WorkingcontrolvalueCalibratecurrenttopressuretransducers(I/P),valvepositioners,andcontrolvalves.Temperature,pressure,level,andflowinstrumentsallsenseaprocessparameterandproduceasignalforindicationorcontrollerinput.

Ifwewanttocontrolaprocessparameter,thecontrolleroutputmustconverttoasignalthatcantranslatetoandsubsequentlydriveacontrolvalve.Thecontrolvalveisafinalcontrolelement.Afinalcontrolelementisanydeviceorelementthatchangesthevalueofamanipulatedvariable.Valvesandheatersarecommonexamples.Let'slookatcontrolvalvesandthedevicesthatprocessthesignalsuppliedtothecontrolvalve.

AchievetheprogrammedInthisillustrationyoucanseethecontrolleroutputsendsanelectronicsignaltothecurrenttopressuretransducer(I/P),whichsendsapneumaticsignaltothecontrolvalve.

Thecontrolvalvepositionchangesinresponsetothesignaltoadjustflowtothesetpoint.Astheflowchanges,itissensedbytheflowtransmitter.Whentheflowsensedisequaltosetpoint,thevalvepositionremainsthesame.Anytimethereisadisturbancetothesystemorachangeinsetpoint,theflowcontrolloopautomaticallyrespondstoachievetheprogrammedsetpoint.Ablockdiagramofthisconceptishere.

ThefinalcontrolelementcanbeproportionalcontrolorONOFFcontrol.ForONOFFcontrol,acontrolleroutputrelaychangesthestateoftherelaycontact,whichcompletesthecircuitforasolenoidvalvetoenergize.Thesolenoidvalveopenstoallowairtoopen(orclose)acontrolvalve.

Thefirstcomponentinthefinalcontrolsubsystemisthesignalconditioner.Thesignalconditioneramplifiesand,ifnecessary,convertsthesignalforcompatibilitywiththeactuator.

Typicaldevicesusedassignalconditionersincludecurrenttopneumatictransducers,currenttovoltage(I/E)transducers,amplifiers(electronicorpneumatic),relays,digitaltoanalogconverters,oranalogtodigitalconverters.ThemostcommonsignalconditionerinaproportionalcontrolloopisanI/Ptransducer.

AtypicalI/Ptransducerisaforcebalancedeviceinwhichacoilsuspendsandhangsinthefieldofamagnet.Currentflowingthroughthecoilgeneratesaxialmovementofthecoil,whichcausesmovementofthebeam.Thebeamcontrolsthebackpressureagainstthenozzlebycontrollingtherestrictionofairflowthroughthenozzle.Thisbackpressureactsasapilotpressuretocontroltheoutletpressure.

Thezeroadjustmentcausesthebeamtomoverelativetothenozzle.Thespanadjustmentisapotentiometerthatlimitsthecurrentthroughthecoil.TheI/Ptransducermustbesuppliedwithinstrumentairwithintherangespecifiedbythemanufacturer,usuallyatleast20psig.

ThetypicalI/Ptransduceriscalibratedfora420mAinput=315psigoutput.MostI/Ptransducerscanbeconfiguredfordirectaction(outputpressureincreasesasinputsignalincreases)orreverseaction(outputpressuredecreasesasinputsignalincreases).

MechanicallytothevalveThenextcomponentinthefinalcontrolsubsystem,ifapplicable,istheactuator.Theactuatorreceivestheconditionedsignal

andchangesittosomeformofmechanicalenergyormotion.

Typicaldevicesusedasactuatorsincludesolenoids,pneumaticvalvepositioners,ACandDCmotors,steppermotors,hydraulicmotors,andhydraulicpistons.Manycontrolvalvesincludeapneumaticvalvepositioner.

Avalvepositionerisadeviceusedtoincreaseordecreasetheairpressure(fromtheI/P)operatingthecontrolvalveactuator.Positionersusuallymounttothecontrolvalveactuatorandconnectmechanicallytothevalvestemforpositionindication.

Apositionerisatypeofairrelay,whichactstoovercomehysteresis,packingboxfriction,andeffectsofpressuredropacrossthevalve.Itassuresexactpositioningofthevalvestemandprovidesfinercontrol.Therearemanytypesofpositioners.Thebasicprinciplesofoperationaresimilarforalltypes.

Theinstrumentpressure(fromanI/P,forexample)actsontheinputmodule,whichcontrolstheflappernozzlesystemoftherelay.Supplypressureappliestotherelayandtheoutputpressureoftherelaygoestothecontrolvalveactuator.

Mostpositionerscansetupandfunctionfordirectorreverseaction.Foradirectactingpositioner,increasingtheinstrumentpressurecausestheinputmoduletopivotthebeam.Thebeampivotstheflapperandrestrictsthenozzle.Thenozzlepressureincreasesandcausestherelayassemblytoincreaseoutputpressuretotheactuator.

Withadirectactingactuator,theincreasedpressuremovestheactuatorstemdownward.Thepositionerconnectsmechanicallytothestemofthevalve.Stemmovementfeedsbacktothebeambymeansofafeedbackleverandrangespring,whichcausestheflappertopivotslightlyawayfromthenozzletopreventfurtherincreaseinrelayoutputpressure.

NotethatsomepositionersacceptamilliampinputandincludeanintegralI/Ptransducer.

Thelastcomponentinthefinalcontrolsubsystemisthefinalcontrolelement.Let'slookatcontrolvalves(Otherfinalcontrolelementsincludeservovalves,heaters,conveyors,augerfeeds,andhoppergates.).

Therearemanydifferenttypes,sizes,andapplicationsforcontrolvalves.Selectingthecorrectcontrolvalveforaspecificapplicationiscrucialtopropersystemperformance.Undersizingandoversizingarecommonproblems.

Therearemanyvaluableresourcesavailabletoassistwithproperselection,nottheleastofwhichisacontrolvalvesalesengineer.Here'satypicalcontrolvalve.

Thepneumaticsignalfromthepositioner(orI/Pifapositionerisnotused)appliesdirectlytotheactuator.Forthiscontrolvalve,theairentersabovethediaphragmandpushesagainstspringpressuretoclosethevalve.Thevalvefullycloseswhentheplugseatstightlyagainsttheseatring.

Asairpressuredecreases,thespringpressurecausesthediaphragm,stem,andplugtomoveupward,openingthevalve.Thismeansalossofpressurewouldcausethevalvetoopen.Thisisafailopenvalve.

Differentconfigurationsofairinlet,springlocation,andvalveseatarrangementresultindifferentfailpositionsanddeterminewhetherthevalveisdirectorreverseacting.Forexample,thissamevalve,withtheplugbelowtheseatring(reverseseated),wouldopenwithincreasedairpressureandwouldfailclosedonlossofairpressure.

So,allcomponentsinthefinalcontrolsubsystemmustbeconfiguredcorrectlyforthesystemtoworkproperly.Thefailsafepositionsmustbecorrectfortheapplication,andtheactionmustproducethedesiredresults.Theseconfigurationsmustbeproperlydocumentedandutilizedduringcalibration,loopchecks,ortroubleshooting.

AttuneI/PtransducerThefigurebelowshowsthesetupforabenchcalibrationofanI/Ptransducer.Theairsupplyconnectedtotheinputmustbeinaccordancewithmanufacturer'sspecification(typicallybetween20100psig).

Thepressurestandardconnectstotheairoutlet,andamAsimulatorconnectstothecurrentinput.ItisimportantfortheI/Ptransducertobeorientedthesamewayastheinstalledpositioninthefield.AchangeinorientationwillintroduceerrorinmostI/Ptransducers.

Ifthecalibrationtakesplaceinthefield,oneusestheexistingsupplyair.Itisconvenienttoteeintotheairoutletsoonecancheckthecontrolvalvepositionatthesametime.Ofcourse,youneedtoensurethesystemisinasafeconditionbeforeyouopenandclosethevalve.

Oncethesetupisestablished,applythemAinputsforeachdesiredtestpoint,suchas4.0,8.0,12.0,16.0,and20.0mA.Recordthecorrespondingoutletpressureateachtestpoint.Fora420mAinput=315psigoutputI/P,thecorrespondingoutputswouldbe3.0,6.0,9.0,12.0,and15.0psig.

Somefacilitiesadjustthe0%testpointsoaslightlyhighermAinputresultsinthe0%output.Forexample,4.10mAmayresultina3.0psigoutput.Thisensuresthevalveisintheclosedstatewithacontrolleroutputof4.0mA.

Uponascertainingtheasfoundreadings,evaluatetheresultsagainsttherequiredspecification.Ifrequired,performzeroandspanadjustmentsuntilnofurtheradjustmentisrequired.Then,repeatalltestpointstorecordasleftreadings.

ManyorganizationsdonotrequireperiodiccalibrationofI/Ptransducers,positioners,orcontrolvalves.Thejustificationisthecontrolsignalwilladjusttheoutputuntiltherequiredsetpointisachievedbasedontheprocessmeasurement.Thisistrue,butyouwanttomakesuretheoutputloopisperformingcorrectly.Thebestwaytodosoistocheckthecalibrationperiodically.

CalibratevalvepositionerCalibrationofthevalvepositionercanbeperformedatthesametimeastheI/Pinaloopcalibration.SimplyteeinthepressuremoduleattheI/PoutletintheI/Pcalibration.Recordthevalvepositionateachtestpoint.

Ifcalibratingthevalvepositionerseparately,connectaninputtestpressureregulatororhandpump,andmonitortheinputpressureappliedwithapressurestandard.Ifthereisnosupplyair,connecttherequiredsupplyairtothepositioner.Applythepressureforthedesiredtestpointsandrecordvalveposition.

Forexample,assumeourvalvepositioneris315psiginput=0100%valveposition.Inthiscase,apply3.0,6.0,9.0,12.0,and15.0psig.Theexpectedvalvepositionsshouldbe0,25,50,75,and100%,respectively.

Thevalvepositionindicatoronthestemusuallymarksoffin5%or10%increments.Therefore,abestestimateofthevalvepositionmaybeallyoucanobtain.Inothercases,avalvepositiondetectorprovidesaremoteindicationtoaDCS.Insuchcases,ensurebothindicatorsareworkingproperly.

Manyorganizationsdonotrequirecalibrationofvalvepositionersforthesereasons.There'smuchdocumentationthatcontrolvalvepositionerperformanceisresponsibleforsignificantlossinsystemefficiencyand,therefore,increasedcosts.

Toprovideguidanceonmethodsfortestingpositionersandcontrolvalveperformance,ISAhasdevelopedastandard,ANSI/ISA75.25.012000,TestProcedureforControlValveResponseMeasurementforStepInputs.

Astocontrolvalvecalibration,theprocessissimilartopositionercalibrationinthatoneappliesapressuresignaltotheactuatorandthentalliestheresultingvalveposition.Thisstepcantakeplacewiththepositionercalibration,ifapplicable,anditcanhappeninconjunctionwithI/Pcalibration.

Remembertoensurethesystemisinasafeconditionifperformingthecalibrationinthefield.Inaddition,knowthecorrectaction,directorreverse,andfailpositionbeforestarting.CE

NicholasSheble(nsheble@isa.org)editstheCertificationdepartmentforInTechmagazine.ThisarticleisfromMichaelCable'sbookCalibration:ATechnician'sGuide,ISAPress2005.CableisaLevel3CertifiedControlSystemTechnicianandisthevalidationmanageratArgosTherapeutics.