高密度聚乙烯管的无损检测.docx

高密度聚乙烯管的无损检测.docx

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高密度聚乙烯管的无损检测

PlasticFantastic?

-AnNDEInspectionSolutionforHDPEButtWeldsDavidMACLENNAN,IreneGPETTIGREWandColinRBIRDDoosanPowerSystemsNuclearTechnologyServices,PorterfieldRd,Renfrew,PA48DJ,UK.Tel:

+44（0）1418853849

Fax:

+44（0）1418853670

E-mail:

david.maclennan@

AbstractThispaperpresentsatechniquethathasbeendevelopedforEDFEnergytodetectbuttwelddefectsinHDPEpipeworkusingphasedarrayultrasonics（超声相控阵）.ThetechniqueusesacustomizedwedgetosupportasoundvelocitydifferencethatcangenerateforwardanglebeamswithinthelowvelocityandhighattenuationmaterialpropertiesofHDPE.

Testblockshavebeenmanufacturedwithavarietyofsubsurfaceimplantedtargets.Allthesetargetshavebeendetectedwithasignaltonoiseratioofgreaterthan12dB.Additionally,theabilitytoresolvetargetedgesisdemonstratedallowingmeasurementofthroughwalldefectsize.

Thecapabilityofdetectinglackoffusiondefectsgreaterthan6mmcircumferentiallengthby2mmthroughwallwithinHDPEpipeworkusingphasedarraytechnologyispresentedtogetherwiththechallengesovercomeduringdevelopmentoftheprobeandmanufactureofrepresentativetestblocks.

Keywords:

HighDensityPolyethylene（HDPE）高密度聚乙烯管,phasedarray

1.0Introduction

HighDensityPolyethylene（HDPE）isanalternativematerialbeingadoptedforthereplacementofcastirontertiarycoolingpipeworkcurrentlyinstatedatnuclearpowerplants[1,2].TheattractivenessofHDPEpipeworkisrelatedtothelong-termstructuralintegritybenefitsthatithastoofferbecauseitdoesnotcorrode,isflexible,lightweightandacosteffectivereplacementformetalpipes.

HDPEbuttweldsaremanufacturedinfourstages:

heating,heatsoaking,heaterplateremovalandjoining-cooling[3,4].TheheatingstageinvolvesthetwopreparedpipesectionsofHDPEbeingpressedagainstaheaterplateunderaspecificpressureandbeingheldthereforaspecifictime（heatsoaking）.Thereafter,theheaterplateisremovedandthetwoendsarealignedexactlyandbroughttogether.Thejoining-coolingprocessinvolvesthecompressionofthetwofacestogetherandbeingheldforaspecifieddurationinordertofusethetwoends.Theexcessmaterialsqueezedoutduringthebuttfusionforceformsabeadontheinnerandoutersurfaceoftheweldleavingvirtuallynoheataffectedzoneintheweld.Therecanbearequirementremovetheweldbeadtoberemovedfrombothsurfacestopreventgrowthandblockagesofinvertebratesorplants.Thisoftenresultsinanirregularandroughweldprofile.

InstallationandjoiningofHDPEpipeworksectionshappensin-situwhereenvironmentalconditionsmayintroduceimpuritiessuchasgrease,dust,dirtandsandtoboththeheaterplateandpipeends.Thistypeofcontaminationcanleadtodefectsinthebuttfusionweld.ThesizeoftheairpocketcreatedbythecontaminantscanproduceaLackofFusion（LoF）wherethereisnobondorapartialbondbetweenthefacesofthe18thWorldConferenceonNondestructiveTesting,16-20April2012,Durban,SouthAfrica

sections.

GenerallyitisconsideredthatanyLoFpresentwilloccurperpendiculartotheradial-axialplanei.e.0°tilt.Kissingbondsorcoldjointscanoccurwhenthecontactthatismadeisnotsufficienttoformbondscapableoftransmittingshearstressresultinginalackofstrength[5].Thiscanbecausedbyseveralfactorsincludingexcessivelylongheaterplateremovaltime,inadequateheattotheendoftheHDPEsectionsormisalignmentofthetwoends.

Currentqualityassurancesystemsinplaceincludeapprovedpersonnel,monitoringofweldingconditionsandvisualinspection.Furthertestsincludehydrostaticallytestingspoolsofpipe,cuttingasampleofweldanddestructivetestingthroughtensile/bendtests.

IthasbeendocumentedthattheTimeOfFlightDiffraction飞行衍射时间（TOFD）techniquehastheabilitytodetectdefectsintheHDPEmaterial[6],howeverthistechniquehaslimitations.ThedeploymentofTOFDrequiresaccessfrombothsidesoftheweldandalevelsurface.Itisrequiredthatthetechniqueshallbeapplicabletopipetopipeweldsandpipetofittingwelds.FittingspreventaccesstoonesideoftheweldandthereforeallthejointscannotbeinspectedwithTOFD.PhasedarrayandmicrowaveinspectionofHDPEmaterialshavebeenreportedelsewhere[1-3,5-8].

ThispaperpresentsaphasedarrayinspectionsolutionfordetectionandsizingofLoF-typedefectsinHDPEweldspecimens.

2.0InspectionSolution

2.1.1InspectionMethod

HDPEisamaterialwithhighdensityandhighattenuationvalues.Shearwaves（横波）arenotsupportedinHDPEduetothematerialpropertiesandhencelongitudinalwaves（纵波）aretheonlypossibleinspectionmode.

ThelongitudinalvelocityofsoundinHDPEisapproximately2150m/s.Thevelocitycanhaverelativelylargevariationsindependingonthespecificbatch,potentiallyvaryingonmaterialeithersideoftheweld.Furthermore,itisbelievedthattherearealsominorfluctuations（小幅波动）invelocitythataredependentontheangleofthesoundtravellingthroughthecrystallinestructure[3].HDPEpipeworkisveryattenuative（0.3dB/mmusinga2.25MHzprobe）andthereforeacquisitionwaslimitedtotheshortestinspectionrangepossible,inpractisethisbeinghalf-skip.

Duringthetesttrialstageofthedevelopment,an‘offtheshelf’32element2.25MHzphasedarrayprobeinpulse-echomodewithasweepof35°upto89°hasbeenusedtoproduceavolumetricinspectionoftheweldandHeatAffectedZone（HAZ）.DatawascollectedusingZetec’sDynaraysystemwhichwasinturndrivenusingUltravision3softwareviaalaptop.ThetechniqueutilisedwaterasawedgemediumresultinginasoundvelocitydifferencethatcangenerateforwardanglebeamswithintheHDPEpipework.

2.1.2BespokeUltrasonicProbeWedge

IthasbeenwidelyreportedthatthechallengepresentedbyHDPEistofindamaterialwithaslowervelocityinordertogenerateforwardsteeringangles,asdeterminedwithSnell’sLaw,buttoalsohavelowattenuation[5].Therefore,togenerateforwardsteeringangles,awedgemediumwithaslowervelocitythanHDPEisneeded.

ConventionalPerspex（2740m/s）andRexolite（2350m/s）shoesareinappropriateasthechangeinvelocityisnotsuitabletogenerateforwardlookinganglebeams.Aphasedarraysystemwithtwolowfrequencyprobesandtwowedges;agelwedge（slowervelocitytogeneratehigheranglebeams）incombinationwithaRexolitewedgeiscommerciallyavailable[1]forinspectionofHDPEpipework.Waterhasbeenusedasastand-offbutthewedgedesignsarebasicandhavenotbeendevelopedandoptimisedforstand-aloneindustrialuseonsiteapplications[8].

Inordertoproduceausefulrefractedangle（折射角）withintheHDPE,abespokewaterwedgehasbeendesigned.Theprobeishousedinthewedgeatanangleof21°inacustom-builtwater-filledacousticstand-offsecuredwitha0.6mmconformablemembraneinterface.AnearlydevelopmentdesignisshowninFigure1.Non-returnvalvespreventwaterlossandmachinedchannelsremoveanyairbuilduponthearrayfaceandmembrane.Themembranepocketatthefrontofthehousingcontainsairtoactasaspringforthemembraneinterface.Themembraneretainsthewateratthefaceoftheprobeanddoesnotaffecttheintendedfrequencypropagatedintothemateriali.e.therubberisacousticallytransparentat2.25MHz.Awearplateandadjustablescrewsonthebottomoftheholderareusedtoavoiddeteriorationofthemembraneandprovidestabilityonthecomponentsurface.

Longitudinalwavesaregeneratedatthemembrane-HDPEinterfacetoallowrefractionat35°upto89°forhalf-skipinspection.Inpractice,thearrayholdercanbyusedinanyorientationandisthereforeversatileforinspectionofallweldsgeometriesandlocations.

Figure1.（Left）Baseofwaterwedge（Right）Side-viewofwaterwedge

3.0ManufactureofTestSpecimens

Toevaluatetheinspectioncapabilityofthephasedarraywaterwedgesystem,anumberofsampleweldsweredesignedandmanufactured.Thetestspecimens（450mmdiameterand38mmwallthicknessPE100toBSEN12201）wereconstructedbyEDFEnergy’schosencontractor.

Inadditiontothis,acalibrationblockwasmanufacturedusingHDPEmaterial.A40mmthickblockmanufacturedfromthepipematerialcontainingØ3mmside-drilledholesandgeometricreflectorswasusedtodemonstratepositionalaccuracyandcalibratethesensitivity（Figure2）.Side-drilledholesareapreferredtargetduetothedifficultyinaccuratelymachiningflatbottomholes.

Testspecimensweredesignedtocontainvariablesizesofsmallstainlesssteelovalinsertstorepresentdefectspresentintheweld（seeFigure2）.Thesuppliedshimswereall0.2mmthickwhichisapproximately¼ofawavelength.ItisconsideredthatthestainlesssteelinsertconservativelyrepresentLoFbecausetheyrepresentabondedimplantwherelackoffusioniseffectivelyairwithagreateracousticimpedancedifference.Occasionallyitissuspectedthattheplasticdidnotbondtotheinsertinthiscasethelackoffusionisreal.Thebottomedgeofthediscscanonlybedetectedviadiffraction（衍射）whereasthetopedgecouldoccasionallybedetectedviapulseechoduetoreflectionfromthelackoffusionoftheplastictotheinsert.Theovaledgeensuresthatthediffractedsignalissmallandrepresentativeofarealirregulardefect.Thedesignofthetestpieceandstainlesssteelinsertsweresuppliedtothecontractorforconstruction.

Figure2.（Left）Phasedarrayinwaterwedgeoncalibrationblockwithtestspecimenand（Right）weldcapremovalsurfacefinish

Themanufacturingofthetest