analytical considerations in code qualification of piping system

8/7/2019 ANALYTICAL CONSIDERATIONS IN CODE QUALIFICATION OF PIPING SYSTEM

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W SRC: MS- 95 -0 0 0 8Analytical Considerations in the Code Qualification of PipingS y s t e m s (U)

byG. A. AntakiWestinghouse Savannah River CompanySavannah River SiteAiken, South Carolina 29808

DISCLAIMERThis report wa s prepared as an account of work sponsored by an agency of the United StatesGovernment. Neither the United States Government nor any agency thereof, nor any of theiremployees, makes any warranty, express or implied, or assumes any legal liability or respnsi-bility for the accuracy, completeness, or usefulness of any information, apparatus, product, orprocess disclosed, or represents that its use would not infringe privately owned rights. Refer-ence herein to any specific commercial product, process, or service by trade name, trademark,manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recom-mendation, or favoring by the United States Government or any agency thereof. The viewsand opinions of authors expressed herein do not necessarily state or reflect those of theUnited States Government or any agency thereof.

A document prepared for ASME PRESSURE VESSELS& PIPING CONFERENCE at Honolulu from 07124/95-07/27/95.

DOE Contra& No. DE-AC09-89SR18035This paper was prepared in connection with work done under the above contract number with the U. S.Department of Energy. By acceptance of this paper, the publisher and/or recipient acknowledges the U. S.Government's right to retain a nonexclusive, royalty-free license in and to any copyright covering this paper,along with the right to reproduce and to authorize others to reproduce all or part of the copyrighted paper.


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Analytical Considerations in theCode Qlalification of Piping SystemsGeorge A. AntakiWestinghouse Savannah River Co.

ABSTRACTThe paper addresses several analyticaltopics in the design and qualificationof piping systems which have a directbearing on the prediction of stresses inthe pipe and hence on the applicationof the equations of NB, NC and ND-3600of the ASME Boiler and Pressure VesselCode. For each of the analytical topics,the paper summarizes the current coderequirements, if any, and the industrypractice.

CHOICE OF ANALYTICAL TOPICSIn 1984 and again in 1990, severalissues related to the analysis of pipingsystems were documented by thePressure Vessel Research Council(PVRC) in WRC Bulletins 300 and 353 [l ,21. More recently, within PVRC, a jointSection I11 and XI Review Committeeprovided further recommendations onthe analysis and qualification ofnuclear piping systems [3]. A TaskGroup on Analytical Methods wasformed within the ASME Section I11Working Group on Piping Design, tocompile the topics addressed by theabove PVRC reports and present, ineach case, the industry practice andrecommendations for improvements toSection I11 where ap pr op ria te .Members of the Task Group includedMesrs. T.M. Adams, K.C. Chang, G.Karshafdjian, J.E. Lucena, M.S. Sills,G.G. Thomas, E. Wais and G.A. Antaki.Following is a summary of the topicsresearched by the Task Group. Severalof these topics are under review by theWorking Group on Piping Design todecide whether they warrant anychanges to Section III.

CONTACT STRESS

Figure 1. Localstressesatpipe - support interface

WRC-300 part 111 and WRC-353Section 2.3.5 recognize that localizedstresses are generated in the contactarea between a pipe and a pipe support.Rules are needed to specify (a ) whenthese stresses should be quantified and(b) how they should be evaluated.WRC-353 Section 2.3.5 states that"industry experience has shown thatcommon forms of attachment, such asstandard clamps and U-bolts andbearing on structural members,produce stresses in the pipe which arelocalized in the pip e wall and aresecondary in nature, and hence can beneglected". The bulletin goes onhowever to say the "in any case, thelocalized stresses must be evaluated andcombined with o ther stresses orcompared to a predefined allowableestablished for localized stress". Thedesigner is cautioned to use load-


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distributing features (such as cradles)when supporting Schedule 10 piping.WRC-300 Part 111 warns that "Highstrength pipe clamp designs should beeither avoided or carefully evaluatedfo r their ef fect on local pipe wallstress".

Section 111 NB, NC and ND address thequestion of interaction between thepressure boundary and its attachments.The Code requires that "the interactioneffects o f attachments on the pressureboundary, producing thermalgradien ts, localized bending stresses,stress concentrations or restraint ofthe pressure boundary shall beconsidered by the piping designer"(NB.3651.3). Requirements for Class 2(NC.3645) and Class 3 (ND.3645) aresimilar to Class 1 although the wordingis somewhat different. The Codeprovides relief to the extent that"standard clamps generally have anegligible e ff ec t on the pressureboundary" [if thick-wall] (NB.3651.3).Subsection 3200 classifies the stress dueto a pipe attachment as secondary.Section NB-3 227.1 addresses bearingloads and states that "the averagebearing stress f o r resistance tocrushing under the maximum load...shall be limited to Sy a t temperature,except that when the distance to a freeedge is larger than the distance overwhich the bearing load is applied [i.e.no risk of shear failure], a stress of1.5Sy a t temperature is pem'tted".The closest Subsection NF comes toaddressing the contact load from thesupport onto the pipe is in NF-3412.4(d)where it states that "design offunctional [support] members such asinterconnections ... shall consider theef fect o f internal pressure, thermalexpansion and vibration loadings'!

Code Cases N-122, N-318, N-391 and N-392 address stresses generated on thepipe by integral welded attachments.The Code Cases provide equations forcalculating stresses from the appliedloads on the welded attachments, whichare added to the code stress equationsand compared to the code allowables.3 )1

PositionThe effect of pipe support loads on thepipe wall has been the subject of earlystudies in piping design. The Kelloggmanual "Design of Piping Systems"(1955) suggests that "when such localstresses are evaluated they should betreated in the category of secondary orlocalized stresses ... the allowable limitfo r such stresses when due to sustainedloadings cannot reasonably be set a tthe limit for sustained primary stressSh; instead it is recommended t h a t alimit of 2Sh be used for designpurposes".Today, there is no uniform commercialnuclear industry practice for theanalysis of contact stresses in pipingfrom non-welded (non-integral)supports. Most design applications didnot explicitly analyze stresses inducedin piping by localized contact loads. Insome cases, mostly a later 1980'spractice, pipe-to-support contactstresses are computed and added to thecode stress equation.Where contact stresses are calculated,they typically cover:(a ) line load from a longitudinalcontact between the pipe andthe flat surface of a support

member.(b ) circumferential loads from acircular line load between thepipe and a circular stiff clamp,strap or collar plate.


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Each of the loads is, in turn, comprisedof four contributions:(a ) preload (such as tight fit ortorque)(b ) internal pressure (constrainedradial expansion of pipe wall)(c ) thermal expansion (constrainedradial expansion and, for Class 1,thermal discontinuity). Notethat in non-nuclear plants, theeffec ts of therm al radia lexpansion are at times accountedfor by construction rules, suchas leaving the pipe clampsufficiently loose to allow for agap around the pipe.support reactor loads.

The pipe stress due to contact load,when calculated, is typically based onsolutions such as "Formulas for Stressand Strain" by R.J. Roark and W.C.Yound or WRC Bulletin 198..The Roark formula for contact stressesis adopted by the American WaterWorks Association (AWWA) "Guide forDesign and Installation" (Manual M l l ) .The AWWA manual states that "theability of steel pipe to resist saddle loadhas sometimes been greatlyunderestimated by designers' and,consistently with Roark, recommendsan allowable of 2Sy for the maximumlocalized stress at the saddle.Surveys of earthquake damage topiping systems (EPRI research projectRP-2635-1) does not indicate failurefrom contact stress effects.The NRC has issued Information Notice83-80 "Use of Specialized Stiff PipeClamps" to warn that "piping designerswho are accustomed to neglectingthese localized [pipe-to-s upportcontact] stresses because of the lowmagnitude stresses associated withconventional pipe clamps mightincorrectly assume that such stressescan be neglected with these new [stifflclamps".

In conclusion, the industry practicevaries from no explicit analysis (inmost cases) to analysis based on stressformulas (mostly in the late 1980's).There is no evidence through tests oroperational experience that contactstresses are a credible source of pipefailure.

DESIGN - BY - RULEFORSMALL BORE PIPING SYSTEMS

WRC-300 P a r t I 1 1 Section 4.7recommends "rather than to continueto insist on complex ana l y t i ca lsolutions, it may be more beneficial totake our experience and developdetailed design by rule requirementsthat control geometry for envelopes ofconditions".WRC-300 Part 11, recommendations4.15 and 4.16state:"4.15 The piping codes should berevised to require a simple analysisrelated to the expected mode o f failurecoupled with specified standard designand fabrication details, and"4.16 Experience and judgment must berelied on as much as complicatedanalytical solutions."Recommendation F-05(3) of the PVRCC o m m i t t e e on R e v i e w of ASMENuclear Codes and Standards (1988-1991) states "Bounding spectra withapplicable limitations as developed byNCIG-EPH should be permitted as thebasis for layout and design o f supportsfor all small bore piping (Do= or < 2 112inches)".


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The Code provides rules for evaluationof stresses resulting from Design andService Loads. However, the definitionof these loads is the responsibility ofthe Owner and certificate holder,through the Design Specification.The Code does require a stress analysisin NB-3625: "StressAnalysis :"A Stress analysis shall beprepared in sufficient detail to showthat each of the stress limitations ofNB-3640 and NB-3650 is satisfied whenthe piping is subjected to the loadingsrequired to be considered by this Sub-article.''Appendix N, Section N-1100, states 'I... adynamic system andysis is required toshow how seismic loading istransmitted...If.In NB-3672.6, the Code allows for somequalificationper comparison:"NB-3672 .6 Method Of Analysis. Allsystems shall be analyzed for adequateflexibility by a rigorous structuralanalysis unless they can be judgedtechnical ly adequate by anengineering comparison withpreviously analyzed systems.If3 ) h d u s t r v and R e n datorv

positionThe original piping Codes (B31 Series)were based on rules for proper layoutand detailing of piping systems. Therules evolved to focus on analysis foroperating loads (thermal expansionstress evaluation introduced by Marklin the 1950's) and, later, analysis foraccident loads. The practice since theearly-1970's and through the 1980's hasbeen to qualify nuclear safety relatedpiping by analysis. For small borepiping (2'' and smaller) various "cookbooks" were developed over time,

which were based on limiting spanlengths and layouts to certain pre-analyzed configurations.Recognizing the excessive costs andthe limited value added of wholesaleanalysis, the nuclear industry,through EPRI, has developed rules forthe seismic evaluation of small borepiping systems.Recently (1990) EPRI and NCIG haveissued a "Procedure For SeismicEvaluation and Design of Small BorePiping (NCIG-I 4) EPRI NP-6628, whichprovides criteria to design againstrealistic seismic failure modes. Thecriteria are based on the investigationof earthquake experience and test data.NP 6628 does not change current use ofcode rules for the evaluation of seismicanchor movements and non seismicloads, but does not require includingseismic inertia stress in thequalification process.NUMARC submitted NP-6628/NCIG14for NRC review, and TVA requestedapproval of use of N P 6628 forBellefonte completion. The NRCresponded with a request for additionalinformation. EPRI developed aresponse to the NRC request consistentwith recent work done by the SG DSpecial Task Group and work done forthe Advanced Reactor Corporation Firstof a Kind Engineering activity relatedto ASME piping. This response wasprovided to the NR C in October 1993with a request from NUMARC for anestimate of review cost to complete theeffort.In Generic Letter 81-14, the NR C hadendorsed the use of an experiencebased approach including "walkdownb y personnel experienced in theanalysis, design and evaluation I' toidentify flagrant weaknesses in theseismic adequacy of auxiliaryfeedwater systems. The Generic Letterstated "Given the time frame...noexplicit analyses are requested to


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demonstrate system qualificationunless deemed necessary by you".SUPPORT STIFFNESS RULESource and DescnxmmQm2Qk

. .WRC-353 Section 2.3.2 providesseveral rules for the treatment of theeffects of support stiffness on a pipingsystem. The Bulletin does not make arecommendation on whether tointroduce this topic into the ASME Code.

Figure2. Supportsare modeledas springstiffhesses

Current Code PositionSection HI NB, NC and ND do not addressmodeling details, such as supportstiffness to be used in determiningstresses for comparison against thelimits in 3600. NF 3122 states that"deformation limits for the supportedpiping or component shall bestipulated in the Design Specifications,ifrequired 'I.3 )1PositionAmong U.S. nuclear plants, a variety ofpiping supports/restraint stiffnesscriteria have been used. They have

included designing pipe supportrestraint to a minimum stiffness or aminimum frequency of either 20HZ or33HZ, modeling the actual stiffness(mostly in Class 1 applications), orusing a deflection criterion rangingfrom 1/32" to 1/8".EPRI Research Project 2967-2developed seismic restraint stiffnesscriteria as part of a study on supportmodeling, based on numerous papers,studies, laboratory tests and reviews ofactual earthquakes.The recommendations of the EPRIproject for ductile piping systems(with no brittle joints and with stablerestraints) is that the total maximumrestraint deformation, under maximumdynamic loads, not exceed a smallnominal predetermined value. Thisvalue could be less than 5% of theaverage piping system dynamicresponse displacement or a fraction ofan inch such as 1/8" or 1/4" totaldisplacement. The Research Project isalso recommending the use ofminimum design loads based on theaverage loads for a subsystem or a fixedvalue based on pipe size.

SUPPORT GAPS

Gap Gap

Figure 3. Small gaps can exist around thepipein the restrainted direction


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W R C 353, Section 2.3.3 providesguidelines for the treatment of pipe-to-supp ort gaps (clearances). TheBulletin does not make arecommendation on whether tointroduce this topic into the ASME Code.. .2) en t Code Positlnn

Section 111NB, NC and ND do not addressmodeling details, such as gaps betweenpipe and restraint to be considered indetermining the s t resses forcomparison against the limits of 3650.In various places, NF 3000 requires thepipe support to provide for movementof the piping or component.3 ) Ipdustrv and Remlatory

PosftionA thorough discussion of pipe supportgaps is provided in WRC-353. Gaps areused to allow unrestrained movementsof piping in the non-load directionswhile being sufficiently small to benegligible in the loaded direction. Theindustry practice for the analysis ofpiping with small gaps (usually 1/16"per side or 1/8" total clearance) is toignore the pipe support gaps wherethey are small.The recent EPRI Research Project 2967-2 identified several studies of theeffects of support gaps. In almost allcases, when a non-linear analysis wasperformed considering small gaps, therestraint loads and pipe stresses wereenveloped by the linear analysismethods. Small sup po rt gaps(including snubber clearances)acquire more importance in thevicinity of equipment nozzles whichare particularly sensitive to small loadredistributions.

SUPPORT WEIGHT

tpipeWeight

Figure4. In certaincasespart of thesupportweight is carried by thepipe.

W R C 353, Section 2.3.4 providesrules for judging the significance ofsupport mass on a piping system. TheBulletin does not make arecommendation on whether tointroduce this topic into the ASME Code.

Section I11 NB, NC and ND 3623 statesthat "piping systems shall provide forthe effects of live and dead weights"and defines dead weight as "the weightof the piping, insulation, and otherloads permanently imposed upon thepiping".

The industry practice is to generallyneglect the weight of the support massin considering piping stresses.Normally, if proportionally sized


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support components are used, the ratioof the support mass to the piping massis sufficiently low to justifyneglecting the effects of the supportcom pone nt. However, whendisproportionally sized components areused, results have shown the mass ofthe support components can have aneffect on the piping stresses andshould be factored into the pipinganalysis. EPRI Research Project 2967-2identified isolated cases where supportcomponent mass was factored into thepiping analysis, particularly forcertain types of configurations andprimarily on small bore pipes.

PIPE BRANCH AND RUNDECOUPLING TECHNIQJJES

Section I11 NB, NC and ND address thequestion of intersections in piping.For class 1 piping, NB-3683.1 requiresthat branch and run moments becombined to calculate the total stress ata branch connection. For Class 2 and 3piping, the branch and run momentsare calculated separately, each with itsown stress intensification factor.There is no explicit Code Criteria onhow to decouple branch lines from runpiping to accomplish this.Application Subsections: NE3 643, NC-3643 and ND-3643.3 )

PositionThere is no uniform commercialnuclear industry practice for pipeWRC-300 Part 111 recommends the bra nc h an d r u n dec oupl ing"use[of3 a moment of inertia ratio of 25 techniques. In dynamic and flexibilityto 1 for analysis decoupling of small analysis, a piping run is typically usedbranch lines from major runs" further as an anchor point for a branch line, ifclarifications an d exceptions are it meets certain criteria. Criteriaprovided in Section 2.2.2 of the usually include that the ratio of run toBulletin. branch diameter, moment of inertia orsection modulus should exceed aThe Bulletin does not make a specified ratio. Decoupling criteria for

rec om me nda tion on whe the r to moment of inertia typically rangedintroduce this topic into the ASME Code. from 1O:l to 25:l and 41 for diameter.A moment in inertia ratio of 25:l wasused in the industry recommendationof WRC Bulletin 300, with the additionalrecommendation that lower valuesshould be considered where applicable.An analysis of a branch pipe thatmeets these criteria typically use theconnection to the run pipe as a rigidanchor. The practice has been not todevelop run pipe amplified responsespectra at the decoupling point, butrather to apply anchor motions and toenvelope run pipe support attachmentpoints response spectra. This approachgets to be less accurate as the run pipeis more flexible-

BranchLine

H&pipe

Figure 5. Certain branch lines are decoupledfromthe headersystem analyticalmodel.


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Run pipe SIF's were typically notincluded in the original pipinganalyses in the late 1960's and early1970's for most branch connectionlocations. This is because the Code didnot provide formulas for calculation ofSIF's except for the connections. TheSIF's for branch connection point, andfor the run pipe at the branchconnection point in the run pipeanalysis (when applicable).Vent or drain lines are seldom includedin the model of the run pipe as theymeet the requirements for decouplingas described in the precedingparagraphs.In a 1980 memorandum, the NR CMechanical Engineering Branchaccepted a moment of inertia ratio of7: l for a specific plant application. Abroader decoupling criterion isprovided in Section 3.7.2 of theStandard Review Plan.

DESIGN TEMPERATURESMALL BORE PIPINGFOR

1:

T desim

tFigure 6. The Design Temperature may beset significantlyhigherthan actualsystem operating temperatures.

WRC Bulletin 300 Part I11 Section2.2.6 advises that "using the systemdesign temperature in place of theoperating temperature for thermalflex ibilit y analyses should beavoided........The design specificationshould reflect the appropriatetemperature for analysis". TheBulletin recommends consideringtemperature decay in stagnant lines;and limiting therm al expansionanalysis to normal and upset operatingmodes. A similar warning is providedin Section 3.3, which states "A commonmistake leading to increasinglyconservative nozzle loads is to use thesystem design temperature rather thanactual operating temperatures whenanal'irzg piping".2) Current C ode PositionASME Code Section I11 addresses designloadings in NCA-2 142.1. The designtemperature is defined as follows:"Design temperature shall not be lessthan the expected maximum meanmetal temperature through thethickness of the part considered forwhich Level A service limits arespecified".The Level A service limits are "thoseset o f limits which must be satisfied forall Level A service loading identified inthe Design Specification to which thecomponent or support m a y besubjected in the performance of itsspecified service function".From the above definitions, it is clearthat design te mp eratu re is themaximum metal temperature which isexpected to occur during normal plantoperations.


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3 )1PositionThe design process requires theidentification of all operatingconditions by system engineers. Thedesign temperature is then defined,typically by the equipment vendor, asa temperature higher than all theoperating temperatures, regardless oftheir service levels. For large bore andASME Class 1 systems, this process maynot be overly conservative. However,for small bore Class 2, 3 and non-nuclear safety piping, whose designdoes not require design specification,this process leads to unnecessaryconservatism in both high thermalexpansion stresses and low allowablestress intensities.The use of unnecessarily large designtemperatures for small bore pipingsystems can also lead to the addition ofexpans ion loops , wi th thecorresponding increase in congestion,material procurement, construction,support structures and risk of leaks atpipe joints.Advancement in nuclear piping designproved that the old design process doesnot necessarily lead to a safer system.Thermal sleeve failures at branchnozzles with reinforcement are typicalexamples.In conclusion, the industry designpractice tends to select conservativelyhigh design temperature which inturn penalize system design with noobvious benefits. This pena lty isespecially severe for small borepiping.

DEFINITION OF "COLD" PIPING1) Source and DescriDtionQmmkThe term "Cold" piping refers to pipingthat operates at low temperature and asa consequence, the stresses associated

with the thermal expansion arerelatively small and not required to beexplicitly calculated. The source of thetopic is two fold(a) recommendation F-05 (4) of thePVRC Committee on Review

of ASME Nuclear Codes andStandards (1988-1991( whichstates: "Thermal Stress analysis(flexibility analysis) of pipingsystems is not required only[SIC] for ASME Class 2 and 3where To= or (150OFand Do= or< 6 inches."(b ) an upcoming PVRC PositionPaper tha t proposes atem peratu re of 150F as aboundary between cold and hotpiping.Current Co2) de P o s m. .

Section I11 NB, NC, ND do not addressthe definition of cold piping. The Coderequirements for evaluation ofthermal expansion are as follows:"The design of piping systems shalltake account of the forces and momentsresulting from thermal expansion andcontraction and from the effects ofexpansion joints'' NC,ND-3624.1."The design of the complete pipingsystem shall be analyzed betweenanchors for the effects of thermalexpansion, weight, and other sustainedand occasional loads". NC/ND-3651 (a)."....piping systems subject to thermalexpansion or contraction... shall bedesigned in accordance with therequirements for the evaluation andanalysis o f flexibility and stressspecified in this paragraph". NC/ND-3672.1 (a)."All systems shall be analyzed foradequate flexibility by a structuralanalysis unless they can be judgedtechnical ly adequate by anengineering comparison with


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previously analyzed systems. I' NC/ND-3673.1Subsec t ion N B has s imi la rrequirements NB-3624.1, 3672 (a) ,3672.7.In summary, the Code requires athermal expansion analysis exceptwhen an engineering comparison withpreviously analyzed systems indicatesthat the system is technically adequate.3 ) Jndustrv and Reeulatory

PositionThere have been several attempts inthe piping industry to developsimplified rules to screen those pipingsystems that require a detailedflexibility analysis. The only one rulethat is part of a code appeared in the1955 edition of the Piping Code (ASAB31.1) and can still be found in thecurrent editions of the ASME B31 codes.This rule is not based on a temperaturethreshold, but on a formula that relatesvarious parameters such as, pipe size,movements to be absorbed, developedlength and minimum length betweenpiping anchors. This formula tries toquantify the relative flexibility of thepiping system in a simplified manner.There are however so many cautionsthat the rule becomes practicallyuseless.Current industry practice in fossil andco-generation plants is to analyzepiping systems with a temperature ator above 2500F.WRC Bulletin #300 refers to cold pipingin the chapter related to the pipingrestraint selection criteria indicatingthat "For piping systems where theanalyst has determined that thermalexpansion stresses, loads anddeflections are minimal, rod or frametype restrain ts in lieu of snubbersshould be specified".

The draft version of the upcoming"PVRC Position Paper-Piping AnalysisTechniques" proposes the followingcriterion: "The definition of "cold"versus "hot"piping as a dividing linebetween piping that is analyzed forthermal expansion and that which isnot, is important. A reasonablecr i t e r ion e s tab l i shes p ip ingcontaining fluids at 15OOF or greater as"hot" and therefore r equirin gexpansion analysis.EPRI Report NP-5 184M "SnubberReduction Program" in page 1-2, whendealing with snubber reductiontechniques for cold piping systems,states: "...Those systems that areessentially cold (


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piping and 1550F = l5WF for stainlesssteels. This appears to be a morereasonable criteria.NOZZLE FLEXIBILITY

Figure 7. Equipmentnozzlesand shellshave flexibilitieswhich canreducepipe reaction loads.

Source and DescriDtionQmmkWRC - 300 Part I11 recommendation 3.5states that "the flexibility analysis o fequipment nozzles and foundationsupports should be considered in theanalysis". Recommendation 3.8 statesthat "A coupled piping analysis,ineluding noz zle flex ibilit y an dfoundation support flexibility, canreduce calculated nozzle loads". TheBulletin does not make arecommendation on whether tointroduce this topic into the ASME Code.

Current Code Positloa. .Section 111 of the Code does not addressnozzle flexibility in piping systemanalysis.3 )1PositionTerminations of piping systems quiteoften consist of nozzles in pressurevessels and tanks or nozzles of rotatingequipment such as pumps or steamturbines in branch connections to run

pipe. The specified allowable loads onnozzles are often so low that theyrequire additional restraints on thepiping. Also, quite often the nozzlesare modeled as rigid anchors in thepiping system analysis.The combination of using lowallowable nozzle loads and ignoringnozzle flexibility may lead to additionalrestraint which by a more rationalapproach, could be shown to beunnecessary. these additionalrestraints may reduce the reliability ofthe piping system and will add to itscost.Typically, the nozzle on a plate or shellis more flexible than the attached pipe.Use of the correct flexibility in static ,thermal and dynamic analyses willfrequently reduce predicted nozzleloads by a significant amount.Flexibility can be computed usingequations from plate and shell theory,with proper considerations ofstiffening members and internals.WRC Bulletin 297 provides data fromwhich the nozzle flexibility can beobtained.WRC Bulletin 300, published in 1984,recommend that flexibility of thenozzle should be included in the pipingmodel.NUREG-1061, published in 1985,recognizes that the design of pipingbranch connections and tank andvessel nozzles do not generally takecredit for nozzle flexibility, resultingin higher calculated stresses.Improving nozzle design procedurescould help reduce the number ofseismic restraints required in currentpiping design.NUREG-1061 report made the followingrecommendations with regard to nozzleloads and flexibility:1) Request that WGPD revise theCode sections addressing pipesystem flexibility calculation to


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3)

also consider tank and vesselnozzle flexibility. results as dynamic analysis, should bepermitted for large bore class 2 and 3piping".Revise Standard Review Plan3.9.2 to consider nozzleflexibility in piping analysis.Develop improved designguidance on nozzle stress limitsand flexibilities.

SEISMICSTATIC m O D SFORPIPING SYSTEMStime history

t

t static tFigure 8. Applied loadings for variousanalyticalmethods.PVRC recommendation F05 "Section I11Analysis Requirements for Piping"proposes that "ASME Section I11 Coderules should be modified to include thefollowing: (1) The use of rigorousdynamic seismic analysis methods(response spectra, time history) forpiping systems is not always required.(2) Simplified static load procedures(e.g., Code Case N-468), which on theaverage give higher or the same

Appendix N. Article N - 1 0 0 "DynamicAnalysis Methods" currently states:"In order to determine the specificseismic designs for each component,dynamic system analysis is required toshow how seismic loadings iftransmitted form the defined groundmotions to all parts of the buildings,s t r u c t u r e s , e q u i p m e n t a n dcomponents I t . The commoninterpretation of the requirement isthat floor spectra will be developed bydynamic analysis of the buildingstructures. However, this requirementmay be interpreted textually to meanthat a dynamic analysis is required for"equipment and components" andtherefore for piping systems.3 ) DUSTRY AND REGULATORYl?QsmQNWhile lateral load design forearthquake resistant buildings wasintroduced into practice in the 1930's,the concept of lateral load design forpiping systems has been introducemuch more recently (.20g static designof nuclear plant piping in the 1960;s)and published in the early 1970's (J.D.Stevenson, 1973; J.M. Gwinn and N.A.Coldstein, 1974; T.R. Simonson and G.Kost, 1976). At the time, it was judgedthat rigorous dynamic analysis was notrequired for the "tens of thousands offeet of conduits consisting of smallerdiameter piping, tubing, electricalconduit raceways and ductwork whichserve a safety function and thereforerequire a determination of seismicdesign adequacy" (J.D. Stevenson andW.S. W a y , 1975). It was estimated thata coefficient of 0.67 to 1.50 could beapplied to the input responseacceleration to determine the lateralload. A coefficient of 1.20 was later


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proposed (C.W. Lin and T.C. Esselman,1982).The NRC Standard Review Plan (SRP)adopted the equivalent static loadmethod with a factor of 1.5 applied tothe peak floor acceleration if "thesystem can be realistically representedby a simple model" (SRP 3.9.2-7, July1981). The "simple model" conditiondoes not make the SRP 1.5 factornecessarily applicable to pipingsystems.Currently, the California UniformBuilding Code specifies lateral loadp rovi sion s l1non -st ru ctu ra1components supported by structures",including "plumbing" and "machineryand associated piping" as Fp = ZICpWp,where Z is a seismic zone factor, theimportance factor I is 1.5 for hazardouscontents and Cp is 2x0.75 = 1.5 forflexible ( f 4 7hz) systems.

for

A similar form of static load factor wasintroduce in the IAEA "EarthquakeResistant Design of Nuclear Facilitieswith Limited Radioactive Inventory"(1985) and has been pursued by theASME Working Group on DynamicAnalysis since 1986. the later effortresulted in ASME Code Case N-468"Alternate Method of EarthquakeDescription for Class 2 and 3 Piping atLow Seismicity Sites Section 111,Division 1" now expired. In its latestform, (as presented in 4-468), the loadcoefficient is developed by a statisticalprocess based on comparative staticand dynamic (response spectra)analysis of representative pipingconfigurations. A review of themethods in N-468 by concluded that "itis not ready for incorporation intoAppendix N" (attachment 10 to 9/14/92WGPD meeting minutes). A revisedproposal is being reviewed by theSpecial Working Group on DynamicAnalysis. The revised proposalincludes static coefficients of SO , .75 or1.0 depending on applied to the peak ofthe floor response spectra. Further

documentation of the method is beingdeveloped through the PVRCSubcommittee on Dynamic StressCriteria.Recently, the Seismic QualificationUtilities Group (SQUG) established thata 1.0 equivalent static coefficient w asto be used for determining seismicinertial equipment loads. The 1.0coefficient for equipment anchorage isbased on closed form solutionsdocumented in EPRI NP-5228 Volume 1''D ev elopm en t An c h orageGuidelines", Appendix D. Pipingsystems are not addressed by the SQUG.

of

NON-LINEAR DESIGNOFDUCTILE PIPING

Recommendation F-05 (5) of the PVRCCommittee on Review of ASME NuclearCodes and Standards (1988 - 1991) states"Limited non-linear behavior o fductile piping and other mechanicalsystems should be permitted for seismicdesign".2) t Code P o s m

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Appendix N, N-1222.2 providesacceptable numerical methods toanalyze nonlinear problems,including:(1) material non-iinearities(plasticity);(2) geometric non-linearities(large displacement);(3) combination ofmaterial andgeometric non-linearities(impact and fiction)".Appendix F provides for the inelasticanalysis of vessels, pumps, valves,piping and core support structuressubject to level D loadings. TheAppendix F, "Inelastic analysis" isdefined as a class of methods whichincludes "limit analysis" (elastic-


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perfectly plastic model) and "plasticanalysis 'I (strain hardening model).Appendix F also allows for the use ofcollapse load methods (unstable hingemechanisms).For piping, Appendix F, F-1430 permitsas an alternative to either elastic orplastic analysis to apply the pipe stressequations of NB-3652 limited to thelesser of 2SM or 2Sy. In this context,the increased allowable is viewed as asimplified alternative (a substitute) forplastic analysis. The allowables arespecified in NX-3655.3) hdustrv and ReeuIatory

PositionIn the late 1960's Newmark and Hallproposed that for "small excursionsinto the inelastic range" the seismicresponse spectrum be decreased by theductility factor (mu)as follows:l/ (m u) below approximately 2 hz;1 d G between 2 hz and 8 hz; andno decrease beyond about 33 hz. Theductility factor (mu) was defined as theratio of the maximum allowabledisplacement of a structure to its elasticdisplacement. They suggested thatequipment and components that candeform "inelastically to a moderateextent" be allowed a ductility factor of 2to 3. The method later evolved into the"inela s t ic response spec trumapproach. This and other analyticalmethods for estimating the non-linearresponse of piping systems arediscussed in the recent WRC Bulletin379 "Alternative method for seismicAnalysis of Piping Systems".In a 1976 commentary on the newlydeveloped faulted stress limits for class2 and 3 components (76-PVP-61),Branch and Gascoyne recognized thatfaulted condition limits were selectedon the basis of "ultimate tensilestrength, rather than yield", allowingtherefore yielding. The faulted stressallowable being the minimum of (0.5SU or 1.25 Sy) for general primary

membrane stress and the minimum of(0.6 Su or 1.6 Sy) when primarybending is included.The non-linear behavior of ductilepiping has been correlated to the ASMEI11 faulted stress allowables byRodabaugh and Moore (NUREG/CR-0261) and more recently by Rodabaughand Terao (NUREG1367).In the Standard Review Plan (NUREG-0800, 3.7.2-5 Rev. 2) the NRC states " m eSRP criteria generally deal with linearelastic analysis coupled with allowablestresses near elastic limits of thestructures. However, for certainspecial cases (e.g., evaluation of as-built structures), the s taff has acceptedt h e c o n c e p t o f l i m i t e dinel as tic /n onlin ear behavior wh enappropriate. The actual analysis,incorpora ting inel as tic/n onlinearconsiderations, is reviewed on a case-by-case basis".The NRC Piping Review Committeeconcluded in 1985 (NUREG-1061) that"since SSE is a low-probability event, itis appropriate to accept some inelasticbehavior in the design o f pipingsystems in order to fully use theircapability to absorb and dissipateen ergy. linear- elas ticestimation methods should bedeveloped and procedures designed toaccountfor inelastic response".

P seud o

REFERENCES1 Welding Research CouncilBulletin 300 "Technical Position onIndustry Practice", December 1984.2 Welding Research CouncilBulletin 353 "Position Paper on NuclearPlant Pipe Supports", May 1990.3 Welding Research CouncilBulletin 370 "RecommendationsProposed by the PVRC Committee onReview of ASME Nuclear Codes andStandards", February 1992.

analytical considerations in code qualification of piping system

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