Experience With Code Cases
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Experience with Code Cases John R Lilley. C Eng. MInstNDT. ASNT Level 3 RT.UT.PT.MT General Manager, Sonomatic Ltd. Biography John Lilley has been engaged in the NDT industry since 1975, and has held certification to ASNT Level 3 since 1984. He has been instrumental in the application of TOFD and other automated ultrasonic inspection technology to industrial applications since 1988. He has written numerous technical publications and has been a regular contributor to codes and standards over the years. He is currently the General Manager of Sonomatic Ltd, and became a Chartered Engineer in 2007. Background Code Case 2235 was originally issued by the American Society for Mechanical Engineering (ASME) Boiler and Pressure Vessel Code (B&PVC) committee in 1996 [Ref 1]. The enquiry asked “Under what conditions and limitations may an ultrasonic examination be used in lieu of radiography, when radiography is required....”, and the code case text goes on to define these conditions and limitations. It essentially addresses the following: i. Material thickness ranges and volumetric coverage requirements. ii. Requirement for a documented examination strategy. iii. The requirement for the examination to be carried out in accordance with Section V, Article 4 [Ref 2]. iv. The requirement for acceptable demonstration of performance of equipment, procedures and personnel on a qualification block(s). v. Acceptance criteria based on a combination of flaw height and length measurements which are derived from a linear elastic fracture mechanics procedure. vi. The requirement for automatic computer based data acquisition with data recorded in unprocessed form. vii. Investigation and analysis criteria. viii. Discrimination between surface and sub‐surface flaws. ix. Rules defining interaction in the case of multiple flaws. Other factors are also addressed, but the above list describes the key elements of the initial code case. There has been extensive discussion through a variety of forums, and the document was revised nine times before being incorporated into the main body of Section VIII in 2008 [Ref 3]. The various revisions have addressed specific refinements although the fundamental document has remained relatively unchanged throughout this process. Along the way, essentially the same code case, with some modification, has been incorporated into the ASME Gas Process Piping Code B31.3 [Ref 4], in the form of Code Case 181, which was issued in January 2007. The generic term ‘Code
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Experience With Code Cases
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ExperiencewithCodeCases
JohnRLilley.CEng.MInstNDT.ASNTLevel3RT.UT.PT.MT
GeneralManager,SonomaticLtd.
BiographyJohnLilleyhasbeenengagedintheNDTindustrysince1975,andhasheldcertificationtoASNTLevel3since1984.HehasbeeninstrumentalintheapplicationofTOFDandotherautomatedultrasonicinspectiontechnologytoindustrialapplicationssince1988.Hehaswrittennumeroustechnicalpublicationsandhasbeenaregularcontributortocodesandstandardsovertheyears.HeiscurrentlytheGeneralManagerofSonomaticLtd,andbecameaCharteredEngineerin2007.
BackgroundCodeCase2235wasoriginallyissuedbytheAmericanSocietyforMechanicalEngineering(ASME)BoilerandPressureVesselCode(B&PVC)committeein1996[Ref1].TheenquiryaskedUnderwhatconditionsandlimitationsmayanultrasonicexaminationbeusedinlieuofradiography,whenradiographyisrequired....,andthecodecasetextgoesontodefinetheseconditionsandlimitations.Itessentiallyaddressesthefollowing:
i. Materialthicknessrangesandvolumetriccoveragerequirements.ii. Requirementforadocumentedexaminationstrategy.iii. TherequirementfortheexaminationtobecarriedoutinaccordancewithSectionV,Article
4[Ref2].iv. Therequirementforacceptabledemonstrationofperformanceofequipment,procedures
andpersonnelonaqualificationblock(s).v. Acceptancecriteriabasedonacombinationofflawheightandlengthmeasurementswhich
arederivedfromalinearelasticfracturemechanicsprocedure.vi. Therequirementforautomaticcomputerbaseddataacquisitionwithdatarecordedin
unprocessedform.vii. Investigationandanalysiscriteria.viii. Discriminationbetweensurfaceandsubsurfaceflaws.ix. Rulesdefininginteractioninthecaseofmultipleflaws.
Otherfactorsarealsoaddressed,buttheabovelistdescribesthekeyelementsoftheinitialcodecase.Therehasbeenextensivediscussionthroughavarietyofforums,andthedocumentwasrevisedninetimesbeforebeingincorporatedintothemainbodyofSectionVIIIin2008[Ref3].Thevariousrevisionshaveaddressedspecificrefinementsalthoughthefundamentaldocumenthasremainedrelativelyunchangedthroughoutthisprocess.Alongtheway,essentiallythesamecodecase,withsomemodification,hasbeenincorporatedintotheASMEGasProcessPipingCodeB31.3[Ref4],intheformofCodeCase181,whichwasissuedinJanuary2007.ThegenerictermCode
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CaseisusedthroughoutthispapertorefertotheuseofUTinlieuofRTasdescribedinSectionVIIIoftheboilerandpressurevesselcode,orinB31.3CodeCase181.Inbothcases,fracturemechanicsbasedacceptancecriteriamaybeusedinlieuofgoodworkmanshipcriteria.
Itisimportanttonotethatthekeydifferencebetweenconventionalandcodecaseacceptancecriteriaisthattheformerisbasedongoodworkmanshipcriteria,andthelatterissupportedbyfracturemechanicscalculation.Adefinitionisofferedherebasedonpersonalexperienceandinteractionswithclients,colleaguesandpartnersoveraperiodinexcessofthirtyyearsinvolvementwithrelevantprojects,standardcommittees,R&Dforums,seminars,workshops,conferencesandlegalwork.
Constructioncodesandstandardsareintendedtoensurethatpressureequipmentandstructuresaredesigned,fabricatedandtestedtoconsistentqualitystandardsintheinterestsofsafetyandreliability.Theearlycodesstipulatedradiographictestingforthedetectionofflawswithintheweldvolume.Radiographictestingishowever,inefficientforthedetectionofplanarflaws,whichmustbea)preferentiallyalignedtotheradiographicbeamandb)withagapethatexceedstheapplicableradiographicgeometricunsharpnessvalue.Radiographyisefficienthowever,forthedetectionofvolumetricflawssuchasslagentrapment,porosity,undercut,andpoorweldprofile[Ref5].Itwillalsodetectcertaingrossplanarflaws,especiallywheretheseareassociatedwithothertypesofvolumetricflaw.Allofthedescribedvolumetricflaws/conditionsareindicativeofpoorworkmanshipandthecodesprovideveryclearlimitsonwhatcanbedefinedasunacceptableintermsofqualitystandards.Rejectionlevelsarenotderivedfromwhatisconsideredtobedetrimentaltotheequipmentonceplacedintoservice,butwhatisconsideredtobepoorpracticeintermsoffabricationquality.Thisqualityculturepervadesthroughouttheentirefabricationprocessinmuchthesamewayasastrongsafetycultureleadstofeweraccidentsoverall.Theinferenceisthatifweldingproceduresareapplieddiligentlyandgoodworkmanshipprinciplesareupheld,thenitfollowsthattheoccurrenceofcrackingandlackoffusionflawsislikelytobeminimised.Thisprocesshasbeenobservedtobeeffectiveinthattheincidenceofboiler/pressurevesselfailureshasdramaticallydecreasedfollowingtheintroductionofcodes.AttentiontodetailinconstructionNDTspillsovertocontrolandcareofweldingconsumables,weldingprocedures,heattreatment,documentation,etc.Itisafundamentalaspectofqualitycontrol(QC).Thereisalsoapsychologicalinfluenceinthatweldersjustifiablytakeprideintheirworkandareinclinedtobeaversetothestigmaofbeingclassifiedaspoorworkmenthroughgeneratingunacceptablelevelsofrepair.
ConstructiongoodworkmanshipNDTdoesnotneedtohaveahighProbabilityOfDetection(POD)becauseindividualflawswhichmaygoundetectedarenotlikelytobedetrimentaltotheuseoftheequipmentinservice.Thisisduetothehighlevelofconservatismembodiedwithinthegoodworkmanshipapproach.ThegoodworkmanshipacceptancecriteriaofSectionVIIIDiv2Part7(2008Addenda)includes,butisnotrestrictedto,thefollowing:
Cracksandlackoffusion/penetrationNotpermitted VolumetricflawsLimitationsdependentonwallthickness
Asanexample,inthecaseofapressurevesselwithawallthicknessof50mm,a20mmlongslaglinewouldberejectabletothesecriteria.Theslaglinemaybelessthan2mmincrosssectionandwouldbeexpectedtoberoundedinprofile.Accordingtothecodecases,asubsurface,39mmlongby
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4mmhigh,verticallyorientedplanarcrackinacomponentofthesamewallthicknesswouldbepermitted.Thedifferenceisthatthegoodworkmanshipcriteriaaredesignedtomaintainqualitystandards,therebyimplyingfitnessforservice,whereasfracturemechanicsbasedacceptancecriteriaaredesignedtoeliminateflawsexceedinggivendimensions.TheperformancecriteriafortheNDTassociatedwiththesetwoapproachesareverydifferent.GoodworkmanshipNDTmaybelessthanperfectbecauseitisdesignedtoflagupwhenthefabricationprocessisgoingoutofcontrolbypickingupsystematicflaws(asopposedtodetectingallflaws)whichareindicativeofunderlyingbreakdownsinqualityprocesses.Thereislessroomformanoeuvrewithfracturemechanicsbasedacceptancecriteria,hencetheadditionalrequirementforqualification.
Adoptionofthecodecaseshassteadilyincreasedovertheyearstothepointwhereincertaincircumstances,radiographictestingofpressurevesselsunderconstructionhasfallenawayaltogether.Thisprocesshowever,hasoccurredatatimewhentheindustryhasgonethrough:
a) aperiodofchangingregulatoryinfluence(lessinfluencefromcertifyingauthoritiesandinsurers),
b) increaseddependencyonformalqualityprocessesandc) increasedpressureonefficienciesofprocurement.
Thesefactorsincombinationhavecreatedtheenvironmentwhereithasbecomeprevalentforfabricatorsand/orinspectionservicecompaniestointerpretandapplythecodecasesaccordingtotheirownunderstanding.Thishasbeenobservedtohaveoccurredwithoutguidanceorexperienceofworkingwithcodesingeneral,andbypeoplewhosenativetongueisnotEnglish.ThesituationisnothelpedbythefactthattheCodeCaseshavenotbeenverywellwrittenorpresented.Intheauthorsexperience,thishasledtosituationswhereanyambiguitiesofinterpretationtendtoswingmarkedlytowardstheinterestsofthefabricatorand/orinspector,oftentothedetrimentoftheendclientorpurchaseroftheplantrequiringinspection.Theconsequentialeffectsofthiscanbeverycostlyinthelongertermduetoprojectdelays,remediationcostandotherprojectrisks.Itissuggestedthatthesituationcouldbeimprovedthroughstrictercontrol,bothintermsofinitialspecificationoftheprocurementofNDTprocesses,butalsotightercontrolthroughoutthefabricationprocess.Thecostsassociatedwithqualityassuranceareverysmallcomparedtotheprojectriskinvolved.
ThepotentialbenefitsofUltrasonicTesting(UT)inLieuofRadiographicTesting(RT)forproductionweldingTherearecertainpotentialadvantagesofUTinlieuofRTduringthefabricationprocess.Hereisanoverviewofthemoreimmediatebenefits[seealsoRef6]:
i. Noradiationhazardpersonnelcanworkinandaroundtheinspectionareaii. Norequirementtotransportpressurevessels/pipespoolstoradiographiccompoundsiii. Speed.Theinspectioniscompletedinashortertimeframeiv. Potentialforimprovedqualityofwelding.Ifappliedatthefrontendofaproject,itcan
beusedattheweldprocedureandwelderqualificationstagetooptimisetheweldingprocess,henceminimisingthelikelihoodofrepair.SeeFigure1.
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v. Depth&positioninginformationprovided.UnlikeRT,preciserepaircoordinatescanbeprovided,minimisingthepossibilityofrerepairsthroughmissedflaws
Other,lessobviousbenefitsinclude:
i. Thedataformsafingerprintforcomparisonwithfutureinserviceinspectiondata.ii. Theacceptancecriteriaofthecodecasesareinmanycasesmoreforgivingintermsof
acceptableflawsize.Thisalsoreducestherepairfrequency,andisespeciallythecaseforvolumetricweldingflaws.
Figure1.ExampleofTOFDdatatakenfromaB31.3CC181project.Theindicationsscatteredthroughouttheweldbodyweretermedbytheweldersasfisheyes.Thesearesmalllackoffusionflawsthatformattheweldbevelfacesastheweldingheadweavesacrosstheweldbody.Theyareaformofcombinedlackofinterrunandsidewallfusionthatcanbealignedintheaxialdirection,butalsocouldpotentiallyextendverticallybylinkingbetweenweldpasses.Byadjustingthewelltimeateachendoftheweave,thisflawtype(whichwasnotdetectableradiographically)waseliminated.NotealsotheinterestingIDfitupandstopstartinterruptionintheweldrootpenetrationallusefulfeedbackforthedesignandweldingengineers!
Takenincombination,thebenefitsintermsofprojectcost,quality,durationandriskcanbeverysignificantindeed.Therehavebeencaseswhere24hourworking(asopposedtofabricationduringthedayandRTatnight)enabledprojectdurationstobehalved,butconversely,whereUTusedin
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lieuofRThasbeenimpactedbylackofplanningandoversight,delaysandcostshavebeenseentoescalatemassively,potentiallyleadingtolitigation.
DisadvantagesofUTinlieuofRTi. CertaingeometricfeaturespresentrestrictionsforUT,e.g.attachments,skirtsornozzles
adjacenttoweldsinthecaseofpressurevessels,orfittingsinthecaseofpipingsystems.ii. Variationsinexpertisemorecapableserviceinspectioncompanies/fabricatorswill
engineersolutionstomany,ifnotallultrasonictestrestrictions.Othersreadilygiveupandseekdispensationtoexcludecertainweldsonthegroundsofdifficultgeometry.ItmaybepermissibletousethetestrestrictionexcuseinthecaseoffabricationQC,butnotinthecaseofFFSbasedonfracturemechanicsprinciples.Inspectionsshouldbeplannedproperlytominimiseand/ordealwiththeseeffects.Fabricators/serviceinspectioncompaniesshouldbevettedduringtheprequalificationprocesstoassesstheirabilitytodealwiththesesituations.
iii. CertainflawtypesaredifficulttodetectwithUT(e.g.excessrootpenetration).iv. CertainmaterialsarenotsuitedtoinspectionbyUT,especiallycoarsegrainedaustenitic
stainlesssteels.v. Thefracturemechanicsbasedacceptancecriteriaofthecodecasesbecomemoreonerous
forthinwallmaterialsandthebenefitsbecomediminishedwithwallthickness.
CommonmisinterpretationsormisapplicationsoftheCodeCasesProceduresThepurposeandintentofultrasonictestingproceduresthemselvesisveryfrequentlymisunderstood.Aprocedureshouldbederivedfromtheconstructioncode(inthiscaseSectionV,Article4),incorporatingtherequirementsofthecode,butreflectingtheequipmentinuseandthespecificitemstobeexamined.Workinstructions/methodstatementsandchecklists/calibrationrecordsshouldbedefinedintheproceduresthatarerequiredtobeusedaslivingdocumentsaseachcontractprogresses.Asthecodesareintendedtobeusedacrossawiderangeofdesignsandsituations,thereisacertaindegreeofflexibilityembodiedwithinthem.TheAmericanSocietyforNondestructivetesting(ASNT)personnelcertificationschemeSNTTC1AorCP189asreferencedbythecodesdefinescompetencylevelsforindividualstobeabletointerpretthecodessufficientlytoextracttheinformationrequiredinordertoconstructaprocedure.Itiscommonpracticehowever,forfabricatorsandserviceinspectioncontractorstocutandpasteblocksoftextdirectlyfromthecodesintotheircompanyprocedures.Classicexamplesincludecoderequirementsforanultrasonictestfrequencyrangeof1MHzto5MHz,ortwobeamanglestobeselectedfrom45,60or70toberepeatedwordforwordintheprocedure.Thisleavestheactualinspectionopentointerpretationandinconsistencieswilloccur.Thesearesimplisticexamplesthatleadtorelativelyminordiscrepancies.
TherequirementtogenerateaprocedureaccordingtoASMEV,Art4,T421(andTIII422inthecaseofTOFDand/orTIV422forPhasedArray),includingtheidentificationandcontrolofessentialandnonessentialvariablesisgenerallymisunderstood,especiallytherequirementfortheproceduretoestablishasinglevalue,orrangeofvalues,foreachrequirement.Thisentiresectionhasbeenobservedtohavebeenpastedverbatimdirectlyintoaprocedure.
Muchmoreseriouscasesareprevalent.Asanexample,thecodecasesstipulatethattheultrasonicexaminationshallincludeavolumeofmaterialtobeincludedoneachsideoftheweld
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(theactualdistanceisdictatedbythematerialthickness).Thedistancemaybereducedtocoveronlytheweld,HeatAffectedZone(HAZ)+6mmofbasematerial,providedTheextentoftheweldHAZismeasuredanddocumentedduringtheweldqualificationprocessand,TheUTtransducerpositioningandscanningdeviceiscontrolledusingareferencemark.....Ithasbeenobservedpracticethatthetextisoftenlifteddirectlyfromthecodecasetotheprocedureandthenignored.Theendeffectisthatinadequatematerialisexamined.
Veryoften,proceduresaredocumentsthataregeneratedforauditpurposesandtechniciansperformingtheworkneverseethemorareevenunawareoftheirexistence.
QA/QC:Qualitystandardshavedevelopedquitestronglyinrecentyearsandtherehasbeenastrongfocusonrelianceonadherencetoaccreditedschemesattheexpenseoftechnicalaudit,specificationandsupervision.Informertimes,certifyingauthoritiesemployingpersonnelwithtechnicalknowledgeandexperienceusedtoprovidethisformofoversight.CurrentQA/QCprocessesensurethatproceduresareadheredtoatthesystemlevelratherthandiggingintothetechnicaldetail.AkeyfailingofthisprocessisthatQA/QCrepresentativestendtohavecompetenciesinQA/QCratherthaninNDT.ThisdoesnotdetractfromthehighlyimportantfunctionofQA/QCasaprocess,butthereisacompetencygapthatiscurrentlynotaddressed.
UltrasonicinspectiontoSectionV,Article4:ThecodecasesdonotstipulatewhichofthetechniquesdescribedinArticle4shouldbeused.ConventionalpulseechoUT(fixedbeam),PhasedArray(PA)andtheTimeOfFlightDiffraction(TOFD)techniquesarealldescribedandtheuserisfreetoselectwhichofthesemaybeused.Thecriteriaofthecodecasesarethatwhichevertechniqueisused,itmustmeetorexceedtheminimumqualificationcriteriaoftheCodeCase.ItisunlikelythataconventionalpulseechotechniquewillmeetthesizingcriteriaasthesearemoresuitedtotechniquesthatmakeuseofthetipdiffractionprocesssuchasTOFDand/orPA.Manyofthefactorsthatapplytointerpretationandapplicationofthecodecasesapplyequallytointerpretationandapplicationofthebasecodeitself,butthiswillnotbeaddressedhereunlessitspecificallyrelatestoapplicationoftheCodeCase.
TOFDSupplementarycoverage(asrequiredbyArticle4,MandatoryAppendixIII):
a) Transverseflaws.AnanglebeamexaminationisrequiredfortransverseflawsunlessthereferencingCodeSectionrequiresaTOFDexamination.Inthesecases,positioneachTOFDprobepairessentiallyparalleltotheweldaxisandmovetheprobepairalonganddowntheweldaxis.Iftheweldreinforcementisnotgroundsmooth,positiontheprobesontheadjacentplatematerialasparalleltotheweldaxisaspossible.Caution:TherequirementtoperformTOFDscanswiththebeamorientedparalleltotheweldaxiswillnotincreasetheprobabilityofdetectionfortransverseflawsanymorethanascanwiththebeamorientedacrosstheweld,possiblylessso.ThisrationalestemsfrompulseechoUTanddoesnotapplytoTOFDinthesameway.AlthoughtheclassicdepictionofTOFDiswhereacrackisperpendiculartotheultrasonicbeam,diffractedsignalsarestillgeneratedatthetipsoftransversecrackswhenthecracksprimaryaxisisorientedparalleltotheTOFDbeam.AnexampleisshowninSectionV,Article4,NonMandatoryAppendixN.Thecodeiswrongonthispoint,whichdoesnothelpmatters.
b) Supplementalshearwaveexamination.WhenTOFDisused,Article4callsforsupplementalshearwaveexaminationsduetothepresenceofthelateralwaveandback
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wallsignals.Comment:Unlessthedetectionandsizingaccuraciesusingthesetechniquescanbesuccessfullyqualifiedusingaqualificationblock(s),thesupplementalshearwaveexaminationtechniquesshouldbeusedinconjunctionwiththegoodworkmanshipacceptancecriteriaofSectionVIII.Thisisoftenoverlookedinpractice.
Qualificationblock(s):ThisispossiblythemostuniversallymisunderstoodsectionofeitheroftheCodeCases.Theblock(s)is(are)requiredtobemanufacturedbyweldingortheHotIsostaticProcess(HIP).Theauthorhasnoexperienceofthelatterbeingusedforfabricationofentirequalificationblocks,althoughthisprocesshasbeenseentohavebeenusedinthenuclearindustrytomanufactureindividualflawsofveryprecisedimensions.Inthiscasetheflawsarecreatedbysparkerodingtherequiredflawdimensionsintooneorbothfacesoftwoblocksofsteel.Thesizeoftheblocksissomewhatlargerthantheintroducedflaw.The(uneroded)matingfacesofthetwoblocksarethenbondedtogetherthroughtheapplicationofintenseheatandpressure,creatingahomogenouspieceofmaterialexceptforthenowembedded,intendedflaw.Theblockisthenmachinedintoabobbin,whichisimplantedintothequalificationblockbywelding.ItisunlikelythatHIPbondingwouldbeusedinthenonnuclearindustryasthereareother,lesscostly,iflessprecisemethodsofsimulatingplanarflawsinwelds.Referencetothisfabricationprocesshowever,hasledtospeculationthatblocksneednotbewelded,andthatartificialflawsmaybeintroducedfromtheendsofunweldedqualificationblocks(presumablybydrilling/ElectroDischargeMachining(EDM).Discussionofthiscanbefoundonwww.ndt.net[Ref7].
TheCodeCasescarriedthestatementregardingqualificationblock(s):andshallcontainaminimumofthreeflaws,orientedtosimulateflawsparalleltotheproductionweldsfusionline....Interpretationofthisdefinitionhasbeencarriedoutinvariousways.Theauthorsinterpretationoftheintentofthisisthattheflawsshouldsimulatethemostdifficulttodetectoffabricationflawsintheformofatight,smooth,planarflawsuchasalackoffusion(inisolation,i.e.notincombinationwithanyotherflaw).Thisissupportedbythefollowingstatementalsopostedonthewww.ndt.netwebsite:WhiletheoriginalASMECC2235wasnotclearonthisissue,theintentwastouseartificialcracksnotsidedrilledholesornotchesasthereflectors.Inaddition,theartificialcracksshouldfollowthebeveltosimulateLackofFusiondefectsorsimilar.ASMEiscurrentlyworkingonclarifyingthissituation.MichaelMoles;Member,ASMESectionVUltrasonicsWorkingGroup.Itispositivetonotethatthe2008AddendatoSectionVIIIrequirementsforqualificationblockflawsnowreads:...andshallcontainaminimumofthreeplanar(e.g.cracklike)flaws,orientedtosimulateflawsparalleltotheproductionweldsfusionline...,althoughB31.3CC181stillretainstheoriginaltext.Theauthorsexperienceisthatwhereaninspectionservicecontractororfabricatordrivestheprocess,qualificationblocksmayormaynotcontainwelds,buttheywillcontaindrilledholesornotches,whereaswhereanenduserdrivestheprocess,qualificationblockswillalwaysbeweldedandwillusuallycontainsimulatedplanarweldingflaws.Thelatterfallintothecategoryofcracklike,inthattheyaretight,planarandpreferentiallyoriented.Suchflawscanbeintroducedbyweldingshimsontoweldbevelfaces(practicerequired!),ortheycanbeprocuredfromaspecialistfirmofsamplemanufacturers.
Caseshavebeenobservedinpracticewhere6mmwidebuttressnotcheshavebeenusedtorepresentsurfaceflawsinconjunctionwithTOFDprocedures.Thisisincorrectontwocounts.Firstly,abuttressnotchrepresentsalarge,stronglyreflectiveareawheninspectedfromtheoppositesurface(quiteunlikeanynaturalplanarflawalignedwiththeweldfusionline),and
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secondly,itisopentothesurface.AccordingtoSectionVIIIMT&PTacceptancecriteria,linearsurfacebreakingflawsareunacceptable,sothereseemslittlepointinqualifyingdetectionandsizingperformanceforflawsthatarenotpermitted.Thisdoesnotapplyhowever,toslightlysubsurfaceflawsthatareclassifiedassurfaceflawsiftheremainingligamenttotheexternalsurfaceislessthanhalftheflawheight.Moreonthislater.
SidedrilledholesandEDMnotcheshavefrequentlybeenobservedasartificialflawsinqualificationblocks.ThisgenerallyspeakingisacarryoverfromtheuseofdrilledholesandnotchesincalibrationblocksdescribedinSectionV,Article4.Inthiscase,theartificialflawsareintendedtocreatereflectorsthatarereproducibleandeasytomanufacture.Theirpurposeistocontrolaccuraciesofcalibration,sizing,sensitivityandcoverage.Thepurposeofimplantedflawswithinqualificationblocksisanentirelydifferentmatter.Heretheonusisnottorepeatthecalibrationprocess,buttoverifytheperformanceofthecalibratedUTsetuponsimulatedplanarweldingflaws.Itshouldbeborneinmindthatatruelackofsidewallfusionisoftenaverytightflawthathasnegligiblewidthorgape.Inthemostextremeofsituations,theweldpoolmaysolidifyalongsideaweldbevelfacewithoutactuallyformingabond,andinthiscaseitisnomorethanamolecularseparation.Suchaflawwillbepartiallyopaqueandthetipswillhavenegligiblewidth.Flawopacityandmorphologyarebothimportantforpulseechotechniques(includingphasedarray),andtipconditionisimportantfortechniquesthatrelyontipdiffractionsuchasTOFDorphasedarray.Drilledholesandnotchessatisfynoneoftheseconditionsandshouldnotbeconsideredforqualificationblocks.
Afurtherconsiderationwithqualificationblocksisthenumberofflawsrequired.Thecodecasesstipulateaminimumofthreeplanar(e.g.,cracklike)flaws,orientedtosimulateflawsparalleltotheweldfusionline.Theminimumcriteriaofonlythreeflawstendstobetheautomaticchoiceinpractice,andintheauthorsview,thisislikelytobeinadequateinmost,ifnotallcases.However,therearetwoalternativeinterpretationstothisrequirement:
i. therequirementforoneflawateachsurfaceandonesubsurfaceflawistodemonstrateoverallsystemperformancethroughthefullthickness,or,
ii. therequirementtodemonstratesystemperformancecoveringeachdepthzoneasdescribedinSectionVArticle4,forallbevelangles.
Inpractice,fabricatorsandinspectionservicecompanieswilloptfori.above,butwouldendusersbemorecomfortablewithqualifyingsystemperformancethroughouteachzoneandbevelangle?
Anotherfactorwhichiswidelyignoredisthequalificationblockgeometry.TableT421ofASMEV,Article4,2008AddendaprovidesmandatoryrequirementsforUTexaminations.Thistabledefinesessentialvariablesforwhichasinglevalueorrangeofvaluesaretobeestablished.ItgoesontosaythatwhenprocedurequalificationisrequiredbythereferencingCodeSection,achangeofarequirementinTableT421identifiedasanessentialvariablefromthespecifiedvalue,orrangeofvalues,shallrequirerequalificationofthewrittenprocedure.Thecodedoesnotspecifytherequiredcalibrationblockgeometry,althoughthe2008AddendatoSectionVIIIdoesstatethatthequalificationblockmustbewithin25%ofthethicknesstobeexamined.Itfollowshowever,thatifthecomponentgeometrylimitsdefinedintheprocedure(essentialvariableaccordingtoT412)areexceeded,thenadditionalqualificationblock(s)willberequired.Thistranslatestoarequirementfor
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multiplecalibrationblockscoveringthefullrangeofthicknessestobeexamined,materialcombinationsandcomponentgeometries.
Otherfactorsinfluencedbyprice/productivitypressuresWeldingquality.Asstatedearlier,a39mmlongby4mmhighsubsurfaceflawwouldbeacceptableina50mmthickpressurevesselorpipeweld.Basedonobservationsinthefielditissuspectedthatsomeweldersorfabricators,onrealisingthatitwouldinfactbehardtoproduceaflawofthesedimensions,wouldbeinclinedtoconcentrateonquantityratherthanquality.Thisisespeciallythecaseiftheyareincentivisedonthebasisofproduction.Thisiscompoundedbythefactthatintheoryatleast,unlimitedquantitiesofvolumetricflawsarepermitted.
NDTtechniciancompetencies.Largenumbersoftechnicianshavebeenrequiredbyindustrytoworkinaccordancewiththecodecasesandtherehasbeenatendencytorushthesethroughthecertificationprocess.Thishasbeenachievedbystreamliningthetraining,examinationandcertificationprocesstoconcentrateonsimplistic,clearcutandunrealisticflawconditions.NonMandatoryAppendixNofSectionV,Article4(TOFDInterpretation)couldbeconstruedasmisleadinginthisregard.Veryclearcutexamplesofclassicflawinteractionsaredescribed.Thesurfacebreakingflawforexample,isanotch,notarealfusionflaworcrack,whichbehaveverydifferently.Completelossoflateralwavesignalisavery,rareevent.TheAppendiximpliesthatTOFDindicationscanbeidentified,characterisedandsizedfromtheTOFDdataalone.Inextremecasesthisissometimespossible,buttheyreallyarerareoccurrences.Inreallife,TOFDindicationsneedtobeinvestigatedusingadditionalTOFDscanningand/orpulseechotechniquestodifferentiateplanarfromvolumetricflawsbasedontheirreflectivity.PleaseseeFigure2asanexampleofcomplexflawformations.
mm 0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320
mm0.0
13.819.924.829.032.736.339.642.745.748.7
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Figure2.TOFDDataComplexflawformations,possiblyarisingasaresultofabuseoftherelaxedacceptancecriteriaoftheCodeCases,butleadingtodifficultieswithinterpretationfortheoperatortrainedonclearcut,idealisticflaws.
Thisisanexampleofwhatoccursregularlyinthefield.Inthiscaseitismanualwelding,andistheresultofpoorworkmanship.Itprobablycomprisesofacombinationofslaginclusions,porosity,lackofsidewallandlackofinterrunfusion.Thepossibilityofcrackingcannotbediscounted,butunderthefracturemechanicsbasedacceptancecriteria,flawtypeisimmaterial,onlyflawdimensionsarerequiredforacceptancepurposes.Intheauthorsexperience,provisionisnotmadeforeithermanualorautomatedpulseechocharacterisation(discriminationbetweenplanarandvolumetricflaws)ofcomplexflawsandthissituationisnotaddressedbycodes,standardsorprocedures.Codesandtrainingmaterialsdohowever,refertoburiedflawsbeingrecognisableduetophasereversals,andfarsurfaceflawsbeingidentifiableduetoeffectsatorafterthebackwall.Intheauthorsexperience,theseidealisticsituationsdonotariseinpractice.Theonlywaytodealwiththeaboveconditionisacombinationofadvancedsignalprocessingandcomprehensive(andtimeconsuming)evaluationbyanexperiencedtechnicianconversantwithbothpulseechoandTOFD.Similarlimitationsapplytophasedarray,wherecomplexinteractionscanoccurwithdirectandmodeconvertedresponsesfromcomplexflawformationswheremaskingcanalsobeanissue.Thereisadangerofaninexperiencedtechnicianbeingtrainedtoonlylookfortops&bottomsorbackwalleffectsasevidenceofplanarflaws.Thisisapotentiallydangeroussituation,anditshouldbedealtwithearlyonintheproductionprocess.Severalcaseshavebeenexperiencedwheresimilarconditionswereexposedonlyafterallwelding/fabricationwascompleted.
ConclusionsItcanbeconcludedthattherearesignificantdisparitiesbetweenthemannerinwhichcodecaseinspectionsare,orcouldbeappliedinpractice.Ontheonehand,theprocesscanbeappliedinthespiritwithwhichitwasintended,possiblywithenhancementsthatenablecomponentqualitytobeoptimised,andontheother,thecodecasescanbeinterpretedtodeliverlowestprojectcost,butwithhighestrisk.
Itcanalsobeconcludedthatifappliedinthespiritwithwhichtheyareintended,thecodecasescanleadtoarequirementforalargequantityofcostlyqualificationblocksandhighlevelsofqualificationactivity,whichcouldhavethepotentialtoimpedeprojecttimescales.Althoughmoretimeandexpenseshouldcertainlybeincurredinthisareathaniscurrentpractice,therearewaysinwhichtomanagethisprocessefficiently.Thisiselaboratedfurtherinthenextsection.
Wherethereisinsufficientattentiontoprequalificationandtechnicalsupervisionthroughouttheproject,successfulbiddersforconstructionNDTprojectsarelikelytobethosethatprovideaminimalistinterpretationofcodecaserequirementsthatleadtounacceptableprojectrisksfortheenduser.
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RecommendationsItisrecommendedthatthefollowingprocessisadoptedinthecaseofnewconstructionprojectswhereultrasonicexaminationiscarriedoutinlieuofRT(SectionVIII,Div2,Part7)and/orASMEB31.3CodeCase181:
i. Theendusertogenerateaspecificationofperformancecriteriaforthefabricator/inspectortofollow.Simplytostatethattheinspectionmustmeetcoderequirementsisinsufficient.Fabricatorsandinspectionservicecompaniesareunderintensepressuretomeettimescaleandcosttargetsandwilladapttheirinterpretationofcodestomeettheseends.
ii. Riskassessmentsshouldbecarriedoutontendersubmissions.Theseshouldaddressfactorssuchastheprobabilityofprojectoverruns,expectedweldquality,andshouldbebasedonreviewsofcurrentcompetencies,demonstratedcapabilities,experienceandproventrackrecord.Dependencyonqualitysystemsaloneisinadequate.
iii. Technicalsupervision.Acompetenttechnicalauthorityshouldbeengagedtomonitortheprocessfromprecontractauditthroughqualification,inspectionandfinaldatareview.Thisshouldalsoinvolveindependentreviewofdatatoassessqualityandreliabilityofinterpretation.
iv. Itshouldbecontractuallyagreedthattheinspectionbodyisresponsibleforworkthatistechnicallynoncompliantwithprovisionforrepeatand/orescalationofinspectionactivity.
v. Thedesignofqualificationblocks,includingthetypeofartificialflawshouldbedefinedintheprojectspecificationandapprovedbythetechnicalauthoritypriortoprojectcommencement.Qualificationblocksshouldreflectactualcomponentgeometries,materialcombinationsandwallthicknesses.
vi. Qualificationflawsshouldaddressallinspectionzones,flaworientations(includingtransverseflaws)andweldbevelangles.
vii. Procedures,trainingandcertificationcriteriashouldmakeprovisionforinterpretationandcharacterisationofcomplexflawformationsandshouldaddresstheabilitytoaccuratelymeasuretheligamentofremainingmaterialbetweenaflawtipandtheexternalsurfaceinthecaseofnearsurfaceflaws.
viii. Theinspectionprocessshouldcommenceatthewelderandweldprocedurequalificationstageinordertooptimiseweldingqualityinadvanceofproductionwelding.Thisshouldbecomeaholdpoint.
ix. Inaccessiblewelds.Inaccessible,orpartiallyinaccessibleweldsareoftenclassifiedasatestrestriction,andexcludedfromexamination.Asdescribedearlier,thiscouldbeamatterofconvenience.Designsshouldbereviewedbyatechnicallycompetentauthoritypriortocommencementofconstruction.
x. Apossiblealternativetotherequirementforextensiveflawedsamplesistoapproachqualificationinthespiritofanuclearindustryqualification.ASMEcodesmakeprovisionforvariationssubjecttotheagreementofallparties,providedthevariationsatleastmeettheminimumcoderequirements.QualificationisrequiredinthenuclearindustryaccordingtoASMEXI,Appendix8,PerformanceDemonstration[Ref8].Inthiscase,theperformancecriteriaofaninspectionaredefined,andtheinspectionservicecontractorisrequiredtoqualifyequipment,proceduresandpersonnelusingacombinationofopenandblindtrialsontestsamplesthroughanindependentbody.TheEuropeannuclearcommunityestablishedtheEuropeanNetworkforInspectionQualification(ENIQ)[Ref9]throughthe
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EuropeanJointResearchCommission(JRC)asamechanismtomaintainqualificationstandardswhilstminimisingtherequirementforqualificationsamples.Thisisachievedthroughavigorousprocessinvolvingexpertjudgement/reasoning,mathematicalmodelling(e.g.CIVA[Ref10]andUMASIS[Ref11])andpracticaldemonstration.Priorexperiencewithqualificationmaybetakenintoaccount.Adoptionofthisprocesscouldenableaninitialbasecasetobequalifiedfollowedbyqualificationofvariablesonacasebycasebasis.ENIQwasdevelopedbythenuclearcommunity,butitisintendedtoapplytononnuclearapplicationsalso.Ithasbeenusedforseveraloilindustryapplicationsinrecentyears[Ref12].
AcknowledgementsThanksareextendedtomycolleagues,PeterConlin,GordonDavidsonandGordonReidofSonomaticLtd,allofwhomprovidedoutofhourssupportbyprovidingmaterialinsupportofthistechnicalpaper.
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