Download - Copy of Steam_Drum_Level_Measurement.pdf

FOSSILPOWERSYSTEMSINC.

1|P a g e

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.


2|P a g e

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|

Anybelocauindiare

SSILPOWERS

P a g e

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


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

FOS

5|

SSILPOWERS

P a g e

muschem(PGstanmayDendenspres1. H

t2. I3. Pa. R

lld

A(c

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

ugesaretypcorrectlyatbedensitycditionalpipeualization).lculatedforoheamountof

egageshavedevelopaWressureistypple.

GURE3

GURE4

vingmultipleuchamanne

paidtoconti

shallcompltingmarkedasedalloys.

picallypositilowpressurcorrectedtconnectedto

onepressurefcorrectiona

egroovedglWhite/Blackipically350p

esections,werthatwille

nuouscontr

ywithaMaontheunit

onedwithoure,withincretherearesevothesteamd

andoneindicatvariousope

assthatappimage,indicpsi.SeeFIGU

whetheroftuensureamin

rolofwater

nufacturers.Themater

utconsideraeasingerrorveralapproadowncomer

catedlevel.eratingpressu

pliestheprinatingthewaURE4that

ubularorothnimumof1

rialsused

ationforrasaches:(referred

ures.

nciplesofater

her(25mm)

FOS

6|

SSILPOWERS

P a g e

oom(

b. Ftlsp

Add(PGmealengstrelevetrancontsourc. P

t

YSTEMSINC.

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

WEBNO

PERMITT

aterlevelmaidreadydetions.e.

RE5.Theseegageglass.hincreases.Armoured,

structuralmrenotcontiTransverseolengthofthekingshadowzedsolidmedfrontlitby

erateatveryconstructed

OT

TED

aybevisibletermination

gagesrelyo.TubularglaDesignpre,canoperat

FIGURE6

members(crnuousovertorcrosswebegageglasswscausedbyembersbehiyeitheramb

yhighsteaminatrapezo

,exceptthaoftheliquid

ontheabilityassgagesopessurewilllimeathigher

osswebbingtheentirevebstructuralwherethelythepresenndtheglassientorproje

pressuresboidshape.T

tporteddlevel,

ytoseeperateatmitthe

g)asaertical

iquidceofthethatareected

becausehe

FOS

7|

SSILPOWERS

P a g e

ii

A(dF

YSTEMSINC.

illuminatorwindicationof

AdditionalA(PG60.1)PodeterminatioFIGURE8illu

willprojectafdrumwate

ASMERequirortedgagesoonoftheliqustratesthis

aREDandGerlevel.

FIG

ements:orreflexgaguidlevel,maoperatingp

REENlightth

GURE7

gesthatuseayomittheprincipleand

FIGUR

hroughthep

refractionorequiremendtheimaged

RE8

portstopro

oflighttoaidntforoverlapddisplayed.

videaRED/G

dreadyppingsectio

GREEN

ns.

FOS

8|

SSILPOWERS

P a g e

(tbNg

2. Watcoluthef

abc

A((Psr

YSTEMSINC.

(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

yoftheleve

onI,figurePdthevisualg

nnectioncenionvalveshegageand

nnectionofbeconnectemn.Whentw.

edoesnotsaiscriminatio

linthebico

PG60.Thesgaugeandp

nters

overthegag

atleastNPS

eddirectlytowogageglas

atisfyon

olour

seprovide

ge

S3/4

othessesare

FOS

9|

SSILPOWERS

P a g e

oso(osc(

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

FOS

10|

SSILPOWERS

|P a g e

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


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|

LINNO123

LINNO4567

SSILPOWERS

|P a g e

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


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.


14|P a g e

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


15|P a g e

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

16|

SSILPOWERS

|P a g e

caoE

AATHT

PRESSPSIA

900300

Thista3

Inadditionfurthererro

YSTEMSINC.

calculatingaaveragewatoperatinglev

Example:Op

AssumethatAssume:SteMaTheheighto

Hg=Hd(Dd

Thetablebe

S TEMPF

532417

ableshowst33.5432.

,thebuoyanor.

acorrectedftertemperatvelsandoth

peratingPres

tthemagneteamdrumleagGaugesubofthewater

Ds)/(Dg

elowshowst

HdINCH

3636

thattheleve24=1.30

ncyoftheflo

floatpositionturewithintheroperating

ssurediffere

ticfloatposevel(Hd)of3bcoolingofinthegauge

Ds)

theindicatio

DdLB/FT3

47.1052.92

elinthefloa

oatwillincre

nbasedonfthechambergpressures.

entthancalc

FIGURE

itionisdesig36inches.100Ffromeis:

onerrorwhe

DsLB/FT3

2.000.65

tchamberw

ease,raising

floatweight,r.However

culation.

E16

gnedforano

thedrumw

enoperated

TEMPGAUGE,

F432317

willchangeb

gtheposition

,floatdisplar,errorsare

operatingpr

watertemper

at300psia

P Dg,Tg LB/F

52.356.7

by:

nofthefloa

acedvolumeintroduced

ressureof90

rature.

g HFT3 INC

36 32.75 33.

t,andaddin

,andatother

00psia.

HgCH.24.54

ng


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.


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

Download - Copy of Steam_Drum_Level_Measurement.pdf

Top Related