TECHNICAL MANUALTES-492B

Hoffman Specialty® Steam Regulating Valves Manual

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I. Introduction ................................................................................................................................... 3

Definitions .............................................................................................................................. 3

PressureMeasurement ........................................................................................................... 3

PropertiesofSteam ................................................................................................................ 4

II. RegulatorConstruction .................................................................................................................. 6

ValveDesigns ......................................................................................................................... 6

Actuators ................................................................................................................................. 8

III. PressureRegulation ....................................................................................................................... 9

SelectingandSizingPressureRegulators .................................................................................... 15

UsingtheFlowCoefficientFormula ..................................................................................... 16

UsingtheManufacturer'sCapacityTables ........................................................................... 20

PressureRegulatorInstallation .................................................................................................... 24

Troubleshooting ........................................................................................................................... 25

NoiseinPressureRegulators....................................................................................................... 27

TypicalApplicationsforPressureRegulators ................................................................................ 28

IV. TemperatureRegulation .............................................................................................................. 29

SelectingaVaporTensionTemperatureRegulator ....................................................................... 32

DeterminingtheValvePressureDrop .................................................................................. 32

DeterminingtheSteamFlowRateRequired ........................................................................ 34

SelectingExternallyPilotedTemperatureRegulators .................................................................. 36

InstallationofTemperatureRegulators ........................................................................................ 38

Troubleshooting ........................................................................................................................... 38

TypicalApplicationsofTemperatureRegulators ........................................................................... 41

V. SpecialProblems .......................................................................................................................... 43

HandlingLargePressureReductions ................................................................................... 43

SuperheatDownstreamofaValve ........................................................................................ 43

HandlingHighlyVariableLoad ............................................................................................. 44

SteamSelectionandSizingTools ........................................................................................ 46

TABLE OF CONTENTS

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SteamregulatingvalvesarefoundinavarietyoflightindustrialandHVACapplications.Theyautomaticallycontroltheflowofsteaminorderto:

1. regulatedownstreamsteampressure,or2. regulatethetemperatureofsomeothersubstancebeingheatedbythesteam.

PRESSURE REGULATORSMostlargesteamplantsgenerateanddistributesteamathighpressure,thenlowerthepressureatthepointofusewithapressurereducingvalve,orpressureregulator.Theydothisbecausehighpressuresteamboilerstendtobemoreefficientthanlowpressureboilers,andhighpressurepipelinescostagreatdeallessthananequivalentpipelineforlowerpressuresteam.Also,lowpressuresteamcontainsmoreusableheatenergyperpound,andcomponentsthatoperateatlowerpressuresdon'thavetobebuilttowithstandthestressofhighpressure.Pressureregulatorsusedownstreampressureasafeedbacksignaltopositionavalvesothatsteamathigherpressureupstreamwillflowthroughthevalveataratethatwillmaintainsomedesiredlowerpressuredownstream.

TEMPERATURE REGULATORSSteamisanexcellentheatingmediumbecauseitcarriesalotofheatperpound,it'snontoxic,inexpensive,andreadilyavailable.Temperatureregulatingvalvescontroltheflowofsteamintoaheatexchangerinordertoheatanotherfluid.Theyusethetemperatureofthatotherfluidtogenerateafeedbacksignaltooperatethevalvethatcontrolstheflowofsteam.This"otherfluid"canbeanyliquidorgas,andthenatureofthatfluidwilldeterminethetypeofheatexchangertobeused.

I. INTRODUCTION

Figure 2showsaverycommontype,ashellandtubeheatexchangerbeingusedtoheatliquidinthetubesbysteamcondensingintheshell.

DEFINITIONSInbothpressureandtemperatureregulators,thereisanimportantandpredictablerelationshipamongthreefactors:

(1) pressureaheadofthevalve,called"supplypressure", or"P1"

(2) pressuredownstreamofthevalve,called"controlpressure","loadpressure",or"P2",and

(3)steamflowrateinpoundsofsteamperhour,"W".

Thesethreequantitiesmustbeknowninordertodesign,operate,ortroubleshootasteamregulatingvalveinstallation.Anotherfactor,"pressuredrop",isthedifferencebetweenupstreamanddownstreampressure.Othertermsusedtodescribepressuredropare"differential"and"ΔP",whichisread"deltaP".

PRESSURE MEASUREMENTThepressureinasystemcanbemeasuredusingeitheroftwostartingpoints,absolutezero,oratmosphericpressure:

In Figure 3 thebottomlinerepresentsanabsolutevacuum-

nopressureatall.Atmosphericpressure,causedbytheweightoftheatmosphere,isabout14.7poundspersquareinchaboveabsolutezero,psia,butapressuregageexposedtoatmosphericpressurewouldreadOpsig(Atsealevel.).ThepressureinSystemAcouldbemeasuredfromabsolutezeroinpoundspersquareinch,absolute,"psia",oritcouldbemeasuredbyapressuregagestartingatatmosphericpressure,inpoundspersquareinch,gage,or"psig".

Figure 1 Pressure Regulator

Figure 2 Temperature Regulator

Figure 3 Pressure Measurement

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PROPERTIES OF STEAMTheamountofheattransferrediscommonlymeasuredin"BritishThermalUnits"or"Btu".ThenumberofBturequiredtobringapoundoficewatertotheboilingtemperatureiscalledthe"sensibleheat"becausewecansensetheeffectoftheheatadditionbyobservingariseintemperatureofthewater.Amoreformaltermforsensibleheatis"liquidenthalpy",measuredinBtu/lb.Theheatthat'saddedtowateratboilingtemperatureiscalled"latentheat",becauseit'seffectonthewaterdoesn'tshowupasanincreaseintemperature.This"latentheat",or"enthalpyofevaporation",causesthewatertochangefromaliquidtoavaporwithnochangeintemperature.Theterm"saturatedsteam"referstothesteamgeneratedinatypicalboiler.It'sthewatervaporthatformswhileheatisaddedtoliquidwaterattheboilingtemperature.Pressureandtemperatureofsaturatedsteamarerelated.Thisrelationship,aswellastheenthalpyandotherpropertiesofsteamatvariouspressuresaretabulatedinthe"SteamTables",ashortenedversionofwhichisshownin Figure 4.

Thesteamtablescanprovideagreatdealofinformationtohelpsolveproblemsinsteamsystemoperationanddesign.Forexample,supposewewanttomoldsomeplasticthathasameltingpointof300°Finaplasticmoldingmachine.

Accordingtothesteamtables,we'llneedatleast55psigsteaminthemachineheatingcoilstomelttheplastic becausethetemperatureofsteamatpressuresbelow 55psigislessthan300°F.Themanufacturerofthemoldingmachinemightinstallapressureregulatortoprovidesomeconstantsteampressureinthecoilsofhismachine,say, 150psigat366°F.Wecan'treducethepressuremuchbelowthis,becauseoursteamtemperaturehasgottostaywellabovetheplasticmeltingpointinordertomelttheplasticquicklyandmaintaintheproductionrate.Ontheotherhand,ifwewanttoheatwaterto140°F,wecoulduseanysteampressure,evensteamat0psig/212°F,wouldbehotenoughtotransferheattothewateratanacceptablerate.Sosteamflowingthroughatemperatureregulatingvalvecoulddroptoalmostanylowpressureandtemperature,butwewouldn'tcare,aslongaswegetthedesired140°Ftemperaturein thewater.

Noticethatthesensibleheatofapoundofwaterathigherpressureisgreaterthanthesensibleheatatsomelowerpressure.Eachpoundofsteamat0psigcarries970oflatentheat,buthigherpressuresteamcarrieslesslatentheat.Finally,noticethatthetotalenthalpyofthesteam,thesumofsensiblepluslatentheat,increaseswithincreasingpressure.

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PROPERTIES OF SATURATED STEAM

Figure 4 Properties of Saturated Steam

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Allregulatorshaveavalvebodyandanactuatortooperateitautomaticallyaccordingtothefeedbacksignalitreceives.

VALVE DESIGNSValvebodiescomeinavarietyofdesigns.They'reusuallymetalcastingswithinternalpassagewaystodirectthesteamflow,openingstoallowthemovingpartsofthevalvetooperate,andflangesorothermeanstoattachthevalvetothepiping.Theyaremostoftenmadeofcastbronzeorcastiron;althoughdesignsforveryhighpressureserviceusestronger,morecostlymaterials.

Thegraphsin Figure 5showtemperatureandpressurelimitationsforsomecommonvalvematerialsfromANSIB16.1,andB16.24.Thepressureratingissometimescastintothevalvebody.The"WSP"orprimaryratingstandsfor"workingsteampressure".TheWSPratingislowerthanthe"WOG"orsecondaryratingwhichappliesto"water,oil,orgas"service.Forexample,avalvecouldcarrythefollowinginformation: 150psiWSP 300psiWOG

whichmeansthatitcanhandleupto300psiwater,oil,orgas,butonly150psisteam.Themanufacturerofsteamregulatorsalwaysspecifiesthemaximumsteampressureforagivenvalvemodel.Ifavalvecanbeusedforavarietyofdifferentfluids,makesurethatyoudon'ttrytouseitathighsteampressures,eventhoughitmightbeusedathigherwater,oil,orgaspressures.Sometimesavalvewillbelimitedtopressuresandtemperatureslowerthanyouwouldexpectforgivenbodymaterials.Thisisbecausetheothermaterialsusedforstempackingorvalvetrimmaynotbeabletowithstandhigherpressures.

"Valvetrim"isthetermthatdescribesallthepartsthatcomeintocontactwiththesteamexceptforthevalvebody.The"valveplug",or"disc"isthepartthatmovestomakecontactwiththevalve"seat"toregulatesteamflowandclosethevalve.Moreaccurately,the"disc"isthesurfaceoftheplugthatactuallytouchestheseat.Oneendofthevalvestemisattachedtotheplug,andtheotherendisattachedtosomekindofactuatorwhichmovesthestemtooperatethevalve.The"valvebonnet"isthepartthatallowsthestemtopenetratethevalvewithoutleaking.Thebonnetmaybethreadedtothebodyandprovidedwithagasket.Itmaybeattachedwithaunionorevenaboltedassemblyinhigherpressurevalves.

Arenewablediscvalveissimilartoaglobevalve.It'sgoodforapplicationsthatrequiretightshutoffofsteamflow,becausethepliablecompositiondiscwillcloseinspiteofgritorscalethatmightaccumulateonthevalveseat.Thediscmaterialwon'tstanduptohighsteamtemperatures,ortheerosioncausedbyhighvelocitysteamflow,sotherearetightlimitsplacedonthesteampressureandtheamountofpressuredropallowedacrossthevalve.Thislimitsthesteamvelocityandthereforetheerosionofthedisc.Ontheotherhand,thevalveisinexpensive,andthedisciseasilyreplaced.

II. REGULATOR CONSTRUCTION

ASTM Al26 Cast Iron ASTM B61 & B62 Cast Bronze

Figure 5 Valve Material Pressure and Temperature Ratings Figure 6

Renewable Disc Valve

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Ametallicdiscvalveisalsoasingleseatedvalve,abletoprovidetightshutoff.Themoredurablematerials,suchasstainlesssteel,orotheralloyswhichareresistanttoerosion,allowittobeusedwithhighertemperaturesteamandwithgreaterpressuredropsthantherenewablediscvalve.Thesevalvesaregenerallymoreexpensivetobuyandrepairthanthecompositiondiscvalves.Whenthesekindsofvalvesareusedinsimplepressureregulators,theyaresometimesdescribedas"unbalanced",meaningthatchangesinupstreampressurewillbereflectedassmallerchangesindownstreampressure.

Doubleseatedvalveshavegreatercapacitythansingleseatedvalvesofthesamesizebecausetheyhavetwoflowpassages.Thevalvestemisalsoeasiertopositionsincethesteamforceonitispartiallyoffset.Thatis,upstreamsteampressureexertsforcesinoppositedirectionsagainstthetwovalveplugs,partiallycancellingeachother,andallowingthestemtobepositionedmoreaccuratelywithlessactuatorforce.Doubleseatedvalvesarelikelytoallowalittleflow(about1%)evenwhentheyaresupposedtobeclosedbecauseofthedifficultyingettingbothdiscstoseatatexactlythesametime.Theyareusedonlywhereamoreorlesscontinuousflowofsteamcanbecondensed,andtightshutoffisnotrequired,forexample,inalowpressuresteamheatingsystem.Theyarealsolikelytobealittlemoreexpensivethanasingleseatedvalveofthesamesize.

Thepistonor"internallypiloted"valveusesthesteamitselftopositionthevalvetoachieveveryaccuratecontroloversteamflow.Thevalvesternopensasmallpilotvalveallowinghighpressuresteamtoexpandthroughthepistonintothechamberbehindthepiston.Thisprovidestheforcetoopenthemainvalvepassage.Byusingtheenergyofthesteamtooperatethevalve,wegetquickandaccuratevalveactiontocontrolsteamflow.Thisisa"balanced"valvebecausesmallchangesinupstreampressurewillnoteffectthedownstreampressure.Theconstructionofthisvalve,requiresmoreengineeringandmanufacturingtime,soyoucanexpectitspricetobesomewhathigherthanothers.

Anothersingleseatedvalvedependsuponaspringloadedballtocloseagainsttheseat.Itisopenedbyavalvestemthatpushesagainsttheballtoallowflow.Becausetheballcanrotatealittleasitclosesagainsttheseat,thisvalvedesignismoreresistanttoscaleordirt,andwearisnotasbigaproblemsincetheballhasanynumberofcontactsurfaces,or"discs"tomatewiththeseat.Thisdesignisoftenusedasalowcostdirectsensingpressurereducerorasanexternalpilotvalvetoalargerregulator.

Figure 7Metallic Disc Valve

Figure 8 Double Seated Valve

Figure 9 Piston Type Valve

Figure 10 Pilot Type Valve

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ACTUATORSAllautomaticpressureortemperatureregulatorsrequiresomekindofactuatortopositionthevalveplugsothatthesteamflowwillbecontrolledtotheproperrate.Oneofthemostcommonkindsofactuatorsisadiaphragm,orbellowswhichflexesasthefeedbackpressuresignalonitchanges.Thediaphragmexertsaforceonthevalvestemandplugthatdependsontheareaofthediaphragmandthesignalpressureonit.

Inapressureregulator,thesignalpressureissimplythedownstreamsteampressurebeingfedbacktotheregulatordiaphragm.

Inatemperatureregulator,thefeedbacksignalcomesfromatemperaturesensingbulborrodwhichisinsertedinthefluidtobeheated.Thetemperaturesensingdevicemaydeveloptherequiredsignalpressureinseveraldifferentways,dependinguponthedesignofthetemperaturesensor,theaccuracyandresponsivenessrequired,andthecostoftheunit.Differenttypesoftemperaturesensingdeviceswillbedescribedinthesectionontemperatureregulators.

Aspringopposestheactionofthediaphragm,allowingtheregulatortoreopenasthesignalpressureisreduced.Theactuatoralwayshassomekindofadjustmenttosetthelevelofsignalpressurerequiredtooperatethevalve.Thatsettingdeterminesthepressureortemperaturesetpointoftheregulator.

Averypopularregulatordesignusesasingleseatedmainvalveoperatedbyoneormoreexternalpilotvalves.Theexternalpilotvalvecontrolstheflowofsteamtoalargediaphragmwhichopensandclosesthemainvalve,sotheexternalpilotvalveandthemainvalvediaphragmacttogetherastheactuator.Thepressureortemperaturesetpointcanbeadjustedatthepilotvalve.Avarietyofpilotvalvesisavailabletoregulatepressure,ortemperature,orbothtemperatureandpressure.

Figure 11 Signal Pressure x Area = Actuating Force

Figure 12 Externally Piloted Regulator

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Thedirectsensingvalvehasaninternalpassagewaythatexposesthebottomofthediaphragmtodownstreampressure.Theremotesensingregulatorhasanexternalfeedbackline,usually1/4"coppertubing,extendingfromthedownstreampipelinebacktothevalvediaphragm.Theremotesensingregulatorisusuallymorecostly,andrequiresmoreinstallationtimeandspace,butitgivesbettercontrolqualitysincethesourceofthefeedbacksignalisremovedfromtheturbulentsteamflowaroundthevalve.Thedirectsensingvalveislessexpensive,morecompact,andgivesadequatecontrolformanyapplications.

III. PRESSURE REGULATION

Figure 13 Types of Pressure Regulators

Theexternallypilotedregulatorhasanumberofdifferentpilotvalvesforpressureregulation.Theyprovideveryresponsiveandaccuratepressureregulation,althoughtheinstallationismorecomplexthanthedirectorremotesensingvalves.Costsofremotesensingandexternallypilotedregulatorsareusuallyaboutthesame.

PRESSURE REGULATOR OPERATIONThevalvebody,spring,anddiaphragmoftheremotesensingregulatorinFigure 14havebeenchosenandinstalledtoaccomplishapressurereductionfrom100psigto50psigatasteamflowrateof1500lbsperhour. Figure 14 showshowthisvalvewillreacttomaintainthedownstreampressureinresponsetodifferentconditionsofsteamdemand.

Pressureregulatorsaremostoftenfoundinoneofthefollowingdesigns:directsensing,remotesensing,orexternallypilotedasshowninFigure 13.

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NO PRESSUREThepressurereducingvalveisinstalledinthesteampipewitharemotepressuresensinglineleadingbacktothebottomofthevalvediaphragm.Beforewecutinsteam,bothpressuregagesshow0psig,andthevalveiswideopen.That'sbecausetheregulatingnut,A,hasbeenadjustedonitsthreadedsleeve,soit'scompressingthespring,causingthediaphragmtobulgedownward.Thevalvestemisconnectedtothebuttonabovethediaphragm,sothisstemmovementpullstheplugofftheseat.

NO FLOWIfwekeepthedischargevalveshutandopenthesteamsupplyvalve,theupstreamgaugewillread100psig,thedownstreamgaugealmost60psig,andthevalvewillcloseasthedownstreampipefillswithsteam.Risingpressureinthedownstreampipingactsthroughthesensinglineonthediaphragm,exertinganactuatingforceupward,furthercompressingthespringandclosingthevalve.Ittakessomewhatgreaterforcetoovercomefrictionandgetthevalveplugtostartmoving.That'swhythedownstreampressureinthis"lockup"ornoflowsituationisalittlehigherthanthedesiredsetpointof50psig.

Figure 14 Pressure Regulation

REGULATINGWhenweopenthedischargevalve,steamflowsoutofthepipe,andpressureinthesensinglinedrops.Decreasedpressurebelowthediaphragmmeanslessforceactingupward,sothespringstartstorelaxdownward,openingthevalveandallowingflow.Steamflowandpressuredropthroughavalvearerelatedinadefinite,predictableway.Asthedemandforsteamdownstreamofthevalverisesandfalls,thepressureinthesensinglinewillalsovary,repositioningthevalveplugtoallowenoughflowtomaintainabout50psigatthesensingpoint.Thisisthenormaloperatingsituationforthevalve-changesinpressureatthesensingpointareconvertedintochangesinactuatorforceatthediaphragm,openingorclosingthevalvetoincreaseorreducetheflowasrequired.

OVERLOADAsthedemandforsteamincreases,thefeedbackprocessworksuntilthevalveiswideopen.Withfurtherincreasesindemand,thewideopenvalvewillnolongerbeabletopassenoughsteamtomaintainthedesiredpressure,anddownstreampressurewillstarttodropoff.It'sgenerallyacceptedthatadropofsteampressuregreaterthan10%belowthesetpointmeansthatthevalveisundersized,ortoosmall,forthatflowrate.

No Pressure No Flow

Regulating Overload

02040

60 80100

120 02040

60 80100

120 02040

60 80100

120

02040

60 80100

12002040

60 80100

12002040

60 80100

120 02040

60 80100

120

02040

60 80100

120

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DIRECT SENSING VALVEDirectsensingvalvesoperateinthesamewayastheremotesensingvalveexceptthatdownstreampressureisledfromwithinthevalvetotheundersideofthediaphragmwithoutneedforaremotesensingline. In Figure 15 steamflowsfromlefttoright.Asmallpassageinthevalvebody,oragenerousfitaroundthevalvestemallowsdownstreampressuretoactagainstthebottomofthediaphragmtobalancethespringforceexertedontopofthediaphragm.Becausethedownstreampressureislikelytobeunsteadyduetoturbulenceassteamflowsthroughthevalve,thequalityofpressureregulationinadirectsensingvalveisnotlikelytobeasgoodasinaremotesensingvalve,althoughthesmallersize,easierinstallation,andlowercostoftensuitthemforsimpleapplications.

EXTERNALLY PILOTED REGULATORSThesameprinciplesofpressuredropandfeedbackcontroloperateinexternalpilotoperatedvalves.

Themainvalveinanexternallypilotedregulatorisinstalledinthesteampipingwiththreaded,orflangedfittingsdependingonthevalvesize.Ithasasinglemetallicseatandplug,the"valvetrim",oftenmadeofhardenedstainlesssteeltoresisterosionandcorrosion.Differentsizevalveseatscanbescrewedintothebodyinordertoprovidesomeflexibility

indesigningthecapacityofthevalve.Moredetailsontheuseofdifferentsizevalvetrimareincludedintheexampleonregulatorsizing.

Themainvalveisheldclosedbyaspringwhichiscompressedbetweenthevalvebodyandthedoublestainlesssteeldiaphragmatthebottomofthevalveaswellassteampressureactingontopofthevalveplug.Inordertoopenthevalve,steampressuremustbeexertedonthebottomofthatdiaphragmtofurthercompressthespringandopenthevalve.Anexternalbalancetubeinsuresthattherewillbenodifferencebetweenthepressureontopofthediaphragmandpressureatthevalveexit,andtappingsareprovidedinthevalvebodytomountthepilotvalveoneitherside.Smaller,externalpilotvalvesregulatesteamflowtothediaphragmtooperatethemainvalve.

Thespringtypeexternalpressurepilotvalvegetsitssourceofsteamfromthehighpressuresideofthemainvalve,andregulatessteamflowtothemainvalvediaphragminordertoprovidereducedsteampressuredownstream.Thepilotvalvealsohasafeedbackconnectionwhichappliessystemsteampressureagainstthebottomofasmalldiaphragm.Forceontopofthediaphragmisadjustedbytheamountofspringcompressionsetbytheoperator.Ifthedownstreampressureislowerthanthesetpoint,thepilotwillopen,allowingsteamtoflowtothemainvalvediaphragm.Boththeoriginaldesignandtheimproveddesignforthespringpressurepilothaveanumberofspringsofdifferentstiffnesstoprovidearangeofdownstreampressures.Pilotvalvesareoftenequippedwithstrainerstoremoveanygritorscalewhichmightdamagethepilotorcauseittostickopen. Figure 18showshowthemainvalveandspringpressurepilotvalvesworktogethertoregulatedownstreampressure.

Figure 15 Direct Sensing Valve

Figure 16 Main Valve

Figure 17 Spring Type Pressure Pilot Valves

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OPENINGIfsystempressureonthebottomofthepilotdiaphragmislessthanthespringcompressionabove,therewillbeanetforceactingdownfromthepilotdiaphragm,forcingtheballoffthepilotvalveseat,andallowingsteamtoflowtothemainvalvediaphragm.

CONTROLLINGAspressurebuildsunderthemainvalvediaphragm,thespringiscompressed,andthevalveopens.Highpressuresteamflowsthroughthevalve,raisingthedownstreampressuretowardthesetpoint.Pressureofthesteamactingunderthemaindiaphragmwillbepartwaybetweenthehighpressureupstreamandthelowpressuredownstreamofthemainvalve.

CLOSINGAsdownstreampressurebuilds,theforceimbalanceacrossthepilotvalvediaphragmiscorrected,allowingthepilottoclose.Assteampressurebelowthemainvalvediaphragmstartstofall,themainvalvespringcanexpand,closingthemainvalveandpushingtheremainingsteamfrombelowthediaphragmtothedownstreamsideofthevalveviaa"bleedorifice",shownat"B"

Thebleedorificemostcommonlyusedisacompressionfittingwithadrilledorifice.It'sinstalledinthetubingfromthemainvalvediaphragminthetappingatthedownstreamsideofthevalve.Anytimethepilotvalveisopen,therewillbesomesmallflowthroughthebleedorifice,butflowthroughthepilotasitopensismuchgreaterthanflowthroughthebleedorifice.Thisallowspressuretobuildunderthemainvalvediaphragmtoopenthevalve.Thebleedorificeisrequiredinordertoallowtheregulatortoclosequicklywhenthepilotcloses.Withoutthebleedorifice,themainvalvewouldn'tcloseuntilthesteambelowitsdiaphragmcondensed,andthatwouldcauseanunacceptablelackofresponsiveness,andwideswingsinpressuredownstream.

Figure 19Typical Bleed Orifi ce

Figure 18External Pressure Pilot Operation

Bleed Orifi ce

Out

Out

Pilot

Pilot

Pilot

In

In

Main Valve

Main Valve

Main Valve

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PNEUMATIC PRESSURE PILOTThespringcontrolledpilotvalveisusefulforreducingsteampressuretosomeconstantlowerpressure,butit's ofteninconvenienttochangethepilotvalvesetting.Pneumaticpilotvalvescanbeeasilyresetifadifferentdownstreampressuresetpointisdesired.Ofcoursealowpressureaircompressormustbeavailabletoprovide"shopair"tosetthepilot.TheoriginalstylepneumaticpressurepilotisidenticaltotheoriginalstylespringoperatedpilotexceptthatthespringandyokeassemblyofthespringpilotisreplacedbyapneumaticdiaphragmasshowninFigure 20.Withpneumaticpilots,thereducedsteampressuresettingcanbechangedsimplybyincreasingordecreasingairpressuretothetopofthepneumaticdiaphragm.Forhigherdownstreampressures,thesingleairdiaphragmcanbereplacedbyasetoftwodiaphragmswhichgivea3:1or5:1arearatiotomultiplythepneumaticforce.

Figure 20 Original Style Pneumatic Pilot

Figure 21 Pneumatic Pilot Valves

Animproveddesignpneumaticpilotvalveoperatesinmuchthesamewayastheoriginalstyle,buthaseitherasinglediaphragmwhichgivesa1:1ratioortwodiaphragms,whichgivea4:1ratioofdownstreamsteampressuretoairsignalpressure.Thelargediaphragmhasanareafourtimesgreaterthanthelowerdiaphragm,soagivenairpressuresignalontopofthelargediaphragmwillrequirefourtimesmoredownstreamsteampressurebelowthesmallerdiaphragmtoachievebalanceandclosethepilot.Theoutletportwhichsuppliessteamtothemainvalvediaphragmandthebleedorificeislocatedjustabovethehighpressuresteaminlet, at90°toit,soitdoesn'tappearinthisview.Thebuttonsandlargediaphragmcaparedesignedtolimittheamountofdiaphragmmovementtopreventdamage.

SOLENOID PILOTSolenoidpilotvalvesareelectricallyoperatedon/offvalveswhichcancontroltheflowofsteamfromthemainvalvetootherpilots,providingasimplemeanstocontroltheregulator.Thesolenoidpilotsareeithernormallyon,requiringanelectricsignaltoclose,ornormallyoff,requiringanelectricsignaltoopen.Theymaybeusedwithanycombinationofotherpilotstoprovideremote,automatic,ortimedoperationoftheregulatorsystem,ortoactassafetyover-ride.

Figure 22 Solenoid Pilot Valve

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SUMMARY

TheperformanceofapressureregulatorissummarizedinFigure 23,althoughthefiguremisrepresentsactualregulatorperformancebycompressingthehorizontalscaleinordertoshowtheeffectofdifferentsizevalvetrim.Anactualregulatorwouldprovideacceptabledownstreampressureoverawiderangeofflowrates.

Figure 23 Pressure Regulator Performance

Legend:A. Steamflowthroughtheregulatorasitincreasesfromzeroattheleft.B. Threecurvesdescribetheperformanceofagivensizeregulatorbody

whendifferentsizevalvetrimisinstalled.C. Theminimumcontrollableflow.D. Pressureriseatnoflow.E. Pressuresetpointminus10%.F. Maximumnormalflowforthegivenmainvalvetrim.

Someadditionaltermsaresometimesusedtodescribetheperformanceofpressureregulators.

Rangeabilityofaregulatoristheratioofmaximumtominimumcontrollableflow.IntermsofFigure 23;

rangeability = flowatE flowatC

Turndownratioistheratioofmaximumnormalflowtominimumcontrollableflow.IntermsofFigure 23;

turndownratio = flowatF flowatC

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Thereareanumberofdecisionsorcalculationsinvolvedinchoosingasteampressureregulator:

(a) Materialsusedforthevalvebody,trim,andactuatormustbecompatiblewiththesteampressuretobeused,thedesigndifferentialpressure,andthedurabilityandprecisionexpectedfromthevalve.

(b) Valvedesignmustprovidesuitableservice.Amongthechoicesavailable:• singleordoubleseatedvalvedesign• balancedorunbalanceddesign• remotesensing,directsensing,orexternally piloteddesign

(c) Thepressurereductionrequiredmustnotbeexcessive.Valveshavedefinitelimitationsonthemaximumpressuredropallowedacrossasinglevalve.Observingtheselimitswillprovidelongerlifeandquieteroperation.Ifyoursystemrequiresapressuredropgreaterthanthatallowedforasinglevalve,thentwoormorevalvescanbeusedinseriestoachievethetotaldropinacceptablestages.SectionVhasmoredetailsonvalvesinseries.

(d) Steamflowrequiredthroughthevalvecanoftenbedeterminedsimplybynotingthesteamrequirementsonthenameplateofthecondensingequipment.Steamflowraterequirementscanalsobecalculatedfromtheheattransferrequired,andthesteamenthalpyofevaporationatthereducedpressure.Seethesectionontemperatureregulatorsizingforadditionaldetailsandasamplecalculation.Maximumandminimumflowratesshouldbeestimatedtoo.Asinglevalvethatislargeenoughtohandlethemaximumflowmaybeoversizedattheminimumflowrate.Inthiscase,valvesshouldbeusedinparallel.SeeSectionVfordetailsaboutdesigningandinstallingvalvesinparallel.

(e) Thevalvesizerequiredcanbedeterminedby,(1) usingsizingformulastocalculatearequiredvalve

flowcoefficient,(2) usingregulatorcapacitytablesprovidedbythe

valvemanufacturer,or(3) usingspecialselectionsoftware.Thevalveflow

coefficient,Cv,foragivenvalveisdefinedastheflowrateingallonsperminuteofwateratstandardtemperature,thatwouldpassthroughthewideopenvalveifaconstantpressuredifferentialofonepsiweremaintainedacrossthevalve.Theflowcoefficientthusbecomesameasureofavalve'scapacityrelativetoothervalves.

SELECTING AND SIZING PRESSURE REGULATORS

Sizing formula(1) Calculatethevalveflowcoefficient,Cv,required

forthegivensteampressuredropandflowratebyusingtheformulasorothermethodsprovidedinthissection.

(2) Selectavalvefromthemanufacturer'scatalogthathasaCvratingequalto,orgreaterthantheonecalculated.Samplevalvedatasheetsgivingtheflowcoefficientsandothertypicaldataforsampleregulatorsarealsoincludedinthissection.

Capacity tables(1) Obtainthecapacitytableforthevalvemodelyou've

chosen.Eachvalvemodelhasitsowncapacitytable,determinedbytestsconductedatthefactory.Enterthetablewiththesteaminletpressure,usuallylistedverticallyalongtheleftsideofthetable.(SeeFigure 28.)

(2) Gotothedesiredoutletpressureandreadacrossthatrowuntilyoufindaflowratethatmeetsorexceedsthecapacityrequired.Thevalvebodysize islistedacrossthetopofthetable.

Computer equipment selection programsFollowtheonscreenpromptstoinputvaluesforinitialandreducedpressurerequired,andforthedesignflowrate.Theprinciplesusedindevelopingtheseprogramsareillustratedinthefollowingexamples.Theseprogramsare"userfriendly".Simplyrefertotheprogramfordetails.

Regardlessofthemethodused,becarefulnottoaddcapacitysafetyfactorssincethatwillresultinselectionofavalvethatisoversizedforyourapplication.Oversizedvalveshavehigherinitialcost,providepoorerqualityofcontrol,andarelikelytowearoutmorequicklythanaproperlysizedvalve.

(f) Choosethevalveactuatororpilotbasedupontheinitialanddownstreampressuresdesired.Theactuatormayconsistofadiaphragm,diaphragmcase,andspring,oritmaybeanexternallymountedpilotvalvethatsuppliessteampressuretothemainvalvediaphragmdependingonthetypeofregulatorchosen.

16

STEAM VALVE SIZING FORMULAS

DEFINITIONS:P1 = pressureatvalveinlet,psiaP2 = pressureatdownstreamsensingpoint,psia∆P = P1-P2W = flowrateofsaturatedsteam,lb/hr

Calculatingthevalveflowcoefficient,Cv WCv = 2.1∆P(P1+P2)

whenP2isgreaterthan0.5P1

WCv = 2.6∆P(P1) whenP2islessthanorequalto0.5P1

Inordertocorrectthesteamflowrateforsuperheatormoistureinthesteamflow,usethefollowingrelationships.

MOISTURE CORRECTION:

Ww = flowrateof"wet"steamatX%quality,lb/hrX = steamquality,inpercent,(X=1-%moisture)

W = Ww X

SUPERHEAT CORRECTION:

Wsh = flowrateofsuperheatedsteamatS°Fsuperheat,lb/hrS = degreesofsuperheat,°F

= superheatedsteamtemperature-saturationtemperatureatthegivenpressure.

W = Wsh(1+0.00065S)

Example:Selectaregulatorusingtheflowcoefficientformula.

Weneedavalvethatwillprovidetightshutoffandreasonablylonglifetosupply3800lb/hrof75psigsteamfromaninitialpressureof150psig.We'llusethevalvesizingformulastodeterminetherequiredsize.

Step 1.Convertthegivengagepressurestoabsolutepressures:

P1=150+14.7=164.7psiaorabout165psiaP2=75+14.7=89.7psiaorabout90psia

Step 2.Choosetherightformulaonthebasisoftheratioof P2 to P1,andcalculatetherequiredvalveflowcoefficient.

P2 = 90 = 0.54 P1 165

Sincetheratioisgreaterthan0.5,wechoosethefirstformula,andsubstitutethepropervaluesasfollows:

WCv = 2.1∆P(P1+P2)

3800 Cv = 2.1(165-90)(165+90) 3800 Cv = 2.1 (75) (255) 3800 Cv = 290

Cv = 13.1

Alternatemethodsforfindingthevalveflowcoefficientareavailable.Somemanufacturerspublishtables,liketheoneshownonpg.17,tohelpyoucalculatetheCvrequiredwithoutusingtheformulas.

TofindtheCvrequiredforapressureregulatordesignedtoreduce150psigsaturatedsteamto75psigataflowrateof3800lb/hrusingFigure 24:

Enterthetableatthetopofthecolumnfor150psiginletpressure,andrundowntotherowcorrespondingto75psigoutletpressure.

Inthiscase,75isnotlisted,butweknowthatitlieshalfwaybetween291and283inthetable,soweinterpolate,or"splitthedifference".

291–283= 8 and 8÷2=4

283+4 =287 or, 291–4=287

Therefore,287isthe"basicsteamnumber"correspondingtoasteampressurereductionof150to75psig.

CalculatetheCvbydividingtheflowratebythebasicsteamnumber.

steamflowrequired Cv = basicsteamnumber

3800 = 287

= 13.2

(Notethesimilaritybetweentheflowcoefficientformulaandtheformulaweusedwiththebasicsteamnumber.Ineffect,theBasicSteamTableallowsustocalculatethedenominatoroftheCvformulawithoutextractingsquareroots.)

Step 3. AfterusingeithermethodtocalculatetherequiredCv,wenowturntothecatalogtofindsomesuitablechoices.Manufacturersprovidesummarysheetsliketheonein Figure 25,tohelpinselectingvalves.Usingthesamplepage,weeliminateallvalvesthatarenotmadeforsteamservicesuchasthemodel758.Next,weeliminateallvalvesthataretoosmallforthisapplicationasindicatedbytheirCv rangesuchasthemodel752.Finally,wenotethatthemodels760and765arebackpressureorreliefvalves,soweare leftwiththemodel710and720valvesaspossibilitiesfor thisapplication.

17

For Illustration OnlyFigure 25

Regulator Summary Chart

TABLE 1 SELECTION CHART

Figure 24

OUTLET PRESSURE

PSIG

INLET PRESSURE PSIG

5 10 15 20 25 30 40 50 60 70 80 90 100 125 150 175 200 225 2502 21 38 52 63 73 82 100 118 136 154 172 191 209 254 300 345 391 436 4825 29 46 57 73 82 100 118 136 154 172 191 209 254 300 345 391 436 482

10 35 50 65 77 100 118 136 154 172 191 209 254 300 345 391 436 48215 37 55 69 96 118 136 154 172 191 209 254 300 345 391 436 48220 40 58 88 110 136 154 172 191 209 254 300 345 391 436 48225 43 77 107 130 154 172 191 209 254 300 345 391 436 48230 67 96 126 150 172 191 209 254 300 345 391 436 48240 74 107 125 162 186 209 254 300 345 391 436 48250 79 115 146 171 197 254 300 345 391 436 48260 82 122 154 182 247 300 345 391 436 48270 90 130 163 230 291 345 391 436 48280 95 130 214 283 345 391 436 48290 96 193 264 332 391 436 482

100 168 245 315 382 436 482115 109 213 289 357 423 482

125 174 267 340 410 472

150 197 266 383 434

BASIC STEAM TABLE (POUNDS PER HOUR)

NOTE: MultiplybasicsteamnumberfromthistablewiththeselectedvalveCvnumbertogetthemaximumpoundsperhourofsteamcapacityofthetypeandsizevalveselected.

18

For Illustration OnlyFigure 26

Model 710 and 715

MODEL 710 and 715PRESSURE REDUCING VALVESfor steam, water, air or oil systemsValve is single seated, unbalanced,tight closing, for dead-end service.

APPLICATION:Hoffman Model 710 and 715 Pressure Reducing Valves aresingle seated, tight closing, recommended for dead-end orcontinuous service. These valves are used for reducing steam,water, or other fluid pressure on applications such as:• Unit Heaters • Laundry Equipment• Pressing Machines • Small Heating Systems

CONSTRUCTION:The Model 710 valves are supplied with bronze body in sizes 1/2" to 1-1/2". The Model 715 2" valves have a high tensile iron body. Standard trim is replaceable PTFE disc and Stainless steel seat on all size valves. The diaphragm is neoprene with a nylon insert. The spring is selected in accordance with the control pressure specified and is made of cadmium plated steel.

OPERATION:The Model 710 and 715 Pressure Reducing Valve is singleseated, with inlet pressure entering under the seating area.The valve is diaphragm actuated, spring loaded to normallyopen the valve. The downstream or control pressure is admitted to the diaphragm through a 1/4" or 3/8" hole at the top of the diaphragm case, which is connected by pipe, not less than 10 pipe diameters downstream from the valve. When the spring is properly adjusted this upward force plus the inlet force under disc area will equal the downward force of the diaphragm when the desired downstream pressure is reached. (Note: this unbalanced valve will deviate. from the set pressure if inlet pressure changes either up or down.)

RECOMMENDATIONS:The Model 710 and 715 Pressure Reducing Valves are suitable for initial pressures up to 250 psi. Control pressures from 5 to 125 psi can be supplied with various diaphragms and springs.

VALVE MATERIAL LISTBody 1/2" to 1-1/2" ................................................................ BronzeBody 2" ............................................................................. Cast IronDiaphragm Case ............................................................. Cast IronDiaphragm ........................................... Neoprene-Nylon InsertedSpring ....................................................... Steel-Cadmium PlatedTrim ....................................... Stainless Steel Seat, PTFE Disc

CAPACITY CHARTSCapacity tables and formulas can be found on pages 22 and 23.

MAXIMUM PRESSURES & TEMPERATURESBronze–Screwed .................................... 250 PSI @ 406°FCast Iron–Screwed ................................. 250 PSI @406°FCast lron–125 lbs. Flanged ................. 125 PSI @ 353°FCast lron–250 lbs. Flanged .................... 250 PSI @406FABOVE ARE NON-SHOCK RATINGS

Cv Factors

Size 1/2" 3/4" 1" 1-1/4" 1-1/2" 2"

Cv 2.0 3.0 6.0 10 14 20

NOTE: The models 710 and 715 are unbalanced single seated valves. Therefore, any fluctuation of the supply pressure (inlet pressure) will be reflected in the control pressure (outlet pressure). The examplebelow shows the changes that occur when the supply of pressure changes. Example: A 1" valve purchased for inlet of 100 psi and control of 8 to 14 psi will control between 5 and 11 psi if used on inlet pressure of 50 psi. The same valve will control between 11 and 16 psi if used on inlet of 150 psi.

Maximum High Pressure: 250 psigReduced Pressure Range: 5-125 psigSizes: Model 710 - 1/2" - 1-1/2" Model 715 - 2" only

VALVE SIZE

Initial Pressure

PSIG

1-1/2"Reduced Pressure

Range PSIGCase Size

InchesSpring

No.

101-200

18-28 8" 227-36 6" 335-54 6" 244-82 6" 1

76-116 5" 296-125 5" 1

19

For Illustration OnlyFigure 27

Model 720

MODEL 720PRESSURE REDUCING VALVESfor steam or air systems

Single balanced seat, internal pilot,dead-end or continuous service.

APPLICATION:The model 720 pressure reducing valves are balanced, singleseated, tight closing, recommended for dead-end or continuous service. They are used for the reduction of air or steam pressure in industrial plants, institutions, and other public buildings on applications such as:

• Hospital sterilizing equipment• Tire vulcanizing equipment• Hot water heaters with regulators• Laundry equipment

CONSTRUCTION:The Model 720 Pressure Reducing Valves are supplied withbronze body in sizes 3/4" to 1-1/2" with screwed ends. The 2"screwed and the 2" and 6" size flanged valves are supplied with high tensile iron body. Bronze and iron body valves are rated up to 250 psig initial pressure at 406°F. Standard trim is bronze with stainless steel trim also available. The diaphragm is neoprene with a nylon insert. The spring is cadmium plated steel.

OPERATION:The Model 720 Pressure Reducing Valves are actuated by adiaphragm and the loading is a spring, which is adjustablewithin the reduced pressure ranges shown on the oppositepage. Downstream or control pressure is admitted to thediaphragm housing through a 3/4" or 3/8" hole at the top of the diaphragm case connected by pipe to the downstream side not less than 10 pipe diameters from the valve. The loading tends to open the valve against the closing force of the diaphragm. When the loading is properly adjusted it counterbalances the pressure on the diaphragm, with the main valve in position to maintain the desired controlled pressure.

RECOMMENDATIONS:For initial pressures of 50 psig or more use stainless steel trim. For pressure drops of 100 psig or more use two stagereduction.

VALVE MATERIAL LISTBody 3/4" to 1-1/2" .............................................. Bronze-Scre~edBody 2" ........................................................... Cast Iron ScrewedBody 2" to 6" ................................................... Cast Iron FlangedDiaphragm Case 3/4" to 1-1/2" .................................... BronzeDiaphragm Case 2" to 6" .............................................. Cast IronDiaphragm ....................................... Neoprene-Nylon InsertedValve Trim ............................ Bronze (Stainless Steel-if required)Spring ..................................................... Steel-Cadmium Plated

Valve Size In. Reduced Pressure Range PSIG

Case Size

InchesSpring

No.

3/4" – 1-1/2" 1-6 6" 53/4" – 1-1/2" 5-20 6" 33/4" – 1-1/2" 15-45 6" 23/4" – 1-1/2" 40-70 6" 13/4" – 1-1/2" 60-125 5" 1

Cv Factors

Size 3/4" 1" 1-1/4" 1-1/2" 2"

Cv 4.5 9.0 13 19 27

Size 2-1/2" 3" 4" 5" 6"

Cv 42 56 70 90 115

Maximum High Pressure: 250 psigReduced Pressure Range: 1-125 psigSizes: 3/4" - 6"

20

Step 4. Turningtothedatapagesforthesevalvemodels,weseethatthemodel710comesinsizesof1/2"through2"withcorrespondingCvratingsof2.0through20.The11/2"valve,withaCvof14lookslikeasuitablechoice.Similarly,themodel72011/4"withaCvof13orthe11/2"withaCv of19mightbesuitable.WecanevaluatethecapacityofeachofthesechoicesbysubstitutingtheactualvalveCvintheexpressionweusedearlier. steamflow basicnumber

Therefore,steamflow=Cvxbasicnumber,andactualsteamflowforthe17101-1/2"valveis14x287,or4018lb/hr.

Wecansummarizetheselectionprocesssofarinatable likethis:

Thiskindofcapacityevaluationcanbevaluableinavoidingtheoversizingthatissooftenthesourceofperformanceproblems.Inourcase,wefounda1-1/4"model720valvethathadaCvofonly13;notquitegreatenoughtomeettherequirementof13.1thatwehadcalculated.Bydeterminingtheactualflowratewecanexpectfromthatvalve,weareinapositiontoevaluateifthatvalvewillbe"closeenough".Forexample,wecalculatethatthemodel720willhaveacapacityof13x287=3731lb/hr.Comparedtothedesignflowrateof3800,wecanseethatthemodel720iswithinabout1.8%oftherequirement.Nowwecanaskifasafetyfactorhasbeenincludedintheoriginalfigureof3800lb/hr.Ifasignificantsafetyfactorhasbeenincludedinthecalculationoftherequiredflowrate,thenwemaychoosethesmaller,lesscostlyvalve.Butifthe3800lb/hrflowrateistrulyrequired,thenwecanconfidentlychoosethelargercapacityvalve.

Agoodruleofthumbfordeterminingifoneofthesevalvesisproperlysizedistocomparethedesignflowratetothetabulatedorcalculatedcapacityofthevalve.Thedesigncapacityshouldbebetween65%and75%ofthetabulatedcapacity.Thisallowssomeexcesscapacityincasethevalveisrequiredtosupplyunusuallyheavydemands,whileinsuringthatthevalvewillbecomfortablywideopenmostofthetime,thusavoidingthehighvelocity,noise,andpoorcontrolthat

resultsfromoversizedvalves.Followingthisruleofthumb,a1-1/2"model720valvethathasaCvvalueof19hasacapacityof5453lb/hr.Ourdesignflowrateis70%ofthisvalvestabulatedcapacity.Manymanufacturerssuggestevenbroaderguidelines.Theysuggestsimplythatthevalvebeat50%ormoreofitstabulatedcapacitywhenit'spassingthedesignflowrate.

Anotherfactorcouldbeevaluatedinchoosingbetweenthemodel710and720.Althoughtheyarebothdescribedassingleseatedvalvessuitablefortightshutoff,andbothareabletooperateintherequiredupstreamandreducedpressureranges,oneofthemisdescribedasa"balancedvalve"whiletheotherisnot.Theunbalancedvalvewillallowvariationsindownstreampressureshouldtheupstreampressurechange.Thiscouldbeanimportantreasontochoosethebalancedvalvedesign.Ontheother hand,iftheapplicationwillnothavemuchvariationinpressureupstreamoftheregulator,orifsmallvariationsindownstreampressurewillnotcauseaproblem,thentheunbalancedvalvemightbeanacceptablechoice,particularlyifithasapriceadvantage.

Step 5.Finally,wemustchoosetheactuatorforthevalve.Assumingwe'vechosenthemodel710,weturntothedatasheetforthatvalve,andselectthe6"diaphragmcaseandthe#1springbasedontheinitialandreducedpressuresintheapplication.

Example:Selectaregulatorforthepreviousapplicationusingthemanufacturer'scapacitytables.

Whenthevalvemanufacturerprovidescapacitytablesforhisvalves,thesizingandselectionprocessismuchfasterandlesscomplicated.Capacitytablesarenotonlyeasiertouse,buttheyalsoprovidemoreaccuratesizingthantheCvcalculationsincethetablesarebasedonactualtestsofasampleofvalvesunderactualupstreamanddownstreampressureconditions,andtheydon'tassumethevalveiswideopenastheCvdefinitiondoes.Undersomeconditionsofdifferentialpressure,themainvalveinanexternallypilotedregulatormayoperateonlypartlyopen.Thecapacitytablesforthoseregulatorsautomaticallytakethisintoaccount.

Supposewehavechosentouseanexternalpilotoperatedpressureregulatorinthesameapplicationwejustdescribed.ThreetablesareprovidedinFigure 28forthisfamilyofvalvestodescribethedifferentcapacitiesavailablefromthereducedport,normalport,andfullporttrim.Thesechoicesarisebecauseeachmainvalvebodycancomeequippedwith

Design Capacity

Inlet Press.

Outlet Press.

Basic Number

Cv Req'd

Possible Valves

Actual Cv

Actual Capacity

3800 150 75 287 13.1 1.50"/710 14 4018

3800 150 75 287 13.1 1.25"/720 13 3731

3800 150 75 287 13.1 1.50"/720 19 5453

Cv=

21

valvetrim,orseatandplugsets,ofdifferentsizestotailorthevalvecapacity.Thiswouldbevaluableforexample,ifavalveisrequiredforalimitedcapacitynow,butfutureplanscallforanincreaseincapacity.Withoutachoiceoftrim,wewouldhavetochoosebetweenasmallercapacityvalvethat'snotoversizednow,butthatwillbecomeundersizedinthenearfuture,oravalvethatwillbeabletohandlethefuturerequirement,butthatwillbeoversizeduntilthatlevelofdemandisreached.Byhavingsomechoicesoftrimavailable,wecaninstallalargersizevalvebodywithreducedtrimnow,thensimplyincreasetheregulatorcapacitylaterbysubstitutingalargercapacitytrimwithoutchangingthevalvebody.

Iffuturechangesincapacityarenotimportantnow,startwiththetablebasedonfullporttrim.Thiswillgivethesmallestvalvebodysize,andmosteconomicalchoice.Findthesteaminletpressurealongtheleftside,150psig,andtheoutletpressureinthenextcolumn,75psig.Followalongtherow,andnoticethatthecapacityfigures,inpoundsperhour,increaseasthevalvesizesincrease.Sinceweneedacapacityof3800lb/hr,wewouldchoosethe1-1/4"mainvalvewithacapacityof4900lb/hr.Thatsamevalvebodysizewithanormalportwouldhaveacapacityof4000lb/hr,andthereducedportwouldgiveonly2760lb/hr.Onceagain,wecanapplythe50%ruleofthumbtogetaproperlysizedvalve.

Forthefullporttrim: 3800 4900

= 0.77

Forthenormalporttrim: 3800 4000

= 0.95

Inthiscase,thenormaltrimwouldnotallowmuchextracapacity,whilethefullporttrimwouldfittherequirementverywell.

22

HOFFMANSPECIALTY®

Series2000PressureandTemperatureRegulators

SteamCapacityTable

(inpoundsperhour, assuming saturatedsteamatvalveinlet)

ForTrainingPurposesOnly

Figure 28 Full, Normal and Reduced Trim Capacity Tables

Reduced Port Normal Port

Full Port

23

Theexternalpilotvalverequiredforthisapplicationisselectedfirstbydeterminingiflowpressureairisavailabletooperateapneumaticpilot.Ifitis,thenwemustchoosebetweenthepneumaticandthespringoperatedpilots.Thepneumaticpilot'ssetpointiseasilychangedcomparedtothespringoperatedpilot.Ontheotherhand,ifthesetpointwillnotbechangedofteninthisapplication,thenthesimplerspringpilotisprobablyabetterchoice.Thespringpilotisselectedonthebasisofdownstreampressure.Ifapneumaticpilotisthechoice,thentheselectionmusttakeintoaccountthecontrolairpressureaswellasthedesireddownstreamsteampressure.

Forexample,manyplantsuse20psigairintheirregulators.Butallofthat20psiisnotavailabletouseasapilotsetpointbecauseoffrictionandinertiainthepilot.Inthesinglediaphragmpilot,airpressureontopofthediaphragmisbalancedbysteampressurebelowit.Ittakesabout9psiairpressuretoovercomethespringforce,friction,andinertia,togetthepilotstarted.Inotherwords,thepilotvalvestaysclosedaswestarttoincreaseairpressureontopofthediaphragm.Onlyaftertheairpressurehasrisentoabout 9psigdoesthepilotstarttooperatetoopenthemainvalve.Afterthatpoint,eachadditionalonepsiofairpressurewillincreasethesetpointbyonepsi,uptoamaximumof:

20-9=11psi(approximately)

Formanylowpressureapplications,that'sgoodenough,butifhighersteampressuresarerequired,weneedadifferentpilotvalve.

The3:1and5:1pilotshavetwodiaphragmsinsteadofone.Pneumaticpressureisexertedontopofthelargerdiaphragm,whiledownstreamsteampressureisbelowthesmallerone.Thisgivesamechanicaladvantagetothepneumaticsignal,sinceapoundofpressureexertedoverthelargerarearesultsinagreaterforce.Thisgreaterforcemeansthatmoreofthe 20psiglowpressureairsignalcanbeusedasthesetpoint,sincelessofitmustbeusedtoovercomefrictionandinertia.Oncefrictionisovercome,anincreaseofpneumaticpressurebyonepsiresultsinasteampressureincreaseof3or5poundsdependingonthearearatio.Assumingapneumaticpressureof20psigtothepilot,the3:1pilotcancontrolsteampressuresinarangeuptoabout50psig,andthe 5:1pilotuptoabout90psig.

Thenewdesignpneumaticpilotshaveeithera1:1ora4:1arearatio.The4:1modelisshownin Figure 30.About9psiairpreloadingpressureisrequired,afterwhicheachadditionalpsiofairpressurewillincreasethedownstreamsteampressureby4psi,soa20psiairsignalwillallowamaximumofabout44psi.Ifhigherpressuresarerequired,simplyincreasethepneumaticpressuretothepilot.

Figure 29 Pneumatic Pilot for Higher Pressures

Figure 30 New Design Pneumatic Pilot

Figure 28 Full, Normal and Reduced Trim Capacity Tables

24

PRESSURE REGULATOR INSTALLATION

Properinstallationiseverybitasimportantasthedesignthoughtprocessthatprecedesitbecausethesystemwillnotworkasplannediftheinstallationisfaulty.Insurethatyoufollowallofthemanufacturer'sinstructionsthatcomewiththeregulator.Thefollowingideasapplyingeneraltoalargenumberof pressurereducingvalves.

Mostregulatorsmustbeinstalledinhorizontalpiperunsinordertoavoidexcessivestressonthevalvebracketanddiaphragmassembly.Valvesthathavealargediaphragmcaseandlongbracketareparticularlyvulnerabletostressthatcancausethevalvestemtobindiftheyareinstalledinaverticalpiperun.Somedirectsensingvalvesmightbeinstalledinaverticalpipebecauseoftheirsmallsize,butthepreferredorientationistoinstallitinahorizontalpipewiththevalvesteminthevertical,ornearverticalposition.Directsensingregulatorsareinstalledwiththediaphragmcaseabovethepipelineinordertokeeptheinternaldownstreampressuresensingpassagewayclearofdirtanddebris.Ontheotherhand,itispreferabletoinstallremotesensingregulatorswiththediaphragmcasebelowthepipelinesothatacondensatesealcanformintheremotesensinglinetoprotectthediaphragmfromhightemperaturesteam.Theremotesensinglineitselfshouldbetakenfromthetopofthemaintoavoidcloggingbysolidgritorrust.Inallcases,insurethatthevalveisinstalledwiththeflowarrow,whichiscastintothevalvebody,pointingintherightdirection.

Planforfutureinspectionandservice.Allowroomabovethevalvetoinspectandremovetheactuator,aswellasaccesstoseethevalvestemasitstrokesduringoperation.

Blowoutallpipingwithsteamorcompressedairbeforeinstallingthevalvetoremovemillscaleanddirt.Installasteamstrainerupstreamofthevalve,andequipitwithablowdownvalvetoperiodicallyremovesoliddebris,orevenasteamtrapinlargesizepipingtoremovecondensate.

Goodpipingpracticemustbeobservedthroughouttheinstallation.Forthreadedandunionvalves:

(a) Wirebrushandinspectallthreadstoinsuretheyarecleanandundamaged.

(b) Usepipesealantonmalethreadsonlytokeepitoutofthesystem.It'sbestnottocoatthefirsttwothreadsatall.

(c) Don'tovertightenthejoint,especiallyifyou'reusingPTFEtape.Overtighteningcanforcethepipetoofarintothevalvebody,warpinganddamagingit.Teflontapeissuchagoodlubricant,it'seasytoforcethepipetoofarinto thevalve.

(d) Alwaysuseawrenchontheflatsofthevalvebodytotightenthejoint,usingthevalvebrackettoturnthevalveontothepipecanbendthestem.Also,usethewrenchonthesideofthevalvethat'sbeingtightened.Ifyouuseitontheotherside,thevalvebodymaybedistorted.

Forflangedvalves:

(a) Cleanandinspectallflangefacesandgaskets.(b) Alignandsupportthepipeproperly.Don'tusethevalve

flangeboltstopullthepipesectionsintoalignment.(c) Usetherightgasketforthetypeofflange.Nevertrytojoin

raisedfaceflangestoplainfaceflanges.(d) Tightenallboltsevenlyusingastarpattern.

Acompletevalveinstallationincludesisolationvalves,abypass,steamtraps,andgagesasshownin Figure 31.

Figure 31 Pressure Reducing Station

Valveandpipesizingaretwodifferentprocesses.Thoughtheydependonthesamefactors,eachaimsfordifferentresults.Insizingpipe,wecontrolpressuredropandkeepsteamvelocitywithinlimitsbychoosinglargepipesizes.Weavoidvalveoversizingbymaintainingacomfortableratioofdesigntotabulatedflow.Thepointis:valvesshouldnotbechosenbypipesizeorviceversa.Ifvalidsizingmethodsareusedforpipingandthevalve,thevalvewillusuallybesmallerthantheupstreampipe,andthedownstreampipewillbelargerthantheupstreampipe.

Allpipingmustbesecurelysupported,andprovidedwithmeanstoallowforthermalexpansionandcondensatedrainage.Thermalinsulationisrequiredtocutdownonheatloss,andprotectplantoperatorsfromburns.

Inasmallplant,eachvalveinstallationmaybetailoredtotheapplication;butinlargerplantsit'sbettertomakeupstandardvalvemanifolddesignstoavoidconstantly"reinventingthewheel".

Isolationvalvesshouldbeinstalledinthesteammainonbothsidesoftheregulator.Thesevalvesshouldpresentlittleresistancetoflow,solinesizevalvessuitableforon/offservicesuchasgatevalvesshouldbeused.Ifaremotesensingfeedbacklineisrequired,itshouldhaveanisolationvalve

25

too,butasmallvalvesuitableforthrottlingservicesuchasaneedlevalveorglobevalvewouldbethebestchoiceheresinceregulatorperformancecansometimesbeimprovedbyrestrictingthepressurefeedbacksignal.

Allowstraightpiperunsupstreamanddownstreamoftheregulatortominimizenoiseandallowsteampressuretosteadyout.Specificpipelengthsintermsofminimumnumberofpipediametersareprovidedinthemanufacture'sinstallationinstructions.Usuallytheyrequireaminimumofthreetofivepipediametersofstraightpipedirectlyupstreamofthevalve,andasmanyas20pipediametersdownstream.Itisespeciallyimportanttoinstallthefeedbackpressuretapfarenoughdownstreamofanyfittingtopermitanaccurateandsteadypressuresignaltothevalve.Tenpipediametersfromthenearestfittingupstreamisusuallyrecommendedasaminimum.

Bypasspipingmustbeprovidedtoallowoperationofthesystemwhentheregulatorisoutofservice.Thebypasspipingandvalveshouldbethesamesizeastheregulator,andthebypassvalveshouldbesuitableforthrottling,forexample,aglobevalve.Innormaloperation,theisolationvalvesarefullyopenandthebypassisclosed.Whentheregulatorisoutofservice,theisolationvalvesareclosedandthebypasswillbecrackedopentoprovidedownstreampressure.

Pressuregagesshouldbeinstalledupstreamanddownstreamoftheregulatorfarenoughfromfittingssothattheywillregisteranaccuratepressurereading.Thegageshouldbeselectedsothatthenormalpressurereadingwillbeatabouthalfscale;andeachgageshouldbemountedona"pigtail"toprovideacondensatetraptoprotectthegagefromfullsteamtemperatureandtoisolateitfromshockandvibration.Thedownstreamgagemustbelocatedwhereitcanbeseenwhentheregulatorisadjusted,andwhenthebypassmustbethrottled.Insurethatitisnotlocatedwhereclosingthedownstreamisolationvalvewillisolateitfromthesystem.

Reliefvalvesarerequiredtoprotectdownstreamcomponentsiftheyarenotratedforfullupstreampressure.ThereliefvalveshouldbeASMEratedtohandlefullsteamflowthroughtheregulatorwithoutexcessivepressurebuilduponthedownstreamsideiftheregulatorshouldfailwideopen.Reliefvalvesareoftensettoopenatfivepsiovernormaldownstreampressure.

Finally,installsteamtrapstodrainthecondensatefromanysectionofpipingwhereitmightaccumulate.Ataminimum,

anyverticallegsofpipingupstreamanddownstreamoftheregulatorshouldbeequippedwithcondensatecollectionpockets,strainers,isolationvalves,traps,andcheckvalves.Undernormaloperation,thesetrapswillcollecttherelativelysmallamountsofcondensateformedasheatescapesfromthepipewalls,buttheymayhavetohandlemuchlargeramountsofcondensateonsystemwarmup.Insurethat thetrapsareproperlyselectedtohandlethecondensate loadexpected.

TroubleshootingCAUTION

(1) Alwaysusecareandcommonsensewhenworkingwithsteamsystems.Seriousburnsorscaldingcanresultfrom:failuretowearproperprotectivegearsuchasgloves,longsleeves,andeyeprotection,failuretorelievepressurebeforeopeningjointsinpipingorsteamcomponentsthathavebeenunderpressure,andfailuretoallowthesecomponentstocooldown.

(2) Pressurecanremaininapipingsystemeventhoughallvalvesare"closed"becauseisolationvalves,checkvalves,andbypassvalvesmayleak.Openallpressurerelieffittingsandstrainerblowdownvalvestorelievepressure.

(3) Whentakingapartaflangedconnection,alwaysmakeitleakbeforeremovingallofthebolts.Gasketsmayhavebeensealedbyheattobothflangefaces,allowingthejointtoholdpressureevenaftertheboltsareremoved.

(4) Alwayswarmupasteamsystemgradually,allowingplentyoftimeforcondensatetodrainandforallcomponentstoreachtheirnormaloperatingtemperature.Monitordrainagetoavoidthebuildupofcondensatethatcancausedamagingwaterhammer.

Dailychecksduringnormaloperation.

Youcanoftenidentifyasmallproblembeforeitgetslargeenoughtodisruptnormaloperation.Forexample:

1. Checkthepressureinheatexchangersandsteammainswhenthepressureregulatorshouldbeshut.Ifpressureisabovethesetpoint,theregulatororbypassmaybefailingtoclose,orleaking.

2. Raiseandlowerthepressuresetpointandobservethattheregulatorvalvestemmovessmoothly,andintherightdirection.Ifyoucan'tseethevalvestem,observethepressuregagesdownstreamofthevalve.

3. Regularlyanalyzetroublecalls,maintenancelogs,androutinegagereadingstoseeifthere'sapatternofexcessiveorinsufficientpressureinaparticularsectionoftheplant.Seeifthatpatternpointstoaspecificvalveorvalvegroupasthesourceoftheproblem.

26

Thetestsfordeterminingiftheregulatorisoversizedorundersizedcanbecarriedoutsimplybyusingthebypassorisolationvalves.Ifyoususpectthattheregulatorisundersized,testitbyopeningthebypassvalveoneoroneandahalfturns.Ifdownstreampressurecanbemaintainedatmaximumloadwiththebypasspartlyopen,theregulatorisundersized.Ifyoususpectthattheregulatorisoversized,partiallyclosetheinletisolationvalvetotheregulator.Ifthedownstreampressuresteadiesoutwiththeisolationvalvethrottled,thentheregulatorisoversized.

EXTERNALLY PILOTED REGULATORS Mostofthetroubleshootingtipsinthetablesaboveapplytoexternallypilotedvalvestoo.Themajordifferenceisthatwithanexternallypilotedregulator,eithertheexternalpilotorthemainvalvecouldbethesourceofaproblem.Here'saquickwaytofindoutwhichvalveismalfunctioning.Themainvalveisnormallyclosed,itneedsasignalfromthepilotbeforeitcanopen.Youcanchecktoseeifthemainvalveisleakingbyclosingtheinletisolationvalve,andcarefullydisconnectingthecoppertubingfromthepilotatthebottomofthemainvalvediaphragm.Observethepressuregagedownstreamoftheregulatororlistenforsteamflowasyouslowlyopentheupstreamisolationvalveagain.Iftheregulatorisleaking,therewillbeanincreaseindownstreampressure,orflownoiseinthemainvalve.Ifthemainvalveisnotleaking,youcancheckthepilotoperationbyslowlyincreasingthepressuresetpointtoobservethatsteamcanflowfromthecoppertubingtoloadthemainvalvediaphragm.Ifsteamwillnotflowthroughthepilotafterthesetpointisincreased, thenthepilotismalfunctioning.Itcanbeanalyzedlikeanyotherdiaphragmoperatedvalve.Often,thecauseoftheproblemissimplycloggingofthestrainerinthepilotvalve.Themostcommoncauseofproblemswithexternallypilotedregulatorsisthebleedorifice.Insurethatitisproperlyinstalled,and not clogged.

IN ALL OF THESE TESTS, BE CAREFUL TO AVOID INJURY BY USING HEAVY GLOVES AND OTHER

PROTECTIVE CLOTHING.

Whenreplacingthevalvetriminanexternallypilotedregulator,makesureyouprovideasourceofpressureunderneaththemainvalvediaphragmbeforeyouremovethenutthatholdsthevalveplugtothestem.Airpressurebelowthediaphragmwillcompressthespring,andallowyoutoremovetheplugwithoutdamagingthediaphragm.Therearestopsbuiltintothevalvebodytopreventexcessupwardmotionofthediaphragm.Aspannertoolwillberequiredtoremovethevalveseat.

Problem Cause Action

Valve won't close.

Scalebuilduponthevalveseatordisc. Inspect&clean.

Seatordiscisdamaged. Regrindorreplaceifpossible.

Internalfeedbackchannelclogged. Cleanorreplace.

Ruptureddiaphragm. Replacediaphragm.

P2 huntsValveisoversized. Testbythrottlinginlet

isolationvalve.Pressuredroplimitsexceeded. Checkpressuredrop.

P2 too high Bypassvalveopen Checkfordamagedbypassvalve.

Valveseatordiscdamaged. Inspectandreplace.

P2 too low

Valveseatorstrainerclogged. Clean.

Upstreamisolationvalvepartiallyclosed.

Inspectisolationvalve,insureit'sopen.

Regulatorisundersizedatheavyloads.

Testbycrackingopenthebypassvalve.

Troubleshooting Remotely Sensed Pressure Regulators

Problem Cause Action

Valve noisyLackofthrottlinginfeedbackline.

Slowlyclosefeedbackvalveuntilregulatoroperatesquietly.

Loosetrimorfittings. Adjustorreplace.

Valve won't close

Stuffingglandnuttootight. Adjustnutfingertight.Stemrustedorscaled. Clean.

Feedbacklineclogged.Insurelineisclear.Itshouldbeconnectedtothetopofthesteampipe.

Dirtorscaleonvalveseat. Clean.

Rupturedorleakingdiaphragm.

Inspectandreplace,ortightennutsarounddiaphragmcase.

P2 hunts

Insufficientthrottlinginfeedbackline.

Slowlyclosevalveinfeedbacklineuntilhuntingstops.

Bypassleaking. Insurebypassisclosedtight.

Regulatorisoversized. Testbythrottlingupstreamisolationvalve.

Downstreampressuretapistooclosetoafitting.

Insureminimumoftenpipediametersfromnearestupstreamfitting.

Downstreampipeistoosmall.

Replacedownstreampipewithlargersize.

P2 falls off at heavy load

Regulatorisundersized.Testbycrackingopenthebypassvalve.Replaceregulator.

Isolationvalvespartiallyclosed.

Insurebothvalvesfullyopen.

Troubleshooting Direct Acting Pressure Regulators

27

NOISE IN PRESSURE REGULATORS NoisepollutionisagrowingconcerntoplantoperatorsandregulatoryagenciessuchastheOccupationalSafetyandHealthAdministration,(OSHA).Noisefromaregulatorcouldbecausedbysimplemechanicalproblemssuchasloosefittingorbrokencomponentsthatvibrateinthesteamflow,orbytheaerodynamicnoisegeneratedwhensteamexpandstoalowerpressureathighvelocity.Steampressureregulatorsmaybethesourceofhazardousaerodynamicnoiseifthesteamflowrate,pressuredrop,andpressuredropratioarelargeenough.(PressuredropratioisdefinedasΔP) P1

Inmostcases,valveslessthan2"insizewillnothaveflowratesgreatenoughtobeofconcern,butlargervalvesmaydevelopahighpitchedscreamingsound,especiallyifthemanufacturer'sinstructionshavenotbeenfollowed.

Noiseismeasuredintermsof"decibels",dB,whichrelatethesoundpressurelevelatthepointofmeasurementtothelowestpressureleveldetectablebythehumanear."Hazardous"noiseexposureasdefinedbytheregulatoryagencies,mustbe avoided,butlowerlevelsofnoiseevenifnothazardous,maystillbeobjectionable.Thefollowingroughrulesofthumbmayhelpinestimatingifaproposedvalveinstallationislikelytodevelopnoiselevelsgreaterthan90db,thecurrentdefinitionofhazardousnoise.

MultiplyP1,inpsia,bythevalveCv.

Iftheresultislessthan500,hazardousaerodynamicnoiseisunlikely,althoughothersourcesofnoisee.g.,loosevalvetrim,arealwayspossible.

Iftheresultisbetween500and1,000,thenaerodynamicnoiseislikely,butitprobablywillnotbeabovethe"hazardous"definition.

Iftheresultisgreaterthan1,000youshouldanticipatehazardousaerodynamicnoise.

Manufacturerssometimesprovidecomputerprogramstohelpinregulatorsizingandselection.Thesecomputerprogramscanprovideanestimateofnoiselevels.

Somepracticesthatwillreduceaerodynamicnoiseinsteampressureregulatorsinclude:

Observethelimitsonpressuredropacrossasingleregulatorasadvisedbythemanufacturer.

Limitvelocitiesofsteamto10,000ft/minatthevalveinletand12,000ft/minattheoutletbysizingthepipinggenerouslyforthemaximumflowrateexpected.

Usetaperedpipeexpansionstoavoidabruptchangesinpipediameter,especiallyinthedownstreampiping.Insurethatcondensateisremovedbytheuseofproperlyinstalled steamtraps.

Allow20pipediametersofenlargedpipedownstreamoftheregulatorbeforethefirstturn.Uselongradiuselbows.

Useheavierschedulepipedownstreamoftheregulator.Aerodynamicnoiseisactuallygeneratedinthedownstreampiping.Heavierpipewallswillreducetheamountofnoisetransmittedtotheatmosphere.

Usespecialacousticinsulationonthedownstreampiping.Eachinchofacousticinsulationwillattenuatenoisebyabout9dB,uptoamaximumof25dB.Heavythermalinsulationcanalsoreducenoise,butislesseffective.Eachinchofheavythermalinsulationreducesnoiseabout4dB,uptoamaximumof14dB.

28

SOME TYPICAL APPLICATIONS FOR PRESSURE REGULATORS

Application:Convenientmanualcontrolofthesteamsystem.

Figure 32System operation:Closingthesteamsupplytothepilotvalveautomaticallyclosesthemainvalve.A1/4"gatevalveatthepilot,orlocatedasfaras50'awayinaloopof1/2"piping,willpermitremotemanualoperationofthesystem.

Application: Electricalcontrolofthesteamsystem.

Figure 33System operation: Asolenoidpilotcanbeopenedorclosedbyanytypeoftwopositioncontrolsuchasapressureswitch,aquastat,ortimer.

Application:Choiceofdownstreampressure.

Figure 34 System operation:Seteachpilottoadifferentpressurerequiredby,forexample,night/dayoperation.Controlsolenoidpilotswithatoggleswitchtoopenonewhileclosingtheothertoprovideaconvenientwaytochangedownstreampressure.

Application: Pressureregulatortomaintainconstantpressureinputtoalowpressureratedtemperatureregulator.

Figure 35System operation: Mainvalvemaintainstightshutoffuntiltemperatureregulatorcallsforsteamtomaintaintemperatureinstoragetank.

Application: Provideautomaticcontrolofroomtemperatureandunitheaterpressure

Figure 36System operation: Thespringpilotissettoprovidespecifiedsteampressuretotheunitheaters.Thesolenoidpilotopensonlywhentheroomthermostatcallsforheat,avoidingheatloss.Afterwarmup,aquastatsstartthefans.

Application:Automaticpreheatingandpressurizationofsteammains

Figure 37System operation:Thetimeropensthesmallerregulatorearlyenoughtowarm-upandpressurizethemainsbythestartoftheworkday.Thepressurestatclosesatoperatingpressure,closingthesolenoidtothewarmupregulator,andopeningthepilotstothelargerregulatorwhichiscapableofhandlingthesystemdemandforsteam.

29

VAPOR TENSION TEMPERATURE REGULATORSAtemperatureregulatorvariessteamflowtoaheatexchangertomaintainasetfluidtemperatureattheheatexchangeroutletinspiteofchangesintheinlettemperatureandflowrateofthefluidbeingheated.Thisresultsinavaryingsteampressureinsidetheheatexchanger,unlikethemoreconstantpressuremaintainedbyapressureregulator.Thetemperatureregulatorrequiresafeedbacksignalbasedupontheoutlettemperatureofthefluidbeingheated.Thereareseveralwaystoachievethistemperaturefeedbackdependinguponthetypeofregulator.Thetwobroadcategoriesoftemperatureregulatorsarevaportension,andexternallypilotedtemperatureregulators.

Inavaportensiontemperatureregulator,thetemperaturefeedbacksignalisgeneratedbymeansofathermalbulbmadeofsomemateriallikecopperthatconductsheatwell.

Figure 38 Vapor Tension Actuator

Thisbulbissurroundedbythefluidtobeheated.Thebulbispartlyfilled,undervacuum,withavolatilefluidselectedtohaveaboilingpointsomewherenearthedesiredfluidtemperature.Asfluidtemperaturerisestowardthesetpoint,thevolatilefluidboils,increasingvaporpressureinthethermalbulbandforcingsomeoftheliquidthroughacapillarytubetoabellowsmountedonthevalve.Asthebellowsexpandsduetothisincreasedpressure,itcompressesthetemperatureadjustingspringandforcesthevalvestemdowntoclosethevalve.Vaportensiontemperatureregulatorscanbeadjustedtoanytemperaturewithintherangedeterminedbythefluidusedinthebulb,howeveritisbesttochoosethetemperaturerangesothatthedesiredtemperaturesetpointisnearthemiddleoftherangeratherthanateitherextreme.Thetemperatureadjustmentwheelisraisedtoincreasethetemperaturesetting.Asthewheelisraised,thespringiscompressed,requiringmorevaporpressureinthebellows,andhighertemperatureinthefluidtoclosethevalve.

Figure 39 Vapor Tension Regulator

Thiskindofvalveis"directacting"becauseitclosestoreducethesteamflowtotheheatexchangerwhenthetemperatureofthefluidrises."Reverseacting"valveswhichopenonariseintemperaturearealsoavailablebuttheyareusedincoolingapplicationstoincreasetheflowofcoolantonariseintemperature.

EXTERNALLY PILOTED TEMPERATURE REGULATORSTheexternallypilotoperatedmainvalvedescribedinthepressureregulationsectioncanalsobeusedtoregulatetemperature.Temperatureregulatorsthatuseanexternalpilotcanuseeitheroftwomeanstosensethetemperatureofthefluidandgenerateafeedbacksignaltothemainvalve:aliquidexpansionprincipleasusedintheselfcontainedpilotorthesolidexpansiondeviceusedwiththepneumatictemperaturepilot.

IV TEMPERATURE REGULATION

30

Figure 40 Self Contained Temperature Pilot

SELF CONTAINED TEMPERATURE PILOTTheselfcontainedtemperaturepilotisinstalledmuchthesameasapressurecontrolpilotatthehighpressuresideofthemainvalvewithasteamconnectiontothemainvalvediaphragm.Itstemperaturesensingbulbisinserteddirectlyinthefluidtobeheated,orinatemperaturesensingwellinthetankorpipingwhereitwillsensethetrueaveragetemperatureoftheliquidinatank,ortheoutlettemperatureofaliquidleavingaheatexchanger.

Thebulbandcapillaryarefilledwithaliquidthatexpandsandcontractswithchangesintemperature,butunlikethevaportensiondevice,itwillnotboilintheapplicationtemperaturerange.Theparticularfluidusedtofillthepilotbulbwilldeterminethetemperaturerangeofthepilot.Asthefluidflowingoverthebulbheatsup,theliquidinsidethebulbexpandsandclosesthepilotvalveagainstthereturnspringatthebottomofthepilot.Thereisatemperatureadjustmentonthetemperaturepilotwhichisusedtosetthepilotclosingtemperature.Thereisalsoanovertemperaturespringtoabsorbexcessiveexpansionforcesinthecapillarywhichcouldbecausedbyexposingthebulbtotemperaturesaboveitsnormalrange.

Figure 41 Self Contained Temperature Pilot Operation

SELF CONTAINED TEMPERATURE PILOT OPERATIONThemainvalveisnormallyclosed,justasinthepressureregulatingapplication.Itisinstalledinthesteamlinetocontroltheflowofsteamtotheheatexchanger.Thetemperaturepilotwillopenasit'sbulbisexposedtolowtemperaturefluidfromtheheatexchangeroutlet.

Steamflowingthroughthepilotfromtheupstreamsideofthemainvalvewillpressurizethemainvalvediaphragm,openthemainvalve,andallowsteamtoflowtotheheatexchanger.

Assteamintheheatexchangerheatsthefluidflowingacrossthepilotbulb,theliquidinthebulbexpandsandclosesthepilot.Whenthedesiredfluidtemperaturesetpointisreachedandthepilotvalvecloses,thesteampressureunderthemainvalvediaphragmisallowedtobleedoffthroughthebleedorifice,andthemainvalveclosesjustasinpressureregulation.

31

PNEUMATIC TEMPERATURE PILOTThemainvalveandpneumaticpressurepilotdescribedinthesectiononpressureregulatorsareoftenusedtogethertoregulatetemperaturebyaddingapneumatictemperaturepilot.

Thepneumatictemperaturepilotrequiresasourceoflowpressureairand,usually,apneumaticpressurepilot.The"shopair"systemisgenerallyadequateaslongasit'sequippedwithanairpressureregulatorandfiltertoremovemostoftheoilandwaternormallyfoundinshopair.

Anairsupplyof18to36psigisconnectedtothereverseactionsupplyportofthepneumatictemperaturepilot,andthedirectactionsupplyportisleftopeninordertogenerateanincreasingairsignalpressureasfluidtemperaturedrops,ora"reverseacting"operation.Thedevicecouldalsobeusedtoprovide"directaction",anincreaseinairsignalpressureonariseoffluidtemperature,whichwouldbeusefulinotherapplicationsorvalveconstructions.

Theouterbrasstemperaturesensingrodexpandsandcontractsasthefluidsurroundingitheatsandcools.Movementofthe"Invar"rodattachedinsidethesensingrodchangesanorificesettinginthecontrolcreatingavariableairpressureatthesignalport.Invarisanalloythatdoesn'tchangelengthverymuchwithchangesintemperature.Thetemperatureissetbymovingtheadjustmentpistontoraiseorlowertheairpressureatthesignalport.Sincethisdevicehasnotemperaturereadout,itmustbesetbyreferringtoathermometerinthefluid.

Thevariableairpressuresignalfromthepneumatictemperaturepilotissenttothetopofthepneumaticpressurepilotwhereit,ineffect,changesthepilotsetpointtocontroltheflowofsteamtotheheatexchangerasdescribedearlier.

Figure 43 Pneumatic Temperature Control

Apressurefeedbackpipefromtheheatexchangertothesystempressuresensingportofthepneumaticpressurepilotopenstheregulatoronstartupandservesasapressureoverridetostoptheflowofsteamifpressureintheheatexchangershouldbuildtoohigh.Thistypeofmodulatingtemperatureregulatorprovidesveryaccurateandresponsivetemperaturecontrol;andispreferredwheneverarelativelylargevolumeofsteamisusedtoheatarelativelysmallvolumeoffluid,orwhendemandonthesystemislikelytobehighlyvariable,asinadomestichotwaterapplication.Thisisthecaseinthecommonshellandtubeor"instantaneous"heatexchanger.

Otherpneumatictemperaturepilots,whichuselessairvolume,andprovideanadjustablethrottlingrangetominimizetemperatureovershootarealsoavailable.Theyrequireasupplyofconditionedair,thatis,oilandwaterfree"controlair".

Sym. DescriptionA KnobB Dial PlateC Adjustment ScrewD Adjustment PistonE O-RingF Sensor Piston AssemblyG BodyH LocknutI OrificeJ Thermal Bulb AssemblyK Sensor Rod

Figure 42 PNT Pneumatic Temperature Pilot

32

Manyofthedecisionsinvolvedinselectingapressureregulatorapplyequallytotheselectionofatemperatureregulator,sowecanconcentratehereontheadditionalconsiderationsthatareuniquetotemperatureregulators.

DETERMINING THE VALVE PRESSURE DROPUnlikeapressureregulator,atemperatureregulatorwillhaveavaryingpressuredifferentialinnormaloperation.Pressureattheinlettoatemperatureregulatingvalveisdeterminedbyboilerpressure,orapressureregulatorlocatedupstream,andbythepressuredropthatoccursduetofrictionlossesasthesteamflowsthroughthepipeandfittingstothetemperatureregulator.Thisvalveinletpressuremustbeallocatedaspressuredropsacrosstheregulator,throughthesteaminletpipingtotheheatexchanger,intheheatexchangeritself,inthecondensatedrainagepiping,andacrossthesteamtrap.

Figure 44 Temperature Regulator Installation

Thepipingpressuredropsinmostinstallationsareminimizedbykeepingtheconnectingpipingshort,straight,andgenerouslysizedaccordingtothedesignflowrate,sowecanusuallyignorethepipingpressuredropsbetweenthetemperatureregulatorandheatexchanger,andtheheatexchangerandtrap.However,unusualinstallationsdoexistwherepipingpressurelossescanbecomesignificant.

Pressureattheregulatoroutletwillbedeterminedbytheheatexchangerandit'sheatload.Steampressureintheheatexchangerisdeterminedbytheheattransferloadimposeduponituptosomemaximumloadcalledthe"designload".Atdesignload,theheattransferrate,thesteamflowrate,andthesteampressureintheheatexchangerareatdesignvalues.Mostheatexchangersoperatemostofthetimeatsomeheatloadlessthanthedesignvalue.Therefore,thesteamflowandsteampressurewillbelessthandesignvalues.Heatexchangersareoftenequippedwithvacuum

breakersinstalledtoopenandintroduceairatatmosphericpressureintheeventthattheheatloaddropssolowastocreateavacuumintheheatexchanger.Vacuumisaproblembecauseitcanallowcondensatetocollect,resultingindamagetotheheatexchanger.

Thesearesomeoftheimportantfactorsinsizingandoperatingheatexchangers.That'swhywecan'tdeterminethevalvepressuredropwithoutconsideringtheheatexchangeroperationaswell.

Let'sassumewe'reusingatemperatureregulatortocontrolsteamflowintothetubebundleofastoragetankwaterheater.

Figure 45 Storage Tank and

Vapor Tension Temperature Regulator

Coldwaterentersthetank,isheatedasitflowsacrossthetubebundle,andleavesatdesigntemperature.Steaminsidethetubescondensesandtransfersheatthroughthetubewallstothewater.Inordertogetanideaofhowsteampressureinthetubesrisesandfallswithchangesinotherfactors,considerwhathappensastheincomingwaterchangesintemperatureandflowrate.

SELECTING AND SIZING A VAPOR TENSION TEMPERATURE REGULATOR

Tube side steam

Pressure

Outlet water

temperature

Temperature regulator

action

Water flow through tank increases Drops Drops Opens

Inlet water temperature increases

Rises Rises Closes

33

Inselectingatemperatureregulator,weusethepressuredropthatwillexistatthedesignconditionsofflowandtemperatureintheheatexchanger,eventhoughweknowthatthispressuredropwillnotalwaysexistinnormaloperation.

Themostcommonmistakeinselectingatemperatureregulatoristooversizethevalve.Forexample,avalvethat'schosentomatchanexistingpipesizewillusuallybetoolarge,aswillavalvethat'sbeenchosenafteralarge"safetyfactor"hasbeenaddedtothedesignsteamflowrate.Avalvethat'soversizedwithrespecttothedesignflowratewillobviouslyhavealowpressuredrop.Itwillbemoreexpensive,willallowgreatervariationsinfluidtemperature,andwillprobablywearoutfasterthanasmaller,properlysizedvalve.

Theinitialcostfortheoversizedvalveisgreatersimplybecausethelargervalvehashighermanufacturingandtransportationcosts.Theoversizedvalvewillbeabletopassthedesignflowrateusingonlyafractionofitsfullstrokebecausetheflowareaissolarge.Astheloaddecreasesfromthedesignpoint,thevalvewilltrytomodulate,butindoingso,itwillovercorrectsinceevenaverysmallmovementofthevalvestemislikelytoresultintoomuchchangeinsteamflow.Finally,ifthevalveoperatesforalongtimejustbarelyopen,thehighvelocityflowofsteamwillerodeand"wiredraw"thevalveseatorplugorboth.Onceavalveis"steamcut",itwillnotbeabletoshuttightwithoutregrindingorreplacementofthedamagedparts.Therefore,it'sbettertoselectavalvethatwillhaveasignificantpressuredropatthedesignflowrate.

Thefollowingguidelinesapplyinchoosingvalvepressuredrop:

(1) Iftheheatexchangerisgravitydrained:

(a) andP1islessthan15psig,choose∆Ptobe100%ofP1inpsig.

(b)andP1isgreaterthan15psig,choose∆Ptobe50%ofP1inpsia.

(2) Iftheheatexchangerisdrainedtoavacuum:(a)andP1islessthan2psig,choose∆Ptobe2psi.

(b)andP1isbetween2-15psig,choose∆PtobeequaltoP1 psig.

Figure 46illustratesthattherearelimitstotheamountofpressuredroprequiredforgoodvalveperformance.WhenP1andP2arethesame,thereis,ofcourse,noflowthroughthevalve.AsP2decreases,thedifferentialincreases,actingasadrivingforcetocauseflow.

Figure 46 Steam Flow Varies with Pressure Drop

In Figure 46,whenP2isalargefractionofP1atdesignflowthenthepressuredropissmall,thevalvecapacityislargerelativetodesignflow,andthesteamflowthroughthevalvecanbecalculatedbytheexpression:

W=CAP12-P2

2

where:

W =steamflowrate

C =aconstantdependingonthevalvedesign

A =crosssectionflowareathroughthevalve

P1 =upstreampressure,psia

P2 =downstreampressure,psia

Inthissituation,closingthevalve,orreducingthevalueofA,inordertoreducethesteamflowratewillalsocauseareduction in P2becauseoftheincreasedvelocityofsteamflowandbecausetheheatexchangerpressuredropswithdecreasingload.ThisreductioninP2willpartlyoffsetthedesiredeffectbyincreasing∆P,actingtoincreasetheflow.Theresultispoorcontroloverflow.

Inthesecondcase,thevalvepressuredropatthedesignflowrateissuchthatthesteamisexpandingasfastasitpossiblycanrightinthenarrowestpartofthevalve.ThishappensinsteamwhenP2isequaltothe"criticalvalue"of0.58P1.Forvaluesof∆Pinthisrange,theoffsettingeffectofincreasing∆Pisnegligiblesowegetbettercontroloverthesteamflow.Reducingtheflowareagivesanimmediatereductioninsteamflow,andthequalityofthevalve'sperformanceimproves.

Inthethirdcase,P2isverylowwithrespecttoP1,belowthecriticalvalue.Inthiscase,furtherreductionsofP2willhavenofurthereffectontheflowrate,becausethesteamisflowingatsonicvelocity.Forthisreason,valvecapacitytablesdonotlist

34

flowratesforP2lowerthanabout50%ofP1.SincereductionsofP2belowthecriticalvaluenolongerhelpimproveperformance,andbecauselargepressuredropsacrossavalvecanleadtorapiderosionandnoiseproblems,it'sagoodideatoavoidpressuredropsgreaterthanthecriticalvalue.Forallpracticalpurposes,weliketochooseapressuredropofabout50%ofP1fortemperatureregulatorapplications.

Acommonmistakeistobasetheheatexchangerdesignonfullsupplysteampressurewithouttakingtheregulatorpressuredroprequirementintoaccount.Inthiscase,chooseaminimumpressuredropforthevalveaccordingtothefollowingguidelines:

SupplyPressure PressureDrop 15psi 5psi 50psi 7.5psi 100psi 10psi over100psi 10%ofP1

Thiswillgiveareasonablebalancebetweentheneedfordesignpressureattheheatexchangerandtheneedforagoodpressuredropacrossthevalvesothatitwillperformwell.Inmostcases,afoulingallowanceisusedtoincreasetheheattransferareaoftheheatexchanger.Thisoffsetstheeffectoftarnishandscalebuildupontheheattransfersurfaceswhichwilleventuallydegradetheheatexchanger'sperformance.Itisoftenthecasethatanewheatexchangercanoperateatdesignflowrates,butatsubstantiallylowerthandesignpressuresduetothefoulingallowanceandconservativedesign

Iftheheatexchangerhasnotbeenselectedyet,thensomeconsiderationshouldbegiventothefactthatsystemefficiencycanbeimprovedbydesigningtheheatexchangerwithlowdesignpressure.

• Wherethefinaltemperatureoftheloadislessthan200°F,lowpressuresteamishotenoughtoprovideatemperaturedifferencetoheattheloadefficientlywithoutbeingsohotthatpoortemperaturecontrolwillresult.Hightemperaturesteamtrappedintheshellofaheatexchangerjustattheinstantthevalvecloses,willhavetotransferheatintothetubesidefluid,possiblyoverheatingthefluidandcausingwideswingsoftemperature.Thisisespeciallyimportantinheatexchangersthatheatasmallvolumeoffluidwitharelativelylargevolumeofsteamlikethetypicalshell andtubeheatexchanger.

• Lowpressuresteamcontainsmoreavailablelatentheatperpoundthanhigherpressuresteam,reducingthesteamflowraterequirement.

• Thetemperatureoflowerpressurecondensateleavingtheheatexchangerwillbelower,reducingtheneedforwastefulcondensatecoolingorflashinginaflashtank.

Determining the Steam Flow Rate RequiredAmajorfactorinsizingatemperatureregulatoriscalculationofthesteamflowrateitmusthandle.Thepurposeofthesteamsystemistotransferheatfromtheboilertotheload.Steamisthemediumthatcarriestheheat,mostofitaslatentheat.Thefollowingsamplecalculationsshowtherelationshipbetweenthesensibleheattransferdesiredintheprocessandthelatentheattransferthatisaccomplishedasthesteamcondenses.

Examples of sensible heat transfer:

Heatingaliquid,"A":

[Btu] galA 8.34lbs 60min BtuQ hr

= min

xgalwater

x hr

x lbA°F

x spgrA x ∆T°F

Heatingasolid,"B":

[Btu] lbB BtuQ hr

= hr

x lbB°F

x ∆T°F

Heatingagas,"C":

[Btu] cuftC lbC 60min BtuQ hr

= min

x cuftC

x hr

x lbC°F x ∆T°F

Example of latent heat transfer:

Steamcondensing:

[Btu] lbsteam BtuQ hr = hr x lbsteam

Rememberthat"sensibleheat"referstothekindofheattransferthatcanbesensedbymeansofathermometer.Ineachofthethreesamplecalculationsgroupedunderthesensibleheatcategory,somesubstanceisbeingheatedfromaninitialtemperaturetoafinaltemperature.Thedifferencebetweenthetwotemperaturesisthe"∆T"ineachexpression.Inthefirstcase,aliquid"A"flowingatsomanygallonsperminuteisbeingheated,inthesecondcase,asolid"B"atsomanypoundsperhour,andinthethirdcase,agas,C,atsomanycubicfeetperminute.Alltheothertermsrequiredtocompletethecalculation,suchasspecificheat,density,andsoonareincludedineachcasetoarriveattheheattransferrate,"Q",inBtu/hr.Alwaysusethemaximumexpectedtemperatureriseandmaterialflowinordertogetthegreatest

35

heattransferrequirementyoucouldreasonablyexpectintheprocess.Onceweknowthis"designheattransferrate",weassumethatalltheheatrequiredtogointotheprocesswillcomeoutofthesteam,andthatthesetwoquantitiesofheatareequal.Thisignoresanylossesofheatthatcouldoccur,butit'scloseenoughinpracticebecausetheheattransferequipmentisdesignedandinsulatedinordertokeeptheselossessmall.

Nowwecanturntothesecondpartofthesampleheattransfercalculations-the"latentheat"transfer.Rememberthatthisreferstothekindofheattransferthatoccurswithnochangeintemperature,suchasthelargeamountofheatthatbecomesavailablewhensteamcondensestoaliquidatconstanttemperature.TheamountofheatavailablefromthesteamisequaltotheweightofsteamflowingtimesthespecificenthalpyofevaporationaslistedintheSteamTables.Nowthatweknowtwoofthequantities,latentheatandtotalheatrequired,wecansolveforthesteamflowrateinpoundsperhour.Thiskindofthoughtprocessisfundamentaltoalargenumberofsteamheatingapplications.

ReturningtoourexampleinFigure 45.Supposeweneed20gpmofwaterheatedfrom40°Fto140°Fatdesignconditions,andthetubebundlehasbeenselectedforthatcapacitywith20psigsteamcondensinginthetubes.Thefollowingfourstepprocedurewillallowustofindtherighttemperatureregulator.

Step 1. Determinethesteamflowraterequired.Iftheheatexchanger,thetubebundleinthiscase,isbeingselectedatthesametimeasthevalve,thenthesteamflowrequirementwillbeavailablefromtheheattransfercalculationscompletedfortheheatexchanger.Iftheregulatorisbeingselectedforsomeexistingheatexchanger,thesteamflowrequiredmightbeobtainedbyreferringtothenameplateofthetubebundle.Ifnoneoftheseareavailable,youcanalwayscalculatethesteamflowratebydividingtherequiredheattransferratebytheenthalpyofevaporationofthesteamatthedesignpressure.

Sinceour"process"issimplyheatingwater,wecanusethefirstofthesensibleheatexamplestocalculatetheheattransferrequiredatdesignconditions.Waterhasaspecificgravityof1andaspecificheatof1.Valuesforthesefactorsareavailableforawiderangeofsubstancesincommonengineeringreferencesorinthevalvemanufacturer'sliterature.Forourproblem,wehave:

aflowrateof20gpm,

andatemperatureriseof140–40=100°F

so,

Q=20gpmx8.34lbs/galx60min/hrx1Btu/lb°Fx1x100°F

=1,000,800Btu/hrintothewater.

Accordingtothesteamtables,thesteamenthalpyofevaporation,hfg,at20psigis939Btu/lb.Therefore,thesteamflowraterequiredis: W = Q hfg = 1000800

939 = 1065lb/hrsteamat20psig

Step 2. Determinethevalvepressuredrop.Upstreampressuremaybeboilerpressureif theregulatorisinstallednearby.Iftheregulatorisfarawayfromtheboiler,thenpipingfrictionlossesmaybesignificant,particularlyinalowpressuresteamsystemwheretheselossesmaybelargecomparedtotheinitialpressure.Downstreampressure,aswe'venoted,dependsuponthecapacityandloadontheheatingunit.Inthisexample,theregulatorpressuredropissimplythesupplypressureof50psigminusthedesigntubebundlepressureof20psig,

or,∆P=30psi.Inthisexample,pressuredropworksouteasily.Noticethatitisabouthalfoftheupstreampressureintermsofpsia.

Step 3. Selectthevalvebodytypeandvalvesize.Inselectingthetemperatureregulator,considertheneedfortightshutoff,theexpenseanddurabilityofthematerialsusedinthevalve,theallowableresponsetimeandaccuracyofcontrolrequired.Alargenumberofdifferentvalvedesignsareavailableforuseinthistypeofregulator.Eachhasacapacitytableforeaseinsizingthevalve.Inthisapplication,weneedtightshutoff,andgoodaccuracyofsteamcontrol.Therefore,we'llselectaninternallypiloted,balanced,singleseatvalve.Themanufacturerhasgiventhisdesigntheidentifyingbodycode03.Aportionofthetableforthisvalveisprovidedas anexample.

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For Illustration Only

Figure 47 Hoffman Specialty Body Code 03 Temperature

Regulating Valve

Thisvalvecomesinsizes3/4"through4"aslistedacrossthetopofthetable.Valveinletandoutletpressuresarelistedalongtheleftside,andthebodyofthetablecontainsthesteamflowrateinpoundsperhour.Inourexample,thepressureattheinlettothevalveis50psig,andthedesignpressureinthetubebundleis20psigsowecanmoverightalongthatlineuntilwefindavalvecapacitythatmeetsorslightlyexceedstheflowweneed,inthiscase1065lb/hr.A1¼"valveundertheseconditionsofinletandoutletpressurewillpass1260lb/hr.Obviously,thevalveislargeenoughforthisapplication,butisittoolarge? Wecanapplythe50%ruleofthumbtoanswerthequestion.

designflowrate shouldbe50%orbetter maximumflowrate 1065 1260 = 0.84

Thistellsusthatthe1¼"valvewillhavetobecomfortablywideopeninordertohandlethe1065lb/hrflowourapplicationrequiresatdesignconditions.Thiswillhelpavoidwear,andwillleadtogoodcontroloverthesteamflowastheloaddropsawayfromdesignconditions.Ontheotherhand,itwillhavesomeadditionalcapacityavailableincaseofunusuallyhighdemand,soitlookslikeagoodchoice.

Step 4.Determineifunusualconditionsmustbesatisfied.Inthisexample,theregulatorbulbwillbeimmersedinwater,sostandardbulbmaterialslikecoppercanbeused.Manyindustrialprocessesrequirebulbmaterialsthatwillwithstandcontactwithotherchemicals.Alwaysconsultwiththemanufacturertoinsurethatthebulbmaterialwillbecompatiblewiththefluidtobeheatedorplantousean"immersionwell"tocontainthebulbwithoutdirectcontactwiththefluid.Thewellisagoodideaevenifthebulbandfluidarecompatiblesincetheuseofthewellallowsremovalofthebulbforservicewithoutdrainingthesystem.Otherunusualconditionsthatmayhavetobeaddressedmightincludehighambienttemperatures,longerthanusualcapillarylengths,orunusualmountingsforthethermalbulb.Someofthesetopicsarecoveredinthesectionontroubleshooting.

Example:Selectingexternallypilotedtemperatureregulators.Thesamemainvalvethatwasusedwithoneofthepressureregulatingpilotscanalsobeusedtoregulatetemperature.Therearetwokindsoftemperaturepilotvalves:selfcontained,andpneumatic.

Themainvalveisselectedforthesteamflowrateandappropriatepressuredropusingthecapacitytablesjustasdescribedinthepressureregulatorsection.

Figure 48 Regulator with Self Contained Pilot

Theselfcontainedpilotisselectedforthetemperaturerangeofthefluidtobeheated.It'sbestiftheapplicationtemperatureissomewherenearthemiddleoftherange,ratherthanateitherextreme.Iftheregulatorisdesignedtomaintaintemperatureonly,withnocontroloverthemaximumpressureintheheatexchanger,themainvalveandselfcontainedtemperaturepilotwillbesufficient.Pressurein

37

theheatexchangerwillbelimitedonlybytheinitialsupplypressure,thecapacityoftheheatexchangerandit'sload.

Figure 49 Temperature Pilot and Pressure Pilot

Addingaspringoperatedpressurepilotinserieswiththetemperaturepilotprovidesamorepositivewaytolimitpressureintheheatexchangerwhilecontrollingtemperature.Ifeitherthetemperatureorthepressuresetpointissatisfied,oneofthepilotswillclose,therebyclosingthemainvalve.

Theexternallypilotedmainvalvewithapneumatic

temperaturepilotandapneumaticpressurepilotwillaccuratelyandresponsivelycontrolbothtemperatureandpressurewithinthefollowingbroadpressurerangesastheairpressuresignalfromthepneumatictemperaturepilotrangesfrom9psigto30psig:

Figure 50 Main Valve with Pneumatic Pressure and Temperature

Pilots and Air Filter/Regulator

Regulator outlet steam

pressure required (psig)

Pilot valves required

0 to 21 Singlediaphragm(1:1arearatio)pneumaticpressurepilotandpneumatictemperature

0 to 84Doublediaphragm(4:1arearatio)pneumaticpressurepilotandpneumatictemperaturepilot.

38

Allofthegeneralideasregardingpressureregulatorinstallationapplytotemperatureregulatorsaswell.Thefollowingideasapplyspecificallytotemperatureregulators.

Thetemperatureregulatormustbeinstalledsothatitisaccessibleforinspectionandmaintenance.Itisparticularlyimportanttoavoidlocatingtheactuatororcapillarytooclosetosourcesofheatsuchassteamorhotwaterpiping,andtheareaaroundtheregulatorshouldbeadequatelyventilatedtopreventexcessiveambienttemperatures.Someofthethermalactuatorsusedinvaportensionregulatorstodayaredesignedtooperateinhotenvironments,a"crossambient"design.Theygenerallyhavemuchlargerthermalbulbsthatcanabsorbthevaporizationofthevolatilefluidcausedbyexternalsourcesofheatwithoutsendingasignaltoclose theregulator.

Alwaysprovideshutoffvalvesorunionstoallowforeasyremovaloftheregulatorwhenitmustbeserviced.Athrottlingbypassvalvecanprovidelimitedheatingcapabilitywhereservicemustbemaintainedwhiletheregulatorisoutofservice.Sincethebypassvalvecannotprovideautomatictemperatureregulation,it'simportanttoinsurethatadequatesafeguardssuchasreliefvalves,areinstalledtopreventoverheatingandoverpressure.Frequentinspectionandadjustmentofthebypassisrequireduntiltheregulatorisbackinservice.

Alwaysprovideastrainerupstreamoftheregulator,andsomeprovisionforremovingcondensatefromthepipingupstreamoftheregulator.Acollectionpocketandsteamtrapisthebestidea,or,forshortpiperuns,simplypitchingthepipebacktowardthemainwillkeepcondensateoutoftheregulator.Ifcondensateisnotremoved,operationoftheregulatorwillbesluggish,andwaterhammermaydamagethebellows.

Locationofthetemperaturesensingbulbisextremelyimportant.Itmustbelocatedwhereatrulyrepresentativetemperatureexists.Inatankwhereliquidisbeingheatedbysteamcondensinginatubebundle,thebulbshouldbeinaregionofaveragetemperature;somewhereabovethecenterofthetank,butneitherdirectlyoverthetubebundle,nordirectlyatthecoldwaterinlet.Inashellandtubeheatexchanger,thebulbmustbelocatedascloseaspossibletothehotfluidoutlet.

Avaportensionbulbmustbeinstalledhorizontallyorverticallywiththemountingflangeatthetop.Ifit'sinstalledverticallywiththeflangeatthebottom,thenvapor,rather

thanliquidwillbeforcedintothecapillarywhenthefluidstartstoboil.Thisvaporwillcondenseinthecapillary,andthevalvebellowswillnotreceivethefeedbacksignaltoclose,resultinginoverheating.Arelatedproblemmayexistinabulbinstalledhorizontally.Thebulbmountingflangemustbeturnedsothatthecurvedendofthecapillaryinsidethebulbissubmergedintheliquid.Inthisway,theliquid,ratherthanthevaporwillbeforcedthroughthecapillarytoclosethevalve.Theportionofthemountingflangemarked"TOP"mustbeatthe12:00o'clockpositiontoinsuretheendofthecapillaryisintheliquidratherthanthevapor.It'sbesttoinstallthebulbhorizontally,andpitcheddowntowardthebulbendtoinsurethatthecapillarywillalwaysbeflooded.

Ifthebulbhasbeeninstalledinaseparatepipingwell,thewellshouldbepackedwithheattransfergreasetoimproveheattransfertothebulb.Alltemperaturesensingbulbsmustbefullyinsertedintothefluidorwell.Ifanexistingimmersionwellistoosmalltoholdtheentirebulb,thewellmustbereplacedwithalargerone.Excesscapillarytubingbetweenthebulbandtheregulatorshouldbewoundintoafourinchorlargerdiametercoiltopreventkinking.

TROUBLESHOOTINGMosttemperatureregulatorsmustbesetbyreferringtoanaccuratethermometerinstalledwhereitcangiveagoodindicationofactualfluidtemperature.Thenewerselfcontainedtemperaturepilothasit'sowntemperaturesetting,butitshouldbecomparedtoathermometerafterinstallationasacheck.Ifnecessary,thepilotcanbeeasilyre-calibratedafterinstallationAfterchangingthetemperaturesetting,alwayswaitseveralminutesforthetemperaturetostabilizebeforemakingfurtheradjustments.Manyproblemsinregulatingtemperatureareduetoimpropersizingorselectionofthevalve.Anundersizedvalvemaybesatisfactoryatlowdemand,butwillallowtemperaturestodropoffathighdemand.Anoversizedvalvewillgiveerratictemperatures,withwideswingsaboveandbelowsetpoint.Inadditiontovalvesize,thevalve'spressureandpressuredroplimitationsmustbeobserved.Forexample,apressuredropgreaterthanthemaximumallowedmayholdthevalveopen,sincetheactuatormaynotbeabletoexertsufficientforcetoclosethevalve,orthevalveseatanddiscmaybewiredrawninashortperiodoftimeresultinginoverheating.

Vapor Tension RegulatorsAnotherpossiblesourceofproblemsinvaportensiontemperatureregulatorscanbethevalvestroke.Valvestrokeismeasuredonthevalvestembymarkingitatthestuffing

INSTALLATION OF TEMPERATURE REGULATORS

39

boxwhenthetemperatureatthesensingbulbisraisedabovethesetpoint.Makeanothermarkafterthebulbiscooledandthebellowsiscompletelycontracted.Thedistancebetweenthetwomarksmeasurestheamountofstemtravelbetweenthewideopenandclosedpositions.Theacceptablestrokeisdeterminedbythesizeofthevalve,rangingfrom1/4"forsmallvalvestoasmuchas7/16"forlargervalves.Themanufacturerliststhestrokevalueinthetechnicalliterature.Ifthestrokeistoolongortooshort,thevalvewillnot operateproperly.

Figure 51 Temperature Regulator Bracket

Theproperstrokecanbeestablishedattheconnectionbetweentheupperandlowervalvestems.Thelowerstemisattachedtothevalveplugandscrewsintotheupperstemjustabovethestuffingbox.Thetwoareheldinpositionbyalocknut.Toincreasethestroke,gripthelocknutwithonepliers,andthelowerstemwithanotherpliers.Holdthelowerstemsoitcannotturn,andloosenthelocknut.Griptheupperstem(abovethelocknut)andturnthelowerstemtotheright.Thiswillraisethestem,increasingthestroke.Tightenthelocknutafterthepropersettingisachieved.Todecreasethestroke,simplyturnthelowerstemtotheleftandthentightenthelocknut.

Figure 52 Valve Stem and Packing Gland

Buildupofscaleorrustaroundthevalvestemcansloworstopthevalveresponsetotemperaturechangesbecauseofincreasedfrictionasthestemtriestoslidethroughthepacking.Periodicallycleanoffthestematthepackingnutwithsomefineemeryclothtoavoidthisproblem.Insurethatthesplitpackingringsarereplacediftheydryoutorbecomehard.Addadropofoiltothepackingoccasionally,andinsurethattheglandnutisonlyfingertight.Iftheglandnutistootight,thestemwillnotslidethroughtheglandasitshouldresultinginpoortemperatureregulation.

Iftheactuatororcapillaryshoulddevelopaleak,theregulatorwillfailopen,sincethetemperatureadjustingwheelandspringwilltendtoopenthedirectactingvalveasbellowspressureisreduced.Thiswillresultinlossoftemperaturecontrol,andhightemperaturesintheheatedfluid.Ifthebulbinthefluidshoulddevelopaleak,theheatedfluidcouldleakintothevacuumfilledbulb,causingthebellowstoexpandandclosethedirectactingvalve.

Whenavaportensionactuatormustbereplaced,insurethatthereplacementunithastherighttemperaturerange.Alwaysclosethesteamsupplyvalves,allowthesystemtocooloffandpressuretodropto0psig.Drainthetankorpipelinetoalevelbelowthebulbinsertionpoint.Ifatemperaturewellisused,it'snotnecessarytodrainthesystem.Loosenthefourbulbbushingscrews,andbreakthebulbconnectionslowlytoinsurethatfluidwillnotleakfromthebushing.Removethefourbulbbushingscrewsandthebulbfromthetank.

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Turnthetemperatureadjustmentwheeltothelowestposition.Coolthebulbandbellowsatleast20°Fbelowthelowlimitofthetemperaturerangeindicatedonthenameplate.Thisisparticularlyimportantforactuatorswithtemperaturerangesatorbelow120°F.Thebellowsmustbecooleduntilitcanbecompressedbyhand.Failuretocoolthebellowswilldestroytheactuatorbecausethebellowswillexpandandrupturewhentherestrainingforceofthebellowshousingscrewsisremoved.Removethebellowshousingnutsandscrewsandliftthebellowsoffthebracket.Make

Figure 53 Vapor Tension Actuator and Bracket

sureyoukeepthebulbandbellowscool.Pushtheuppervalvestemplatedownuntilthevalveisshut.Markthestematthestuffingboxsothatthestrokecanbecheckedafterthenewactuatorisinstalled.Coolthenewactuatoratleast20°Fbelowit'sminimumtemperaturerating,andmakesureyoucancompressthebellowsbyhandbeforeremovingtheshippingclip.Removetheclip,andattachthenewactuatortothebracketusingallthescrewsandnutsprovided.Workquickly,andkeepthebulbcoolwhiletheactuatorisbeingattachedtothebracket.Checkthevalvestemstrokeasdescribedabove.

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Application:Tankandexternalheatexchangercombinedtomeetdemandfordomestichotwater.

Figure 54

System operation: Theheatexchangerhaslittlevolumetoaccomplishthemixingandprovidestoragetomeettheshort,heavydemandstypicalindomesticwatersystems.Thestoragetankactsasabuffertomeetpeakloadsandallowmixingtoaveragetemperaturesandpreventwidetemperatureswings.Thevaportensiontypetemperatureregulatorprovidesadequateresponsivenessandaccuracyoftemperaturecontrolinspiteofirregularandheavypeakdemandsforhotwater.

Notes:Thepumpwhichprovidescirculationfromtanktoheatexchangermustbecapableofhandlingoxygenrich,corrosivewater.Recirculationfromthesystemshouldflowacrossthethermalbulbinordertoprovideinstanthotwaterthroughoutthesystemandtubeheatexchanger.

Application: Temperatureregulatorcontrolsfluidtemperaturefromashellandtubeheatexchanger.

Figure 55

System operation:Steamflowintotheheatexchangeriscontrolledbyavaportensiontemperatureregulator.

Note: Vaportensionregulatorsarenotrecommendedforapplicationswhereashellandtubeheatexchangerwillbeexpectedtomeetsuddenchangesinflowrate,becausewideswingsintemperaturemayresult.

Application: Controloftemperatureinastoragetankwithsteampressurelimitation.

Figure 56

System operation: Thetemperatureregulatorcontrolssteamflowtomaintaintanktemperature.Thelargetankvolumeallowsthesystemtomeetheavydemands,andallowsmixingtoachieveasatisfactoryaveragetemperatureattheoutlet.Thespringpilotpreventsexcesspressureinthetubebundle.

Application:Steamtohotwaterconverterforhydronicheatingsystemusingspringandtemperaturepilots.

Figure 57

System operation:Thespringpilotcontrolssteampressuretotheconverteronstart-up.Asthethermalsensingbulbsensestheincreaseinwatertemperature,thetemperaturepilotsignalsthemainvalvetomodulateandmaintainaconstanttemperatureattheconverteroutlet.

SOME TYPICAL APPLICATIONS OF TEMPERATURE REGULATORS

42

Application: Heatexchangerhighlimitsafetycontrol.

Figure 58

System operation:Asolenoidpilotiswiredinserieswithanaquastatsetat5-10°Fabovethecontroltemperaturetolimittheoutlettemperature.

Notes:Thisautomaticsystemwouldbeusedwherethetemperaturelimitsforaprocessarecriticalorasasafetyoverrideforothersystemssuchasdomestichotwater.Aflowswitchmountedinthehotwateroutletfromtheheatexchangercanbeusedtoshutoffthesteamsupplyatperiodsofnoflow.

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HANDLING LARGE PRESSURE REDUCTIONS Manufacturersestablishalimitforpressuredropacrossasingleregulatorinordertominimizenoiseandprematurewear.Whenapressurereductiongreaterthanthatlimitisrequired,wemustusetwoormoreregulatorsinseries.

Thetotalpressuredropistakeninstages.Eachpilotvalveissettoachieveitsportionoftheoverallpressuredrop,andthepressuresensingpointmustbeatleast10pipediametersdownstreamofthenearestupstreamfitting.Eachmainvalvemustbeabletohandlethetotalrequiredsteamflowrateatthepressuredropitwillaccomplish.Eachregulatorisequippedwithabypassandisolationvalvestoallowmanualoperationwitheitherregulatoroutofservice,aswellasstrainers,andsteamtrapsasdescribedearlier.Thepipingwhichconnectsthetworegulatorsmustbelongenoughforsteadypressuretodevelopattheinlettothedownstreamvalve,andlargeenoughtoreducethesteamvelocityandavoidexcessivenoise.Twentypipediametersoftheexpandedpipesizeisusuallysufficient.

SUPERHEAT DOWNSTREAM OF A VALVEWheneverlargepressuredropsaretaken,thepossibilityofsuperheatingthesteamexists,becausesteamthatexpandswithoutdoinganyworkwillhavethesametotalenthalpyatthelowerpressurethatithadatthehigherpressure.Thisiscalleda"throttlingprocess".

Forexample,supposethatsteamat200psigexpandsto100psigacrossthefirstvalve,thento15psigacrossthesecond.We'llassumethatthesteamenteringthefirstvalveisatsaturationconditions,purevaporat200psig.Sincethere'snoworkbeingdoneinthevalve,thetotalenthalpyattheoutletof thefirstvalvewillbethesameasitwasattheinlet,butwhatwillbethetemperatureandspecificvolume?Saturatedsteamtablessaythatthetemperatureof100psigsteamshouldbe338°F,it'sspecificvolumeshouldbe 3.89ft3/lb,andit'senthalpyshouldbe1189Btu/lb. Comparethefiguretothesteamtabledataandyou'llnoticethattheactualenergycontentofthe100psigsteamis 10Btu/lbgreaterthanthesaturatedsteamtableswouldpredictforthatpressure.This"excess"energyshowsupasanincreaseinthesteamtemperatureabovesaturationtemperature.Inordertodeterminehowmuchthetemperaturehasincreased,weneedtolookatthetables forsuperheatedsteam,asmallportionofthesetablesis providedbelow.

Properties of Superheated Steam

Temperature

Pressure 350°F 354°F 360°F

100psigh=1196.8 h=1199.0 h=1202.7

v=3.996 v=4.022 v=4.060

hstandsforenthalpy,inBtu/lb vstandsforspecificvolumeinft3/lb

Theonlyconditionwherewefindapressureof100psigandanenthalpyof1199Btu/lbisatatemperatureof354°F.Atthiscondition,wesaythatthesteamissuperheatedto354°F,orthatithas354-338=16degreesofsuperheat.Noticethatthesteamtemperaturedidnotincreaseasitexpandedthroughthevalve,infactitdroppedfrom388°Fto354°F.The"superheating"referstothe16degreeincreaseabovethetemperaturewewouldexpectatsaturationconditions.

AquickwaytodeterminetheamountofsuperheatresultingfromagivenpressuredropistousetheMollierdiagramthatisoftenpartofthesteamtables.Onthisdiagram,horizontallinesrepresentconstantenthalpy,soallyouhavetodoisfindtheintersectionoftheinletpressureandthesaturationconditionlines,thenmovehorizontallytotherightuntilyoureachthelowerpressure.Theamountofsuperheatcanbereadrightoffthechartasshownwhere"a"representstheinletand"b"theoutletpressure.

V. SPECIAL PROBLEMS

Figure 59 Pressure Regulators in Series

Figure 60 Regulators in Series

Provide Two Stage Pressure Reduction

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Inthisexampleweassumedthatthesteamenteringthevalvewasatsaturation,butsteammayactuallycarrysomeliquidalongwithitasitleavestheboiler.Additionally,steamlosesheatthroughthepipewallsandinsulationasitflowstothevalve.Thisheatlosscondensessomeofthesteamandaddstothepercentageofliquidinthesteamflow.Therefore,thesteamenteringthefirstvalveprobablyisnotatpoint"a"onthediagram,butatpoint"c",carryingsomeamountofliquid.Ifthat'sthecase,thentheamountofsuperheatingachievedforthegivenpressuredropwillbemuchlessthanthetheoreticalmaximumshownatpoint"b".Ifthere'senoughliquidattheentrancetothefirstvalve,wemayhavemerelysaturatedsteamattheoutlet-nosuperheatatall,becausethe"excess"energywillmerelyevaporatetheliquidwaterdropletsratherthansuperheatthesteam.

Whatdifferencedoesthissuperheatingmake?Noticethatthespecificvolumeofthesteamatthesuperheatedcondition,4.022ft3/lb,isgreaterthan3.89wewouldexpectforsaturatedsteamatthesamepressure.Thisincreaseinvolume,ordecreaseindensity,meansthattheflowthroughthesecondvalvewillnotbequiteasgreataswewouldexpectifwehadsaturatedsteamandthesamepressuredrop.Sooneofthemostcommonconsequencesofsuperheatingisthecorrectionofflowratesforthesecondvalve.Aruleofthumballowsustocalculateacorrectiontothevalvecapacitytables,whicharealwaysbasedonflowofsaturatedsteamatthevalveinlet.

Correctionfactor=1+(0.00065xsuperheatdegrees)

Thiscorrectionfactorisappliedtotheflowrateinthevalvecapacitytableasfollows:

TabulatedflowrateFlowofsuperheatedsteam = correctionfactor

Inourexample,thecorrectionfactorwouldbeabout1.011,soitwouldn'tmakeagreatdealofdifferenceinsizingthesecondvalve.Infact,thesizeofthatcorrectionfactorrarelymakesmuchdifferencesofarasvalvecapacityisconcerned.Itmaybemoreimportanttocalculatethesuperheatsothatwecaninsureallofthecomponentsdownstreamofthevalveareratedtohandlethehigherthanexpectedtemperature,orthattheheatexchangerwillhaveadequatecapacitytodesuperheatthesteamandthencondenseitatan acceptablerate.

Handling Highly Variable LoadsWesawin Figure 23 thatallpressureregulatorshaveacharacteristiccapacitycurvewhichdescribesthewaythatdownstreampressurechangeswithincreasingloadthroughtheregulator.Noticethatthemajorpartofthecurveisnotquitehorizontal.Ithasadefinitedrooporslopeindicatingthatdownstreampressuredropsoffasloadincreases.Inthesectiononpressureregulators,wedefinedtheregulator'smaximumcapacityastheflowwhichcorrespondstoadropof10%fromsetpointpressure.Obviously,ifthesystemrequiressteamflowgreaterthanthat,we'llneedtoinstallalargercapacityregulator,orperhapsalargersizetrim.Ontheotherhand,we'vediscussedtheproblemsofcontrolquality,excessiveinitialcost,andearlyfailureduetosteamcuttingthatarisewhenaregulatorisoversizedwithrespecttoitsdesignflowrate.

Asystemthatrequiresawiderangeofflowratescannotbesuppliedbyasingleregulatorwithoutencounteringtheseproblems.Ifthevalveislargeenoughtohandlethemaximumexpectedflowratewithoutexcessivedropindownstreampressure,itwillbesolargethatcontrolquality

Figure 62 Pressure Regulators in Parallel

Figure 61 Mollier Diagram

45

andwearwillbeaproblematthelowflowrate.Whenasystemmustbeabletosupplyawiderangeofflowrates,thesolutionistouseregulatorsinparallel.

Eachregulatorhasthesameinletandoutletpressure,buttheregulatorcapacitieswillbeselectedsothatoneofthemwillbeabout30%ofthemaximumexpectedsystemflowrate,theother,about70%.Thelargerofthetworegulatorswillbesettomaintainapressureslightlylowerthanthesettingofthesmallervalve.

Asdemandforsteamflowincreasesfromzero,thesmallerregulatorisabletomeetthedemanduntilit'sfullyopen.Thelargerregulatorisclosedduringthisperiodbecauseitssetpointissatisfied.Asthesmallerregulatoropenswide,furtherincreasesindemandwillcausetheoutletpressuretodroopto

thepointwherethelargerregulatorwillbegintoopen.Fromthatpointon,theincreasingdemandcanbesatisfiedbythewideopensmallervalveplusthemodulatinglargervalveallthewaytomaximumcapacitywhenbothvalveswillbewideopen.

Theexactproportionusedtoestablishtheregulatorcapacitiesisnotsignificant,a1/3 -2/3proportionisoftenused.Similarly,thedifferencebetweenthesetpointsisnotcritical.Withinthelimitsofprecisionestablishedbythevalvemanufacturingprocess,theycanbesetascloseaspossibletominimizethechangeinpressureasthesecondregulatorbeginstoopen.

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