otsg water side dynamics 2017-11-21 - kevindorma.ca€™s begin with a quick review of drum...

PhDinFluidMechanics(modelling)18yearsasaconsultant,10yearsexperiencewithSAGD.Someunderstandingofdrumboileroperationandcontrol.ExtensivetroubleshootingofSAGDwaterhammerissuesProblemsoriginatefromtheOTSG:Why?

UnderstandtherootcauseofoperatingproblemsUnderstandOTSGdynamicsbyapplyinggoodmodelling

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KevinDormaConsulting,Ltd.

OTSGWaterSideDynamics

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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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Anothercommondisturbanceisasuddendropinthefeedwater temperature.Thiscausesasuddenquenchingofthesteamdrum,andinterruptsthesteamproduction.Withasuddendropofsteamflowtothesuperheatersection,thesteamtemperatureexitingthesuperheater canbecomeveryhigh.

Thisunderstandingofdrumleveldynamicshasleadtostandard,sophisticatedcontrolsystemstomanagedrumlevelduringfiringratechanges

SingleelementTwoelementThreeelement

Theresultisthatdrumboilersareveryreliableanddonotexperiencespurioustrips.Adrumboilerisexpectedtotripmaybeonceayear,andthisisduetoanactualmechanicalorinstrumentfailure,andmustbefixed.

OTSGWaterSideDynamics 2017-11-21

KevinDormaConsulting,Ltd. 4

Thiscouldarise ifawell-meaningProcessControlpersonsays“Icanmakethatfaster”.

Suddendecreasein steamdemandcausespressuretoincrease.Lessboilingintubes.Morewaterheldupinrisers.Drumleveldrops.Thesuperfastlevelcontroladdsabigshotofcoldwaterandquenchesthedrum.Killthesteamproduction.Lesssteamthroughsuperheater,cookthesuperheater.

Thisiswhyweneedtounderstandwhenthingsareoperatingproperly.

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Reallyawful water.Looksliketea.

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4or6passiscommon250MMBTU,150t/hwaterrateiscommonforthelargeunits.70MMBTU/hforthesmallerunitsaroundLloydminster.

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IhaveheardafewcommonfrustrationsinSAGDcontrolrooms.OperationsteamsfeelthatOTSGsseemtodosomeweirdthings.Someteamshaveseensafetyvalvesroutinelyliftonstartup.Othershaveseentheoddtriponsuperheat,butthemeasuredsteamqualitywaslow.MostoftheOperationsteamshaveexperiencewithdrumboilers.Ihaveheardthatdrumboilersareconsideredtobemorereliable.

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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).



Ingeneral,ifsomethingcausesthemassflowoutofthetubetobedifferentfrommassflowin(or,there iswateraccumulation)thenthesteamqualityventuri readsfalse.

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Highlightthewater volumeintheOTSG.ThisisTHEmostimportantconceptforanOTSG.Thetimeneededtogothroughtheliquidsectionasthesecondmostimportantconcept.

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Highlighthowtheliquidfilledvolume changes.

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Explainthateveryonegoesonestepforward,oneover.Crossthesaturationlineandexpand.

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WelcometoLecture 2.IwillshowyouwhatgoesoninsidetheOTSGtubes,andsharesomeobservations.

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Nofaceshot

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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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OTSGstartupwithoutthehugerushofwater.Higherwaterflowrate,crosstheboilingpointfurtherdownthetubes.Lesswateriskickedoutwheneverythingboils.

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The steadystateflowrateoutishigher,butthepeakismuchsmaller.ThestartupcontrolvalvewillhaveamuchbetterchanceofprovidingenoughcapacitytokeeptheOTSGpressurebelowthePSVsetpressure.• CommonpracticeistostarttheOTSGathigherflowrates

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.

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ThisisSLOW

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• SuddendropinBFWflowratecausesoutletenthalpytoincreaseslowlyThesteamqualitymeasurementreadsFALSEfor5- 6minutesThislooksalotmoreseriousthanitreallyis.SpuriousHHqualitymeasurementmaycausefrequentfiringtrips

ConsidersofteningHHqualitytripsorHHpassflowdeviation

Increaseinfiringrateisnearlyidenticalforchangeinsteamquality

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A suddendropinwaterflowrateisnotahazardaslongastheflowcontrollercantakeaction.Nodifferentthanlevelcontrolinadrumboiler.LevelcontrollersetstheBFWflowsetpoint.AblipintheBFWheaderismanagedbecausetheBFWflowiskeptontrack.

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MyYouTubelecturedemonstrateswhyapumpstartchangesthefeedwatertemperature.

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Startwithabout150C feedtemperature,or600kJ/kgenthalpy.Thenwewatchahotfrontmovethrough.Thehotfrontspreadsoutbecauseofthethermalmassofthepiping.Steelhastowarmup.

Waitforthehotfronttobarelytouchtheboilingline.Whatwillhappennext?

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Theactualsteamqualitydropsbecausea largemassofwaterispushedout.ThemeasuredqualityincreasesbecausethemeasuredDPbecomeshighforashorttime.Thelevelintheseparatorjumpsup,andthenslidesdown.

ThiswasthekeyobservationthatconfirmedwhatisgoingoninsidetheOTSG.A10Criseinthewatertemperaturecausedtheseparatorleveltohopupbyameter,thenslidedown.

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Nofaceshot.There isabout8m3ofwatertostartwith.Thenthereis10m3ofwater.Inbetween,theOTSGfilledwithwater.Thismeansthatthewaterflowratedownthetubesdecreasedduringthefillingstage.

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Themeasured steamqualitydrops,buttheactualqualityincreasesandmaysuperheat.Thetrustedmeasurementisthedrumlevel.Levelcrashesabout1m,thenrisesslowly.

Anotherkeyobservation.Someoperatorshaveseensuperheatassociatedwiththis.Difficulttocorrelatesuperheattripwithadropinwatertemperaturebecausea) Thisisnotintuitive,b) The5minutetimedelaymakesitdifficulttoseetheconnection.

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• A dropintemperaturecouldleadtodryout.Thisneedstobeavoided.• Payattentiontoproceduresforstartingpumps.• MostOTSGhavefairlytightpassflowdeviationtrips.• Therevisedpumpstartmethodavoidspassflowtrip,butpotentiallydamaging

dryout.• Anormalpumpstartwouldcausechangeinwaterflow(trip),butnochangein

temperature.• Softening thepassflowdeviationtripwouldpermitthenormalpumpstart,and

avoiddryout.

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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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otsg water side dynamics 2017-11-21 - kevindorma.ca€™s begin with a quick review of drum...

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