what performance do you really need
TRANSCRIPT
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Specifying Required Performance when Purchasing Reciprocating Compressors
Purchasing a new compressor can be an involved and expensive process. Therefore, it is prudent that
companies require a full array of compressor performance from bidding compressor packagers/OEMs.Otherwise, buyers may end up with equipment that may not be useable over the full operating maps for
which the customers expect to operate.
OEMs and packagers have been delivering simple point performance to end-users for more than half acentury. While these are important reports to receive and review, there are additional types of
compressor performance reports available today. Not only do these new reports give further insight into
compressor operations and safety, but they also allow end-users to quickly review the compressorsabilities across entire envelopes of operating ranges, instead of just a handful of limited operating points.
A full performance review of a packagers proposed solution to a purchasers compression needs willhelp identify where the unit is truly useful, and where the proposed unit may have potential operating
limitations. Unfortunately, the standard deliverable of just a few printouts of performance at the
specified design points, along with a few printouts of select performance curves, may not always
identify potentially serious operating issues with reciprocating compressors. In general, as a unitsoperating map expands, it becomes even more important to fully review that units performance across
its entire operating map all pressures, all temperatures, all speeds, variations in gas being compressed,
and all load steps.
To meet the needs of end-users and to help clarify to packagers and OEMs what compressor
performance end-users really need, a useful check list for identifying various types of performance hasbeen co developed. This concise list is detailed on the following page. End-users can simply specify
which items are required and which items are highly recommended in any bid proposals. Items of nointerest are easily deleted from the list.
During the biding process, required/suggested items should be based on modeling of the proposed
unit(s), albeit with allowances for certain yet-to-be-determined parameters, such as more accurate
pressure drops via acoustic pulsation modeling. However, after the unit is purchased, all items shouldreflect the final compression system as delivered to the end-user.
Finally, the remaining sections of this document detail explicit types of compressor performance end-users should require/request from their packagers/OEMs when specifying new equipment, along with
justifications for why those types of performance are required, and what type of responses the end-user
might consider if a bidding packager/OEM cannot provide the required performance. Finally, eachsection contains a sample report, plot, curve, or data table of format similar to what the buyer should be
specifying.
Note: The various sections in this document are written so as to be independent of other items. Thus an
end-user can extract just the items desired and create a list of performance required by their company.
Electronic versions of this document in MS Word format can be downloaded from either
www.CompressorTech2.com or www.ACIServicesInc.com.
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Check List for Required Compressor Performance for New Units
Point Performance Runs
o All Guarantee and Design Points OEM Compressor Modeling File
Full Unit Informationo Detailed unit diagram, geometries, unloading device types/volumes, weights, etc.
Expected Pressure Drops
Safe Startup Maps
Maps Where Rod Load Issues Existo Include all pressure combinations, all load steps and full range of speeds
Maps Where Pin Non-reversal Issues Existo Include all pressure combinations, all load steps and full range of speeds
Highest Expected Discharge Temperature Mapso Include all pressure combinations, all load steps and full range of speedso Include associated Flow Rates
Highest Expected Discharge Pressure Mapso Include all pressure combinations, all load steps and full range of speeds
Sidestream Review
Unit Robustness Map
Performance Curveso Sets must include varying speed, when appropriateo Suction vs. Load, and Suction vs. Flow Curves:o Discharge vs. Load, and Discharge vs. Flow Curves:
Flow Maps
Unit (and/or System) Isentropic Efficiency Maps
Compression Ratio Maps
Load Step Curvature Review
Acoustic Review across Entire Operating Map
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Point Performance Runs for All Guarantee Points and All Design Points
Point Performance Runs Required Item
Item(s) Point Performance runs for all Guarantee Points and all Design Points.
Reason(s) Used as part of warranty for the unit being considered; used to verify that unit is correctlysized to meet specified flow rates at the specified guarantee and design points.
Response If data is not provided, then buyer should seriously consider an alternate
packager/OEM.
In general, point performance reports are always supplied. Usually there are from one (1) to five (5)guarantee points, and anywhere from one (1) to thirty (30) design points.
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Compressors Modeling File
Compressor Modeling File Required Item
Item(s) File of compressor model used in OEM software of unit, such as an eAjax file, an Ariel
RUN file, a Superior Cascade or Compass file, a DR-Size file, a KWPerformance file, or a
GE EZ Size Pro file.
Reason(s) Used for identifying compressor performance after unit is brought online. Used to identify
units hardware configuration. Used by end-user and third parties to create models of unitfor automation purposes, in-house modeling, for use in reservoir models, etc.
Response If file is not provided, then buyer should consider an alternate packager/OEM.
Each of the major compressor OEMs has their own performance software for modeling their line of
compressors. Thus, when a new unit is ordered, specify that the electronic file used for the model is to
also be delivered.
OEM Software Files
Ajax (Cameron Compression Systems) eAjax *.eaj
Ariel Ariel Performance Software *.run
Dresser-Rand DRSize *.inp
Gemini (GE Energy) EZSize Pro *.ezs
Knox Western KW-Performance Software *.kps
Superior (Cameron Compression Systems) Cascade *.cas
Superior (Cameron Compression Systems) Compass *.ccs
Compressor performance software is used to predict loads, flow rates, andareas where high discharge temperatures, high interstage pressures, rodloads, pin non-lubrication, low volumetric efficiencies, and other issues,
such as formation of liquids, may occur. These predictions are based ontheoretical modeling, which should be reasonably close to measured data.
Therefore, somewhere in the OEM software, there should be a section of
text detailing within what tolerance levels the OEM predictions typically
compare with measured data, and also specifications for when thosetolerance levels actually apply.
Note: Attenuation devices, piping changes,
cooler changes, different gas analyses, etc. all
can impact system pressure drops and gas
pulsations. These in turn affect measured loads
and flows. Therefore, those items need to be
compensated in the compressor model if you
expect predicted loads and flows to be consistent
with measured data.
Theoretical P-V Card Actual P-V Card
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Full Unit Information
Complete Unit Geometry Required Item
Item(s) Driver ratings, compressor frame stroke, frame and cylinder ratings, weights and limits
used for rod load and pin reversal calculations, cylinder bore and rod diameters, connecting
rod length, cylinder volumetric clearances (base clearances per end, actual clearances withunloading devices per end, volumes of all volume bottles), and valve information for each
type of valve.
Reason(s) Necessary information if additional load/flow devices are to be added later to the unit or if
new types of valves are later used. Required information to create automation routines to
effectively and safely control the unit.
Response If data is not provided, then buyer should consider an alternate packager/OEM.
A full unit information report should include a layout of the unit as sold and installed. Furthermore,there should be clear listings of:
Driver information and rating, Frame rating, Length of connecting rods, Cylinder bore sizes, Individual cylinder staging, Initial, fixed volumetric base clearances for
each end (i.e. clearances that do not reflect
added fixed volumes from valves and/orunloading devices),
Actual base clearances used (i.e. clearancesthat do reflect added fixed volumes from valvesand/or unloading devices),
Description of how tandem cylinders are setup, Cylinder pressure ratings, Piston and Tail Rod diameters, Clear limits used for rod loads (gas and/or inertia), Clear limits for pin non-reversals, Weights used per throw for net rod load determination, Weights per throw used for pin reversal determinations, Clear temperature limits per cylinder, Clear low volumetric efficiency limits, Valve information:
o Quantity,o Size (i.e. nominal diameters),o Effectiveness (flow areas, lift areas, resistance factors, etc.).
While much of this information may be found in the supplied OEM modeling file, some of it may not be
clearly identifiable to the end-user often items that involve weights and valves are obfuscated and thus
are difficult to find in some of the OEM software.
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Sample
Unit
Arrangement
Diagrams
Access to the correct unit information is critical to others who may need to accurately model units. Themost common reasons for modeling a unit are:
Unit Automation, Converting OEM models for use with in-house and/or third-party software, such as:
o Performance: eRCM, HTS, RCS Excalibur, RecipWin/Recipak-PC, etc.o Fleet Monitoring Software: Enalysis, HPMax, RECON, MAO, and others.o Piping Software: Stoner, WinFlow, etc.
If a new unit cannot be properly modeled in other software used by the end-user, then the ability to fully
utilize that unit may suffer. Or even worse, the unit may be allowed to operate at unsafe conditions.
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Expected Pressure Drops
Stage Pressure Drops Required Item
Item(s) Expected system pressure drops (bottles, filters, separators, piping, etc.) into and out of
each stage. As a minimum, these need to be specified as fixed drops and/or percent drops.
Ideally, pressure drops are given as a function of expected flow rates. When a detailedacoustic study is performed, better estimates of pressure drops due to inclusion of
attenuation devices are determined.
Reason(s) Used to better model unit performance, and hence improve accuracy of load and flow
predictions. Also, can affect rod load and pin reversal calculations and hence the models
ability to keep the unit safe.
Response If pressure drops are not provided, then buyer can work with their own analysts to
measure some actual pressure drops through the system.
In general, whenever an acoustic study is performed, pressure drops are determined throughout the
system. For compressor performance in particular, the collective pressure drops between where the inletpressure is sensed and the inlet cylinder flange, and from the cylinder discharge flange to the where the
final discharge pressure is sensed are the pressure drops required.
Ideally, these pressure drops will be given as a function of flow rates. However, for simplicity, most
packagers/OEMs model their units based on pressure drops given as fixed pressure drops, or as a percent
of stage inlet and outlet pressures (absolute).
A table of these pressure drops per stage (into and out of that stage) should be given to the end user.
Furthermore, a diagram should be provided that details between what locations the estimated pressured
drops apply.
Note: Often, the use of orifice plates, especially on high-speed units that single-act, can contribute to
significant pressure drops (especially dynamic pressure drops). In some cases, such as low compression
ratios, use of orifice plates can affect unit performance by as much as 30%! This is important
information to know prior to ordering a driver and/or getting emission permits.
Supplied performance curves, in-house models, third-party models, and monitoring software should all
reflect the appropriate pressure drops estimated via the pulsation study after appropriate attenuationdevices have been selected and modeled.
Stage-1 Pressure Drops (sample):
Load Step
Speed
(rpm)
Suction
Pressure @
Sensor
(psiA)
Suction
Pressure @
Flange
(psiA)
Discharge
Pressure @
Flange
(psiA)
Discharge
Pressure @
Sensor
(psiA)
Flow
Rate
(MMscfd)
1 1000 145 140 361 345 380.9
12 950 195 191 539 520 260.8
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Safe Startup Maps
Safe Startup Maps Required Item
Item(s) Safe Startup Maps
Reason(s) Identifies when the unit can be brought online at minimum speed, least-load load step, andpressures experienced when closing the recycle valve. If the least-load load step cannot
always be used, then the map needs to identify which load step must be selected prior to
closing the recycle valve to safely start the unit. At times, the unit may only be safelystarted by first reducing the suction pressure so as to prevent rod loads and/or pin non-
reversals.
Response If the packager/OEM cannot specify under what pressure and speed conditions it is
safe to start the unit, then consider an alternate packager/OEM. Or, make sure their
warranty covers all startup operating conditions.
It is important to know when a unit can be safely started. During the startup process, the inertia effect
from the lower speed combined with the gas pressures experienced when closing the bypass valve tobegin the compression event may lead to rod load, and/or pin non-reversal issues on certain load steps.
Occasionally, the load step affected may be the least-load load step (i.e. all pockets open, all applicable
ends deactivated). Most automated units are usually started in their least-load load step. This cansometimes create a serious problem.
Therefore, it may be necessary to change to a different load step, or even decrease the suction pressure
to an acceptable pressure prior to starting the unit.
In the above example, suction pressure should be decreased to less than 29 psig. Furthermore, where the
least-load load step (#25) is not indicated (yellow regions in the table), the units hardware configuration
should be set to the required load step (number in the cell) prior to closing the bypass valve.
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Rod Load and Pin Reversal Maps
Full Rod Load Review Required Item
Item(s) Review of where rod load issues exist for each load step, across all pressure combinations,
and all speeds. Rod load reports should be based on using identical or similar methods used
by OEM for unit warranty.
Reason(s) Identifies where unit can experience rod load issues. These change (often very quickly)
with varying pressures, speeds and load steps, and even suction temperatures. Very oftenoverlooked leaving end-users stuck with a unit that has a very limited operating map.
Response If the packager/OEM cannot identify for what combinations of pressure and speed
and load steps the unit will experience rod load issues, then either consider another
packager/OEM or have these determined by in-house or third-party software.
Full Pin Non-reversal Review Required ItemItem(s) Review of where pin non-reversal issues exist for each load step, across all pressure
combinations, and all speeds.
Reason(s) Identifies where unit can experience pin non-reversal issues. These change (often very
quickly) with varying pressures, speeds and load steps, and even suction temperatures.
Very often overlooked leaving end-users stuck with a unit that has a very limited operating
map.
Response If the packager/OEM cannot identify the combinations of pressure and speed and
load steps for which the unit will experience pin non-reversal (failure to lubricate
crosshead pin) issues, then either consider an alternate packager/OEM or have these
determined by in-house or third-party software.
An initial review covering all load steps, all speed, all throws, and all stages should be determined acrossthe entire defined operating map of suction and discharge pressures. If any issues with rod loads or pin
reversal exist, then all load steps that might contribute to those problem areas need to be reviewed in
more detail.
For each load step having rod load, or pin reversal issues, a detailed map of where the issues exist is
required before automating the unit. Otherwise, the automation algorithms may allow the unit to operate
in unsafe territory. For fixed-speed units, a single speed is sufficient. For variable speed units, acomplete review of all speeds (orat least maximum, medium, and minimum speeds) is required.
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Individual Load Steps Rod Load Plot
Details Worst Case of All Operating Speeds
PS vs. PD vs. Percent of Rod Load Rating
Individual Load Steps Pin Reversal Plot
At One Particular Speed
PS vs. PD vs. Minimum Degrees of Reversal
Do Full Unit Reviews
ALL Load Steps, ALL Speeds, ALL Pressure Combinations
The adjacent plot details where the unit experiencesits worst cases for Pin Non-reversals, when the unit
is running between 60% and 100% of Rated Load.
This one plot covers all load steps, all speeds, and
all pressure combinations.
PS vs. PD vs. Minimum Degrees of Reversal
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Individual Load Steps Safety Plot for Rod Load
and Pin Non-reversal Issues
Plot Results are Based on a Single Load Step:
Suction Pressure vs. Discharge Pressure, All
Speeds, and Various Suction Temperatures perStage Combinations.
Red Areas: Always Unsafe,and Why
Green Areas: Always Safe
Yellow Areas: Some Areas are Unsafe,and Why
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Highest Expected Discharge Temperature and Discharge Pressure Maps
Discharge Temperature and Pressure Maps Required Item
Item(s) Highest Expected Discharge Temperature and Discharge Pressure Maps per stage, at least
for average and maximum suction temperatures.
Reason(s) Used to verify coolers are sufficient to cool the volumes of gas being discharged from the
expected discharge temperatures to the required aftercooler temperatures, and to avoid
potentially adverse operating conditions.
Response If packager/OEM cannot provide these maps, then they need to provide runs
reflecting operations at conditions where highest discharge temperatures and highest
interstage discharge pressures are expected. These select runs need to include
information required to verify interstage coolers and aftercoolers performance.
To verify that the supplied cooler(s) is(are) sized properly, a full review of possible dischargetemperatures across the entire operating map is required.
Tables should be created for each stage and shouldreflect the highest discharge temperatures possible
considering all safe load steps (across all speeds), and
should be done at least for the minimum and maximumgas suction temperatures per stage. To properly review
coolers, it is also necessary to know the flow rates
associated with each of the expected maximum discharge
temperatures.
Discharge Temperature (F) Maps:
Stage Table View 3D View
Stage-1
Max=276 F, Min=105 F
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Stage-2
Max=307 F, Min=292 F
Sometimes, especially for systems with under-sized coolers, sidestreams in/out, or refrigeration
processes between stages, it is necessary to determine the maximum discharge pressure per stage. Ifrequired, tables and plots similar to the ones above should be generated for discharge pressures per
stage.
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Sidestream Reviews
Sidestream Review Required Item if Sidestreams are Present
Item(s) Review methods used to model units with sidestreams (fixed flow rate method or fixed
pressure method).
Reason(s) Varies based on purpose of sidestreams. Required to predict unit flow, unit load, composite
gas composition, etc.
Response If the packager/OEM cannot identify loads, flow rates, and gas thermodynamics for
sidestreams involved, then it may be critical that these are accurately determined by
in-house or third-party software.
OEM software packages allow for inclusion of
sidestreams into and out of the gas system between
compressor stages. Sidestreams may come from aprocess, go to fuel engines, result from dropouts of
heavier gases, etc. In any case, the interstage fromwhich the sidestream emanates can normally be
modeled in one of two ways: interstage pressure
fluctuates or interstage pressure is held fairlyconstant.
Case I: Static Flow Rates / Varying Pressures:
For sidestreams out, a static flow rate istypically specified.
For sidestreams in, a static flow rate istypically specified, along with the gastemperature and a gas analysis of the
sidestream.These types of models assume that the sidestream volumes per stage are constant and the sidestream
pressures fluctuate as needed to maintain the indicated static flow rates. Most OEM software can
effectively model these types of sidestreams.
Case II: Static Ratios / Varying Flow Rates / Varying Pressures:Sidestream flow rates are varied so as to maintain a constant ratio of compressed gas with sidestreamgas. This is often done by precisely specifying the ratios of the gases, or indicating that the
sidestream gas is a fixed percent (mole weight) of the total gas to be compressed on each stage.
Sidestream gas temperatures and gas analyses are specified per each stream in.
Case III: Varying Flow Rates / Static Pressures:If the pressures to the interstages are being held fairly constant via sidestream sources (in or out), the
end-user needs to clarify this to the packager/OEM. Thus, in this situation, the flow rates of thesidestreams vary while the pressures stay relatively constant. End-users need to consult with their
packager/OEM to see if these types of sidestreams are effectively modeled.
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For simple and complex systems, individual performance points are useful in any of the above cases.
However, full performance (curves, tables, plots, etc.) over a units entire operating range can be verymisleading if the wrong case is modeled. In general, full performance can readily be completed for
Cases I and II.
Case III can present problems as final required loads and flows are further dependent upon dynamic
sidestream flows required to maintain constant interstage pressures. End-users should specify exactly
what types of plots, curves, tables, etc. they desire. In many cases, the end-user may have to create theirown reports based on using the OEM software to generate multiple point performance runs. Customized
software can be ideal for these more complex sidestream applications.
Formation of Liquids
In general, liquids should never form inside the
compressor. However, depending upon the
thermodynamic properties of the fluid being
compressed, this can happen. In general, theOEM/packager can review the range of pressures
and temperatures for which the gas is exposed to
make sure that liquids do not form within thecylinder.
By reviewing the pressures and temperaturesexperienced as the gas migrates through the
compression process, the software can determinewhen, and how much, liquid is formed from thegas as dropout.
For exotic gases, much care needs to be taken to make sure that the thermodynamic model used in the
compressor performance software can properly model that gas across the pressure and temperature
envelopes. If not, then a high-end thermodynamic model for gas properties may be required. Operationsnear critical areas (such as the triple point or phase transitions) need to be carefully designed and
meticulously implemented.
Also, prodigious care should be taken to ensure that the
thermodynamic model being used is valid for each gas mixture being
reviewed. Otherwise, any performance results may be completely
baseless.
Hi h CO2 Natural Gas
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Unit Robustness Map
Unit Robustness Map Required for Automated Operations: Recommended for Manual
Operations
Item(s) Unit Robustness Map across all pressure combinations indicates relative ease for
automating the unit.
Reason(s) Used by automation groups to identify how difficult and time-consuming the process of
automating this unit will be. Identifies how sensitive the units safety is to changes inspeed, temperatures and load step. It can also serve to identify how difficult it can be to
keep the unit safe while in manual operation.
Response If the packager/OEM cannot create these maps, then have these determined by in-
house or third-party software.
These maps review all load steps, across all speed ranges, across various combinations of suction gastemperatures per stage, for all combinations of suction and discharge pressures permitted for this unit.
As such, these maps can take hours to generate (often from 1 million to 150 million performance pointsare calculated to render these types of maps). However, the resulting plot can help the end-user evaluate
how effectively the unit can be used in certain areas of the operating map, as well as assessing the
complexity of automating the unit.
Areas of bright green (always
safe) and bright red (always
unsafe) are regions for whichrelatively straightforward PLC
algorithms can safely control the
unit. Areas of darker green andorange indicate the need for more
complex controls. In general, themore bands of color, the more
complex the unit will be to safely
automate darker bands indicatethat some of the normally valid
and safe load steps will become
unsafe simply due to changes inspeed or suction temperatures
(number in cell indicates the
maximum number of load steps
that can possibly be used).
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Performance Curves
Performance Curves Highly Recommended
Item(s) Suction Pressure vs. Load Suction Pressure vs. Flow Discharge Pressure vs. Load Discharge Pressure vs. Flow Suction Pressure vs. Discharge Pressure, per load step. Sets of above curves from maximum speed to minimum speed in decrements of no
more than 50 RPM (for high speed) and 20 RPM for low speed.
Reason(s) Used by operators and/or simple control panels to safely control the unit across its definedoperating map. Curves help to identify where safety issues exist that render certain load
steps unsafe for use at current operating conditions.
Response If curves are not provided, then buyer can often create them with their own software,
or via third-party software. However, curves may not exactly match OEMs
performance predictions and cutoff for safety. If the OEM curves cannot be tuned to
real-world measured data, then their importance is diminished, and third-party
performance curves that can be tuned are favored.
End-users should not allow packagers/OEMs to go skimpy on curves. This can be done by making sure
all generated curves are based on at least fifty (50) performance points per load step curve. Sometimes
the supplier only uses five (5) to ten (10) points per curve. Use of a low number of points can lead not
only to jagged plots, but more importantly can lead to unsafe areas being shown as safe, or safe areas
being shown as unsafe. An exception would be if the unit does not have a lot of cutoffs due to rod loads
and/or pin non-reversals, then the use of at least twenty (20) points per curve is usually OK.
For units utilizing a variable speed driver, each set of curves should be generated at various speeds from
maximum speed down to minimum speed in decrements of no more than 50 RPM (for high-speed) and20 RPM for low-speed units. Failure to cover speed ranges can lead to situations where rod loads and/or
pin non-reversal issues are not properly disclosed. Curves that look good at rated speed may be seriously
clipped due to pin non-reversal at minimum speeds. Curves that look good at minimum speeds may beseriously clipped due to rod load issues at maximum speeds. There are also cases where curves that look
good at rated speeds and minimum speeds may be seriously clipped at in-between speeds due to rod
loads and/or pin non-reversal issues.
End-users need to specify which types of curves the supplier is required to deliver.
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Suction Pressure vs. Load Curves Discharge Pressure vs. Load Curves
Suction Pressure vs. Flow Curves Discharge Pressure vs. Flow Curves
For units with fixed-speed drivers (and not a wide range of suction gas temperatures) thumbnail plots
(a.k.a. thumbprints) may be an ideal alternative to the standard performance curves. These curves, one
per each load step, detail safe regions as well as required power for all combinations of suction anddischarge pressures. Curves can highlight required power (top plot), deliverable flow rates, or both
(bottom plot).
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Regardless of what types of curves are specified, all curves generated must reflect pressure drops based
on changing operating conditions, and not just on a single, fixed pressure drop used across the entire
operating map unless the specified operating map is quite narrow.
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Flow Maps
Flow Maps Suggested Item
Item(s) Flow Maps from minimum to maximum desired flows, in user-specified increments.
Reason(s) Used by Gas Control/Dispatch to quickly identify when specific units can be used toachieve companys flow requirements.
Response If the packager/OEM cannot create these maps, then these can often be created by in-
house or third-party software.
Flow Maps, from the units minimum achievableflow rate to its maximum achievable flow rate in
user-specified increments, are ideal for Gas
Control/Dispatch. These plots allow for a quick
lookup of where the compressor can effectivelyflow a certain amount of gas. Gas
Control/Dispatch rarely are concerned withparticulars such as unit speed and load step, but
rather they are more concerned with the volume
of gas moving through the system based uponavailable inlet and discharge pressures.
In the adjacent example, suction pressure (x-axis)
is plotted against discharge pressure (y-axis). Thegreen areas indicate where the unit can achieve
the desired flow rate of 30 MMscfd, white areas
are where it is unsafe to run the unit, yellow areas are where it cannot flow the desired flow rate, and redareas indicate that the unit cannot be turned down enough to meet the desired flow rate.
A series of these plots can then be generated for 20, 25, 30, 35, 40, 45 and 50 MMscfd to cover the
range of flow rates possible with this unit. Now, Gas Control can just reference these plots to quickly
decide if this unit can be used to help achieve specific flow rates.
Alternatively, some users may prefer Flow versus Discharge Pressure plots of specific Suction
Pressures. While this type of plot is not quite as complete or informative as Flow Maps, it can besignificantly more compact as it details ranges in suction pressure, discharge pressure, and flow all in
one graph.
30 MMscfd
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Unit Isentropic Efficiency Map
Isentropic Efficiency Map Suggested Item
Item(s) Unit Isentropic Efficiency Map (at least at rated speed) across all pressure combinations.
Reason(s) Identifies efficiency of compressor. If the data from an acoustic study and the resultingcorrections to required unit loading due to attenuation devices are included, then the map
gives the efficiency of the system from header to header. Note: use of certain attenuation
devices, such as orifice plates, can sometimes significantly reduce system efficiency evenif the compressors themselves are quite efficient.
Response If an isentropic efficiency map is not provided, then packager/OEM should at least
identify the unit and overall system efficiencies at the design and guarantee points.
While there are different ways to measure the effectiveness of a compressor, one common method is the
use of Isentropic Efficiency. This method gives a percent of effectiveness by taking the amount oftheoretical power that should be required to compress the gas and dividing it by the amount of power a
unit eventually needs to compress the gas.
Part of the total power losses is related to the compressor (gas passageways, valves, frictional losses,
etc.) and part is related to the acoustic dampening systems (bottles, orifice plates, choke tubes, etc.).(Readers may want to review a few GMC papers available at www.GMRC.org for more details on
differences on compressor specific methods versus system-specific methods.)
The supplier should make it clear whether the supplied map is just for the compressor, or for the entiresystem. In general, the entire system map is best as it details the actual situation. However, if the
pulsation dampening system is very restrictive to gas flow, the resulting isentropic efficiency map may
look as if the compressor is not very efficient. The best situation is to generate both models. Asignificant difference between them indicates a very costly (in terms of power and reduced flows)
acoustic dampening system. For high-speed units needing a lot of single-acting compression, this isoften the price paid to utilize a wide range of unloading capabilities (often the case for units used for
injection and withdrawal services).
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PS vs. PD vs. Unit Isentropic Efficiency (%)
Isentropic Efficiency =GasCompresstoRequiredPowerActual
GasCompresstoRequiredPowerlTheoretica
A similar graph that details the
amount of required load per unit
of flow (often referred to asBHP/MM) can highlight areas
where the unit is less efficient.
Since these plots are based on
how much power is required to
compress each unit of flow,
they are often used in part fordollar-based decisions since
many end-users have historic
data available for estimatinghow much they spend per
horsepower used.
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Compression Ratio Maps
Compression Ratio Maps Suggested Item
Item(s) Provide Compression Ratio Maps across all pressure combinations per stage. Report
should generate maximum (and if requested, minimum) compression ratios.
Reason(s) Used to help balance compression ratios across stages, and sometimes used to maintain
safety related to rod load issues. Also, higher ratios usually lead to higher discharge
temperatures.
Response If the packager/OEM cannot create these maps, then have these determined by in-
house or third-party software.
Ratios of compression per stage may be required for controlling rod loads, controlling temperatures, or
balancing unloading across stages. Tables of data should be created for each stage, and should reflect the
highest and lowest ratios possible considering all safe load steps (across all speeds).
Compression Ratio Maps:
Stage Table View Ranges
Stage-1Max = 4.0
Min = 1.8
Average = 2.1
Stage-2Max = 4.4
Min = 1.0
Average = 2.1
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Load Step Curvature Reviews
Load Step Curvature Suggested ItemItem(s) Load Step Curvature Review is used to identify which load steps (and where) are quite
steep in nature, and thus small changes in pressure can have significant changes in load andflow. Also, steep load step curves tend to be more difficult to tune and automate.
Reason(s) Identifies units that have unreasonably steep load curves, which in turn create problems
with operations and tuning. Note: Units with steep load curves are often not sized correctly.
Response If the packager/OEM cannot create these maps, then have these determined by in-
house or third-party software.
Often, when reviewing performance based on just simple performance points, the end-user may notnotice how fast load and flow can change based on small changes in pressure. A review of the steepness
of the proposed load curves can quickly identify this type of issue. Often, steep load curves may indicateimproper sizing of cylinders.
In the adjacent plot, green
identifies areas of relative
flatness, while yellow identifyareas of normal curvature to
be expected, and orange areas
reflect where the curves arebecoming a bit too steep. Red
areas are unsafe areas for this
load step. Large areas ofgreen and yellow are ideal.
Some areas of orange are to
be expected, but these areasshould only represent a small
portion of the overall safe
operating map.
Units with steep load step
curves can prove difficult to
automate and tune. This is
due to the inherent nature ofthose load steps generating
large changes in load andflow for relatively small
changes in pressure.
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Full Acoustic Review
Full Review of Unit Acoustics Across Entire Operating Map Suggested Item
Item(s) Acoustic Report. Detailed acoustics reports of performance at design and guarantee points,
review of thousands of potential operating points with identification of areas in the
operating map where vibration and acoustic issues may still be present even withattenuation devices in place. Provide data in a useable format (for use in OEM/third-party
compressor modeling software) for tuning load, flow and pressure drops across entire
operating map.
Reason(s) Often, acoustic reviews are performed centered around the design and guarantee points.
However, the attenuation devices specified by this review may not fully dampen allpulsations within API guidelines, especially the pulsations that affect loads and flows.
Thus, unforeseen shaking forces may be experienced at certain combinations of pressure,
speed and load step. Furthermore, initial software/PLC predictions of load and flow maynot adequately reflect load and flow accurately enough for safe unit operations.
Response If the company performing the acoustic review (and thus specifying attenuation
devices to add to the system to prevent dangerous levels of vibration) cannot perform
a review across the entire map, then:
1. Accept report as-is and accept possible consequences, or2. Include additional points in the original specification to at least potentially
identify some problem areas, and to learn more about what may happen
within the specified operating map. or
3. Specify a company that is capable of performing a full acoustic review.
These reports vary a lot and can be quite voluminous. In regards to
performance, the end-user needs to review areas in the report thatindicate how static and dynamic pressure drops affect both the
required power and the subsequent flow rates. Failing to peruse theseareas may lead to compressor installations with insufficient driver
power.
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OriginalCompressorPerformance
CompressorPerformance withDynamic Effects
Included
Original CurvesOverlaid in Red to
Highlight PotentialDifferences.
The following sources have a plethora of recent information about the effects from pulsations on
reciprocating compressors:
Integrating Compressor Performance with the Effects of Pressure Pulsation across a UnitsEntire Operating Map, GMC-2007 Paper (ACI/Ariel/Beta Machinery)
ARCT & GMRC Pulsation Control Project Update, GMC-2007 Paper (SwRI) An Investigation of the Application of Finite Amplitude Wave Tuning Technology for ControllingReciprocating Compressor Pulsations, GMC-2007 Paper (ACI/Optimum Power) Evaluation of Pulsation Levels at Reciprocating Compressor Installations, GMC-2007 Paper
(SwRI)
Advanced Pulsation Control Devices Ready for Field Tests, GM Journal, 2nd Quarter, May2007
GMRC Advanced Reciprocating Compressor Technology (ARCT), CompressorTechTwo, June& July 2007, (SwRI)
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Implementation of Compressor Performance
While requiring a full review of unit potential before purchase can quickly identify the strengths and
shortcomings of that unit when compressing within a specified operating map, many end-users also needthe ability to implement similar-to-OEM performance modeling in PLCs (programmable logic
controller). This is required so that the PLC can safely control the compressor while optimizing it for
load, flow, or fuel usage. The failure to model a unit similar to how the OEM would model that unit can
not only lead to warranty disputes, but also to operations at potentially unsafe operating conditions.
The main safety-related items are: high pressures, high temperatures, rod loads, and pin non-reversals.
Of these, standard real-time sensors can readily identify high pressures and high temperatures. There aredevices currently available for measuring dynamic, internal gas pressures and thus inferring real-time
rod load forces and crosshead pin forces. However, these devices are not prevalent in the gas
compression industry at this time.
Therefore, it becomes very important that
each compressor OEM properly discloseexactly how they warrant their equipment
in regards to determining rod load and
crosshead pin forces: what equations are
used, what pressures (if any) are used,what weights (if any) are used, what
other OEM-specific criteria are
implemented, etc.
In short, for these types of safety items,end-users not only need to know requiredweights and limits, but how those items
are used in determining if the unit can be
safely operated at a specific operatingpoint, at a specific speed, and at a
specific load step configuration.
Note: While a simple pressure differential can be useful in keeping reciprocating compressors out of rod
loads, this method is usually insufficient for high-speed units. Most high-speed OEMs base rod load and
pin reversal calculations on internal gas pressures, which vary based on speed due to the pressure
drops associated with valve velocities and gas passageways. Furthermore, regardless of whether theOEM uses inertia forces for rod load calculations, all high-speed OEMs use inertia for pin non-reversal
calculations. Inertia forces are a function of reciprocating weights and rotation speed. Thus, simple
pressure differentials fail to cover safety issues related to pin non-reversals.
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Building upon the Solid Cornerstone of Compressor Performance
From the beginning of large scale reciprocating compressors, there has been a need to predict unit
performance and operational safety. Some of the industrys fundamental research and development usedextensively as the thermodynamic basis for many compressor performance models were developed and
written by William Bill Hartwick. Two key papers are Efficiency Characteristics of ReciprocatingCompressors (December 1968, ASME technical paper 68-WA/DGP-3), and Power Requirements and
Associated Effects of Reciprocating Compressor Cylinder Ends Deactivated by Internal Bypassing(April 1975, ASME technical paper 75-DGP-9). Together, these papers set many standards for
effectively predicting reciprocating compressor performance. Bills thermodynamic talents were
paralleled, referenced and expanded upon by others, such as Ed Miller (thermodynamics, softwaredevelopment, and automation) and Derek Woollatt (thermodynamics, valve dynamics, and hardware
innovation).
Bill HartwickCooper
Ed MillerACI Services
Derek WoollattDresser-Rand
Once formal mathematical models based on the actual physics of gas compression were developed,
computer software models naturally evolved. Hartwicks paper On-Site Computer PredictedCompressor Performance (March 1982, ASME technical paper 82-DGP-26) covers an actualimplementation of computer-aided compressor performance for optimizing and safely controlling
compressors. Performance software continued to mature and thus provided engineers with the abilities to
review unit performance with better proficiency, and across a broader spectrum, which in turn allowed
engineers to more completely review and evaluate how hardware changes subsequently affected unitperformance and safety.
History tells us that compressor performance started with hand-calculated point performance, advancedto software-calculated point performance, then advanced to software-generated performance curves, and
now has evolved to allow engineers and end-users to render 3-D plots, complex tabled-data, animated
compressor performance plots and more. Nevertheless, whether doing a few points by slide rule, or afew million points by computer, the thermodynamic-based equations have not changed a lot in the last
fifty years, thanks to the well-defined and flexible mathematical models developed by individuals such