pump zone article 1.docx
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pump zone article 1.docx
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pumpzonearticle1
Gravity…amostreliablepartner
By
GarySugar,SeniorRotatingEquipmentSpecialist;ScotTroxell,StaffMaintenanceEngineerandFranKludt,AssociateRotatingEquipmentEngineer,CelanesePampa,TommyThomas,SouthernRegionMechanicalSealProductManagerandChrisKowalski,ConsultingEngineer,A.W.ChestertonCo.,withBradWilson
Thesealingoftemperature-sensitivepolymerizingliquidsintraditionalANSIpumpspresentedmanyuniquedifficultiesthatledtoalonghistoryofunsatisfactoryperformanceatoneTexaschemicaloperation.Thesedifficultiesseemedtodefysolutionasmanyvarietiesofsealsandarrangementswereunsuccessfullyapplied.Thepumpedfluidwasafinishedproduct(makingaflushunacceptable),andtemperature-inducedpolymerizationfoiledbothsealedandseallesssolutions.SuccessfulresolutionfinallywasachievedwiththeinnovativeapplicationofagassealinaverticalANSIpump.Thisarrangementutilizesandcontrolsthenon-ventingcharacteristicsoftheverticalconfigurationtoprovidethe“environment”forthedesiredseallifeandreliability.Today,aftermorethantwoyearsofsuccessfuloperation,thissolutioncontinuestoprovidepaybacktothecompany.
“GoodChemistryintheTopO’Texas”
In1952,totakeadvantageofalargebutanesourcenearby,CelaneseChemicalsestablishedanewplantinPampa,Texas.Locatedabout50milesnortheastofAmarillo,intheTexasPanhandle,thePampaplantusesaliquid-phaseoxidationprocessintheproductionofaceticacidandawidevarietyofacetic-acidderivativesandco-products.Additionally,thefacilityproducesacrylateesters,aswellasmultifunctionalmonomersforanothercompany.
Thereactivityofthesespeciesmakesthemusefulandflexibleintermediates.Inparticular,themultifunctional(morethanoneunsaturated“activesite”permolecule)monomersallowforsolvent-free,“cure-in-place”polymerizationofprintinginks,coatingsandlaminates.Whenirradiatedwithultraviolet(UV)lightinthepresenceofaphotoinitiator(or,alternatively,exposedtoacceleratedelectronbeams),themultipleactivesiteschemicallybondtoeachotherandtootherspecieswithinthemixture,cross-linkingthestructuretogether(ref.1).
Thesamereactivitythatmakesthesematerialssousefultotheirend-userscancauseheadachesfortheirmanufacturers.Withreferencetoshelflifeofthematerials,theHealthandSafetyCommitteeofRadTechInternational,NorthAmerica,said,“Thehigherthetemperature,theshorterthetimeamaterialwillbestable.Tomaintainqualityandavoiduncontrolledpolymerization,avoidhightemperaturesforextendedperiodsoftime”(ref.2).
Manyofthesemonomersreallyseemtowanttobepolymers.Inadditiontothermalinstabilities,theyalsoshowshearsensitivity.Concernsaboutprematurepolymerizationofthesematerialsduringstorageprovidejustatasteofwhatproblemsmayariseinapump.
GoodEnough?
TheproblemareawasinFinishedProductTransfer.Manyvarietiesofpolymerizingfluidswereproducedinbatchoperations,whichmadeforfrequentcycling.Transferringfinished,revenue-producingmaterialsoutoftheBatchHoldingTank,beforethetankwastobeusedforothersuchmaterials,oftenrequiredthatthetankbepumped“dry.”
TheproductsweretransferredfromtheholdingtankwithanANSIpumpfittedwitha“dynamicseal,”thepreferredchoiceofmanyintheindustry.Inthisarrangement,anauxiliaryimpelleror“repeller”isfittedtotheshaftbehindthepumpimpellerinthesealingspace.Thesealingchamberisdesignedsothat,duringoperation,therepellercentrifugallyslingsliquidtotheoutsideofthechamber,providingadynamic“liquidring”attherepellertipsthatsealsthepumpageandpreventsleaks.Airisdisplacedtowardtheshaftandfillstheinteriorofthechamber.Uponshutdown,lossofcentrifugalforceallowsthepumpedfluidtofillthesealingchamberand,byitspressure,seatastaticsealagainstthebackwallofthechamber.
ThedynamicsealingarrangementdidnotprovidethereliabilityandidlesealingperformancedesiredbyCelanese.Eventhoughthearrangementworkedwellwhilethepumpwasrunning,batchproductionrequiredintermittentoperations.Whenthepumpwasshutdownandthepumpagerefilledthesealchamber,itinevitablycameincontactwithhotsurfacesatthestatic-sealface.Asitwentontheseal,itpolymerized.Ifitpolymerized,thesealperformancewasdegraded.Historically,seallifewaslessthandesirableandledtohighmaintenancecosts.
Acrylateestersingeneralproduceastrong,acridodor,detectableatextremelylowconcentrations.Thoughtheypossessamuchlowervaporpressurethanthatoftraditionalsolvents,odorcontrolfortheUV-curablemonomersisalsoadvised.Regulatoryandsafetyconcernsoverthematerialsarelargelylimitedtotheircharacterizationasskin/eyeirritants(ref.s3&4)buttheirpotential(sp.g.~1.1,viscosity~150cP)tosplashandproduceaerosolsrequiredconsideration.
Inatimeofculturalchange,ofchangingdefinitionsastowhatwasacceptable,theindustry-standardchoicewasre-addressed.Itwastimetolookforsomethingbetter.
LimitedOptions
Thesensitivityoftheproductstoshearandhightemperaturesprecludedanyseriousconsiderationofproduct-lubricatedpackingormechanicalseals.Cure-in-placebehaviorisobviouslyinappropriatewhenthe“place”isbetweenmechanical-sealfaces.Likewise,thefactthatthematerialsrepresentfinishedproductsexcludedanyliquid-flushedalternatives.Flushdilutionofmonomersthatwerereadyforshipmentwasunacceptable.
Fromthebeginning,theeffortfocusedongasseals.Theideaofahydrodynamicliftingfilmofgas,preventingdirectcontactbetweenthesealfacesandminimizingfrictionalheatgeneration,wasappealing.Useofagastocoolthesystemwouldeliminateanyworriesaboutdilutingtheproduct.
Two“bubbletheories,”describingnormalgas-sealoperationanddifferingwithrespecttowhathappenedtothegasfilm/flushasitlefttheseal,wereconsidered.Inthefirstview,smallbubbleswerethoughttotrailoffthesealfacesanddirectlyintothesurroundingfluid.Liquid,circulatingincontactwiththeseal,wouldcarryawayheatduringoperation,andthesealwouldclosequicklyduringstarts/stopstoexcludeliquidfrombetweenthefaces.Inthesecondview,gasescapingfrombetweenthesealfaceswasthoughttocreatealargebubblethatenclosedthesealassembly.Ifthiswerethecase,coolingwouldlikelybelesseffective(heatwouldhavetobecarriedawaybygas,asthelargebubbletrailedoffsmallerbubblesintotheprocessstream),butthepumpedfluidwouldnevercontactthesealduringoperation.Eithermodelseemedencouraging,andgassealsshowedagreatdealofpromiseforthischallengingapplication.Itwasworthatry.
IfatFirst…
Inthefirstattempt,apopular-modelgassealwasinstalledonanANSIpump.Thismodelwasconfiguredwithproductontheinsidediameterofthesealfaces.Duringoperation,seal-facegeometryactingongasintroducedattheouterperimetergeneratedtheliftingforcetoseparatethesealfaces.Thesealsdegradedinsevenstraightapplicationattempts,withrun-timesvaryingbetween10minutesandtwodays,andthemanufacturergaveup.
Afterconversionbacktodynamicsealsforaperiodoftime,anothersealmanufacturerofferedtoapplyadifferenttypeofseal.Typicalgasseals,gatheringgasfromtheedgeofthesealbymeansofseal-facegeometry,onlyforcethegasbetweenthefaceswhilethepumpshaftisturning.If,atanypointduringstart-uporshutdown,thepressureofthegasfilmissufficienttoopenthesealfacesbutisnotsufficienttoexcludepumpage,polymerizingfluidscouldenterthespacebetweenthefaces.Reasoningthatthepreviousfailuresmightrelatetotheestablishmentorcollapseofthehydrodynamiclayer,thisnewmanufacturerproposedadualsealintowhichgascanbehydrostaticallyinjectedintoanannularspacebetweenthetwo(concentricandcoplanar)seal-faceareas.Thisshouldmaximizeperformanceofthegasasabarrierfluidaswellasalubricantandcoolant(Figure1).
Withrun-timesbetween2hoursand2weeks,thenewsealconfigurationwasadecidedimprovement,butwasstillnotthedesiredresult.Thistime,however,nobodygaveup.
Concernsaboutprematurepolymerizationofthesematerialsduringstorageprovidejustatasteofwhatproblemsmayariseinapump.
…Try,TryAgain
Allofthetrialsealsindicatedthattheproducthadpolymerizedonandacrossthesealfaces.O-ringswellingandotherproblemswerealsonotedand,withalargenumberofpotentialcontributingfactors,failureanalysiswasdifficult.
Onethingthatdidseemclear,however,wasthatpolymerizingmaterialswerebeingexposedtohightemperatures.Eitherthesealwasgeneratingtoomuchheat,orthesealenvironmentwasnotremovingenoughheat.Fallingbackonthefirstoftheoriginal“bubbletheories,”thesmall-bubblemodelofgas-sealoperation,itseemedreasonabletomodifythesealenvironmenttomaximizeheatdissipation.
Undertheassumptionthatthegas-liquidinterfacewasatorneartheseal,Celaneseandthesealmanufacturerapplieda“LiquidinContact”approachtomodifyingthesealandstuffingbox.Similartooptimizingthecoolingofconventionalliquid-lubricatedseals,thisapproachconsistedofmodifyingthesealandstuffingboxtoassurecompleteliquidcontactaroundthesealbodyandrapidventingofanygasesfromtheimmediatesealenvironment.Thoughstillplanningontheexclusionofliquidsfromthesealfacesthemselves,theteam:
•Modifiedtheseal,openingtheareaaroundtheseal-faceedgestoallowfreecircu
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