Post on 01-Mar-2020
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.
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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.
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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
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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
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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
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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.
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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
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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
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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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