Uniaxial Tensile and Creep Behavior of Omnisil Membranes in Membrane Based Wet Electrostatic Precipitator

UNIAXIAL TENSILE AND CREEP BEHAVIOR OF OMNISIL MEMBRANES IN MEMBRANE BASED WET ELECTROSTATIC PRECIPITATOR

A thesis presented to the faculty of the Russ College of Engineering and Technology of Ohio University

In partial fulfillment of the requirements for the degree Master of Science

Pavan Kumar Valavala August 2005

This thesis entitled UNIAXIAL TENSILE AND CREEP BEHAVIOR OF OMNISIL MEMBRANES IN MEMBRANE BASED WET ELECTROSTATIC PRECIPITATOR

BY PAVAN KUMAR VALAVALA

has been approved for the Department of Mechanical Engineering and the Russ College of Engineering and Technology by

David J. Bayless Associate Professor of Mechanical Engineering

Dennis Irwin Dean, Russ College of Engineering and Technology

VALAVALA, PAVAN K. M.S. August 2005. Mechanical Engineering

Uniaxial Tensile And Creep Behavior Of Omnisil Membranes In Membrane Based Wet

Electrostatic Precipitator (140 pp.)

Director of Thesis: David J. Bayless

ElectroStatic Precipitators (ESP) are widely used in coal fired power plants for control of particulate matter and other toxic gases. Traditionally, ESP’s used metal plates as collecting surfaces for capture of particles in flue gas. However, back corona and re- entrainment of particles into flue gases hinder with the performance of ESP in collection of fine particulate matter (PM2.5). Metal plate ESP’s also suffer from problems due to corrosion. Researchers at Ohio University developed a patented wet membrane based

ESP to overcomes these difficulties. Wet membrane based ESP replaces the metal collecting surfaces with woven fabrics. Omnisil 1000, a silica fabric (98.5% silica), is found to be suitable for this application. The replacement of metal collecting plates with fabric requires application of tensile loads on the fabric during operation of ESP. A study of tensile and creep behavior is presented. A test facility for uniaxial tension and creep testing of Omnisil fabric is developed. The uniaxial tensile behavior of the fabric is typical to plain woven fabrics and the failure strength bears a linear relation to fabric width. The creep elongation of the fabric is negligible in the experimental conditions.

Approved: David J. Bayless

Associate Professor of Mechanical Engineering

Dedicated to my parents, Ganapathi Rao and Saraswathi Valavala

Acknowledgements

Before getting into thick of things, I would like to express heart felt words to the people who were a part of this research in numerous ways, people who showed unrelenting support during my work at Ohio University.

My sincere thanks to Dr. David J. Bayless, for advising me on academic and personal issues throughout my master’s program; to Dr. Ben J. Stuart and Dr. Gregory G. Kremer for their valuable advice and guidance on numerous occasions and for accepting to be on my thesis committee. I am also indebted to Dr. Frank F. Kraft for his valuable advice on a variety of topics.

I am grateful to all the staff and students working at Ohio Coal Research Center (OCRC) for their support and assistance at various stages of this research. I would also like to thank many researchers and friends for their advice and encouragement. I would like to express my appreciation for all the readers of this manuscript, who I hope will find it useful and carry on the work with same dedication and integrity I have valued in all above mentioned people.

Finally, I wish to express my deepest gratitude and indebtedness to my family for supporting and encouraging me to pursue my dreams.

vii Table of Contents

Page Abstract...... iv Dedication...... v Acknowledgements...... vi List of Tables ...... x List of Figures...... xi

Chapter 1 Introduction ...... 1 1.1 Pollution Health Risks ...... 3 1.2 Particulate Air Pollution ...... 4 1.3 Pollution: Trends and Predictions...... 7 1.4 Technologies in Pollution Control...... 9 1.5 Wet Membrane ESP...... 10 1.6 Research Objective ...... 12 1.7 Significance of Membrane Creep in ESP...... 12

Chapter 2 Literature Review...... 15 2.1 Air Pollution Control Techniques...... 15 2.2 Electrostatic Precipitation ...... 15 2.3 Woven Fabric Mechanical Behavior ...... 22 2.4 Load-Extension Behavior of Woven Fabrics...... 22 2.5 Modeling Techniques for Prediction of Tensile Behavior...... 23 2.6 Short Term Tests...... 26 2.7 Long Term Tensile Testing ...... 32 2.8 Creep Testing with Discrete Load Increments...... 32 2.9 Relations between Creep Compliance and Stress relaxation Modulus...... 34 2.10 Accelerated Creep Testing...... 35 2.11 OmniSil...... 37

viii Chapter 3 Experimental Methodology...... 38 3.1 Tensile Tests on Omnisil ...... 38 3.2 Tinius-Olsen Machine ...... 39 3.3 Yarn Tensile Test...... 40 3.4 Strip Tensile Test ...... 41 3.5 Plane Strain Test ...... 43 3.6 Tensile Tests on MTS Machine ...... 44 3.7 Plane Strain Tests on MTS ...... 48 3.8 Creep Test on Omnisil...... 50 3.9 Creep Test Set-up...... 53 3.10 Mullen Burst Tests...... 64

Chapter 4 Results and Discussions ...... 67 4.1 Yarn Tensile Tests ...... 67 4.2 Strip Tensile Tests on Tinius-Olsen...... 68 4.3 Short Term Uniaxial Tensile Tests on MTS...... 70 4.4 Short Term Plane Strain Testing...... 79 4.5 Transducers in Creep Tests...... 81 4.6 Creep Tests: 8% Rupture Strength...... 84 4.7 Mullen Burst Tests...... 87 4.8 Creep Tests: 4% Rupture Strength...... 88 4.9 Creep Master Curves...... 93 4.10 Interpretation of Results...... 94 4.11 Voltage-Deformation Relations...... 98

Chapter 5 Conclusions ...... 101 5.1 Yarn Tensile Tests ...... 101 5.2 Strip Tensile Tests...... 102 5.3 Plane Strain Tension Tests...... 104 5.4 Failure of Creep Samples...... 105

ix 5.5 Modulus-Time Behavior of Viscoelastic Material ...... 108 5.6 Concluding Remarks...... 108

Chapter 6 Recommendations ...... 111 6.1 Tensile Testing...... 111 6.2 Creep Testing...... 114

References...... 118

List of Tables

Table Page

1.1 Estimation of deaths prevented by PM2.5 standard ...... 7 4.1 Summary of short term tensile tests ...... 78 6.1 Test matrix for determination of tension loads for keeping membrane taut ...... 114 6.2 Test matrix for creep tests ...... 116

xi List of Figures

Figure Page 1.1 Energy consumption by an individual per year ...... 2 1.2 Sources of energy production ...... 3 1.3 Particulate distributions in ambient air in urban areas ...... 6 1.4 Forecast of harmful emissions from power plants ...... 8 1.5 Predictions in increase of electricity production from various sources ...... 9 2.1 Principle of capture in ESP ...... 16 2.2 Schematic of a gas channel in a single stage ESP ...... 17 2.3 Schematic of an ESP ...... 18 2.4 Voltage-Current characteristics of corona discharge ...... 21 2.5 Load-elongation curve for two woven fabrics ...... 23 2.6 Geometry and mathematical model of unit cell in plain weave ...... 25 2.7 Three dimensional model of a unit cell for plain weave ...... 26 2.8 The concept of a digital chain ...... 26 2.9 Geometry and deformation pattern of sample in strip tensile test ...... 27 2.10 Geometry and deformation in a grab tensile test ...... 28 2.11 Schematic and deformation of fabric in Manchet test ...... 29 2.12 Biaxial tensile testing machine for fabrics ...... 30 2.13 Specialized equipment for biaxial tension test of fabrics ...... 31 2.14 Schematic of burst test on fabrics ...... 31 2.15 Principle involved in creep test ...... 32 2.16 Principle involved in a stress relaxation test ...... 33 2.17 Application of discrete stress increments in creep test ...... 34 2.18 Time-Temperature superposition applied to modulus prediction ...... 36 3.1 Tinius-Olsen machine (far right) with vertical specimen ...... 40 3.2 1 inch wide vertical sample in Tinius-Olsen machine with wooden tabs ...... 41 3.3 Schematic of Omnisil sample used for strip tensile tests ...... 42 3.4 Sample of Omnisil after being tested in Tinius-Olsen ...... 42

xii 3.5 Attachment for plane strain testing in Tinius-Olsen Machine ...... 43 3.6 Schematic of the MTS 810 test facility ...... 45 3.7Attachment to MTS 810 to facilitate testing of fabrics ...... 45 3.8 Half cylindrical rods used to grip fabric in the fixture during testing ...... 46 3.9 Fabric sample secured in the the cylindrical grips before test ...... 47 3.10 Uniaxial test of fabric in MTS 810 ...... 47 3.11 U-shaped bottom fixture to restrict cross-contraction ...... 48 3.12 Eight inch wide sample with loops stitched on sides to restrict cross-contraction ...49 3.13 Eight inch wide sample during plane strain tensile test ...... 49 3.15 Schematic for creep test of Omnisil fabric ...... 52 3.16 Creep test set-up ...... 53 3.17 The top frame of the outer structural member of the creep rig ...... 54 3.18 The bottom frame of creep rig ...... 54 3.19 High temperature polypropylene tank ...... 55 3.20 Resistance heater used to heat the water in the tank ...... 56 3.21 Magnetic drive pump used for pumping hot water to the samples ...... 56 3.22 Headers that deliver hot water to the samples and valves to control the flow ...... 57 3.23 The inside of the tank and the bulk heads that lead connecting rods to the weight hanger ...... 58 3.24 Weights used in the experiment (under the table that seats the creep rig) ...... 58 3.25 Clevis-pin arrangement which houses the clamps of the sample ...... 59 3.26 Clamps used for gripping samples ...... 59 3.27 Sample ready to be tested with LVDT’s attached ...... 60 3.28 Nut welded to the top clamp to which the LVDT is attached ...... 60 3.29 Screw welded to the bottom clamp to facilitate mounting an LVDT core connecting rod ...... 61 3.30 Double slotted plate in bottom clamp to hold the core of LVDT ...... 61 3.31 Sample ready to be tested with LVDT’s attached from the clamps ...... 62 3.32 Sample (in the back) during a test ...... 63 3.33 Header for supplying hot water to the samples ...... 63

xiii 3.34 Creep rig set-up insulated and sensors connected to DA system ...... 64 3.35 Picture of Model ‘A’ mullen tester ...... 65 3.36 Polypropylene samples after being tested for burst strength ...... 66 4.1 Load-elongation curve for yarns of Omnisil fabric ...... 67 4.2 Load-elongation for 1 inch uniaxial samples in fill direction of fabric ...... 68 4.3 Load-elongation for 1 inch uniaxial samples in warp direction of fabric ...... 69 4.4 Load-elongation curve for 1 inch uniaxial samples in fill direction of fabric ...... 71 4.5 Load-elongation curves for 2 inch uniaxial samples in fill direction of fabric ...... 72 4.6 Load-elongation curves for 4 inch uniaxial samples in fill direction of fabric ...... 73 4.7 Load-elongation curves for 8 inch uniaxial samples in fill direction of fabric ...... 74 4.8 Scatter in the rupture strength of fabric at various widths ...... 74 4.9 Standard deviation of rupture strength in sample with different widths in fill direction ...... 75 4.10 Load-elongation curve for 8 inch uniaxial samples in warp direction of fabric ...... 76 4.11 Load-elongation behavior of 4 inch wide plane strain samples in longitudinal direction of fabric ...... 80 4.12 Failure of plane strain sample due to shear forces ...... 80 4.13 Calibration curve of the LVDT used in creep test ...... 82 4.14 Calibration curve of the LVDT used in creep test ...... 82 4.15 Calibration curve of the LVDT used in creep test ...... 83 4.16 Calibration curve of the LVDT used in creep test ...... 83 4.17 Time-elongation data (long term) of fabric sample with 100 lbf load ...... 84 4.18 Time-elongation plot (short term1) of fabric sample with 100lbf load ...... 85 4.19 Time-elongation plot of (short term 2) sample at 100lbf load ...... 86 4.20 Burst strength of Omnisil fabric ...... 87 4.21 Time-elongation plot of (short term1) sample at 50lbf load ...... 88 4.22 Time-elongation plot of (short term2) sample with 50lbf load ...... 89 4.23 Time-elongation plot of (short term 4) sample with 50lbf load ...... 90 4.24 Time-elongation plot of (short term 5) sample with 50lbf load ...... 91 4.25 Time-elongation of (long term) fabric sample at 50lbf load ...... 92

xiv 4.26 Consolidated data five creep tests at 50 lbf load ...... 93 4.27 Log-Log plot of master creep curve for sample 5 for first 45 hours ...... 94 4.28 Log-Log plot of master creep curve for sample 5 for first 45 hours ...... 95 4.29 Two models of membrane deformation after elongation: Model I (left) & Model II (right) ...... 96 4.30 Deformed state of membrane for Model I after elongation ...... 97 4.31 Deformed state of membrane for Model II after elongation ...... 99 5.1 Regions in the load-elongation plot of Omnisil ...... 102 5.2 Yarn interactions at various stages of tensile test ...... 103 5.3 Linearity of the rupture strength with width of test sample in fill direction of fabric 104 5.4 shows the modulus temperature curve for viscoelastic materials ...... 106 5.5 Interaction between silica and water molecules ...... 107 5.6 Variation of modulus of a viscoelastic material at constant temperature with time..109 6.1 Recommendation for tensile testing ...... 111 6.2 Schematic for plane strain tensile test ...... 112 6.3 Schematic of a biaxial tension test on woven fabric ...... 113 6.4 Schematic of experimental set-up for tensile load determination ...... 116

1 1.0 INTRODUCTION

Energy is an essential ingredient in meeting basic needs, extending life expectancy and providing comfortable living. We took our first steps in the use of mechanical energy with the harnessing of wind and waterpower. Later, the industrial revolution lead to the use of coal and steam power, these technologies laid the foundations for today’s scientifically advanced society, with significant developments such as the internal combustion engines and large-scale power generation of electricity.

Energy is one of the most important basic inputs for economic and industrial development. There is an ever-increasing energy demand in developing countries. Demand for electric power, in particular, is increasing by far more than the general energy demand because of its versatility, high efficiency and ease of utilization. In the current industrialized countries of the world, the energy demand is between 150 and 350 gigajoules per person each year, an increasing of it in the form of electricity [2]. Figure 1.1 shows a classification of various forms of energy consumed by individuals. It is also seen that the energy consumption per person in developed countries is higher when compared to the rest of the world. Unites States of America (U.S) consumes around 25% of the world-wide electricity production [1]. In addition to the increase in energy demand per person each year, the increasing population results in an increased energy demand. According to a NSW/EPA report the average annual growth rates in the consumption of electricity have been about 6% in the 1970’s, about 4% in the 1980’s and about 1.5% in the 1990’s [4]. And the world population growth rate was recorded to be 1.4 percent in 2000 [5].

Coal was the first to be widely used industrially, on account of it ease of availability and low price, it played an important role in economic development and remains central for power generation, with a world production of 3.5 billion tones per year, most of this being used for electricity. It dominates the scene of with approximately 50% of electricity world wide being produced from coal. For many years, the U.S. has depended on coal-

2 fired power plants to meet the ever increasing demands for electricity [10-13]. However, coal based power generation has received wide spread criticism as one of the major causes of air pollution.

Figure 1.1 Energy consumption by an individual per year [2]

3 Newer air emissions standards over the past few years have resulted in the development of advanced technologies for reducing coal fired power plant emissions [8,15]. Air pollution has adversely affected the people leading to poor health and in some cases death.

Figure 1.2 Sources of energy production [3]

1.1 Pollution Health Risks

Many studies have shown that pollution has a great impact on the general health of people in the exposed areas. Coal burning for power generation leads to various kinds of

4 pollution. These forms of pollution can be broadly classified into particulate matter, mercury, carbon dioxide, sulfur and nitrogen oxides. Particulate Pollution – The particles produced during combustion of coal get suspended in the air resulting in increase of particulate matter in ambient air [1-6]. Carbon dioxide – Coal is a highly carbonaceous fuel and burning of coal results in formation of oxides of carbon like carbon monoxide and carbon dioxide. Power sector is the largest source of CO2 pollution [22]. Mercury – Coal-fired power plants are the nation's largest source of mercury contamination. Coal power plants account for more than 40% of mercury air pollution in the U.S. Mercury is highly volatile. and leads to bioaccumulation as methyl mercury in the environment. Mercury interferes with the development and function of the central nervous system [9-12]. Sulfur and Nitrogen Oxides – Sulfur and Nitrogen oxides produced during coal combustion lead to acid rain. EPA announced a new regulation cutting emissions of sulfur dioxide by about 73% and oxides of nitrogen pollution by 61% from power plants in East and Midwest states over the next 10 years in 1990 [8].

1.2 Particulate Air Pollution

Many health studies have reported link between particulate pollution to increased hospital and emergency room admissions, reductions in lung function, and premature deaths [6]. Studies have also revealed that people living in more polluted cities had an increased risk of premature death compared to those in cleaner cities. According to a NRDC estimate, the current levels of pollution will result in approximately 64,000 premature deaths from cardiopulmonary causes each year. These deaths will constitute around 6.5% of all cardiopulmonary deaths per year. It was found that the elderly and people with heart and lung disease are at greatest risk of premature mortality due to particulate pollution. According to an estimate their lives might be shortened by one to two years on average in more polluted areas [21]. Although, the exact toxicological mechanisms involved in health effects are not well understood, but researchers have a number of theories. For

5 instance, some studies have shown that particulate matter in air causes respiratory symptoms, changes in lung function, alteration of mucociliary clearance, and pulmonary inflammation which can lead to increased permeability of the lungs. Increased permeability of the lungs can result in precipitation of fluid in the lungs in people with heart disease [21]. In addition, mediators released during an inflammatory response can increase the risks of blood clot formation and strokes. Particulate exposure can also increase susceptibility to bacterial or viral respiratory infections, leading to severe cases of pneumonia in vulnerable members of the population. Some potential mechanisms can be related to impairment of clearance mechanisms or immune system function. In case of people with pre-existing heart disease, acute bronchiolitis or pneumonia induced by air pollutants can result in congestive heart failure. Expose to particulate matter in air can also aggravate the severity of underlying chronic lung disease, causing more frequent or severe exacerbation of airways disease or more rapid loss of lung function [6].

1.2.1 PM2.5 Vs PM10

Air quality and sampling standards for particulate matter in the U.S. were first set in 1971. The sampling is done through the determination of total suspended particulate matter in a high-volume sample that is drawn from ambient air through a large filter at about 50 cubic feet per minute. The air quality standard required an annual average of less than 75 µg/m3. Figure 1.3 shows the distribution of particle sizes typically found in urban areas [7].

3 The PM10 standard requires an annual average concentration of less than 50 µg/m . PM- 10 refers to particles smaller than 10µm in diameter. In 1997 EPA adopted new standards and sampling methods for particles smaller than 2.5µm, called PM2.5. The standard for 3 PM2.5 is an annual average of 15 µg/m . The cutoff at 2.5µm is approximately the diving size between the coarse and fine particle modes as shown in figure 1.3 [7].

6 According to a report larger particles are mostly filtered out by the nose. Mid-sized particles are deposited in the airways are generally trapped on a layer of mucus which carries them to the throat, where they are either coughed up or swallowed. Only particles less than 2µm (approx.) reach the lungs, where they must be dealt with macrophages from the immune system of the body [23].

The particles in ambient air are mixed together. They can be characterized into three modes: the nuclei, accumulation, and coarse particle modes. The particles in each mode have different composition and are generally from different sources. The nuclei and accumulation modes constitute the fine particulate matter. The nuclei mode consists primarily of combustion particles. These are very fine particles and attach rapidly to other fin particles in the accumulation mode. The accumulation mode includes combustion and photochemical smog particles and attached nuclei mode particles [7].

Figure 1.3 Particulate distributions in ambient air in urban areas [7]

7 The coarse particles consist of windblown particles and other mechanically generated particles such as from construction sites. These large particles settle out relatively quickly. There is little mass exchanged between the accumulation and coarse particle modes. Particle size is important for health effects because it controls where in the respiratory system a given particle deposits. Particles that are a few micrometers in size are able to reach and deposit in the sensitive, gas-exchange region of the lung [7]. Table 1.1 gives an estimate (range) of the number of deaths that can be prevented with various prescribed regulation for PM2.5 standard.

Table 1.1 Estimation of deaths prevented by PM2.5 standard [6] Estimate of PM Annual Standard 2.5 Regulation Cardiopulmonary Deaths (µg/m3) Prevented Point Estimate NRDC Recommended 10 33,570-76,874 Level New Lower End of EPA 12.5 22,599-50,957 Range Former Lower End of 15 13,160-29,151 EPA Range New Upper End of EPA 20 2,895-6,134 Range Former Upper End of 30 70-137 EPA Range

1.3 Pollution: Trends and Predictions

Energy Information Administration (EIA) was created by Congress in 1977; it is a statistical agency of the U.S. Department of Energy. It provides independent data, forecasts, and analyses to promote policy making, and public understanding of energy market and its interaction with the economy and the environment. The data presented in this section is extracted from the United States General Accounting Office (GAO) report

8 to congressional requesters (2002). Figure 1.4 presents a forecast of emission of harmful gases from power plants by year 2020 based on reference data of year 2000 [27].

Figure 1.4 Forecast of harmful emissions from power plants [27]

According to the data modeled by EIA there will be an overall increase in electricity production from 3.4 trillion kilowatts to 5 trillion kilowatts resulting in a 42% increase as shown in figure 1.5. It can also be seen from figure 1.5 that there is an increase in use fossil fuels to meet this energy requirement which in turn will result in more particulate pollution in addition to the toxic gases as shown in figure 1.4.

Figure 1.5 Predictions in increase of electricity production from various sources [27]

1.4 Technologies in Pollution Control

“As deregulation of the utility market proceeds and the nation's energy requirements increase, the need for cost-effective and environmentally compliant technologies will also increase” [1]. During recent years, much emphasis has been placed on the development of air pollution control technologies that will allow the continued use of coal as an energy source while meeting the stringent requirements of the Clean Air Act (CAA) Amendments of 1990 [8]. Coal is an extremely polluting and carbon-intensive energy source. Burning coal for energy significantly contributes to acid rain and greenhouse gas build-up in the atmosphere, as well as mercury and soot pollution. Electrostatic

10 Precipitators (ESP) account for the large majority of utility particulate controls in the United States (EPRI 1991). ESP’s have become popular with the utility industry on account of various advantages they have over any other competent technologies. ESP’s are very efficient (up to 99% efficiency), even for small particles. They are designed to handle wet and dry gas compositions for a wide range of gas temperatures (250F-750F) and also handle large volumes of gas flow with low-pressure drop. In addition to all the other advantages ESP’s are economical than other particulate control devices.

1.5 Wet Membrane Electrostatic Precipitator (ESP)

The conventional ESP's use metal plates for collection surfaces. Particles in carrier gas entering the ESP are separated with an electrostatic charge. An ion field generated by high-voltage corona charges the particles, which migrate and are accumulated on grounded plates. The plates are cleaned via rapping, in dry or the conventional ESP’s. In a wet ESP the collection surfaces are continuous wetted to wash the particulates into a hopper. The dry ESP’s have certain drawbacks as compared to the wet ESP. Some of these have an adverse effect on the performance. Particle accumulation (Back Corona) on the collecting plates and re-entrainment of particulates during rapping of electrodes inhibit to attain maximum efficiency in operational conditions. In addition, the presence of toxic gases can result in corrosion of the collecting metal surfaces. In view of the above mentioned drawbacks, dry ESP’s may not exhibit high operational efficiencies in capturing very fine particulate matter.

The recent research conducted at Ohio University (OU) focused on replacing the carbon steel and stainless steel collecting plates with woven membranes made of carbon fiber and silica-fiber, Omnisil [23-26]. The use of woven membrane for collecting surface can overcome some of the problems associated with use of metal plates. These woven membranes have high mechanical strength, operate at high temperatures and are corrosion resistant. In addition they possess high dust collection efficiency and have monetary advantages. The fact that the woven membranes are wetted with a continuous

11 film of water assures an ideal collecting surface without the problems of particulate accumulation or particle re-entrainment. In this system higher flow rates of circulating water can be schieved since the woven fabric effectively distributes water uniformly over the entire surface of the membrane without the splashing or dripping as experienced in case of metal collecting surface that could disrupt the electric field, causing spark-over and eventual grounding of the field.

Over the last few years, researchers at Ohio University have been investigating into the use of membrane based dry and wet ESP’s for capture of heavy metals (like Hg0 and Hg2+) and collection of very fine particulate matter (PM 2.5) [23-26]. The current research at OU includes testing a bench scale wet membrane based ESP for evaluating its efficiency in capture of elemental mercury in conjunction with high oxidizing systems, this ESP is also being used for the capture of soluble oxidized Hg. In addition, a novel bench scale Laminar ESP is also being developed for efficient capture of the fine particulates, which utilizes carbon fabric in the pre-charger and silica fabric in the collection area. Laminar ESP aims at maintaining a laminar flow of the flue gas through the ESP reducing turbulence in the collection area to avoid re-entrainment of the fine particulate into the gas stream.

The electric field in an ESP plays a very important role in the high efficiencies of particle capture. Unlike metal plate ESP’s the membrane based ESP’s would rely on tensioning of membranes for maintaining a constant distance between the collecting surfaces and the charging electrodes. Since the operating conditions of the ESP would typically be above the room temperature, elongation of the membranes as a result of creep can culminate in reduced spark over voltages or field strengths and thereby, poor collection efficiencies. It is thus imperative that we understand the creep behavior of membranes for efficient operation of membrane based ESP’s.

12 1.6 Research Objective

The structural properties of the membrane material has a great influence on maintaining the optimum distance between the charging electrodes and the collecting surface, which determines the performance of membrane ESP. The objective of this research is to investigate into uniaxial tensile behavior of the silica fabric, namely Omnisil™ 1000 and to perform an accelerated (baseline) creep test on the fabric to determine elongation behavior over longer periods of time. This work also involves design and development of test facilities for studying the behavior of Omnisil™ 1000 fabric under uniaxial tension and elongation behavior under constant load (tensile) at elevated temperatures over extended periods of time.

1.7 Significance of Membrane Creep in ESP

The importance of the creep behavior is critical to the performance of an ESP. The creep of the membrane has a direct effect on the spacing between the charging electrodes and collection surface. The influence of the electrode-collecting surface spacing on the particulate capture efficiency of the ESP can be established mathematically as shown below [32-34, 36, and 37]: C η = 1− exit Cinlet

qhere η is the collection efficiency of ESP, Cexit is the concentration of particles at outlet, and Cinlet is the concentration of particles at inlet

C  −  exit = wA exp  Cinlet  Q  − η = −  wA  1 exp   Q 

13 where w is the migration velocity, A is the cross sectional area of the duct, and Q is the volume flow rate.

The expression for the migration velocity w can be obtained by equating the forces acting on the particle to be captured, namely, electrostatic force and the drag force. The electrostatic force is due to the field present in the ESP that is responsible for capture of particle and the drag force is a consequence of the fluid (or) flue gas. = Fe qE 1 F = C A ρ w2 D 2 D P f p 24 where,C = D C Re 24 Re = ρ  C f DP w     µ  Also, = Fe FD qEC w = πµ 3 DP where Fe and FD are the electrostatic force and drag force acting on dust particle respectively, Re is the reynolds number, Dp is the diameter of particle, E is the electric field Strength, Ap is the projected area of particle, ρf is the density of fluid/flue gas, V is the voltage applied to the ESP, and d is the spacing between the electrode and collecting surface. η = f (w) w = f (E) ⇒ η = f (E)

But typically the field in an ESP is controlled by voltage applied, so V= constant; V E = d 1 Eα d ⇒ η = f ()d

From the above relations, we can see that the field strength follows an inverse relation with spacing between electrode and the collecting surface. However, at a critical minimum spacing the medium in between the electrode and the collecting surface breaks down electrically and the medium starts to conduct electricity thereby grounding the field. The critical field strength at which this breakdown occurs is known as the dielectric strength of the medium. So the performance of an ESP is dependent upon the distance between the electrode and collecting surface. A study of the electrode-collecting surface spacing can help maintaining a safe field strength resulting in efficient performance of ESP.

15 2.0 LITERATURE REVIEW

2.1 Air Pollution Control Techniques

There exist a variety of pollution control devices for control of specific types of emission from coal power plants. The following techniques are widely used for emission controls from combustion [28]: • Cyclones -- Used for capture of coarse particulate matter. • Fabric filters -- Used for capture of coarse and fine particulate matter. • Electrostatic precipitators –Used for capture of coarse and fine particulate matter. • Venturi-scrubbers -- Used for both particulate matter and acidic gases. • Wet scrubbers -- Used primarily for control of acidic gases, ionizing wet scrubbers control particulate matter as well as acid gases. • Spray dryers and dry scrubbers -- Used for acid gas control. • Hybrid wet/dry scrubbing systems -- Used for both acid gas and particulate control. • Flue gas cooling -- Includes techniques such as water quench, air dilution, waste heat boilers, or heat exchangers. • Other metals control techniques -- Including activated carbon used for volatile metals and organics control, and specific mercury control techniques including selenium coated filters, sodium sulfide injection, and mercury scrubbers. • Catalytic oxidation -- Used for organics control. • Sulfur-based control -- Used for organics control, particularly for PCDD/PCDF.

2.2 Electrostatic Precipitation

An electrostatic precipitator (ESP) is an air pollution control device that removes suspended particles and aerosols from a flowing gas stream with electric forces. The electrical force acts for the most part on the particles and not the gas. Thus, the pressure drop is very low compared to other control devices [35]. ESP’s are used in industry in steel mills, pulp/paper plants, cement kilns, waste incineration, coal-fired electric power

16 plants, and indoors in homes and offices. In 1906, Dr. Frederick Cottrell, a professor of physical chemistry at Berkeley, designed the first operational ESP. Electrostatic precipitation occurs in a series of steps which can be classified into four distinct categories. The process of capture of particulate matter can be categorized into electrical charging of suspended particles and later particle migration toward collecting electrodes due to electrostatic attraction and deposition of particles on collecting plates and finally removal of the collected particles by either knocking them off or washing them off with water [32-34]. Figure 2.1 shows the top view schematic of the working principle in an ESP indicating the process of capture of particles.

Figure 2.1 Principle of capture in ESP [29]

As the flue gas enters the electrostatic precipitator, it encounters a high-voltage direct- current field, the high voltage maintained between electrodes charges the suspended particles [33]. In Cottrell’s single stage precipitators, the discharge of electricity (corona) is emitted from the discharge electrodes such as thin wires, which are maintained at a high negative potential. Once the particles in the gas stream become highly charged due to the presence of corona, they are driven to the collecting electrodes, usually plates, by the intense electric field. Figure 2.2 shows the schematic of one of the gas channels in Cottrell’s single stage ESP. However, in practice a large number of these channels make an ESP.

Figure 2.2 Schematic of a gas channel in a single stage ESP [30]

To obtain high efficiencies, precipitators often comprise three or more fields in series, each being electrically energized to optimize individual field performance, this is also known as the multistage ESP [31]. The final step is removal of the particles from the collecting plates to an external receptacle. The collecting plates are mechanically rapped at set time intervals to dislodge the collected particles. The particles fall as agglomerates into hoppers below the plates and are removed. Re-entrainment of these particles significantly affects electrostatic precipitator efficiency. Figure 2.3 shows the schematic of a typical ESP with some of the accessories like hoppers for collecting the entrained particles from gas stream after rapping the collecting plates.

During the last two decades, development and use of electrostatic precipitators in the United States have been influenced by federal legislation. The Clean Air Act Amendments of 1977 and 1990 give the U.S. Environmental Protection Agency the power to enforce more stringent regulations on industrial pollutants such as acid rain precursors (SOx and NOx), numerous hazardous materials like carcinogenic organic and heavy metals; volatile organic compounds and ozone precursors like CO and NOx; and total suspended particulates (TSP). Particles of particular concern are those less than 10

18 µm in diameter (PM 10) which reduce visibility and those less than 2.5 µm in diameter (PM 2.5) which adversely affect human health as described in Chapter 1. The need to provide effective removal of both particles and gaseous pollutants from flue gas introduces new features in the determination of the best technology for industrial air pollution control and has complicated the design of the ESP’s.

Figure 2.3 Schematic of an ESP [29]

ESP’s account for majority of utility particulate control devices in the United States. In 1991, EPRI reported that 95% of all utility air pollution controls include ESP’s. Present precipitators are cold-side, wire/plate designs with rigid discharge electrodes and 12 inch to 16 inch plate spacing. Hot-side precipitators were installed as a first attempt at solving the problem of collecting low-sulfur/high-resistivity fly ash. Structural problems associated with the extreme heat (800 F). The wire/plate configuration consists of an array of high-voltage discharge wires located midway between grounded, parallel-plate collecting electrodes. A large ESP for a power plant would consist of many of these flow channels. Electrodes are either hung from the top of the ESP or supported in a frame. A

19 corona discharge is established by applying a high dc voltage. Since the radius of curvature of the wire is small compared to the distance from the wires to the plates, the electric field strength (E) is very high near the wire and an electrical breakdown of the gas near the wire is possible without sparking over to the grounded plates.

Industrial precipitators typically use negative corona because higher voltages can be applied before spark over. The negative corona is the source of negative ions, which migrate across the inter-electrode spacing along the electric field lines. Transport of the particles to the collector plates is determined by the combined influences of the electric force and the interaction of the particles with the turbulent gas stream. Particles moving with the gas stream, as the gas passes between the wires and plates, acquire a negative charge by field and diffusion charging. The charge on the particles results in an electric force on the particles. Electrical forces on large particles ( 10 µm) are much greater than on smaller particles. The mathematical relationship between the applied voltage and the efficiency is derived in Chapter 1.

Particle collection is disrupted if back corona occurs in the collected material. With high resistivity particles, localized areas of high electric field in the collected dust layer can cause electrical breakdown. Back corona is associated with low-sulfur coals, which have relatively high resistivities. For this reason, many utilities in the southwestern U.S. use bag houses. Since resistivity decreases with decreasing temperature, temperature- controlled pre-charging is one promising technique for reducing back-corona in ESP.

Once the particles are collected, either mechanical rapping of the plates or continuous or intermittent washing accomplishes removal of particulate matter from collecting surface. Mechanical rapping dislodges the collected material, which then falls along the plates into hoppers located below. The disadvantage of this method is that some of the particles dislodged from the plates are re-entrained in the bulk gas flow and must be recollected. Low frequency rapping and reduction of gas turbulence can reduce rapping-induced re- entrainment. The re-entrainment problem is most critical in the final sections of the

20 precipitator just before the outlet to stack. The problems associated with re-entrainment are eliminated in a wet precipitator and also back corona is reduced.

The electric field strength at which corona begins has been studied extensively. Ideally the field required to initiate corona would be equal to that which will produce electron energies sufficient to cause ionizing collisions in the gas. The field required for the initialization of corona discharge will depend on the ionization potential of the gas and the mean free path between collisions. Peek established a semi-empirical equation for the required electric field to initiate corona discharge in air [38].  δ  E = 3×106 f δ + 0.03 V / m c    a  where, f is the surface roughness of the electrode, f=1 for smooth surfaces; δ  is the

To P relative density of the air, which is defined as ; To is a reference temperature, 293 K; TPo

T is the actual temperature of the air, K; Po is the reference atmospheric pressure, 760 Torr; P is the actual atmospheric pressure in Torr and a is the radius of the discharge electrode in meters.

The electrical characteristics of a precipitator are very important to the process of charging particles and in collecting the particles on the grounded or collection plates. These electrical characteristics can be described in part by the voltage-current characteristic. A typical voltage–current curve of a corona discharge is shown in figure 2.4.

Figure 2.4 Voltage-Current characteristics of corona discharge [40]

Once the applied voltage is higher than the corona onset point, the corona current will gradually increase with the increase of the voltage. The maximum possible voltage produces a spark discharge. The stable corona discharge lies between the initiation point and the spark over point. It is necessary to know the electric field and current in order to determine the ability of a particular design to remove particles from the air stream as it requires a stable corona at all times.

The spacing between the collecting surfaces and the discharge electrode is of importance, as any decrease in spacing would lead to spark over thereby grounding of charge. Unlike the conventional ESP’s, which consist of rigid metal collecting surfaces, the membrane based ESP’s use a fabric as a collecting surface. The fabric is tensioned to maintain a uniform distance from electrode, the operating conditions of the ESP can result in creep in the fabric resulting in decrease in the distance from the discharge electrode thus a potential for spark over.

22 2.3 Woven Fabric Mechanical Behavior

The physical behavior of woven fabrics can be determined by a variety of experimental and modeling methods. The experimental techniques encompass determinations of large number of physical properties like uniaxial and biaxial tensile behavior, tearing and burst strength, water permeability, resistance to chemicals etc. The tensile behavior of woven fabrics can be modeled by different approaches. For small deformations, an approach based on Castigliano’s theorem could be used and for large deformations a force deformation or an energy approach are generally used. The following sections will describe some of the experimental and modeling techniques used for determination of tensile properties of woven fabrics.

2.4 Load-Extension Behavior of Woven Fabrics

A typical load-extension curve of a woven fabric consists of two distinct regions. The initial region non linear region caused by resistance to bending of yarns in the transverse direction to application of load, produced by friction between the fibers along the direction of load and the direction perpendicular to it. After this region, the yarns in the fabric extend and are likely to be governed by the ease with which the yarns bend. In the final section the yarns have straightened and the fabric modulus is affected mainly by the extension of the yarns. Figure 2.5 shows the typical load-elongation behavior of a woven fabric. The tensile properties of woven fabrics are thus determined by the tensile and bending behavior of the yarns constituting the fabric [41-53].

Figure 2.5 Load-elongation curve for two woven fabrics [74]

2.5 Modeling Techniques for Prediction of Tensile Behavior

Woven fabrics are subjected to tensile extensions and deformations in their practical use and the prediction of the behavior under tensile loads is important for fabric application. The textile woven fabrics can be considered as continuous medium for all analysis however the fabrics do not obey Hookean laws but follow a non-linear stress-strain relationship in the deformation analysis.

Kawabata has reported the possibility of applying the linear elasticity theory to textile fabrics with a simple modification [41-44]. In this theory he suggested that strains could be transformed into “linearized strain” to obtain linear relation between stress and the linearized strain [45].

When the area of application of stress is large compared to the granularity of a material, the material can be treated as a continuum. Fabrics can be considered two dimensional

24 continua. The continuum mechanics literature distinguishes three classes of two dimensional bodies: plates, shells and membranes [62]. Plates are planar in the unrestrained state and resist bending and twisting. Shells are plates with a curved unrestrained state. Membranes are plates with perfect flexibility i.e. they do not offer resistance to bending or twisting. Some researchers have proposed a pseudo continuum model, in which the mechanical properties of the fabrics are formulated by introducing a new strain-displacement relationship instead of conventional material modeling based on the constitutive law [54]. This model has the ability of being used with finite elements.

The force-elongation behavior can be evaluated through tensile experiments or by theoretical models representing the behavior of fabrics under similar loading. Pierce was the first to attempt develop a theoretical model for fabric based on its microstructure [51]. His model assumed an initial structure of a fabric composed of uniform yarns with circular cross-section, incompressible and perfectly flexible. Figure 2.6 shows the mathematical model of repeat unit in plain weave fabric as described by Pierce. This model has some short-coming in describing shear behavior. Like most of the models it does not take hysteresis into account. Although this model could not describe the tensile behavior of fabrics, it constitutes the basis of mathematical representation of fabric. Several improved models were developed later which described tensile behavior of fabrics.

Olofsson and Leaf et.al. developed a model that took into account that the cross section of the yarn may not be perfectly circular and that the yarns can be in a complex state of deformation [52, 53, 58]. He introduced the form factor as a new parameter in the modeling procedure. Grosberg et.al. [55-56, 58] considered two extreme cases of tension in fabrics: a state where the yarns are initially straight and the state in which the crimped state is set into yarns. This model was applicable only to biaxial tension. This model took into account of the rigidity of yarn bending to calculate fabric properties. Kawabata’s model used the structure of the fabric identical to that of Pierce, except that he represented it in a different way to solve the biaxial tension problem [41-43]. Kawabata

25 used the same structure as for biaxial tension, to solve the uniaxial case. He used an empirical approach to evaluate the behavior in uniaxial and biaxial tension.

Figure 2.6 Geometry and mathematical model of unit cell in plain weave [51]

The behavior of fabrics on macro scale can be modeled from their fibrous mesoscale models. Figure 2.7 shows the 3D model of a unit cell of a plain weave. Meso-scale modeling involves 3D finite element analysis of a unit cell from the fabric of interest. The advantage of meso-scale modeling is that non-linear modeling can be performed on the unit cell to predict the behavior of the fabric. Sagar et al. used energy methods to model uniaxial and biaxial behavior of woven fabrics using meso-scale modeling with the help of ABAQUS software [58].

A new technique of modeling was introduced by Wang et al. [61] for textile processing called “Digital Element Analysis”. This technique assumes a hierarchical approach of modeling that fabric constitutes of yarns and yarns constitute of fibers. Each fiber is considered as a frictionless pin connected through rod element. The rod elements with frictionless pins are known as “digital chain”. Figure 2.8 shows the representation of a

26 digital element. Wang et al. used this technique for simulation of 2D weaving and 3D braiding process.

Figure 2.7 Three dimensional model of a unit cell for plain weave [58]

Figure 2.8 The concept of a digital chain [60, 61]

2.6 Short Term Tests

The tensile properties of the fabrics can be evaluated from different kinds of test each resulting in different properties. Some of the properties that can be determined from a tensile test are maximum strain, maximum stress or load, tangent modulus, secant

27 modulus, burst strength etc. A test can be chosen depending on the property of interest. The following are some of the tests used with fabrics [63]:

Yarn Tensile Test: Short term tensile tests can be performed on the yarns of the fabric. Many semi-empirical mathematical models use the data obtained from the yarn tensile test for predicting the behavior of the fabric. The load on the yarn is generally applied through a constant displacement rate of the fixtures. Different characteristics can be obtained from this test i.e. stress and strain at failure, tangent and secant moduli. “Tangent Modulus” is the slope of the stress–strain curve at any point and “Secant Modulus” is the slope of the stress-strain curve between two points. Strip Tensile Test: In this test a relatively narrow strip of a textile is stretched between two parallel clamps with at a constant displacement rate along its length until failure [66]. Figure 2.9 shows the geometry and deformation pattern in a strip test. In some cases, the sample fails in the grip. These values may not be a good representation of the strength of the fabric as stiff clamping mechanisms lead to development of lateral forces next to the clamps and resulting in failure. Specialized clamps can be used to avoid this problem. This test is not suitable for fabrics that are highly sensitive to cross-contraction.

Figure 2.9 Geometry and deformation pattern of sample in strip tensile test [63]

28 Grab Tensile Test: This test is similar to the strip tensile test in principle except that the fabric sample extends wider than the clamps [66]. The deformation pattern is complicated due to presence of shear forces. Figure 2.10 shows the geometry and deformation pattern in a grab tensile test. Interpretation of the results from this test is complicated and is not widely used for testing textile fabrics.

Figure 2.10 Geometry and deformation in a grab tensile test [63]

Manchet Tensile Test: Unlike the previous tests this test employs a tubular specimen with a water tight membrane enclosed inside it. The water tight membrane inside the sample applies an internal pressure during the test. This test largely is dependent on the longitudinal seam in the sample. This test is very tedious to carry out however the results show more important characteristics with regard to the tensile properties in diagonal

29 direction. Figure 2.11 shows the geometry and deformation in a manchet test. A state of plane strain is induced in the sample to some extent [64].

Figure 2.11 Schematic and deformation of fabric in Manchet test [63]

Plane Strain Tensile Test: This test involves application of uniaxial strain along the length of the sample while restricting the cross-contraction. This type of deformation induces a state of plane strain the fabric. This test is very time consuming and complex as restricting cross-contraction leads to local relocation and damage of fibers at the lateral restraints [65]. Wide Width Tensile Test: This is a uniaxial tensile test. Unlike the strip tests this test involves a testing a wide sample typically 200mm wide. This test can also be used as an alternative to machete test or plane strain test for some ratios of width to length of the sample [66, 67]. Wide width test is suitable for an index for non-woven fabrics. However,

30 for high strength fabrics this test is impractical due to the limitations of maximum achievable load on test equipment. Biaxial Tensile Test: In practice, many applications result in biaxial load under operational conditions of the fabric. Although some of the previous tensile tests attempt to produce plane strain to some extent, they cannot be considered true biaxial tests. Biaxial tests are conducted on cross shaped specimens. Both strain controlled and force controlled tests can be conducted with specialized biaxial test equipment. Figure 2.12 and 2.13 show two different configurations of biaxial tensile test equipment. A high degree of alignment is required in these machines to avoid shear forces.

Figure 2.12 Biaxial tensile testing machine for fabrics [69]

Figure 2.13 Specialized equipment for biaxial tension test of fabrics [68]

Burst Test: The sample is clamped in a ring shaped jaws and a membrane applies pressure on the fabric until failure through the opening in the ring. The burst strength does not provide any useful design data but it can provide valuable information about the strength of the fabric.

Figure 2.14 Schematic of burst test on fabrics [63]

32 2.7 Long Term Tensile Testing

Most fabrics exhibit a viscoelastic behavior. The tensile characteristics of viscoelastic materials are highly time dependent. The long term tensile properties can be determined through creep or stress relaxation experiments. Creep tests involve study of deformation as a function of time under the action of constant load. Figure 2.15 shows the schematic of the principle involved in a creep or constant load experiment.

Figure 2.15 Principle involved in creep test

Stress relaxation involves study of load relaxation as a function of time under the influence of constant deformation. Figure 2.16 shows the principle involved in a stress relaxation experiment. The equipment required for stress relaxation experiments are more complicated than the ones used for creep tests.

2.8 Creep Testing with Discrete Load Increments:

The actual operational condition in an ESP might demand re-tensioning of the membranes after a definite period of operation. In which case, the principle of superposition by Boltzmann can be used to evaluate the creep behavior of the fabric. Figure 2.17 shows the representation of Boltzmann superposition principle in case of stress increments after definite time interval in a creep experiment.

Figure 2.16 Principle involved in a stress relaxation test [63]

The creep compliance relating the stress and strain in a creep experiment is given by the equation below; γ ()= σ () t 0 J t

The stress σ0 is applied instantaneously at time equal to zero. According to Boltzmann principle of superposition if the stress is applied at a time when t 0, but at some other arbitrary time, u1. Then the above equation would yield γ ()= σ (− ) t 0 J t u1 In case of multiple stress increments at different times after starting the experiments,

n γ ()= σ (− ) t ∑ i J t ui i=1 ∞ ∂J()a γ ()t = J (0 )σ ()t + σ (t − a ) da ∫ ∂ 0 a Similarly, for a constant strain experiment

∞ ∂G()a σ ()t = G (0 )γ ()t + γ (t − a ) da ∫ ∂ 0 a

Figure 2.17 Application of discrete stress increments in creep test [73]

2.9 Relationship between Creep Compliance and Stress Relaxation Modulus

Creep experiments are simple in operation and generally do not involve complex equipment unlike stress relaxation experiments. Creep compliance is the modulus (slope of load-elongation graph) for a constant load test while stress relaxation modulus is the modulus for a constant strain test. However, a relationship can be used to relate the creep compliance and stress relaxation modulus of a material. This relation is one of the direct consequences of the Boltzmann superposition principle. In time-independent material behavior, compliance and the modulus are simply the reciprocals of each other. This simple relationship does not hold in the time-dependent case as in viscoelastic materials. The relationship between creep compliance and stress relaxation modulus, J(t) and G(t) respectively, can be obtained as follows [73],

∞ − pt ∞ ∂J ()a L()γ ()t = J ()()0 L()σ t + e σ (t − a ) dadt ∫ ∫ ∂ 0 0 a

35 ∞ where, L()f ()t = ∫ e − pt f ()t dt is the laplace transform of the function, f(t) 0 L()γ ()t = pL()()σ ()t L()J t Similarly, L()σ ()t = pL()γ ()t L()G ()t The above equations become, 1 = L[]G()t L[]J ()t p 2 Applying Borel’s theorem,

t t = ∫G()(τ J t −τ )dτ 0

2.10 Accelerated Creep Testing

The main design concern for long-term stability of membrane based ESP is the prediction of creep behavior of membranes under the operational conditions. The long term elongation should not affect the distance between the charging electrode and the collection surface. Creep performance for long duration can be predicted in accelerated creep tests with tests lasting for shorter period of time. In these test loads are applied at a temperatures higher than the operational conditions to accelerate the material deformation. Then using time-temperature superposition principle the deformation at the operational temperatures of ESP can be calculated [69, 70]. Mathematically, it can be represented as follows:

 t  ()= εε   0 TtT ,,   aT  where, ε is creep strain, To is an arbitrary reference temperature, and T is the shifted temperature. aT is the horizontal distance shifted along the time t (or) shift factor

36 The results from the accelerated tests can be extrapolated using shift factors calculated from the relation developed by Williams, Landel and Ferry [71]. This mathematical relation is also known as WLF equation. C (T − T ) log()a = 1 g T + ()− C2 T Tg where, C1 and C2 are constants that change slightly according to type the type of polymer and Tg is the reference temperature.

Figure 2.18 Time-Temperature superposition applied to modulus prediction [73]

Figure 2.18 shows the application of time-temperature superposition principle applied to prediction of modulus of a viscoelastic material. The curves at reference temperature T3 is predicted by from the data obtained from tests conducted at T1-T7 for shorter periods of time. The horizontal shift factors are obtained from WLF equation to construct the complete modulus-temperature behavior at T3.

37 2.11 Omnisil

Omnisil is a silica fabric with 98.5% of SiO2 and is fire retardant. Omnisil fabrics are inert to most chemicals with the exception of HF, H3PO4 and most alkaline solutions. The applications of Omnisil include use as welding and fire blanket, furnace curtains, thermocouple insulation, high temperature composites etc. Omnisil 1000 is a 12HS woven fabric with 43 yarns/inch in warp direction and 33 yarns/inch in fill direction. Omnisil fabric can be used at continuous temperatures of 2000°F and the fabric vaporizes at 3100°F [76].

38 3.0 EXPERIMENTAL METHODOLOGY

As mentioned earlier in Chapter 1, the distance between the collecting membranes and the discharge electrodes plays an important role in maintaining a constant electric field in the ESP and thus help in charging and consequent capture of the particulates in the flue gas stream. This necessitates the need for the membrane to be taut at all times while under operation in an ESP to avoid decrease in electrode-collecting surface spacing and eventual grounding of the electric field. In order to keep the membranes taut there is a need for application of tensioning load on the membranes in longitudinal as well as transverse direction i.e. biaxial in-plane load.

Since the ESP’s operate above the room temperature (typically at 50-60oC) and are subjected to loads, creeping of the collecting surface can be expected to effect the tension in the membranes over period of time. Thus it is essential to understand and study the creep behavior of membrane materials. Omnisil (silica based fabric) has proven to be suitable for application in membrane based ESP on account of its excellent wetting and re-wetting characteristics and good resistance to corrosion. This chapter will discuss the various experimental procedures adopted to study some of the structural characteristics of Omnisil.

3.1 Tensile Tests on Omnisil

Before the study of creep behavior of the fabric is attempted, it is essential to understand the short term tensile behavior of Omnisil at room temperature as it would form the basis of comparison of the tensile behavior at elevated temperatures. The emphasis of this study is to understand the uniaxial tensile behavior of the fabric.

A number of uniaxial tensile tests were performed on Omnisil™ fabric on samples of 1 inch, 2 inch, 4 inch and 8 inch wide samples in the transverse direction of weave (fill direction of fabric) and 1 inch and 8 inch wide samples in the longitudinal direction of the

39 fabric weave (warp direction of fabric). The short tensile testing of yarns and 1 inch samples (or the strip tensile test) were performed on a Tinius-Olsen machine and the other samples were tested on a MTS 810 machine. The samples considered for all tests were based on the guidelines from the ASTM D-35 committee; however, some modifications were incorporated to facilitate the use of available equipment.

3.2 Tinius-Olsen Machine

Tinius-Olsen machine was used for tensile testing of yarns and 1 inch wide samples. An attempt was made to plane stain tension tests on 3 inch wide samples by the use of an attachment to restrict cross-contraction, in which transverse contraction was restricted with the help of side rods which were secured in place using a nut and a bolt arrangement and load in the longitudinal direction was applied by the clamps on the machine with were moved at a constant rate to apply load. The plane strain testing using Tinius-Olsen was not very practicable and did not yield any useable results as most of the samples failed due to seam slippage at the lateral restraints or failure at the clamps.

A Model H5K – UTS Tinius-Olsen machine with a PC based data acquisition system was used for these tests. Figure 3.1 shows the picture of the Tinius-Olsen machine. It is a single column materials testing machine with a frame capacity of 5kN or 1,000lbf. This machine is designed to test a wide range of materials from paper, plastics to medical products in tension, compression, flexure, shear and peel. Figure 3.2 shows the picture of the clamping mechanism and a sample being tested in the machine. The application of load is achieved through a screw driven mechanism. A motor on the machine drives the screw at different speeds to achieve variable rates of loading. The top clamp of the fixture is fixed to the structure and is stationary during the test. The bottom clamp moves at a constant speed resulting in application of load. The fixture in the machine clamps the wooden tabs on the sample and the primary mode of load transfer from the clamps of the machine to wooden tabs of the sample is through friction. The load cell on the machine

40 records the load on the sample while the elongation is obtained from the displacement of the bottom clamp.

Tinius-Olsen

Data Acquisition

Figure 3.1 Tinius-Olsen machine (far right) with vertical specimen

3.3 Yarn Tensile Test

A tensile test (modified ASTM D2256) was performed on the yarns of Omnisil fabric. The yarns were obtained from the fabric. The yarns were clamped in the Tinius Olsen machine with the help of a special arrangement using two rollers in each of the clamps. The load was applied through the constant rate displacement of the clamps. A Data Acquisition system was used to obtain the load-elongation data till failure. These results can be used in the modeling of tensile behavior of fabric.

Figure 3.2 1 inch wide vertical sample in Tinius-Olsen machine with wooden tabs

3.4 Strip Tensile Test

A modified strip test (ASTM 15.03) was used for tensile testing of 1 inch wide Omnisil™ 1000 samples Figures 3.3 shows the top and side views of the sample used for strip tensile tests. The objective of this test was to obtain a load-elongation behavior of the sample. For this purpose, four samples of 1 inch width were cut along the direction perpendicular (transverse) and parallel (longitudinal) to the length of fabric.

Figure 3.4 shows the picture of a sample after tensile test. It can be seen that the sample failed in the test section and thus the results obtained can be considered as a representative of the strength of the fabric. The failure occurred due to breakage of the yarns in the direction of test.

Figure 3.3 Schematic of Omnisil sample used for strip tensile tests

Figure 3-4Sample of Omnisil after being tested in Tinius-Olsen

The test and sample specifications are as follows Sample length: 10 inch x 1 inch

43 Actual test length: 153 mm x 2.54 mm (6 inch x 1 inch approx) Loading rate: 20 mm/min

3.5 Plane Strain Test

An attachment to the Tinius-Olsen machine was developed to facilitate plane strain testing, but this arrangement did not produce any useful results. Figure 3.5 shows the picture of the attachment developed to Tinius-Olsen machine to facilitate plane strain loading. It consists of two parts: a top fixture and a bottom fixture, a similar set up was used with MTS 810 machine for further short term tensile testing of Omnisil, with a modification in the fabric gripping or clamping technique to avoid damage to the sample due to stress concentration and development of shear forces during testing. The bottom fixture is U-shaped to facilitate the arrangement of lateral restraining mechanism. The restraining mechanism is fastened to the parallel vertical sides of the U-shaped bottom for structural rigidity.

Top Fixture

U-Shaped Bottom Fixture

Figure3.5 Attachment for plane strain testing in Tinius-Olsen Machine

44 3.6 Tensile Tests on MTS Machine

Due to the limitations of the Tinius-Olsen machine to accommodate samples larger than 1 inch width and the maximum applicable load, MTS 810 machine was used for samples with greater width; 2 inch, 4 inch and 8 inch samples. 1 inch samples were also tested on the MTS machine but due to the low magnitude of these loads when compared to the maximum load of the machine there was noticeable noise levels in the results. This was overcome by considering the medians values of every five consecutive data points from the force-elongation data obtained from a data acquisition system to plot the force- elongation curve, which smoothened some of the irregularities.

The MTS 810 system is completely integrated to facilitate testing of a wide variety of materials for their mechanical properties. Figure 3.6 shows the schematic of MTS 810 with the accessories like data acquisition system and the hydraulic equipment. The system’s data acquisition and controller TestStar II are graphical interface [72].

An attachment to the MTS 810 test facility was developed to facilitate gripping of the fabric samples in the machine and load them during the test. The attachment also has the provision for restraining cross-contraction of the samples with the help of eyebolt and nut arrangement. A picture of the attachment is shown in figure 3.7, which is used for this purpose. This attachment consists of two parts: a top fixture and a bottom fixture. The top fixture is fixed to the top grip in the MTS machine and the bottom to the bottom grip. The bottom fixture is U-shaped to facilitate transverse restraining of sample, which will be discussed in a greater detail in section 3.7. A pin joint was used in the top grip to account for any misalignment during the loading of sample to avoid development of shear forces that lead to failure in the clamps.

Figure3.6 Schematic of the MTS 810 test facility [72]

Top Fixture Pin Joint

U – Shaped Bottom Fixture Half Cylindrical Rods

Figure 3.7Attachment to MTS 810 to facilitate testing of fabrics

Figure 3.8 Half cylindrical rods used to grip fabric in the fixture during testing

The Omnisil fabric is placed in between the two cylindrical halves and then wrapped around both of the cylindrical rods which serve as clamps for the fabric during the tensile test. Figure 3.8 shows the arrangement for gripping the sample in the fixture. After initial slip of fabric from this arrangement of gripping mechanism, the fabric tightens itself. During the fabric becomes very taut around the cylinders and results in very high frictional forces that further slip is not possible. And this serves as a clamp for rest of the loading. Figure 3.9 shows the arrangement after the fabric sample is secured in the grips of the top fixture.

Figure 3.9 Fabric sample secured in the the cylindrical grips before test

Figure 3.10 Uniaxial test of fabric in MTS 810

48 3.7 Plane Strain Tests on MTS

As explained earlier the attachment developed for the MTS machine facilitated plane strain testing of the fabric. The U-shaped bottom fixture can accommodate a vertical stainless steel rod of ½ inch diameter which can be fastened to the parallel sides of the U- bottom to restrict cross-contraction of the fabric during the test. The ½ inch stainless steel rods are connected to the U-shaped bottom by means of two eyebolt and nut arrangement as shown in the figure 3.11 below. The steel rods slip into the fabric looped around it and stitched such that tightening of the nuts on the eyebolts results in the restraining the fabric from contracting in the lateral direction.

A horizontal metal bar is welded in the middle of the U-section joining the two vertical sides of this section to provide rigidity to the structure of this fixture against curving in or bending of the two vertical sides of the U-section due to transverse loads. Figure 3-11 shows the horizontal metal bar welded in between the two vertical sides of the U-fixture.

Welded horizontal metal bar

Eyebolt and nut arrangement

Figure 3.11 U-shaped bottom fixture to restrict cross-contraction

Loop Stitched for stainless steel rods for lateral load application

Figure 3.12 Eight inch wide sample with loops stitched on sides to restrict cross- contraction

Figure 3.13 Eight inch wide sample during plane strain tensile test

50 Figure 3.12 shows an 8 inch wide sample modified to facilitate lateral restraining durin the tensile test. The loops stitched parallel to the sample’s test length house the ½ inch stainless steel rods which restrict the lateral contraction due to application of load along the length direction. Figure 3.13 shows a sample being tested in plane strain conditions.

3.8 Creep Test on Omnisil

The objective of a creep test is to obtain time-elongation behavior for Omnisil fabric. A creep test at a constant load for 10,000 hours can be performed to understand the time- elongation characteristics of the fabric; instead, an accelerated creep test of shorter time periods at a higher temperature can be used to obtain the elongation behavior. This data can be used to extrapolate the deformation or elongation characteristics for 10000 hours of creep test. The relation between elongation data for shorter periods at higher temperatures to that at lower temperature for longer periods is given by an empirical relationship described in chapter 2.

The environment inside an operational ESP consists of a wide composition of acidic and basic gases (or) aerosol. In the presence of continuous supply of water these gases will form aqueous solutions of acids, bases and salts. So, the simulation of real time conditions would demand a series of creep tests under various concentrations and combinations of these chemicals. Nevertheless, there is a need to have a baseline creep data to which all these other creep results could be compared. A membrane based wet ESP has a continuous flow of water over the membranes at all times in addition to the tensioning loads, so the baseline test should consist of samples under continuous wetting. Hence, the baseline test would consist of obtaining time-elongation behavior of Omnisil fabric with water at a temperature higher than the actual operating temperatures of an ESP. The flow of water over the membranes in the baseline test also ensures that the additional load due to the water flow is applied at all times in addition to the dead load. The operational temperatures of wet membrane ESP are typically around 50˚C-60˚C, so a temperature of 80˚C was chosen for accelerated baseline test as the accelerated test

51 requires higher temperatures to facilitate extrapolation elongation data to longer durations of time than the test duration. Also, any higher temperature lead to significant amounts of evaporation leading to troubles in continuous water supply to the water reservoir for wetting samples and potential problems of rust and sealing the test rig to avoid heat losses.

An accelerated baseline creep (~80◦C ) test of was performed for all the creep tests as it would reduce the run time of the test eliminating the potential practical problems that can be encountered in a relatively longer continuous run, accelerated test also provides a financial advantage in terms of the resources required for the test.

A test facility was developed to facilitate the creep testing of Omnisil with continuous flow of hot water over the fabric. This test is based on the assumption that the fabric will attain the temperature of the water flowing over it. The water will be heated in a tank using a resistance heater and maintained at a constant temperature of approximately 80°C. The elongation of the fabric was measured with the help of four Linear Variable Displacement Transducers (LVDT). The fabric was fixed on one side and a dead weight is hung from the fabric sample on the other side. Two LVDT’s were used on each sample to measure the elongation on either side of the fabric sample. The use of two LVDT’s reduces the chances of errors in recording elongation in fabric samples, in case of friction between the sliding core and LVDT transformer.

A modified creep test based on the ASTM D-35 committee recommendations was performed to obtain creep data. Four inch wide samples of Omnisil in the fill direction were used for this creep test. Loads of approximately 4% and 2% of the failure loads were used for creep test.

The results from the accelerated creep test would be used to predict the behavior at a lower temperature than the test by using shift factors (aT). The shift factors will be

52 calculated using empirical equation developed by Williams Landel and Ferry. The mathematical form of this relation is given in chapter 2.

The schematic shown in figure 3.15 describes the principle in creep testing of the Omnisil fabric. A series of tests would be conducted to evaluate the elongation behavior of Omnisil fabric. These tests would constitute of one long term test (approximately 700 hours) and a number of short term (approximately 100 hour) tests. The objective of the long term test would be to obtain the time-elongation data, while, the short term test are performed to ensure independence and repeatability.

Fixed Sides Header for Water Flow

Omnisil Membrane

LVDT LVDT W1

Figure 3.15 Schematic for creep test of Omnisil fabric

53 3.9 Creep Test Set-up:

The creep rig constitutes of six different systems: the outer frame, the clamping mechanism, the water header system, the sensors, water reservoir system and the weight mechanism. Figure 3.16 shows the creep rig with all the systems except the weight hanging mechanism.

Make-up Water Access Panel Water Tank Header

Water Heater

Water Pump

Figure 3.16 Creep test set-up

Figure 3-16 shows the polypropylene tank inside the metal frame and the lexane sheet in the front serves as an access panel to mount the samples and sensors. The top and bottom frame are connected to each other by means of fasteners. As can be seen from Fig. 3-18 the system has a make-up tank to maintain water level in the system. The set-up has a water heater which helps maintain the temperature of water and a water pump that pumps the hot water to the sample through the plumbing to water headers. This set up sits on a wooden table that helps hang the weights from sample at the bottom.

54 The outer frame is the structural member which supports the load and houses the top clamp. The outer frame consists of two parts: the top frame and the bottom frame. Figure 3.17 shows the top frame of the creep rig. The sample is hung from the top frame of the creep rig. The top frame also houses the polypropylene tank.

Figure 3.17 The top frame of the outer structural member of the creep rig

Figure 3.18 The bottom frame of creep rig

55 Figure 3.18 shows the bottom part of the creep frame. The purpose of the metal sheet in this frame is to provide support the base of the polypropylene tank which sits inside the frame. The rectangular portions are cut from the metal sheet in the bottom frame to facilitate the attachment of weight hangers to the samples. The metal rods of the weight hanger that connect the lower clamp to the weights pass through these holes cut in metallic sheet.

Figure 3-19 shows the polypropylene tank with some holes drilled at necessary points to facilitate plumbing of water lines and installing pump and the immersion heater and the front panel is for accessing samples.

Figure 3.19 High temperature polypropylene tank

Figure 3-20 shows the close up view of the 2KW immersion heater used in the creep rig for heating water to the required temperature. A resistance type heater with variable setting was used for this purpose to adjust the temperature of the water.

Immersion Heater

Figure 3.20 Resistance heater used to heat the water in the tank

Magnetic Drive Pump

Water Supply from make up tank

Figure 3.21 Magnetic drive pump used for pumping hot water to the samples

Figure 3.21 shows the magnetic drive pump used in the rig for pumping water to the sample through the headers. Figure 3.21 also shows the plumbing from the make-up tank to the water reservoir in the main tank for maintaining water level.

Control Valves

Access Port for sensor wiring

Figure 3.22 Headers that deliver hot water to the samples and valves to control the flow

Figure 3.22 shows the valves on the water lines to the headers which deliver hot water to the samples. It also allows adjusting water flow to the samples. The access ports for the sensor wiring can also be seen; the wires are drawn from these ports and connected to the signal conditioners before they are hooked up to the data acquisition system.

Figure 3.23 shows the inside of the polypropylene tank with bulk heads facilitate connecting the metal rods to the lower clamp and the weight hanger for loading the samples. An extension pipe is fitted into these bulk heads so that they lead the connecting rods above the water level to the clamps of the samples.

Figure 3.24 shows the weights hanging from the sample from under the table. The numbers on the cement blocks indicate the weight in lbf of the cement blocks. The rig is set on a table so as to facilitate loading of samples by gravity.

Immersion Extension Heater Pipes

Bulk Heads to facilitate attaching metal rod from clamp to weight hanger

Figure 3.23 The inside of the tank and the bulk heads that lead connecting rods to the weight hanger

Connecting Hole cut in the rod to weight wooden table for hanger connecting rods to pass through

Weights

Figure 3.24 Weights used in the experiment (under the table that seats the creep rig)

59 Figure 3.25 shows the clevis of the clevis-pin arrangement that houses the clamps of the sample. The top clevis is fastened to the top part of the outer frame as shown in figure 3.17; a similar arrangement is used at the bottom but the bottom clevis-pin arrangement is attached to the rod connecting to the weight hanger.

Figure 3.25 Clevis-pin arrangement which houses the clamps of the sample

Clamps

Figure 3.26 Clamps used for gripping samples

60 Figure 3.26 shows the clamps that are used for gripping the samples. The fabric sample is sandwiched between the two metal plates and then nut and bolt arrangement provides the necessary clamping force to avoid slipping of the sample from these grips.

Sensors- LVDT Omnisil Sample

Wiring to the Sensors

Figure 3.27 Sample ready to be tested with LVDT’s attached

Figure 3.28 Nut welded to the top clamp to which the LVDT is attached

Figure 3.29 Screw welded to the bottom clamp to facilitate mounting an LVDT core connecting rod

Figure 3.30 Double slotted plate in bottom clamp to hold the core of LVDT

62 Figure 3.27 shows the sample mounted in the rig. With the sensors hooked up to the clamps. It also shows the wiring to the sensors enclosed in a tubing to avoid the hot environment inside the rig from affecting the wires.

Figures 3.28, 3.29 and 3.30 show the modifications made to the top and bottom clamps to facilitate mounting of the LVDT’s on the clamps so that they measure the elongation of the sample in between the clamps. The core slides into the bore of the LVDT and it is connected to the bottom plate or to the bottom clamp through the core connecting rod and the core connecting rod is fixed with respect to the bottom clamp. As the bottom clamp moved due to elongation of sample so does the core in the LVDT and it can measured as a change in voltage using the data acquisition system. Figure 3.31 shows the bottom portion of the sensor and clamp arrangement on a sample ready to be tested.

Core connecting rod

Plate connecting core connector to bottom clamp

Figure 3.31 Sample ready to be tested with LVDT’s attached from the clamps

Figure 3.32 shows two samples in place and sample in the rear is already is being tested, while the sample in the front is ready to be tested with all the sensors installed. The black

63 tape at the top of the sensors is electrical insulation tape under which there is a silicone sealant to keep the steam away from the wires to the sensor.

Electrical Insulation Tape

Figure 3.32 Sample (in the back) during a test

Headers inside the tank

Figure 3.33 Header for supplying hot water to the samples

64 Figure 3.33 shows picture of the water headers inside the rig. The headers are offset from the samples by a few inches and utilize the pressure head of water to spray hot water on the samples.

Figure 3.34 shows the picture of creep rig covered with insulation to prevent heat losses from the system and the thermocouple can also be seen which was used to measure the temperature of water in the reservoir.

Thermocouple

Insulation

Figure 3.34 Creep rig set-up insulated and sensors connected to DA system

3.10 Mullen Burst Tests

Mullen burst test are conducted on the fabric to determine the tearing strength of the fabric. Figure 3.35 shows the picture of a mullen tester. Mullen burst serves as a quick test to evaluate changes in strength of the fabric. Mullen tester consists of a horizontal

65 base plate that houses the sample of a rectangular shape. The base plate has a counterpart “tripod”, ring shaped plate that applies pressure on the edges of the sample holding it in place. The base plate has a circular hole from which a rubber diaphragm can be inflated to apply spherical load on the sample. A rectangular sample of the test material is clamped between two horizontal plates. Hydraulic fluid is displaced from the chamber of the tester by a piston moving at a constant rate driven by an electric motor, inflating a heavy rubber diaphragm to expand through the base plate opening and against the unsupported area of the sample. A pressure gauge connected to the cylinder indicates the pressure rise in the hydraulic fluid of the tester. When the sample bursts, the pressure drops, leaving the gauge pointer stationary and the pressure gauge shows the force required to burst the sample. A model ‘A’ mullen tester was used for this purpose with a maximum capacity of 1000psi. Figure 3.36 shows pictures of six samples of polypropylene after being tested for burst strength on mullen tester.

Pressure gauge Electric Tripod Motor

Base Plate

Figure 3.35 Picture of Model ‘A’ mullen tester

Figure 3.36 Polypropylene samples after being tested for burst strength

67 4.0 RESULTS AND DISCUSSIONS

4.1 Yarn Tensile Tests:

The purpose of the yarn tensile strength was to provide a first estimate of the tensile strength of the fabric. The failure strength of a yarn when used with the yarn count in the direction of test can provide a rough estimate of the failure strength of the fabric. In addition, these results can also be used for comparing the results for any mathematical model for tensile behavior of the fabric.

Figure 4.1 shows the force-elongation data for Omnisil yarns. These tests were performed on Tinius Olsen machine. Seven yarns drawn from the Omnisil fabric were used for this test.

Yarn Tensile Test

70.00

60.00 Sample 1 50.00 Sample 2 40.00 Sample 3 30.00 Sample 4

Force (N) Force 20.00 Sample 5 Sample 6 10.00 Sample7 0.00 0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 -10.00 Elongation (mm)

Figure 4.1 Load-elongation curve for yarns of Omnisil fabric

68 The yarn tensile strength was found to be consistent with most peaks around 56N ~ 12.4lbf with a standard deviation of 2.38N ~ 0.5lbf. The results obtained from this test are higher than the reported values from the manufacturer which reports the minimum tensile strength of the Omnisil yarns as 11 lbf. The yarn tensile strength is generally assumed to be Poisson distribution so the experimental values of the tensile strength obtained from this test which are higher than the reported minimum values by 12.7% are acceptable.

4.2 Strip Tensile Test on Tinius-Olsen Machine:

The purpose of these tests was to investigate into the relationship between the sample width (number of yarns) and failure strength of the fabric. 1 inch wide samples in the transverse and the longitudinal directions were used for this purpose. Four samples in each direction were tested up to failure.

1 inch Transverse - Uniaxial

1200

1000

800 Sample 1 Sample 2 600 Sample 3 Force (N) 400 Sample 4 200

0 0 5 10 15 20 25 30 35 Elongation (mm)

Figure 4.2 Load-elongation for 1 inch uniaxial samples in fill direction of fabric

69 Figure 4.2 shows the load-elongation curves obtained from the transverse tensile test on 1 inch wide samples. The average failure strength of the samples in the transverse direction was 695N ~ 156lbf with a standard deviation of 251N ~ 56lbf. The standard deviation of the results is more 30% of the average. The average rupture strength is 57% lower than the rupture strength calculated from 11 lbf/yarn strength and 62% lower than the strength calculated from 12.4lbf/yarn.

1 inch Longitudinal - Uniaxial

2500

2000 Sample 1 1500 Sample 2 1000 Sample 3 Force (N) Force Sample 4 500

0 0 10203040 Elongation (mm)

Figure 4.3 Load-elongation for 1 inch uniaxial samples in warp direction of fabric

Figure 4-3 shows the load-elongation curve for tensile test on samples in longitudinal direction. The average rupture strength was approximately 423lbf for the longitudinal samples with a standard deviation of 71lbf i.e. 17% of the average. This value is pretty close to the values expected from the minimum yarn failure strength from the manufacturer i.e. 473lbf. However, the experimental value is 20% less than the strength expected based on the yarn failure strength of 12.4lbf/yarn i.e. 532 lbf.

70 The results from 1 inch wide samples tested on the Tinius-Olsen machine have considerable scatter as there has been a lot of variability in the samples and the samples were not truly in the longitudinal and transverse direction of the weave. A new procedure for making samples was devised to overcome this variability which ensured samples in true warp and fill direction.

It can be seen from the results the fabric tends to be stronger in the longitudinal direction than the transverse direction. This could be a consequence of the fact that the yarn count in the transverse direction is 33 yarns/inch, whereas in the longitudinal direction the yarn count is 43 yarns/inch.

4.3 Short Term Uniaxial Tensile Tests on MTS

MTS 810 machine was used to perform uniaxial tensile test on Omnisil samples of various widths in transverse direction and 8 inch wide samples in longitudinal direction. The purpose of these tests was to understand the load-elongation characteristics of fabric and to establish the relationship between rupture strength and sample width.

Figure 4.4 shows the load elongation curves for 1 inch wide samples in the transverse direction tested on the MTS machine. Twelve samples along the fill direction of the fabric were tested. The rupture load on these samples is a very small percentage of the MTS machine capacity; which can lead to poor resolution in the data obtained.

The average rupture strength of these samples was found to be 428 lbf with a standard deviation of 31 lbf. This value is higher than the rupture strength predicted from yarn rupture strengths of 11 lbf/yarn (i.e. 363 lbf) and 12.4 lbf/yarn (i.e. 408lbf). The higher value of rupture strength of Omnisil™ for 1 inch wide samples in the transverse direction might be due to low loads of failure when compared to the maximum capacity of the MTS machine; which could have lead to poor resolution at these low loads. The predicted failure strength of fabric based on 11 lbf/yarn and 12.4 lbf/yarn and yarn count in the

71 direction of test are the expected lower and upper bounds of the rupture strength based on yarn failure properties.

MTS 1 inch Transverse - Uniaxial Sample 1 600 Sample 2 Sample 3 500 Sample 4 400 Sample 5 300 Sample 6 Sample 7 200

Force (lbf) Sample 8 100 Sample 9 0 Sample 10 -4 -3 -2 -1 0 1 2 -100 Sample 11 Sample 12 Elongation (in)

Figure 4.4 Load-elongation curve for 1 inch uniaxial samples in fill direction of fabric

Figure 4.5 shows the load elongation curves obtained from the tensile test of 2 inch wide samples in the transverse (fill) direction. Ten samples were tested with 2 inch width. Although these loads are approximately twice the rupture strength of the 1 inch wide samples, they rupture loads are small when compared to the capacity of the MTS machine capacity. Hence, it can be seen that these tests have lead to a slight over- prediction of the strength of fabric.

2 inch Transverse - Uniaxial

1200 Sample 1 1000 Sample 2 Sample 3 800 Sample 4 600 Sample 5

400 Sample 6 Force (lbf) Sample 7 200 Sample 8 0 Sample 9 -3.5 -3 -2.5 -2 -1.5 -1 -0.5 0 -200 Sample 10 Elongation (in)

Figure 4.5 Load-elongation curves for 2 inch uniaxial samples in fill direction of fabric

The average rupture strength of the 2 inch wide samples was found to be 812 lbf with a standard deviation of 108 lbf. The calculated rupture strengths in this case are 726 lbf (10 lbf/yarn) and 817 lbf (12.4 lbf/yarn).

Figure 4.6 shows the load-elongation graph for 4 inch wide samples. Eight samples were tested in the fill direction for this width. Results for sample 5 were discarded, as the failure was initiated due to concentration of load to a localized region. The load was unevenly distributed and the failure started on the right side of sample and propagated to the other end leading to ultimate failure of the sample.

The average strength of the samples was found to be 1561 lbf with a standard deviation of 228 lbf. The calculated rupture strengths in this case are 1452 lbf (11 lbf/yarn) and 1633 lbf (12.4 lbf/yarn).

4 inch Transverse - Uniaxial

2500

2000 Sample 1 Sample 2 1500 Sample 3 Sample 4 1000 Sample 5 Force (lbf) 500 Sample 6 Sample 7 0 Sample 8 -4 -3 -2 -1 0 1 2 -500 Elongation (in)

Figure 4.6 Load-elongation curves for 4 inch uniaxial samples in fill direction of fabric

Figure 4.7 shows the load-elongation graph for an 8 inch wide sample in the transverse/fill direction. Sample 8 was not considered for any results as it had an observable damage in the sample. The average rupture strength of the samples was found to be 2995 lbf with a standard deviation of 327 lbf. The calculated rupture strengths in this case are 2904 lbf (11 lbf/yarn) and 3267 lbf (12.4 lbf/yarn).

The plots for load-elongation show some negative elongations, it is due to the coordinate system the MTS machine follows. The elongation values start from the coordinate of the starting position of the moving clamp of the machine. It can be observed from figure 4.8 and 4.9; it can be seen that the scatter in data obtained from the tensile tests has been increasing with the increase in width of the samples.

8 inch Transverse - Uniaxial

4000 3500 Sample 1 3000 Sample 2

) 2500 Sample 3 2000 Sample 4

1500 Sample 5

(lbf Force 1000 Sample 6 500 Sample 7 0 Sample 8 -3 -2 -1-500 0 1 2

Elongation (in)

Figure 4.7 Load-elongation curves for 8 inch uniaxial samples in fill direction of fabric

Width - Rupture Strength Scatter

3750

3000

1" Wide Transverse 2250 2" Wide Transverse 4" Wide Transverse 1500 8" Wide Transverse 8" Wide Longitudinal Rupture StrengthRupture (lbf) 750

0 0123456789 Sample Number

Figure 4.8 Scatter in the rupture strength of fabric at various widths

Standard Deviation - Rupture Strength

350

300

250

200

150

100 Standard Deviation (in lbf)

0 0123456789 Sample Width (in Inch)

Figure 4.9 Standard deviation of rupture strength in sample with different widths in fill direction

Figure 4.8 and 4.9 show a comparison of the tensile strengths of samples of various widths. The data above shows that the scatter has increased with increase in width which could be attributed to two reasons. The first reason being it has always been assumed that the fibers which constitute the yarn follow a Poisson distribution for rupture strength, this is a consequence of the process of their manufacture and every fiber produced may not have the same strength. This is due to the fact that process of producing fibers essentially results in thin and thick points on a fiber and this non-uniformity can play a considerable role in the strength properties. The yarns which are combinations of fibers also follow a poisson distribution. The fabric eventually is composed of constituents whose strengths follow a poisson distribution, which leads us to the conclusion that more the number of fibers in our sample higher the scatter. Secondly, as the width of the sample increases the influence of the alignment of the system becomes very important and the load is now applied on a wider sample and any variation of loading along the width can lead to higher

76 loads at one point when compared to the rest of the sample and the yarns in such regions are subjected to higher loads and fail at lower loads than in case of a more uniform loading along the width of the sample. This can also be applied when there is an increase in yarn count for the same width of the sample. This can be seen in case of samples along warp or longitudinal direction of the fabric.

Figure 4-10 shows the load elongation graphs for 8 inch wide samples in longitudinal samples. There have been only 5 successful tests of the 8 samples. Three of the samples did not yield any useful results as one of the samples had a noticeable damage and the other two tests were aborted due to serious misalignment of the sample.

8 inch Longitudinal - Uniaxial

4000 3500 Sample 1 3000 Sample 2 2500 Sample 3 2000 Sample 4 1500 Sample 5 Force (lbf) Force 1000 Sample 6 500 Sample 7 0 Sample 8 -4 -3 -2 -1-500 0 1 2 Elongation (in)

Figure 4.10 Load-elongation curve for 8 inch uniaxial samples in warp direction of fabric

The average strength of the samples was found to be 3070 lbf with a standard deviation of 136 lbf. The expected rupture strengths in this case are 3784 lbf (11 lbf/yarn) and 4257 lbf (12.4 lbf/yarn). One of the possible reasons for the low tensile strength of the fabric

77 could be due to the degree of alignment/misalignment in the tensile testing apparatus. The yarn count as reported by the manufacturer in the longitudinal direction is 43 yarns per unit width as opposed to 33 yarns per unit width in the transverse/fill direction of fabric. There is a 30% increase in the yarn density in the longitudinal direction when compared to transverse. This significant increase in yarn densities is likely to make the tests in longitudinal direction very sensitive to the slightest misalignment in the test rig. The misalignments can lead to introduction of shear forces resulting in a complex state in place of pure uniaxial tensile stresses leading to failure at lower loads.

As can be seen from Table 4-1, the rupture strength of the longitudinal sample is much lower than the calculated values i.e. 23% below the manufacturer’s reported value and 28% below the expected value from yarn strength. And this could be accounted by two reasons. The increase in number of yarns also increases the potential sites of weak links in the fabric. And it has been observed during the tensile testing of the fabric that the failure process is essentially a cascade effect, once the failure has initiated at a point its travels quickly across the width of the sample leading to failure at lower loads. Due to an increased yarn count any misalignment in the system becomes prominent as this would lead to localization of load at that point and thus the initiation of the failure. A pin joint was provided on the top clamp of the fixture used in MTS machine to facilitate some degree of freedom to align the fabric in the plane of application of load.

Table 4.1 Summary of Short term tensile tests

Manufacturer’s Value Calculated Experimental Value * Average Rupture Strength Standard Deviation Sample Width (lbf) (in lbf) Expected Percent Expected Value Percent Value (lbf) Deviation (lbf) Deviation Yarn Test 12.4 0.5 11 -12.7 12.4* - 1 inch 429 31 363 18.2 408 5.1 2 inch 812 108 726 12.1 817 -0.61 4 inch 1560 228 1452 7.4 1635 -4.5 8 inch 2997 327 2904 3.2 3267 -8.2 8 inch 3070 136 3784 -23.3 4257 -27 (Longitudinal) * - Experimental yarn rupture strength was used in calculated experimental values

The rupture strength for the fabric were calculated from the yarn strength using 33 yarns/ inch for transverse sample and 43 yarns/inch in case of longitudinal sample.

Yarns lbf ExpectedRuptureStrength = ×Width(inInch) ×YarnStrength( ) Inch in

79 4.4 Short Term Plane Strain Testing

The purpose of the plane strain test was to distinguish tensile behavior without cross- contraction from pure uniaxial tension behavior of the fabric. It is important in case of an ESP because the collecting surface being a membrane requires some lateral tension in addition to longitudinal tension order to maintain the constant distance from the charging electrode. In the event of significant difference in tensile behavior, an eventual plane strain characterization of the fabric would also be required to completely describe the behavior of fabric in an operational ESP.

Figure 4-11 shows the load-elongation plot for a plane strain tensile test on a 4 inch wide sample by restricting the lateral contraction of fabric. A modified cross-shaped sample as shown in figure 3.12 was used for these tests. Sample labeled “Uniaxial” indicated in the figure 4-11 was a pure uniaxial test on a plane strain sample, the objective of this test was to establish that the modification in the sample to facilitate the plane strain loading does not influence the strength of the sample. As can be seen this sample showed a rupture strength of approximately 2100 lbf which is ~2% less than calculated value i.e. from 12.4 lbf/yarn using 43 yarns/inch in longitudinal direction. In case of plane strain tests, the loops stitched on the sides failed (figure 4.12) during the test and the test turned into uniaxial and most of such tests (sample 2, 3 and 4) resulted in rupture strengths close to the uniaxial values indicating that those could essentially be uniaxial tension tests. The failure of the stitched loops can be due to the effect of shear forces resulting from restrain in free sliding of the looped fabric around the stainless steel rods due to friction. The samples under test are prevented from cross-contraction, which results in a high normal force on the looped fabric around steel rods avoiding any sliding of fabric due to friction while it is being stretched in the longitudinal direction of test. These frictional forces lead to shear of the seam of the looped fabric at the edges.

Plan4e inch Strain Longitudinal – 4 inc -h Biaxial wide

2500

2000 Sample 1 Sample 2 1500 Sample 3 Sample 4 1000 Sample 5 Force (lbf) Force 500 Sample 7 Uni-axial 0 Sample 6 -4 -2 0 2 4 -500 Elongation (in)

Figure 4.11 Load-elongation behavior of 4 inch wide plane strain samples in longitudinal direction of fabric

Figure 4.12 Failure of plane strain sample due to shear forces

81 4.5 Transducers in Creep Test

A number of creep tests were performed to observe the elongation of fabric at constant load at 80°C (approx.). In order to measure the elongation of the samples over period of time Linear Voltage Displacement Transducer (LVDT) was used to facilitate continuous monitoring of the elongation behavior of fabric. AC powered HSA 750-250-010 by MacroSensors™ were used for this purpose. A micrometer with an accuracy of 10 micro inches was used for calibration of these LVDT’s. A 13 point calibration was done on these sensors against the micrometer after null adjustment. The core of the LVDT was attached to the spindle of the micrometer with help of a connecting rod; the spindle is moved back and forth with the help of dial on micrometer. The core was moved by a known distance from the null position and corresponding voltages were obtained using a DC multimeter. Signal conditioners were used with the LVDT’s and the gain on these signal conditioners was set to ±10V DC output for the full scale motion. Figure 4-13 to 4- 16 show the calibration curve for the four LVDT’s used in the creep test. Sensor 1-1 and 1-2 were used on long term sample on right and left side of the sample respectively. Similarly sensor 2-1 and 2-2 were used on short term samples.

Sensor 1-1

y = 39.95x - 0.0089 10 R2 = 1

Voltage -0.3 -0.2 -0.1 0 0.1 0.2 0.3

-5

-10

-15 Displacement

Figure 4.13 Calibration curve of the LVDT used in creep test

Sensor 1-2

y = 40.028x + 0.0008 10 R2 = 1

Voltage -0.3 -0.2 -0.1 0 0.1 0.2 0.3

-5

-10

-15 Displacement

Figure 4.14 Calibration curve of the LVDT used in creep test

Sensor 2-1

y = 40.053x - 0.0076 10 R2 = 1

Voltage -0.3 -0.2 -0.1 0 0.1 0.2 0.3

-5

-10

-15 Displacement

Figure 4.15 Calibration curve of the LVDT used in creep test

Sensor 2-2

y = 40.019x + 0.001 10 R2 = 1

Voltage -0.3 -0.2 -0.1 0 0.1 0.2 0.3

-5

-10

-15 Displacement

Figure 4.16 Calibration curve of the LVDT used in creep test

84 4.6 Creep Test: 8% Rupture Strength

A constant load creep test was performed to evaluate the time-elongation behavior of the fabric. A series of tests were conducted which included one sample run for a long period of time (long term test) and a number of tests for rather shorter periods of time (approx. 100 hours: short term tests). A load of ~ 8% transverse rupture strength was initially used for these experiments. The experiments with the 8% rupture strength lead to failure of samples after sometime into testing. The long term sample failed after approximately 230 hours of testing. One of the short term tests was completed without failure of the sample at 8% rupture strength. The second short term sample failed after 65 hours of testing. In view of the failure of samples at this load; the load for creep testing was lowered to approximately 4% rupture strength of the fabric.

0.5

0.45

0.4

0.35

0.3

0.25 Elongation (in Inch)

0.2

0.15

0.1 0 50 100 150 200 250 Time (in Hours)

Figure 4.17 Time-elongation data (long term) of fabric sample with 100 lbf load

85 Figure 4.17 shows the time-elongation graph for the long term sample with 100lbf (approx.) load for the first 230 hours of testing till failure of the sample. The loading of the sample preceded raising the temperature of the sample to the desired test temperature in this particular test. The initial curvature in the data could be due to the fact that the rig and the sample were not at the test temperature when the sample was loaded. The sample was loaded at room temperature and then heated to the test temperature. As observed from this test this method of testing leads to considerable slope in time-elongation as the sample creeps considerably with increase in temperature for the first 60 hours of test. In subsequent tests, the sample was first brought to temperatures close to test temperature and then load was applied to avoid the initial creep as observed in this test.

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2 0 20 40 60 80 100 120

Figure 4.18 Time-elongation plot (short term1) of fabric sample with 100lbf load

86 Figure 4.18 shows the time-elongation graph for the short term sample with approximately 100lbf load for 100 hours of testing. The discontinuity in the elongation data from 35 to 65 hours of testing is due to the noise in one of the LVDT’s on the sample which lead to a sudden change in the signal. This change in the output of the sensor could have been due to close proximity of the lead wires from the connector which could influence the voltage output. All these wires were wrapped with electrical insulation tape to avoid physical contact. A sensor safe RTV gasket maker was filled in between the wires so that there would be no electrical interference between these sensor wires.

Short Term 2 (100 lbf)

0.8 0.7 0.6 0.5 0.4 0.3 0.2 Elongation (in Inch) 0.1 0.0 0 20 40 60 80 100 Time (in Hours)

Figure 4.19 Time-elongation plot of (short term 2) sample at 100lbf load

Figure 4.19 shows the time-elongation graph for the second short term sample with 100 lbf load till failure at 80 hours. The irregularities in the output from the sensors are again due to the noise in output from the sensors. The possible reasons could be due condensation of water at the connector, the wires connect to the sensor in the BNC connector provided with the sensor. For later tests the sensor was sealed with a sensor

87 safe RTV gasket maker to avoid steam into the BNC connector of the sensor. And this set-up was wrapped with PTFE tape and electrical insulation tape over it.

4.7 Mullen Burst Tests

Five samples of virgin Omnisil fabric and five samples of Omnisil swatches soaked in the creep rig for 500 hours were tested for burst strength. Results from one of the tests on virgin Omnisil samples was discarded based on chauvenet criterion as its value (400 psi) was much lower than the average of other four samples (565 psi). Figure 4.22 shows the burst strength of Omnisil fabric in virgin state and after being soaked in water at 80°C for 500 hours. It was seen that the burst strength of the fabric reduced to almost half after soaking in hot water in creep rig.

Burst Strength

700

i 600

500

400 Virgin Strength Strength after 500 hours 300 Burst Strength(in ps Strength(in Burst 200

100

0 0246 Sample Number

Figure 4.20 Burst strength of Omnisil fabric

88 Five samples of virgin Omnisil fabric and five samples of Omnisil swatches soaked in the creep rig for 500 hours were tested for burst strength. Results from one of the tests on virgin Omnisil samples was discarded based on chauvenet criterion as its value (400 psi) is much lower than the average of other four samples (565 psi). Figure 4.20 shows the burst strength of Omnisil fabric in virgin state and after being soaked in water at 80°C for 500 hours.

4.8 Creep Test: 4% Rupture Strength

Following the failure of the samples at 100lbf, the load on the samples was reduced to half the initial values to approximately 4% of tensile rupture strength (50 lbf). Figure 4- 21 shows the time elongation behavior of the first short term sample over 110 hours with a constant 50lbf load. There has not been considerable creep in the fabric after the initial elongation during application of load on the fabric. The occasional irregularities in data are due to the noise in system which is a result of moisture interfering with the sensor wiring.

Short Term 1

0.25

0.2

0.15

0.1

Elongation (in Inch) 0.05

0 0 20 40 60 80 100 120 Time (in Hours)

Figure 4.21 Time-elongation plot of (short term1) sample at 50lbf load

Short Term 2 (50 lbf)

0.2

0.15

0.1

Elongation (in Inch) 0.05

0 0 20406080100120140 Time (in Hours)

Figure 4.22 Time-elongation plot of (short term2) sample with 50lbf load

Figure 4-22 shows the time elongation behavior of the first short term sample over 120 hours with a constant 50lbf load. There has been considerable noise in the system during the measurement of elongation during this test. This was probably due to the flow of adhesive from the insulation tape, which was used to avoid electrical contact between the wires. After this test the electrical insulation tape from the wires (that connect to the sensor) was replaced with PTFE tape to avoid electrical contact between the wires. And the sensors were again sealed with PTFE tape and insulating material at the elbow expander joint and the heat shrink joint at the elbow connector as shown in figure 3.32. Creep test on short term sample 3 did not yield any usable results as the noise in the sensors was too high to distinguish between the elongation data and the noise. In addition, the sample showed some highly localized deformation around the clamps due to very high clamping forces damaging the sample.

Short Term 4

0.16

0.14

0.12

0.1

0.08

0.06

Elongation (in Inch) (in Elongation 0.04

0.02

0 0 20406080100120140 Time (in Hours)

Figure 4.23 Time-elongation plot of (short term 4) sample with 50lbf load

Figure 4.23 shows the time-elongation behavior of short term 4 sample at 50 lbf load. The noise levels during this test were very low after the new moisture tight sealing procedure adapted after short term 3 test. As it can be seen from the graph, there is little elongation in the sample over the duration of test.

Figure 4.24 shows the data obtained from the short term 5 sample with a load of 50 lbf. This test was continued to 250 hours of operation unlike the other short term tests where the tests were aborted shortly after 100 hours of operation. A dielectric paste was used in the connector of the sensor to avoid the mutual influence of voltage in neighboring wires in the sensors. This test showed very little noise in the elongation data.

0.18

0.16

0.14

0.12

0.1

0.08

Elongation (in Inch) 0.06

0.04

0.02

0 0 50 100 150 200 250 Time (in Hours)

Figure 4.24 Time-elongation plot of (short term 5) sample with 50lbf load

Figure 4-25 shows the elongation of the fabric with 50lbf load over a longer duration of time than the short term creep tests. As can be seen from figure 4.25, plot of time- elongation there has not been significant creep of the fabric during the initial phases of testing. For the first 230 hours of operation, the output from the sensors has been fairly noise free. After the first 230 hours of operation the seals on the sensors leaked moisture into the sensor connector and the wiring leading to immense noise in the data obtained. After numerous efforts to fix the leakage as explained as explained previously in this section, a moisture tight seal was used devised leading to reduction in noise from 600 to 750 hours of operation.

0.18

0.16

0.14

0.12

0.1

0.08 Elongation (in Inch)

0.06

0.04

0.02 0 100 200 300 400 500 600 700 800 Time (in Hours)

Figure 4.25 Time-elongation of (long term) fabric sample at 50lbf load

Figure 4.26 shows the time-elongations plots of all the fabric samples tested with 50lbf load. The data obtained from the long term sample (figure 4.25) has been truncated after the first 250 hours of operation as there has been a significant influence of noise on the data obtained after that. Although some of the samples exhibit noise in their data, it can be seen from the plot that the slope of the time-elongation plot is almost similar for all the creep tests. The data obtained from the short term sample 5 and that obtained from long term sample coincide till almost 250 hours of testing. The reason for the vertical shift in the data from different samples is due to the inconsistency in point of resetting the sensors immediately following the instantaneous elongation in fabric at the time of application of load on the samples. It can be inferred, within the confines of the limited data that the fabric creep behavior is almost repeatable from the data obtained from five

93 samples. The initial region of instantaneous elongation is also eliminated from the plotted data for better comparison of creep elongations.

0.22

0.2

0.18

0.16

Elongation (in Inch) 0.14

0.12

0.1 0 50 100 150 200 250 Time (in Hours)

Figure 4.26 Consolidated data five creep tests at 50 lbf load

4.9 Creep Master Curves

As discussed in chapter 3, the data obtained by accelerated creep is used with the shift factors calculated from WLF equation. The creep data of first 45 hours from short term sample 5 at 50 lbf load (fig. 4.24) at 80°C was used with shift factors to plot the log strain vs log time and elongation vs time at 50°C. The log-log plot can be seen in figure 4.27. Figure 4.28 shows the elongation vs time plot for the two temperatures. For the

94 calculations of aT, it was assumed that the fabric has a Tg of 350°C and the values of C1 and C2 were taken to be -17.44 and 51.66 respectively.

Master Creep Curve

-1.2 00.511.522.5 -1.25

n -1.3 Strain @ 80degC -1.35 Strain @50degC

Log Strai Log -1.4

-1.45

-1.5 Log Time (in Hours)

Figure 4.27 Log-Log plot of master creep curve for sample 5 for first 45 hours

4.10 Interpretation of Results

The results from this research would be the time-elongation curve for Omnisil fabric at operational temperatures (50°C ~ 60°C) of a wet ESP. These results will help in determining the time after which the membranes in an ESP need to be re-tensioned, as the elongation of the membrane result in reduced distance between electrode and membrane. Failure to maintain the optimum distance between charging electrodes and collecting membranes will lead to collapse of the electric field and thus, no collection of particulates from flue gas.

Master Creep Curve

0.18 0.16 0.14 h 0.12 0.1 0.08 0.06 Elongation (inc Elongation 0.04 0.02 0 0 20 40 60 80 100 120 140 160 Time (in Hours)

Figure 4.28 Log-Log plot of master creep curve for sample 5 for first 45 hours

The elongation at any time can be obtained from the time-elongation data from the creep test. And this data will be used to curve fit for the two models (figure 4.29) and using the curve equation and beginning distance between the charging electrodes and collecting surface, the minimum distance between the two after deformation at any given time can be calculated. Thus, the creep results can be used to calculate the minimum distance between electrode and collecting surface at any given time. These results can be used with a relation between spark-over voltage and the electrode-collection surface spacing and also to calculate the re-tensioning interval for a given spark-over voltage.

4.10.1 Model I The figure below shows the one of possible deformed states of membrane after elongation due to high temperatures. As can be seen from the figure, it is assumed that the deformed membrane would take the shape of a circular arc (Fig. 4-30). This model would constitute the upper extreme of power throughput to the wet ESP. This model

96 results in minimum decrease in the electrode-collection surface distance as the circular profile of the deformation results in a rather uniform distribution of the elongation of the membrane there by results in minimum deviation from taut position. Since, the decrease in distance between the electrode and collection surface is minimum, assuming the current is constant; the maximum attainable voltage without spark-over would be higher and result in higher power to the wet ESP and best collection efficiency for a given time.

Membranes after elongation due to high temperatures

Charging Electrodes

Figure 4.29 Two models of membrane deformation after elongation: Model I (left) & Model II (right)

where, PSQ is the membrane after deformation, PTQ is the membrane before any deformation (taut), ST is the maximum deformation after creep, e is the elongation at any time. R is the radius of the circle which constitutes the curve PSQ, δ is the decrease in distance between electrode and collection surface due to creep, L is the initial length of membrane, θ is the angle made by the arc at the center of the circle.

Figure 4.30 Deformed state of membrane for Model I after elongation

The elongation data from the creep test would be used with the initial length to find the final length of membrane any time. The final length of the membrane would constitute the length of the arc PSQ. Using the equations below, the separation of electrode and collection surface can be found at any point of time.

PSQ = R ⋅θ = L + e

θ  OT = R ⋅ cos   2  δ = ST ⇒ δ = OS − OT

98 θ  ⇒ δ = R − R ⋅ cos   2 

4.10.2 Model II The figure below shows another possible deformed state of membrane after creep. As can be seen from the figure 4.31, it is assumed that all the elongation is concentrated at one place on the membrane. This model would constitute the lower extreme of power throughput to the wet ESP. This model results in maximum decrease in the electrode- collection surface distance as it assumes the concentration of the elongation at a single place. Since, the decrease in distance between the electrode and collection surface is maximum, assuming the current is constant; the maximum attainable voltage without spark-over would be lower and result in lowest power to the wet ESP and least collection efficiency for a given time. Since the deformation is concentrated at a single place, the decrease in the distance between the electrode and the collection surface ‘δ’ would be half the elongation of the membrane. e i.e. δ = 2 where, ‘δ’ is the decrease in distance between electrode and collection surface due to creep and ‘e’ is the elongation of membrane at any time.

4.11 Voltage – Deformation Relation

The maximum possible attainable voltage/power throughput can be expressed as a function of the electrode-collection surface separation. The electrode can be considered as a straight long charged object. When the field due to this charged object at a distance equal to the electrode-collection membrane separation, exceeds the dielectric strength of air, spark-over occurs. The maximum attainable voltage can be calculated using this optimum spacing between the electrodes and collecting surface.

99 V E = d V E = new d − δ

Figure 4.31 Deformed state of membrane for Model II after elongation

θ  where, δ = R − R ⋅ cos  for Model I  2  e δ = for Model II 2

100 Where, d is the electrode-collecting surface separation, e is elongation in membrane, θ is the angle made by the deformed circular arc shaped membrane at the arbitrary center of the circle, E is the field strength, and Enew is the field strength after deformation

But in most ESP’s the field is controlled by means of voltage, therefore, V = Constant, this leads to the conclusion that decrease in distance between electrode and membrane results in increase in field strength. Also, P = V ⋅ I where, ‘P’ is the power input to the wet ESP and ‘I’ is the current in the system.

Example: In a typical ESP the membrane is 15ft long and the voltages are around 45KV with a membrane electrode separation of 1.5 inch.

From the above equation, the field strength is given by V E = d

45000 V V E = = 30000 1.5 in in

Assuming a 10% increase in field strength would result in sparkover,

V E = 1.1× E = 33000 critical in

= V = d min 1.364in Ecritical i.e. when the separation between the membrane and electrode falls less than 1.364 inc, the electric field in the ESP would be grounded.

⇒ δ .15.1 364 =−= .0 136inch i.e e = .0 272inch for Model II where ‘e’ is the maximum allowable elongation in a 15ft membrane.

101 5.0 CONCLUSIONS

The work presented in this report attempt to characterize the tensile and creep behavior of Omnisil fabric. However, the techniques presented can be extended to all woven fabrics. As mentioned in the previous chapters, a series of tensile and creep tests were performed on Omnisil fabric to determine its mechanical properties. The primary objectives were to evaluate the uniaxial tension and creep behavior of the fabric. The tensile behavior was determined from yarn tension tests and strip tension tests warp and fill direction. The purpose of tension tests with varying widths was to establish a relationship between the strength of fabric and the width of the test sample. Also, plane strain tension tests were conducted on the fabric. Finally, creep tests at constant load of 4% rupture strength were conducted to understand the elongation behavior of fabric over longer durations of time when exposed to operational condition of ESP.

5.1 Yarn Tensile Tests

The purpose of the yarn tensile tests was to provide the basis for relating the strength of the fabric to the yarn count in the woven fabric in the direction of loading. The load- elongation characteristics of the yarn were found to follow a linear relationship till failure. The yarn tensile behavior follows a typical linear relationship as most of the homogeneous materials. These tests also indicate that the primary load bearing members in a woven fabric are the yarns in the direction of load.

The yarn density in the warp/longitudinal direction is 30% higher when compared to yarn density in the fill/transverse direction of the fabric weave. It can be seen from the results the fabric tends to be stronger in the longitudinal direction than the transverse direction. This could be interpreted as majority of the load is taken by the yarns in the direction of load and that the yarns in the perpendicular direction do not bear any significant load.

102 5.2 Strip Tensile Tests

It can be seen from figure 5.1 that the load elongation graph can be divided into three regions. The first region of the load-elongation curve is almost parallel to the elongation axis as in this region no significant loading is observed most of the elongation comes from the slip of fabric from the grips and hence, a negligible increase in load. This region is known as the “toe” which can essentially be excluded from the tensile behavior as “toe compensation”. The region after the toe (Region II) is non-linear due to the sliding of the yarns over the ones in the direction perpendicular to that of load application. The non- linearity is due to the presence of friction between the sliding yarns. In this region the crimp of the yarns in the direction of load is taken up. At the end of the non-linear region the yarn is essentially straight and free of crimp further increase in load leads to extension of yarn and thus a higher modulus. This region generally exhibits a linear relationship (Region III) between load and elongation up to failure when the yarns fail in some regions at the weakest link and lead to the failure of fabric as a whole. Figure 5.2 shows the actual mechanism involved in the three different regions of the tensile-elongation of a woven fabric that leads to the initial non-linear region followed by a linear region till failure in short term tension tests.

Figure 5.1 Regions in the load-elongation plot of Omnisil

103

Figure 5.2 Yarn interactions at various stages of tensile test [63]

Figure 5.3 shows the plot of average rupture strength of transverse direction samples against the sample width. It can be inferred from the plot that the fabric exhibits a linear relationship between strength to width. This could be interpreted as, the primary load bearing member in fabric tensile test is the yarns in the direction of load. The yarns in the direction perpendicular to the direction of loading do not have a significant contribution to the tensile strength, although these yarns influence the load-elongation characteristics as evidenced by region II of the tensile elongation plot. A linear relationship between yarn count/sample width and rupture strength is also expected in the warp direction but there is a 30% increase in the yarn density in the longitudinal direction when compared to transverse. This significant increase in yarn densities is likely to make the tests in

104 longitudinal direction very sensitive to the slightest misalignment in the test rig. The misalignments can lead to introduction of shear forces resulting in a complex state in place of pure uniaxial tensile stresses leading to failure at lower loads.

Average Rupture Strength (Transverse) - Width

3500 y = 366.02x + 76.551 R2 = 0.9998 3000

2500

2000

1500

1000 Rupture Strength (in lbf)

500

0 0246810 Width of Sample (in Inch)

Figure 5.3 Linearity of the rupture strength with width of test sample in fill direction of fabric

5.3 Plane Strain Tension Tests

The loops stitched on the side (figure 3.12) to facilitate lateral load often served as pivots due to increased load in lateral direction resulting in increase in frictional force between the loops of fabric and the vertical tensioning rods and as the load was applied on the samples theses pivoted points did not have much motion resulting in shear force rather than plane strain loading (figure 4.12). Specialized equipment as discussed in chapter 2

105 are required for plane strain and biaxial testing of fabrics which involve special grips which have special arrangements to avoid the frictional effects.

5.4 Failure of Creep Samples

The failure of the samples at the loads could have been caused due to following two possible reasons or combinations of these effects. Firstly, reduction in modulus/strength with time and second, plasticizing effect due to interaction with water.

5.4.1 Modulus Temperature Behavior Failure can be attributed to reduction in modulus of material with increased temperature or any misalignments in the system. The temperature dependence of the modulus in viscoelastic material is quite significant. Drastic change in modulus for the viscoelastic material is evidenced at higher temperatures; the material becomes softer and weaker, generally associated with decrease in modulus. Figure 5.4 shows the change in modulus of a material as a function of temperature. The change in modulus with temperature can be differentiated into four regions. In the glassy region, thermal energy is not sufficient to overcome to result in any considerable reduction in modulus. The chain segments in the fabric are essentially “frozen” or fixed in position with segments vibrations restricted about the fixed mean positions. With increase in temperature, the amplitude of vibration increase and when the temperature is sufficiently high, the thermal energy overcomes the energy barriers to result in a drastic decrease in modulus. In this region, the polymer is at the glass transition temperature where the mobility in the chains increases and short- range diffusion motions begin. Segments have short lived conformations and jump from one lattice site to another; the glassy material turns resilient. This phenomenon is evidenced by a huge decrease in the modulus of material.

As temperature is further increased, the modulus reaches a plateau region. The material exhibits rubber like behavior in this region. This rubbery plateau is characterized by a different modulus. In this temperature range, the time constants associated with short- range diffusion motion of the polymer segments are of the order of seconds. However,

106 long-range motion of chains molecules is still restricted as the thermal energy is not sufficient to overcome the local high energy interactions between neighboring chains. In the case of the crosslinked polymer, the long range motion is restricted due to chemical bonding between chains. In a linear polymer, high energy interactions between neighboring chains are a consequence of entanglements.

Figure 5.4 shows the modulus temperature curve for viscoelastic materials [73]

In a cross-linked network, as temperature increases, the crosslinks between chains consisting of physical bonding prevent the chains from moving relative to one another. In case of a linear polymer, the absence of chemical bonding leads to large-scale motion resulting in translational motion of polymer molecules relative to each other. When the temperature is sufficiently high, local chain interactions do not posses sufficient high energy to prevent translation.

5.4.2 Effect of Water The reduction in strength could be the result of plasticizing effect of water on the fabric. It was seen that the burst strength of the fabric was reduced to half (approx.). Water acts

107 as a plasticizing agent in some materials. Materials like nylon exhibit reduction in modulus and strength when exposed to water due to strong interaction between the polar group in amide and water. Omnisil is a silica fabric and it is known that silica has a strong affinity for water. Figure 5.5 shows a graphic representation of affinity of water to silica grains.

Figure 5.5 Interaction between silica and water molecules [75]

The reduction in modulus in modulus of the Omnisil fabric is evident from the instantaneous elongation data. The elongation of the fabric at 100 lbf load at 80°C was observed to be 0.75 inch. In a short term tensile test at room temperature a 0.75 inch elongation of 4 inch wide Omnisil corresponds to a approximately 50% rupture strength (~ 700 lbf) of the fabric. Thus, the fabric elongation has been significantly influenced by temperature and water. The increased elongation could be a consequence of the reduction in modulus of the material and plasticizing effect of water; the reduction in modulus is often associated with a decrease in strength.

108 5.5 Modulus –Time Behavior of Viscoelastic Material

Figure 4.28 and 4.29 demonstrate the use of accelerate creep data for prediction of elongation at relatively lower temperature. It has been observed that with the limited data and the given loading and the temperatures of interest the fabric creep is almost negligible. However, a complete characterization of the creep elongation should be performed with the use of additional experimentation. The shift factors however reveal that longer period (3000 hours approx.) of experimentation is required for extrapolating the elongation behavior to 10,000 hours based on the universal values of constants used in calculation of shift factors. However, a more specific material constant for C1, C2 and

Tg in WLF equation might lead to concluding otherwise.

Figure 5.6 is a schematic plot of modulus-time behavior for viscoelastic material. The modulus is seen to fall from its initial high value to a lower modulus over a time period and, after evidencing a plateau, falls again. In case of Omnisil, it has been seen that the elongation after the initial application of load is fairly constant. Although it has been seen that significant reduction in strength of fabric occurs at 80°C in presence of water, Omnisil does not experience considerable reduction in modulus in these conditions over the length of time involved in creep testing.

5.6 Concluding Remarks

Test facilities for studying uniaxial tensile and creep behavior of OmniSil fabric were developed. The tensile behavior of OmniSil is typical to most woven fabrics and the failure strength exhibits a linear relationship to width of the sample. The work presented in this thesis shows the trend in fabric elongation at constant load of 4% failure strength up to 250 hours of testing. The creep (baseline) elongation of the fabric was found to be almost negligible for the first 250 hours of testing. The creep tests of shorter duration (100 and 250 hours) indicate that the fabric creep is fairly repeatable as shown in figure

109 4.26. Although creep test was performed for 800 hours (approx.), the data obtained after 250 hours of operation could not be distinguished from noise in the data acquisition system. A complete 1000 hour test should be performed before any conclusions on the long-term creep behavior of fabric can be drawn.

Figure 5.6 Variation of modulus of a viscoelastic material at constant temperature with time

The research presented so far constitutes the preliminary testing involved with structural characterization of OmniSil fabric for use in an ESP. In order to develop a complete understanding of the fabric behavior in an operational ESP, additional tests need to be performed including a study to estimate the magnitude of tensioning loads in ESP. Further characterization of the fabric should include tests that help understand the plane strain and biaxial tensile behavior of the fabric. A 4% rupture strength (approx.) was used for determining creep behavior of OmniSil, this load was estimated based on assumed weights of the frame that holds the membrane in an ESP and an estimated 3.5% failure strength (approx.) tensioning load to keep the membrane taut. Tests need to be performed to accurately measure the tensioning loads required to keep the membrane taut. Also, creep behavior of the fabric in acidic and basic environments needs to be studied to

110 evaluate the effects of chemicals on fabric creep. Chapter 6 describes some of the methods that can help better understanding the fabric behavior.

111 6.0 RECOMMENDATIONS

The work presented in this report attempt to characterize the some of the uniaxial tensile and creep behavior of Omnisil fabric. In order to develop complete understanding of the behavior of the fabric in operational conditions of ESP, a number of experiments have to be performed. In addition, a mathematical model can be developed to analytically characterize most of the tensile and shear behavior of woven fabrics.

6.1 Tensile Testing

Tensile Characterization

Experimental Modeling: Characterization Numerical/Analytical

• Geometrical Model – 12 HS weave • Analytical Modeling • Plane Strain Tensile Test • FEM/Mesoscale Models • Biaxial Tension Test • Continuum Models

Figure 6.1 Recommendation for tensile testing

It has been established that the fabric exhibits a linear relationship with rupture strength and width in case of pure uniaxial tension. In practice ESP membranes would be biaxially tensioned to maintain a constant separation from the charging electrodes. There is a need to understand the tensile behavior in presence of lateral loading. Figure 6.1 shows the various approaches that can be taken to understand the tensile behavior of fabric in such

112 cases. The tensile behavior can be understood by either experimentation or through modeling the behavior with data obtained from yarn tensile behavior. The test equipment used in the presented research is not equipped to perform tests that involve lateral loads. However, specialized equipment is available for such testing. Alternately a mechanism as shown in figure 6.2 should be developed as an attachment to MTS machine to facilitate application of lateral restraining loads without introducing any frictional forces that would influence testing of fabric. The clamps along the height of the sample should be able to move vertically without any obstructions to avoid introduction of shear dominated failure.

Figure 6.2 Schematic for plane strain tensile test [74]

Biaxial test on fabrics can be performed with a crucifix sample but the application of loads in longitudinal and transverse directions requires the use of specialized equipment. A schematic of the biaxial tension test can be seen in figure 6.3. The biaxial behavior of the fabric can also be determined from a geometric model of the fabric weave. A representative volume element or a repeat unit has to be established through careful study

113 of 12HS weave of Omnisil. The geometric model of a plain weave can be considered as a good approximation of Omnisil weave for all approximate studies. Derivation of equations of equilibrium for the unit cell under the action of biaxial loading can lead to prediction of tensile behavior of the fabric. However, additional yarn properties like yarn consolidation, yarn bending, yarn flattening etc. would be required. Many analytical models for textile mechanics are available in literature. Some of the widely accepted analytical models for prediction of tensile behavior are developed by Kawabata et.al., Realff, Oloffson, Grosberg and Kedia etc [41-53, 77]. In additional to the analytical models, a number of numerical models can also be developed by the use of finite element modeling or mesoscale modeling. Sagar et.al have used FEM for characterizing biaxial tensile behavior of a plain weave fabric [57]. Some researchers have also reported use of continuum models for prediction of tensile properties [54].

Figure 6.3 Schematic of a biaxial tension test on woven fabric [74]

114 6.2 Creep Testing

A 1000 hour creep testing with water at 80˚C should be performed (Test III; table 6.2) at 4% rupture strength to complete baseline creep characterization of OmniSil fabric. In order to make the results from the creep tests more applicable for prediction of elongation behavior of membranes in an operational ESP, a study of the magnitude of loads required to keep membrane taut needs to be performed. A series of tests can be performed to study the effect of tensioning force on maximum achievable field strength in a wet ESP. A rig similar to that used in Laminar ESP [78] with single air channel consisting of two membranes can be used with necessary modifications. Table 6.1 shows a recommended matrix for determination of the tensile loads to be used in creep tests. The principle behind using samples with different dimensions is to characterize the dependence of tensioning load on membrane shape and dimensions. Three samples for each dimension mentioned in table 6.1 the sample would ensure independence and repeatability of results obtained. The output from this test would be a plot of field strength/power through-put to ESP vs. tensioning force. The average tensioning load per unit width above which there is no noticeable increase in field strength can be used in all creep tests for determination of long-term creep behavior.

Table 6.1 Test matrix for determination of tension loads for keeping membrane taut Electrode- Water Voltage * Test Sample membrane Flow (in KV) Number Dimensions (ft) Spacing * (inch) (gal/width) 1 1×1 1 0.5 50 2 3×3 1 0.5 50 3 4×3 1 0.5 50 4 3×2 1 0.5 50 * - Adjustable parameters to achieve desired field strength/power through-put to ESP

115 Figure 6.4 shows the schematic for experimental set-up that can be used for carrying out the experiments mentioned in table 6.1. These experiments will give better estimates of optimum tensioning loads for understanding long term elongation behavior in an ESP.

Table 6.2 shows the test matrix that can be used for creep experiments. Tests I, II, IV and V serve to establish independence and repeatability in data obtained from the creep experiments. In addition, Tests I and II also serve to provide an estimate of the magnitude of instantaneous elongation upon application of loads and choosing the point of re- zeroing the sensors to collect creep elongation following instantaneous elongation. The following test matrix (Table 6.2) is recommended for all future creep tests for complete characterization of time-elongation behavior of OmniSil. Also, the liquid medium for

each set of creep tests need to be varied to H2SO4 solution and ammonium salt solution to characterize the behavior of the fabric in presence of chemicals of acidic and basic nature. When conducting experiments with acidic or basic medium, all the metallic components inside the creep rig needs to be painted with anti-corrosive paint. The sensors and other component of it need to be enclosed in a chemical resistant material like heat shrink Teflon tubing with high shrink ratios to avoid any damage from the chemicals that can alter the voltage readings.

116

Omnisil Membranes Electrodes

Air Duct

Figure 6.4 Schematic of experimental set-up for tensile load determination

Table 6.2 Test matrix for creep tests Duration of Test Test No. Temperature Load (in Hours) I 80°C 100 II 80°C 100 Determined III 80°C 1000 from Table 6.1 IV 80°C 200 V 80°C 500

The core of the LVDT should be inspected often ensure proper working of the sensor. In the event of changing LVDT core due to damage from chemicals, a complete calibration of the sensors needs to be performed. Alternately, the core can be sprayed with Teflon but care should be taken to maintain free motion of the core inside the LVDT and

117 recalibrated before use. The resistance heater for heating the liquid needs to be replaced with a chemical resistant heater preferably Teflon coated.

A better moisture proof sealing procedure should be devised to eliminate the effects of noise in the output from sensors. This can be achieved by using a sensor safe dielectric paste in the connectors of the sensors and then sealing the connector with an electrical insulation tape and finally using heat shrink tubing with high shrink ratios at the elbow expander joint and other places of wiring.

118 References

[1] Carolyn B. Doty, "Air Pollution Control Technologies for Coal-Fired Power Plants,” Reports-Business Communications Company, Inc., E-116, June 2002.

[2] www.uic.com.au/whyu.pdf

[3] Marilyn Nemzer, "Energy for Keeps: Electricity from Renewable Energy," 1st ed., October 2003.

[4] “State of the Environment 1997,” NSW/EPA Report, 1997.

[5] “World Population Prospects,” Population Reference Bureau.

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