corrosion, the boiler water should be kept moderately alkaline; a pH value of about 10.5 usually provides the necessary alkalinity. A sodium hydroxide solut on is of en used to maintain proper alkalinity within the boiler and may be fed directly to the boiler drum or to the feedwater before it enters the drum. Phosphate compounds may also be used to increase the pH value. Scale Prevention To prevent calcium and magnesium compounds from forming scale within the boiler, it is necessary to precipitate these impurit es in the form of a sludge, which can be removed from the boiler by means of the blow-off . The chemicals added to the boiler water to cause this precipitat on include: • Sodium hydroxide • Sodium carbonate • Sodium phosphate • Sodium aluminate Sludge Conditioning The sludge produced by the precipitat ng of the scale forming compounds in the boiler must be condit oned so that it will stay fl uid and well dispersed. In this state, the sludge will be easily removed from the boiler through the blowoff connect ons. The chemicals used for this purpose are usually organic compounds such as: starch, tannin, lignin and alginates. Starch is produced from corn or potatoes, lignin and tannin from wood and alginates from seaweed. These organic materials coat the scale precipitates, prevent ng them from ad- hering to the boiler plates and tubes. Chemical Deaeration Mechanical deaerat on is used as the primary method for removing corrosive dissolved gases from the feedwater,. There are usually some traces of oxygen remaining in the water af er mechanical deaerat on, they can be removed by the use of chemicals fed to the boiler feedwater line or to the storage space of the deaerator. These chemicals have the ability to absorb the dissolved oxygen, a process called oxygen scavenging.

Two chemicals commonly used for this purpose are sodium sulphite (Na2SO3) and hydrazine (N2H4.) Sodium sulphite reacts with oxygen in the water producing sodium sulphate according to the equat on:

2Na2SO3 + O2 → 2Na2SO4

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In this way, the oxygen is removed from the water. Sodium sulphite is commonly used in boilers operat ng below 5000 kPa. One disadvantage is that this chemical added to the water to remove oxygen results in increasing the dissolved solids concentrat on in the boiler. This is undesirable for very high pressure boilers. Also, at high pressures, and therefore higher temperatures, sodium sulphate can break down and form weak solut ons of sulphuric acid result ng in corrosion.To avoid this, hydrazine is of en used for high pressure boilers. When react ng with water, hydrazine forms nitrogen and water as shown in the following equat on:

N2H4 + O2 → N2 + 2H2O Therefore, the dissolved solids concentrat on is not increased and the format on of acids is not possible with the addit on of hydrazine. However, hydrazine is a carcinogenic substance and must be handled with extreme care using proper safety gear. Prevention of Foaming A high concentrat on of dissolved and suspended solids in the boiler water will tend to cause foaming within the boiler. Organic materials in raw water can pass through external treatment and also cause foaming within the boiler. This result is very undesirable, as it can make the water level in the boiler diffi cult to determine. Foaming may cause the boiler to prime: carry over water with the steam leaving the boiler outlet. The high concentrat on of solids in the boiler water can be reduced by the use of the blow-off , while the tendency for the water to foam can be reduced by adding ant foam compounds to the boiler. Prevention of Caustic Embrittlement A disadvantage of adding sodium hydroxide (caust c soda) to the boiler for scale prevent on and pH control is that it may cause embrit lement and cracking of the boiler metal, part cularly at riveted seams and tube ends. This cracking occurs when metal under stress is at acked by a concentrated caust c solut on. For this problem to occur, there must be a point where steam may escape and cause the boiler water in that locat on to become concentrated. It has been found that the addit on of certain materials, such as sodium nitrate, lignins and tannins, to the boiler water is eff ect ve in prevent ng caust c embrit lement. Prevention of Return Line Corrosion Corrosion of return and steam lines is caused by oxygen and/or carbon dioxide in the presence of moisture. If both these gases are present, the corrosion will take place much more rapidly. But both carbon dioxide and oxygen can be removed from the feedwater by raising the water temperature to the boiling point in a deaerator. However, there is a possibility that more carbon dioxide may be released within the boiler by the decomposit on of bicarbonates contained in the water. In addit on, air may leak into condensate return lines, part cularly in vacuum and gravity systems, result ng in oxygen being absorbed by the condensate. Therefore, to prevent corrosion of return lines due to these gases, chemicals which pass off with the steam may be fed to the boiler. They either neutralize and raise the pH value of the condensate or else form a fi lm over the return line surfaces and thus prevent corrosion. These chemicals, known as amines are of the neutralizing or fi lming amine types.

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OBJECTIVE 2

Describe internal boiler feedwater chemical feed systems.

INTERNAL TREATMENT FEED SYSTEMS

The method used to supply the necessary chemicals to the boiler water is usually the employment of a small, posit ve displacement, motor driven pump. This pump may be situated to pump the chemicals directly to the boiler drum or, in some cases, to the feedwater line or the deaerator storage compartment. The pump, a high pressure, low capacity type, is usually designed so that the volume pumped may be varied by adjust ng the stroke of the pump. The chemical feed line to the boiler should be equipped with a shut-oﬀ valve next to the boiler, and a check valve. Also, as the pump is a posit ve displacement type, a relief valve is installed on the pump discharge line to prevent damage in case the shut-oﬀ valve is closed while the pump is running. Figure 1 shows the general arrangement of a chemical pump having an adjustable crank pin by means of which the pump stroke can be changed.

Figure 1 Adjustable Stroke Chemical Pump

Figure 2 illustrates the act on of the pump plunger within the cylinder during the suct on (a) and discharge (b) strokes.

Figure 2 Chemical Pump Cylinder

(a) Suction Stroke (b) Discharge Stroke

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Chemical Feeders Chemical feeders are required to feed the water treatment chemicals into either the feedwater line, the supply or return line of a hot water circulat ng system, or directly into the boiler. In general, the feeders used for closed heat ng systems with low makeup are quite simple since, af er the init al treatment of the water, only small quant t es of chemicals have to be fed into the system periodically. The bypass feeder, shown in Figure 3, is used to feed chemicals, supplied in briquet e or ball form, into the feedwater or circulat ng line. The briquet es or balls are placed in the pot and a slipstream of water ﬂ ows through the pot feeder. The water slowly dissolves the briquet e and carries the chemicals into the feedwater line.

Figure 3 Closed Type Bypass Feeder

The gravity drip feeder, Figure 4, is used to feed powdered or liquid chemicals into feed or circulat ng lines. The chemical solut on slowly drips by gravity into the pipeline and gradually washes into the system.

Figure 4 Closed Type Gravity Drip Feeder

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OBJECTIVE 3

List and describe the standard boiler water tests and what they measure.

TESTING METHODS

To determine the nature and amount of the impurit es present in the boiler water, or in the feedwater being supplied to the boiler, it is necessary to obtain samples of this water and subject them to various tests. The sample containers must be clean and should be rinsed out with water from the sampling line. Also, water should be allowed to run from the sampling line for suffi cient t me to ensure that any stagnant water in the line is not taken as the sample. The sample should be allowed to cool to room temperature and suspended solids should be fi ltered or allowed to set le out. The test ng of the sample should be done as soon as possible af er obtaining it. In the automat c sampling arrangement sketched in Figure 5, the sample line is connected to the cont nuous blowdown line and the sample water is obtained from this source. The sample water passes through a cooler where its temperature is reduced to about 24°C; it then fl ows to a sample header. From the header, the individual sample lines run to automat c analyzers which cont nuously test the samples for dissolved solids content, excess sulphite and excess phosphate.

Figure 5 Sampling and Cooling Arrangement

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However, in many plants, the tests are carried out manually rather than by automat c analyzers. The following are the most common types of tests: • Hardness Alkalinity• Alkalinity• • Dissolved solids pH• pH• • Sodium sulphite • Phosphate Hardness Test Hardness of the water is caused by impurit es or salts, such as those of calcium and magnesium, which are dissolved in the water. These will cause scale in the boiler unless properly treated. There are two methods to determine hardness: a soap test and a t trat on test. Titrat on refers to a chemical test where an indicator of some type is used to show a react on is completed and some material (such as hardness) has been used up. The react on is produced by adding a reagent (chemical) to the solut on being tested unt l the indicator changes colour (or some other indicat ng point is reached). The amount of reagent added is then determined and the amount of substance being tested for can be calculated. Generally, if the reagent is acidic, the solut on tested will be basic (or vice versa). Alkalinity Test The alkalinity of the water may be due to hydroxides, carbonates and bicarbonates of calcium, sodium and magnesium. The water can be tested to determine the amounts of these alkalies by using the following methods: • Phenolphthalein alkalinity • Total alkalinity or methyl orange Phenolphthalein Alkalinity Test This test indicates alkalinity due to carbonates and hydroxides dissolved in the water. It does not indicate alkalinity due to bicarbonates. Phenolphthalein is a liquid used as an indicator of carbonate and hydroxide alkalinity. When a drop of this indicator is added to a sample of the water, it will cause the water to become pink in colour, providing the water contains carbonates or hydroxides or both. To conduct the test, a small amount of phenolphthalein (indicator) is added to a measured sample of the water. If a pink colour appears, sulphuric acid (reagent) is added drop by drop (t trated) to the water unt l the pink colour just disappears. While this test is being done, the sample must be st rred constantly. The amount of acid required to make the pink colour just disappear will indicate the amount of alkalinity, known as phenolphthalein or “P” alkalinity. The sample is then put aside for the following test. Total Alkalinity or Methyl Orange Test This test is used to determine the amounts of all the dissolved materials which cause alkalinity of the water. The indicator used for this test is methyl orange; it will give a yellow colour to alkaline water. To perform this test, a small amount of methyl orange is added to the same sample as used in the previous test. If a yellow colour is produced, the sample is st ll alkaline due to bicarbonates present. Sulphuric acid is then added drop by drop unt l the yellow colour turns to a salmon pink. This change indicates that all the alkalinity has now been neutralized by the sulphuric acid and the total amount of acid used, namely, that used for the methyl orange test plus that used for the phenolphthalein test, will indicate the total alkalinity of the sample. The methyl orange alkalinity is called “M” alkalinity.

4th Class • Part A2 Unit 15 • Chapter 67 • Internal Feedwater Treatment 438

Dissolved Solids Test To determine the amount of dissolved solids in the water, the ability of the water to conduct an electric current is measured. The greater the amount of dissolved solids present in the water, the greater will be the conductance of the water. To perform this test, a sample of water is taken and a small amount of phenolphthalein is added. If a pink colour appears, sulphuric acid is added drop by drop unt l only a faint t nge of pink remains. This procedure is done to neutralize the hydroxide alkalinity as it has a very high conduct vity compared to neutral salts and, if not neutralized, would render the conductance test inaccurate. The conductance of the neutralized sample is now measured by means of an electrical instrument and the reading, when mult plied by a conversion factor, will give the concentrat on of dissolved solids, which will largely determine the amount of blow-off required. pH Testing The pH value of a water sample may be determined by the use of an electrical instrument known as a pH meter. Two electrodes are immersed in the sample and a voltage is supplied to the electrodes by means of a bat ery or a power pack. The voltage between the electrodes will vary according to the acidity or alkalinity in the sample. This voltage is indicated on the pH meter which is calibrated to read in pH numbers. Sodium Sulphite Test In cases where sodium sulphite is fed to a boiler to prevent pit ng due to dissolved oxygen, it is necessary to ascertain that suffi cient amounts of sulphite are supplied. Usually, if there is an excess of sulphite maintained in the boiler water, then the complete removal of any dissolved oxygen will be assured. In determining the amount of excess sulphite in the boiler water, the following procedure is used: Obtain a sample of boiler water without allowing it to come in contact with the air. Cool the sample to room temperature; but do not fi lter it. A measured amount of the sample is then put in a porcelain dish and turned slightly acidic by the addit on of sulphuric acid. A small amount of starch solut on is added to the sample and then a potassium-iodide-iodate solut on is added drop by drop unt l a permanent light blue colour is at- tained. During this procedure, the sample is st rred constantly. The amount of the potassium-iodide-iodate solut on necessary to produce the permanent light blue colour will indicate the excess sodium sulphite in the boiler water. Phosphate Test A common form of internal treatment for the prevent on of scale in a boiler is the addit on of sodium phosphate compounds to the boiler water. These will precipitate the scale forming materials as a sludge, which may be blown off from the boiler through the blow-off line. To ensure this precipitat on of the scale forming materials, it is necessary to have an excess of phosphate in the boiler. To determine the amount of excess phosphate present, a sample of boiler water is thoroughly fi ltered. A measured amount is then poured into a mixing tube and molybdate reagent is added. The tube is then stoppered and vigorously shaken. The next step is to add dilute stannous reagent, which has been freshly prepared from concentrated stannous reagent and dist lled water, to the mixture in the tube. This reagent will produce a blue colour and the lightness or darkness of the blue will indicate the amount of phosphate in the water. The tube is compared with standard coloured glass slides which are marked in amounts of phosphate. The apparatus used in all of the foregoing tests should be thoroughly cleaned af er using and then rinsed again with dist lled water or with part of the water to be tested just before the test ng is carried out. The test ng room or laboratory should be equipped with a sink, hot and cold running water, electrical outlets, equipment cabinets and the necessary desks and tables.

4th Class • Part A2 Unit 15 • Chapter 67 • Internal Feedwater Treatment & Testing Methods 439

CHAPTER 67 - QUESTIONS INTERNAL FEEDWATER TREATMENT TESTING METHODS

1. Two common chemicals used for deaerat on are Na2SO3 and a) NaOH.

b) H2SO4.

c) N2H4.

d) Na3 PO4.

2. Excess sulphite is necessary to prevent a) pit ng. b) hardness. c) carry over. d) acidity.

3. When conduct ng a TDS, or Total Dissolved Solids, test, the impurity which has a high conduct vity and must be neutralized is the a) acidic solut on. b) calcium alkalinity. c) magnesium chloride. d) hydroxide alkalinity.

4. Chemicals are usually fed to a boiler a) with a small centrifugal pump. b) into the top drum only. c) with a small posit ve displacement pump. d) into the bot om drum only.

5. Which of the following is an operat ng problem that could occur as a result of improper boiler water treatment? a) caust c embrit lement of the boiler metal b) foaming of the boiler water c) sludge deposits d) all of the above

6. A normal temperature for the sample on the exit of a sample cooler is a) 15°C. b) 10°C. c) 35°C. d) 24°C.

Fourth Class • Part A2 Unit 15 • Chapter 67 • Internal Feedwater Treatment & Testing Methods 440

CHAPTER 67 - ANSWERS INTERNAL FEEDWATER TREATMENT TESTING METHODS

1. (c)

2. (a)

3. (d)

4. (c)

5. (d)

6. (d)

Fourth Class • Part A2