Somesealsandgaskets1.oring2.fiberwasher3.papergaskets4.cylinderheadgasket

Polytetrafluoroethylene(PTFE)gasket

GasketFromWikipedia,thefreeencyclopedia

Thisarticleisaboutmechanicalseals.Forotheruses,seeGasket(disambiguation).

Agasketisamechanicalsealwhichfillsthespacebetweentwoormorematingsurfaces,generallytopreventleakagefromorintothejoinedobjectswhileundercompression.

Gasketsallow"lessthanperfect"matingsurfacesonmachinepartswheretheycanfillirregularities.Gasketsarecommonlyproducedbycuttingfromsheetmaterials.

Gasketsforspecificapplications,suchashighpressuresteamsystems,maycontainasbestos.However,duetohealthhazardsassociatedwithasbestosexposure,nonasbestosgasketmaterialsareusedwhenpractical.

Itisusuallydesirablethatthegasketbemadefromamaterialthatistosomedegreeyieldingsuchthatitisabletodeformandtightlyfillsthespaceitisdesignedfor,includinganyslightirregularities.Afewgasketsrequireanapplicationofsealantdirectlytothegasketsurfacetofunctionproperly.

Some(piping)gasketsaremadeentirelyofmetalandrelyonaseatingsurfacetoaccomplishtheseal;themetal'sownspringcharacteristicsareutilized(uptobutnotpassingy,thematerial'syieldstrength).Thisistypicalofsome"ringjoints"(RJ)orsomeothermetalgasketsystemssuchasthosemadebyGrayloc(anOceaneeringInternationalcompany).ThesejointsareknownasRconandEconcompressivetypejoints.[1]

Contents

1Properties1.1Gasketdesign1.2Sheetgaskets1.3Solidmaterialgaskets1.4Spiralwoundgaskets1.5Constantseatingstressgaskets1.6Doublejacketedgaskets1.7Kammprofilegaskets1.8Flangegasket

2Improvements3Reasonsforfailure

3.1Unevendistributedpressingforce3.2Stressrelaxationandtorqueloss3.3Surfacenotsmooth

4Seealso

Compressedfibergasket

4Seealso5Sources6Externallinks

Properties

Gasketsarenormallymadefromaflatmaterial,asheetsuchaspaper,rubber,silicone,metal,cork,felt,neoprene,nitrilerubber,fiberglass,polytetrafluoroethylene(otherwiseknownasPTFEorTeflon)oraplasticpolymer(suchaspolychlorotrifluoroethylene).

Oneofthemoredesirablepropertiesofaneffectivegasketinindustrialapplicationsforcompressedfibergasketmaterialistheabilitytowithstandhighcompressiveloads.Mostindustrialgasketapplicationsinvolveboltsexertingcompressionwellintothe14MPa(2000psi)rangeorhigher.Generallyspeaking,thereareseveraltruismsthatallowforbestgasketperformance.Oneofthemoretriedandtestedis:"Themorecompressiveloadexertedonthegasket,thelongeritwilllast".

Thereareseveralwaystomeasureagasketmaterial'sabilitytowithstandcompressiveloading.The"hotcompressiontest"isprobablythemostacceptedofthesetests.Mostmanufacturersofgasketmaterialswillprovideorpublishtheresultsofthesetests.

Gasketdesign

Gasketscomeinmanydifferentdesignsbasedonindustrialusage,budget,chemicalcontactandphysicalparameters:

Sheetgaskets

Thepremiseissimpleinthatasheetofmaterialhasthegasketshape"punchedout"ofit.Thisleadstoaverycrude,fastandcheapgasket.Inprevioustimesthematerialwascompressedasbestos,butinmoderntimesafibrousmaterialsuchashightempgraphite(http://macrosealinc.com/hightemperaturegraphitegasketmaterials/)isused.Thesegasketscanfillmanychemicalrequirementsbasedontheinertnessofthematerialusedandfitmanybudgetaryrestraints.Commonpracticepreventsthesegasketsfrombeingusedinmanyindustrialprocessesbasedontemperatureandpressureconcerns.

Solidmaterialgaskets

Theideabehindsolidmaterialistousemetalswhichcannotbepunchedoutofsheetsbutarestillcheaptoproduce.Thesegasketsgenerallyhaveamuchhigherlevelofqualitycontrolthansheetgasketsandgenerallycanwithstandmuchhighertemperaturesandpressures.Thekeydownsideisthatasolidmetalmustbegreatlycompressedinordertobecomeflushwiththeflangeheadandpreventleakage.Thematerialchoiceismoredifficult;becausemetalsareprimarilyused,process

contaminationandoxidationarerisks.Anadditionaldownsideisthatthemetalusedmustbesofterthantheflangeinordertoensurethattheflangedoesnotwarpandtherebypreventsealingwithfuturegaskets.Evenso,thesegasketshavefoundanicheinindustry.

Spiralwoundgaskets

Spiralwoundgasketscompriseamixofmetallicandfillermaterial.Generally,thegaskethasametal(normallycarbonrichorstainlesssteel)woundoutwardsinacircularspiral(othershapesarepossible)withthefillermaterial(generallyaflexiblegraphite)woundinthesamemannerbutstartingfromtheopposingside.Thisresultsinalternatinglayersoffillerandmetal.Thefillermaterialinthesegasketsactsasthesealingelement,withthemetalprovidingstructuralsupport.

Thesegasketshaveproventobereliableinmostapplications,andallowlowerclampingforcesthansolidgaskets,albeitwithahighercost.[2](http://www.bing.com/images/search?q=spiral+wound+gaskets&id=8C264F3FFBAA6AC593C155D7075E811633CDF20F&FORM=IQFRBA)

Constantseatingstressgaskets

Theconstantseatingstressgasketconsistsoftwocomponents;asolidcarrierringofasuitablematerial,suchasstainlesssteel,andtwosealingelementsofsomecompressiblematerialinstalledwithintwoopposingchannels,onechanneloneithersideofthecarrierring.Thesealingelementsaretypicallymadefromamaterial(expandedgraphite,expandedpolytetrafluoroethylene(PTFE),vermiculite,etc.)suitabletotheprocessfluidandapplication.Constantseatingstressgasketsderivetheirnamefromthefactthatthecarrierringprofiletakesflangerotation(deflectionunderboltpreload)intoconsideration.Withallotherconventionalgaskets,astheflangefastenersaretightened,theflangedeflectsradiallyunderload,resultinginthegreatestgasketcompression,andhighestgasketstress,attheoutergasketedge.

Sincethecarrierringusedinconstantseatingstressgasketstakethisdeflectionintoaccountwhencreatingthecarrierringforagivenflangesize,pressureclass,andmaterial,thecarrierringprofilecanbeadjustedtoenablethegasketseatingstresstoberadiallyuniformacrosstheentiresealingarea.Further,becausethesealingelementsarefullyconfinedbytheflangefacesinopposingchannelsonthecarrierring,anyinservicecompressiveforcesactingonthegasketaretransmittedthroughthecarrierringandavoidanyfurthercompressionofthesealingelements,thusmaintaininga'constant'gasketseatingstresswhileinservice.Thus,thegasketisimmunetocommongasketfailuremodesthatincludecreeprelaxation,highsystemvibration,orsystemthermalcycles.Thefundamentalconceptunderlyingtheimprovedsealabilityforconstantseatingstressgasketsarethat(i)iftheflangesealingsurfacesarecapableofattainingaseal,(ii)thesealingelementsarecompatiblewiththeprocessfluidandapplication,and(iii)thesufficientgasketseatingstressisachievedoninstallationnecessarytoaffectaseal,thenthepossibilityofthegasketleakinginserviceisgreatlyreducedoreliminatedaltogether.

Doublejacketedgaskets

Doublejacketedgasketsareanothercombinationoffillermaterialandmetallicmaterials.Inthisapplication,atubewithendsthatresemblea"C"ismadeofthemetalwithanadditionalpiecemadetofitinsideofthe"C"makingthetubethickestatthemeetingpoints.Thefillerispumpedbetweentheshellandpiece.Wheninusethecompressedgaskethasalargeramountofmetalatthetwotipswherecontactismade(duetotheshell/pieceinteraction)andthesetwoplacesbeartheburdenof

Copperflangegasketsusedforultrahighvacuumsystems

sealingtheprocess.Sinceallthatisneededisashellandpiece,thesegasketscanbemadefromalmostanymaterialthatcanbemadeintoasheetandafillercanthenbeinserted.Thisisaneffectiveoptionformostapplications.

Kammprofilegaskets

Kammprofilegasketsareusedinmanyoldersealssincetheyhavebothaflexiblenatureandreliableperformance.Kammprofilesworkbyhavingasolidcorrugatedcorewithaflexiblecoveringlayer.Thisarrangementallowsforveryhighcompressionandanextremelytightsealalongtheridgesofthegasket.Sincegenerallythegraphitewillfailinsteadofthemetalcore,Kammprofilecanberepairedduringlaterinactivity.Kammprofilehasahighcapitalcostformostapplicationsbutthisiscounteredbylonglifeandincreasedreliability.

Flangegasket

Aflangegasketisatypeofgasketmadetofitbetweentwosectionsofpipethatareflaredtoprovidehighersurfacearea.

Flangegasketscomeinavarietyofsizesandarecategorizedbytheirinsidediameterandtheiroutsidediameter.

Therearemanystandardsingasketforflangesofpipes.Thegasketsforflangescanbedividedinmajor4differentcategories:

1. Sheetgaskets2. Corrugatedmetalgaskets3. Ringgaskets4. spiralwoundgaskets

Sheetgasketsaresimple,theyarecuttosizeeitherwithboltholesorwithoutholesforstandardsizeswithvariousthicknessandmaterialsuitabletomediaandtemperaturepressureofpipeline.

RinggasketsalsoknownasRTJ.Theyaremostlyusedinoffshoreoilandgaspipelinesandaredesignedtoworkunderextremelyhighpressure.Theyaresolidringsofmetalindifferentcrosssectionslikeoval,round,octagonaletc.Sometimestheycomewithholeincenterforpressure.

SpiralwoundgasketsarealsousedinhighpressurepipelinesandaremadewithstainlesssteelouterandinnerringsandacenterfilledwithspirallywoundstainlesssteeltapewoundtogetherwithgraphiteandPTFE,formedinVshape.InternalpressureactsuponthefacesoftheV,forcingthegaskettosealagainsttheflangefaces.

Improvements

Manygasketscontainminorimprovementstoincreaseorinferacceptableoperatingconditions:

Acommonimprovementisaninnercompressionring.Acompressionringallowsforhigherflangecompressionwhilepreventinggasketfailure.Theeffectsofacompressionringareminimalandgenerallyarejustusedwhenthestandarddesignexperiencesahighrateoffailure.

Acommonimprovementisanouterguidingring.Aguidingringallowsforeasierinstallationandservesasaminorcompressioninhibitor.InsomealkylationusesthesecanbemodifiedonDoubleJacketedgasketstoshowwhenthefirstsealhasfailedthroughaninnerliningsystemcoupledwithalkylationpaint.

Reasonsforfailure

Unevendistributedpressingforce

Unevenpressingforceiscausedbyavarietyoffactors,firstisthehumanfactor:asymmetricconstructionofthepreloadbolt,thisfactorcaneliminateconstruction;theoryontheflangepressed,thesealingsurfaceisabsolutelyparalleltothepractice,however,thecenterlineofapipelinecannotbeabsolutelyconcentric,andthustightentheboltsontheflangemoment,sothattheflangediscontinuity.Asymmetricalconnection,thesealingsurfacemoreorlessdeformed,sothatsealedthepressingforceisreduced,therunningload,pronetoleakage.Third,thedensityofboltarrangementonthepressuredistributionmoreobviousimpact,thecloserthebolts,themoreuniformthepressure.

Stressrelaxationandtorqueloss

Tightenboltsontheflange,duetothevibrationofthebodies,thetemperatureincreasedordecreasedandotherfactors,theworkingprocessofthespiralwoundgasketsstressrelaxation,thebolttensionwillbegraduallydecreased,resultinginlossoftorque,causingaleak.Ingeneral,longbolts,theremnantsofthetorque,thesmallerthediameterthemoreadvantageoustopreventthelossoftorque,withlong,thinboltisaneffectivewaytopreventtorqueloss.Heatingacertainperiodoftimetomakeitstretchthebolt,andthentomaintainagiventorque,isveryeffectivetopreventthelossoftorque.Thereisagasketisthinnerandsmallerthelossoftorque.Inadditiontopreventthestrongvibrationofthemachineandthepipeitself,andexcludetheimpactofadjacentequipmentvibration,theimpactofthesealingsurfaceisnotmeaningless,nottobeattheboltstightened,canpreventthelossoftorque.

Surfacenotsmooth

Itisimportanttomakethesealingfinishproperlyotherwiseitwillcauseleakage.

Seealso

OringOzonecrackingPolymerdegradationVacuumflangeWasher(mechanical)

Sources

1. [1](http://www.oceaneering.com/subseaproducts/grayloc/)

1. Bickford,JohnH.:AnIntroductiontotheDesignandBehaviorofBoltedJoints,3rded.,MarcelDekker,1995,pg.5

2. Latte,Dr.JorgeandRossi,Claudio:HighTemperatureBehaviorofCompressedFiberGasket

Materials,andanAlternativeApproachtothePredictionofGasketLife,FSApresentedPaper,1995,pg.16

Externallinks

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Categories: Seals(mechanical) Enginetechnology

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