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STEAMDRUMWATERLEVELMEASUREMENT

A. Introduction.Boilersteamdrumwaterlevelisoneofthemostimportantpowerplantparameterstobothmeasureandcontrol.Controloftheproperwaterlevelintheboileriscriticalforsafeoperationoftheboiler.Ifthelevelistoolow,boilertubeswillbedamagedbyoverheating.Ifthelevelistoohigh,steamseparatorswillnotfunctionproperly,temperaturecontrolwillbedifficult,andthesuperheatertubesandturbinecouldbedamagedbymoistureorwatertreatmentchemicalcarryover.Inaddition,poorlevelcontrolwillalsoadverselyaffectthedrumpressurecontrol.Theslidingoperatingpressureofmodern3drumHeatRecoverySteamGenerators,alongwithfrequentstartupandshutdown,hasaddedtothechallengeofselectingthepropermixofinstrumentsandmaintainingcorrectwaterlevelsunderallconditions.Althoughinstrumentsfordrumwaterlevelmeasurementhavebeenaroundforwelloverahundredyears,itisimportanttounderstandtheoperatingprinciples,installationrequirements,strengthsandweaknessesofeachtechnology.Toignoretheseconsiderationscanleadtomisapplication,increasedmaintenance,poorinstrumentperformance,andunsafeoperation.TheASMEBoilerandPressureVesselCodeSectionIestablishestherequirementsforsteamdrumwaterlevelmeasurementinfiredsteamdrums.Theprimaryfocusoftheserequirementsissafeboileroperation.Maintenance,performance,andotherspecificapplicationissuesarenotaddressed.Thereareadozenormoreleveltechnologiesthatcouldbeconsideredforthisapplication.Thepurposeofthispaperistoreview5oftheproventechnologiescurrentlyavailableforhighpressuresteamdrumwaterlevelmeasurement.ThesewillbecomparedwiththecurrentASMESection1requirements(2010codeedition),andalsoevaluatetheinstallation,performancecharacteristics,strengthsandweaknessesofeach.

B. ASMERequirements.TheASMEBoilerandPressureVesselCodeSectionIparagraphPG60lists

themajorityoftherequirementsforwaterlevelmeasurementinstruments.Theprimaryfocusissafeandreliabledrumlevelindicationatalltimes.Thisisanimportantconsiderationinordertounderstandwhychangestothecodearemadeslowlyanddeliberately.OtherrequirementspertainingtolevelinstrumentsarelistedinPG5(materialsofconstruction)andPG12(waterlevelindicatorsandconnectormaterial).TheASMEcoderequirementshavecontinuouslyevolvedovertheyears.Instrumentuse,incidentreportsandperformancehistoryareevaluated.Inaddition,InterpretationsarepublishedannuallytoanswerquestionssubmittedtotheASMEcodecommitteepertainingtovariouscodesections.TheresultsofthemostimportantInterpretationsarelaterwrittenintothevariouscodeparagraphstopermanentlyclarifysectionsthatmighthavebeenconfusingandmisapplied.Belowarelistedthemostimportantcoderequirements.Thisisnotacompletelisting.Iwilllaterlistanddiscussspecificrequirementsandinterpretationsapplicabletoparticularinstrumenttypes.

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TheprimaryrequirementsinASMEPG60(2009)areasfollows:1. (PG60.1)Allboilershavingafixedwaterlevel(steamandwaterinterface)shallhaveatleast

onegageglass(atransparentdevicethatpermitsvisualdeterminationofthewaterlevel).2. (PG60.1.1)Boilershavingamaximumallowableworkingpressureexceeding400psi(3MPa)

shallhavetwogageglasses.Insteadofoneofthetworequiredgageglasses,twoindependentremotewaterlevelindicators(twodiscretesystemsthatcontinuouslymeasure,transmit,anddisplaywaterlevel)maybeprovided.

3. (PG60.1.1)Whenthewaterlevelinatleastonegaugeglassisnotreadilyvisibletotheoperatorintheareawherecontrolactionsareinitiated,eitherafiberopticcable(withnoelectricalmodificationoftheopticalsignal)ormirrorsshallbeprovidedtotransfertheopticalimageofthewaterleveltothecontrolarea.Alternatively,anycombinationoftwoofthefollowingshallbeprovided:

a. Anindependentremotewaterlevelindicatorb. Anindependentcontinuoustransmissionanddisplayofanimageofthewaterlevelina

gageglass.4. (PG60.1.1.2)Whentwoindependentremotewaterlevelindicatorsareinreliableoperation

(continuouslyindicatingwaterlevel),theonerequiredgageglassmaybeshutoff,butshallbemaintainedintheserviceablecondition.

INTERPRETATIONSAPPLICABLETOTHESEREQUIREMENTS

NotethattheASMEcodeatonetimedistinguishedbetweenlevelindictorstypeswiththedefinitionDirectReadingforvisuallevelgagesandIndirectIndicationforallothertypesofindicators.Thatlanguagehasbeenreplacedinthecurrentcodewithsimplygageglassandremotewaterlevelindicator.

1. I8313Allowsgaugeglasstobeisolatedwhentworemoteindicatorsused2. I8602AnewTechnologylevelindicatordoesnotreplacethevisualgage3. I8650Gageindicationfortherovingoperatorisnotequaltothecontrolroomoperator.Low

wateralarmandtriparenotconsideredindirectindication.4. I8912Theoperatorsworkareaiswherecontrolactionsareinitiated5. I8972Anindirectleveldevicewithnopowerisnotagageglass,butcanbeconsidereda

remotelevelindicator6. I9215Viewingagageglasswithmirrorsisconsideredadirectreading7. I9269Amagneticlevelindicatormayprovideanindirectlevelreading8. I9296Amagneticlevelindicatorisnotconsideredtobeagageglass9. I9504Apressure/temperaturecompensateddpleveltransmitterisaremotelevelindicator10. I9507ADCSCRTscreencanbearemotelevelindication.Twoareneededtoomitthegageglass,

andtheindicationsmustbecontinuous.11. I9814Boilerswithdrumsafetyvalvessetunder400psiarenotpermittedtoshutoffthevisual

gage,evenwithtworemoteindicatorsinoperation.

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Thewaterdensityintheexternalinstrumentishigherthanthesteamdrumwaterdensity.TheexternalmeasurementinstrumentwillalwaysindicatelowerthantheTrueDrumLevel.Themagnitudeofthelevelerrordependsonoperatingpressureandheightofthewatercolumn.Thedensityofboilerwaterdecreasesasoperatingpressureincreases.Theerrorcanbesignificant,especiallyasoperatingpressuresexceed2000psi,andonlargesteamdrumswithlongvisibilitygagessuchasusedinHRSGapplications.Althoughsomeinstrumentmanufacturesmaystatethattheerrorsareinsignificantandchoosetoignorethem,theerrorsaresignificantunderconditionsofhighsteampressureandhighwaterlevels,andcannotbeignored.Thereareonlythreebasicmethodsavailabletocorrectfortheseerrors.Dependingontheleveltechnologyinuse,somemethodsaremoreorlessapplicabletovariousinstruments.Thesemethodsare:1. Heatthewaterinsidetheinstrumenttoatemperatureclosetothedrumwatertemperature.2. Physicallymovetheinstrumenttoalowerlocation.Thiswillonlyprovideacorrectionforone

operatingpressureandforonelevelposition.Althoughformultipointlevelindicatorssuchasconductivityprobeindicators,eachprobecanbeindividuallycorrectedforagivenoperatingpressure.Correctionsarebasedonanestimateoftheinstrumentoperatingtemperature.

3. Correcttheelectricaloutputsignal.Thisistheapproachusedforpressurecompensateddpleveltransmitters.Itcouldalsobepotentiallyusedtocorrectradarorothertypesorremoteindicatorswithanelectricaloutput.Anadditionalpressure/temperaturecorrectionsignalisusedtocorrecttheinstrumentoutputtoindicatethetruedrumlevel.

D. LEVELINSTRUMENTSANDASSESORIES

1. VisualLevelGages,DirectlevelMeasurementAdditionalASMERequirements:(PG60.1)Thelowestvisiblewaterlevelinagageglassshallbeatleast2abovethelowestpermissiblewaterlevel,asdeterminedbytheboilerManufacturer.(I9250)Theboilermanufacturerdeterminesthelowestpermissiblelevel(I9266)SamePG60.1definition(I8906)GageglassesarepartofBoilerExternalpiping,subjecttoPG60.WeldedgagebodiesmaybeprovidedbyamanufacturerwithoutaSection1certificate.(PG5.5)Theuseofaustenitic(series300SS)alloysteelispermittedforboilerpressurepartsthataresteamtouchedinnormaloperation.ExceptasspecificallyprovidedinPG9.1.1,PG12,andPEB5.3,theuseofsuchausteniticalloysforboilerpressurepartsthatarewaterwettedinnormalserviceisprohibited.(PG5.5note1).Austeniticalloysaresusceptibletointergranularcorrosionandstresscorrosioncrackingwhenusedinboilerapplicationsinwaterwettedservice.Factorsthataffectthesensitivitytothesemetallurgicalphenomenaareappliedorresidualstressandwaterchemistry.Susceptibilitytoattackisusuallyenhancedbyusingthematerialinastressedconditionwithaconcentrationofcorrosiveagents(e.g.chlorides,causticorreducedsulfurspecies).Forsuccessfuloperationinwaterenvironments,residualandappliedstresses

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AdditionalASMERequirements(I9263)Acomputerterminalleveldisplaycanbeselectedondemand,providedthesecondremotelevelindicatoriscontinuous,withonevisualgaugeinserviceandvisibletotheoperator.(I0119)ThesamesignalalgorithmcanbeusedfortworemoteindicatorsDensityErrorConsiderations:FIGURE12Thedrumlevelindicatedtotheoperatorisacalculatedvaluebasedonthefollowingmeasurementsandassumptions:

a. dptransmittermeasurementb. Pressuretransmittermeasurement(requiredforpressurecorrection)c. Referencelegheightd. Referencelegtemperaturee. Variablelegtemperaturef. Steamandwaterdensitycalculationattheoperatingpressure

FIGURE12

Significantlevelindicationerrorsareintroducedwhentheassumptionsusedinthelevelcalculationformulasdonotagreewiththeactualparameters.Errorsarealsointroducedduetoinstallationmistakes.Thesensinglinesfromtheboilertothedpinstrumentshouldbeslopedtopreventformationofgaspocketsanderroneouslevelreadings.Toobtainthebestaccuracy,temperaturesinthereferenceandvariablelegsmustaccuratelyestimated.FIGURE12showstheidealdptransmitterinstallationthatintroducestheminimumnumberofvariablesintotherequiredequations.

LP= H1D1 + H2D2 + H4D4HP= H3D3 + H4D4DP= HP - LPDP= (H3D3 + H4D4) - (H1D1 + H2D2 + H4D4)DP= H3D3 - H1D1 - H2D2

D1= STEAM DENSITYD2= DRUM WATER DENSITYD3= REFERENCE LEG DENSITY NOTE: DISTANCE X MUST BE SUFFICIENT SO

THAT T IS AT AMBIENT TEMPRATURE

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STM STMPRESS TEMPPSIA DEGF2500 66820 2282500 668

REFLEG STMTEMP PRESSDEGF PSIA120 250050 2500120 20120 2500

FIGURE13bachotherascalculation.

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belowillustrapressureinc

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FromFIGURE12,Theformulaforthetransmitterdpis:1) Dp=H3D3H1D1H2D1Notethat:H1=H3H2ReplacingH1intheDpformula,theformulacanbesimplifiedas:2) Dp=[H3(D3D1)+H2(D1D2)]/62.3IftheDpisknown,theheightofwaterinthedrumcanbecalculatedfrom:3) H2=[62.3(DP)H3(D3D1)]/D1D2

Theunitsintheseformulasare:DpinchesofwaterpressureHeight(H):InchesDensity(D):lb/ft3Conversionfactor:62.3

EXAMPLE1:Referencelegtemperaturelowerthanestimated.Comparelines1,4,andline5intheTABLE1.Theassumptionsare:Steamdrumpressure:2500psiaReflegheight,H3,48inchesReferencelegtemperature:120FDrumwaterlevel,H2,36inchesThecalculateddpmeasurementis25.86inchesofwater(TABLEline1).Thisisconfirmedinline4.Ifthemeasureddpis25.86,thecalculateddrumlevel(H2)is36.01But,iftheactualreferencelegtemperatureis50Finsteadoftheassumed120F,theeffectofthiserrorisshowninline5ofthetable.Withthedptransmittermaintainingdrumlevelcontrolat25.86,theactualdrumlevelwillbeat37.24.Anerrorof1.23.EXAMPLE2:Nopressurecompensationofthedptransmitter.Assumethesameconditionsasabove,withthedptransmittersettomaintainadrumlevelof36whentheboilerisat2500psi.(Line1andline4).Ifnopressurecompensationwasavailableduringboilerstartup(Tableline6),atadrumpressureof20psithedrumwaterlevelwouldbeat22.72,anerrorof13.3.

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EXAMPLE3:DPtransmittervariablelegconnectedtothebottomofthedrum.TheFIGURE14belowshowsthisinstallationmethod.

FIGURE14.Variablelegoffbottomofdrum

Assumingthesameboilerconditionsasthefirsttwoexamples,butH2drumlevelnowincludestheYcomponentthatwillbeatalowertemperaturethanthesteamdrumwatersaturationtemperature.AssumeforthisinstallationthatYis12inches,andthiswaterinthislineissubcooled100Fbelowthesaturateddrumwatertemperature(waterat565F).TheeffectofthischangeisshowninLines#3and#7.ThedrumlevelfromLine#7willbethetotalofYandthecalculatedH2,or28.25.Thisisanerrorof7.75fromthesetpoint(36).

5. MagneticFloatIndicators.FIGURE15.Thesedevicesconsistofastainlesssteel(orothernon

magneticmaterial)pipechamberandaninternalfloat.Thefloatisalsononmagnetic,butcontainsaringofmagnets.Themagnetswithinthefloatoperateamagneticindicatorlocatedontheoutsideofthepressurechamber.Theindicatoriseitherasinglefollower,oraseriesofflagsthatrotateandchangecolourasthefloatmagnetspassby.Thefloatmagneticringwillcollectcorrosionparticles(iron/steel)fromthewaterandthefloatmustberemovedandcleanedperiodically,dependingonwaterquality.Floatsmustbecomelargeraspressureincreasesduetothereductioninwaterdensity.

LP= H1D1 + H2D2 + H4D4HP= H3D3 + H4D4DP= HP - LPDP= (H3D3 + H4D4) - (H1D1 + H2D2 + H4D4)DP= H3D3 - H1D1 - H2D2

D1= STEAM DENSITYD2= DRUM WATER DENSITYD3= REFERENCE LEG DENSITY

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FIGURE15

AdditionalASMERequirements:(PG60.1.1.4)IndependentremotelevelindicatorsthatcontainsensingdevicesthatincludeamagneticallycoupledfloatinsideanonmagneticcylindricalpressurechambertoutilizethroughthewallsensingoffloatpositionshallberestrictedtotherequirementsofPG12.2.Thedesignandconstructionofsuchdevicesshallincludeprovisionsforeaseofcleaningandmaintenance.Attachmentofanycontroldevicesforuseotherthanindicatingwaterlevelisprohibited.(PG12.2)Boilershavingamaximumallowableworkingpressurenotexceeding900psi(6MPa)mayusealternativemethodsforindependentremotewaterlevelindicatorsorwaterlevelsensingdevices(seePG60).Thesensingdevicesmayincludeamagneticallycoupledfloatinsideanonmagneticcylindricalpressurechambertoutilizethroughthewallsensingoffloatposition.ThepressurechamberstressesshallmeettheappropriaterequirementsofPG27andPartPW,andshallberestrictedtothematerialgradeslistedinPG12.3DensityErrorConsiderations:Aswithothermanometertypeindicators,thedifferenceindensitybetweenthewaterinthedrumandthewaterinthemagneticfloatcolumnwillcausealevelindicationerror.Theerrorincreasesasthemeasuredlevelincreases.Higherpressuresandlongervisibilitieswillcreatelargererrors.Itispossibletocorrectforthiserroratoneoperatingpressureandoneindicatedlevel(typicallythezeroposition)by

INDICATINGFLAGS

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FOSSILPOWERSYSTEMSINC.

17|P a g e

6. GuidedWaveRadarLevelIndicators.FIGURE17Comparedwiththeothertechnologiesinuse,thisoneisrelativelynew.Guidedwavetechnologysendsaradarpulsedownaprobeinstalledinanexternalcolumn.Thepulsereflectsoffthesurfaceofthewaterbackuptheprobeandtothesensor.Thetransittimeistranslatedintoadistance,andbasedondimensionsoftheinstallation,thewaterleveliscalculated.Thisdeviceproducesacontinuousreading(420mAsignal)ofthewaterlevel.Theadvantageofthismethodoverothertypesofradarsensorsisthattheprobeguidestheradarpulseandconcentratesboththepulseandthereturnsignalenergy.Effectivelytransmittingtheradarpulsethroughthelongguideandthroughthepressurecontainmentboundarytotheelectronicsrequiresspecializedsealsandconstruction.

FIGURE17

DensityErrorConsiderations.Aswithotherdevicesusingawatercolumnconnectedtothesteamdrum,thewaterinthecolumnwillbecolderandconsequentlyatalowerlevelthanthesteamdrumtemperature.Iftheactualwatertemperatureinthecolumnismeasured,itwouldbepossiblefortheelectronicstocalculateandapplyacorrectionfactortotheindicatedlevel.Anadditionalconsiderationaffectingindicationaccuracyisthesignificantchangethatoccursindielectricconstantsofwaterandsteamaspressureandtemperaturechanges.Theradarpulsereflectionisactuallycausedbydetectingtheimpedancechangebetweenthewaterandsteam.Anychangeinthesevalueswillaffectthepulsetransittime,andtheindicatedlevel.Thetablebelowshowsthisaffect.

FOSSILPOWERSYSTEMSINC.

18|P a g e

TEMP(F) PRESSURE

(PSIA)LIQDENS.(LB/FT3)

VAPORDENSITY(LB/FT3)

DIELECTRICCONS,LIQ

DIELECTRICCONS,VAP

ERROR %

100 0 61.99 0.0029 73.95 1.001 0.0400 247 53.65 0.537 34 1.069 3.4600 1543 42.32 3.74 18.04 1.461 21

ThedielectricconstantsofWaterandSteamapproacheachotheraspressureincreases.Programswithintheelectronicscancompensateforthischangeandmaintainthelevelindicationaccuracy.

E. CONCLUSION

TheASMEBoilerandPressureVesselCodeliststherequirementsforsteamdrumlevelgaugesandindicatorstoensuresafeoperationoftheboiler.Directreadingvisuallevelgaugesarerequiredoneveryboiler.Thesecanbecomplimentedwithotherindirectinstrumentstoattainthegoalofsafeandefficientboileroperation.Buttoattainthisgoal,instrumentsmustbeproperlyselectedforthedesignconditions,installedproperly,calibratedproperly,andmaintained.Ifanyoneoftheserequirementsisignored,performancewillbedegraded.Allinstrumentshaveinherentstrengthsandweaknessesbasedonthetechnologyemployedtomeasurelevel.Theseinstrumentcharacteristicsshouldbethoroughlyunderstoodbytheuser,orindicationswillbemisleading.Aprudentandconservativeapproachistousesomecombinationofthedifferentproventechnologies.Theindependentindicationstheseinstrumentsgivewillthenprovideconfidencethattheboilerisalwaysoperatedinasafecondition.

F. REFERENCES1. Liptak,BelaG.,InstrumentEngineersHandbook,ThirdEdition19692. Kalix,D.A,DensityLevelErrorandItsCorrectioninBoilerDrumLevelIndication,ISA

ConferenceOctober19953. Boyes,W.SomeThingsdoexactlywhattheyaresupposedto,ControlMagazineFeb20044. Evely,D.P.,HeatRecoverySteamGeneratorDrumLevelMeasurementSourcesofError,ISA

jointPOWID/EPRIconference,20045. Gilman,J.BoilerDrumLevelControl,ISAJuly/August2010

DavidKalix,P.E.CopyrightC2011FossilPowerSystems,Inc.FossilPowerSystemsInc.AllRightsReserved10MosherDrive,Dartmouth,27July2011NovaScotia,CanadaB3B1N5(902)4682743