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DEPARTMENTOFMECHANICALENGINEERING,COLLEGEOFENGINEERING,
UNIVERSITYOFAGRICULTURE,ABEOKUTA.
LECTURENOTE
COURSETITLE:MECHANICALMAINTENANCE
COURSECODE:MCE509
COURSEUNITS:2
9/4/2015 COURSETITLE:MECHANICALMAINTENANCE
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S.O.ISMAILA,PhD
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CourseContents:Machineinspection,rateofwearandreplacementtimeprediction.Basic
technologiesandequipmentforrepairsofinternalcombustionengines,pumpsandsmalloutput
powergeneratingplants,machinetools,vehicles,earthmovingequipmentandliftingdevices.
Specialtechniquesinmachinerepairs.Planningandorganizationofserviceandmaintenanceshops.
Planningofthesparesstockandrelatedproblems.
1.0INTRODUCTION
Theprimaryaimofanyestablishmentshouldbetomanufactureandsellaproductornumberof
productsinordertosatisfyademand.However,mostestablishmentsusuallystatetheirprimary
objectivesas:
(i)makeprofit(ii)maximizeprofit(iii)satisfyasocialneed/employlabour(iv)provideuseful
commodity.
Theseenumeratedobjectivesshouldbesecondaryasifacompanycansufficientlyidentifieda
marketandsufficientlyservicedit,allotherobjectivesshallbemet.
Toachievetheprimaryobjectiveofsatisfyingademand,itisveryimportantthatproductsare
producedintherightquantity,quality,attherighttimeandrightcost.Fortheserequirementstobe
met,goodsmustbemadeinthemostefficientandeconomicmanner.Themachinesandequipment
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mustoperateefficientlyandeffectivelyattherequiredlevelofproduction.Theremustbeveryfew
ornostoppagesonproductionlineswhichrequireeffectiveplanning,schedulingandgood
administrationofmaintenanceactivities.
Maintenancerequirementshaveanimpactonproductionschedulingandotherfunctionsperformed
bytheproductioncontroldepartment.Timelostduetomaintenancemayinterferewithschedules
fromtheproductiondepartment.Therefore,maintenancerequirementsshouldbeconsideredin
choosingmachinesorequipmentforreplacementorincreasingthecapacityofinstalledmachines
andequipment.
Themaintenancedepartmentinanyorganizationissaddledwiththeresponsibilityofthe
maintenanceoffacilities,equipment,andmachines.
1.1DEFINITIONOFMAINTENANCE
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Maintenanceisanyactivitythatiscarriedoutonanyfacilityeithertorestoretoortoretainthe
facilityinagoodandacceptableworkingconditions.Maintenanceinvolvesalltechnicalandother
proceduresperformedinordertoretainthesatisfactoryworkingconditionofamachineorpartor
restoringittoanacceptableworkingconditionsothatthesettaskscanbeperformedatthe
scheduledtimeandundergivenconditions.
Maintenanceisoftennotgiventhepriorityitdeservesintheoveralloperatingstrategyofafacility.
Maintenanceprogramsaremanagedandfundedbypeople,andhumannatureseemstoabidebythe
oldtenet,Ifitaintbroke,dontfixit.Comparedtootherdepartments,maintenancedepartments
havenorealproductandassuchproducenorealincome.Manymanagersviewmoneyspenton
maintenanceasmoneythrowndownablackhole.Inspiteofanylifecycleproofstothecontrary,
managerslookforwaystocutmaintenancebudgetsfirstwhenanyotherneedarises.
Adejuyigbe(2002)statedthatthemaintenancefunctionsstematsupportingroletokeepequipment
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(i)Tobeabletooperateeffectively
(ii)Tomaintainqualitystandardatalltimes
(iii)Tomaintainthequantitativeandcoststandardsofoutput.
Heenumeratedtheobjectivesofplantmaintenancetoincludethefollowing
Toachieveminimumbreakdownandtokeeptheplantingoodworkingconditionatthe
lowestpossiblecost
Tokeepthemachinesandotherfacilitiesinoperationallevel,andusedactoptimum(profit
making)capacity.
Toensuretheavailabilityofthemachinesbuildingsandservicesrequiredbyothersection,
buildingsandservicesrequiredbyothersectionofthefactoryforefficientperformance.Themost
importantresponsibilityofplantengineeringisthatofmaintainingtheplantfacilitiesandequipment.
Itisonlywhentheequipmentisadequatelymaintainedthatitcanbeexpectedtooperateand
performproperly,andtherebyyieldingahighqualityproductatareasonablecost.
1.2NEEDFORMAINTENANCE
Oneofthefactorsthatcanensureavailabilityofinstalledfacilitiesforefficientuseisaneffective
andefficientmaintenanceengineeringsystem.Gonewerethedayswhenmaintenancewasnotgiven
adequateattention.Foranycompanywithmechanizedandautomatedsystems,moreattentionis
nowgiventomaintenancefunction.Therefore,theneedformaintenanceincreaseswith
technologicaladvancementinproductionfacilities.Otherfactorswhichseemtoemphasizetheneed
foreffectivemaintenancesystemare:
(i)strongcompetition(ii)tightproductionschedules(iii)increasedmachineutilization
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(iv)increasedproductionlevel
Inadequateorlackofeffectiveandefficientmaintenancesystemespeciallyinamanufacturing
enterprisegivesrisetoseveralundesirableconsequences.Theseconsequencesinclude:
(i)Excessivemachinebreakdown
(ii)Frequentemergencymaintenancework
(iii)Shortenedlifespanofthefacility
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(iv)Pooruseofmaintenancestaff(v)Lossinproductionoutput
(vi)Inabilitytomeetdeliverydates
(vii)Excessiveovertime
(viii)Lossoflives
Thesefactorsmaycontributetohighcostsofproductionandconsequentlylossinprofitability.
1.3FUNCTIONSOFMAINTENANCEENGINEERING
Thefunctionofmaintenanceengineeringcanbedividedintoprimaryandsecondary:
Theprimaryfunctionsofmaintenanceare:
(i)Maintenanceofexistingmachinesandequipment
(ii)Maintenanceofexistingbuildings
(iii)Inspectionandlubricationofmachineandequipment
(iv)Generationanddistributionofutilitiese.g.water,electricityetc.
(v)Installationofnewmachinesandequipment
(vi)Modificationsofexistingmachines,equipmentandbuildings
Thesecondaryfunctionsincludethefollowing:
(i)Sanitation(ii)Disposalofuseditems(iii)Storekeeping(iv)Fireprotection
(v)Janitorialservice
1.4TEROTECHNOLOGY
TerotechnologyisawordderivedfromtheGreekrootword"tero"or"Icare",thatisnowusedwith
theterm"technology"torefertothestudyofthecostsassociatedwithanassetthroughoutitslife
cyclefromacquisitiontodisposal.Thegoalsofthisapproacharetoreducethedifferent
costsincurredatthevariousstagesoftheasset'slifeandtodevelopmethodsthatwillhelpextend
theasset'slifespan.
Terotechnologyusestoolssuchasnetpresentvalue,internalrateofreturnanddiscountedcashflow
inanattempttominimizethecostsassociatedwiththeassetinthefuture.Thesecostscaninclude
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engineering,maintenance,wagespayabletooperatetheequipment,operatingcostsandevendisposalcosts.Alsoknownas"lifecyclecosting".Terotechnologyismultidisciplinaryapproachto
obtainingmaximumeconomicbenefitfromphysicalassets.DevelopedintheUKintheearly1970s,
itinvolvessystematicapplicationofengineering,financial,andmanagementexpertiseinthe
assessmentofthelifecycleimpactofanacquisition(buildings,equipment,machines,plants,
structures)ontherevenuesandexpensesoftheacquiringorganization.Practiceofterotechnologyis
acontinuouscyclethatbeginswiththedesignandselectionoftherequireditem,followsthrough
withitsinstallation,commissioning,operation,andmaintenanceuntiltheitem'sremovaland
disposalandthenrestartswithitsreplacement.
Theactivitiesofthemaintenancedepartmentcannotbetotallydescribedbythetermmaintenance.
Inordertodescribevividlythefunctionsofthemaintenancedepartment,thewordTerotechnology
wascoined.
Thepractiseofterotechnologyisconcernedwiththespecificationanddesignforreliabilityand
maintainabilityofplant,machinery,equipment,buildingsandstructurestogetherwiththeir
installation,commissioning,maintenance,modification,andreplacement,andwithfeedbackof
informationondesign,performance,andcosts.
Terotechnology'sdivisionistheoperationsteamthatcariesoutthedailyoperationsofFacilities
ManagementServiceswhichoptimizeassetswithprofessionalFacilitiesManagementandOperation
&Maintenance(O&M)ServicesforMechanical&Electrical(M&E)Systems,CivilWorks,
Landscape,Janitorialandotherfacilityspecialization.Ratherthanbearingadditionbusiness
burdens,a'singlepointresponsibility'throughdirectmaintenance,subcontract&vendor
managementwillsavetime,moneyandeffort.
2.0TYPESOFMAINTENANCE
2.1BreakdownMaintenance:
Breakdownmaintenanceisreferredtobymanydifferentnames:reactivemaintenance,repair,fix
whenfail,andruntofailure(RTF)maintenance.Whenapplyingthismaintenancestrategy,apiece
ofequipmentreceivesmaintenance(e.g.,repairorreplacement)onlywhenthedeteriorationofthe
equipmentsconditioncausesafunctionalfailure.Thestrategyofbreakdownmaintenanceassumes
thatfailureisequallylikelytooccurinanypart,component,orsystem.Thus,thisassumption
precludesidentifyingaspecificgroupofrepairpartsasbeingmorenecessaryordesirablethan
others.Themajordownsideofbreakdownmaintenanceisunexpectedandunscheduledequipment
downtime.Ifapieceofequipmentfailsandrepairpartsarenotavailable,delaysoccurwhilethe
partsareorderedanddelivered.Ifthesepartsareurgentlyrequired,apremiumforexpedited
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deliverymustbepaid.Ifthefailedpartisnolongermanufacturedorstocked,moredrasticand
expensiveactionsarerequiredtorestoreequipmentfunction.Cannibalizationoflikeequipmentor
rapidprototypingtechnologymaysatisfyatemporaryneedbutatsubstantialcost.Also,thereisno
abilitytoinfluencewhenfailuresoccurbecauseno(orminimal)actionistakentocontrolorprevent
them.Whenthisisthesoletypeofmaintenancepracticed,bothlabourandmaterialsareused
inefficiently.
Labourresourcesarethrownatwhateverbreakdownismostpressing.Intheeventthatseveral
breakdownsoccursimultaneously,itisnecessarytopracticeakindofmaintenanceinanattemptto
bringallthebreakdownsundercontrol.Maintenancelabourisusedtostabilize(butnot
necessarilyfix)themosturgentrepairsituation,thenitismovedontothenextmosturgentsituation,
etc.Replacementpartsmustbeconstantlystockedathighlevels,sincetheirusecannotbe
anticipated.Thisincurshighcarryingchargesandisnotanefficientaytorunastoreroom.Apurely
reactivemaintenanceprogramignoresthemanyopportunitiestoinfluenceequipmentsurvivability.
2.2PreventiveMaintenance(PM):
Maintenancerepairsperformedonaregularscheduletominimizecomponentdegradationand
extendthelifeofequipment.Preventivemaintenanceisperformedafterasetamountofelapsed
calendartimeormachineruntime,regardlessofwhethertherepairisneeded.Whilemorecost
effectivethanreactivemaintenance,preventivemaintenancestillrequiressubstantialhuman
resourcesandreplacementpartsinventories.
Itmaybeadailymaintenance(cleaning,inspection,oilingandretightening),designedtoretainthe
healthyconditionofequipmentandpreventfailurethroughthepreventionofdeterioration,periodic
inspectionorequipmentconditiondiagnosis,tomeasuredeterioration.Itentailsunderstandingand
maintainingallthephysicalelementsofmanufacturingmachinecomponents,equipment,and
systemssothattheyconsistentlyperformatalllevelsrequiredofthem.Suchmaintenanceisusually
scheduledbyprovidingformonitoringinspectionsandspecialoperatingprocedures.
TheintentofPMistopreventmaintenanceproblemsorfailuresbeforetheytakeplaceby
followingroutineandcomprehensivemaintenanceprocedures.Thegoalistoachievefewer,shorter,
andmorepredictableoutages.
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AdvantagesofPM1.Itispredictable,makingbudgeting,planning,andresourcelevellingpossible.
2.Whenproperlypracticed,itgenerallypreventsmostmajorproblems,thusreducingforced
outages,reactivemaintenance,andmaintenancecostsingeneral.
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3.Itassuresmanagersthatequipmentisbeingmaintained.
4.Itiseasilyunderstoodandjustified.
DisadvantagesofPM
1.Itistimeconsumingandresourceintensive.
2.Itdoesnotconsideractualequipmentconditionwhenschedulingorperformingthemaintenance.
3.Itcancauseproblemsinequipmentinadditiontosolvingthem(e.g.damagingseals,stripping
threads).
Itisfurtherdividedintoperiodicmaintenanceandpredictivemaintenance.Justlikehumanlifeis
extendedbypreventivemedicine,theequipmentservicelifecanbeprolongedbydoingpreventive
maintenance.
a.PeriodicMaintenance(TimebasedmaintenanceTBM):
Timebasedmaintenanceconsistsofperiodicallyinspecting,servicingandcleaningequipmentand
replacingpartstopreventsuddenfailureandprocessproblems.
b.PredictiveMaintenance:
Thisisamethodinwhichtheservicelifeofimportantpartispredictedbasedoninspectionor
diagnosis,inordertousethepartstothelimitoftheirservicelife.Comparedtoperiodic
maintenance,predictivemaintenanceisconditionbasedmaintenance.Predictivemaintenance
programsmeasureequipmentonaregularbasis,trackthemeasurementsovertime,andtake
correctiveactionwhenmeasurementsareabouttogooutsidetheequipmentoperatinglimits.
Repairingequipmentasneededrequiresfewermanhoursandpartsthanpreventivemaintenance.
However,trackingthemeasurementsrequiresnewtools,training,andsoftwaretocollectand
analyzethedataandpredictrepaircycles.Itmanagestrendvalues,bymeasuringandanalyzingdata
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aboutdeteriorationandemploysasurveillancesystem,designedtomonitorconditionsthroughan
onlinesystem.
2.3CorrectiveMaintenance:
Itimprovesequipmentanditscomponentssothatpreventivemaintenancecanbecarriedout
reliably.Equipmentwithdesignweaknessmustberedesignedtoimprovereliabilityorimproving
maintainability.
2.4ConditionBasedMaintenance
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Theconditionoftheequipmentorsomecriticalpartsoftheequipmentarecontinuouslymonitored
usingsophisticatedmonitoringinstrumentssothatfailuremaybepredictedwellbeforeitoccursand
correctivestepsaretakentopreventfailure.
2.5DesignOutMaintenance
Adesignoutmaintenanceisadesignorientedcurativemeansaimedatrectifyingadesigndefect
originatedfromimpropermethodofinstallationorpoorchoiceofmaterialsetc.Itcallsforstrong
designandmaintenanceinterface.Designoutmaintenanceaimstoeliminatethecauseof
maintenance.
2.6OpportunisticMaintenance
Whenequipmentistakendownformaintenanceofoneoffewwornoutparts,theopportunitycanbe
utilizedtochangeormaintainotherpartswhicharewearingouteventhoughtheyhaveyettofail.
Thismaintenancestrategyisfornonmonitoredcomponents.
2.7.ProactiveMaintenance
Unlikethethreetypeofmaintenancestrategieswhichhasbeendiscussedearlier,proactive
maintenancecanbeconsideredasananothernewapproachtomaintenancestrategy.Dissimilarto
preventivemaintenancethatbasedontimeintervalsorpredictivemaintenancethatbasedon
conditionmonitoring,proactivemaintenanceconcentrateonthemonitoringandcorrectionofroot
causestoequipmentfailures.Theproactivemaintenancestrategyisalsodesignedtoextendthe
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usefulageoftheequipmenttoreachthewearoutstagebyadaptationahighmasterylevelofoperatingprecision.
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Figure1:DiagrammaticrepresentationofTypesofMaintenance
MAINTENANCE
PlannedMaintenance
UnplannedMaintenance
PreventiveMaintenance
CorrectiveMaintenance
PredictiveMaintenance
BreakdownMaintenance
CorrectiveMaintenance
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2.7ReliabilityCenteredMaintenance(RCM)Recently,reliabilitycenteredmaintenancehasbeendefinedasanapproachtomaintenancethat
combinesreactive,preventive,predictive,andproactivemaintenancepracticesandstrategiesto
maximizethelifethatapieceofequipmentfunctionsintherequiredmanner.RCMisanapproach
thattriedtocreateanoptimummixtureofanintuitiveapproachandarigorousstatisticalapproachto
decidinghowtomaintainfacilityequipment.
3.0FAILURERATEANDRELIABILITY
3.1FailureRate
Failureisanyeventthatadverselyaffectssystemcriteria.Forexample,thecriteriacouldinclude
outputinasoldoutcondition,ormaintenancecostorcapitalresourcesinaconstrainedbudget
cycle,environmentalexcursionsorsafety,etc.
Failurerateisthetimerateofchangeoftheprobabilityoffailure.Sincethelatterisafunctionof
time,failurerateisalsoafunctionoftime.However,intermsoffailurerate,onecanobtainphysical
informationastowhichfactoriscontrollingthefailurebehaviorand/orwhenitiscontrollingthe
failurebehavior.Thefailurerateisabasiccomponentofmanymorecomplexreliability
calculations.Dependinguponthemechanical/electricaldesign,operatingcontext,environment
and/ormaintenanceeffectiveness,amachinesfailurerateasafunctionoftimemaydecline,remain
constant,increaselinearlyorincreasegeometrically(Figure2)
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Figure2:FailurePatternsofMachineryinrelationwithtime
Example1:ATVproducertested1000unitsinanacceleratedreliabilityevaluationprogram.Inthatprogram,eachunitisturnedonandoff16timeseachdaytomimicatypicalTVusageforaweek.Basedonafailuretoperformcriterion,failuredataareobtainedforthefirst10daysoftest:_____________________________________________________________________day1day2day3day4day5day6day7day8day9day10
1812107 6 54 30 1Thefailurerateisdefinedasthe"probabilityoffailureperday",denotedbyi,i=1,10:
Forthefirstday(i=1):(1)=18/1000/day
Forthesecondday(i=2):(2)=12/(100018)=12/982/day.
Forthethirdday(i=3):(3)=13/(10001812)=13/970/day.
Notethatthefailurerateforday1isbasedonatotalof1000TVsets,inwhich18failedduringthe
daythefailurerateforday2isbasedonatotalof(100018)setsandforday3,atotalof(100018
12)setsetc.Inthisway,wecanobtain(t)uptot=10days.Clearly,forthisproceduretoyield
reliable(t),thenumberoftheTVsetstestedeachdaymustbelargerelativetothenumberof
failuresinthatday.However,wealsonotethatthetimerequiredingatheringthedataisonly10
days,arelativelyshorttimeperiodcomparedtowhatmightbeneededtogenerateasetoftimeto
failuredata{ti,i=1,N).
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Figure3:ProfileofEquipmentFailure(BathTubCurve)
Failuresdonotgenerallyoccuratauniformrate,butfollowadistributionintimecommonly
describedasa"bathtubcurve."Thelifeofadevicecanbedividedintothreeregions:
(i) InfantMortalityPeriod:Thisperiodisthatofrunningin,wherethefailurerate
progressivelyimproves.Thefailurerateisgenerallyhighbutshortbeforedecreasingdueto
designormanufacturingerrors,defectiveparts,defectsinmaterials,misuse,misapplication,
outofmanufacturingtolerance.Failureatthisperiodcanbeavoidedbysubjectingthe
producttospecifiedperiodofsimulatedtests,inhopethatmostearlyfailuresareweededout,
vigoroustestsduringcommissioning,designimprovement,strictermaterialselection,
tightenedqualitycontrol,andtheuseofredundancy,whichisbuiltintotheproductto
provideafailsafefeature.
(ii) UsefulLifePeriod:Atthisperiod,failurerateisatitslowestandremainsconstantfor
productsthatdonotcontainfataldefectsorthathavesurvivedtheinfancyperiod.
Thisconstantratemodeisgenerallyduetorandomeventsfromwithout,ratherthanby
inherentfactorsfromwithin.Sucheventsarebeyondthecontrolduringtheperiodsofdesign,
prototypedevelopment,manufacturing,etc.butmayberesultthrougheitheraccidentorpoor
operationormaintenance.Failuresmaybereducedbyfollowinggoodoperatiogand
maintenanceprocedures.Theconstantrateperiodisoftenusedtoformulatethepricing,
Guaranteeandservicingpoliciesoftheproductthelatterisofparticularimportancein
commerce.Productwithaconstantfailureratehastheuniqueattributethatitsprobabilityof
failureisindependentoftheproductspastservicelifethisaspectaidstheeaseof
mathematicalmodellinginrepairfrequency,sparepartinventory,maintenanceschedule,etc.
(iii)WearOutPeriod:Theperiodoccurstowardthetailendoftheproductusefullifeandis
associatedwithincreasingfailurerate.Thefailureisbecauseofoldageoftheequipment
materialfatigue,corrosion,contactwear,insulationfailure,andsoon.Productswithrapidly
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increasingfailureratesrequirecorrectivemeasuressuchasregularityofinspection,
maintenance,replacement,etc.Thecentralconcerninthewearoutperiodistheabilityto
predicttheprobableservicelifewithasuitablemodel,sothataprudentschedulefor
preventivemaintenancecanbeformulated.
Generally,theinfantmortalitymodeisaqualitycontrolissue,whilethewearoutmodeisa
maintenanceissue.Therandomfailureorconstantratemode,ontheotherhand,iswidelyused
asthebasisforproductreliabilityconsiderations.
3.2Failurepatterns
Threetypesoffailurepatternscanbeidentifiedasfollows:
1.RandomFailurePattern:Thisisasituationwhentheprobabilityoffailureofacomponentis
constantandindependentoftime.Suchcomponentsdonotwearoutordeteriorateundernormal
operatingconditions.Anexampleisthefusethatcanbeblownoffwhetheroldornew.
Theprobabilitydensityfunctionisgivenby:
X(t)=ei,whereistheaveragefailurerateand1
istheaveragetimetofailure.The
probabilitydensityfunctionisshowninFigure3.
X(t)
FailureRate
=Constant
t(Time)
Figure3:ExponentialProbabilityDensityFunction
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2.RunninginFailurePattern:
Thistermisusuallyreferstotheperiodofstartingamachinerywhenitisneworafteramajor
overhaul,whichnormallyinvolvesthechangingofprincipalworkingcomponentsandparts.
Problemsofrunninginmaybearesultofhumanimperfectionseitherindesign/manufactureor
duringinstallation.TheprobabilityfunctionofsuchafailureisshowninFigure4.
X(t)
FailureRate
1
12
2
t(Time)
Figure4:HyperExponentialProbabilityDensityFunction
3.WearoutFailurePattern:Deteriorationofmachineryisexpectedtoincreasewithuseanditsage.
Machinerywouldthereforefailaftersomeoperatingconditionsatanage.Failurepatterns,which
exhibitwear,arerepresentedbytheNormalProbabilityFunction,whichisgivenby:
X(t)=
22
2
1
mtt
e ,wherem tisthemeanandisthevariance.
3.3Reliability
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Thetermreliabilityinengineeringreferstotheprobabilitythataproduct,asystemoraparticularcomponentwillperformwithoutfailureunderthespecifiedconditionandforaspecific
periodoftime.Thus,itisalsoknownastheprobabilityofsurvival.Toquantifyreliability,atest
isusuallyconductedtoobtainasetoftimetofailuresampledatasay{ti,i=1,N).Thesamplecan
thenbefittedtoaprobabilitydensityfunction,f(t),ortoaprobabilitycumulativefunction,F(t).
Thereliabilityfunctionisdefinedas:R(t)=1F(t).HencethebehaviorofR(t)isconjugatetothat
ofF(t),thecumulativeprobabilityoffailureintime.However,failureofanengineeringproduct,or
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system,maystemfromsuchrandomfactorsasmaterialdefects,lossofprecision,accidentalover
load,environmentalcorrosion,etc.Theeffectsonfailureofthetheserandomfactorsareonly
implicitinthecollecteddata{ti,i=1,N)anditisdifficulttoascertainwhichfactorispredominant
andwhenitispredominant,fromusingF(t).
3.3.1IndicesofReliability
Reliabilitycanbespecifiedbytwoparametersnamely:
1.Meantimebetweenfailures(MTBF)
MTBFisthecriticalcharacteristicforrepairablesystemandisthemeanoraveragetimebetween
twosuccessivefailuresofthesystem.MTBFcanbeobtainedbyrunninganitemorequipmentfora
predeterminedlengthoftimeunderspecifiedconditionsandcalculatingtheaveragelengthoftime
betweenfailures.Ifforexample,anitemfailssixtimesinanoperatingperiodof60,000hrs,MTBF
is10,000hrs.However,iftheidenticalitemsoperatingundersimilarconditionsarestudied,MTBF
isgivenby:
MTBF=(Totaloperatinghoursofallitems)/Totalnumberoffailuresthatoccur
Forexampleif20identicalitemsoperatefor5,000hrsduringwhich40failuresoccurandare
rectified,
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MTBF=40
205000 =2,500hrs
MTBFcanalsobeexpressedastheinverseoffailurerate,asfollows:
MTBF=1
Theexponentialdistribution,themostbasicandwidelyusedreliabilitypredictionformula,models
machineswiththeconstantfailurerate,ortheflatsectionofthebathtubcurve.Mostindustrial
machinesspendmostoftheirlivesintheconstantfailurerate,soitiswidelyapplicable.Belowisthe
basicequationforestimatingthereliabilityofamachinethatfollowstheexponentialdistribution,
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wherethefailurerateisconstantasafunctionoftime:
Where:
R(t)=Reliabilityestimateforaperiodoftime,cycles,miles,etc.(t).
e=Baseofthenaturallogarithms(2.718281828)
=Failurerate(1/MTBF)
Ifforexample,weassumeaconstantfailurerateof0.1foraprimemoverandrunningforsixyears
withoutafailure,theprojectedreliabilityis55percent,whichiscalculatedasfollows:
R(6)=2.718281828(0.1*6)
R(6)=0.5488=~55%
Inotherwords,aftersixyears,about45%ofthepopulationofsimilarprimemoveroperatingin
similarapplicationcanbeexpectedtofail.Itisworthreiteratingatthispointthatthesecalculations
projecttheprobabilityforapopulation.Anygivenindividualfromthepopulationcouldfailonthe
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firstdayofoperationwhileanotherindividualcouldlast30years.Thatisthenatureofprobabilisticreliabilityprojections.
2.Meantimetofailure(MTTF)
Thisisusedforcomponentsoritemsthatarenotrepairablesuchasfilamentlamps,fuses,resistors,
capacitors,etc.ThevalueofMTTFcanbecalculatedfromlifetestresults,whichcanbeobtainedby
stressingalargenumberofcomponentsunderknownconditionsforaperiodandnotingthenumber
offailures.
MTTF=(Lengthoftesttime)/(Numberoffailures).
Anothermethodwhichthoughismoreaccuratebutcostlyisruntofailurespecifiednumberof
componentsunderspecifiedconditions.
MTTF==
=
ni
i
iT
1
1
WhereTi=lengthoftimetakenbytheithspecimentofail
n=totalnumberofspecimens.
MTTF=1
whereisfailurerateandisindependentoftime
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3.3.2CalculatingSystemReliability
Systemreliabilitydependsonthereliabilitiesofthevariouscomponentsinthesystem.Therefore,to
calculatethereliabilityofasystem,thesystemshouldbedividedintosubsystems.Asystemmaybe
connectedinseriesorparallel.
SystemsinSeries
Intheseriessystem,theabilitytoemploysubsystemBdependsupontheoperatingstateof
subsystemA.IfsubsystemAisnotoperating,thesystemisdownregardlessoftheconditionof
subsystemB(Figure5).
Tocalculatethesystemreliabilityforasystemisseries,thereistheneedtomultiplytheestimated
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reliabilityofsubsystemAattime(t)bytheestimatedreliabilityofsubsystemBattime(t).Thebasic
equationforcalculatingthesystemreliabilityofasimpleseriessystemis:
Where:
Rs(t)Systemreliabilityforgiventime(t)
R1n(t)Subsystemorsubfunctionreliabilityforgiventime(t)
So,forasimplesystemwiththreesubsystems,orsubfunctions,eachhavinganestimatedreliability
of0.90(90%)attime(t),thesystemreliabilityiscalculatedas0.90X0.90X0.90=0.729,orabout
73%.
Figure5:SimpleSerialSystem
SystemsinParallelDesignengineersattimesincorporateredundancyintocriticalmachines.Reliabilityengineerscalltheseparallelsystemsandmaybedesignedasactiveparallelsystemsorstandbyparallelsystems.TheblockdiagramforasimpletwocomponentparallelsystemisshowninFigure6.
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Figure6:Simpleparallelsystem
Tocalculatethereliabilityofanactiveparallelsystem,wherebothmachinesarerunning,usethefollowingsimpleequation:
Where:Rs(t)Systemreliabilityforgiventime(t)R1n(t)Subsystemorsubfunctionreliabilityforgiventime(t)
Thesimpleparallelsysteminourexamplewithtwocomponentsinparallel,eachhavingareliabilityof0.90,hasatotalsystemreliabilityof1(0.1X0.1)=0.99.Therefore,thesystemreliabilitywassignificantlyimproved.
3.3.3AvailabilityMaintenanceManagers/EngineersusuallyemployavailabilityRatioforplanningpurposes.Thereliabilityofasystemgivestheprobabilityacertainjobcanbedonewithoutsystembreakdown,however,themanagerneedstoknowhowmuchtimethesystemwouldbeavailableoveraplanningperiod.Themaintainability,whichisafactorofthetimerequiredandresourcesneededtorestoreequipmentincaseoffailure,inconjunctionwithreliabilitydeterminetheavailabilityofamachine.IftheaveragerepairtimeisT,theavailabilityofthemachine,AVintermsofMTBFis
AV=TMTBF
MTBF+
AvailabilityRatioistheportionofthetotaltimeamachineshouldfunctiontothatthemachineactuallyfunctions.IfthetotalisThoursandthemachineisactuallyinworkingconditionforUhourswhileitisdownforDhours,thenT=U+D
AvailabilityRatio,AR=DU
U+
UnavailabilityRatio,UR=DU
D+
Thetotaltime,Tdoesnotincludeplannedoperationalshutdownsduetoproductionschedulesorroutinepreventivemaintenance.
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4.0.TROUBLESHOOTINGGUIDEFORSMALLENGINE
Thefollowingchartlistsavarietyofcommonproblemsandnearlyallpossiblecauses.Diagnosticprocedureswillthenbeneededtodeterminewhichactuallyapply.The'possiblecauses'arelistedin*approximate*orderoflikelihood.
Whilethischartlistsmanyproblems,itisdoesnotcovereverythingthatcangowrong.However,itcanbeastartingpointforguidingyourthinkingintheproperdirection.
Problem:Enginewillnotstartorishardtostart.Possiblecauses:
1.Fueltankisemptyorshutoffvalveisclosed,orfuellineorfueltankcapventisclogged.
2.Thereiswaterinthefuel.3.Carburetorisoverchoked.4.Carburetorisimproperlyadjustedorneedsservice.5.Ignitionsystemoritswiringisdefectiveorignitionswitchisoff,ortiming
wayoff(e.g.,brokenflywheelkey).6.Deadmanorothercutoffswitchisopenordefective.7.Sparkplugisfouled,improperlygapped,ordamaged.8.Enginecompressionispoor.9.Operatorneedstoreadusermanual.:)
Problem:Enginestartseasilybutdiesafterafewseconds.Possiblecauses:
1.Fueltankisemptyorshutoffvalveisclosed,orfuellineorfueltankcapventisclogged.
2.Carburetorisoverchoked.3.Carburetorisimproperlyadjustedorneedsservice.
Problem:Engineidlesroughly,unevenly,orsurges.Possiblecauses:
1.Carburetorisdirty.2.Airleakincarburetororintakemanifold(e.g.,badOring,gasket,primer).3.Carburetorisimproperlyadjustedorneedsservice.
Problem:Enginemissesunderload.Possiblecauses:
1.Sparkplugisfouled,improperlygapped,ordamaged.2.Breakerpointsarepittedorimproperlygapped,breakerarmissluggish,or
condenserisbad.3.Carburetorneedsadjustmentorservice.4.Fuelline,fuelfilter,orfueltankcapventisclogged,orfuelshutoffvalve
partiallyclosed.5.Valvesnotadjustedproperlyorvalvespringsweak.6.Exhaustportsblocked(2stroke).
Problem:Engineknocks.Possiblecauses:
1.Magnetoisnottimedproperly.2.Carburetorissettoolean.3.Enginehasoverheated.4.Carbonbuildupincombustionchamber.
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5.Flywheelisloose.6.Connectingrodislooseorworn.7.Cylinderisexcessivelyworn.
Problem:Enginevibratesexcessively.Possiblecauses:
1.Engineisnotmountedsecurely.2.Bladeorotherdrivenequipmentisunbalanced.3.Crankshaftisbent.4.Counterbalanceshaftisnottimedcorrectly.
Problem:Enginelackspower(possiblyafterwarmup).Possiblecauses:
1.Oldgas,badsparkplug,verythick/dirtyoil.2.Chokeispartiallyclosed.3.Carburetorneedsadjustmentorservice.4.Ignitionnottimedcorrectly.5.Airfilterisclogged.6.Thereisalackoflubrication.7.Valvesarenotsealingproperly.8.Pistonringsarenotsealingproperly.9.Headlooseorheadgasketblownordamaged.10.Exhaustportsblocked(2stroke).
Problem:Engineoperateserratically,surges,andrunsunevenly.Possiblecauses:
1.Fuellineorfueltankcapventisclogged.2.Thereiswaterinthefuel.3.Fuelpumpisdefective.4.Governorisnotsetproperly,sticking,orbinding.5.Carburetorneedsadjustmentorservice.6.Loosecarburetor/intakepiperesultinginvacuumleak.
Problem:Engineoverheats.Possiblecauses:
1.Magnetoisnottimedproperly.2.Carburetorsettoolean.3.Airintakeorcoolingfinsareclogged.4.Shroudorblowerhousingmissing.5.Excessiveload.6.Insufficientorexcessiveoil.7.Improperoilviscosity(4stroke)ormixture(2stroke)8.Valveclearanceistoosmall.9.Excessivecarbonbuildupincombustionchamber.
Problem:Crankcasebreatherpassingoil.Possiblecauses:
1.Toomuchoilincrankcase.
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2.Enginespeedisexcessive.3.Oilfillcaporgasketisdamagedormissing.4.Breathermechanismisdirtyordefective.5.Pistonringgapsarealigned.6.Pistonringsareworn.
Problem:Enginebackfires.Possiblecauses:
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1.Carburetorsettoolean.2.Magnetoisnottimedcorrectly.3.Valvesaresticking.
5.0MODERNTECHNIQUESINMAINTENANCE
Themoderntechniquesinmaintenanceincludethefollowing:
5.1UseofConditionMonitoring(CM)Technologies
Thefollowingmethodsareavailabletoassesstheconditionofsystems/equipment,todeterminethe
mosteffectivetimetoschedulemaintenance:
(i)vibrationmonitoringandanalysis
(ii)thermography
(iii)lubricantandparticlewearanalysis(oilanalysis)
(iv)nondestructivetesting.
(v)AcousticEmissionmonitoring
5.1.1VibrationMonitoringandAnalysis
Theory,Applications,andTechniques
AnalysisofsystemandequipmentvibrationlevelsisoneofthemostcommonlyusedCM
techniques.Vibrationmonitoringhelpsdeterminetheconditionofrotatingequipmentandstructural
stabilityinasystem.Italsohelpsidentifynoisesources,asseverelyvibratingequipmentisnoisy.
BasicVibrationTheory
Vibrationissimplythemovementofamachineormachinepartbackandforthfromitspositionof
rest.Aweighthangingonaspringisthesimplestexampleofhowvibrationworks.Untilaforceis
appliedtotheweighttocauseittomove,wehavenovibration.Byapplyinganupwardforce,the
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weightmovesupward,compressingthespring.Ifwereleasedtheweight,itwoulddropbelowits
neutralpositiontosomebottomlimitoftravel,wherethespringwouldstoptheweight.Theweight
wouldthentravelupwardthroughtheneutralpositiontothetoplimitofmotion,andbackagain
throughtheneutralposition.Themotionwillcontinueinexactlythesamemannerastheforceis
reapplied.Thus,vibrationistheresponseofasystemtosomeinternalorexternalforceappliedto
thesystem.Withafewexceptions,mechanicaltroublesinamachinecausevibration.Themost
commonproblemsthatproducevibrationare:
unbalanceofrotatingparts
misalignmentofcouplingsandbearings
bentshafts
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worn,eccentric,ordamagedparts
baddrivebeltsanddrivechains
badbearings
torquevariations
electromagneticforces
aerodynamicforces
hydraulicforces
looseness
rubbing
resonance
Theamountoftimerequiredtocompleteonefullcycleofavibrationpatterniscalledtheperiodof
vibration.Ifamachinecompletesonefullcyclein1/60thofasecond,theperiodofvibrationissaid
tobe1/60thofasecond.Theperiodofvibrationisasimpleandmeaningfulcharacteristicoftenused
invibrationdetectionandanalysis.Anothersimplecharacteristicisthefrequency.Frequencyis
relatedtoperiodbythefollowingformula:
frequency=1/period
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Frequencyistheinverseofperiod.Inreality,frequencyisameasureofthenumberofcomplete
vibrationcyclesthatoccurinaspecifiedamountoftime.Thefrequencyofvibrationisusually
expressedincyclesperminute(CPM).SpecifyingvibrationfrequencyinCPMmakesiteasyto
relatethischaracteristictoanotherimportantspecificationofrotatingmachinery:revolutionsper
minute(RPM).So,ifyouhavepieceofmachinerythatoperatesat3600RPM,youcanexpect
certainproblemstocreatevibrationatafrequencyof3600RPM.Frequencyissometimesexpressed
incyclespersecond,orHertz(Hz).TherelationshipbetweenHzandCPMisexpressedbythe
followingequation:
CPM=Hzx60
Vibrationdisplacementisdefinedasthetotaldistancetravelledfromoneextremelimittotheother
(thepeaktopeakdisplacement).Peaktopeakvibrationdisplacementisusuallyexpressedinmils,
where1milequals1/1000thofaninch(0.001in.).Sinceavibratingpieceofmachineryismoving,
ithasavelocity.Thevibrationvelocityconstantlychanges.Atthetoplimitofthemotionthespeed
iszerosincetheweightmustcometoastopbeforeitcangointheoppositedirection.Thespeedof
velocityisgreatestastheweightpassesthroughtheneutralposition.Sincethevelocityofthepartis
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constantlychangingthroughoutthecycle,thehighestpeakisselectedformeasurement.Vibration
velocityisexpressedininchespersecond.Sincevibrationvelocityisdirectlyrelatedtovibration
severity,forthemostgeneralpurposevibrationmeasurements,itisthepreferredparameterfor
measurement.Asaruleofthumb,vibrationsoccurringinthe600to60,000CPMfrequency
rangearegenerallybestmeasuredusingvibrationvelocity.Underconditionsofdynamicstress,
displacementalonemaybeabetterindicationofseverity,especiallywhenthemachinepartexhibits
thepropertyofbrittleness,thetendencytobreakorsnapwhenstressedbeyondagivenlimit.For
example,consideraslowlyrotatingmachinethatoperatesat60RPM,andthatexhibitsvibrationof
20milspeaktopeakdisplacementcausedbyrotorunbalance.Intermsofvibrationvelocity,20mils
at60CPMisonly0.0585in./sec,whichwouldbeconsideredgoodforgeneralmachineryand
littlecauseforimmediateconcern.However,keepinmindthatthebearingofthismachineisbeing
deflected20mils.Undertheseconditions,fatiguemayoccurduetostress(resultingfromthe
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displacement)ratherthanduetofatigue(causedbythevelocityofdisplacement).Generally,themostusefulpresentationofvibrationdataisagraphshowingvibrationvelocity(expressedin
inches/second)ontheverticalaxisandfrequencyonthehorizontalaxis.Byanalyzingthisdata,a
trainedvibrationtechniciancanascertainwhatkindsofproblemsexist.Thetrainedtechnicianhas,
ineffect,learnedtoreadvibrationsignatureshehaslearnedtointerpretwhatthedifferent
peaksinthedifferentfrequencyrangesindicate.
Allrotatingmachinerywillexhibitacertaindegreeofvibration.ThequestionthenbecomesHow
muchistoomuch?Therearenorealisticfiguresforselectingavibrationlimit,which,ifexceeded,
willresultinimmediatemachineryfailure.Theeventssurroundingthedevelopmentofamechanical
failurearetoocomplextosetreliablelimits.However,therearesomegeneralguidelinesthathave
beendevelopedovertheyearsthatcanserveasgeneralindicationoftheconditionofapieceof
machinery.
Whensettingupavibrationmonitoringprogramthatuseshandheldvibrationinstrumentation,itis
necessarytoensurethatthemeasurementsaretakenconsistently.
Aslightvariationinthelocationwhereameasurementistakenonapieceofmachinerycan
significantlyalteritsaccuracy.Thisissuebecomesespeciallydifficulttopolicewhenseveral
technicianstakemeasurementsatdifferenttimesonthesamepieceofmachinery.
InformationObtainedthroughVibrationMonitoring
Ifappliedbyatrainedprofessional,vibrationmonitoringcanyieldinformationregarding:wear,
imbalance,misalignment,mechanicallooseness,bearingdamage,beltflaws,sheaveandpulley
flaws,geardamage,flowturbulence,cavitation,structuralresonance,andmaterialfatigue.
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Themaintenancesupervisor/managermustmakethedecisionwhetheritmakeseconomicsenseto
performthisfunctionwithinhouselaborforcesorwhetheritshouldbeoutsourcedtoacontractor
specializinginvibrationmonitoringandanalysis.Inmakingthisdecision,maintenance
supervisors/managersshouldconsiderwhethertheyhavesufficientinhouselabortodedicateto
vibrationmonitoring.Vibrationmonitoringtheoryiscomplicated,theequipmentisexpensive,
andtheanalysisofthedatacollectedisaskillthatmustbepracticedregularly.Oncehehas
completedthebasicvibrationtraining(costingseveralthousandsofdollars),amaintenance
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technicianmustbecommittedandallowedtoworkatleast3dayspermonthinvibrationanalysisif
heistostaycompetentwiththetechnologyandanalysistechniques.
DetectionInterval/AmountofDataCollected
Narrowbandvibrationanalysiscanprovideseveralweeksormonthsofwarningofimpending
failure.Inestablishingavibrationmonitoringprogram,onemustfirstdeterminehowoftentotake
samplingdata.Differentvibrationfrequenciesforebodedifferentupcomingfailures.Thefrequency
ofdatacollectiondependsonmachinetypeandfailurecategory.Typically,itisnotcosteffectiveto
takerealtimevibrationdataspotcheckingfacilityequipmentoncepermonth(oronceperquarter)
withhandheldvibrationmonitoringequipmentusuallyprovidessufficientwarningofimpending
problems.Facilityrotatingequipment(e.g.,fans,pumps)doesnotdeterioratefastenoughtowarrant
continualrealtimedatacollection.
Maintenancetechniciansshouldrealizethataccumulatingmoredataisnotnecessarilyindicativeofa
bettervibrationmonitoringprogram.Evenafterthefirstcostsofthevibrationmonitoringanddata
acquisitionsystemhavebeenabsorbed,thereisanoverheadassociatedwithdatacollection.The
datamustbeanalyzedandinterpreted.Evenwiththesophisticatedsoftwareavailabletoassist
themaintenancetechnicianwiththesetasks,ittakesanongoingtimeinvestment.
SpectrumAnalysisandWaveformAnalysis
Spectrumanalysisisthemostcommonlyemployedanalysismethodformachinerydiagnostics.In
thistypeofanalysis,thevibrationtechnicianfocusesonanalyzingspecificslicesofthevibration
datatakenoveracertainrangeofCPM.Spectrumanalysiscanusedtoidentifythemajorityofall
rotatingequipmentfailures(duetomechanicaldegradation)beforefailure.Waveformanalysis,or
timedomainanalysis,isanotherextremelyvaluableanalyticaltool.Whilenotusedasregularlyas
spectrumanalysis,thewaveformoftenhelpstheanalystmorecorrectlydiagnosetheproblem.
TorsionalVibration
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Torsionalvibrationisoftenusedtodetectthevibrationassociatedwiththemeasurementofgear
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vibrationandtorque.Itprovesmosthelpfulinsituationswhere,duetotransmissionpathattenuation,thecasingvibrationsignalhasasignaltonoiseratioinsufficienttodetecttheproblem(i.e.,thenoise
obscuresthesignal).Torsionalvibrationisespeciallyeffectiveinsituationswhereunsteady
forcesexcitetheresonanceofthestructureorhousing.Measuretorquebyusingpairsofmatched
sensorsspacedatasonicintervaltotakeadvantageofthephasedifferenceinthesignals.
Limitations
Theeffectivenessofvibrationmonitoringdependsonsensormounting,resolution,machine
complexity,datacollectiontechniques,andtheabilityoftheanalyst.Thislastfactor,theabilityof
theanalyst,isprobablythemostimportantaspectofestablishinganeffectivevibrationmonitoring
program.Theanalystmustbesomeonewhopossessesathoroughunderstandingofvibrationtheory
andtheextensivefieldexperiencenecessarytomakethecorrectdiagnosisofthevibrationspikes
thatmayappearinthedataacquired.Complex,lowspeed(
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performingoutdoorinspectionsinareassuchastransformers,motorcontrolcenters,switchgear,
substations,switchyards,orpowerlines.Insuchcases,sunlightreflectedfromshinysurfacesmay
makethosesurfacesappeartobehotterthantheadjacentsurfaceswhentheyreallyarenot.Tobe
effectiveinfacilitiesapplications,IRTinstrumentsmustbeportable,sensitivetowithin0.20Cover
arangeoftemperaturesfrom100to+3000C,andaccuratewithin+/3percent.Inaddition,the
instrumentmustbecapableofstoringanimageofthethermogramforlateranalysis.IRTinspections
areidentifiedaseitherqualitativeorquantitative.Thequantitativeinspectionattemptstheaccurate
measurementofthetemperatureoftheitemofinterest.Toperformaquantitativeinspectionrequires
detailedknowledgeandunderstandingoftherelationshipoftemperatureandradiantpower,
reflection,emittance,andenvironmentalfactors,aswellasthelimitationsofthedetection
instrument.Thisknowledgeandunderstandingmustbeappliedinamethodicalfashiontocontrol
theimagingsystemproperlyandtoobtainaccuratetemperaturemeasurements.Quantitative
measurementsoftemperatureareextremelytimeconsuming,andarerarelyneededinfacilities
applications.
Thequalitativeinspectionisinterestedinrelativedifferences,hotandcoldspots,anddeviations
fromnormalorexpectedtemperatureranges.Theknowledgeandunderstandingdiscussedaboveis
neededtoperformameaningfulqualitativeinspection.However,qualitativeinspectionsare
significantlylesstimeconsumingbecausethethermographerisnotconcernedwithhighlyaccurate
temperaturemeasurement.Inqualitativeinspectionsthethermographerobtainsaccuratetemperature
differences(DT)betweenlikecomponents.Forexample,atypicalmotorcontrolcenterwillsupply
threephasepower,throughacircuitbreakerandcontrollertoamotor.Ideally,currentflowthrough
thethreephasecircuitshouldbeuniformsothecomponentswithinthecircuitshouldhavesimilar
temperatures.Anyunevenheating,perhapsduetodirtyorlooseconnections,wouldquicklybe
identifiedwiththeIRTimagingsystem.Becausethemanyvariablesthatinfluencethequantitative
inspection(reflection,emittance,etc.)arethesamebetweenlikecomponents,thethermographercan
quicklyfocusonthetemperaturedifferences.Thefactorssoimportanttoahighlyaccurate
quantitativetemperaturemeasurementhaveverylittleinfluenceonthetemperature
differencesbetweenlikecomponents.
TheoryandApplications
IRTcanbeusedtoidentifydegradingconditionsinfacilitieselectricalsystemssuchastransformers,
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motorcontrolcenters,switchgear,substations,switchyards,orpowerlines.Inmechanicalsystems,
IRTcanidentifyblockedflowconditionsinheatexchanges,condensers,transformercooling
radiators,andpipes.IRTcanalsobeusedtoverifyfluidlevelinlargecontainerssuchasfuelstorage
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tanks.IRTcanidentifyinsulationsystemdegradationinbuildingwallsandroof,aswellas
refractoryinboilersandfurnaces.Temperaturemonitoring,infraredthermographyinparticular,isa
reliabletechniqueforfindingthemoistureinducedtemperatureeffectsthatcharacterizeroofleaks,
andfordeterminingthethermalefficiencyofheatexchangers,boilers,buildingenvelopes,etc.
Deepprobetemperatureanalysiscandetectburiedpipeenergylossandleakagebyexaminingthe
temperatureofthesurroundingsoil.Thistechniquecanbeusedtoquantifygroundenergylossesof
pipes.IRTcanalsobeusedasadamagecontroltooltolocatemishapssuchasfiresandleaks.In
solicitingconsultantstoperformthermography,oneshouldrememberthat(unlessrequested
otherwise)thethermographerwillnormallyprovideonlyanexceptionreportthatidentifies
finds/faults(i.e.,hisanalysiswillbeofqualitativetemperaturedifferences).
Insummary,IRTcanassesstheinserviceconditionofelectricalandmechanicalsystems.Oncethis
isdone,themaintenancesupervisor/managercanprioritizeworkbasedonthetemperaturedifference
criteria.ThegreatertheDT,themoreurgenttheproblem.
Limitations
Thermographyislimitedtolineofsight.Errorscanbeintroducedduetocolorofmaterial,material
geometry,andbyenvironmentalfactorssuchassolarheatingandwindeffects.
Logistics
EquipmentRequired
Equipmentrangesfromsimple,contactdevicessuchasthermometersandcrayonstofullcolor
imaging,computerbasedsystemsthatcanstore,recall,andprintthethermalimages.
Thedeepprobetemperaturetechniquerequirestemperatureprobes,analysis
softwareandequipmenttodeterminethelocationofpipingsystems.
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OperatorsOperatorsandmechanicscanperformtemperaturemeasurementsandanalysisusingcontacttype
deviceswithminimaltrainingonhowandwheretotakethetemperaturereadings.
Becausethermographicimagesarecomplexanddifficulttomeasureandanalyze,trainingis
requiredtoobtainandinterpretaccurateandrepeatablethermaldataandtointerpretthedata.With
adequatetrainingandcertification,electrical/mechanicaltechniciansand/orengineerscanperformed
thistechnique.
Maintenancepersonnelcanapplydeepprobetemperaturemonitoringafter
beingtrained,althoughthisserviceisoftencontracted.
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TrainingAvailable
Trainingisavailablethroughinfraredimagingsystemmanufacturersandvendors.
TheAmericanSocietyofNondestructiveTesting(ASNT)hasestablishedguidelinesfor
nondestructivetesting(NDT)thermographercertification.
Theseguidelines,intendedforuseinnondestructivetesting,maybeusedasguidelinesfor
thermographyinCMifappropriatelyapplied.Certificationisnoteasilyobtained.Whendeciding
whichmaintenancetechnicianshouldbecertifiedasathermographer,themaintenancemanager
shouldconsidergeneralbackground,workexperience,andanypreviousthermographicexperience
orthermographictraining.
5.1.3LubricantandWearParticleAnalysis
Purpose
Lubricatingoilanalysisisperformedforthreereasons:
1.Todeterminethemachinemechanicalwearcondition
2.Todeterminethelubricantcondition
3.Todetermineifthelubricanthasbecomecontaminated.
Awidevarietyoftestscanprovideinformationregardingoneormoreoftheseareas.Thetestused
willdependonthetestresultssensitivityandaccuracy,thecost,andthemachineconstructionand
application.Thethreeareasarenotunrelatedchangesinlubricantconditionandcontamination,if
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notcorrected,willleadtomachinewear.Becauseoftheimportantrelationships,commercial
analysislaboratorieswilloftengroupseveraltestsincosteffectivetestpackagesthatprovide
informationaboutallthreeareas.
MachineMechanicalWearCondition
Thecriteriaforanalyzingthelubricatingoiltodeterminethemachinesconditionaregenerallythe
sameasforperformingvibrationanalysis.Thisanalysisisapplicabletoallmachineswithmotors
7.5HPorlarger,criticalmachines,orhighcostmachines.Generallytheroutinesamplingand
analysisperiodicitywillbethesameasthevibrationanalysisperiodicity(whenusingaportable
vibrationdatacollector).Formachineswithaconditionhistory(ayearormoreofdata),thisis
typicallyperformedquarterly.
LubricantCondition
Lubricatingoiliseitherdiscardedorreconditionedthroughfilteringand/orreplacingadditives.
Analyzingtheoiltodeterminethelubricantconditionis,therefore,drivenbycosts.Smallmachines,
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thosewithoilreservoirs1galorless,havetheoilchangedonanoperatingtimebasis.An
automobileisthemostcommonexampleoftimebasedlubricatingoilmaintenance.Inthisexample,
thecoststoreplacetheautomobileoil(thereplacementoil,labortochangetheoil,anddisposal
costs)arelowerthanthecosttoanalyzetheoil(i.e.,thecostofsamplematerials,labortocollectthe
sample,andtheanalysis).Inthecaseofautomobileoil,timebasedreplacementischeaperthan
analysisduetocompetitionandtheeconomiesofscalethathavebeencreatedtomeettheconsumer
needforreplacingautomobileoil.
Inthecaseoflubricatingoilusedinfacilityequipment,simplyreplaceanddiscardthemachine
lubricatingoilifitischeaperthananalyzingit.Whenmakingthisdecision,themaintenance
managermusthavefirmpricesformaterialsusedtotakesamplesandthelaborhoursitwilltaketo
collect,package,andsendthesamplesoutforanalysis.Remember,though,thatoneoilsampleis
sufficientformanytests.
LubricantContamination
Lubricatingoilcanbecomecontaminatedduetothemachinesoperatingenvironment,improper
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fillingprocedures,orthroughthemixingofdifferentlubricantsinthesamemachine.Ifamachineis
toppedoffwithoilfrequently,themaintenancetechnicianshouldsendtheoiloutforanalysis
periodicallytocheckthemachineforanyseriousproblems.
StandardAnalyticalTests
Lubricatingoilandhydraulicfluidanalysisshouldproceedfromsimple,subjectivetechniquessuch
asvisualandodourexaminationthroughmoresophisticatedtechniques.Themoresophisticated(and
expensive)techniquesshouldbeusedwhenconditionsindicatetheneedforadditionalinformation
andtheequipmentcostorcriticalityjustifiesthecost.
VisualandOdour
Simpleinspectionscanbeperformedweeklybytheequipmentoperatortolookatandsmellthe
lubricatingoil.Avisualinspectionlooksforchangesincolour,hazinessorcloudiness,andparticles.
Thistestisverysubjective,butcanbeanindicatorofrecentwaterordirtcontaminationand
advancingoxidation.Asmallsampleoffreshlubricatingoilinasealed,clearbottle,canbekepton
handforvisualcomparison.Aburnedsmellmayindicateoxidationoftheoil.Otherodorscould
indicatecontamination.Odorismoresubjectivethanthevisualinspectionbecausepeoples
sensitivitytosmellvaries,andthereisnoeffectivewaytocomparetheodorbetweensamples.The
operatormustbecarefulnottointroducedirtintothesystemwhentakingasample.
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Viscosity
Viscosityisameasureofoilflowrateataspecifiedtemperature.Achange(increaseordecrease)in
viscosityovertimeindicateschangesinthelubricantcondition,oritmayindicatelubricant
contamination.Viscositycanbetestedusingportableequipment,oritcanbetestedmoreaccurately
inalaboratoryusingtheASTMD445procedure.Viscosityismeasuredincentistoke(cSt),and
minimumandmaximumvaluesareidentifiedbytheISOgrade.Testingoilviscosityisusuallypart
ofacommerciallaboratorystandardtestpackage.
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Water
Waterinlubricatingoilandhydraulicfluidcontributestocorrosionandformationofacids.Small
amountsofwater(lessthan0.1percent)canbedissolvedinoilandcanbedetectedusingthecrackle
testorinfraredspectroscopy(minimumdetectableis0.05percentorapproximately500ppmbyboth
methods),theASTMD95distillationmethod(minimumdetectableis0.01percent/100ppm),the
ASTMD1744KarlFischermethod(minimumdetectableis0.002percent/100ppm).Ifgreaterthan
0.1percentwaterissuspendedoremulsifiedintheoil,theoilwillappearcloudyorhazy.Freewater
inoilcollectsinthebottomofoilreservoirsandcanbefoundbydrainingthemfromthebottom.
PercentSolids/Water
Asimple,inexpensivetestisusedtoprovideagrossestimateofsolidsand/orwaterintheoil.A
sampleiscentrifugedinacalibratedtubeandtheresultingvolumeismeasured.Thetestiseffective
foramountsintherangeof0.1to20percentofvolumeandisusuallypartofacommercial
laboratorystandardtestpackage.
TotalAcidNumber(TAN)
TotalacidisanindicatorofthelubricatingoilconditionandismonitoredrelativetotheTANofnew
oil.Insomesystems,theTANwillalsobeusedtoindicateacidcontamination.TANismeasuredin
milligramsofpotassiumhydroxide(KOH)pergramofoil(mgKOH/g).KOHisusedinatitration
processandtheendpointisindicatedbycolorchange(ASTMD974)orelectricalconductivity
change(ASTMD664).
TotalBaseNumber(TBN)
SimilartotheTANtestmethod,theTBNtestmeasuresalkalinity(abilitytoneutralizeacid)ofoil
sample.Thistestisusedonoilwithhighdetergentadditivessuchasdieselandgasolineengines.
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KOHisusedinatitrationprocessandtheendpointisindicatedbyelectricalconductivitychange
(perASTMD664orASTMD2896).Whencomparingtestresultsfromyouroilagainstbaseline
datafromtheoilsupplier,makesurethatthesametestmethodwasusedforyouroilaswasusedin
generatingthebaselinedata.Resultscanvarysignificantlybetweentestmethods.
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SpectrometricMetals
Alsoknownasemissionspectroscopy,thistechniqueexaminesthelight(spectrum)emittedfromthe
sampleduringtesting,andidentifiesupto21metals.Metalsarecategorizedaswear,contaminate,or
additivemetals.Theprocedureidentifiesbothsolublemetalandmetalparticlesupto5to10
microns(510mm).
Thetestcostismoderate,andisusuallypartofacommerciallaboratorystandardtestpackage.Other
techniques(e.g.,absorptionspectroscopyandXrayspectroscopy)areusedbysomelaboratoriesto
identifymetals.
InfraredSpectroscopy
Thistechniqueisalsoknownasinfraredanalysis,infraredabsorptionspectroscopyor
spectrophotometry,andFourierTransformInfrared(FTIR)spectroscopy.Thetechniqueexamines
theinfraredwavelengththatisabsorbedbytheoilsample.Thetestisusedtoidentifynonmetallic
contaminationandlubricantconditions(e.g.,oxidation,antioxidant,otheradditivedepletion).Inthe
future,itmaybecomepossibletocouplecomputerexpertsystemanalysiswithknownoilspectrums,
inanefforttoproducehighlyaccuratediagnosisofsmallchangesintheoilcondition.Costs
vary,dependingonthelevelofsophisticationrequired.Infraredspectroscopyisusuallypartofa
commerciallaboratorystandardtestpackage.
AnalyticalFerrography
MoredetailedthanDirectReading(DR)ferrography,analyticalferrographyisofteninitiatedbased
onchangesinDR,spectrometricmetalincreases,orincreasedparticlecount.Theanalysisis
sometimesperformedonaregularbasisonexpensiveorcriticalmachines.Thetestprocessislabor
intensiveandinvolvesthepreparationofsampleandexaminationundermagnification.Results
varywiththeanalystscapability,buttheprocedurecanprovidedetailedinformationregarding
wear:e.g.,weartype(rubbing,sliding,cutting),color,particletypes(oxide,corrosive,crystalline),
andothernonferrousparticles.Thisdetailedinformationcanbecriticalinfindingtherootcauseof
wearproblems.Costsaremoderatelyhighthetestisperformedonafixedpricebasis(persample)
fromacommerciallaboratory.
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SpecialTests
Specialtestsaresometimesneededtomonitorlubricantconditionsonsomeexpensiveorcritical
Systems.Usuallythespecialtestisusedtomonitoralubricantcontaminate,acharacteristic,or
additivedepletion.Thissectionidentifiessomeofthespecialtestsavailable.Specialtestsarerarely
neededforroutinemonitoringoflubricants.Thelistofspecialtestpresentedhereisnotmeantto
beallinclusiveonlyalistofsamples.Testproceduresareconstantlybeingdevelopedandrefined.
TheannualASTMStandardsprovidesadescriptionofcurrenttestmethods,
GlycolAntifreeze
Glycolcontaminationcanbedetectedusinginfraredspectroscopy(seeInfraredSpectroscopy,
discussedearlier)atlevelsgreaterthan0.1percent(1,000ppm),whichisusuallyadequatefor
conditionmonitoring.However,additionaltestscanbespecifiedtoidentifyifsmallamountsof
glycolarepresent.ASTMD2982willindicateiftraceamountsarepresent.ASTMD4291usesgas
chromatographytoquantifysmallamountsofglycol.
KarlFischerWater
Watercontaminationcanbedetectedusinginfraredspectroscopy(seeInfraredSpectroscopy,p74)
atlevelsgreaterthan0.05percent(500ppm),whichisusuallyadequateforconditionmonitoring.
UsingatitrationprocesswithaKarlFischerreagent,lowlevelsofwatercanbedetectedand
quantified.Thetest,ASTMD1744,isusefulwhenacceptingnewoilorevaluatingcleanupefforts.
Costofthetestismoderate.
Foamlug
Someoilmayhaveantifoamagentsaddedtoimprovethelubricationcapabilityinspecific
applicationssuchasgearboxesormixers.ASTMtestD892canbeusedtotesttheoilsfoam
characteristics.Thetestblowsairthroughasampleoftheoilandmeasuresthefoamvolume.Costof
thetestismoderatelyhigh.
RustPrevention
Somesystemsaresusceptibletowatercontaminationduetoequipmentlocationorthesystem
operatingenvironment.Inthosecases,thelubricatingoilorhydraulicfluidmaybefortifiedwithan
inhibitortopreventrust.TheeffectivenessofrustpreventioncanbetestedusingASTMD665(or
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ASTMD3603).Resultsarepass/failandthecostofthetestishigh.
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RotatingBombOxidationTest(RBOT)
AlsoknownastheRotaryBombOxidationTest,ASTMD2272isusedtoestimateoxidation
stabilityandtheremainingusefullifeofoil.Thetestsimulatesaging,identifyingwhenrapid
oxidationtakesplaceandindicatingthatantioxidantshavebeendepleted.Thetestisnotaonetime
testitmustbeperformedovertime,startingwithabaselinetestofthenewoil.Subsequenttests
arenecessarytodevelopthetrendline.Becauseofthehighcostandthemultipletestsrequired,this
testisusuallyonlyperformedonlargevolumereservoirsorexpensiveoil.
Application
Typically,lubricatingoilanalysisshouldbeperformedonaquarterlybasisonallmachineswith
motors7.5HPorlarger,andonallcriticalorexpensivemachines.Theanalysisscheduleshouldbe
adjustedinthesamewaythatthevibrationanalysisscheduleisadjusted.Analyzemorefrequently
formachinesthatareindicatingemergingproblemslessfrequentlyformachinesthatoperateunder
thesameconditionsandarenotrunonacontinuousbasis.Anewbaselineanalysiswillbeneeded
followingmachinerepairoroilchangeout.Allhydraulicsystems,exceptmobilesystems,shouldbe
analyzedonaquarterlybasis.Mobilesystemsshouldbeconsideredforanalysisbasedonthe
machinesizeandthecosteffectivenessofperformingtheanalysis.Generally,itismorecost
effectiveinmobileequipmenttomaintainthehydraulicfluidbasedonthefluidcondition.
However,forsmallsystems,thecosttoflushandreplacethehydraulicfluidonatimebasismaybe
lowerthanthecosttoanalyzethefluidonaroutinebasis.Greaseisusuallynotanalyzedonaregular
basis.Althoughmostofthetestingthatisdoneonoilcanalsobedoneongrease,thereisaproblem
gettingarepresentativesample.Togetarepresentativesamplethatisahomogeneousmixture
ofthegrease,contaminants,andwear,themachinemustusuallybedisassembled.Onceamachine
hasfailedandmustbedisassembled,analysisofthegreasetodiagnosethefailurecansometimesbe
useful.Aconcerncommontoallmachineswithlubricatingoilsystemsiskeepingdirtandmoisture
outofthesystem.Commoncomponentsofdirt,suchassilica,areabrasiveandnaturallypromote
wearofcontactsurfaces.Inhydraulicsystems,particlescanblockandabradetheclosetolerancesof
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movingparts.Waterinoilpromotesoxidationandreactswithadditivestodegradetheperformance
ofthelubricationsystem.Ideally,therewouldbenodirtormoistureinthelubricant
this,ofcourse,isnotpossible.Thelubricantanalysisprogrammustthereforemonitorandcontrol
contaminants.Largesystemswithfilterswillhavesteadystatelevelsofcontaminates.Increasesin
contaminatesindicatebreakdowninthesystemsintegrity(leaksinseals,doors,heatexchangers,
etc.)ordegradationofthefilter.Unfilteredsystemscanexhibitsteadyincreasesduringoperation.
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Operatorscanperformaweeklyvisualandodorcheckoflubricatingsystemsandprovideafirst
alertofcontamination.Somebearinglubricatingsystemshavesuchasmallamountofoilthata
weeklycheckmaybeimpractical.
Motors,Generators,Pumps,Blowers,Fan
Formachineswithlessthan5galinthelubricationsystem,theanalystismostlyconcernedwith
machinecondition.Lubricantconditionandcontaminationareofinterestbecausetheyprovidesome
indicationofmachinecondition.
Routinelymonitorviscosity,percentsolids/water,andspectrometricmetals.Monitortrendsand
discardorrefreshtheoilwhenviscositychanges10percentfromthebaseline.Viscositynormally
increasesabovethebaselinewiththeoilservicetime.Iftheviscositydecreasesbelowthebaseline,it
usuallymeansthattheoiliscontaminated,probablyfromaddingthewrongtypeofmakeupoil.
Thereshouldbenowaterpresent(minimumdetectablewateris0.1percent).Ifthereiswater,the
sourceofthewaterneedstobeidentifiedandcorrected.Formachineswithmorethan5galofoilin
thesystem,addinfraredspectroscopy(minimumamountofwaterdetectableis0.05percent)and
particlecounting.Changesinparticlecountcanindicateincreasedcontaminationorincreased
wear.Correlateparticlecountwithspectrometricmetals.Therateofparticlecountchangeindicates
howquicklythelubricantisdegrading.Visualparticlecountingcanbeusedtoidentifythesourceof
thecontamination.Inaddition,performDRferrographyforexpensiveorcriticalmachines.Inall
machines,changesinspectrometricmetalsorDRshouldbeinvestigatedfurtherusinganalytical
ferrographyandcorrelatedwithvibrationanalysis.
Gearboxes
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Sameasabove,exceptforgearboxeswithlessthan5galofoil,addparticlecounting.Implement
DRferrographyforhighcostorcriticalgearboxes.Monitortrendsandcorrelatewithvibration
readings.
Chillers
Inadditiontotheitemsidentifiedabove,addTotalAcidNumber(TAN)andDRferrography.
DieselEngines
UsethesameprocedureasforchillersexceptsubstituteTotalBaseNumber(TBN)forTANwhen
oilhashighdetergentadditives.Adecreaseinviscositybelowthebaselinemayindicatefuel
contamination.Coolantleakage(glycolandothercharacteristics)isidentifiedfromtheinfrared
spectroscopyanalysis.
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Compressors
Centrifugalcompressorsshouldbetreatedthesameaschillers.Reciprocatingcompressorsshouldbe
treatedthesameasdieselengines.
HydraulicSystems
Performthesameoilanalysisasthatperformedongearboxes.MonitorparticlecountbyISO
category.Eachhydraulicsystemwillhavelimitingclearancesthatwilldeterminecriticalparticle
sizes.Notethatsomehydraulicsystemsusefluidsotherthanoil(waterorglycol).Forthesesystems,
oilanalysisdoesnotapplyhowever,performparticlecontrolthesameasforoilfilledhydraulic
systems.
LargeReservoirs
Forreservoirsover500gal,considerperforminganRotatingBombOxidationTest(RBOT)to
assesstheoxygenstability.Costisusuallythedecidingfactor.Atleastthreetestsareneededto
developatrend.Oncethetrendhasbeenestablished,additionalretestingshouldbeperformedat
leastonceayear.Maintenancedollararesavedwhenreplacementorrefreshingofalargevolumeof
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oil(orsmallervolumeofexpensiveoil)canbedeferred.
LubricationAnalysis
Asonecanseefromreadingtheabove,therearenumerouslubricationtests.
Commerciallaboratoriesperformingthetestshavechartsavailablethatsummarizethevarious
lubricanttests,monitoringinterval,andapplication.
Sampling
Oilsamplesmustbecollectedsafelyandinamannerthatwillnotintroducedirtandother
contaminatesintothemachine/system,orintothesample.Itmaybenecessarytoinstallpermanent
samplevalvesinsomelubricatingsystems.Theoilsampleshouldberepresentativeoftheoilseenin
themachine.Thesampleshould,therefore,becollectedfromamidpointinreservoirsandupstream
ofthefilterincirculatingsystems.Samplecollectionbottlesandtubingcanbeprocured
throughtestinglaboratories.Thetestinglaboratorycanalsoprovideguidanceasregardstothe
cleanlinesslevelneeded.Oilsamplepumpsforextractingoilfromreservoirsmustbeusedproperly
toavoidcontamination.Samplesmustbecollectedfromthesamepointinthesystemtoensure
consistencyinthetestanalysistherefore,themaintenanceproceduremustprovidedetailed
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directiononwhereandhowtocollectsamples.Theequipmentoperatorscancollectsamples.Each
sampleismarkedwiththesystem/machinename,samplelocationpoint(thesystemmayhave
multiplesamplepoints),date,elapsedoperatingtimeforthesystem/machine,andothercomments
suchaslasttoppingofforfilteringoperation.Theanalystwillalsoneedtoknowtheamountofoil
inthereservoirtomakerecommendationstocorrectabnormalities.
5.1.4NonDestructiveTesting
NonDestructiveTesting(NDT)evaluatesmaterialpropertiesandqualityofmanufacturefor
expensivecomponentsorassemblieswithoutdamagingtheproductoritsfunction.Insteadof
statisticalsamplingtechniquesthatuseonlysurfacemeasurementsorrequirethedestructivetesting
ofselectedcomponentsfromaproductionlot,NDTisusedwhenthesetestingtechniquesarecost
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prohibitiveorineffective.Typically,NDThasbeenassociatedwiththeweldingoflargehighstress
componentssuchaspressurevesselsandstructuralsupports.Processplantssuchasrefineriesor
chemicalplantsuseNDTtechniquestoensureintegrityofpressureboundariesforsystems
processingvolatilesubstances.
Techniques
ThefollowingsectiondiscussedvariousNDTtechniques:
(i)Radiography
Radiographyisperformedtodetectsubsurfacedefects.RadiographyorXrayisoneofthemost
powerfulNDTtechniqueavailableinindustry.Dependingonthestrengthoftheradiationsource,
radiographycanprovideaclearrepresentation(radiograph)ofdiscontinuitiesorinclusionsin
materialseveralinchesthick.Xrayorgammaraysensitivefilmisplacedononesurfaceofthe
materialtobeexamined.Theradiationsourceispositionedontheoppositesideofthepiece.The
sourcemaybeeitheranaturalgammaemitterorapoweredXrayemitter.Thesourceisaccurately
alignedtoensuretheproperexposureanglethroughthematerial.Whenallpreparationsandsafety
precautionsarecomplete,theradiationsourceisenergizedorunshielded.GammaorXrayspass
throughamaterialandexposefilmplacedunderthematerial.Bydevelopingthefilminamanner
similartophotographicfilm,animageofdefectsorinclusionsinthematerialisproduced.More
advancedradioluminescentfilmdoesnotrequirephotographicprocessing.Multipleshotsfrom
varyinganglesprovideacompletepictureofthethicknessofthematerial.Dualanglesarerequired
todeterminethesizeandorientationofaninclusion.
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Oncethetype,size,andorientationofeachinclusionaredefined,thesecanbeclassifiedaseither
acceptableinclusionsorunacceptabledefects.Defectsinthematerialmustbeaccuratelylocatedto
facilitateminimalmaterialremoval,yetensurethedefecthasbeencompletelyeliminated.
Minimizingmaterialremovalalsominimizesrepaircostandreducesthelikelihoodofadditional
defectscreatedbytherepair.Therepairisthenreevaluatedtoensurethedefectremovaland
subsequentrepairwereconductedproperly.
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Radiography,thoughaversatiletool,islimitedbythepotentialhealthrisks.Useofradiography
usuallyrequiresthepiecebemovedtoaspecialshieldedarea,orthatpersonnelbeevacuatedfrom
thevicinitytoavoidexposuretothepowerfulradiationsourcerequiredtopenetrateseveralinchesof
densematerial.Temporaryshieldingmayalsobeinstalled,buttheinstallationandremovalof
thousandsofpoundsofleadislaborintensiveandrarelyworththeexpense.Radiography
techniciansaretrainedinradiationhealthphysicsandmaterialproperties.Thesetechnicianscan
visuallydistinguishbetweenweldingslaginclusions,porosity,cracking,andfatiguewhenanalyzing
radiographicimages.
(ii)UltrasonicTesting(Imaging)
Ultrasonictestingprovidesdetectionofdeepsubsurfacedefects.Ultrasonic(UT)inspectionof
weldsandbasematerialisoftenanalternativeorcomplementaryNDTtechniquetoradiography.
Thoughmoredependentontheskilloftheoperator,UTdoesnotproducetheharmfulradiation
entailedwithradiography.UTinspectionisbasedonthedifferenceinthewavereflectingproperties
ofdefectsandthesurroundingmaterial.Anultrasonicsignalisappliedthroughatransducerintothe
materialbeinginspected.Thespeedandintensitywithwhichthesignalistransmittedorreflectedto
atransducerprovidesagraphicrepresentationofdefectsordiscontinuitieswithinthematerial.A
couplantfluidisoftenusedtoprovideauniformtransmissionpathbetweenthetransducer,receiver
andthematerialofinterest.Transducerconfigurationsdifferdependingonthetypeofsystemused.
Somesystemsuseasingletransducertotransmitandreceivethetestsignal.Othersuseatransmit
transducerinconjunctionwithaseparatereceivetransducer.Dualtransducersystemsmaybe
configuredwithbothtransducersonthesamesurfaceofthematerialorwithtransducers
ontheoppositesurfacesofthematerial.
Threescantypesaremostcommonlyused:AScan,BScanandCScan.AScansystem
analyzessignalamplitudealongwithreturntimeorphaseshiftsthesignalstravelbetweenaspecific
surfaceanddiscontinuities.BScansystemsaddsignalintensitymodulationandcapabilitytoretain
videoimages.CScanSystemsincludedepthgatingtoeliminateunwantedreturns.UTinspectionis
adeliberateprocesscoveringasmallarea(4to8sqin.)ateachsampling.Consistencyintest
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methodandinterpretationofresultsiscriticaltothereliabletestresults.Surfacepreparationisalso
criticaltoreliableUTresults.Anysurfacedefectssuchascracks,corrosion,orgougeswilladversely
affectthereliabilityofUTresults.Duetothetimeandeffortinvolvedinsurfacepreparationand
testing,UTinspectionsareoftenconductedonrepresentativesamplesofmaterialssubjected
tohighstresslevels,highcorrosionareasandlargewelds.Byevaluatingthesamesitesatregular
intervals,onecanmonitortheconditionofthematerial.OnehundredpercentUTinspectionis
typicallyreservedfororiginalconstructionofhighstresscomponentssuchasnuclearreactorvessels
orchemicalprocessvessels.
(iii)MagneticParticleTesting
TheNDTtechniqueusesmagneticparticledetectionofshallowsubsurfacedefects.
MagneticParticleTesting(MT)techniquesareusefulduringlocalizedinspectionsofweldareasand
specificareasofhighstressorfatigueloading.MTprovidestheabilitytolocateshallowsubsurface
defects.Twoelectrodesareplacedseveralinchesapartonthesurfaceofthematerialtobeinspected.
Anelectriccurrentispassedbetweentheelectrodesproducingmagneticlines.Whilethecurrentis
applied,ironinkorpowderissprinkledintheareaofinterest.Theironalignswiththelinesofflux.
Anydefectintheareaofinterestwillcausedistortionsinthelinesofmagneticflux,whichwillbe
visiblethroughthealignmentofthepowder.Surfacepreparationisimportantsincethepowderis
sprinkleddirectlyontothemetalsurfaceandmajorsurfacedefectswillinterferewithsubsurface
defectindications.Also,goodelectrodecontactandplacementisimportanttoensureconsistent
strengthinthelinesofmagneticflux.AmajoradvantageforMTisitsportabilityandspeedof
testing.Thehandheldelectrodesallowtheorientationofthetesttobechangedinseconds.This
allowsforinspectionofdefectsinmultipleaxesoforientation.Multiplesitescanbeinspected
quicklywithoutinterruptingworkinthevicinityTheequipmentisportableandispreferredforon
siteorinplaceapplications.TheresultsofMTinspectionsarerecordablewithahighquality
photographortransfertotape.Fixingcompoundsareavailabletogluetheparticlepatterninplace
onthetestspecimen.Interpretationofresultsdependsontheexperienceoftheoperator.
(iv)DyePenetrant
DyePenetrantisusedtodetectsurfacedefects.Dyepenetrant(DP)inspectionsprovideasimple
methodfordetectingsurfacedefectsinnonporousmaterials.DPallowslargeareastobequickly
inspected.Oncethesurfacehasbeencleaned,apenetratingdye(magentaorfluorescentcolor)is
sprayedliberallyontheentiresurface.Thedyeisallowedtopenetrateforseveralminutes.The
excessdyeisthenwipedfromthesurfaceleavingonlythedyethathasbeendrawnintosurface
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defects.Adeveloper(usuallywhite)issprayedontheentiresurface(sameareaasthedye
application).Thedeveloperdrawsthedyefromthedefects,producingavisualindicationofthe
presenceofsurfacedefects.Thedefectiveareasarethenidentifiedforrepairandtheremainingdye
anddeveloperareremoved.
(v)HydrostaticTesting
HydrostaticTesting(Hydro)isanNDTmethodfordetectingdefectsthatcompletelypenetrate
pressureboundaries.Hydrosaretypicallyconductedpriortothedeliveryoroperationofcompleted
systemsorsubsystemsthatactaspressureboundaries.Asthenameimplies,hydrostatictestsfillthe
systemtobetestedwithwaterortheoperatingfluid.Thesystemisthensealedandthepressure
isincreasedtoapproximately1.5timesoperatingpressure.Thispressureisheldforadefinedperiod.
Duringthetest,inspectionsareconductedtofindvisibleleakstowellasmonitorpressuredropand
makeupwateradditions.Ifthepressuredropisoutofspecification,theleak(s)mustbelocated
andrepaired.Theprincipleofhydrostatictestingcanalsobeusedwithcompressedgases.Thistype
oftestistypicallycalledanairdroptestandisoftenusedtotesttheintegrityofhighpressureairor
gassystems.
Applications
1.Radiography.Radiographictechniquesarereadilyapplicabletometalcomponents,including
welddeposits.Specializedapplicationsforplasticsorcompositematerialsarepossible,though
typicallythesematerialsarenotmosteconomicallyinspectedwithradiography.Forthickcross
sections,radiographyisoftentheonlyreliablemethodforinspection.
2.Ultrasonics.UTtechniquesareapplicabletometalcomponentsincludingwelddeposits.
Specializedapplicationsforplasticsorcompositematerialsarecommon.Whenpossible,UTisa
preferredmethodoverradiographyforinplaceapplications,duetoexpenseandsafetyprecautions
requiredbyradiography.UTisespeciallyusefulsinceitonlyrequiresaccesstoonesurfaceofthe
material.Ultrasonictechniquesprovideexcellentpenetratingpowerforthickcrosssections.
3.MagneticParticle.MTtechniquesareapplicableonlytomaterialsthatconductelectriccurrent
andmagneticlinesofflux.OnlyshallowdefectsaredetectablewithMTinspection.Typically,these
techniquesaremosteffectiveonweldedareas.Thespeedoftestingallowsmultipleinspectionstobe
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conductedalongdifferentaxestodetectdefectsindifferentorientationplanes.
4.DyePenetrant.DPinspectionsareapplicableforanynonporousmaterialthatischemically
compatiblewiththedyeanddeveloper.ThisisthesimplestNDTtechniqueinwhichtogain
proficiency.
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5.HydrostaticTesting.Hydrostesttheintegrityofpressureboundariesforcomponentsand
completelyassembledsystemsthatcontainpressurizedfluidsorgases.Identificationofdefectsthat
penetratetheentirepressureboundaryistheprimaryapplicationforhydrostatictesting.
Limitations
1.MagneticParticle.MTtechniquesareapplicableonlytomaterialsthatconductelectricalcurrent
andinfluencemagneticlinesofflux.Thedifferenceintheinfluenceofthelinesoffluxbetweenbase
materialandthedefectisthebasisforMTinspection.Onlysmallareas(30sqin.)betweenthetwo
electrodescanbeinspected.Surfacepreparationisimportant,thoughnotascriticalaswithUT.
Consistentelectrodecontactiscritical.Loosecontactwillweakenthemagneticlinesoffluxtothe
pointwheretheinfluenceofadefectmaynotbevisibleinthefilingpattern.Operatorskillis
important,thoughthisisarelativelysimpletechnique.Nohistoricalrecordisproducedforeachtest,
unlessspecificstepsaretakentophotographtheresultofeachtest.
2.Ultrasonics.UTtechniquesareonedimensional.Unlessspecialtechniquesareapplied.,defects
thatparalleltheaxisofthetestwillnotbeapparent.Componentsconstructedusinglaminate
techniquesorlayeredconstructionpresentspecialproblemsforUTtechniques,sincetheboundary
betweeneachlayermaybeinterpretedasadefect.Thethickerthelayersofbasematerial,themore
likelyUTwillprovideusableresults.
3.Radiography.Effectiveuseofradiographymandatesexpensiveequipment,extensivesafety
precautionsandskilledtechnicianstointerprettheimages.Expensivetrackingandsecurityfor
radiationsourcesismandatory.Safetyprecautionsoftendemandevacuationofareasadjacenttothe
piecebeingexaminedorinstallationofextensiveshielding.Evenwiththeselimitations,radiography
isoftenthemosteffectivemethodofassuringintegrityofcriticalwelds,structuralmembers,and
pressureboundaries.Asmaterialthicknessincreases,radiographyisoftentheonlyacceptable
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methodtoachievea100percentpenetration.4.DyePenetrant.Minutesurfacediscontinuitiessuchasmachiningmarkswillbecomereadily
apparent.Theinspectormustbetrainedtodistinguishbetweennormalsurfacediscontinuitiesand
defectsthatmustberepaired.Thedyeanddeveloperareusuallysprayedorpaintedonthepieceto
beinspected,sooversprayandprotectionofinternalsurfacesareprimeconcernsforsystemswith
stringentchemistryandcleanlinesscontrol.ProductcleanlinessstandardsmayprohibittheuseofDP
inspection.
5.HydrostaticTesting.Cleanlinessandchemistrycontrolofthefluidmustbeconsistent
withtheoperatingstandardsofthesystem.Closeattentionshouldbegiventocontrollingsystem
thermodynamicparametersduringthetesttopreventoverpressurizationofthesystem.
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Overpressurizationcouldleadtounintendeddamagetothesystem.Individualcomponenthydrosdo
notensuresystemintegrity.Afinalhydroofthecompletedsystemisusedtoensuretheintegrityof
theassembledsystemspressureboundary.
6.Hydroswillnotidentifydefectsthatarepresent,buthavenotcompletelypenetratedapressure
boundary.Thepressureappliedtothesystemisgenerallynotsufficienttoenlargeexistingdefectsto
thepointofdetectionbythetest.Hydrostatictestingrequiresapressuresourcecapableof
expeditiouslyfillingandpressurizingthesystem,extensiveinstrumentationandmonitoring
equipment,alongwithasufficientquantityoffluidtofillthesystem.Amethodofisolatingpressure
reliefdevicesandconnectingthepressuresourcetothesystemmustbeprovided.
6.0MAINTENANCEPLANNING
Aneffectivemaintenanceplanningisessentialinanorganization.Itisgoodpracticetoconduct
someformofanalysistoidentifytheappropriatemaintenancetaskstocareofequipment.
Theanalysiswillresultinalistoftasksthatneedtobesortedandgroupedintosensiblechunks,
eachformingthecontentofachecklist.Themostobviousnextstepistoscheduletheworkorders
generatedintoaplanofworkfortheworkshopteams.Lesscommon,however,istousethis
checklistdatatocreatealongrangeplanofforecastedmaintenancework.
Thisplanservestwopurposes:
(i)Theresultscanbeusedtodeterminefuturelabourrequirements,and
(ii)Theyfeedintotheproductionplan.
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