Troubleshooting of Mechanical Seals

Troubleshooti roubleshootin n of Mecha Mechanical nical Seals

-1-

3)  Amechanicalseal isexpectedtoachieveits isexpectedt oachieveits 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 afterthesealhasachie afterthesealhasachieveditsdesi veditsdesignedlifeexp gnedlifeexpectancy. ectancy.  Afailureof  A failureof aseal maybe amajor contributorto rotary equipmentfailureanddowntime.  Anunderstandingo  Anunderstandingofthemode fthemodeofsealfailu ofsealfailurecanle recanleadto adto extendingthelifeofrotaryequipmentbyimprovingseal designandmaterialsele designandmaterialselection,installationan ction,installationandoperatin doperating g procedures,andenviro procedures,andenvironmentalcon nmentalcontrols. trols.

Thefailureofasealmaybetheresultof: ofsealdesignormaterialsforthe  applicationtemperature,pressure,speeds,andfluid 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 partstobecomedirty.  incl includ udin ing g asse assemb mbly ly, , seal seal setting,orplacingofcompone setting,orplacingofcomponentsinthecham ntsinthechamber. ber. dry running, or failure of  environmentalcontrols. operation.  of the sealin sealing g fluidwith fluidwith abrasi abrasive ve or  corrosivematerials. withexcessiveshaftrunout,deflection,  vibration,orwornbearings. The The seal seal achi achiev eved ed its its norm normal al life life  expectancy.

 A singlestage mechanicalseal mayleak may leakalongone alongone of fourpaths.SeeFigure1.

Figure1.PossibleSealLeakagePoints

1)

leakageisvisibleatthepointwherethe shafte shaftexit xitst sthe hegl glan ando dora ratth tthed edrai rainc ncon onne necti ction ons. s. 2) leakageisalsovisibleat the pointwher pointwheretheshaft etheshaft exitstheglan exitstheglandor dor atthe drainconnections.

www.easterny.com   www.easterny.com

leakageisalsovisibleat the point point where where the shaftexits shaftexits the gland gland or atthe drainconnections. 4) leakag leakage e is visibl visible e at the gland gland seal seal chamberinterface.

Much Much can can be lea learne rned by a com complete lete docum cumente nted analysisincludingthefollowing: beforeshutdownand  record: equipmentinletandoutle equipmentinletandoutletpressures tpressures pumpflowvscurvebestefficiencyp pumpflowvscurvebestefficiencypoint(BEP) oint(BEP) equipmenthistory productandsealchambertemperatures leakagepoints,see1.3 leakagerates typeofleakage(produ typeofleakage(productorflush) ctorflush) flushflow,pressure,andtem flushflow,pressure,andtemperature perature noisetypeandintensity vibrationlevels during during disass disassemb emble le  andrecord: equipmenttype,model,an equipmenttype,model,andserialnum dserialnumber ber evidenceofpitting(cavitation) evidenceofpitting(cavitation)inpumpbowl inpumpbowl conditionofbearings alignmentmeasurements conditionofsealchamber,d conditionofsealchamber,deposits,wear eposits,wear conditionofshaft(sleeve),fre conditionofshaft(sleeve),frettingcorrosion ttingcorrosion during during disassem disassembly bly of  therotaryequipmentandrecord: sealmanufacturer,type,a sealmanufacturer,type,andidentification ndidentification sealhistory conditionoftheseal,discolorin conditionoftheseal,discoloring,corrosion g,corrosion buildupofsediments,prod buildupofsediments,product,coke,etc. uct,coke,etc. thatistobereturnedtothe  manu manufa fact ctur urer er for for anal analys ysis is or reco recond ndit itio ioni ning ng and and includetheaboveinformation. componentsduringdisassemblyof  the the seal seal and and reco record rd obse observ rvat atio ions ns deta detail iled ed in the the followingportionsofthissection. may may often often explai explain n the causesofsealfailure.Mostsealfaceassemblieshave one narr narrowfaceandonewideface. owfaceandonewideface. 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 wideface. 

Figure2,areconcentricwith the the oute outer r and and inne inner r diam diamet eter ers s and and are are the the same same widthasthenarrowface.Contactislight,even,and consistent.

[email protected]

Troubleshootin of Mechanical Seals

-2-

face is only touchingthe 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 mountedstationary. Figure2.NormalFaceWearPattern 

Figure3,arethinnerthanthe narrowfaceandcanbetheresultofoperatingthe sealatpressuresaboveitsdesignlimits.Reducethe pressureinthesealchamberorselectasealdesign withadequatepressurelimits. Figure6.IntermittentWearPattern 

Figure 7, indicate that the shaft is out of alignment or that the seal was not properlycentered.Realigntheequipment,checkthe seal and gland centering, and inspect for buildup andaccumulationaroundtheseal.

Figure3.NarrowWearPattern 

Figure 4, are wider than the narrowfaceandcanbetheresultofproblemswith 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 designconditions.

Figure7.UnevenWearPattern 

onthehardsealface,Figure8,is the result of inadequate lubrication or abrasives embedded in the soft (carbon) face grinding a groove inthehard face. Provide adequate flush to exclude abrasives from the seal chamber, change seal design, use hard faces, or prevent crystallizationofthesealingfluid.

Figure4.WideWearPattern 

Figure5,indicatesthattherotary face is not turning against the stationary face. Eliminateslippingof therotarydriveor interference of the rotary with the seal chamber or gland. Use properinstallationtechniques.

Figure8.DeepWearPatternontheHardFace

may indicate other possiblecausesofsealfailure. 

Figure5.NoWearPattern  

Figure 6, indicate unevencontactaround the face, where thenarrow

www.easterny.com 

isseenasfracturesinthesealface thatradiatefromthecenterofthesealring,Figure9. They may be caused by localized expansion resulting from severetemperaturechanges 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

[email protected]

Troubleshootin of Mechanical Seals temperature by flushing or cooling the chamber, changesealfacematerials,or changesealdesign. Eliminatedryrunning.



-3-

ofahardsealfacecoating, Figure 12, istheresult ofa defective coatingor of chemicalorthermalattackof theinterfacebetween thecoatingandthebasematerial.Provideadequate cooling,changematerials,orchangetoasolidface material.

Figure9.HeatChecking 

Figure10,istheresultofthermal or mechanical shock of a seal ring. Ceramic seal ringsare sometimes subject to this type of failure, but other materials can also crack. Use caution in handlingsealrings,avoidovertighteningofclamped units,avoiddryrunning,providecooling,orchange facematerials.

Figure12.FlakingorPeeling 

ofasealring,Figure 13,istheresultchemicalattackofthesealringorof sealing fluid being forced from a porous seal ring due to rapid temperature changes. Provide adequatecooling,changematerials,andavoidrapid temperature changes (dry running) when using viscoussealingfluids. Pitting and blistering usually occur on the wearing face. Corrosion and spalling mayoccuronallexposedsurfacesofthesealring.

Figure10.CrackedHardFace 

Figure 11, may result from mishandling or rapid separation and closure of the faces. Causes include pump cavitation, excessive vibration,shaftdeflection,andflashing(popping)of thesealingfluidasitcrossesthesealface.Usecare in handling seal rings, return equipment to manufacturer’sspecifications,avoidcavitation,cool thesealchamber,provideadequatepressureinthe seal chamber to avoid flashing, or change face materials.Inweldedmetalbellowsdesigns,damage tothefacecanresultfromnutationorcoiningofthe seal rings. Improve face lubrication, use a seal design that includes a vibration dampener, or changesealfacematerials.

Figure13.PittingofSealFace

used as O-rings, boots, bellows, gaskets,andothersecondarysealsshouldbecarefully examinedindividually. 

Figure14,istheresultofchemicalattack. Changetoaresistantelastomer.

Figure14.O-ringSwelling 

Figure11.ChippedEdges

www.easterny.com 

Figure 15, are the result of installationerrorsorroughcontactsurfaces.Ensure that contact surfaces meet seal manufacturer’s specificationsforsmoothness.Removesharpedges andprovidechamfers.

[email protected]

Troubleshootin of Mechanical Seals

-4-

chamber dimensions, seal design, installation, set screw settings, temperature limits, product buildup, and vibration levels. Change seal metallurgy. Figure15.O-ringCutsandNicks 

elastomers, Figure 16, are the resultof thermalorchemicalattack.RolltheO-ring to examine the inside diameter. Cool the seal chamberorselectaresistantelastomer.

Figure19.BellowsFracture

Figure16.HardorCrackedO-ring 

elastomers,Figure17,maybetheresultof high pressure, high temperature, excess vibration, or improper shaft and O-ring dimensions. Reduce pressure,coolthesealchamber,useabackupring, changesealdesignordimensions,orchangeO-ring material.

Figure20.MarksonCasingO.D.

Figure17ExtrudedO-ring 

Figure 18, may be the result of high temperature or pressure, or chemical attack. Coolthesealchamberorchangeelastomer.

Figure21.MetalCorrosion 

maybeincludedinsomesealdesignsand can be subject to fatigue (Figure 22), corrosion (Figure23),andstresscorrosion(Figure24)failure. Check equipment alignment. Changemetal, spring design,orsealdesign.

Figure18.CompressionSet

usedinmechanicalseals as well as metal bellows, set screws, glands, various clips,centeringtabs,etc.,aremadeofmetalandother materialsthataresubjecttochemical,mechanical,and thermaldamage. 





shouldbeexaminedforfracture(Figere19) thermal discoloration, and chemical corrosion. Check equipment alignment and condition, installation, pump cavitation, vibration levels, and sealoperatinglimits. shouldbeexaminedfor rubbingon itsoutside diameter (Figure 20)thermal discoloration, and chemical corrosion and pitting (Figure 21). Check equipment alignment, seal and

www.easterny.com 

Figure22.SpringFatigue

Figure23.SpringCorrosion 

Figure24.SpringstressCorrosion 

[email protected]

wear may be

Troubleshootin of Mechanical Seals evidentontheparts,thebodyofdrivecasing(Figure 25), orshaft (sleeve). Check equipment alignment, runout of shaft, vibration, and torque limitations of sealdesign.

Figure25.DrivePinWear 

is a buildup of varnish or sludge on the atmosphericsideoftheseal,Figure26,andisthe result of chemical or thermal breakdown of a hydrocarbonsealingfluidasitcrossesthesealface. Cokecanclogthesealandwearcarbonsealfaces.  Also check for deposits on the rotary casing body andinsideoftheseal,flushfromacleancoolsource, cool the seal chamber, change face materials, or change to a heat transfer fluid with higher temperaturelimits.

Figure26.Coking

www.easterny.com 

[email protected]

-5-