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TheScienceBehindEddyCurrentandRemoteFieldTes8ng:
forCondensersandHeatExchangers
ConcoServicesCorpora4on
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Introduc4on
• WhatIsEddyCurrentTes4ng(ECT)andRemoteFieldTes4ng(RFT)?
• WhathasDriventheAdvancementofECTandRFTTechnology?
• ProbeConstruc4on• FillFactor• Applica4ons• Calibra4on• Singlevs.Mul4-Frequency• Conclusions
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WhatisECTandRFT?
• Non-destruc4veinspec4ontechniques.
• Basedoninducingelectricalcurrentsinmaterialsbeinginspectedandobservingtheinterac4onbetweenthosecurrentsandthematerials.
• Usedtomonitorcondenserandheatexchangertubeintegrity.
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WhatisECTandRFT?
• ECTandRFTtechnologycantraceitsrootsbacktoMichaelFaraday.
• Modernelectromagne4ctechniquesrelyontheexperimentsperformedbyFaradayinthemid1800s.
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WhatisECTandRFT?
• Eddycurrentsaregeneratedbyelectromagne4ccoilsinatestprobe.
• Astheprobemovesthroughthetube,defectsinthetubewallwillinterruptoraltertheamplitudeandpaYernoftheeddycurrent,changingitsmagne4cfieldandcausingavarianceinthecoilselectricalimpedance.
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WhatisECTandRFT?
• Bymeasuringtheprobeelectricalimpedance,atrainedtechniciancan“see”defectsinthetubewall,suchaspi]ng,cracking,erosion,corrosion,grooving,dentsandchangesinwallthickness.
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WhathasDrivenTheAdvancementofECTandRFTTechnology?
• Worldwideincreaseinthedemandforpower.• Increasedfocusonpowerplantequipmentreliabilityandefficiency.
• Needtofindsmallerandsmallerdefectsthatcanleadtotubefailure,causingadropinMWoutputortriggeringaforcedoutage.
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ProbeConstruc4on
• ECTandRFTProbesconsistofacoilorcoilswrappedaroundastructure.
• Designwilldeterminehowtheeddycurrentsareinducedandhowflawsaredetected.
• Builtonanonconduc4vebody(aircore)orusingferritecoresandconduc4veshieldingtohelpshapetheeddycurrentfieldforspecialapplica4ons.
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Probe(Pancake)Coils
• Designedtotestthesurfaceofmaterials.
• Canbeappliedeffec4velytoplatesandwelds.
• WhenfixedtoaspinningdeviceitiscalledaMotorizedRota4ngPancakeCoil(MRPC)andisveryeffec4veontubes,butis4meconsumingandexpensivetooperate.
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BobbinCoils
• Allowfortheinspec4onofinstalledtubesfromtheID.
• Interrogatestheen4recircumferenceofatube.
• Themostwidelyusedcoil.• Consideredthe“workhorse”ofthetubetes4ngindustry.
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Encircling(Feedthrough)Coils
• Allowfortheinspec4onofroundobjectssuchastubes,wiresandrodsfromtheOD.
• Interrogatetheen4recircumferenceofthematerial.
• Usedmostlyinproduc4onmonitoringac4vi4es.
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AbsoluteArrangement
• Coilworksindependently,makingnoreferencetoanyothercoil,andisaffectedbyallchangesinthematerial.
• Usuallylimitedtousebyconduc4vitytesters,coa4ngthicknessgaugesandsmallsurfaceridingpancakecoilsforsurfacescanning.
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Differen4alArrangement
• Twoormorecoilselectricallyconnectedtoopposeeachotherandlookforanimbalanceor“difference”betweenthetwocoilimpedanceswhenaflawisencountered.
• Subcategorizedintotwotypes:• SelfComparison.• ExternalReference.
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SelfComparisonDifferen4al
• Twoormorecoilselectricallyconnected,placedincloseproximitytoeachotherandwoundinopposi4on.
• Verysensi4vetosmallvolumeflaws.• Minimizenoiseduetoprobemo4on(wobble),temperaturevaria4onsanddeposits.
• Effec4veindetec4ngabruptchangesinwallthickness.• Cannotdetectgradualwalllossassociatedwithsteamerosionortube-to-tubewear.
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ExternalReferenceDifferen4al
• Twoormorecoilselectricallyconnected.• Coilscanbeonthesametestpartseparatedbyadistance
toavoiddirectcouplingbetweenthetwocoils.• Onecoilcanbeonthetestpartwhiletheothercoilsitsin
afixedloca4ononareferencesamplethatrepresentsnominalmaterialcondi4ons.
• Sensi4vetoallmeasurablechangessimilartoabsolutecoils.• Datacanbeerra4c–muchlikeabsolutecoils.• Typicallyreservedfordefectconfirma4onagainsttheself
comparisondifferen4alchannelsandfordetec4onofspecificdamagelikeerosionandtube-to-tubewear.
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HybridCoilsDriverPickup/Reflec4on
• Havethewidestrangeofconfigura4ons.• Basicconfigura4onu4lizesanexcita4oncoilandanindependentsensing(pick-up)coilorsetofsensingcoils.
• Theexcita4onandsensingcoilscanbeincorporatedintoeachotherorseparatedbycertaindistance.
• Hybridcoilshaveendlessconfigura4onstomeetspecialneedsoftheinspec4onindustry,butarenotnecessarilythemostaffordablebreeds.
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HybridCoilsArrayProbes
AdvancedArrayTechnology-DetailedDisplay
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ECTvsRFTProbes
• MostECTProbesareBobbinProbesopera4nginthedifferen4almodes,selfcomparisonandexternalreference.
• SomeECTProbesfallintotheHybridcategoryforspecializedconfigura4ons.However,assensi4vityisgainedforonespecificdamagemechanism,othermechanismsmaybemissed.
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ECTvsRFTProbes
• RFTProbesareHybridProbes,(Driver/Pickup).
• RFTProbesfeaturealargeexcita4oncoiltogeneratethemagne4cfieldandapickupcoillocatedtwotothreetubediametersfromtheexcitercoiltodetectthechangesinfieldstrength.
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FillFactor
• FillFactor(FF)isthepercentagethetubeIDfilledbytheprobefortubinginspec4onsfromtheID.
• Whencalcula4ngfillfactor,usetheformula:FF=d2/D2wheredistheoutsidediameterofthecoilandDistheinsidediameterofthetube.
• ECTpreferredtohaveaFFof85%orbeYer.• RFTpreferredtohaveaminimumFFof70%.• SmallerFFvaluesmaybeacceptablebasedonsignal
responsefromacalibra4onstandard.• SameformulaforODencirclingcoils,reversethevalues.
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ECTApplica4ons
• Idealfornon-ferromagne4cmaterialssuchascopper,brass,copper-nickel,austeni4cstainlesssteelsandsimilaralloys.
• Reliesondirectcouplingbetweentheinspec4oncoilandthetestmaterial.
• CanbeusedonmildlypermeabletubeslikeMonelandferri4cstainlesssteelswiththeuseofmagne4cbiasprobes,to“zero”outthepermeabilityeffectsandlettheeddycurrentalterna4ngmagne4cfieldsrunfreetodotheirjob.
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RFTApplica4ons
• Idealforpermeableandhighlypermeablematerialslikeferri4cstainlesssteel,carbonsteelandsimilaralloys.
• Designedtoovercomethepermeabilityeffectsinferromagne4ctubing.
• Couplingtakesplaceindirectlythroughthegenera4onofeddycurrentsandtheirresul4ngmagne4cfield.
• RFTcanworkonnonpermeablematerialsbutitisnotasaccurateoreffec4veasconven4onalECT.
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Calibra4on
• AllECTandRFTequipmentmustbecalibratedonaregularbasistoensurefunc4onalityandsensi4vityareadequatefortheintendedinspec4on.
• TheASMEBoilerandPressureVesselCode,Sec4onV,ar4cle8andar4cle17specifytherequirementsforEddyCurrentTes4ngandRemoteFieldTes4ng.
• ASMEsec4onValsodefinescer4fica4onrequirementsforinspec4onpersonnel.
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SingleFrequencyvsMul4FrequencyTes4ng
• ForECT,themorefrequenciesthebeYer.• Eachfrequencyhasitsstrengthfordetec4onandallowsforsignal
mixingtoeliminateunwantedinterferenceliketubesupportplatesignals.
• Useofbothdifferen4alandabsolutemodesisalsorecommended.• Thevarietyofanumberofeddycurrentchannelsandmode
combina4onsallowsanextensiveanalysisofdefect(flaw)depthandcharacteriza4on.
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SingleFrequencyvsMul4FrequencyTes4ng
Morechannelsofdataensuretheanalystismakingavaliddefectcall,notiden4fyingandpluggingfalseindica4ons.
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SingleFrequencyvsMul4FrequencyTes4ng
• RFT,fewerfrequenciesthanECT.• RFTdoesnoteasilylenditselftomul4frequencyinspec4onand
signalmixing.• Duetothecharacteris4clowfrequencyopera4on,oneortwotest
frequenciesaretypicalforaRFTinspec4on.• Addingtoomanylowfrequencieshasanimpactonproduc4onby
reducingthesamplerateandinturnforcesslowerscanningspeeds.
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Conclusions
• ECTandRFTareeffec4vetoolsformeasuringtubeintegrity.• ECTwithmul4plefrequenciesfornon-ferromagne4ctubes.• RFTwithoneortwofrequenciesforferromagne4ctubes.• Armedwithdetailedinforma4onaboutaHTX’stube
integrity,plantmanagerscantakeproac4vestepstoeitherrepair,replaceorplugdamagedtubesbeforetheyfail,preven4ngaforcedoutage.
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Ques4ons
JamesKocherEddyCurrentLevelIIIACONCOSERVICESCORP.1-800-345-3476jkocher@conco.net