January 22, 2021 | Author: Anonymous | Category: N/A
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Troubleshooti roubleshootin n of Mecha Mechanical nical Seals
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3) Amechanicalseal isexpectedtoachieveits isexpectedt oachieveits maximum perf perfor orma manc nce e and and life life when when it is oper operat ated ed with within in its its desi design gn spec specif ific icat atio ions ns. . A seal seal is cons consid ider ered ed to have have failed failed when when leakag leakage e excee exceeds ds envir environ onmen mental tal or plant plant site site operat operating ing limits limits. . The The failur failure e may may occur occur befor before e or afterthesealhasachie afterthesealhasachieveditsdesi veditsdesignedlifeexp gnedlifeexpectancy. ectancy. Afailureof A failureof aseal maybe amajor contributorto rotary equipmentfailureanddowntime. Anunderstandingo Anunderstandingofthemode fthemodeofsealfailu ofsealfailurecanle recanleadto adto extendingthelifeofrotaryequipmentbyimprovingseal designandmaterialsele designandmaterialselection,installationan ction,installationandoperatin doperating g procedures,andenviro procedures,andenvironmentalcon nmentalcontrols. trols.
Thefailureofasealmaybetheresultof: ofsealdesignormaterialsforthe applicationtemperature,pressure,speeds,andfluid properties. of the seal seal compo compone nents nts befor before e instal installat lation ion including including chipping, chipping, scratching scratching, , nicking, nicking, or allowing allowing partstobecomedirty. incl includ udin ing g asse assemb mbly ly, , seal seal setting,orplacingofcompone setting,orplacingofcomponentsinthecham ntsinthechamber. ber. dry running, or failure of environmentalcontrols. operation. of the sealin sealing g fluidwith fluidwith abrasi abrasive ve or corrosivematerials. withexcessiveshaftrunout,deflection, vibration,orwornbearings. The The seal seal achi achiev eved ed its its norm normal al life life expectancy.
A singlestage mechanicalseal mayleak may leakalongone alongone of fourpaths.SeeFigure1.
Figure1.PossibleSealLeakagePoints
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leakageisvisibleatthepointwherethe shafte shaftexit xitst sthe hegl glan ando dora ratth tthed edrai rainc ncon onne necti ction ons. s. 2) leakageisalsovisibleat the pointwher pointwheretheshaft etheshaft exitstheglan exitstheglandor dor atthe drainconnections.
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leakageisalsovisibleat the point point where where the shaftexits shaftexits the gland gland or atthe drainconnections. 4) leakag leakage e is visibl visible e at the gland gland seal seal chamberinterface.
Much Much can can be lea learne rned by a com complete lete docum cumente nted analysisincludingthefollowing: beforeshutdownand record: equipmentinletandoutle equipmentinletandoutletpressures tpressures pumpflowvscurvebestefficiencyp pumpflowvscurvebestefficiencypoint(BEP) oint(BEP) equipmenthistory productandsealchambertemperatures leakagepoints,see1.3 leakagerates typeofleakage(produ typeofleakage(productorflush) ctorflush) flushflow,pressure,andtem flushflow,pressure,andtemperature perature noisetypeandintensity vibrationlevels during during disass disassemb emble le andrecord: equipmenttype,model,an equipmenttype,model,andserialnum dserialnumber ber evidenceofpitting(cavitation) evidenceofpitting(cavitation)inpumpbowl inpumpbowl conditionofbearings alignmentmeasurements conditionofsealchamber,d conditionofsealchamber,deposits,wear eposits,wear conditionofshaft(sleeve),fre conditionofshaft(sleeve),frettingcorrosion ttingcorrosion during during disassem disassembly bly of therotaryequipmentandrecord: sealmanufacturer,type,a sealmanufacturer,type,andidentification ndidentification sealhistory conditionoftheseal,discolorin conditionoftheseal,discoloring,corrosion g,corrosion buildupofsediments,prod buildupofsediments,product,coke,etc. uct,coke,etc. thatistobereturnedtothe manu manufa fact ctur urer er for for anal analys ysis is or reco recond ndit itio ioni ning ng and and includetheaboveinformation. componentsduringdisassemblyof the the seal seal and and reco record rd obse observ rvat atio ions ns deta detail iled ed in the the followingportionsofthissection. may may often often explai explain n the causesofsealfailure.Mostsealfaceassemblieshave one narr narrowfaceandonewideface. owfaceandonewideface. The The narro narrow w face face create creates s a circul circular ar mark mark called called a wear wear patte pattern rn on the wideface.
Figure2,areconcentricwith the the oute outer r and and inne inner r diam diamet eter ers s and and are are the the same same widthasthenarrowface.Contactislight,even,and consistent.
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Troubleshootin of Mechanical Seals
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face is only touchingthe wide face off and on. Ensure that the seal faces are flat, check the flatness of the chamber face, tighten gland bolts evenly, and make sure that the stationary is not cocked. Consider a seal design with a flexibly mountedstationary. Figure2.NormalFaceWearPattern
Figure3,arethinnerthanthe narrowfaceandcanbetheresultofoperatingthe sealatpressuresaboveitsdesignlimits.Reducethe pressureinthesealchamberorselectasealdesign withadequatepressurelimits. Figure6.IntermittentWearPattern
Figure 7, indicate that the shaft is out of alignment or that the seal was not properlycentered.Realigntheequipment,checkthe seal and gland centering, and inspect for buildup andaccumulationaroundtheseal.
Figure3.NarrowWearPattern
Figure 4, are wider than the narrowfaceandcanbetheresultofproblemswith the equipment misalignment, bad bearings, shaft deflection, bent shaft, or pump cavitation. Restore the equipment to the manufacturer’s standards or select a seal designed to handle off designconditions.
Figure7.UnevenWearPattern
onthehardsealface,Figure8,is the result of inadequate lubrication or abrasives embedded in the soft (carbon) face grinding a groove inthehard face. Provide adequate flush to exclude abrasives from the seal chamber, change seal design, use hard faces, or prevent crystallizationofthesealingfluid.
Figure4.WideWearPattern
Figure5,indicatesthattherotary face is not turning against the stationary face. Eliminateslippingof therotarydriveor interference of the rotary with the seal chamber or gland. Use properinstallationtechniques.
Figure8.DeepWearPatternontheHardFace
may indicate other possiblecausesofsealfailure.
Figure5.NoWearPattern
Figure 6, indicate unevencontactaround the face, where thenarrow
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isseenasfracturesinthesealface thatradiatefromthecenterofthesealring,Figure9. They may be caused by localized expansion resulting from severetemperaturechanges caused by dry running, vaporization of the sealing fluid, inadequate cooling, or excessive pressures or velocities. Heat checking may be accompanied by discoloring or bluing of the seal ring. Reduce the
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Troubleshootin of Mechanical Seals temperature by flushing or cooling the chamber, changesealfacematerials,or changesealdesign. Eliminatedryrunning.
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ofahardsealfacecoating, Figure 12, istheresult ofa defective coatingor of chemicalorthermalattackof theinterfacebetween thecoatingandthebasematerial.Provideadequate cooling,changematerials,orchangetoasolidface material.
Figure9.HeatChecking
Figure10,istheresultofthermal or mechanical shock of a seal ring. Ceramic seal ringsare sometimes subject to this type of failure, but other materials can also crack. Use caution in handlingsealrings,avoidovertighteningofclamped units,avoiddryrunning,providecooling,orchange facematerials.
Figure12.FlakingorPeeling
ofasealring,Figure 13,istheresultchemicalattackofthesealringorof sealing fluid being forced from a porous seal ring due to rapid temperature changes. Provide adequatecooling,changematerials,andavoidrapid temperature changes (dry running) when using viscoussealingfluids. Pitting and blistering usually occur on the wearing face. Corrosion and spalling mayoccuronallexposedsurfacesofthesealring.
Figure10.CrackedHardFace
Figure 11, may result from mishandling or rapid separation and closure of the faces. Causes include pump cavitation, excessive vibration,shaftdeflection,andflashing(popping)of thesealingfluidasitcrossesthesealface.Usecare in handling seal rings, return equipment to manufacturer’sspecifications,avoidcavitation,cool thesealchamber,provideadequatepressureinthe seal chamber to avoid flashing, or change face materials.Inweldedmetalbellowsdesigns,damage tothefacecanresultfromnutationorcoiningofthe seal rings. Improve face lubrication, use a seal design that includes a vibration dampener, or changesealfacematerials.
Figure13.PittingofSealFace
used as O-rings, boots, bellows, gaskets,andothersecondarysealsshouldbecarefully examinedindividually.
Figure14,istheresultofchemicalattack. Changetoaresistantelastomer.
Figure14.O-ringSwelling
Figure11.ChippedEdges
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Figure 15, are the result of installationerrorsorroughcontactsurfaces.Ensure that contact surfaces meet seal manufacturer’s specificationsforsmoothness.Removesharpedges andprovidechamfers.
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chamber dimensions, seal design, installation, set screw settings, temperature limits, product buildup, and vibration levels. Change seal metallurgy. Figure15.O-ringCutsandNicks
elastomers, Figure 16, are the resultof thermalorchemicalattack.RolltheO-ring to examine the inside diameter. Cool the seal chamberorselectaresistantelastomer.
Figure19.BellowsFracture
Figure16.HardorCrackedO-ring
elastomers,Figure17,maybetheresultof high pressure, high temperature, excess vibration, or improper shaft and O-ring dimensions. Reduce pressure,coolthesealchamber,useabackupring, changesealdesignordimensions,orchangeO-ring material.
Figure20.MarksonCasingO.D.
Figure17ExtrudedO-ring
Figure 18, may be the result of high temperature or pressure, or chemical attack. Coolthesealchamberorchangeelastomer.
Figure21.MetalCorrosion
maybeincludedinsomesealdesignsand can be subject to fatigue (Figure 22), corrosion (Figure23),andstresscorrosion(Figure24)failure. Check equipment alignment. Changemetal, spring design,orsealdesign.
Figure18.CompressionSet
usedinmechanicalseals as well as metal bellows, set screws, glands, various clips,centeringtabs,etc.,aremadeofmetalandother materialsthataresubjecttochemical,mechanical,and thermaldamage.
shouldbeexaminedforfracture(Figere19) thermal discoloration, and chemical corrosion. Check equipment alignment and condition, installation, pump cavitation, vibration levels, and sealoperatinglimits. shouldbeexaminedfor rubbingon itsoutside diameter (Figure 20)thermal discoloration, and chemical corrosion and pitting (Figure 21). Check equipment alignment, seal and
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Figure22.SpringFatigue
Figure23.SpringCorrosion
Figure24.SpringstressCorrosion
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wear may be
Troubleshootin of Mechanical Seals evidentontheparts,thebodyofdrivecasing(Figure 25), orshaft (sleeve). Check equipment alignment, runout of shaft, vibration, and torque limitations of sealdesign.
Figure25.DrivePinWear
is a buildup of varnish or sludge on the atmosphericsideoftheseal,Figure26,andisthe result of chemical or thermal breakdown of a hydrocarbonsealingfluidasitcrossesthesealface. Cokecanclogthesealandwearcarbonsealfaces. Also check for deposits on the rotary casing body andinsideoftheseal,flushfromacleancoolsource, cool the seal chamber, change face materials, or change to a heat transfer fluid with higher temperaturelimits.
Figure26.Coking
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