course title_ mechanical maintenance

9/4/2015 COURSETITLE:MECHANICALMAINTENANCE

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DEPARTMENTOFMECHANICALENGINEERING,COLLEGEOFENGINEERING,

UNIVERSITYOFAGRICULTURE,ABEOKUTA.

LECTURENOTE

COURSETITLE:MECHANICALMAINTENANCE

COURSECODE:MCE509

COURSEUNITS:2



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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.

Page18



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.



18

Page19

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.



19

2.Enginespeedisexcessive.3.Oilfillcaporgasketisdamagedormissing.4.Breathermechanismisdirtyordefective.5.Pistonringgapsarealigned.6.Pistonringsareworn.

Problem:Enginebackfires.Possiblecauses:

Page20

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



20

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



21

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

Page22

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



22

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.

Page23

Themaintenancesupervisor/managermustmakethedecisionwhetheritmakeseconomicsenseto

performthisfunctionwithinhouselaborforcesorwhetheritshouldbeoutsourcedtoacontractor

specializinginvibrationmonitoringandanalysis.Inmakingthisdecision,maintenance

supervisors/managersshouldconsiderwhethertheyhavesufficientinhouselabortodedicateto

vibrationmonitoring.Vibrationmonitoringtheoryiscomplicated,theequipmentisexpensive,

andtheanalysisofthedatacollectedisaskillthatmustbepracticedregularly.Oncehehas

completedthebasicvibrationtraining(costingseveralthousandsofdollars),amaintenance



23

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

Page24

Torsionalvibrationisoftenusedtodetectthevibrationassociatedwiththemeasurementofgear



24

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(



Page25

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,



25

motorcontrolcenters,switchgear,substations,switchyards,orpowerlines.Inmechanicalsystems,

IRTcanidentifyblockedflowconditionsinheatexchanges,condensers,transformercooling

radiators,andpipes.IRTcanalsobeusedtoverifyfluidlevelinlargecontainerssuchasfuelstorage

Page26

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.



26

OperatorsOperatorsandmechanicscanperformtemperaturemeasurementsandanalysisusingcontacttype

deviceswithminimaltrainingonhowandwheretotakethetemperaturereadings.

Becausethermographicimagesarecomplexanddifficulttomeasureandanalyze,trainingis

requiredtoobtainandinterpretaccurateandrepeatablethermaldataandtointerpretthedata.With

adequatetrainingandcertification,electrical/mechanicaltechniciansand/orengineerscanperformed

thistechnique.

Maintenancepersonnelcanapplydeepprobetemperaturemonitoringafter

beingtrained,althoughthisserviceisoftencontracted.

Page27

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



27

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,

Page28

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



28

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.

Page29

Viscosity

Viscosityisameasureofoilflowrateataspecifiedtemperature.Achange(increaseordecrease)in

viscosityovertimeindicateschangesinthelubricantcondition,oritmayindicatelubricant

contamination.Viscositycanbetestedusingportableequipment,oritcanbetestedmoreaccurately

inalaboratoryusingtheASTMD445procedure.Viscosityismeasuredincentistoke(cSt),and

minimumandmaximumvaluesareidentifiedbytheISOgrade.Testingoilviscosityisusuallypart

ofacommerciallaboratorystandardtestpackage.



29

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.

Page30

KOHisusedinatitrationprocessandtheendpointisindicatedbyelectricalconductivitychange

(perASTMD664orASTMD2896).Whencomparingtestresultsfromyouroilagainstbaseline

datafromtheoilsupplier,makesurethatthesametestmethodwasusedforyouroilaswasusedin

generatingthebaselinedata.Resultscanvarysignificantlybetweentestmethods.



30

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



31

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



32

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



33

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



34

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



35

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.



36

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

Page37



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



37

Page38

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



38

conductedalongdifferentaxestodetectdefectsindifferentorientationplanes.

4.DyePenetrant.DPinspectionsareapplicableforanynonporousmaterialthatischemically

compatiblewiththedyeanddeveloper.ThisisthesimplestNDTtechniqueinwhichtogain

proficiency.

Page39

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



39

methodtoachievea100percentpenetration.4.DyePenetrant.Minutesurfacediscontinuitiessuchasmachiningmarkswillbecomereadily

apparent.Theinspectormustbetrainedtodistinguishbetweennormalsurfacediscontinuitiesand

defectsthatmustberepaired.Thedyeanddeveloperareusuallysprayedorpaintedonthepieceto

beinspected,sooversprayandprotectionofinternalsurfacesareprimeconcernsforsystemswith

stringentchemistryandcleanlinesscontrol.ProductcleanlinessstandardsmayprohibittheuseofDP

inspection.

5.HydrostaticTesting.Cleanlinessandchemistrycontrolofthefluidmustbeconsistent

withtheoperatingstandardsofthesystem.Closeattentionshouldbegiventocontrollingsystem

thermodynamicparametersduringthetesttopreventoverpressurizationofthesystem.

Page40

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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