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FOSSILPOWERSYSTEMSINC.
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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.
FOSSILPOWERSYSTEMSINC.
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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.
FOS
3|
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C. OPERATLevelganotinaambientcondensthrough
ywaterlevelowtheactuasealevelericatelowertaffected.F
YSTEMSINC.
TINGTEMPEagesandothtotallystatit.RefertoFsateflowsinhthegaugea
lmeasuremealdrumstearrorwithinththanthetruIGURE2bel
ERATUREERerremoteleccondition.FIGURE1.Antothegaugandpiping.
entdevicecam/watersahemeasuremedrumwateowillustrate
RORCORREevelindicatoIftheywerssteamconeandheats
onnectedtoaturationtemmentdeviceerlevel.Waesthiseffect
FIG
ECTION(DENorsconnectee,thewaterdensesinthit.Asmalla
FIGURE1
othesteammperature.Te.Themeasuaterlevelgagtandthefor
GURE2
NSITYLEVELedtotheboirtemperatuhegageandamountofw
drumwillopThislowerourementinstgesandalltyrmulasused
ERROR)iler(exceptreinthegagconnectingwaterisalwa
perateatatoperatingtemtrumentwillypesofremtocalculate
dptransmittgewouldbepiping,theaysflowings
temperaturemperaturewlthenalwayotelevelindetheerror.
ters)arenear
slowly
ethatiswillysdicators
FOSSILPOWERSYSTEMSINC.
4|P a g e
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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muschem(PGstanmayDendenspres1. H
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YSTEMSINC.
stbeminimizmistry.12.1)Gagegndardthatdeyincludeaus
sityErrorCositylevelerrssureincreasHeatthewatetoassteamhnstallthegauProvideacha
ReflexGagesightreflectievel.Maximdemonstrate
AdditionalA(PG60.1)Gaconstruction
zedandcare
glassbodyaefinestheprsteniticstain
onsiderationror.Thesewses.Ifthegerinthegaugeatingortemugeinalowertatthegaug
s.SeeFIGURonandtransmumoperatiesthisopera
ASMERequirageglassassn,shallbede
efulattentio
andconnectoressuretemnlesssteelsa
ns:Visualgawillindicategaugemustbgewithanadmperatureeqrposition,cageshowingth
RE3.Thesesmissiontodngsteampratingprincip
FIG
FIG
ements:semblieshavesignedinsu
onmustbep
ormaterialsperatureratandnickelba
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egageshavedevelopaWressureistypple.
GURE3
GURE4
vingmultipleuchamanne
paidtoconti
shallcompltingmarkedasedalloys.
picallypositilowpressurcorrectedtconnectedto
onepressurefcorrectiona
egroovedglWhite/Blackipically350p
esections,werthatwille
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onedwithoure,withincretherearesevothesteamd
andoneindicatvariousope
assthatappimage,indicpsi.SeeFIGU
whetheroftuensureamin
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utconsideraeasingerrorveralapproadowncomer
catedlevel.eratingpressu
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ubularorothnimumof1
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overlapofalorREFLEXgamayomitth(I0438)Des
FlatglassGagthewater/stowpressuresafeglasslenpressures.
FIG
itionalASME60.1)Gageganstostrenggthofthegangtheningmlcannotbesversememtinuousoverrcesasappli
PortedGagethewindow
lladjoiningsagesthatuseerequiremescribessame
gesandTubteaminterfaesandbecomngth.Flatgla
GURE5
ERequiremeglassdesigngthenthebogeglass,arememberswilreadilydete
mbers.Reflexrtheverticacable,areno
es.FIGURE7areaisredu
sectionsinwerefractionentforoverlerequireme
ularGlassGace(meniscumefragileasassgauges,a
ents:FIGUREsthatutilizeodyofthegaenotpermittlcauseareaerminedbecaxtypedesigllengthofthotprohibite
.Thesegauuced.Thega
whichthewaoflighttoaappingsectientsasabove
ages.FIGURus)insidethesglasslengtalsocalled
E6etransverseage,whichated.Note:Tsalongthelauseofmasnsthatutilizheglass,andd.
gescanopeagebodyisc
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aterlevelmaidreadydetions.e.
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structuralmrenotcontiTransverseolengthofthekingshadowzedsolidmedfrontlitby
erateatveryconstructed
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aybevisibletermination
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FIGURE6
members(crnuousovertorcrosswebegageglasswscausedbyembersbehiyeitheramb
yhighsteaminatrapezo
,exceptthaoftheliquid
ontheabilityassgagesopessurewilllimeathigher
osswebbingtheentirevebstructuralwherethelythepresenndtheglassientorproje
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illuminatorwindicationof
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ASMERequirortedgagesoonoftheliqustratesthis
aREDandGerlevel.
FIG
ements:orreflexgaguidlevel,maoperatingp
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GURE7
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videaRED/G
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(I0708)AhtherequirembetweenwaNote:AREDgaugeMUST
terColumns.mnsareusefollowingbea. Allowsfob. Providesc. Prevents
window,
AdditionalA(PG60.2.3)E(DN20)toinPG60.3.1)Gshellordrumrequired,bo
handheldligmentthatthaterandvapo/GREENillumTbeusedto
.FIGURE9.edasaninteenefits:ormismatchsafixedandsexcessivecreducingor
ASMERequirEachwatercstallavalveGageglassesmoftheboilothmaybec
ghtsource(flegagebeeqorintheindminatordessatisfythisA
Thisillustraterfacebetwe
hbetweendrrigidpointt
condensatefrobscuringt
FIG
ements:columnshalddraintoathatarereqlerortoaniconnectedto
lashlight)hequippedtopdividualsectiignedforprASMErequir
tionisfromeenthestea
rumandvisutoinstallthefromflowingthevisibility
GURE9
lbefurnishesafepointoquiredbyPGnterveningwoasinglewa
eldbehindaprovideobviions.operdisplayrement.
ASMESectiomdrumand
ualgageconegageisolatgthroughth.
edwithacoofdischarge.G60.1shallbwatercolumatercolumn.
portedgageiousvisuald
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nnectionofbeconnectemn.Whentw.
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atisfyon
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othessesare
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3. Condtheclikecoluopencondindic
YSTEMSINC.
(PG60.3.2)orausteniticshallnotbeoffsetinthe(PG60.3.3)Torausteniticshallnotbeconnections(PG60.3.4)C
ductivityProconductivityanon/offswmnbody.Tncircuit,whductivityprocationsofdr
ThelowerecstainlessstbelowthehpipingthatTheupperecstainlessstabovetheloshallbeabovConnections
obeSystemsyofwateranwitch.AlowThereturnsighilewatershobesarelocarumwaterle
edgeoftheseelornickehighestvisiblwillpermittdgeoftheweelofnickelowestvisiblevethepointsfromthebo
s.FIGURE10ndsteamarewvoltageisagnalismeasortstheproatedatvarioevel.
FIG
steamconnelbasedalloyewaterlevetheaccumulwaterconneclbasedallowewaterlevetofconnectioilertothew
0Thesedevicesignificantlppliedtothsuredbyaneobetogrounouslocations
GURE10
ectionbetweywaterleveelinthegaglationofwatctionbetwewwaterlevelinthegageionatthewawatercolum
cesmeasurelydifferent.eprobetiptelectroniccindandappeasinaprobe
eenawaterlsensingdeeglass.Theter.enawatercelsensingdeeglass.Nopatercolumnmnshallbea
elevelusingTheconducthatisinsulaircuit.Steamarsasaclosecolumntop
column,gagviceinthebereshallben
column,gageviceandthepartofthispn.atleastNPS1
theprinciplctivityprobeatedfromthmappearsasedcircuit.Tprovidediscr
geglassboilernosagor
eglass,eboilerpipe
1(DN25)
lethateactshesanTherete
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A((DcPdtpc
4. Diffeindicmeaastawateandchalmeaachioperdensabsowith
YSTEMSINC.
AdditionalA(I0122)Aus(I0125)Con
DensityErrocolumnwillProbelocatiodrumlevelathanprobespressuresotconditions.
erentialPrescatedrumleasuringthedaticcolumn,erlevelsprorepeatablelengingsincasureonlyineveaccuracratingpressusitycompenolutepressuhinonedevic
ASMERequirsteniticmatenductivitypr
orConsiderabelowerthaonscanbeiataspecificoatlowleveltherthanthe
ssure(dp)Trevelandaredifferenceinalsocalledt
oducehighdmeasuremeetheinstrumchesofwatey,highpressureismeasusatedtransmre,andprocceshouldre
ements:erials(seriesrobetypele
ations.Asdianthedrumndividuallycoperatingprs.Measureenormalop
ransmitters.mostcommpressurebetheRefereneltap,andhntsthatarementsoperaerpressuresuresystemuredandthemittersareacessorwetlduceerrors.
FIG
s300SS)canvelcolumns
iscussedearmwatertempcorrectedtoressure.Promenterrorseratingpres
FIGURE11.monlyusedfoetweenthelnceLeg.Ashighdrumwunaffectedateatthousadifferential,smustbePeindicateddalsoavailableegtemperat.
GURE11
nnotbeusedscannotuse
rlier,watertperature,caoimprovethobesathighsareintrodussure,andin
TheseprodordrumlevelevelofwatescanbeseewaterlevelspbychangesandsofpsiboftenlesstPressureComdrumlevelisethatmeasture.Measu
dforlevelinausteniticm
emperatureusingadenseaccuracyolevelsreceivucedwhenonparticularw
uctsprovideelcontrol.Terinthedrunfromthedproducelowinboilerpreboilerpressuhanonepsimpensatedscorrected.uredifferenurementsan
ndicatorsmaterials.
eintheprobsitylevelerroftheindicavemorecorroperatingatwhenatcold
ea420mATheyoperateumandthewdiagram,lowwdeltap.Acessureareure,butmusdifferential.,meaningthMultivariab
ntialpressurendcalculatio
beror.atedrection
dstartup
signaltoebywaterinwdrumccurate
st.Tohattheblee,ons
FOSSILPOWERSYSTEMSINC.
11|P a g e
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
FOS
12|
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LINNO4567
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Iaw
Ti
REFLEG
E TEMP PO. DEGF
120 2120120 2
E DP TO. INCHES D
25.8625.8625.8625.86
YSTEMSINC.
Inaddition,approacheawaterlevelc
TABLE1belointroducedb
STM STMPRESS TEMPPSIA DEGF2500 66820 2282500 668
REFLEG STMTEMP PRESSDEGF PSIA120 250050 2500120 20120 2500
FIGURE13bachotherascalculation.
owwillbeusbyinaccurat
H1 DINCH LB/12 7.612 0.012 7.6
STMTEMP DDEGF LB/668 7.6668 7.6228 0.0668 7.6
belowillustrapressureinc
sedtoillustreassumptio
D1 H2/FT3 INCH658 360498 36658 24
D1 D2/FT3 LB/FT3658 34.97658 34.970498 59.42658 34.97
atesthatthecreases.Th
FIGURE
rateanumbons.
D2 H3LB/FT3 INC34.97 4859.42 4834.97 48
H3 D3INCH LB/F48 61.48 62.48 61.48 61.
edensityofiswillexace
E13
berofexamp
3 D3CH LB/FT38 61.718 61.718 61.71
3 YFT3 INCH.7 0.4 0.7 0.7 12
waterandderbateanye
plesofdptra
TEY T
INCH DE0012 5
DY HLB/FT3 IN
0 360 370 22
44.96 16
densityofsterrorsinthed
ansmitterer
EMPTY DYEGF LB/FT30 00 0565 44.96
H2NCH6.017.242.726.25
eamdrum
rrors
DPINCHES25.8613.2022.46
FOSSILPOWERSYSTEMSINC.
13|P a g e
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.
FOSSILPOWERSYSTEMSINC.
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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
FOSSILPOWERSYSTEMSINC.
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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
STAINLESS STEELCHAMBER WALL
RINGFLUX
LEVELLIQUID
FLOATMAGNETS
FLOAT
FOS
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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