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
Slug Catchers
Effluent
Liquids
Solids
Yes
100%>50 microns and larger
100%>50 microns and larger
Dependent of Shell Velocity
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
Gas Scrubbers
Impingement Type Separation
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).
The design typically is horizontal with
inlet and outlet connections located on
the upper portion of vessel heads. The
primary method of separation is gravity.
Some designs incorporate impingement
plates, mesh pads, and vane packs to
facilitate separation. Figure 1 depicts a
typical knock out drum or slug catcher
design.
LLC
Gravity or Knock Out Drum
LLC
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
Centrifugal Separator
LLC
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.
Pocket
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.
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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
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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
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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 crosssectional 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
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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.
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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)
L e s s p r e c i s e t e m p e r a t u r e c o n t r o l ( c o m pa r e d t o i n d u c e d d r a ft )
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
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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)
or on the same end (even number of passes). This
Hot
Air
Hot Air
can be specified up front to best fit your piping or
design needs.
Cool
Air
Tube
Bundle
Figure 7
CONSTRUCTION CONSIDERATIONS
The main components of an ACHE consist of the
plenum, tube bundle and the drive system.
Maintenance walkways, louvers, hail screens, and
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
1 FAN
4
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
Tube
Nozzle
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
20
EMBEDDED
EXTRUDED
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
Pass Plate
Top Plate
End Plate
Bottom Plate
Fins
Tube
Removable
Coverplate
BOX HEADER
Tubesheet
End Plate
Stud Bolt
Bottom Plate
Nozzle
Top Plate
REMOVABLE COVER
Pass Plate
Fins
Removable
Coverplate
Tube
PLATE HEADER
(STUD BOLT)
Tubesheet
End Plate
Stud Bolt
Bottom Plate
REMOVABLE COVER
PLATE HEADER
(THRU BOLT)
21
Figure 15
Nozzle
Fins
Gasket
Tube
Plug
Nozzle
Through Bolt
Billet
Fins
Tube
Bonnet
BILLET HEADER
Pass Plate
Nozzle
Tubesheet
REMOVABLE BONNET
HEADER
Manifold
Fins
Billet
Tube
Nozzle
Plug Gasket
(Optional)
Fins
Tube
Pipe Headers
MANIFOLD BILLET
HEADER
MANIFOLD HEADER
22
Figure 16
16
9
14
10
3
13
1
4
5
2
11
7
8
6
12
3
15
16
16
18
9
3
17
PLUG HEADER
1
10
13
5
18
14
11
17
6
12
4
3
15
COVER PLATE HEADER
1.
2.
3.
4.
5.
6.
Tube Sheet
Plug Sheet
Top and Bottom Plates
End Plate
Tube
Pass Partition
7. Stiffener
8. Plug
9. Nozzel
10. Side Frame
11. Tube Spacer
12. Tube Support Cross-member
13.
14.
15.
16.
17.
18.
Tube Keeper
Vent
Drain
Instrument Connection
Cover Plate
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
6
4
8
3
9
8
6
3
8
5
4
9
5
RIGHT ANGLE GEAR FORCED DRAFT
DIRECT CONNECTED
RIGHT ANGLE GEAR INDUCED DRAFT
(FAN ABOVE TUBE BUNDLE SECTION)
2
1
6
8
3
8
Legend:
1. Fan Ring
2. Fan
3. Fan Shaft Bearing
4. Driver
5. Support Structure
6. Fan Shaft
7. Belt and Sheaves
8. Coupling
9. Right Angle Gear
4
9
5
RIGHT ANGLE GEAR FORCED DRAFT
WITH FAN SUPPORT
(FAN BELOW TUBE BUNDLE SECTION)
Figure 18
24
2
2
1
1
3
7
6
3
4
5
3
3
6
7
5
BELT FORCED DRAFT
(FAN BELOW TUBE BUNDLE)
4
BELT INDUCED DRAFT
(FAN ABOVE TUBE BUNDLE)
1
2
1
2
5
4
3
6
6
4
DIRECT CONNECTED
FORCED OR INDUCED DRAFT
7
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.
Technical
Definitions
Soil:
Spray
Mist
Atterberg or International Std. Classification System
adopted by Internat. Soc. Sci. Since 1934
Clay
Common Atmospheric
Dispersoids
O2
CO2
F2
C6H6
Carbon Black
CO
H2O
Zinc Oxide Fume
Colloidol
Silica
SO2
C4H10
HCl
Molecular diameters calculated
from viscosity data at 0°C.
Drizzle
Cement Dust
Sulfuric
Concentrator Mist
Contact
Pulverized Coal
Sulfuric Mist
Paint Pigments
Flotation Ores
Insecticide Dusts
Cl2
CH4
Mist
Ground Talc
Spray Dried Milk
Alkali Fume
Aitken
Nuclei
Rain
Fertilizer, Ground Limestone
Fly Ash
Coal Dust
Rosin Smoke
Oil Smokes
Tobacco Smoke
Metallurgical Dusts and Fumes
Ammonium Chloride Fume
Gas
Molecules
N2
Clouds and Fog
Gravel
Coarse Sand
Fine Sand
Silt
Smog
H2
Typical Particles
and
Gas Dispersoids
Dust
Fume
Solid:
Gas
Dispersiods
Beach Sand
Copyright by
Stanford Research Institute
Menlo Park, California
1959
Plant
Spores
Pollens
Milled Flour
Atmospheric Dust
Nebulizer Drops
Sea Salt Nuclei
Lung Damaging
Combustion
Pneumatic
Dust
Nuclei
Nozzle Drops
Hydraulic Nozzle Drops
Red Blood Cell Diameter (Adults): 7.5
0.3
Bacteria
Human Hair
Viruses
Ultrasonics
Settling Chambers
(very limited industrial application)
Centrifugal Separators
Liquid Scrubbers
Cloth Collectors
Packed Beds
Common Air Filters
High Efficiency Air Filters
Impingement Separators
Types of
Gas Cleaning
Equipment
Thermal Precipitation
Mechanical Separators
(used only for sampling)
Electrical Precipitators
*Stokes-Cunningham
factor included in
values given for air but
not included for water
0.0001
2
3
4 5 6
8
0.001
(1m
)
2
3
4 5 6
8
0.01
2
3
4 5 6
8
0.1
2
3
4 5 6
8
1
Particle Diameter, Microns ( )
2
3
4 5 6
8
10
2
3
4 5 6
8
100
2
3
4 5 6
8
1,000
(1mm.)
2
3
4 5 6
8
10,000
2
3
(1cm.)
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
AVERAGE
PARTICLE
SIZE
(microns)
PROPORTIONATE
QUANTITIES
by PARTICLE
COUNT
PER CENT
BY
PARTICLE
COUNT
PER CENT
BY
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
CHANGE OF
PARTICLE
SIZES
(microns)
Table 3
Images 12, 13, & 14 depict various environments with very different dust concentrations.
Image 13
Image 12
30
Image 14
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
31
Image 20
Image 21
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 minipleated micro-fiber mats.
The pre-filter in a static two stage barrier filter is typically 24" x 24" x 4" thick disposable glassfiber 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.
Image 22
Images 23 & 24 depict the two different types of self cleaning
air filter systems.
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
32
“Inertial & Puff”
Image 24
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
35
Erosion, corrosion
Primarily erosion, sometimes corrosion
and/or fouling
In this environment, the widest possible
variations in dust particle concentration and
type exist.
Erosive, fouling, and corrosive dust (gas) are
seen.
Fouling, corrosion, erosion
The particle size and concentrations will vary
dramatically depending on the immediate
surroundings.
In many cases the dust concentrations will be
quite low (0.04 grains/1000 cubic feet) and
require minimal filtration. However, in other
situations where equipment is located near
power stations or light industrial complexes
the concentrations will increase and/or the
amount of hydrocarbon suspended in the air
will require a high efficiency filter system.
Corrosion
Airborne salt crystals contribute to hot section
corrosion within the gas turbine. This occurs
after the salt combines with sulfur and/or
oxygen during the combustion phase and is
deposited on the hot section parts. Other
metals, primarily potassium, vanadium, and
lead, either as sulfates or oxides will also
contribute to the corrosion. Typically, 0.01
PPM of sodium chloride is considered to be
the maximum concentration.
Minimal
Is area primarily forest, where foliage
minimizes dust load ?
Is area agriculture, where wind blown dust
(especially during plowing or harvesting
seasons) may introduce high concentrations
of erosive dust ?
Effect On Equipment
Considerations
Materials
90% ASHRAE final filter with waterproof
media.
Pre-filter/coalescer combination.
Filter selection is typically a two stage.
The most critical factor in filter media selection
is treatment of the salt particulate.
Of prime importance is weather protection use high efficiency weather louvers.
The filter housing is generally constructed of The filter housing is generally constructed of
carbon steel appropriately coated with an all carbon steel that includes a corrosion
protective coating appropriate for the site
weather paint system.
conditions. In many cases stainless steel or
aluminum materials are utilized.
Self-cleaning systems should be considered
if equipment is located in areas subjected to
hoar frost or ice fogs.
Self-cleaning systems should be considered
if equipment is located in farming areas where
blowing dust during plowing and harvesting
season subject system to high levels of
erosive particulate.
Filter selection is typically a two stage. Prefilter and static barrier final filter. 60% ASHRAE
final filter.
Of prime importance is weather protection use rain hoods or weather louvers, (Snow
hoods if located in northern climate).
Insect or bug screens optional.
1.0 to 500* microns *(during severe sand
storms)
0.01 to 50.00* microns * (in emission areas
of chimneys)
0.01 to 20.00 micronst
0.01 to 3.00 microns
0.01 to 3.00 microns
Particle Size
Equipment Selection
0.10 to 500.00 gr./1000 cu. Ft.
0.05 to 4.50 gr./1000 cu. Ft.
0.01 to 0.13 gr./1000 cu. Ft.
0.01 to 0.30 gr./1000 cu. Ft.
0.01 to 0.05 gr./1000 cu. Ft.
Dust Concentration
Sodium chloride is usually present in the form
of a solid salt nuclei of about one micron
equivalent diameter. However, with elevated
humidity levels (above 75% RH) the salt will
be carried in solution. When this condition
occurs it is critical that the filter medium be
waterproof to insure that saline droplets do
not pass downstream of the filter with the risk
of evaporation that allows by-passing of dry
salt particulate and leaching when humidity
levels decrease.
0.004 to 0.10 gr./1000 cu. Ft.
Dry, erosive in sand storm areas; fine talc-like
in areas of non-sand storm.
Sooty-oily (hydrocarbons), corrosive gases,
erosive dust
Dry, non-erosive (insects, airborne fibers)
Types of Dust
Where dust concentrations exist above 0.05
grains/1000 cubic feet a self-cleaning system
would be recommended.
The filter housing is generally constructed of
carbon steel appropriately coated with a high
quality paint system which has minimal
corrosion protection that will provide for long
life and minimal maintenance.
Augmented self-cleaning that combines the
pulse and inertial type self-cleaning systems.
The filter housing is generally constructed of The filter housing is generally constructed of
carbon steel appropriately coated with a high carbon steel, painted with a high quality paint
system which provides corrosion protection.
quality paint system that will provide the
protection necessary for long life and minimal
maintenance. In cases where high
concentrations of corrosive gas and/or dust
exist, material selection may require specific
grades of stainless steel.
Note:
High levels of hydrocarbon prevent
dislodgement of agglomerated dust in selfcleaning systems and thereby reduce the
ability of the filter to clean itself.
Inertial self-cleaning with a high efficiency
static final filter.
Oil wetted pre-filter.
Oil wetted pre-filter in areas with dust
concentrations greater than 0.06 gr./1000 cu.
ft.
If extended freezing conditions exist anti-icing
system should be utilized.
Pulse-type self-cleaning.
Filter selection is typically a two stage. Prefilter and static barrier final filter. 90% ASHRAE
final filter.
Filter selection is typically a two stage. Prefilter and static barrier final filter. 60% ASHRAE
final filter.
There are several different arrangements to
achieve the necessary results:
Weather protection - use rain hoods or
weather louvers, (Snow hoods if located in
northern climate).
Of prime importance is weather protection use rain hoods or weather louvers, (Snow
hoods if located in northern climate).
Self-cleaning systems have proven to be the
optimum means of inlet protection due to their
ability to provide continuous, clean air without
excessive differential pressure.
Field experience shows that high dust
concentrations remain in suspension many
hours after the initial dust storm has expired.
Removal of salt is necessary if satisfactory
life on hot parts is to be achieved. Airborne
particulate concentrations in excess of 500
grains/1000 cubic feet of air can exist.
Aggregate dust particulate (i.e. silica and
sodium chloride) is common in desert
locations, specifically in the Middle East.
Inlet locations are recommended to be
elevated to reduce the effects of small gusts
of wind and movement from vehicular traffic.
+41 to +113 Degrees F
Non-erosive
+20 to +120 Degrees F
-4 to +95 Degrees F
-4 to +95 Degrees F
Sooty-oily (hydrocarbons), corrosive mist,
erosive dust
-15 to +80 Degrees F
Dry, non-erosive, but salt particles exist:
corrosive mist
-4 to +90 Degrees F
Temperature Range
Special paint systems are generally used to
prevent corrosion. However frequent use of
assorted grades of stainless steel can reduce
long term maintenance costs.
60% ASHRAE final filter with waterproof
media.
Combination pre-filter and coalescer.
Filter selection is typically a two stage.
Of prime importance is weather protection use both weather louvers and rain hoods.
Extended surface Insect screens.
Protection of insect swarms. Extended area
screens are employed minimizing the air
velocity through the screen open area, which
allows the insects to move from the screen;
there by preventing obstruction of the inlet air
flow.
Torrential rains must be considered in the
design. Insure minimal updraft velocities and
protect from horizontal rain.
Fouling
0.01 to 10.00 microns
Long, dry, sunny periods; high winds; sand and Hot, high humidity, monsoons, high winds,
dust storms; occasional heavy rain
insect swarms
Tropical
Inland
Sun, rain, snow, hail, smog, hoar frost, mist
Deserts
Sand Storms & Dusty Ground
Sun, rain, snow, hail, smog
Industrial Areas
Steel Works, Petro-Chemical, Cement Works,
Power Stations & Mining
Dry and sunny, rain, snow, sea mist, fog, ice
fog, insects
Large Cities
Power Stations & Chemical Plants
Sun, rain, snow, and some fog
Marine
Coastal & Off Shore Platforms
Inlet Air Filter System Selection Chart Based on System’s Environment
Rural
Inland
Weather Conditions
Table 4
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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 selfcleaning 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.
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Image 25
Image 26
Image 27
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:
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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?
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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:
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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
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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?
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Capacity Curve: Is a capacity curve supplied with the quote and is it specified
what pressure drop is assumed?
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Performance: What are the liquid and particle removal efficiencies and for what
micron size are they specified?
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Closure: What type and size closure is included?
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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?
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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
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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
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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?
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Performance: What is the filter arrestance by weight? Are all bids using the same
39
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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
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Price: Does the price include the options and cleaning and testing you required
in your specification or is it priced separately?
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Drawing Delivery: Does the drawing delivery meet your design requirements?
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Equipment Delivery: Does the equipment delivery meet your construction schedule?
Is the delivery guaranteed?
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Guarantee: Exactly what does the manufacturer guarantee? Does the guarantee
period start from shipment or from in-service?
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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?
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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.
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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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