ReciprocatingCompressorPulsationControlUsingPANTMFilters
Pulsationcontrolachievedwithoutpressuredrop.______________________________________________________________________________
AwhitepaperpublishedjointlybyACIServicesInc.andOPTIMUMPumpingTechnologyRev0IssuedSeptember9,2016
_____________________________________________________________________________________________
THEPROBLEMPulsation ismost often created by reciprocating compressors and resonance in system/yard piping.Traditionalpulsationcontrolsystemsemploycombinationsofprimaryand/orsecondaryvolumebottles,oftenwithcomplex internalchoke tubes,baffles,andchambers,aswellasexternalchoke tubesandvarious orifice plates installed at specific locations in the system piping. These devices accomplishpulsationcontrolbyaddingresistance,ordamping,tothesystem;andtheycancausesignificantsystempressurelossesupstreamanddownstreamofthecompressorcylinders.Althoughthesesystemsmaybewelldesigned,theresultingpressurelossesreduceoverallsystemefficiency.
Thereductioninpulsationvs.theincreaseinpressurelosstradeoffistolerableformanyhighpressureratio compressor applications. However, for common pipeline transmission applications having lowpressure ratios (in the range of about 1.1 to 1.6), system pressure losses can severely degrade thecompressoroperatingefficiency,especiallywhenhigherspeed(>600rpm)compressorsareused5,6.Forexample,a20%lossofefficiencyonan8,000HPcompressorwastes1,600HP.
This loss of efficiency has becomemore significant in recent years as theU.S. pipeline industry hasexpandeditsuseoflargerhighspeedreciprocatingcompressors.Withtheworldsincreasedawarenessoftheneedforhigherenergyefficiencyandareducedcarbonfootprint,newmoreefficientpulsationattenuationsystemsarerequirednow.THESOLUTION
Arelativelynew,fieldproventechnology,PANTMFilters,canbeusedtosolvepulsationproblemswithoutcausingpressuredropor compromising the systemefficiency3.PAN Filters canbeused to attenuatepulsation in virtually any piping system, including reciprocating compressor suction and dischargeheaders,upstreamofflowmeteringstations,andupstreamordownstreamofcentrifugalcompressors.Theycanbedesignedtoreplacemostpipelinepulsationcontrolbottles inservicetoday,andtheyareworthyofconsiderationasanalternativetobottlesfornewcompressorapplications.TheapplicationofPANFilterstoreciprocatingcompressorsuctionanddischargeheadersisthefocusofthisparticularpaper.PANFilterscanbeapplied to reduceharmfulpulsationsandassociated shakingforcesandpipestressesinreciprocatingcompressorsuctionanddischargeheaders,withoutcausinganysignificantpressurelosses.InaPANFilter,theflowstream,aswellasthepulsationenergy, iscarefullyandequallysplit intotwopaths.Asexplainedindetailinapreviouspaper3,acarefullyengineeredTSTcollectordirectshalfofthe
pulsatingflowthroughaloopofpipeandtheotherhalfofthepulsatingflowpassesstraightthroughtheTSTcollector.ThePANFiltersprimarycancellationfrequencyisthefrequencythathasawavelengthoftwicethedifferenceinthetwoflowpathlengths,i.e.,thepaththatincludestheloopofpipeandthepathstraightthroughtheTSTcollector.Whenthetwowavesofequalamplitude,atthisprimaryfrequency,travelthroughthetwoflowpathsandthencarefullyrejoined180outofphasebytheTSTcollector,onewavecompletelycancelstheother,eliminatingthepulsationatthatfrequencyinthedownstreamflow.In addition to the primaryfrequency, all odd harmonics ofthat frequency are also completelycancelled.Additionalloopscanbeaddedinseriestoproducebroad bands of pulsation attenuationthatfillinthecancellationgapsbetween theoddandevenordersof theprimary frequency.Two or more loops may benecessary for applications withvariable speed compressors,varying gas compositions andtemperatures, varying flowvelocitiesandflowstreamswith
Figure1:2LoopPANFilterLoopsinseries
complexpulsationsignatures.A2loopPANFilterisshowninFigure1. Notethat,whennecessary,thefootprintcanbereducedbyfoldinglooppipesoverthemselves.Figure2showstheextensivebandofpulsationfrequencycancellationthatcanbeachievedwitha4LoopPANFilter.Theseparticularloopsaredesignedtocancelprimaryfrequenciesof4,8,16,and32Hz.Notethat4Hzandall14harmonicsof4Hzupto60Hzarecompletelycanceled. Inaddition,notethatallfrequenciesfrom11Hzto53Hzareattenuatedbymorethan90%.
Figure2:Exampleof4LoopPANFilterpulsationcancellation
Whilethepressurelossesofconventionalbottlesandpulsationdampingorificeplatesaresignificant,asmuchasoflinepressureinextremecasesthathavebeendocumented6,pressurelossesinPANFiltersandPANHiPerformanceCompressorManifoldsarenearlyzero4,even inthehighesttransmission lineflowcases.Asaresult,PANFilterswillincreaseoperatingefficiency,sothatmoreflowcanbeproducedforagivenhorsepowerinput,orlesshorsepowerisrequiredforagivenflowrate.Eitherway,theuseofaPANFilterreducesoperatingcostandgreenhousegas(GHG)emissions. EXAMPLESOFPANFILTERAPPLICATIONSFORRECIPROCATINGCOMPRESSORSThree examples are presented to show how PAN Filters can be retrofitted to existing reciprocatingcompressorstosolvepressuredrop,pulsationandpulsationrelatedvibrationproblems.Assuggestedbytheseveralexamples,PANFilterscanbeusedwithcompressors inanyoperatingspeedrange.Longerpipe loop lengthsare typicallyneeded forPANFiltersapplied toslowerspeedcompressors,and looplengthsare typicallyshorter forhigherspeedcompressors.PANFilterscanalsobedesigned intonewcompressorpackagestoreducethelossesassociatedwithtraditionalmeansofpulsationcontrol.Example1In2009,collaborationwithamajorgastransmissioncompanyledtotheinstallationofaproofofconceptPANsystemonacompressoratagasstoragesiteinNorthernCentralPennsylvania4.AcombinationPANManifoldandPANFilterwasdesignedandretrofittedtothedischargeofonesideofa4throw,singlestageSuperiorMH64compressorhaving9.5in.cylinders.The6in.strokecompressoroperatedfrom750to1000rpmandtheexistingsystememployedavolumechokevolumepulsationcontrolsystemthatalsoincludedmultipleorificeplates.Atlowpressureratios,pressuredropintheexistingsystemwasexcessive,andathighpressureratios,thepulsationcontrolwasnotsufficientlyeffectiveoveratleastsomeoftheoperatingrange.The primary objective of thePAN systemwas to eliminatethe excessive pressure dropassociated with effectivepulsation control. The resulting PAN system for thisapplicationcombinedasimplePANManifoldwithatwointoonewye TSTcollector, and atwoloopPANFilteroperatinginseriesasshown inFigure3.Theendusersrequirementtominimize changes to thepackageskidandtomakethePAN removable in case it didnotmeetexpectations,ledto
Figure 3: Proofofconcept PAN system, including a twoloop PAN Filter,installedonthedischargeofonetwothrowsideofafieldcompressorin2009.
mountingthePANFilterinastackedarrangementinaprefabricatedsteelframethatwaslaggedboltedtoa4in.thickconcrete floor,i.e.,nofoundationunderit,asshowninFigure3.Resultsfromthefieldtest2ofthisarrangementshowedthattherewasnegligiblepressuredropacrossthetwoloopPANFilter.
And, as shown inFigure 4, the PANFilter,notincludingtheadditional benefit ofthe upstream PANManifold, reduced thepulsation by 51% to87% over the testedspeed rangeof860 to1000rpm.
Speed(rpm)
PulsationBeforePANFilter(P/Ppsi)
PulsationAfterPANFilter(P/Ppsi)
PulsationReduction
(%)
FinalPulsation%ofLinePressure
860 9.8 4.8 51 0.5900 27.0 3.6 87 0.4950 14.8 4.0 73 0.4980 20.6 4.8 77 0.51000 20.8 5.3 75 0.6
Figure 4: Proofofconcept discharge PAN pulsation measurements at 886 psigaveragedischargepressureand1.35pressureratio.
Example2In late 2015, an existing compressor stationencounteredhigh vibration and related failures insystempiping loosepipe clamps,brokenanchorbolts,etc.arounda3000HP,300rpmClarkTLADintegral engine compressor. As often happens,operatingconditionshadchangedovertimeasnewshale gas supplies increased the demands on thecompressor.The 19 in. stroke twostage compressor had two18.5in.firststagecylinderswithtwoheadendandtwocrankendfixedvolumeclearancepockets,alongwith two12.0 in. secondstage cylinderswith twoheadendandonecrankendfixedvolumeclearancepocket.Thisprovided17loadstepsforoperatingthecompressorwithsuctionpressuresrangingfrom100to260psigandfinaldischargepressuresof900to1180psig.A detailed field analysis found that vibration andpulsationsatvariouslocationsinthecompressorpipingwere in excess of acceptable limits at discretefrequenciesof10Hzand20Hz,whichare2xand4xmultiples of the compressor rated running speed.Figure5showsthattypical10Hz(2x)pulsationlevelis4to5%ofaveragepressureinthe1ststagesuctionline.Figure6showsthatthe10Hz(2x)pulsationisnearly
Figure5:1ststagesuctionlinepulsationLoadstep12
\
Figure6:2ndstagesuctionlinepulsationLoadstep12
5%and20Hz(4x)pulsationisabout2%ofaveragepressureinthe2ndstagesuctionline.Pulsationsmeasuredinthe1ststagedischargeweregenerallyacceptable,withthehighestamplitudeoccurringat20Hz(4x).Figure7showsthatthe10Hz(2x)pulsationlevelismorethan3%ofaveragepressureinthe2ndstagedischargeline.Installingorificeplates tocontrolpulsationsmetwithvery littlesuccess,and itwasdetermined thatnewpulsationbottleswouldbe required inorder to reduce thepulsation levels to safe levels. In lieuof thisexpensiveoption,thepossibilityofapplyingPANFilterswasinvestigated.Closeexaminationofthefieldtest
datashowedthatpulsationwasonlyhigherthangenerallyacceptablelimitsatadiscretefrequencyof10Hzonthe1ststagesuctionlineandonthedischargelinesofbothstages.Thepulsationinthe2ndstagesuctionline,however,wasaboveacceptablelevelsatboth10Hzand20HzfrequenciesasshowninFigure6.PANFilters,designedwithonelooptocancelpulsationcompletelyatafrequencyof10HZ,wereproposedforthe1ststagesuctionand2ndstagedischargelines.AsshowninFigure8,thisPANFiltercancels100%ofthepulsationat10Hzandallitsoddharmonicfrequencies(10Hz,30Hz,50Hz,etc.).Thereisnocancellationatevenharmonicsof10Hz(20Hz,40Hz,60Hz,etc.).ThereispartialcancellationatfrequenciesinbetweenasshowninFigure8.For the 2nd stage suction line, a twoloop PANwasproposedtocompletelycancelpulsationatboth10Hzand20H
