otsg water side dynamics 2017-11-21 - kevindorma.ca€™s begin with a quick review of drum...
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PhDinFluidMechanics(modelling)18yearsasaconsultant,10yearsexperiencewithSAGD.Someunderstandingofdrumboileroperationandcontrol.ExtensivetroubleshootingofSAGDwaterhammerissuesProblemsoriginatefromtheOTSG:Why?
UnderstandtherootcauseofoperatingproblemsUnderstandOTSGdynamicsbyapplyinggoodmodelling
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2017-11-21
KevinDormaConsulting,Ltd.
OTSGWaterSideDynamics
Let’sbeginwithaquickreviewofDrumBoilers.
AllPowerEngineersarefamiliarwiththewatersidebehaviourofdrumboilers.ThedynamicsofthewaterlevelinadrumboilerisdominatedbyaphenomenathatiscalledShrinkandSwell.Itisthisphenomenathatmakesteamdrumlevelcontrolsointeresting.
Considerthissketchofadrumboilerwithheataddedtotherisers.Steamiscreated,andmakesbubblesintherisers.Thebubblesintherisersoccupyacertainamountofvolume.Thedrumlevelisheldconstantatthenormallevel,andthesupplyoffeedwaterintothedrumequalstheflowrateofsteamthatisproduced.Then,wedecreasethefiringrate.Thismakeslesssteam,andtherearenowlesssteamvolumeintherisers.Waterinthedrumaccumulatesintherises,andthedrumlevel“shrinks”.
DrumBoilerdynamicsarewellunderstoodChangesinfiringratecauseschangesinliquidvolumeinrisers,andchangesthe
drumlevel.Thisis”Shrink”and“Swell”.
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2017-11-21
KevinDormaConsulting,Ltd.
OTSGWaterSideDynamics
Anothercommondisturbanceisasuddendropinthefeedwater temperature.Thiscausesasuddenquenchingofthesteamdrum,andinterruptsthesteamproduction.Withasuddendropofsteamflowtothesuperheatersection,thesteamtemperatureexitingthesuperheater canbecomeveryhigh.
Thisunderstandingofdrumleveldynamicshasleadtostandard,sophisticatedcontrolsystemstomanagedrumlevelduringfiringratechanges
SingleelementTwoelementThreeelement
Theresultisthatdrumboilersareveryreliableanddonotexperiencespurioustrips.Adrumboilerisexpectedtotripmaybeonceayear,andthisisduetoanactualmechanicalorinstrumentfailure,andmustbefixed.
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KevinDormaConsulting,Ltd. 4
Thiscouldarise ifawell-meaningProcessControlpersonsays“Icanmakethatfaster”.
Suddendecreasein steamdemandcausespressuretoincrease.Lessboilingintubes.Morewaterheldupinrisers.Drumleveldrops.Thesuperfastlevelcontroladdsabigshotofcoldwaterandquenchesthedrum.Killthesteamproduction.Lesssteamthroughsuperheater,cookthesuperheater.
Thisiswhyweneedtounderstandwhenthingsareoperatingproperly.
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4or6passiscommon250MMBTU,150t/hwaterrateiscommonforthelargeunits.70MMBTU/hforthesmallerunitsaroundLloydminster.
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2017-11-21
KevinDormaConsulting,Ltd.
OTSGWaterSideDynamics
IhaveheardafewcommonfrustrationsinSAGDcontrolrooms.OperationsteamsfeelthatOTSGsseemtodosomeweirdthings.Someteamshaveseensafetyvalvesroutinelyliftonstartup.Othershaveseentheoddtriponsuperheat,butthemeasuredsteamqualitywaslow.MostoftheOperationsteamshaveexperiencewithdrumboilers.Ihaveheardthatdrumboilersareconsideredtobemorereliable.
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2017-11-21
KevinDormaConsulting,Ltd.
OTSGWaterSideDynamics
RELIABILITYandSAFETY.Pureandsimple.Yeah,higher steamqualityisnice,andworthafairbit.Ithinkreliabilitycanalsoimprovebyunderstandingthewaterside.ButifyoucanrunlikehellandrelighttheOTSGin3minutes,withoutcheckingwhyitwentdown,youcanbebackinbusinessin15minutes.Lossofproductionistrivial.Butrelightsarehazardous.Thankfully,Ihavenotseenalotofseriousproblems(yet).
IfweunderstandhowtheOTSGbehaves,thenwecanexpecttooperateitinasafermanner,andwecanexpecttocontrolitbetterformorereliableoperation.Andincreaseproductionrates.Thewatersidebehaviourdetermineshowweshouldcontrolthefeedwaterflowrateandthefiringrate.ThisiswhyitisvaluabletounderstandthewatersidebehaviourofanOTSG.
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ThisisaschematicforanOTSG.
Onevalvetocontrolthetotalflowofwater.Thenpassbalancingvalves.Winddownthroughtheconvectionsection,andgetwarmer.Boilingstartsattheexitoftheconvectionsection.Thenafewlapsbackandforthacrosstheradiantsection.
Makeabout78%qualitysteam.Wetsteamflowsthroughthepassventuri,whichmeasuresapressuredropandcalculatesthesteamquality.Thenthepassescombine.
Normalflowisthroughacheckvalveandtoaseparatorvessel,withthesteamgoingtothewellpadandhotblowdownusedtopreheatBFWinaheatexchanger.Startuppathisthroughacontrolvalve,andthentowardsapondortank.
Thereareanumberoftripsthatareoftenpresent.Onthewaterside,PassFlowdeviationisafrequenttripinseveraloperatingplants.Steamqualitytripsarelesscommonnowthantheywereafewyearsago.Onthefireside,lowO2orhighcombustiblesinthefluegasisacommontrip.
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Onthefiringtrain,Lowfuelgassupplypressureisacommontripthatpreventsreliablelight-off.
Thecontrolsystemisoftenverycomplex,andthereareseveralPIDcontrollersthatcanfightwitheachother(ie,passflowcontrolandthetotalBFWflowcontrol).
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Ingeneral,ifsomethingcausesthemassflowoutofthetubetobedifferentfrommassflowin(or,there iswateraccumulation)thenthesteamqualityventuri readsfalse.
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2017-11-21
KevinDormaConsulting,Ltd.
OTSGWaterSideDynamics
Highlightthewater volumeintheOTSG.ThisisTHEmostimportantconceptforanOTSG.Thetimeneededtogothroughtheliquidsectionasthesecondmostimportantconcept.
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2017-11-21
KevinDormaConsulting,Ltd.
OTSGWaterSideDynamics
Highlighthowtheliquidfilledvolume changes.
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2017-11-21
KevinDormaConsulting,Ltd.
OTSGWaterSideDynamics
Explainthateveryonegoesonestepforward,oneover.Crossthesaturationlineandexpand.
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2017-11-21
KevinDormaConsulting,Ltd.
OTSGWaterSideDynamics
WelcometoLecture 2.IwillshowyouwhatgoesoninsidetheOTSGtubes,andsharesomeobservations.
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2017-11-21
KevinDormaConsulting,Ltd.
OTSGWaterSideDynamics
Thesudden increaseinoutletflowrateisareflectionoftheaddedheatdecreasingthemassofwaterinthetubes,andforcingwaterout.ThentheradiantsectionboilsallatonceandthereisahugeExodusoutofthetubes.
Theentireradiantsectionboilsallatonce.Thinkaboutourfavourite jokeinvolvingtwocoldbeer,andanunsuspectingfriend.Bonkthebottle,andaquarterofthebeergushesout.
ThiscausesthepressuretobuildintheOTSGbecauseitisdifficulttogetthelargemassofwaterthroughthestartupvalve.AndthePSVmaylift.OnoneOTSGstartup,thepressurewasheldat8000kPag.Whenthetemperaturereachedboiling,thepressureshotupto11,000kPa,justshortofthePSVsetpressure11,700kPa.ItdidnotliftthePSVthistime.Hotshowerswerecommon.
That’sgreat.Whatcanwedoaboutthis?Remember,wearealreadyattheloweststablefiringratethatwecanget.
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2017-11-21
KevinDormaConsulting,Ltd.
OTSGWaterSideDynamics
OTSGstartupwithoutthehugerushofwater.Higherwaterflowrate,crosstheboilingpointfurtherdownthetubes.Lesswateriskickedoutwheneverythingboils.
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2017-11-21
KevinDormaConsulting,Ltd.
OTSGWaterSideDynamics
The steadystateflowrateoutishigher,butthepeakismuchsmaller.ThestartupcontrolvalvewillhaveamuchbetterchanceofprovidingenoughcapacitytokeeptheOTSGpressurebelowthePSVsetpressure.• CommonpracticeistostarttheOTSGathigherflowrates
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2017-11-21
KevinDormaConsulting,Ltd.
OTSGWaterSideDynamics
• SuddendropinBFWflowratecausesoutletenthalpytoincreaseslowlyThesteamqualitymeasurementreadsFALSEfor5- 6minutesThislooksalotmoreseriousthanitreallyis.SpuriousHHqualitymeasurementmaycausefrequentfiringtrips
ConsidersofteningHHqualitytripsorHHpassflowdeviation
Increaseinfiringrateisnearlyidenticalforchangeinsteamquality
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2017-11-21
KevinDormaConsulting,Ltd.
OTSGWaterSideDynamics
A suddendropinwaterflowrateisnotahazardaslongastheflowcontrollercantakeaction.Nodifferentthanlevelcontrolinadrumboiler.LevelcontrollersetstheBFWflowsetpoint.AblipintheBFWheaderismanagedbecausetheBFWflowiskeptontrack.
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2017-11-21
KevinDormaConsulting,Ltd.
OTSGWaterSideDynamics
MyYouTubelecturedemonstrateswhyapumpstartchangesthefeedwatertemperature.
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2017-11-21
KevinDormaConsulting,Ltd.
OTSGWaterSideDynamics
Startwithabout150C feedtemperature,or600kJ/kgenthalpy.Thenwewatchahotfrontmovethrough.Thehotfrontspreadsoutbecauseofthethermalmassofthepiping.Steelhastowarmup.
Waitforthehotfronttobarelytouchtheboilingline.Whatwillhappennext?
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2017-11-21
KevinDormaConsulting,Ltd.
OTSGWaterSideDynamics
Theactualsteamqualitydropsbecausea largemassofwaterispushedout.ThemeasuredqualityincreasesbecausethemeasuredDPbecomeshighforashorttime.Thelevelintheseparatorjumpsup,andthenslidesdown.
ThiswasthekeyobservationthatconfirmedwhatisgoingoninsidetheOTSG.A10Criseinthewatertemperaturecausedtheseparatorleveltohopupbyameter,thenslidedown.
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2017-11-21
KevinDormaConsulting,Ltd.
OTSGWaterSideDynamics
Nofaceshot.There isabout8m3ofwatertostartwith.Thenthereis10m3ofwater.Inbetween,theOTSGfilledwithwater.Thismeansthatthewaterflowratedownthetubesdecreasedduringthefillingstage.
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2017-11-21
KevinDormaConsulting,Ltd.
OTSGWaterSideDynamics
Themeasured steamqualitydrops,buttheactualqualityincreasesandmaysuperheat.Thetrustedmeasurementisthedrumlevel.Levelcrashesabout1m,thenrisesslowly.
Anotherkeyobservation.Someoperatorshaveseensuperheatassociatedwiththis.Difficulttocorrelatesuperheattripwithadropinwatertemperaturebecausea) Thisisnotintuitive,b) The5minutetimedelaymakesitdifficulttoseetheconnection.
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2017-11-21
KevinDormaConsulting,Ltd.
OTSGWaterSideDynamics
• A dropintemperaturecouldleadtodryout.Thisneedstobeavoided.• Payattentiontoproceduresforstartingpumps.• MostOTSGhavefairlytightpassflowdeviationtrips.• Therevisedpumpstartmethodavoidspassflowtrip,butpotentiallydamaging
dryout.• Anormalpumpstartwouldcausechangeinwaterflow(trip),butnochangein
temperature.• Softening thepassflowdeviationtripwouldpermitthenormalpumpstart,and
avoiddryout.
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2017-11-21
KevinDormaConsulting,Ltd.
OTSGWaterSideDynamics
AsmallOTSGhas150Cfeedwater,andgenerateswetsteamat7,000kPaa.Feedwaterrateis70,000kg/hr,Heatinputfromfiringis125GJ/hr.Tubevolumeis9m3,with4tubepasses.
1) Whatisthewaterinletenthalpy?Whatistheenthalpyofboilingwater(0%quality),andsaturatedsteam(100%quality).Sketchonagraphofenthalpyvstotaltubevolume.Usesteamtables.
2) Whatisoutletsteamquality?Assumetheheatisaddeduniformlyalongthetube.Drawtheoperatingline.
3) Whatistheliquidfilledvolume?Howmuchtimeisneededforwatertotravelfromtheinlettotheboilinglocation.Thisisthetimeneededtoreachsteadystate.
4) Firingrateisincreasedfrom1125GJ/hto140GJ/h/Whatisnewsteamquality?Showthenewoperatinglineandsaturationline.Whatisthenewliquidfilledvolume?HowmuchwatermasswasgainedorlostbythetimetheOTSGlinesout?
5) AssumethewatermassisremoveduniformlywhiletheOTSGlinesout.Whatisthetotaloutletmassflowofwater+steam duringthetransient?DoestheVenturi DPincreaseordecrease,andwhy?WilltheestimatedsteamqualityreadafalseHIGHorafalse LOW?
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2017-11-21
KevinDormaConsulting,Ltd.
OTSGWaterSideDynamics