CRC Handbook of Lubrication and Tribology, Volume 3 PDF
Preview CRC Handbook of Lubrication and Tribology, Volume 3
CONDITION MONITORING SENSORSAND SYSTEMS EskoVenalainen Duetoincreasedspeedsofmachineryandsubstantialcostsofunexpectedshutdowns,general interestinparametersinfluencingbearingperformancehasgreatlyincreased.Thisissuewas extensively discussed at the Tappi Engineering conference in Boston, 1984. Further development of data communication and computer hardware since 1984 has offered cost- effective means to continuously monitor certain conditions of the machinery and its oil to avoid unexpected bearing failures and to obtain more efficient and organized maintenance workthroughscheduledshutdowns. Sensortechnologytomeasurecertainparametershasbeenavailableforquitesometime, but these applications have lacked the opportunity to communicate cost effectively into centralizedcontrolrooms.Withoutpresentdatacommunication,everysensorhadtobehard- wiredfromitslocationallthewaytothecontrolroomorannunciatorpanel. OILFLOWSENSORS Whilecommonintheindustry,conventional“sightglasses”cannotbeconsideredasoil flowsensorsformonitoringpurposesbecausetheycannotprovideanyelectronicoutput.Quite alargevarietyofoilflowmeteringorsensingdeviceshave,however,beenofferedoverrecent years. Measurement of oil flow is rather specific and requires special attention to the requirements of each application.This means that compromises may have to be made.An idealoilflowmeteringdeviceorsensorshouldhavethefollowingfeatures: • Noleaks. • Easytomaintain,i.e.,takeapart. • Relativelyaccurateandrepeatable. • Easytoread,evenwithdark,dirtyoil. • Built-invalveforflowadjustment. • Influenceofoilviscosityshouldbeminimal. • Reliableincontinuousoperationtoprovideflowindicationwithno“sticking”behavior. Several types of oil flowmeter for continuous measurement of oil flows to the bearing are available. These can be grouped basically as the two following types: (1) variable area rotameter,and(2)meterswithacontinuouselectronicoutputsignal(on/offorcontinuous). VariableAreaFlowMonitors Differenttypesofvariableareameters,knownasrotametersorsimplyas“sightglasses” are shown in Figure 1.Traditionally, they are made of different materials, including steel, aluminum,brass,glass,andplastic.Theindicatingdeviceisafloatinsidetheflowtubewhich canbeglass,plastic,ormetal.Connectingpartsarealwaysplasticormetal.Thewholemeter can also be made of metal, in which case it is called an armored flowmeter, which is very rarelyusedbecauseofitshighprice.Themajorbenefitisfiresafetyandfreedomfromleaks. Useofplasticcomponentshasbeenincreasing.Onereasonistheircapabilitytoendure highertemperatures;somearenowavailableforroutineuseupto150°C.Anotherreasonis their elasticity: with proper construction plastic components can take some stress and are relativelyeasytomakeleak-proof. 3 Copyright © 1994 CRC Press, LLC 4 CRCHandbookofLubricantsandTribology FIGURE1.Variableareaoilflowsensors. SensorswithContinuousElectronicOutput Thefollowingtypesofoilflowsensorsgenerateanelectronicsignal,analogordigital,for continuousmonitoringofoilflows. Positive Displacement Meter.The most common of this type is the so-called oval gear meter,developedduringWorldWarItoaccuratelydetectoilquantityinsubmarineperiscope control.AsshowninFigure2,themeasuringelementsareovalshapedandtheflowisrotating them.Thepistontypepositivedisplacementmeterhasalsooccasionallyappliedinlubrication. A major drawback is the back and forth moving piston, which easily becomes a high maintenanceitem. Turbine Meter. Turbine meters are also applied for continuous oil flow measurement. Theyarenotasaccurateaspositivedisplacementmeters.Whiletheyareviscositysensitive, athigherviscositiesthesensitivityerrorisnotveryhigh,andthistypeofmeterhasbeenfound tobegenerallyacceptable. Gear Meter. While gear meters are also available, they have not often been used for lubricationpurposes.Theyareveryaccurateandcanbemadetotakehighpressures. PressureDifferentialOilFlowMeter.Inthesemeters,asillustratedinFigure2,thesharp- edged orifice plate generates pressure differential which is proportional to the flow. It is nonlinear,butviscosityimmune.Ifsupplypressureisconstant,thendownstreampressurecan bemeasuredasanindicationofoilflow. OILFLOWSENSORS PROVIDING ON/OFFANDALARM SIGNALS Thesearetypicallyvariableareaflowmeterswithasignalfromafloatusedtoindicatethe lack of adequate oil flow. Of on-off sensors in this category, Figure 3 illustrates the free- floating type and shows three types of detectors used, and Figure 4 illustrates the technical characteristicsofinductiveproximitysensors.Themaindifferencebetweenfree-floatingand spring-loadedunits(seeFigure5)isthemounting.Thefree-floatingonehastobemounted vertically,butthespring-loadedflowmetercanbemountedinotherpositionsaswell. Anotherbasicdifferencebetweenthesetwotypesofsensorsistheirviscositysensitivity. Free-floatingoilflowmetersaremoreviscositysensitivethanspring-loadedones.Thefloat onafree-floatingunitisoftenmadewithsharpedgestoreducetheviscositysensitivity.Ifwe Copyright © 1994 CRC Press, LLC VolumeIII 5 FIGURE2.Ovalandpressuredifferentialtypesofflowmeteringdevices. FIGURE3.Differenttypesofdetectorstoprovidelowflowalarmwithfree-floatingdevices. trytomaximizetheflow,however,thenasharpedgemayhavetoberemovedfromthefloat andthemeteristhenmoreviscositysensitive. Afree-floatingdevicealsocannotbecomestuck,whileaspring-loadedfloatneedssupport from tight side clearances which introduces the possibility of getting stuck. This sticking Copyright © 1994 CRC Press, LLC 6 C R C H a n d b o o k o f L u b r ic a n ts a n FIGURE4.Electricalfunctionandcharacteristicsofaninductiveproximitysensor. d T r ib o lo g y Copyright © 1994 CRC Press, LLC VolumeIII 7 FIGURE5.Spring-loadedvariableareaflowmeterattop,swingingvanetypeatbottom. behavior may cause a false indication of adequate oil flow, even when the bearing may be starving. An alarm arrangement with a spring-loaded float is often controlled by micro-switch. Tightclearanceconstructionofthespring-loadedswitchcallsforcleanoiltomakethismeter reliable. Figure3illustratesthefollowingthreedifferentmeanstodetectthepositionofthefloat: • Magneticreedrelay • Infrared • Inductiveproximitysensors Copyright © 1994 CRC Press, LLC 8 CRCHandbookofLubricantsandTribology FIGURE6.Standardovalgearmeterscoverflowrangesupto100l/min(25gpm). Themagneticreedrelayislowincost,butitsreliabilityislessthanthatofinductiveproximity sensors.Inductiveproximitysensorsdonotattractsteelparticleslikemagneticones.Infrared israrelyusedbecauseitissensitivetooilcolorandcontamination. TheinductiveproximitysensorshowninFigure4hasproventobethemostreliableand accuratedevicetodetectthepositionoftheindicatingfloatinavariableareaflowmeter.At the tip of the sensor, there is an electromagnetic field. Presence of the metallic float will “overload”thiselectromagneticfieldandinthatwayprovideon/offindication. Anotherbasicproximitysensoristhecapacitivetype.Itsfunctionsarereversed,butthe basicoperationprincipleisthesameasthatoftheinductivesensor. Thebasicfunctioninallthesetypesofflowalarmsistodetectthepositionofthefloator pointerintheindicatingmechanism.Wheneverthefloatenterstheproximityofasensor,then analarmwillbeprovided,asillustratedinFigure3.Also,aninductiveproximitysensoris often applied as a low flow alarm in the bottom of the meter. Basic features of inductive proximitysensorsaregiveninFigure4. OILFLOWSENSORS PROVIDING CONTINUOUS DIGITALOR ANALOG SIGNALS Thesetypesofsensorswillprovideremoteindication.Alarmingwiththesesensorsisnormally arrangedinreceivingelectronics,abasicdifferencefromthesensorsmentionedabove.For continuousoilflowmeasurement,fourbasictypesareavailable: • Ovalgearmetersorpistonmeters • Gearmeters • Turbinemeters • Orificeplate/pressuredifferentialmeter OvalGearMeters AtypicalexampleofanovalgearmeterisillustratedinFigure6.Thistechnologywas developedseveraldecadesagoforsubmarineperiscopecontrol.Itwasappliedforthefirsttime Copyright © 1994 CRC Press, LLC VolumeIII 9 in lubrication applications almost 10 years ago in Finland, and today there are tens of thousands of these meters employed successfully in oil flow measurement. The meter generates a pulse increment proportional to flow and in practical terms provides reliable viscosity-immune measurements. Lubrication oval gear meters typically use inductive proximity sensors to detect rotation: in every rotation, the meter gives two pulses.Table 1 givesatabulationofdifferentsizesofovalgearmeterswiththeirpulserateperU.S.gallon andperlitersperminute.Theovalgearmeternormallyhasacapabilityof1:10flowratio;for instance,asize2meteriscapableoffrom200cm3/minupto2l/minatitsnormalspeedlimit of900rpm. GearMeter The gear meter gives a more accurate measurement than the oval gear meter, because gearmetersnormallygiveapulseoneachtoothofthegear.Gearmetersarealsoavailablefor highpressurehydraulicapplicationsrangingupto9000psi.Inprinciple,thereisnodifference between these two, except that today the oval gear meter dominates the market because of price.Theelectronicpickupisaspecialcrystalwhichwillveryreliablypickupmagnetization ineachtoothofthegear.Thisprovidesoutstandingaccuracyascomparedtoanyothermeter. TurbineMeter The turbine meter is not a positive displacement meter, although it does provide continuous indication of the oil flow. The basic limit for a turbine meter in lubrication applications is its sensitivity to viscosity variations above 30 cSt. There have been developmentsandtrials,butthismeterhasnotachievedpopularityinlubricationmonitoring. While piston-type metering devices have found some use, they are declining in popularity becausetheyaredifficulttomaintain. Orifice-TypeOilFlowmeter Anothertypeofcontinuousflowmeasurementisthewell-knownandreliableorificeplate. AsindicatedinFigure2,thesharp-edgedorificeplatewillprovidelowpressuredifferential proportionaltotheflow.Becauseofthesharp-edgedorificeplate,itisconsideredpractically viscosityimmune. Sinceconstantsupplypressurecanbearranged,itispossibleinoilflowapplicationsto useapressuregaugeonthedownstreamsideoftheorificeplateasasecondarymeasurement. Obviously,thereisapotentialerrorifsupplypressurevaries.Oftenthisapproachisusedonly foralarmingbymountingthepressuregaugewithanalarmcontactonthedownstreamside. Copyright © 1994 CRC Press, LLC 10 CRCHandbookofLubricantsandTribology FIGURE7.Thermowellandsensorsusedformonitoringbearingtemperatures. OILAND BEARINGTEMPERATURESENSORS Asmentionedbefore,excessivebearingtemperatureshavebeenacknowledgedasamain cause or indication of bearing damage.At certain temperatures, thermal stresses occur and bearingswillbedamagedandtheirlifeshortened.Also,athightemperatures,theviscosityand lubricatingeffectivenessoftheoilaresignificantlyreduced. Extraheatcancomefromseveralsources.Ifsteamispresent,thenobviouslyinsulation ofthebearingandtheoilisveryimportant.Often,inadequatedesuperheatingcanintroduce overheatedsteamadjacenttothebearing. Bearings can also be overheated because of excessive friction as from overloading. Obviously, lack of sufficient oil flow also will lead to increased friction and reduced heat transferfromthebearing,resultinginoverheating. To sense temperatures, several technologies are available. Heat-sensitive paints which changecolorsatcertaintemperaturehavebeenused.Forcontinuousmonitoring,thefollowing arepossibilities: • Resistancetemperaturedetectors(RTDs) • Thermocouples • Semiconductors • Infraredtemperaturesensors The electrical resistance of all metals will vary as the temperature varies. For this kind of temperaturesensor,typicallyplatinumandnickelareused.Thesesensorsaretheninsertedinto thethermowellshowninFigure7.WhenapplyinganRTDfortemperaturemeasurement,a varying resistance signal has to be converted into an electronic signal acceptable for the monitoring systems. The required converter often increases the price of the system and thereforeisnotused. Copyright © 1994 CRC Press, LLC VolumeIII 11 FIGURE8.Anexampleofinfraredtemperaturedetectionsystemwithafocusedlensusedtoviewheated surfacefrompredetermineddistanceawayfromR.F.field. Thermocouples Inathermocouple,thejunctionoftwodifferentmetalswillprovidealowmillivoltsignal whichisafunctionofitstemperature.Thissignalagaincanbeconvertedintoacurrentsignal andthenappliedtothemonitoringsystems.ThissensorhasthesamerequirementasRTD,i.e., itrequiresaconvertertogenerateamilliamperesignalformonitoringpurposes. SemiconductorSensor The semiconductor sensor utilizes a tiny transistor having an accurately known temperaturecoefficient.Thistypeofsensorprovidesalinearsignalfrom0to150°C(32to 302°F).Agoodfeatureofsemiconductorsensorsisthatreceivingelectronics,normallydata acquisition,willacceptthislowmilliampsignalasaninputwithoutaconverter.Inpractice, thismeansthateliminationofconverterswillmakethistypemostcosteffective. InfraredSensors Infrared temperature measurement as illustrated in Figure 8 detects radiation from the object for which temperature is being measured. The infrared thermometer can measure temperaturefromadistancewithoutbeingincontactwiththeobject.Thesignalprovidedcan betransmittedelectronicallyorthroughfiberoptics.ThemainadvantagesofanIRsystemare fastdetectionoftemperaturevariation,thefactthatthesensordoesnotneedtobephysically incontact,andhighreliabilityinahostileenvironment.Itisalsoreliableineliminatingso- callednoiseatthepick-uppoint.Goodexamplesarebearingtemperaturemeasurementson inductionmotorsoronheat-generatingdevicesthatcaninterferewithothertypesofsensing devices. OILCONTAMINATIONANALYZERS Oilcontaminationhasaproveneffectonbearinglife.Oilcanbecontaminatedbyparticles orbyotherliquids,typicallywater.Particlescanbeofdifferenttypes,andiftheparticlesize is larger than the thickness of the oil film, its effect on bearing life through wear and extra stress can be significant. Harder metal particles are known to be more damaging than nonmetallicparticles.Sensorshavebeendevelopedforbothparticlesandwatercontamination. WaterContamination Watercontaminationcanbeverydamagingtoballandrollerbearings,gears,andother machinecomponents,althoughthereisafairamountofdebateastowhatisactuallyaffected Copyright © 1994 CRC Press, LLC 12 CRCHandbookofLubricantsandTribology FIGURE9.Effectofwatercontaminationonballbearinglife.(DatafromR.E.Conlley,ASLETrans.,20,244– 248,1977.) bywaterintheoil.Figure9givesanindicationofhowwateraffectsbearinglifeincertain conditions.Verylowconcentrations,downto0.01%(100ppm)canreducebearinglifetoone half.Ontheotherhand,watercontaminationhasverylittleeffectinreasonableamountson tinbabbittbearingsinpowerplantequipment(turbines,etc.). Whilemanytraditionalmeasurementsdonothavesufficientsensitivity,waterabsorption of infrared light through the oil sample will provide measuring sensitivity below 100 ppm. These units have an oil sample flow through a measuring cell between two glass surfaces, after which the sample flow is directed back to the system. Two different wavelengths of infrared are commonly applied, one for measuring and the other as a reference.While the measuringwavelengthisbeingabsorbedbywatermolecules,thereferenceoneisunaffected by water.These two signals are then amplified and processed by the monitoring system to indicatewatercontaminationlevel.Theseunitsnormallyofferthefollowingfeatures: • Measuringrangefrom100to5000ppm • Accuracy±50ppm • Outputs:analog4to20ordigitalRS422 • Alarmoutput:dryrelaycontactnormallyopenornormallyclosed A water evaporation technique is also used. Its drawback is insensitivity for low concentrations.Itisreliableforhighconcentrationsonly. MetallicParticleSensors Quite recently, sensors to detect metallic particle distribution in lubrication oil have becomecommerciallyavailable.Theyarebasedoninductivebehaviortodetectthequantity of contamination. They are relatively low cost and easily applicable for lubricating oil applications. VIBRATION SENSORS Vibration measurement and vibration analyses can become very complex and are discussedinalaterchapterofthishandbook.Whileveryeasytoincludetheminacontinuous monitoringsystem,detailedcontinuousanalysison-lineinvolvesacostlysystemandisoften difficulttojustify.Continuousmeasurementfromseveralhundredbearingsuptoafrequency of30to60kHzrequiresverysophisticatedelectronics. Copyright © 1994 CRC Press, LLC
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