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Jaan TaageperaChevron Energy Technology Co.,100 Chevron Way,Richmond CA 94802e-mail: taageperajTrevor G. SeippFluor Canada Ltd.,55 Sunpark Plaza, SE,Calgary, AB, Canada T2Y 3X6e-mail: seipptSlip Blinds at Pressures CausingPermanent DeformationSlip blinds are frequently used for hydrotesting piping. In addition, when maintainingequipment or piping, the equipment or piping must be isolated to ensure a safe workingenvironment. Separating flange pairs and inserting a blind flange against the process sideprevents hazardous substances from entering the work area. Slip blinds are often used forthis type of service. However, slip blinds are generally limited to low-pressure servicesince at excessive pressures the blind will become dished and may leak or becomeimpossible to remove. For this paper, slip blinds of various sizes and thicknesses werehydrostatically tested to determine their deformation as a function of pressure. Nonlinearfinite element analysis (FEA) was used to analytically determine the deformation of slipblinds. The goal of the testing and FEA was to determine allowable pressures that wouldlimit permanent deformation of the blinds to specified values. ?DOI: 10.1115/1.2716428?IntroductionPiping spools are often isolated for hydrotesting in the field bybreaking a flange pair and inserting a blind to provide the isolationrequired for the hydrotest. Slip blindsround plates which areplaced inside the bolt circle of standard piping flangesare fre-quently preferred over blind flanges for several reasons: Blindflanges are more costly than slip blinds. Slip blinds are lighter andthus are easier and safer to handle, particularly in piping acces-sible only from scaffolding. Finally, slip blinds can fit whereflange pairs cannot be separated enough for standard blind flangesto fit.Vessels and heat exchangers are routinely taken out of servicefor inspection and maintenance. A safe working environment mustbe provided for personnel entering the equipment. One aspect of asafe working environment is breathable, nonhazardous air. Ensur-ing this environment requires a means of isolating the equipmentfrom the piping normally attached to it. If a piping spool piececannot be removed, other approaches for equipment isolation mayhave to be utilized.Block valves, when present, do not provide a 100% leak-freeguarantee. Maintenance personnel working in a vessel isolated byonly a single block valve is at risk of being exposed to potentiallytoxic chemicals. If the piping system is sufficiently flexible, ablind flange can be installed downstream of a single block valveto provide the required isolation.If a blind flange cannot be installed, a “double-block-and-bleed” setup may be used. A double-block-and-bleed setup con-sists of two block valves with a bleeder valve in between. Thebleeder is left open to ensure that the downstream block valvesees no pressure. This provides reasonable assurance that no fluidwill enter the isolated equipment.In some cases, a pipe spool cannot be removed and the flangescannot be spread far enough apart to fit a fully rated pressure blindflange into the gap. Only a single block valve is present or perhapsone of the two block valves required for a double block and bleedis inoperable. In these situations, a relatively thin slip blind maybe used. Slip blinds, when properly installed, are recognized bythe U.S. Occupational Safety and Health Administration ?1? as alockout device for “Lockout/Tagout” programs. However, thinslip blinds are not usually considered to have adequate thicknessto contain even moderate pressures.Slip blinds are used for both hydrotesting and equipment isola-tion. However, hydrotesting in the field, as shown in Fig. 1, isprobably the greatest cause of dished slip blinds. Although notlikely to burst when exposed to moderate pressures, the slip blindmay permanently deform by yielding at the inside diameter of theflanges and become dished. A dished slip blind may be impossibleto remove when the time comes to return the piping or equipmentinto service.When engineers are consulted for allowable pressures on slipblinds, limited resources are available to make this determination.The available resources include textbooks and piping codes thatprovide formulas to ensure that the blind does not yield. In aneffort to determine allowable pressures higher than those allowedby linear calculations for 0.250 in., 0.375 in., and 0.500 in. thickslip blinds, five sizes of slip blinds, 4 in. nominal pipe size, 8 in.NPS, 14 in. NPS, 20 in. NPS, and 24 in. NPS were hydrostati-cally tested. Nonlinear FEA was utilized to model the blinds thatwere hydrotested. Matching the modeling to the physical testsvalidated the FEA models.Allowable pressures for 0.250 in. thick blinds have been devel-oped that limit the deformation to 0.125 in. and 0.250 in. Thetarget deformation of 0.125 in. was chosen since the deformedthickness of the blind would become 0.375 in. This is roughly theminimum gap between flanges, which is reasonable in order toinsert a gasket and slip blind. When the equipment is to be placedback into service, the gasket can be removed first, thus makingroom to remove the now deformed blind. The 0.250 in. deforma-tion target was chosen simply to match the plate thickness. For the0.375 in. and 0.500 in. thick blinds, target deformations were cho-sen as 0.125 in. ?based on gasket considerations?, half the platethickness, and plate thickness.The slip blinds in this effort are assumed to be fabricated fromcommonly available carbon steels with a specified minimum yieldstrength ?SMYS? of at least 35 ksi. For this effort, these steels arelimited to SA36, SA515-65 and 70, and SA516-65 and 70 ?2?.Furthermore, the blinds are assumed to be used at ambient tem-peratures. This temperature assumption is clearly valid for hy-drotest situations. For equipment isolation uses, a slip-blind loca-tion would typically be downstream of a valve. Even if the valvewere to leak and pressurize the slip blind, there would not beenough flow to heat up the blind significantly above ambient tem-peratures.Linear AnalysisEngineers asked to provide maximum allowable pressures forslip blinds would typically use a formula originally solved byPoisson and presented by Timoshenko ?3? for a circular plate withContributed by the Pressure Vessel and Piping Division of ASME for publicationin the JOURNAL OFPRESSUREVESSELTECHNOLOGY. Manuscript received January 23,2006; final manuscript received November 10, 2006. Review conducted by DennisK. Williams. Paper presented at the 2005 ASME Pressure Vessels and Piping Con-ference ?PVP2005?, Denver, CO, July 1721, 2005.248 / Vol. 129, MAY 2007Copyright 2007 by ASMETransactions of the ASMEDownloaded From: / on 01/27/2018 Terms of Use: /about-asme/terms-of-useflat edges. This formula is incorporated into the ASME B31.3piping code ?4? and this same equation from B31.3 is used byB16.48 Steel Line Blanks ?5? in developing the thicknesses usedfor blinds in that standard. For a uniformly loaded circular platewith clamped edges, and assuming that the plate remains linear ingeometry and materials, the maximum deflection w occurs in thecenter of the plate withw =qa464D?1?where q is pressure, a is radius, and D is the plate constant definedasD =Et312?1 v2?2?where E is Youngs modulus, t is the plate thickness, andvisPoissons ratio.The maximum bending stress occurs at the boundary of theplate:?=34qa2t2?3?Equation ?3? is based on the plate resisting the pressure prima-rily through bending. As the blind deforms, however, it begins toresist the pressure load through membrane stress in addition to thebending stress. Young ?6? recommends limiting Eq. ?3? for stressto situations where the deflection does not exceed half of the platethickness. Alternative, iterative formulas are provided for thosesituations where the deflection exceeds half of the plate thickness.A comparison of results using Timoshenkos approach ?3?,Youngs ?Roarks? approach ?6?, and hydrotesting results was pre-sented in a paper by Taagepera and LaBounty ?7? in 2004.HydrotestingSix test rigs were fabricated in order to run hydrostatic tests onvarious sizes of slip blinds. Figure 2 shows the test rigs used forthe 24 in. and 14 in. tests. The rigs were fabricated as shown inFig. 3 of carbon steel components. The slip blinds were cut fromSA516-70 material with mill test reports ?MTRs?. Some of thematerial had yield stresses of 65 ksi, substantially higher than thespecified minimum yield strength of 38 ksi ?2? for that material.These high yield stresses were incorporated in validating the FEAmodels.Two raised face weld neck flanges were used to sandwich a1/16 in. thick corrugated metal graphite-coated ?CMGC? gasketand the slip blind. One flange had a pipe cap with pressure testconnections welded to it. The optimal flange bolt torque to use asrecommended by the gasket supplier ?8? results in a bolt stress of?60 ksi. The hydrotest rigs were torqued to a bolt stress of?45 ksi. This is a more realistic value for flanges bolted up in thefield. Deflection was measured directly with a dial indicator fromthe nonpressured side of the blind through the flange.Each blind was then pressurized, depressurized, and the re-sidual deflection measured. The test pressure was incremented up?increments varied for different sizes?, and the process repeated.Pressure was increased until a permanent deformation of betweenhalf the original plate thickness and 1 in. was achieved. In gen-eral, later testing was carried out to higher deflections than earliertesting.Fig. 1Slip blind used for hydrotesting an exchangerFig. 224 in. and 14 in. NPS slip-blind hydrotest rigs with Cl-300 flangesFig. 3Typical slip-blind hydrotest rigJournal of Pressure Vessel TechnologyMAY 2007, Vol. 129 / 249Downloaded From: / on 01/27/2018 Terms of Use: /about-asme/terms-of-useNonlinear Analysis: FEA ValidationA nonlinear finite element analysis was performed in order toprovide better insight into the behavior of blinds in pipe sizes thatwere not hydrotested. The sizes that were tested were modeledfirst in order to validate the FEA.Flange configurations are notoriously difficult to model accu-rately. Flange rotation, gasket material nonlinearities, bolt loadingat assembly, the mechanical properties of the plate, and severalother factors could all be accounted for in the model. In this case,however, some simplifications are possible. The blind is clampedbetween two flanges that are substantially stiffer than the blinditself. Only one side of the blind has a gasket; the other side is indirect contact with the other flange. Thus, it is reasonable to modelthe blind as clamped on both edges without consideration for theflanges and gasket. Furthermore, since the deflection of the blindis toward the side without the gasket, it is unlikely that the gasketwill contribute much to the overall stiffness of the blind.Therefore, the models consisted of only the blind. The blindwas modeled as two-dimensional ?2D? axisymmetric using qua-dratic elements. The mesh for each model was chosen such thatthe results were independent of the mesh within 2%. On the topand bottom of the blind, vertical deflection is restrained from theflange i.d. to the gasket o.d. Radial deflection is not restrained. Atypical model, showing typical mesh discretization and boundaryconditions is shown in Fig. 4.The nonlinear material model that best fit the hydrotest datawas a linear elasticperfectly plastic ?EPP? material model, withthe von Mises yield criteria used. The yield stress used was equalto the yield stress from the mill test report ?MTR? for each of thehydrotests, as shown in Table 1.Results of the FEA and the hydrotest for the 24 in. blind areshown in Fig. 5. The results for the 14 in. also closely tracked thedata in a similar manner, as shown in Fig. 6, while the 4 in. resultsfollow the general trend of the experimental data, as shown in Fig.7. Although the 4 in. FEA results consistently underestimate theexperimental results, at these pressures the deflection as deter-mined both experimentally and through FEA is well over an orderof magnitude below the deflection limits for the recommendedpressure listed in Fig. 8.As can be seen from the results shown in Figs. 68, the finiteelement model provides a reasonable approximation of the hy-drotest deflection. Therefore, a high degree of confidence can beplaced in the FE model.Application of Validated FEA ModelHaving established the validity of the FE model using the MTRyield stress values, the yield stress used in the modeling was re-duced for the models that were run in order to establish maximumpressure recommendations. In order to capture a large portion ofthe available carbon steel plate material that might be used for slipblinds, a yield stress of 35 ksi was used for these models. Thisvalue would be appropriate for SA36 ?which has a SMYS of36 ksi? and SA515-65 and SA516-65 ?which have a SMYS of35 ksi?. It would also be applicable for SA515-70 and SA516-70?which have a SMYS of 38 ksi?.Results and RecommendationFigures 810 provide recommended allowable pressures forslip blinds fabricated from SA36, SA515 grades 65 or 70, orSA516 grades 65 or 70 plate installed between raised face flanges.These recommended pressures were derived by reducing the FEAcalculated pressure required to permanently deform a blind to theindicated amount by a factor of two-thirds. A design margin of 1.5?equivalent to reducing the pressure by 2/3? was chosen because itis the same design margin used against yield in the typical refineryASME standards ?B31.3, Section VIII, Division 1 and 2? ?4?.Exposing blinds to the pressures presented in Figs. 810 shouldresult in residual deflections, after depressuring, of less than thedeformation shown. These pressures are compared to allowablepressures as calculated in accordance with Eq. ?15? of paragraph304.5.3 of the B31.3 ?4? piping code. This calculation is based onSA516-70 or SA515-70 material and uses an allowable stress of23.3 ksi and a blind diameter based on the inside diameter of thegasket. The allowable stress is determined as the lesser of 1/3specified minimum ultimate strength ?SMUS? or 2/3 SMYS. Inthis case, the values are 23.3 ksi and 25.3 ksi and the allowablestress is governed by the SMUS.The use of different design criteria leads to the significant dif-ference in allowable pressures between this paper and B31.3 ?4?.Although the B31.3 criteria ?4? is to avoid yielding of the blind,the criteria used in this paper accepts yielding but limits perma-nent deformation. Accepting different design criteria would beexpected to yield different results. It should be noted that theB31.3 formula ?3? compares well to the experimental results whenusing the yield stress of the material as the allowable stress andfirst yield as a design criteria.Burst TestingAfter sufficient pressure and deflection data had been collectedin the initial phase, attempts were made to determine the burstpressure of the 0.250 in. thick 14 in. and 24 in. slip blinds. Thesetests were conducted in a remote area with restricted access forsafety. The dial indicator, which has a 1 in. range, was removed.Since the 24 in. rig had a convenient cross bar, which was usedfor both rigging and as a dial indicator mount, a simple guide wasattached to the cross bar. This allowed a ruler to be placed on theblind in order to gather deflection data while the blind was beingpressurized.The first attempt was limited not by the burst pressure of theblinds but by the available compressor that powered the hydrotestFig. 4Typical FEA modelTable 1Actual mill-test-report yield and ultimate stresses for slip-blind material0.250 in., 4 in., some 14 in.,and 24 in. NPS0.250 in., 8 in., some 14 in.,and 20 in. NPS0.375 in.0.500 in.Yield stress ?ksi?61625455Ultimate stress ?ksi?76827475250 / Vol. 129, MAY 2007Transactions of the ASMEDownloaded From: / on 01/27/2018 Terms of Use: /about-asme/terms-of-usepump. This testing setup was able to generate between 1000 psiand 1100 psi pressure. Even with a blind thickness of only0.250 in., however, this pressure was insufficient to burst theblinds. Residual deflection was measured at ?2.7 in. for the 24 in.blind. Next, the 14 in. blind was tested to similar pressures withresidual deflection measured at ?0.8 in.A second burst test was then performed using a more powerfulcompressor. In this case, flange leakage limited the testing. The0.250 in. thick 24 in. blind was pressurized to 1200 psi, whichresulted in a residual deflection of over 3 in. The 0.250 in. thick14 in. blind withstood 2500 psi before the test rig experienced aflange leak. This resulted in a residual deflection of roughly 1.7 in.FEA Burst EvaluationTo further ensure the acceptability of the pressure recom-mended in Figs. 810, several additional FEA models were cre-ated. As with the models used to generate Figs. 810, the yieldstress was set at 35 ksi. However, in the burst model FEAs, anonzero tangent modulus was used and was set to 0.100 times theelastic modulus. The von Mises yield criteria and the isotropichardening rule were also used, and geometric nonlinearities werealso considered. Since all of the physical nonlinearities are in-cluded, upper-bound behavior would be expected, and these mod-els would be reasonable models against burst.Using a local strain limit of 5%, the maximum burst pressureswere determined and compared to the values presented to obtain adesign margin against burst. The design margin against burst wasfound to be the lowest in the 4 in.?0.250 in. thick blind at avalue of 5. The margin against burst increased to 8 for the 6 in.?0.250 in. blind and was a maximum at 24 times for the 24 in.?0.250 in. blind. However, as demonstrated by the physical bursttests, flange rotation leading to leakage is the more likely failurescenario and is much more likely to occur before a blind wouldburst.ConclusionAccepting some permanent deformation in slip blinds allowsthem to be used at pressures significantly higher than those deter-mined by traditional linear elastic calculations. Figures 810 pro-vide pressures at which slip blinds of various thicknesses and NPS4 in.NPS 24 in. will suffer permanent deformation less than theindicated amount. The pressures provided were based on a SMYSof 35 ksi. Thus, the recommendations are valid for blinds fabri-cated of commonly available SA36, SA515 grade 65 or 70, andSA516 grade 65 or 70 steel.Depending on the tolerable permanent deformation, these pres-sures are on the order of 3.5 times higher than those that would beallowed using traditional methods using Timoshenkos formula?3? and the outside diameter or reaction diameter of the gasket.The pressures are 2.5 times higher than those allowed by theB31.3 piping code ?4?, which uses the same formula but with theinside diameter of the gasket. Although quite a bit higher than theFig. 5Comparison of hydrotest and nonlinear FEA residualdeflections for 24 in. blindFig. 6Comparison of hydrotest and nonlinear FEA residualdeflections for 14 in. blindFig. 7Comparison of hydrotest and nonlinear FEA residualdeflections for 4 in. blindJournal of Pressure Vessel TechnologyMAY 2007, Vol. 129 / 251Downloaded From: / on 01/27/2018 Terms of Use: /about-asme/terms-of-useallowable pressure determined per B31.3, they still have at least a2.5 factor against burst as determined by the limit load finite ele-ment analysis.The results presented in Figs. 810 are applicable to 4 in. NPS24 in. NPS slip blinds sandwiched in flanges with standard sched-ule bores. Since the allowable pressures will increase with reduc-tions in diameter, these results may be conservatively used forflanges with heavier wall bores.Although the testing and FEA were limited to 4 in. NPS24 in.NPS slip blinds in between raised face flanges, certain extrapola-tions may be made as follows:Flange rating is not expected to impact the behavior of theblind. Flanges with higher ratings may have a gasket with alarger outside diameter, but the raised face inside dimen-sions will remain the same ?9?. Thus, the behavior of the slipblind will remain the same.Since the allowable pressures increase with decreases in di-ameter, the recommended value for a 4 in. or 6 in. NPSblind may also be used for blinds in smaller flange pairs.The following items must be kept in mind when using slip blindsat pressures higher than those typically allowed:Using slip blinds at these pressures should be contemplatedonly if the use of a standard blind would be infeasible.Applicable legislation and regulations should be reviewedprior to using slip blinds at these pressures.Fig. 8Recommended maximum pressure for 0.250 in. thick slip blindsFig. 9Recommended maximum pressure fo