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6/12/2015 ProperlySizeControlValves|ChemicalProcessing
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Home/Articles/2015/ProperlySizeControlValves
Properly Size Control
ValvesOversizingafflictsmanyplantsbutisavoidableByMichaelMcCarty,EmersonProcessManagement
Dec16,2014
Asurveyofmorethan500individualsinvolvedinprocessengineering,procurement,operationsandmaintenanceatover200plantsworldwideidentifiedoversizingasthenumberonecontrolvalveproblem.Inafewinstances,oversizingisintentional(e.g.,toprepareforfutureproductionrateincreases).However,inmostcases,oversizingresultsfromwellmeaningbutmisguideddecisionsduringthevalveselectionprocess.Unfortunately,anoversizedvalvecanincurasizableeconomicpenaltyandcausesignificantoperatingproblems.So,letslookathowtoavoidoversizing.
REASONSFORCONCERNAtfulloperatingconditions,controlloopsoperateoveraverynarrow,essentiallysteadystatethrottlingrangewheresmallinputsignalchangestothecontrolvalveresultinsmallvalvestemorshaftmovements.Asyoumightexpect,asmallpositionchangebyavalvethatsoversizedgivesalargerthandesiredchangeinflow.Dependingupontheaccuracyoftheelementsintheloop,thecontrolsystemthenrespondstocorrectthesituation,whichcanresultinathrottlingsequencethatoscillatesbackandforth,causingcontinuousvariationinprocessconditions.
Whileahigherperformancedigitalvalvecontrollercanmaskthisditheringbythecontrolvalvetogivetheappearanceofacceptableloopperformance,oversizingproblemsremain.Infact,anincorrectlysizedcontrolvalvecanresultinproblemseventhoughtheplantappearstoberunningsmoothly.
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FLOW COEFFICIENT VERSUS
TRAVELFigure1.Oversizedvalve(NPS4)cantprovideadequateperformance.
Duringasystemstartuporaturndownto25%maximumcontinuousrating(MCR),operatingconditionswillfalloutsideoftheoversizedvalvesabilitytoadequatelycontrolbecauseoftheneedforextremelysmallvalvemovements.Processcontrolinthisrangemaybenearimpossibledependingupontheinherentflowcharacteristicofthevalve.Highgaincharacteristics(i.e.,theamountofchangedividedbytheamountofinput)canresultininstability,againcausingthevalvetocycle.
Inaddition,theseoffcase,lowflowconditionscanleadtovalvethrottlingoccurringessentiallyrightattheseatorseal.Theresultanthighvelocityflowacrossthenarrowopeningcauseswearanderosion.Impingementoftheacceleratedprocessmediacancutlinesintosealingsurfaces,aneffectthatreducesthecontrolelementsabilitytopreventorminimizeleakagewhenthevalvecloses.Erosiontothecontourofthevalvesflowcontrolelementaltersitsflowcharacteristicandabilitytocontrolasintended.Moreseverecasescanpromptvibrationthatcausesadditionaldamageand,ultimately,equipmentfailure.
Avalvegivesthebestcontrolwhenitssizedtooperatearound60%80%openatmaximumrequiredflowandnotlessthan20%openatminimumrequiredflow.Usingalargerthannecessaryvalvecompromisesperformance,asindicatedinFigure1,whichcomparesflowcoefficient,C
,versustravelforNominalPipeSize(NPS)4andNPS3valvesforaserviceneedinganNPS3valve.
Becauseoftheircompromisedrangeability,improperlysizedcontrolvalvesalsocancauseproblemsduringprocesstransients.Atypicalcontrolvalvewithanequalpercentageflowcharacteristichasabouta30:1turndownratio(i.e.,theratioofmaximumC
tominimumC
).However,whenthevalveisoversizedandthrottlingatthelowend,itsturndownratiofallsto3:1orless.
Also,installingtoolargeavalveamplifiesmechanicalproblemssuchasstictionandhysteresis,makingthesystemdifficulttocontrolandpotentiallycausingprocessupsets.
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IMPACT ON
HORSEPOWERFigure2.Anoversizedvalverequiresadramaticincreaseincompressororpumphorsepower.
Oversizingofcontrolvalveshasadominoeffect.Safetyreliefvalvesmustbesizedtomatchthecapacityofthecontrolvalve.Withinabypassconfiguration,isolationvalves,bypassvalvesanddrainvalvesallmustbelarger,whichcanimpactthesizeofpipingandstructuralpipelinesupports.
Consideralsothattoachieveormaintainflowvelocityinaloopequippedwithanoversizedvalverequiresadramaticincreaseincompressororpumphorsepower(Figure2).Sizingissuescanpropagatebeyonderectioncostsintoperpetualoperationalcostincreasesaspumpinghorsepowerincreasesbyapowerofthreetomaintainthedesiredfluidvelocitywithinthepiping.
SOLVINGTHEVALVESIZINGPUZZLEWhentryingtoavoidorcorrectvalvesizingproblems,itsimportanttounderstandthecausesofvalvesizingerrors.Emersonresearchhasidentifiedseveralmajorcontributors:multiplesafetyfactors,selectinglinesizevalves,andoutofdateprocessdataresultingfromchangesinprocessconditionsorconditionsthatdifferfromtheoriginaldesign.
Tocorrectsystemswithimproperlysizedequipment,itsvitaltoobtainaccurateprocessdataatallexpectedoperatingconditions.Then,sizethevalvetoperformoptimallyattheseconditions.
Multipleengineeringdisciplinesimpactvalvesizing,witheachcontributingtheirspecializedflowcontrolknowhowtovalveselection.
Forexample,processengineersdeterminethefluidflowrates,temperaturesandpressuresthatmustbeestablishedandthenmaintainedbasedonthefeedstockbeinghandledandthedesiredsystemoutput.
Theirgoalistomaximizeoutputwithminimuminputand,todoso,theydesigntheprocessfora100%MCR.
Theyalsodevelopcontrolstrategiesforprocessturndownandturnupscenariosaswellasthosetobeencounteredatstartupandshutdown,takingintoconsiderationfluidflowratesandprocessconditions.
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COST
IMPLICATIONSFigure3.Adecisiontoaddalittlemoremarginoftenwillhitequipmentlimits,forcingastepchangeinsizeandcost.
Pipingengineersdeterminesystemlayoutsandequipmentsupportsnecessarytomeetstructuralandmechanicalneeds.Theysizepipingforappropriatefluidvelocitiesandlinelossesat100%MCR.Theseengineersalsoconsiderthepotentialimpactofcorrosiveorfoulingfluidsupondesignmargins.Theultimategoalistobalanceenergycosts(i.e.,pressurelosses)andpiping/equipmentcosts(drivenbysize).Asvalvesizeincreases,sodoesweight(Table1),whichcandramaticallyaffectassociatedpipingarrangementandsupportsso,oversizinginthisphaseofsystemdesignimpactscostconsiderably.
Instrumentengineerssizecontrolvalvestopasstherequiredflowratesyetmaintainappropriateloopcontrollability.Theunderlyinggoalistomakeeconomicyetappropriateequipmentspecifications.
GLOBE VALVE SIZE VERSUS
WEIGHTTable1.Goingupavalvesizeincursasubstantialpenaltyinweight.
Theinstrumentengineersnotonlyestablishequipmentrequirementsandspecificationsfornormalsystemoperationbutalsosizeequipmentforworstcasescenarios.Theymayaddextrasafetyfactorsatthispointsoyoucanbegintoseehowflowcontrolandmeasuringequipmentstartstobecomeoversizeforthetaskathand.
Itscriticalthatinstrumentengineerscommunicatewithprocessandpipingengineerstoensurethat
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valves,metersandcontrollersnotonlymeetbutalso,importantly,dontexceedsystemflowandpressuredroprequirements.
Valvesupplierapplicationengineersconvertprocesscontrolrequirementsintodetailedequipmentspecificationsthatleadtodeterminingtherightvalveforthejob.Theapplicationengineermustensurethevalveoperateswithinreasonablethrottlinglimits,beingabletopasstherequiredflowrates.Borderlineapplicationsalmostalwaysarepushedintothenextlargersize,resultinginasignificantstepchangeinequipmentcost(Figure3).
Forexample,whenanNPS6valvecanmeetadesignsexpectedprocessflowrequirementsbutoversizingbytheplantsengineerspushtheparameterspastthatvalvescapacity,forcingamovetoanNPS8,thencosttakesasignificantjumpupward(Table2).
FINANCIAL IMPACT OF GOING FROM NPS 6 TO NPS
8Table2.Increasingthecontrolvalvesizeleadstoanumberofotherchangesthatraisetotalcost.
Itsappropriateandnecessaryfortheapplicationengineertoquestionanapparentoversizingofacontrolvalve.Areviewofvalverequirementsbeforethepurchaseorderisplacedsavesbothcostandtimeforallinvolved.
Dontignorecontrolvalvesalreadyinstalled.Youoftencanidentifyapoorlysizedcontrolvalvebyitsdiscorplugposition.Duringnormaloperations,avalvethrottlingatlessthan10%openorgreaterthan90%ofvalvetravelisinappropriatelysized.Anotherindicatorofavalvebeingoversizediswhena1%changeincontrolleroutputcausesagreaterthan3%changeintheprocess.
Globevalveshaveanumberofavailableportsizesforagivenbodysize.So,changingthetrimmayalterthethrottlingpositionoftheflowelementintoamoreoptimalrange.Othercontrolvalves,suchasrotaryones,dontoffersuchflexibilityandmustbechangedoutforaproperlysizedversion.
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THE RIGHT
APPROACHFigure4.Closecollaborationamongthedesigngroupsandthesupplierisneededtooptimizesystemcomponentsandminimizeunintentionalequipmentoverspecification.
ADDRESSOVERSIZINGTheuseofmultiplesafetyfactorssomeinherent,somediscretionarybydifferentdesignfunctionscanleadtospecifyinganoversizedcontrolvalve.Itsimportanttoknowthataseeminglysmallamountofaddedcapacitydoesntresultinaproportionallysmallincreaseinequipmentcost.Mechanicalpipingcomponentshavediscretesizes.Adecisiontoaddalittlemoremarginoftenwillhitequipmentlimits,forcingastepchangeinsizeandcost.Eventheoversizingofasmallvalvecansignificantlyimpactsystemcost,performanceand,ultimately,maintenance.
Avoidingoratleastalleviatingoversizingrequiresclosecollaborationamongtheengineeringgroupsandtheirequipmentsuppliersaseachappliesbestpracticesinprocesssystemdesign(Figure4).
MICHAELMCCARTYisvicepresidentoftheFisherSlidingStemandBaumannValvesBusinessUnitofEmersonProcessManagement,Marshalltown,[email protected].
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