COMPRESSOR STATION ANCILLARY EQUIPMENT: KNOW WHAT YOU ARE BUYING PART 1 Frederick J. Mueller Mueller Environmental Designs Michael A. Smith, P.E. Texas Gas Transmission LLC
ABSTRACT Compressor station design engineers and other design professionals are usually "systems" designers. These professionals design, specify and procure equipment that is engineered and fabricated by others. This course will address the basics of some of the ancillary equipment required to create a natural gas compressor station. This will help the designer to make more informed decisions. The equipment covered in Part 1 will be Natural Gas Separation / Filtration Equipment, Air Cooled Heat Exchangers (ACHE) and Intake Air Filters. The following are the basic areas covered for each type of equipment:
¨ What (is in the pipeline; are you cooling; is in the air) ¨ Types and/or designs available and their application; ¨ Construction considerations; ¨ Specifying your equipment; and ¨ Analyzing your options
Successful completion of the course will allow design personnel to create more detailed specification sheets and to better analyze the proposals received from vendors.
INTRODUCTION An introduction to a paper should normally begin with telling the reader what they are going to be told. This introduction, however, will start by telling you what you are not going to be told. This paper is not intended to teach all of the requirements for gas separator, air-cooled heat exchanger and air filter design and fabrication. You will not be able to completely design this equipment and you certainly will not be able to go into business for yourself making this equipment, at least, not because of this paper.
In today's gas pipeline industry, companies are striving to do more work with less people. As a result of that trend, gas pipeline design professionals are less able to target a specific area of expertise and less "expert" help is available as backup within the Company. As a group that designs, specifies and purchases equipment for all areas of a compressor station, it is only practical that we adopt the "jack-of-all-trades, master of none" mentality.
What you will gain from this paper is the ability to write a specification for this equipment to insure that it meets your specific needs and, as a result, you will be able to analyze the bids you receive to be sure that the equipment offered meets your needs. 1 Most of the information in this paper is "industry standard" and some might even be termed "common sense", at least to the technically minded. Much of the information is based on the experiences of the authors who have 43 years of combined experience in the gas pipeline industry. If you have several years of experience in this industry, your experiences may vary from some of those presented in this paper since design requirements and philosophies can vary greatly in different parts of the country. If you are relatively new to the gas pipeline industry, you will certainly have many experiences ahead of you that can build upon what you learn here. Remember, we usually learn much more from our mistakes than from our successes.
NATURAL GAS SEPARATION / FILTRATION EQUIPMENT Separation/filtration is the removal of any unwanted liquid or solid that may cause damage to equipment or contaminate the end product.
Some of the separation/filtration applications in the transmission of natural gas include:
¨Compressor station suction to protect the compressor from liquid slugs and prevent cylinder wear from solids.
¨Removal of oil from reciprocating compressors to improve pipeline efficiency.
¨Removal of liquid hydrocarbons, water, sand, and pipe scale from the gas at metering stations and city gates.
¨Protection of desicant beds, to keep liquids and solids from fouling desicant.
¨Gas storage to prevent injection or withdrawal of liquids and solids.
¨Removal of solids and liquids in fuel lines to power plants, industrial plants and engines.
The equipment used in the removal of liquid and/or solids from the gas stream are commonly referred to as slug catchers, scrubbers, filter separators, and coalescers. Each of these devices have unique separation/filtration techniques.
Problems encountered in the gas transmission industry are as varied as the contaminants in the pipeline.
WHAT IS IN THE GAS PIPELINE BESIDES NATURAL GAS? The most common contaminants in natural gas pipelines include water, lubricating fluids, amine, glycol, drilling fluids, liquid hydrocarbon, salts, chlorides, sand, dirt and black powder.
Black powder is a catchall term that describes a material that can be wet, with a tar-like appearance, or dry and be a fine powder. Components of black powder can be chemically broken down into several forms of iron oxide and iron sulfide. It can be pyrophoric when exposed to air. Black powder mixes with most or all of the above stated contaminants. Images 1 & 2 are examples
2 Dry Black Powder
Image 1
Wet Black Powder
Image 2
3 TYPICAL SEPARATION/FILTRATION EQUIPMENT (See Table 1 for performance)
Separator Type Slugs Solids Liquids Effluent Slug Catchers Yes 100%>50 microns and larger 100%>50 microns and larger Dependent of Shell Velocity Gas Scrubbers Impingement Type Separation Mesh Pad No No 100%>8 microns and larger <0.10 Gallons per MMSCF Vane Packs No No 98%>10 microns and larger <0.10 Gallons per MMSCF Centrifugal No 98%>10 microns and larger 99%>10 microns and larger <0.10 Gallons per MMSCF Multi-Cyclone Yes-Intermittent 99%>5 microns and larger 100%>5 microns and larger <0.10 Gallons per MMSCF Helical Coil Yes-Continuous 99.95%>5 microns and larger 100%>5 microns and larger <0.10 Gallons per MMSCF Filter Separator No 99.5%>5 microns and larger 100%>3 microns and larger <0.01 Gallons per MMSCF Coalescing Separator No 99.98%>.03 microns and larger 99.98%>.03 microns and larger <0.01 Gallons per MMSCF Table 1
Slug catchers - This particular separator design is able to absorb sustained inline flow of large liquid volumes at irregular intervals (slug and annular flow regimes). LLC The design typically is horizontal with inlet and outlet connections located on the upper portion of vessel heads. The Gravity or Knock Out Drum primary method of separation is gravity. Some designs incorporate impingement plates, mesh pads, and vane packs to facilitate separation. Figure 1 depicts a LLC typical knock out drum or slug catcher design. Figure 1
Applications include withdrawal out of storage before compression, sections of pipeline prone to liquid slugging, gathering systems and two phase pipeline systems.
Advantages ¨ Ability to absorb large volumes of fluid ¨ Lower initial cost when compared to other separator designs
Disadvantages ¨ Not very efficient, particularly if concerned with mist flow regime
Gas Scrubbers - The term gas scrubber typically is applied to a class of separators that utilize centrifugal and impingement separation techniques to remove liquid from gas in a mixed phase gas stream. They utilize mesh pads, centrifugal elements, and vane packs for separation. Figure 2, depicts centrifugal, mesh pad, and vane type separators.
4 LLC LLC
Centrifugal Separator Wire Mesh Separator Vane Type Separator Figure 2
Applications include removal of liquid hydrocarbons, water, lubricating and other fluids ahead of compressor suction, absorption plants, gathering lines, metering stations and gas storage.
Mesh pad type separators employ impingement wire mesh pads, (Image 3) rely on downward force of gravity to overcome the upward gas velocity and surface tension of liquid for separation.
Advantages ¨ Removes gross liquid flow and mist particles ¨ Lower initial cost when compared to other scrubber designs ¨ Can be used in slug catchers when configured properly
Disadvantages ¨ Sensitive to design flow conditions; at low velocity, efficiency is reduced due to drift and at high velocity due to flooding. ¨ Applicable for liquids only. Dirt, solids, and sticky viscous liquids will plug mesh pad. Image 3
Centrifugal type separators impart a change in the direction of flow, moving liquids and solids to the periphery of the shell. Gravity will separate these particles from the gas flow. Centrifugal separators are velocity dependent for separation performance. Illustration 1 is a centrifugal type separator.
5 Illustration 1 Advantages ¨ Removes mist particles ¨ Removes small amounts of solids ¨ Lower initial cost when compared to vane type scrubber design
Disadvantages ¨ Sensitive to design flow conditions, poor turn down rate when compared to vane type scrubbers ¨ Particle removal rate and size is dependent on inlet velocity and the number of turns the particulate makes within the separator
Vane type separators make use of a labyrinth formed from sinusoidal parallel plates with pockets. Flowing gas and liquids change direction a number of times causing liquids to become captured in side pockets. Once captured, liquids drain downward to a liquid holding sump due to gravity. Figure 3 is vane type separator elements.
Hook Figure 3 Advantages ¨ Removes both gross liquids flow and small mist particles. ¨ Good down turn ratio (not velocity sensitive) ¨ Can be used in slug catchers when configured properly
Disadvantages ¨ Applicable for liquids only. Dirt, solids, and sticky viscous liquids will plug drain pockets. Filter Separators - Filter separators are designed to provide removal of both solids and liquids from the gas stream. The filter separator employs filter elements and vane packs to achieve impingement, diffusion and interception separation/filtration methods to reach its efficiency. Illustration 2, is representative of a filter/separator design. 6 Illustration 2 Advantages ¨ Removes both liquids and solid particles to one micron in size. ¨ Good turn down ratio (not velocity sensitive)
Disadvantages ¨ Can not handle intermittent slugs ¨ High maintenance requirements ¨ Replacement filter costs
Gas Liquid/Solid Separator - Gas liquid/solid separator refers to a type of separator that makes use of cyclone tube and helical coil separators to remove both solids and liquid from gas in a mixed phase gas stream. Illustration 3 depicts a multi-cyclone type separator.
Advantages ¨ Removes both liquids and solid particles to six micron in size. ¨ Good turn down ratio (not velocity sensitive) ¨ Can be used as a slug catcher ¨ Low maintenance requirements
Disadvantages ¨ Initial cost higher when compared to filter separator
Construction Considerations
Wear plates - Wear plates are added to the shell wherever internal components would normally attach to the inner shell. The purpose of the wear plates is to minimize shell degradation in the event that hydraulic forces or corrosion would create a condition that internal parts would shear at the inner shell weld attachment. Without wear plates the shell could possibly fail. With wear plates the shell probably would remain usable.
7 Illustration 3 Separation components - Separation component design and materials of construction are primarily a function of the manufacturers proprietary design, and secondarily the environment in which the separating components are subjected to in the gas stream. Separation elements can be as simple as a tangential baffle plate, illustration 4, welded to the inner shell adjacent to the inlet or as complex as a helical coil tube sheet. See images 4 & 5. For the most part, separating elements that utilize thin materials of construction, such as vane or coalescer packs and wire mesh pads, should be fabricated from 304 or 316 stainless steel. Separating elements such as cyclone tubes or helical coils that are in a solids removal application should be manufactured from erosion resistant alloy steel.
Illustration 4
Image 4 Image 5 Filter elements - Filter separators utilize filter elements to capture solids and condition small aerosol liquid particles for eventual removal by the final separating element. Due to the potential of high-pressure differential, gas filter elements should be designed with a high collapse strength. The collapse strength is a function of the center tube fabrication. Filter manufacturers use perforated tubes or louvered slot tubes that are spiral wound and "locked" or seam welded. In either case, the filter element usually has a collapse strength of 75 to 100 PSI.
The next important aspect of the filter element is the media. The media has two functions, first is to remove solid particles from the gas stream, and secondly is to condition liquid aerosol particles to a size suitable for removal by the final separating element. Solid particles are removed from the gas stream by the filter media due to impingement, diffusion and straining. Liquid aerosols are coalesced by the same media. Coalescence is the mechanism where small droplets (aerosols) are agglomerated on a fiber mat or surface, and forms a continuous liquid film that periodically shears and releases large droplets back into the gas stream for eventual removal by the final separating element.
Media materials differ by manufacturer, and are typically considered proprietary. However, most are manufactured with fiberglass or synthetic fibers of graduated diameters and progressive 8 density. They are typically considered to be depth loading type filters. Depth loading is a function of particle size, fiber diameter and density; large particles captured on the "coarse" gas entering side and smaller particles are captured through-out the filter as the density becomes higher. Gaskets and end cap sealing is as important as the media. If the gasket or end cap seal fails, the entire separation system is compromised. A typical gasket material used is Buna-N. Gasket and end cap sealant must be able to withstand liquid hydrocarbons and other liquid contaminants in the gas stream. A typical sealant is a heat cured urethane adhesive.
Inspection Openings/Manways - ASME requires vessel inspection openings, however those specified by ASME are too small and inadequate for viewing critical components of the separator. It is advisable to require the vessel manufacturer to provide 6" blind flanged inspection openings where possible. Additionally, manways should be considered wherever separating devices such as vane packs, cyclone tubes, helical coils, etc. are employed in order to facilitate cleaning in the event of unforseen pipeline conditions.
Full / Partial opening closures - The type of filter holding devices used in the filter separator design determine whether or not a partial opening closure can be utilized. See image 6 depicting the necessary clearance required for element removal. See image 7, 8, 9, and 10 for the various types of closures. The use of partial closures may also be constrained by operating company policies.
Image 6
9 Image 7 Image 8
Image 9
Image 10
10 SPECIFICATIONS Now that we know everything there is to know about gas separation and filtration, we need to write a specification to tell our vendors what we need to accomplish with this piece of equipment. Almost every application is different in terms of what contaminants we have in the gas and what facilities are downstream of the proposed equipment. You may have done a good bit of research and know exactly whether you need a vertical gas scrubber or a horizontal filter separator, or you may only know that you have a problem that must be addressed. In the latter case, you may create your specification to only address the gas conditions encountered ahead of the proposed equipment and the gas conditions required downstream of the equipment. Be aware that this scenario can result in many different proposals being submitted for your request. One vendor may be overly conservative and propose a filter separator where a vane type separator would have sufficed. Another vendor may decide that an inexpensive slug catcher is all you really need since it is mainly for pigging operations when the pressure regulation downstream really demands that a vane-type scrubber be installed.
As a minimum, the specification for your gas conditioning equipment should contain the following information:
¨ Required Codes: Most vessels specified for high-pressure gas service are constructed according to ASME Section VIII. While this is not required by D.O.T., it is relatively cheap insurance. Of course the vessel must meet the minimum requirements of the Federal Safety Standards, CFR Title 49, Part 192. There may be some times you want to specify requirements that are in excess of the codes such as stress relieving and 100% x-ray. ¨ Configuration: Do you have plenty of room to place this equipment? If not, then a vertical vessel may be required. If you require the vessel to be able to store a large amount of liquid, a horizontal vessel may be a better choice. Slug catchers and scrubbers usually have process gas connections at, or near the top of the vessel. Does your piping design enable you to adequately support this elevated piping or should you require the vessel to have piping supports welded to the shell? Does the gas stream contain abrasive particles that may cause excessive wear on a vane pack? If so, but a vane type separator seems the best choice, make sure it is removable. Is this vessel a permanent installation at this location or is it possible it may be moved to another site at a later date? If this is the case and you know the type of equipment you want, you may want to create a drawing that specifies "hold" dimensions for certain vessel parameters. This way, you can standardize your piping design, which enhances the ability to move the equipment around. ¨ Gas Design Data: Your specification sheet will have to specify the parameters of your process fluid both ahead of and downstream of the vessel if you want the vendor to specify the correct equipment. Obviously, the data needs to include pressure, flow and temperature ranges. What is the allowable pressure drop? If this equipment is immediately ahead of pressure regulation, it may not be of great importance to you. It may be of some importance to the design of the equipment though, depending on the type. If you know what type of gas conditioning equipment will work best for you, then you only need to specify the upstream conditions. It is still best to include the downstream condition in the specification, but let the vendor fill in the information. This gives you a check on your choice of equipment. ¨ Design and Construction: Do special conditions warrant special materials of construction? If you do not specify, you will get a carbon steel vessel and maybe some stainless steel internals such as vane sections and mesh pads. Even if you do not have special needs, 11 it is best to include a section in your specification on materials so that you can best analyze the bids you receive. If the gas stream contains substantial amounts of solids you will want to verify that the internals will resist the erosion. Will this equipment be located on the pipeline or in a compressor station setting? If in a compressor station, are vibrations and pulsations a concern? If so, you may want to specify that internals be attached to wear plates and you may want to specify that internal components are made from heavier gauge materials. If you have actual pulsation data from your piping system, you may want to include this information with your specification. The vendor may or may not be able to utilize the data. If your vessel has a vane pack or mesh pad and if your gas contains measurable solids, you may want to specify that these items be removable and replaceable. If your vessel is a filter separator with a large number of filters, you will want to know about the vendors filter retaining system. Replacing filters in a filter separator is a time consuming job. If you have a 60" vessel with 120 filter elements, it will take quite some time to remove them if you have to unbolt each filter individually. Most vendors today have systems to release many filters by removing only 2 or 3 bolts. The size of the closure through which you remove the filters is of great importance also. Even if the closure is full diameter (the same size as the vessel shell), the removal of the filters is usually considered a confined entry on a large vessel. While a 30" closure on a 60" vessel allows access to all of the filter elements, ask operations how much complexity this adds to perform that maintenance with a breather pack on. The two most common types of closures are the screw type and the clamshell type. A bolted flange is obviously an option, but is very maintenance intensive. If you do not specify the exact closure you want, you want to make very sure that the proposed closure cannot be opened under pressure. Information on the filter elements themselves is also important to request in the specification for obvious reasons. On large vessels, you may want to specify a manway to facilitate inspection or cleaning. Most likely you will be hanging quite a bit of instrumentation from these vessels, so it is best to insure up front that the connections you need are included. Some of the most common instrument connections include:
¨ Thermal Relief ¨ Vent ¨ Liquid Level Controller(s) ¨ Dump Valve(s) ¨ Drain(s) ¨ Gauge Glass(s) ¨ Differential Pressure
For a slightly easier installation, you might also want to specify jacking bolts for the vessel. Other requirements to consider are liquid holding capacity, total filter surface area, vane pack cross- sectional area and fabrication tolerances. ¨ Coatings: Surface preparation should be specified since coating manufacturers recommendations can vary widely. Be sure to specify if you want the inside of the skirt painted on a vertical vessel. If you have the vendor paint the vessel and it is part of a large project, you may want to leave the top clear-coat (if used) to be performed in the field since clear-coat is hard to touch-up. ¨ Accessories: Accessories for gas filters and separators usually include liquid level controllers, automatic dump valves, gauge glasses, differential pressure gauges, high level alarms, thermal relief valves and vent valves. Most vendors will supply this equipment for you 12 or you can supply it yourself. The greatest benefit of having the vendor supply this instrumentation is that you can have the vendor supply all of the piping, valves, tubing and fittings required to connect this instrumentation. It will be removed for shipping, but is usually easy to re-assemble on site. If your vessel does not have a liquid collection bottle, then you need to have an exterior chamber to install the level controller and the float switch. Inserting the floats of these instruments into the gas stream may damage them. Make sure that you install filters ahead of your differential pressure gauge to avoid damage to it also. If you install a magnetic type level gauge, you can install a switch or switches to take the place of a separate high-level switch. Be careful that your dump valves are designed to work properly with the contaminants you expect to encounter. A dump valve with a 3/8" orifice will not work for a system that has black powder in it. A plug valve with a pneumatic operator may work better in this instance. ¨ Inspection: The vessel specification should specify any inspection points that are desired. Be careful to specify these as inspection points and not hold points unless that is what you truly want. Hold points can cost you money. ¨ Special Requirements: Documentation required and payment and delivery schedules should be included in the specification as well if not included elsewhere in the request for quote (RFQ). It is a good idea for this type of equipment to require a sketch of the vessel, including internals, and a capacity curve at a given pressure drop. If your project is on a tight schedule, you may also want to require written progress reports of the fabrication at specified intervals.
13 AIR COOLED HEAT EXCHANGERS (ACHE)
Air cooled heat exchangers, also known as "aerial coolers" or "fin fans", are used to cool process fluids by moving ambient air over the exterior of "finned" tubes through the use of a fan or blower, Figure 4. Natural draft type coolers, image 11, are not common in the gas transmission industry and will not be covered. Obviously, ACHE performance is greatly dependent on ambient air temperature. ACHE first appeared in the 1940s and matured into the current product by the 1960s. The application of ACHE in a typical gas compressor station include jacket water cooling, combustion air / lube oil cooling-water cooling, lube oil cooling, and high pressure gas cooling. There are other cooling applications, such as in gas processing plants, which will not be directly addressed in this paper.
Exhaust Air Hot Fluid
Cold Fluid
Ambient Air Figure 4
WHAT IS BEING COOLED AND WHAT ARE THE CHARACTERISTICS The characteristics of the process fluid are obviously of great importance to the system designer as well as the ACHE manufacturer. What additives are in the process fluid and how do these change the heat transfer characteristics? Can the additives react with the cooler materials? Is the process fluid a viscous material or is it compressible? Is the outlet temperature tightly controlled or are you in a situation of "the cooler the better"? Are the process fluid's properties greatly dependent on temperature? Do the maximum thermal design requirements coincide with the maximum flow and pressure drop requirements?
Image 11 14 TYPICAL CONFIGURATIONS Aerial coolers have various design configurations. The primary configurations used within the natural gas compression industry are induced and forced draft with a horizontal or vertical core. Forced draft aerial coolers with a horizontal core are the most prevalent of these in the gas transmission industry. Horizontal core, forced draft, and vertical discharge coolers, Figure 5, are coolers in which the tube bundle is on the discharge side of the fan and the tube bundle is in the horizontal position. In this configuration, the air flow is "pushed" across the tube bundle.
Air Flow
Air Flow Air Flow
Figure 5
Air Flow Air Flow
Air Flow
Figure 6 15 As previously stated, this is the most common type of ACHE found at natural gas compressor stations. This is true for several reasons stated below:
¨ Slightly lower horsepower is necessary since the fan is in the cold air; ¨ Better accessibility of mechanical components; ¨ Initial cost is lower compared to other configurations; ¨ Easy to replace tube bundle
However, this configuration is not without it's drawbacks. The disadvantages of the forced draft configuration are as follows:
¨ Poor distribution of air over the tube bundle; ¨ Greatly increased possibility of hot air re-circulation due to low discharge velocity and no stack; ¨ Low natural draft capability on fan failure due to small stack effect; ¨ Total exposure of tube bundles to climate conditions resulting in operational problems and poorer process control. ¨ Slightly higher noise levels (compared to induced draft) ¨ Less precise temperature control (compared to induced draft)
Another popular configuration for ACHE is the horizontal core, induced draft, vertical discharge (Figure 6). The difference here is that the tube bundle is on the suction side of the fan. In this configuration, the airflow is "pulled" across the tube bundle. The advantages of this type of configuration are listed below:
¨ Better distribution of air across the tube bundle; ¨ Less possibility of air re-circulation because of high discharge velocity; ¨ Better process and temperature control; ¨ Better protection of tube bundle from climate conditions; ¨ Ability to provide some cooling in a fan failure mode due to natural draft stack effect; ¨ Lower noise levels at grade (compared to forced draft).
In addition, the disadvantages and limitations are:
¨ Possibly higher horsepower if air temperature rise is high; ¨ Outlet air temperature must be limited to prevent damage to drive system; ¨ Drive system is less accessible for maintenance and working conditions may be hot; ¨ Higher initial cost; ¨ Bundle replacement requires disassembly of unit.
The decision to use forced draft or induced draft coolers comes down to site specific conditions, past experience, and operational requirements. Other draft configurations include natural and re-circulation. Horizontal or vertical core refer to the orientation of the tube bundle. Horizontal tube bundles are generally the most economical. Vertical tube bundles are used because of space constraints or when maximum drain back and/or head are required such as for condensing
16 service. Tube bundles can also be arranged in an "A" or "V" configuration, Figure 7, in order to save space. The disadvantages of this type are higher horsepower for a given capacity and decreased performance due to ambient winds on the exposed sides inhibiting air movement. The discharge direction of the cooling air is usually perpendicular to the length of the tube bundle. However, the cooler can be designed to force or direct the air discharge as needed (Figure 8). The configuration of the inlet and outlet connections is another factor to consider when specifying an Angled Cooler ACHE. The connections can be located on the top or the bottom of the header or both and they can be located on opposite ends (odd number of passes) Hot or on the same end (even number of passes). This Air Hot Air can be specified up front to best fit your piping or design needs. Cool Tube Air Bundle CONSTRUCTION CONSIDERATIONS The main components of an ACHE consist of the plenum, tube bundle and the drive system. Maintenance walkways, louvers, hail screens, and Figure 7 other optional equipment will be covered under the specifications section as will the support structure.
PLENUM DRAFT TYPES
17 Figure 8 PLENUM COMPONENTS
3 4
3 5 4
3 3 4 1 FAN
1
2 5
2 5 Legend: 1. Mid-Panel 2. Fan Deck 3. End Panel 4. Side Panel 5. Fan Ring (Not Shown) 4 3
2 FANS Figure 9 The plenum, Figure 9, is essentially the ducting that directs the flow of air over the tube bundle. They can be designed as a box type, Figure 10, or a transition type, Figure 11. The transition type plenum gives the best distribution of air over the tube bundle, but is normally used only for induced draft coolers. This is due to the fabrication difficulties encountered when trying to apply the design to a forced draft cooler. The plenum is normally of carbon steel, but stainless steel may be warranted in corrosive atmospheres or on an offshore platform. BOX TYPE PLENUM
Figure 10
TRANSITION TYPE PLENUM
18 Figure 11 The fan ring, Figure 12, which is attached to the plenum, defines the tip clearance of the fan blades. The fan ring plays an important role in re-circulation, fan power usage and noise control. By minimizing the clearance between the tip of the fan blade and the fan ring, re-circulation is minimized. This is extremely important since the outermost 10% of the fan blade typically does over 50% of the air moving work. By incorporating a bell-mouth entry to the fan ring to smooth the inlet air flow, the dynamic energy losses can be minimized and the fan power requirements lowered. Both of these can also result in a reduction of noise level.
FAN RING TYPES
TAPERED INLET EASED INLET
STRAIGHT FLANGED INLET
CHANNEL Figure 12
19 The tube bundle, Figure 13, is the actual heat transfer device and consists of many tubes covered with fins and is attached to fabricated headers. The tubes are usually layered in offset rows forming a triangular pitch with the fin tips of adjacent tubes either touching or separated by 1/16 inch to 1/4 inch. The tubes are either rolled or welded into the tube sheets of the headers. Depending on the required service, tubes can be made of carbon steel, stainless steel or admiralty brass. Aluminum fins are normally applied to the tubes to provide an extended surface area of 12 to 25 times the outside surface area of the base tubes.
TUBE BUNDLE COMPONENTS (Exploded View)
Lifting Lug Air Seal
Tube Keeper (Top) Header
Air Seal Nozzle Tube Fins Header Tube Spacer Side Frame Tube Support (Bottom) Notes: 1. Side Frame Flanges May Be Toed In or Out. 2. Structural Shapes Shown May Vary. Figure 13 The fins can be tension wrapped on the tubes, embedded in the tube or extruded from a sleeve pressed on the tube, Figure 14. Tension wrapped fins are most common for continuous service with temperatures below 300 °F due to economics. Within practical limits, the use of longer tubes and a greater number of rows usually results in less costly designs compared to shorter tubes and fewer rows.
FIN ATTACHMENT METHODS (Most Commonly Used)
L - FOOTED TENSION OVERLAPPED FOOTED TENSION EMBEDDED EXTRUDED 20 Figure 14 The headers for the tube bundle may be pipe, billet or box-type headers, Figures 15 and 16, with box-type comprising the majority in the gas transmission industry. The box-type header consists of a tube sheet, top, bottom, end plates, and a cover plate that may be welded or bolted on, Figure 17. If the cover is welded on, holes must be drilled and threaded opposite each tube for maintenance of the tubes. A plug is screwed into each hole and this cover plate is then called the plug sheet. Bolted removable cover plates are used for improved access to the headers and tubes in severe fouling services. Partitions are welded in the header(s) to establish the flow pattern.
HEADER TYPES
Nozzle Top Plate Pass/Stay Plate Gasket Fins Tube
Plug or
Tubesheet Nozzle Plugsheet
Top Plate Pass Plate End Plate Bottom Plate Fins
Removable Tube Coverplate BOX HEADER Tubesheet End Plate
Stud Bolt
Bottom Plate Nozzle
Top Plate Pass Plate REMOVABLE COVER Fins PLATE HEADER Removable (STUD BOLT) Coverplate Tube
Tubesheet End Plate
Stud Bolt
Bottom Plate
REMOVABLE COVER PLATE HEADER (THRU BOLT)
21 Figure 15 Nozzle
Gasket Fins
Tube
Plug Nozzle Through Bolt
Billet Fins Tube
BILLET HEADER Bonnet
Pass Plate Nozzle Tubesheet
REMOVABLE BONNET HEADER Manifold
Fins Billet
Tube
Plug Gasket Nozzle (Optional)
Fins Tube
Pipe Headers MANIFOLD BILLET HEADER
MANIFOLD HEADER
22 Figure 16 16 9 10 14 3 13 1 5 4
2 11 7 8 6
12
3 15 16 PLUG HEADER
16 18 9
3 10 18 17 13 14 1 5 11 17
6
12
3 4 15
COVER PLATE HEADER
1. Tube Sheet 7. Stiffener 13. Tube Keeper 2. Plug Sheet 8. Plug 14. Vent 3. Top and Bottom Plates 9. Nozzel 15. Drain 4. End Plate 10. Side Frame 16. Instrument Connection 5. Tube 11. Tube Spacer 17. Cover Plate 6. Pass Partition 12. Tube Support Cross-member 18. Gasket Figure 17 23 The drive system, Figures 18 and 19, main components are the driver, speed reducer, fan and support. The driver can be an electric motor, a hydraulic motor, gas engine driven or driven off of a PTO shaft. The electric motor is by far the most common. A speed reducer, if used, can be a right angle gear, a V-belt drive or a cog-belt drive. The air mover for an ACHE is commonly an axial flow fan. Fans can be made out of aluminum, reinforced plastic, steel or even wood. Fans can also be hollow or solid. The blades can be of fixed pitch or adjustable pitch with the pitch adjustment being manual or automatic. The drive system support can be structural steel, supported from the plenum or it can have it's own concrete foundation to minimize vibration transfer.
DRIVE COMPONENT ARRANGEMENTS
2 2 1 1
4 6 6 8 3 3 9 8 4 5 8 9
5 RIGHT ANGLE GEAR FORCED DRAFT DIRECT CONNECTED RIGHT ANGLE GEAR INDUCED DRAFT (FAN ABOVE TUBE BUNDLE SECTION) 2 1
6 3 Legend: 8 8 4 1. Fan Ring 2. Fan 9 3. Fan Shaft Bearing 4. Driver 5 5. Support Structure 6. Fan Shaft 7. Belt and Sheaves 8. Coupling RIGHT ANGLE GEAR FORCED DRAFT 9. Right Angle Gear WITH FAN SUPPORT (FAN BELOW TUBE BUNDLE SECTION) Figure 18
24 2 2 1 1
3 3 4 7 6 3 5 6 3 7 5
BELT FORCED DRAFT 4 (FAN BELOW TUBE BUNDLE) BELT INDUCED DRAFT (FAN ABOVE TUBE BUNDLE)
1 2 1 2 3 6 6 5 4
4 7
DIRECT CONNECTED FORCED OR INDUCED DRAFT 5 VERTICAL BELT FORCED OR INDUCED DRAFT Figure 19
SPECIFICATIONS Now that we are "experts" in the area of air-cooled heat exchangers, we need to write a specification to tell our vendors what we need and what we want for our ACHE. Philosophies and procedures are different for different companies and specifications can range from a single page with site / process fluid data only to 30 or more pages with the majority of the information being company "boilerplate". In many of the areas listed below, you, the designer may not have a preference. That doesn't mean that someone else doesn't. You will deal with this equipment for perhaps six months to a year during the design, procure and installation phase. The men and women in the field (Operations) are stuck with it for the next 15 to 20 years. They probably have some opinions, preferences and experiences to share. 25 As a minimum, the specification for your ACHE should contain the following information ¨ Required Codes: Most ACHE specified for gas compressor stations specify ASME Section VIII and API 661. Both of these codes apply to ACHE, but neither is required by regulation. In order to cut stocking and fabrication costs, manufacturers standardize much of their equipment design. As a result, even if you do not specify one of these codes, you will most likely get many of the benefits because much of the manufacturers standard design incorporates the code requirements already. While it is considered cheap insurance to require a cooler to be built to ASME, especially on high pressure gas coolers, the API 661 specification may have many requirements that are not as important to a particular application in a gas compressor station since the code is specified for refinery service. Cost savings by not requiring API 661 could range from as low as 3% to as high as 12%. If you are uncertain, don't be afraid to ask your vendor the differences in the cost and the final product. ¨ Configuration: Do you have plenty of room to place the cooler? If so, then a horizontal core cooler will work. Do you need precise control of the process fluid temperature? If not, then a forced draft will work fine. Is your piping existing or has the design already been determined? Do you need both the inlet and outlet connections on the same header or do you need one on each end of the cooler? Is this an oil cooler for a turbine in which case you want the oil to be able to drain back to the sump? If this is the case, connections on top of the header will not work for this application. Is this a cooler for a reciprocating engine requiring two bundles, one for jacket water and one for auxiliary water? Which bundle needs to be on which side? It is possible that you may not have requirements in some of these areas when you go out for bids. It doesn't hurt to ask upfront, so that, if your design requirements become stricter later, you have an idea of the potential changes required. ¨ Site / Process Fluid Design Data: Your specification sheet will have to specify the design parameters of both your process fluid and the ambient air conditions since that is your cooling medium. What are the minimum and maximum ambient temperatures? At what elevation will the equipment be installed? What is the process fluid? Does it have any additives that may change its heat transfer properties? Does it have any additives that might react with potential cooler materials? Obviously, the process fluid flow rates, pressures, temperatures and viscosity must be specified. Any other fluid properties you know could also be helpful in the cooler design. What is the allowable pressure drop? What is the velocity of the process fluid in the tubes and is it in line with industry standards? Will the maximum thermal design occur at the same time as the maximum flow condition? If not, required pressure drop may ultimately control the size of the cooler. You may want to provide a table of process flow conditions and let your bidders determine the critical design. ¨ Mechanical Equipment and Material: Do you know what material you want your tubes made from? What about the headers? What type of fin attachment do you want? For temperatures below 300 °F and not in a marine environment, wrap-on fins should work fine. For higher temperatures and/or marine environments, double-overlap wrap-on fins may work before deciding on the considerably more expensive extruded fins. Are vents and drains required for each header and do you need a special vent connection on your lube oil cooler to allow drain back? If you have a black powder problem at your station, it may be wise to invest in removable cover plate headers. How much fluid does the cooler hold? This is important for drain back calculations and for ordering additives. Do you require a minimum of two fans for the cooler in case one fails to allow curtailed operation? What fan material and design do you want? Stainless steel bolts in the fan will minimize failures due to corrosion. What is the maximum fan tip speed you will allow? If noise is an issue in this application, you should greatly limit fan tip speed and 26 use an electric motor with a v-belt drive. Hydraulic motor systems generate quite a large amount of noise. Cog-belt speed reducers generate a little more noise than a v-belt. In general, gear drives are louder still. Bell mouth inlets to the fan ring can also be specified to lower noise. How will you control the outlet temperature? Louvers? Two-speed motors? VFD motor drive? Thermostat or mixing valve? Automatic variable pitch fan(s)? Is the cooler vendor supplying the VFD? If not, you need to make sure that the electric motor is suitable for variable speed application. If the speed reducer is a right angle gear and the driver is a VFD, you need to make sure an exterior oiler is supplied for the gear. Most right angle gears rely on splash lubrication. If installed in a VFD system that is run less than 1/3 speed, a splash lubrication system will not operate properly. If this is a gas cooler, should the motor be explosion-proof? Do other applications require explosion-proof motors or will TEFC or even ODP suffice? Do you care if the motor driver is horizontal or vertical? Does Operations care? Ask! ¨ Structure Design: Materials of construction of the cooler plenum and support system can vary greatly among manufacturers, mostly in material thickness. If you know that you want the plenum to be made of 7-gauge steel (~3/16" thick) due to past experience, put it in the specification. Since the cooler is a structure exposed to the elements, a wind speed or loading design should be specified as well as a seismic zone. Do you want the drive system supported from the cooler, or will you provide a concrete foundation for it? If your piping layout requires a certain height for the cooler nozzles or if the cooler will be surrounded by other structures, specify a cooler or nozzle height. Support legs for coolers come in all shapes and sizes also. Some designs have standard length legs, two to three feet long, extending below the plenum. The height of the cooler is determined by adding a support column to this standard short leg. Because of the weakness of this "flanged" connection, much external cross bracing is required between the support legs. One-piece support legs do a much better job of providing cooler support than the "add-on" type. ¨ Coatings: Specifications for coatings usually boil down to galvanizing or painting. Hot-dipped galvanizing is a very good coating and is relatively inexpensive. The drawback is that there are no good cold patching coatings for galvanizing. If the galvanizing gets scratched during construction, it will be a continuous fight to keep the scratched area from bleeding rust. Painting is more expensive than galvanizing, but if a good job is done to start and a clear coat applied, the cooler will look good and be protected for many years. Surface preparation should be specified since coating manufacturers recommendations can vary widely. Be sure to specify if you want the inside of the plenum coated. If you paint the cooler and it is part of a large project, you may want to leave the top clear-coat (if used) to be performed in the field since clear-coat is hard to touch-up. ¨ Accessories: Accessories are many and varied as with any large piece of equipment. Common accessories include shutters (louvers), hail screens, fan guards, walkways and ladders, vibration switches, jacking bolts, expansion tanks, pneumatic controls, etc. For tube bundles exposed to the elements, hail screens should be specified. Fan guards should also always be specified. Shutters may not be required if a VFD is specified, but in regions prone to snow and ice, this practice should be re-evaluated. If your cooler does have shutters, where do you want the control located, at ground level or up on the walkway? Walkways and ladders are convenient on the header(s) of a cooler but certainly not always warranted. If you have a walkway across the headers, have you made sure they will not interfere with your piping? If your cooler has multiple bays with a walkway between the bays, the walkway should be solid (i.e. not open grating) to prevent re-circulation. But solid checker plate may not be the ideal solution if the location is prone to ice and snow due to accumulation and safety concerns. If a vibration 27 switch is specified, indicate if it is to be explosion-proof. ¨ Shop Cleaning, Testing and Inspection: The fabrication, assembly and coating of an ACHE leave many chances to have debris inside the tube bundle. Even the hydrostatic test water can leave residue in the cooler bundle. It is a good idea to include a procedure in the cooler specification for cleaning and testing to minimize this problem. A procedure to circulate water through the cooler and a filter after testing until the filter elements remain clean should alleviate this problem. The flow of water should be reversed periodically during this process. Purging the water from the cooler and following up with a nitrogen purge and immediate sealing of all connections should help minimize oxidation inside the tube bundle. The cooler specification should also specify any inspection points that are desired. Be careful to specify these as inspection points and not hold points unless that is what you truly want. Hold points can cost you money. ¨ Special Requirements: If the ACHE is to be a lube oil cooler, an oil flush of the cooler should be specified. Either specify a procedure to the cooler manufacturers or evaluate their standard procedure, which should be included in their proposal. Be sure that their standard procedure will meet the minimum requirements of the OEM if they are available. Documentation required and payment and delivery schedules should be included in the specification as well if not included elsewhere in the request for quote (RFQ).
28 INLET AIR FILTERS
Rotating equipment used in the natural gas transmission industry are primarily air compressors, reciprocating engines, gas turbines and electric motor drives. They all require clean air for optimum performance and life expectancy. Dirty intake air will cause erosion, fouling, corrosion and cooling air passage plugging.
WHAT IS IN THE AIR? Ambient air has many foreign components that are sticky, abrasive, wet or any combination of the three. The components consist of mineral dusts, sand, airborne salt, hydrocarbon aerosols, organic matter, rain, snow, and fog. They range in particle size from 0.3 to 30 microns in size. Table 2 provides particle characteristics.
Gas Solid: Fume Dust Technical Dispersiods Mist Spray Definitions Soil: Atterberg or International Std. Classification System adopted by Internat. Soc. Sci. Since 1934 Clay Silt Fine Sand Coarse Sand Gravel
Common Atmospheric Smog Clouds and Fog Mist Drizzle Rain Dispersoids Rosin Smoke Fertilizer, Ground Limestone Oil Smokes Fly Ash Tobacco Smoke Coal Dust Metallurgical Dusts and Fumes Ammonium Chloride Fume Cement Dust O CO C H 2 2 6 6 Sulfuric H F Cl Beach Sand 2 2 2 Concentrator Mist Contact Gas Carbon Black Pulverized Coal Molecules Sulfuric Mist Paint Pigments Flotation Ores Copyright by Typical Particles Zinc Oxide Fume Insecticide Dusts CH Stanford Research Institute and N2 4 SO2 Colloidol Ground Talc Menlo Park, California Gas Dispersoids CO H2O HCl C4H10 Silica 1959 Spray Dried Milk Plant Molecular diameters calculated Spores from viscosity data at 0°C. Alkali Fume Pollens Aitken Nuclei Milled Flour Atmospheric Dust Sea Salt Nuclei Nebulizer Drops Hydraulic Nozzle Drops Combustion Lung Damaging Pneumatic Nuclei Dust Nozzle Drops Red Blood Cell Diameter (Adults): 7.5 0.3 Viruses Bacteria Human Hair
Ultrasonics (very limited industrial application) Settling Chambers Centrifugal Separators Types of Liquid Scrubbers Cloth Collectors Gas Cleaning Packed Beds Equipment Common Air Filters High Efficiency Air Filters Impingement Separators
Thermal Precipitation Mechanical Separators (used only for sampling) Electrical Precipitators
*Stokes-Cunningham 234568 234568 234568 234568 234568 234568 234568 234568 23 factor included in 0.0001 0.001 0.01 0.1 1 10 100 1,000 10,000 values given for air but (1m ) (1mm.) (1cm.) not included for water Particle Diameter, Microns ( ) Prepared by C.E. Lapple STANFORD RESEARCH INSTITUTE Table 2
Table 3 represents a typical atmospheric dust sample based on size distribution. Dust concentration levels can vary between 0.01 to 300 grains per thousand cubic feet. 29 SIZE DISTRIBUTION OF A TYPICAL ATMOSPHERIC DUST SAMPLE
CHANGE OF AVERAGE PROPORTIONATE PER CENT PARTICLE PARTICLE QUANTITIES BY PER CENT SIZES SIZE by PARTICLE PARTICLE BY (microns) (microns) COUNT COUNT VOLUME
30-10 20 1,000 0.005% 28%
10-5 7 1/2 35,000 0.175 52
5-3 4 50,000 0.25 11
3-1 2 214,000 1.07 6
1 1/2 1/4 1,352,000 6.78 2
1/2-0 1/4 18,280,000 91.72 1
Table 3
Images 12, 13, & 14 depict various environments with very different dust concentrations.
Image 12 Image 13
Image 14 30 TYPICAL FILTRATION CONFIGURATIONS Heavy-duty type air filter systems used in the natural gas transmission industry typically have two configurations - static or self-cleaning. Images 15, 16 and 17 are typical inlet air filter housings.
Image 16
Image 15 Image 17
Most static air filtration systems include two stages of filtration, a pre-filter and high efficiency final filter. The pre-filter removes large dust and dirt particles from the entering air stream preventing premature loading of the high efficiency final filter. There are other static type air filtration systems that employ a single stage high efficiency "bag type" barrier filter, Image 18. Images 19, 20, & 21 are typical of high efficiency barrier type filter elements.
Image 18 Image 19 Image 20 Image 21
31 Air filter manufacturers have, for the most part, standardized the cross section face area of static type air filters to 24" X 24" (nominal dimension). The length or depth of the filter is dependent on service, dust holding capacity, filtration efficiency, and manufacturer's proprietary design.
High efficiency barrier filters commonly use pleated fiberglass, micro-fiberglass, synthetic fibers or paper media. Some manufacturers utilize corrugated aluminum separators or v-shaped mini- pleated micro-fiber mats.
The pre-filter in a static two stage barrier filter is typically 24" x 24" x 4" thick disposable glass- fiber pad. The purpose of the pre-filter is to lengthen the life of the high efficiency barrier filter. Image 22 is a typical pre-filter element. There are two types of self-cleaning filters. Both types utilize high pressure, reverse pulse, bursts of air to dislodge dust, dirt, ice, snow and other debris from the face of the filter. One type utilizes gravity settling for the removal of the dislodged material from the inlet air stream; the other type utilizes a secondary air circuit blower to create a vacuum to pneumatically remove the dislodged material away from the inlet airflow. Additionally, in this type of self-cleaning air filter, inlet air and debris separate due to inertial separation. The inertially separated debris is pulled into the secondary air circuit and pneumatically removed from the inlet airflow.
Images 23 & 24 depict the two different types of self cleaning air filter systems. Image 22 Both static and self-cleaning air filter systems have advantages and disadvantages. Many factors influence the selection of one over the other. For the most part, selection is based on the operating experiences of the end user, if it is a manned or un-manned facility, climatic conditions of the facility in which it is located, and the equipment for which it is protecting.
Huff & Puff Image 23 Inertial & Puff Image 24 32 Static Air Filters
Advantages of the static two-stage barrier filter versus self-cleaning types ¨ Conservation of utilities, air and electricity ¨ No maintenance requirements of a pulsing system and blower system ¨ Can readily adapt to new filtration technologies
Disadvantages of the static two-stage barrier filter versus the self-cleaning types ¨ Maintenance costs associated with removing, replacing, and disposing of used filters ¨ Material cost of replacement filter elements ¨ Higher initial cost
Self-cleaning Air Filters
Advantage of the gravity settling type self cleaning filter versus static and secondary air circuit type self-cleaning filters: ¨ Lower initial cost.
Disadvantages of the gravity settling type self-cleaning filter versus static and secondary air circuit type self-cleaning filters:
¨ Re-entrainment of the dislodged dust and dirt back onto the face of the filter element after reverse pulsing. ¨ Large footprint area required for placement ¨ Maintenance of the pulsing circuit ¨ Utility usage
Advantages of the secondary air circuit type self-cleaning air filter versus the static and gravity settling type self-cleaning filter: ¨ Pneumatically conveys dislodged dust and dirt away from the inlet air stream ¨ Incoming air and debris are inertially separated reducing the loading on the filter element, extending the life of the filter and reducing the reverse pulse cycles. ¨ Small footprint
The disadvantages of the secondary air circuit type self-cleaning air filter: ¨ High initial cost ¨ Utility usage ¨ Maintenance of the pulsing circuit, electric blower motor and blower
FABRICATION PARAMETERS There are many manufacturers of inlet air filtration systems. Each manufacturer has their own unique design and fabrication standard. The following are some of the significant fabrication differences between manufacturers that you should be a aware of:
33 Welded or Bolted Assembly Is the air filter housing a welded or bolted assembly? Bolted assemblies are susceptible to leakage due to gasket shrinkage, manufacturing imperfections, and loosening of the bolted connections. The issues associated with leakage are, (1) dust and dirt by-passing the air filter elements, (2) water and ice ingestion. Water ingestion typically does not pose as serious a concern as ice, both can cause harm to rotating machinery, but in the case of gas turbine applications, ice can be catastrophic
Angle or Formed Flanges Does the air filter housing have welded angle flanges or formed angles? Formed angle flanges are susceptible to leakage due to weak connections and manufacturing imperfections. The issues associated with leakage are the same as above.
Filter Holding Frame What is the filter holding frame design and construction? The filter frame is of primary importance. Without a good sealing surface of the filter element, bypassing occurs, producing a general failure of the filtration system.
Filter Element Access How is the filter element accessed? Air filter elements are maintenance items. If they are difficult to access, remove or replace, chances are they will not be properly maintained. On small air filter housings, are the weather protection devices lift off or hinged and latched doors? On large filter houses, are access doors large enough to accommodate personnel ingress and egress? Are access doors large enough to handle replacement filters with ease? Does the filter maintenance area facilitate filter removal and replacement without personnel discomfort or injury? Are filter elements easy to reach?
Materials of Construction What are the materials of construction? The air filter house construction should be compatible in relative strength of materials as the rotating machinery its protecting. Properly designed and fabricated air filtration equipment should have a life expectancy of twenty to thirty years.
SPECIFICATIONS The following issues should be considered when developing the specifications for the inlet air filtration system.
¨ Environment and Local Climactic Conditions: What is the facility's environment? What is the facility's climatic conditions? Environmental and climactic considerations include its setting; rural or agricultural, coastal or marine, large cities or industrial, desert or tropical. Each setting has its unique set of parameters. Table 4 lists many of the environmental issues to consider when specifying an inlet air filtration system. ¨ Trash Screens: Trash screens protect rotating parts of the machinery from any damage due to catastrophic failure of the filter element. They are placed downstream of the filter element(s) and should be manufactured such that they can sustain any impact forces due to flying filter element debris.
34
Inland
, monsoons, high winds, high monsoons, ,
Tropical
13 Degrees F Degrees 13
.
orrential rains must be considered in the in considered be must rains orrential
Fouling
0.004 to 0.10 gr./1000 cu. Ft. cu. gr./1000 0.10 to 0.004
+41 to +1 to +41
long term maintenance costs. maintenance term long assorted grades of stainless steel can reduce can steel stainless of grades assorted prevent corrosion. However frequent use of use frequent However corrosion. prevent Special paint systems are generally used to used generally are systems paint Special
media.
60% ASHRAE final filter with waterproof with filter final ASHRAE 60%
Combination pre-filter and coalescer. and pre-filter Combination
Filter selection is typically a two stage. two a typically is selection Filter use both weather louvers and rain hoods. rain and louvers weather both use Of prime importance is weather protection - protection weather is importance prime Of
Extended surface Insect screens. Insect surface Extended Non-erosive
0.01 to 10.00 microns 10.00 to 0.01
flow
there by preventing obstruction of the inlet air inlet the of obstruction preventing by there
allows the insects to move from the screen; the from move to insects the allows velocity through the screen open area, which area, open screen the through velocity screens are employed minimizing the air the minimizing employed are screens Protection of insect swarms. Extended area Extended swarms. insect of Protection protect from horizontal rain. horizontal from protect design. Insure minimal updraft velocities and velocities updraft minimal Insure design. T
insect swarms insect Hot, high humidity high Hot,
to be the be to
Deserts
, sunny periods; high winds; sand and sand winds; high periods; sunny ,
Sand Storms & Dusty Ground Dusty & Storms Sand
, erosive in sand storm areas; fine talc-like fine areas; storm sand in erosive ,
pulse and inertial type self-cleaning systems. self-cleaning type inertial and pulse
Augmented self-cleaning that combines the combines that self-cleaning Augmented
static final filter. final static Inertial self-cleaning with a high efficiency high a with self-cleaning Inertial
Pulse-type self-cleaning. Pulse-type
achieve the necessary results: necessary the achieve There are several different arrangements to arrangements different several are There excessive differential pressure. differential excessive ability to provide continuous, clean air without air clean continuous, provide to ability optimum means of inlet protection due to their to due protection inlet of means optimum Self-cleaning systems have proven proven have systems Self-cleaning
in areas of non-sand storm. non-sand of areas in Dry
hours after the initial dust storm has expired. has storm dust initial the after hours
concentrations remain in suspension many suspension in remain concentrations
Field experience shows that high dust high that shows experience Field
grains/1000 cubic feet of air can exist. can air of feet cubic grains/1000
particulate concentrations in excess of 500 of excess in concentrations particulate life on hot parts is to be achieved. Airborne achieved. be to is parts hot on life
Removal of salt is necessary if satisfactory if necessary is salt of Removal
locations, specifically in the Middle East. Middle the in specifically locations,
sodium chloride) is common in desert in common is chloride) sodium Aggregate dust particulate (i.e. silica and silica (i.e. particulate dust Aggregate of wind and movement from vehicular traffic. vehicular from movement and wind of elevated to reduce the effects of small gusts small of effects the reduce to elevated Inlet locations are recommended to be to recommended are locations Inlet
0.10 to 500.00 gr./1000 cu. Ft. cu. gr./1000 500.00 to 0.10
+20 to +120 Degrees F Degrees +120 to +20
storms) 1.0 to 500* microns *(during severe sand severe *(during microns 500* to 1.0
Erosion, corrosion Erosion,
system which provides corrosion protection. corrosion provides which system carbon steel, painted with a high quality paint quality high a with painted steel, carbon The filter housing is generally constructed of constructed generally is housing filter The
dust storms; occasional heavy rain heavy occasional storms; dust Long, dry Long,
ASHRAE
. 90% 90% .
Industrial Areas Industrial
Power Stations & Mining & Stations Power
teel Works, Petro-Chemical, Cement Works, Cement Petro-Chemical, Works, teel
and/or fouling and/or Primarily erosion, sometimes corrosion sometimes erosion, Primarily
0.05 to 4.50 gr./1000 cu. Ft. cu. gr./1000 4.50 to 0.05
grades of stainless steel. stainless of grades
exist, material selection may require specific require may selection material exist,
concentrations of corrosive gas and/or dust and/or gas corrosive of concentrations maintenance. In cases where high where cases In maintenance. protection necessary for long life and minimal and life long for necessary protection quality paint system that will provide the provide will that system paint quality carbon steel appropriately coated with a high a with coated appropriately steel carbon The filter housing is generally constructed of constructed generally is housing filter The
ability of the filter to clean itself. clean to filter the of ability
cleaning systems and thereby reduce the reduce thereby and systems cleaning
dislodgement of agglomerated dust in self- in dust agglomerated of dislodgement
High levels of hydrocarbon prevent hydrocarbon of levels High
Note:
would be recommended. be would grains/1000 cubic feet a self-cleaning system self-cleaning a feet cubic grains/1000
Where dust concentrations exist above 0.05 above exist concentrations dust Where
Oil wetted pre-filter. wetted Oil
final filter. final filter and static barrier final filter final barrier static and filter Filter selection is typically a two stage. Pre- stage. two a typically is selection Filter northern climate). northern weather louvers, (Snow hoods if located in located if hoods (Snow louvers, weather erosive dust erosive Weather protection - use rain hoods or hoods rain use - protection Weather Sooty-oily (hydrocarbons), corrosive gases, corrosive (hydrocarbons), Sooty-oily
S
seen. Erosive, fouling, and corrosive dust (gas) are (gas) dust corrosive and fouling, Erosive, type exist. type variations in dust particle concentration and concentration particle dust in variations In this environment, the widest possible widest the environment, this In
Sun, rain, snow, hail, smog, hoar frost, mist frost, hoar smog, hail, snow, rain, Sun,
of chimneys) of 0.01 to 50.00* microns * (in emission areas emission (in * microns 50.00* to 0.01
-4 to +95 Degrees F Degrees +95 to -4
ASHRAE
. 60% 60% .
and concentrations will vary will concentrations and
Large Cities Large
Power Stations & Chemical Plants Chemical & Stations Power
0.01 to 20.00 micronst 20.00 to 0.01
0.01 to 0.13 gr./1000 cu. Ft. cu. gr./1000 0.13 to 0.01
will require a high efficiency filter system. filter efficiency high a require will
amount of hydrocarbon suspended in the air the in suspended hydrocarbon of amount the concentrations will increase and/or the and/or increase will concentrations the
power stations or light industrial complexes industrial light or stations power
situations where equipment is located near located is equipment where situations
require minimal filtration. However, in other in However, filtration. minimal require quite low (0.04 grains/1000 cubic feet) and feet) cubic grains/1000 (0.04 low quite In many cases the dust concentrations will be will concentrations dust the cases many In surroundings. dramatically depending on the immediate the on depending dramatically The particle size size particle The
-4 to +95 Degrees F Degrees +95 to -4
Sun, rain, snow, hail, smog hail, snow, rain, Sun,
erosive dust erosive Sooty-oily (hydrocarbons), corrosive mist, corrosive (hydrocarbons), Sooty-oily
life and minimal maintenance. minimal and life corrosion protection that will provide for long for provide will that protection corrosion quality paint system which has minimal has which system paint quality carbon steel appropriately coated with a high a with coated appropriately steel carbon The filter housing is generally constructed of constructed generally is housing filter The
Fouling, corrosion, erosion corrosion, Fouling,
system should be utilized. be should system
If extended freezing conditions exist anti-icing exist conditions freezing extended If
ft.
concentrations greater than 0.06 gr./1000 cu. gr./1000 0.06 than greater concentrations
Oil wetted pre-filter in areas with dust with areas in pre-filter wetted Oil
final filter. final filter and static barrier final filter final barrier static and filter Filter selection is typically a two stage. Pre- stage. two a typically is selection Filter hoods if located in northern climate). northern in located if hoods use rain hoods or weather louvers, (Snow louvers, weather or hoods rain use Of prime importance is weather protection - protection weather is importance prime Of
, 0.01 ,
, with elevated with ,
, sea mist, fog, ice fog, mist, sea ,
. However .
Marine
, rain, snow rain, ,
Coastal & Off Shore Platforms Shore Off & Coastal
, non-erosive, but salt particles exist: particles salt but non-erosive, ,
levels decrease. levels
salt particulate and leaching when humidity when leaching and particulate salt
of evaporation that allows by-passing of dry of by-passing allows that evaporation of
not pass downstream of the filter with the risk the with filter the of downstream pass not
waterproof to insure that saline droplets do droplets saline that insure to waterproof
occurs it is critical that the filter medium be medium filter the that critical is it occurs
be carried in solution. When this condition this When solution. in carried be
humidity levels (above 75% RH) the salt will salt the RH) 75% (above levels humidity equivalent diameter equivalent of a solid salt nuclei of about one micron one about of nuclei salt solid a of
Sodium chloride is usually present in the form the in present usually is chloride Sodium
the maximum concentration. maximum the
PPM of sodium chloride is considered to be to considered is chloride sodium of PPM
contribute to the corrosion. Typically corrosion. the to contribute
lead, either as sulfates or oxides will also will oxides or sulfates as either lead,
metals, primarily potassium, vanadium, and vanadium, potassium, primarily metals, deposited on the hot section parts. Other parts. section hot the on deposited oxygen during the combustion phase and is and phase combustion the during oxygen after the salt combines with sulfur and/or sulfur with combines salt the after corrosion within the gas turbine. This occurs This turbine. gas the within corrosion Airborne salt crystals contribute to hot section hot to contribute crystals salt Airborne
media.
90% ASHRAE final filter with waterproof with filter final ASHRAE 90%
Pre-filter/coalescer combination. Pre-filter/coalescer Filter selection is typically a two stage. two a typically is selection Filter is treatment of the salt particulate. salt the of treatment is The most critical factor in filter media selection media filter in factor critical most The use high efficiency weather louvers. weather efficiency high use Of prime importance is weather protection - protection weather is importance prime Of
Corrosion
0.01 to 3.00 microns 3.00 to 0.01 0.01 to 0.30 gr./1000 cu. Ft. cu. gr./1000 0.30 to 0.01 -15 to +80 Degrees F Degrees +80 to -15
aluminum materials are utilized. are materials aluminum conditions. In many cases stainless steel or steel stainless cases many In conditions. protective coating appropriate for the site the for appropriate coating protective carbon steel that includes a corrosion a includes that steel carbon The filter housing is generally constructed of constructed generally is housing filter The
corrosive mist corrosive Dry fog, insects fog, Dry and sunny and Dry
ASHRAE
. 60% 60% .
generally constructed of constructed generally
Inlet Air Filter System Selection Chart Based on Systems Environment Systems on Based Chart Selection System Filter Air Inlet
Rural
Inland
, non-erosive (insects, airborne fibers) airborne (insects, non-erosive ,
0.01 to 0.05 gr./1000 cu. Ft. cu. gr./1000 0.05 to 0.01
weather paint system. paint weather
carbon steel appropriately coated with an all an with coated appropriately steel carbon The filter housing is is housing filter The
hoar frost or ice fogs. ice or frost hoar
if equipment is located in areas subjected to subjected areas in located is equipment if
Self-cleaning systems should be considered be should systems Self-cleaning
erosive particulate. erosive
season subject system to high levels of levels high to system subject season
blowing dust during plowing and harvesting and plowing during dust blowing
if equipment is located in farming areas where areas farming in located is equipment if
Self-cleaning systems should be considered be should systems Self-cleaning
final filter. final
filter and static barrier final filter final barrier static and filter Filter selection is typically a two stage. Pre- stage. two a typically is selection Filter hoods if located in northern climate). northern in located if hoods use rain hoods or weather louvers, (Snow louvers, weather or hoods rain use Of prime importance is weather protection - protection weather is importance prime Of Insect or bug screens optional. screens bug or Insect
of erosive dust ? dust erosive of
seasons) may introduce high concentrations high introduce may seasons) (especially during plowing or harvesting or plowing during (especially
Is area agriculture, where wind blown dust blown wind where agriculture, area Is minimizes dust load ? load dust minimizes
Is area primarily forest, where foliage where forest, primarily area Is Minimal
0.01 to 3.00 microns 3.00 to 0.01 Dry -4 to +90 Degrees F Degrees +90 to -4
Sun, rain, snow, and some fog some and snow, rain, Sun, ypes of Dust of ypes
emperature Range emperature Table 4
Particle Size Particle
Equipment Selection Equipment
Considerations
Materials
Effect On Equipment On Effect
Dust Concentration Dust T T Weather Conditions Weather 35 ¨ Pressure Drop: Pressure drop should be specified considering the type of application, i.e., reciprocating engine, air compressor, gas turbine, or electric motor drive. Intake system pressure drop analysis should allow 1" to 11/2" for dirty filters. ¨ Lighting: Lighting is a safety issue to be considered. Particularly on gas turbine installations where filter housings are large, filter maintenance areas, filter staging platform areas, and clean air plenums are all locations that lighting, though not essential, makes the installation a safer work environment. ¨ Instrumentation: Minimum instrumentation should include a pressure differential gauge across atmosphere to clean air plenum to measure filter loading. Pressure differential gauges for both pre-filters and final filters are good to use as maintenance indicators. Pressure differential switch alarms should be set for filter change out time, high-pressure differential shut down, etc. ¨ Access Stairs and Platforms: Access stairs and platforms may be required for some gas turbine applications. They are necessary for ingress and egress from the filter house. Platforms at the filter maintenance door facilitates filter element staging, thereby reducing filter change out time. ¨ Electric Motors: Electric motors are used for the secondary air circuit of one type of self- cleaning air filter. The electric motors should be TEFC, wired for the available electric service, and suitable for Class 1, Group D, Division 1 or 2 (dependent on pipeline operating procedures) service. ¨ Structural Support: The structural support should include all necessary columns, beams, braces, and erection bolts necessary to complete the assembly. All structural components should have match markings to facilitate assembly. ¨ Lifting Lugs: Lifting lugs are an essential item used to facilitate assembly of large gas turbine intake housings. Often systems ship as "erector sets". Utilization of straps or chains without the benefit of lifting lugs often damages finishes and creates time consuming rigging problems extending construction time. ¨ Vibration Isolation Joints: Vibration isolation joints isolates rotating equipment from stationary static ancillary equipment such as ducts, silencers, and air filter housings. It eliminates any axial or lateral loading from ancillary equipment to the rotating equipment and minimizes acoustic energy (external noise) transfer from rotating equipment to ancillary equipment. ¨ Combination filter/silencer: Air compressors and reciprocating engines sometimes can benefit from combining the intake silencer and air filter. The combination of these two applications results in reduced costs for purchased equipment, reduced construction costs due to handling one device instead of two, and reduced area in and around the rotating equipment. ¨ Backfire Relief (reciprocating engine application) Backfire relief devices protect engine intake components from damage due to engine intake explosions (backfire). Most intake explosions are caused by mechanical malfunction of the engine or by operator error. The magnitudes of intake explosions vary depending on the air/fuel mixture in the intake upon ignition. See Images 25, 26, 27, and 28 of damaged intake air filters due to backfire in the intake. ¨ Guarantee: Guarantee should cover both performance criteria and workmanship. As a minimum, pressure drop and filtration guarantees should be a part of the written specification. Additional language in the specification should include workmanship and craftsmanship.
36 Image 25
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37 Image 28 BID ANALYSIS OK. Your bids have come in. Several of the manufacturers ignored your specification sheet and sent in their standard design sheet without filling out your data sheets. You've explained to them that you took the time to create the data sheets; they need to take the time to fill them out if they want their bids evaluated. You've also addressed the bids that essentially said "I know what you said you wanted, but here is what you need". You've explained to them that you more than welcome new information and alternate bids that might save you some money, but you need primary bids that match the requirements of your specification sheet so that you can evaluate all bids equally. You are tired of talking to people on the phone. You are now down to three bidders from the original seven you sent an RFQ.
From your remaining quotes, you read carefully through the data sheets to verify that all bids truly match your requirements. At this time you are looking to see that your design requirements are met and the list of exceptions don't cause you concern that your requirements cannot be met and maintained. It's a good idea at this same time to have Purchasing review the standard terms and conditions so they can highlight the problem areas from their perspective. Once the design requirements are satisfied, it's time to look at price and delivery. Not just delivery of the equipment, but also delivery of the approval and certified drawings. Now the true "Bid Analysis" starts. At this point you have determined that your remaining bidders are supplying a unit that will meet your requirements. But how are they going to accomplish it? Some important parameters to investigate while analyzing your gas filter or separator bids are as follows:
¨ Is the type of vessel what you expected for the service? ¨ Does the claimed removal efficiency match the proposed type of vessel? ¨ How has the vendor balanced diameter, height (or length) and required volume?
How does the design of the proposed ACHE compare to these industry standards?
¨ Maximize tube length while maintaining 40% or more fan coverage. ¨ Cooler dimensions should have about 1 to 3 ratio (i.e. 10' wide x 30' long). ¨ Minimize tube rows to increase heat transfer effectiveness and minimize header height. ¨ Minimize tube diameters.
On your air intake proposals, look for the following:
¨ Is the intake filter designed for easy maintenance? ¨ Is it properly designed for potential ambient conditions? ¨ For reciprocating engine applications, has a backfire relief valve been included? ¨ What is the dust loading capacity of the filter elements?
Has your low priced bidder reduced the heat transfer area to reduce the cost of the equipment, but his horsepower requirements are double that of the others? This design will cost you more in power consumption for the life of the equipment. Is your low cost scrubber small in diameter but tall in height causing you to spend extra dollars on piping supports? Is it still a good deal? Do you agree with the thermal, pressure drop or velocity calculations of the vendors? How does
38 the calculated specific heat, heat exchanged and total transfer surface compare between the bidders for the ACHE? If they are not comparable, you probably need to get out the thermo book and back check the calculations. The key areas to cover in your bid analysis are:
NATURAL GAS SEPARATION / FILTRATION EQUIPMENT ¨ Pressure Drop: Are the pressure drops stated in the quotes within the limits set in your specification? Does the actual pressure drop specified include inlet and outlet losses? ¨ Capacity Curve: Is a capacity curve supplied with the quote and is it specified what pressure drop is assumed? ¨ Performance: What are the liquid and particle removal efficiencies and for what micron size are they specified? ¨ Closure: What type and size closure is included? ¨ Construction: Are internals made of erosion and corrosion resistant materials? Are wear plates and/or internal stiffeners included? How do the filtration area and vane section cross-sectional areas compare? ¨ Accessories: Are all required accessories included in the price or priced separately? Are the accessories tubed and piped up in the shop for easy installation in the field? Are float type accessories out of the path of the flowing gas stream? Are all of the connections included for the accessories specified, including those you may supply yourself? Can the accessory connections be moved to best fit your location and application?
ACHE ¨ Thermal Design: Do the thermal factors and calculations agree between the bidders and do they agree with your calculations? If not, find out why. ¨ Pressure Drop: Are the pressure drops stated in the quotes within the limits set in your specification? Does the actual pressure drop specified include inlet and outlet losses? ¨ Utility Consumption: Do some bids increase utility usage to reduce up-front costs while increasing long-term costs? Is calculated pressure drop less than or equal to allowable pressure drop? Does the stated velocity coincide with the calculated pressure drop? ¨ Construction: Does the sketch furnished with the quote provide enough detail to insure it will work with your piping design? Does the design look easy to maintain? Are turbulators included for a lube oil cooler? Will the volume of the tube bundle work with your drain-back requirements? Is the fan tip speed at or below your requirement? How many fans are used? What is the horsepower of the motors? Does the ACHE have one piece or two piece support legs? Is cross bracing supplied? If a gear drive speed reducer is supplied in a VFD application, is an auxiliary oiler supplied?
INLET AIR FILTERS ¨ Pressure Drop: Are the pressure drops stated in the quotes within the limits set in your specification? Is the pressure drop stated at the rated maximum airflow? Does the actual pressure drop specified include the piping or ducting? ¨ Performance: What is the filter arrestance by weight? Are all bids using the same 39 parameters when stating filter efficiencies? What are the initial and recommended final resistances of the filter? What is the dust holding capacity of the filter system? Are cut sheets supplied for the filter elements? ¨ Construction: Is the filter housing a durable, all-welded unit or is it made of bent sheet metal? How is the ducting connected? Are the filter elements easily replaced? Are all required supports and bracing included? What about required gaskets? Are insect screens supplied and removable? Is a trash screen provided to protect from ingesting failed filters? What is the inlet velocity of the air stream into the filter housing? Will it suck up rain and snow? Is a backfire relief valve or implosion door provided? Are manways provided to access the ducting? How much real estate does the filter take up? Are there any chances of physical or temperature interference with a nearby exhaust system? Does the support system remove all weight from the engine or turbine being supplied? ¨ Accessories: Are all required accessories included in the price or priced separately? What is the cost of replacement filters? Are all of your accessory connections present? Is any required lighting provided and conduits brought to a single junction box? Is a platform and stair/ladder provided? Do they meet OSHA requirements?
ALL EQUIPMENT ¨ Price: Does the price include the options and cleaning and testing you required in your specification or is it priced separately? ¨ Drawing Delivery: Does the drawing delivery meet your design requirements? ¨ Equipment Delivery: Does the equipment delivery meet your construction schedule? Is the delivery guaranteed? ¨ Guarantee: Exactly what does the manufacturer guarantee? Does the guarantee period start from shipment or from in-service? ¨ Terms & Conditions: Do the manufacturer's terms and conditions conflict with those of your company's standard purchase order? Are the conflicts simple enough to be resolved in a short time frame to keep the project on schedule? Should Legal be involved or just Purchasing? Does the manufacturer require a payment schedule? Does it conflict with your payment schedule? ¨ Exceptions and Clarifications: Self-explanatory.
The specification/data sheets for the equipment covers many areas that may or may not be useful to you in the bid analysis. But all of the information can be very useful in the future when it comes to troubleshooting, checking for possible upgrading and future replacement of the equipment.
40 References : Handbook of Separation Techniques for Chemical Engineers, Philip A Schweitzer, 3rd ed, ©1997 McGraw Hill Company, Inc. Fundamentals of Fluid Mechanics, Bruce R. Munson, Donald F. Young, Theodore H. Okiishi, 4th ed, ©2002 John Wiley & Sons, Inc.
Gas Engineers Handbook, 1st ed, ©1965 Industrial Press Inc.
Separation Handbook, E. J. Halter, 1st ed., ©1966, Burgess Manning Company
Engineering Data Book, Gas Processors Suppliers Association, Volumes 1 and 2, 11th ed., ©1998, Gas Processors Suppliers Association.
Flow of Fluids, Engineering Department, Crane Valves, ©1988, Crane Co.
Air Filter Selection Guide for Gas Turbines, D. G. Hill, and J. Le Merchant, AAF Co. Inc., Bulletin 157. Air Pollution, C. J. Regan, Heating and Ventilating Engineering and Journal of Air Conditioning. Environmental Factors -- Airborne Dust and Sound, W. B. Moyer, General Electric Co., U.S.A., Gas Turbine Reference Library No. GER2232 Gas Turbine Inlet Air Treatment, R. L. Loud, and A. A. Slaterpryce, General Electric Co., Schenectady, New York Black Powder in the Gas Industry, Richard M. Baldwin, Southwest Research Institute
Special Thanks for the use of additional information, graphics, photos, and illustrations from; GEA Masters Customer Presentation Hudson Products Corporation Hammco Air Coolers Burgess-Manning Incorporated Mueller Environmental Designs Inc.
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