flange_gasket_basics.pdf

7/28/2019 Flange_Gasket_Basics.pdf

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ProductLiteratureSheet RP11/2004

Central Plastics Company39605 IndependenceShawnee, OK 74801PH: 800.654.3872FX: 800.733.5993www.centralplastics.com

Flange InsulationGasket Basics

Why Gaskets Are Used:Gaskets are used to create a static seal between two stationary members of a

mechanicalassemblyandtomaintainthatsealunderoperatingconditionswhichmayvarydependent upon changes in pressures and temperatures. If it werepossible tohaveperfectlymated flangesand ifitwerepossibletomaintainanintimatecontactofthoseperfectlymatedflangesthroughouttheextremesofoperatingconditions,agasketwouldnotberequired.Thisisvirtuallyanimpossibilitybecauseof: 1. thesizeofthevesseland/ortheflanges2. the difficulty in maintaining such extremely smooth flange finishes during

handlingandassembly3. corrosionanderosionoftheflangesurfacesduringoperations.

As a consequence, relatively inexpensive gaskets are used to provide the sealingelementinthesemechanicalassemblies.Inmostcases,thegasketprovidesasealbyexternal forces flowing the gasketmaterial into the imperfectionsbetween thematingsurfaces. It follows then, that in a properly designed gasket closure, three majorconsiderationsmustbetakenintoaccountinorderforasatisfactorysealtobeachieved. 1. Sufficientforcemustbeavailabletoinitiallyseatthegasket.Statingthisanother

way,adequatemeansmustbeprovidedtoflowthegasketintotheimperfectionsinthegasketseatingsurfaces.

2. Sufficient forcesmustbeavailable tomaintaina residual stresson the gasketunderoperatingconditionsto ensurethat the gasketwillbe in intimatecontactwiththegasketseatingsurfacestopreventblow-byorleakage.

3. The selection of the gasket material must be such that it will withstand thepressuresexertedagainstthegasket,satisfactorilyresisttheentiretemperaturerangetowhichtheclosurewillbeexposedandwithstandcorrosiveattackoftheconfinedmedium.


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Effecting A Seal:A seal is effected by compressing the gasketmaterial and causing it to flow into theimperfectionsonthegasketseatingsurfacessothatintimatecontactismatedbetweenthegasketandthegasketseatingsurfacespreventingtheescapeoftheconfinedfluid.Basically there are four different methods that may be used either single or incombinationtoachievethisunbrokenbarrier. 1. Compression.Thisisbyfarthemostcommonmethodofeffectingasealona

flangejointandthecompressionforceisnormallyappliedbybolting.SeefigureA.

2. Attrition. Attrition is a combination of a dragging action combined withcompressionsuchasinasparkpluggasketwherethesparkplugisturneddownonagasketthatisbothcompressedandscrewedintotheflange.SeefigureB.

3. By Heat,suchasinthecaseofsealingabellandspigotjointoncastironpipebymeansofmoltenlead.Note,however,thatafterthemoltenleadispoured,itistampedintoplaceusingatampingtoolandahammer.

4. Gasket lip expansion. This is a phenomenon that would occur due to edgeswellingwhenthegasketwouldbeaffectedbyconfinedfluid,asinthecaseofelectrometriccompoundsaffectedbyconfinedfluids,suchassolvents,causingthegasketmaterialtoswellandincreasetheinteractionofthegasketagainsttheflangefaces.

Ingeneral,itisconsideredthatonlythegasketsurfaceprovidesthesealinthejoint.Itisassumedthatthegasketmaterialisimperviousandwillnotpermitanypassageoffluidthroughthegasketseal.


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Gasket Seating:There are twomajor factors to be consideredwith regard togasketseating. The firstis thegasketmaterialitself.TheASME *UnfiredPressureVesselCodeSectionVIII,Division1definesminimum design seating stresses for a variety of gasket materials. These design seatingstresses range from zero psi for so-called self-sealing gasket types such as durometerelastomersandO-Ringsto26,000psitoproperlyseatsolidflatmetalgaskets.Betweenthesetwoextremes thereareamultitudeofmaterialsavailable tothedesignerenablinghim/her tomakeaselectionbaseduponthespecificoperatingconditionsunderinvestigation.Forces Acting on a Gasket Joint::

1. The Internal Pressure: These are the forces continually trying to unseal agasketed jointbyexerting pressure against the gasket (blowoutpressure) andagainsttheflangesholdingthegasketinplace(hydrostaticendforce).

2. The Flange Load:Thetotalforcecompressingthegaskettocreateaseal,i.e.theeffectivepressureresultingfromboltloading. 3. Temperature: Temperature creates thermo-mechanical effects, expanding or

contracting the metals, affecting the gasket material by promoting creeprelaxation which is a permanent strain or relaxation quality of many soft

* ASME American Society of Mechanical Engineers


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materials under stress. The effect of certain confined fluids may become

increasingly degrading as temperature rises and attack upon organic gasketmaterialsissubstantiallygreaterthanattheambienttemperatures(about75oF).Asarule,thehigherthetemperature,themorecriticaltheselectionofthepropergasket.

4. Medium: Theliquidorgasagainstwhichthegasketistoseal. 5. General Conditions: The type of flange, the flange surfaces, the type of boltmaterial,thespacingandtightnessofthebolts,etc.

Eachofthesefactorsrequireconsiderationbeforeaneffectivegasketmaterialisfinally

chosen. However, thepropergasketmayoftenberejectedbecause failure occurredduetopoorlycleanedflangefaceorimproperbolting-uppractice.Thesedetailsrequirecarefulattention,butifcompliedwith,willhelpeliminategasketblowoutorfailure.Therearethreeprincipalforcesactingonanygasketedjoint.Theyare: 1. Bolt Loadand/orothermeansofapplyingtheinitialcompressiveloadthatflows

thegasketmaterialintosurfaceimperfectionstoformaseal. 2. The hydrostatic end force, that tends to separate flangeswhen the system ispressurized.

3. Internal pressureactingontheportionofthegasketexposedtointernalpressure,tending to blow the gasket out of the joint and/or to bypass the gasket underoperatingconditions.

There are other shock forces that may be created due to sudden changes intemperatureandpressure.Creeprelaxationisanotherfactorthatmaycomeintothepicture.Theinitialcompressionforceappliedtoajointmustserveseveralpurposes. 1. It must be sufficient to initially seat the gasket and flow the gasket into the

imperfectionsonthegasketseatingsurfacesregardlessofoperatingconditions.

2. Initial compression force must be great enough to compensate for the totalhydrostaticendforcethatwouldbepresentduringoperatingconditions.

3. Itmustbesufficienttomaintainaresidualloadonthegasket/flangeinterface.Fromapracticalstandpoint,residualgasketloadmustbeXtimesinternalpressureifatightjointistobemaintained.TheunknownquantityXiswhatisknownasthem


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factorintheASMEUnfiredPressureVesselCodeandwillvarydependinguponthe typeof

gasketbeingused.Actuallythemvalueistheratioofresidualunitstress(boltloadminushydrostaticendforce)ongasket(psi)tointernalpressureofthesystem.Thelargerthenumberusedform,themoreassurancethedesignerhasofobtainingatightjoint. Selecting the Proper Gasket Material:The optimum gasket material would have the following characteristics. It would have thechemicalresistanceofPTFE,theheatresistanceofgraphite,thestrengthofsteel,requirezeroseatingstresssuch assoft rubberand be inexpensive. Obviously there isnoknowngasketmaterialthathasallthesecharacteristicsandeachmaterialhascertainlimitationsthatrestrictitsuse.Itispossibletoovercomelimitationspartiallybyseveralmethodssuchasincludingthe

useofreinforcinginserts,combiningitwithothermatrials,varyingtheconstructionordensity,orbydesigningthejointitselftoovercomesomeofthelimitations.Obviously,mechanicalfactorsare important in the design of the joint but the primary selection of a gasket material isinfluencedbythreefactors. 1. the temperature of the fluid or gas to be contained2. the pressure of the fluid or gas to be contained3. the corrosive characteristics of the fluid or gas to be contained.Some of the factors that can influence the corrosion resistance of a particular material atoperatingconditionsare:

1. Concentration of the corrosive agent. (Full strength solutions are not necessarily morecorrosive than those of dilute proportions and, of course, the reverse is also true.)2. The purity of the corrosive agent. For example, dissolved oxygen in otherwise purewater may cause rapid oxidation of steam generation equipment at high temperatures.3. The temperature of the corrosive agent. In general, higher temperatures of corrosiveagents will accelerate corrosive attack.

Asaconsequence,itisoftennecessarytofield-testmaterialsforresistancetocorrosionundernormal operating conditions to determine if the material selected will have the requiredresistancetocorrosion.

Environmental Considerations:Obviously,materialselectiondependsonthetotalenvironmentalspectruminwhichnaygivensealwillperform.Thoroughanalysisofsealfunctionshouldalwaysbeginwithconsiderationofthefour major environmental conditions.


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1. Media Alwaysconsiderallmediathatmaycomeincontactwiththesealandretainer.Example: ifthesystem is to be cleaned or purgedperiodically, be sure toanticipatewhatcleaning fluidswillbeused. These secondary fluidswillplayasmucha roleinselectingthemostcompatiblecompoundaswilltheprincipleoperatingmedia.

2. Pressure Properretainerdesigncoupledwithasoundchoiceofmaterialswillprovideamolded elastomer seal capable of withstanding extremely high pressures (e.g. somemoldedelastomersealshavebeentestedover100,000psi).Similarly,reliablemoldedelastomersealsconfigurationscanbeachievedforextremelylowpressurelevelsthatis,ultrahardvacuumsdowninthe10 -9range.Itisessentialthatcompletepressuredatabesubmittedwithallinquirestoassureoptimumsealdesign.Thisincludesmaximumandminimumpresusures,proofpressures,burstpressure,cyclingconditionsandpulsations. Note: Extremely high pressure situations demand adequately structured matingassemblies to maintain metal to metal contact and prevent extrusion of the elastomer.

3. Temperature Temperature, though seemingly elementary is often the mostmisunderstoodandexaggeratedof theenvironmental parameters. Hence, it isalltoooftenOVERSPECIFIED.Itshouldalwaysbeborninmindthattemperatureatthesealitselfmayvarywidelyfromtheambientcondition(sometimesbyhundredsofdegrees). For example: The seal surroundingawater coolingportmayneverget over200oF,whereasthetemperatureinacombustionchamberamereawaymayeasilyexceed1,000o F! Low temperature performance, on the other hand, involves a somewhatdifferent considerationnamely the inherent flexibility of the chosen material and itsabilitytomaintainanadequatelineofcontact.

4. Time The fourthmajor determinant inmaterial selection is time. Brief temperatureexcursionsup toa temperature level higher than those recommended in thematerialspecifications are often tolerable. Example: The standard Buna-N (Nitrile) isrecommendednormallyforserviceupto225oF.However,Nitrilecanwithstandamuchhighertemperatureforbriefperiodsoftime.

Types Of Gaskets:Type E Full Face Gaskets

Fullfacegasketscompletelycovertheflangefacefromflangebedtooutsidediameter.Fullfacegasketsareusedonflatfacedflanges,however,canbeusedwitharaisedflatfaceflangetoeliminatedebrisbuildingupinraisedareacausingashort.


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Type E Flange and Gasket

Type F Ring GasketRingGasketsaredesignedtosetinsidetheboltcircleandaremostcommonlyusedwithraisedfaceflanges.

Type F Flange and Gasket


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Type D Ring Type Joint (RTJ) The RTJ is an oval phenolic gasket designed towork specifically withRTJ grooved

Flanges.RTJgasketsaresizedbyR-number.AneoprenefillergasketissometimesusedtosetontheI.D.oftheRTJgaskettotheI.D.oftheflangebore.Thisistoeliminatedebrisbridgingacrossandcausingashort.TheRTJsaremadefromplatesofphenolic,therefore,thegaspressurewouldbeparalleledtothegrainallowingtheRTJtoworkwithpressuresupto2,500PSI.

RTJ Gasket

Type D BXBXgasketsareanoctagonalshapephenolicgasketdesignedtoworkspecificallywitha

BXgroovedflange.BXgasketsaremachinedfromphenolictubing;therefore,thegaspressurewouldbeperpendiculartothegrain,allowingthegaskettowithstandpressuresupto15,000PSI.BXgasketsaresuppliedwithanasbestos-fillgasketwhichisplacedfromtheO.D.oftheBXtoinsidetheboltcircle.

BX Gasket


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Jock O-Ring TypeThe Jock O-Ring gasket can be a full face (Type E) or ring gasket (Type F)configuration.TheJockhasahigh-energyO-Ringelastomersealthatcompressesandexpands to fill anyvoidsas the flangeis tightened. The totallyencapsulatedsealingelementpreventsgasketblow-out. TheO-Ringprovidesapositive,constantsealwithzero environmental leakage. Complete contact of flange and retainer prevents firefailureandreducesexposureofthesealtoservicefluidsandenvironment.TheconfinedO-Ringsealallowszeromandyfactorstobeachieved.Onspecialrequest,theJockgasketcanreplacemostsizesofRTJandBXgaskets.With theversatilityofchangingthesealingelementortheretainermaterial,the Jockcanbeutilizedinmostanyapplication.

Jock Gasket

Insulating WashersCentrals standard insulating washers are manufactured from CE Phenolic. Themaximumoperatingtemperatureisapproximately225oFwithacompressionstrengthof36,400 PSI and a dielectric strength of 300 VPM. Our high temperature insulatingwasherhasamaximumoperatingtemperatureof450 oF.

Insulating Sleeves


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Centraloffersinsulatingsleevesinavarietyofmaterials.Ourmoststandardinclude

Mylar,HighDensityPolyethylene,PhenolicandNomex.Theyaredesignedtoeasilybeinsertedintheboltholeleavingampleroomforthebolt,evenwhentheboltholesaremisaligned.Thewallthicknessis1/32andcanbeprovidedforboltsizesfromto3-1/2.

Molded One-Piece Sleeve & WasherTheone-piecesleeveandwasher reduces handling (many looseparts)andmake-upproblemsinthefield.Oneglanceandaninspectorcaneasilytelliftheflangehasbeenproperlyinsulated.AMinlonmaterialshouldbeusedbecauseof itshighcompressionstrengthandsuperiortoughness.Aone-piecesleeveandwasherismostgenerallyusedforone-sideflangeinsulationinthatwemolditinasleevelength,thatisonanaverage,significantlylongerthanthethicknessofasingleflange.Theycanbecuttosizefortwo-sideinsulation.Themaximumoperatingtemperatureis250 oF,dielectricstrength420VPMandcompressionstrength19,000PSI.

flange_gasket_basics.pdf

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