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Bolt preload scatter and relaxation behaviourduring tightening a 4 in-900# flange joint withspiral wound gasketM Abid* and S Hussain

Faculty of Mechanical Engineering, Ghulam Ishaq Khan Institute of Engineering Sciences and Technology,

Topi, Pakistan

The manuscript was received on 16 August 2007 and was accepted after revision for publication on 20 November 2007.

DOI: 10.1243/09544089JPME160

Abstract: Gasketed bolted flange pipe joints are found prone to leakage during operating con-ditions. Therefore, performance of a gasketed flange joint is highly dependent on the properjoint assembly with proper gasket, proper gasket seating stress, and proper preloading in thebolts of a joint. For a gasketed flange joint, the two main concerns are the joint strength andthe sealing capability. To investigate these, a detailed three-dimensional non-linear finite-element analysis (FEA) of a gasketed joint is carried out using a spiral wound gasket. FEA resultsare compared with the experimental results and are found to be in good agreement, hence vali-dating the FE model developed. Bolt scatter, bolt bending, and bolt relaxation are considered tobe the main factors affecting the joint performance. In addition, the importance of proper bolttightening sequence and number of passes and the influence of elastic and elasto-plasticmaterial modelling on joint performance are also presented. Stress variation in the flangedue to flange rotation highlighted joint strength, and gasket contact/seating stress variationhighlighted the sealing performance during bolt up. Modelling the non-linearity of thespiral wound gasket using non-linear elements highlights reduced joint strength and sealingperformance. Summarizing, a dynamic mode in a gasketed joint is concluded, which is themain reason for its failure.

Keywords: bolt, relaxation, gasketed, joint, dynamic, tightening, sequence, spiral wound,gasket

1 INTRODUCTION

Gasketed pipe flange joints are widely used inindustry to connect pipe to pipe or pipe to equip-ment. These are used in a wide variety of differentapplications from water supply to high pressure andhigh temperature processes. In a gasketed pipejoint, problems of bolt bending, bolt preload scatter,and joint relaxation is observed during joint assem-bly, concluding its dynamic mode-of-load [1–3],which results in poor sealing and strength. Some

experimental and numerical investigations [4, 5] lim-ited to the linear elastic material modelling areperformed to estimate bolt preload scatter due tothe elastic materials interaction in the process ofsuccessive bolt tightening. However, these do notconsider bolt bending behaviour, flange rotation,and flange stress variation. Detailed experimentalstudies are performed by Abid et al. [1–3] to highlightthese with special emphasis on joint strengthand sealing capabilities. In this study, a detailedthree-dimensional non-linear finite-element analysis(FEA) of a gasketed joint is carried out using a spiralwound gasket, of the typemostly used in the industry.FEA results compared with the experimental resultsare found to be in good agreement, thereby validatingthe FE model developed. Bolt scatter, bolt bending,and bolt relaxation are concluded to be the main

*Corresponding author: Faculty of Mechanical Engineering,

Ghulam Ishaq Khan Institute of Engineering Sciences and

Technology, Topi, NWFP 23460, Pakistan. email: abid@giki.

edu.pk; [email protected]

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JPME160 # IMechE 2008 Proc. IMechE Vol. 222 Part E: J. Process Mechanical Engineering

factors affecting the joint’s performance. Importanceof proper bolt tightening sequence and number ofpasses is discussed. Stress variation in the flangedue to flange rotation highlighted the joint strength,and gasket contact/seating stress variation high-lighted the sealing performance during bolt up.Modelling the non-linearity of the spiral woundgasket using non-linear elements highlights reducedjoint strength and sealing performance. Summar-izing, a dynamic mode in a gasketed joint is con-cluded, which is the main reason for its failure. Aflange joint of 4 in 900# class is used in the presentstudy.

2 FINITE-ELEMENT ANALYSIS

2.1 Modelling

Abid and Baseer [6] investigated the joint strengthand sealing capability under combined loading foran axisymmetric three-dimensional model wherethe preload of each bolt was the same using a solidplate gasket. An angular portion of flange was mod-elled with a bolt hole at required position. In the pre-sent study, a full 3608 flange joint model is developedto study joint relaxation behaviour during assembly.For a spiral wound gasket, only the seal ring thatremains in contact with the flange was modelled.The gasket is modelled by rotating area about they-axis through 3608 in 16 numbers of volumes. Thehalf thickness of the gasket due to its symmetry ofgeometry and loading conditions is modelled. Thebolt is modelled by rotating area about its axisthrough 3608 in four numbers of volumes and theremaining seven bolts are generated by using sym-metry in z-axis. The half-length of the bolt is mod-elled by its planar symmetry. Only a small portionof pipe is modelled to reduce computational time.The resulting flanged joint model is shown inFig. 1(a). Commercial FEA software ANSYS [7] isused during the analysis.

2.2 Material properties

Allowable stresses and material properties for flange,pipe, and bolt [8] are given in Table 1. An model isconstructed using elastoplastic material and it con-sists of two sections each having a linear gradient.The first section, which models the elastic material,is valid until the yield stress is reached. The gradientof this section is Young’s modulus of elasticity. Thesecond section which functions beyond the yieldstress, and models the behaviour of the plasticmaterial, has a gradient of the plastic tangent mod-ulus, which for this study was 10 per cent of theYoung’s modulus of elasticity [2].

2.2.1 Spiral wound gasket material modelling andsimulation

Spiral wound gaskets are manufactured by winding aV-shaped metal strip of stainless steel and a soft non-metallic filler (asbestos fibre) under pressure. Itsinner and outer centring rings are made of stainlesssteel. Gasket stiffness in the thickness direction isdetermined by conducting a compression test. Gas-kets can be modelled by two different methods: oneis a detailed model that shows non-linearity ofgasket stress–strain relation, and the other is a sim-plified modelling that approximates gasket non-linearity using two different elastic moduli accordingto the condition of loading and unloading. Thedetailed non-linear method is more complex, incor-porating non-linear loading and unloading curveswith no limitation on data points (Fig. 2(a)). Asimple modelling technique is adopted in the presentwork (Fig. 2(b)), which defines unloading behaviourby specifying a straight line as in reference [9].

Fig. 1 (a) Full gasketed flanged joint (3608); volumetric mesh, (b) flange, (c) bolt, and (d) gasket

Table 1 Material properties

Parts As per code y Yield stress (MPa)

Flange/pipe ASTM A350 LF2 0.3 248.2Bolt ASTM SA193 B7 0.3 723.9

124 M Abid and S Hussain

Proc. IMechE Vol. 222 Part E: J. Process Mechanical Engineering JPME160 # IMechE 2008

Stress–strain relationships of a spiral wound gasket,given in reference [9], for the calculation of elasticunloading and non-linear loading are used. Table 2illustrates the elastic moduli in loading and unload-ing during each pass. For example, for pass 1, boltpreload against 210 Nm torque is calculated as37 kN, and the total bolt preload for eight bolts is296 kN. Nominal average gasket stress defined bythe total bolt preload divided by the gasket bearingsurface area calculated is 30 MPa. Elastic modulusin loading and unloading is calculated as 319 and3186 MPa, respectively. Gasket material loading andunloading curves using simplified method are plottedin Fig. 2(c), where ‘L’ represent loading/compressionand 1, 2, 3, and 4 represents linear unloading in pass1–4, respectively. Since the torque applied at pass 5is the same as that for pass 4, the gasket undergoescompression and unloading during pass 5, which isthe same compression/unloading line defined forpass 4.

2.3 Element selection

An eight-node structural SOLID45 lower order isoperi-metric element is used for themodelling of flange, bolt,and pipe. Three-dimensional ‘surface-to-surface’CONTA174 contact elements, in combination withTARGE170 target elements are used between flangeface and gasket, bolt shank and flange hole, and thetop of the flange and the bottom of the bolt to simulatecontact distribution. No friction was employed

between any of the surfaces, since it can reasonablybe assumed that the forces normal to the contactsurfaces would be far greater than the shear forces.Due to the non-linearity of the gasket, INTER195which is a three-dimensional, eight-node linear inter-face element, is used in conjunction with the three-dimensional linear structural element SOLID45 tosimulate the gasket using commercial softwareANSYS [7].

2.4 Meshing

Before volume mesh generation, an area mesh is cre-ated on one side of the flange, bolt, and spiral woundgasket by a specified number of divisions and spaceratio for each line. The hub-flange fillet and raisedface areas of the flange are fine meshed due tostress concentration. The areas of bolt head, that isin contact with the flange top is meshed with smallsize elements, giving a fine mesh. The unmeshedvolume of flange is then filled with elements bysweeping the mesh from the adjacent area throughthe volume. A complete 3608 flange mesh is gener-ated from the angular portion of the flange by sym-metrical reflection, as shown in Fig. 1(b). For thebolt, volumetric mesh is generated by sweeping themesh from an adjacent area through the volume(Fig. 1(c)). In order to simulate non-linear gasket inANSYS, INTER195 interface elements are definedand generated by meshing gasket volume ensuringone element through thickness with correct node

Fig. 2 (a) Non-linear modelling technique, (b) simplified modelling technique, (c) gasket

material loading and unloading curves (stress versus strain) using simplified technique

Table 2 Elastic modulus, gasket stress, bolt preload, and bolt target stress against the torque applied in

each pass

Pass Torque (Nm) Bolt preload (kN) Target bolt stress (MPa) Average gasket stress (MPa)

Elastic modulus (MPa)

Loading Unloading

1 210 37 57 30 319 31862 310 55 86 45 390 44073 400 70 112 60 450 55374 505 89 145 75 500 6598

Bolt preload scatter and relaxation behaviour 125


numbering. The stress–strain curve is input tocharacterize the through thickness response forgasket material parameters. A three-dimensionalmesh of a spiral wound gasket is shown in Fig. 1(d).

2.5 Contact generation

To define the contact pair between flange and bolthead, flange face areas are taken as the target surface,and the bottom areas of the bolt head are taken ascontact surface. The same real constant numbersare assigned to both the target and contact elements.For contact pair generation between flange bottomsurface and gasket, flange bottom surface areas aretaken as target surface while gasket top surfaceareas are taken as contact surface. The contact con-dition is applied and friction is taken into consider-ation at the interfaces between the flange and thegasket. Friction coefficient is varied from 0.1 to 0.2,however, its effects on interface stress distributionsis observed to be very small. To prevent rigidmotion of the flange during bolt up, contact isdefined between bolt shanks and the bolt holes inthe flange. As there is a gap between the two surfaces,a contact surface offset of 1.61 mm is specified tooffset the entire contact surface (bolt hole) towardsthe target surface (bolt shank).

2.6 Boundary conditions

The flange and the gasket are free to move in both theaxial and radial directions, providing flange rotationand stress variation in flange, bolt, and gasket. Sym-metry conditions are applied to the lower portion ofthe gasket. Bolts are constrained in both the radialand tangential directions along the neutral axis ofthe bolt. An axial displacement is applied to thebottom area of the bolt shank to obtain the requiredprestress (Fig. 3(a)).

2.6.1 Bolt preloading

To ensure a proper preload in the joint during jointassembly, the sequence in which bolts are tightened

during a pass has a considerable importance, asjoint relaxation mostly depends on this factor. Inthe present work, following two sequences are used:

(a) sequence 1: 1,5,3,7,2,6,4,8 [2] (Fig. 3(b));(b) sequence 2: 1,2,3,4,5,6,7,8 [2] (Fig. 3(c)).

Bolts are tightened as per sequence 1 during the firstfour passes and as per sequence 2 during the lastpass. Bolts are tightened one by one with the torquecontrol method [5], i.e. each bolt is tightened to atarget stress for a given pass. In the experimentalwork [2], the author tightened the joint in incrementsof torque 210, 310, 400, and 505 Nm as persequence 1. Finally, all the bolts were tightenedagain to 505 Nm in one pass as per sequence 2 toachieve uniform preload values. Target torques areconverted to the bolt preloads for each pass. In a sim-plified form, for lubricated fasteners, the relationshipof bolt preload achieved against a given torque with0.2 as factor of load loss due to friction is calculatedas per reference [10], i.e. T ¼ 0.2 � F � D. Averagebolt stress is then calculated by dividing the bolt pre-load by the nominal area of the bolt shank. The jointis tightened to the target stress for each pass calcu-lated as above. For this purpose, an optimization rou-tine is developed and used in amanner that each timeaxial displacement (UY) is applied to the bolt, and theresulting stress on the midnode of the bolt shank(close to the strain gauge location) [2] is comparedwith the target stress. In the case of difference, UYis incremented and comparison is made again. Simi-larly, UY is incremented till it reaches an optimumvalue for which the target stress in the bolt isachieved. Table 2 shows the bolt preloads andtarget stress calculated against the applied torques.The magnitude of the axial displacement UY appliedto the bottom area of the bolt shank to prestress eachbolt to the target stress in each pass is given inTable 3. Maximumdisplacement is applied to achieve30 per cent of the yield of the bolt. Although this isconsidered very low, it avoids gasket crushing, andhence, maximum recommended torque applied bythe gasket suppliers is 505 Nm [2].

Fig. 3 (a) Boundary conditions; bolt tightening (b) sequence 1 (c) sequence 2



3 RESULTS AND DISCUSSIONS

3.1 Bolt stress variation during bolt up

To determine bolt relaxation and bolt bending beha-viour during tightening the bolts as per sequence 1and 2, four nodes are selected at an angle of 908 onshank of each bolt. B1/1 and B1/2 represent theinner and outer nodes respectively, B1/3 and B1/4represent the side nodes, and B1/M represents themidnode on the bolt shank. Similar nomenclatureis used for all other bolts. For average bolt stress,midnode on the shank of the bolt is selected(Figs 4(a) and (b)). Figure 5(a) shows relaxation ofbolt 2 and bolt 4, when bolt 1, 3, and 5are tightened. Flange areas beneath bolt 2 andbolt 4 are compressed, resulting in their relaxation.Tightening all other bolts, bolt preload of bolt 1increases and is concluded as a result of the flangejoint opening/rotation, hence setting the bolt in ten-sion. Figure 5(b) shows how bolt 5 comes in tensionwhen bolt 1 is tightened during pass 1. Similarly, aslight increase in the stress is observed in the caseof tightening all the other bolts, except neighbouringbolts. It is also noted that bolt 1, which is initially

tightened to 57 MPa, gets almost relaxed at the endof pass 1, with a bolt stress of only 5 MPa, resultingin 90 per cent of the preload relaxation. Figure 6(a)shows the preload variation of bolt 1, which is tigh-tened first, during the tightening of other bolts inpass 1. Bolt stress reduces when neighbouring bolts,bolt 2 and bolt 8, are tightened. This is due to theelastic interaction [3, 4] of the flange, as it deformsin the axial direction during bolt preloading, resultingin the relaxation of the bolt closest to the bolt beingtightened. Figure 6(b) shows the effect of tighteningof one bolt, i.e. bolt 1 on all the other the bolts forfirst pass. It is clear that tightening any bolt relaxesits neighbouring bolts, although a slight rise in thestress is observed in the remaining bolts. Similarly,an increase in the preload of the bolts on the oppositeside is observed. At the end of the first pass, i.e. whenbolt 8 is being tightened, 27–38 per cent of preloadrelaxation is observed in bolt 3 and bolt 7, respect-ively. Bolts 2, 4, 6, and 8 remain completely relaxedduring tightening of bolts 3, 5, and 7.Figure 7 shows the detailed bolt preload variation

when bolts are tightened as per sequence 1 andsequence 2. For all the bolts, almost same stress vari-ation pattern is observed for the first four passes asper tightening sequence 1. A uniform bolt preloadvariation is observed during the last pass as persequence 2. Maximum preload variations areobserved for bolt 5, which is observed completelyrelaxed for pass 1 to pass 3. The preload variationsfor bolt 5 during each bolt 5 tightening are 56 MPafor the first pass, 70 MPa for the second, 83 MPa forthe third, 110 MPa for the fourth, and 35 MPa forthe last pass. Similarly, for all the bolts, an increasein the stress is observed when tightening three boltslocated on the other side. For example, preload ofbolt 3 increased when bolts 6, 7, and 8 (located at

Fig. 4 Nomenclature (a) side nodes (b) midnodes

Table 3 Axial displacement UY applied in each bolt for

different passes

Bolt Pass 1 Pass 2 Pass 3 Pass 4 Pass 5

B1 0.176 0.270 0.367 0.445 0.540B5 0.083 0.170 0.247 0.315 0.443B3 0.224 0.324 0.440 0.560 0.610B7 0.070 0.158 0.230 0.283 0.450B2 0.265 0.366 0.475 0.582 0.605B6 0.080 0.157 0.221 0.290 0.410B4 0.222 0.333 0.455 0.540 0.500B8 0.170 0.278 0.275 0.450 0.540



the opposite side of bolt 3) are tightened. Tighteningneighbouring bolts relaxes the target bolt.

3.2 Scatter in bolt stress

Figure 8(a) shows a visible bolt scatter at the end ofeach pass after tightening each bolt to a targetstress using the torque control method by Fukuokaand Takaki [4] using FEA. This is due to the highlynon-linear behaviour of spiral wound gaskets duringloading and unloading. Bolt scatter variation at 69,88, 121, 131, and 72 MPa is observed duringpasses 1 to 5. The scatter in bolt stress is observedto increase remarkably with each pass and the pre-load variations are obvious at pass 4. Relativelyhigher and uniform bolt stresses are achievedduring pass 5 (almost half of the pass 4) whenbolts are tightened as per sequence 2. This high-lights the importance of the last pass as persequence 2. Bolts 1, 3, 5, and 7 are observedalmost completely relaxed for the first threepasses. This is due to the reason that these boltsare tightened in advance of the neighbouring twobolts. Relaxation of bolts 1, 3, 5, and 7 at the

completion of first pass is observed, whereas pre-load of bolts 2, 4, and 8 is greater than the bolttarget stress. The reason is obvious, these bolts aretightened as last four bolts in a pass and theirneighbouring bolts are tightened already, so thesehave greater stresses than the target stresses. Themaximum difference in the bolt stress is observedbetween bolt 4 (maximum) and bolt 5 (minimum)for the first four passes and between bolt 3 (maxi-mum) and bolt 6 (minimum) for the last pass.The scatter in bolt stresses is substantial andincreases with the pass operation. Maximum scatteramong bolt preloads is observed at the fourth passwith 133 MPa preload difference between bolts 4and 5.FEA results are compared with the experimental

results by Abid [2], for the same size of the pipeflange joint. Figure 8(b) shows the bolt scatterobtained experimentally. The same trend is observedfor all the passes with a slight difference in the mag-nitude of the bolt preloads for average bolt stress.Some of the factors that might cause the differencebetween the FEA and experimental results are asfollows.

Fig. 5 Exaggerated deformation plots (a) bolt relaxation phenomena (b) flange opening

phenomena

Fig. 6 (a) Bolt 1 stress variation during pass 1, (b) stress variation effect on other bolts during

tightening of bolt 1 in pass 1



1. By converting the target torques to the boltpreloads for each pass, a load loss factor of 0.2was incorporated in the expressions in reference[9], indicating that 20 per cent of the load is con-verted into useful axial bolt load due to thethread friction. However, in reality only a part ofthe load (10–15 per cent) is converted into theaxial bolt load [1, 2].

2. In FEA, gasket’s non-linearity is calculated usingthe analytical equations. However, the actual gas-ket’s stress–strain curve might be differentbecause of the complex behaviour during loadingand unloading and no experimental data wereavailable.

3. During experiments, flange was partially con-strained in the axial direction, whereas in FEA,no axial constraint was applied.

4. Manufacturing error of the flange and gasket, sur-face roughness, misalignment of the gasket withthe flange, bolt, lubricant, and gasket quality[1, 2] might lead to the difference between exper-imental results and the FEA.

However, overall, FEA results are in good agreementwith the experimental results, which prove thevalidity of the FE model and analysis results.

3.3 Bolt bending behaviour

Bolt bending behaviour along the four locations, i.e.inner, outer, and side on each bolt shank at the endof each pass is shown in Fig. 9. Bending behaviourof each bolt is observed to be different from theothers, even for the same joint. Bolt 1, which is tigh-tened first, B-1/1 (node on inner side) is in tensionand B-1/2 (node on outer side) is in compression,but as the torque increases, difference in axial stressbetween these two locations is increased and B-1/2still show compressive stress, identifying increase inbolt bending. Difference in axial stresses betweenbolt nodes at side locations is also noticed, whichmeans that bolt 1 not only bends outward but alsosidewise. For bolt 5, the difference among inner andouter nodes is little, however, B-5/4 is found in com-pression and B-5/3 in tension, indicating that bolt 5bending is mainly sidewise. With bolt up, the bolt 3inner node goes to tension and the outer node intocompression; the difference increases rapidly withincreasing torque, and a smaller difference amongside nodes is noted. For bolt 7, sidewise bending isnoted, but as B-7/3 is in tension and B-7/4 in com-pression, the difference between the inner andouter nodes is negligible. The difference betweeninner and outer nodes for bolt 2 is remarkable andincreases rapidly with each pass. The differencebetween side nodes is smaller as compared to thedifference between the inner and outer nodes.When B-6/4 is in tension and B-6/3 in compression,the difference between side nodes is observed to begreater than that between the inner and outernodes, and bending is maximum at pass 4 and isminimized at last pass. Bolt 4 shows tensile stressesat all bolt locations for all passes, and there is anoticeable difference between the inner and outeras well as side nodes with rapid increase due toincreasing torque, indicating bending in both thedirections, bolt 8 also shows with tensile stresses atall locations, with an almost equal difference between

Fig. 8 Scatter in bolt stress at the completion of each pass: (a) FEA (b) experimental

Fig. 7 Bolt stress variation in all the bolts during

tightening for all passes



inner and outer and side nodes with maximum bend-ing at pass 5. Bending of all the bolts is increased withincreasing torque and is observed maximum duringthe last two passes.

3.4 Gasket behaviour during joint assembly

Nomenclature of selected nodes on gasket close toeach corresponding bolt is shown in Fig. 10.

3.4.1 Gasket deformation during bolt up

In order to understand the bolt relaxation and boltscatter behaviour during each pass, gasket conditionismonitored at the end of each pass. Plots of displace-ments UX, UY, and UZ at the last pass are shown inFig. 11(a). From axial displacement (UY) plot it isobserved that the gasket shape is changed and thegasket no longer retained its original shape. Gasketdeformation is remarkable after the last pass as itssurface being in contact with the flange is observedsheared resulting in its permanent deformation. Vari-ation of axial displacement along gasket circumfer-ence with maximum at locations closer to bolt 2 and3 and minimum at 1808 location is observed. For thelast pass, maximum axial displacement observed is0.236 mm (near bolt 3) and minimum 0.023 mm(near bolt 6). This concludes that the gasket under-goes uneven compression circumferentially. It is

also observed that due to the elongation, outersealing ring has changed more or less to octagonalshape.Figure 11(b) shows the radial displacement (UX) of

the gasket with respect to flange at the end of lastpass. The maximum radial displacement noted isalmost 3 mm (at locations closer to bolt 1 and 5).UX close to bolt 1 is 3 mm, while on the oppositeside (at 1808), it is 23 mm, which shows that duringbolt up, gasket stretches and shears in the radialdirection. Figure 11(c) represents the circumferential(hoop) displacement (UZ) of the gasket at the end oflast pass. Maximum UZ of almost 3.5 mm (close tobolt 3), shows that gasket during bolt up undergoesrotation with respect to the flange, thus disturbingthe initial alignment with the flange and might poss-ibly lead to leakage. Figure 12(a) shows the maximumvalues of UX, UY, and UZ for all the five passes.FEA results are compared with the detailed exper-

imental results by Abid [2], who has used four differ-ent gaskets of the samematerial and dimensions usedin FEA. He has reported the bending of the centringring during joint assembly of one of the gaskets.The spiral wound portion was reasonably com-pressed, and along one location it was found morecompressed. The same gasket was used for thesecond time to make the other joint. After makingthe joint, it was disassembled and the gasket wastaken out to see its condition this time. This time itwas found to be more compressed less than the cen-tring ring thickness on one side, i.e. some dents werefound along about 908 on one side and centring ringwas found more bent. A new gasket was used forthe third time to make the joint. After bolting up itscentring ring was not found bent. The gasket wastaken out after dismantling the joint to note its phys-ical condition. Its spirals were found damaged fromone side along with a dent at about 1808. Thenanother gasket was used for other series of tests.After performing the experiments for different load-ing, gasket was found reasonably compressed withno damage to the spirals, however, seepage wasFig. 10 Nomenclature of selected nodes on gasket

Fig. 9 Bolt bending behaviour of all eight bolts in the joint



observed from the joint during internal pressureloading. Fourth gasket was used in the joint andafter dismantling the joint, the gasket was found ina good condition.

3.4.2 Gasket stress variation during bolt up

Contact stress variations in the circumferential direc-tion during bolt up at a node closest to bolt 1, markedas G1 is shown in Fig. 12(b). It is observed that duringthe tightening of the neighbouring bolts of bolt 1, i.e.bolt 2 and bolt 8 and bolts at 908 to bolt 1 (bolt 3and 7), there is an increase in stress at G1. While tigh-tening bolt 4, 5, and 6, reduction in contact stress isobserved due to spiral wound gasket’s flexibility.Figure 13(a) shows contact stress variation along theouter diameter of the sealing ring during bolt up forpass 1. Contact stress distributions pattern does notshow conspicuous changes after tightening fourthbolt, i.e. bolt 7. This is because gasket is seated toits minimum thickness during the tightening of firstfour bolts. It is observed that at the end of the firstpass, the magnitude of contact stress significantlyvary in the circumferential direction in a sinusoidalmanner. Figure 13(b) shows contact stress distri-bution along the outer diameter of the sealing ringat the end of each pass. Contact stress variations areobserved to increase with each pass and are the

maximum at pass 4. These variations are substantialeven at the last pass when all the bolts are tightenedin clockwise sequence. Thus it is concluded that thescatter in axial bolt stresses deeply affects the sealingperformance of a joint. Contact stresses significantlyvary in the circumferential direction and the vari-ations are increased remarkably with each pass.During all the passes, the maximum contact stressdifference is between bolt location G2 and G6.Gasket areas along G2 and G3 are at higher stresslevels as compared to location G6 and G7. This indi-cates the possible location of leakage from theflange joint.

3.5 Flange stress variation during bolt up

FEA stress variation results are compared with theexperimental results by Abid [2] for the same size ofpipe flange joint. Nodes close to the strain gaugelocation at 908 locations (top, bottom, right, and leftside) are selected and behaviour of the flange at thetop, bottom and side locations during the bolt up isinvestigated. Principal stress variation in the axialdirection during bolt up at hub flange fillet and hubcentre are evaluated. Stress variation at hub pipefillet is not considered due to negligible small vari-ation and is concluded due to the free flange

Fig. 11 Gasket displacement at completion of last pass: (a) UY, (b) UX, and (c) UZ

Fig. 12 (a) Maximum displacements (UX, UY, UZ) of gasket at the completion each pass,

(b) gasket contact stress variation along G1 during bolt up at pass 1



movement in the axial direction, whereas in theexperimental results by Abid [2] axial movement isconstrained.

3.5.1 Stress variation at hub flange fillet

Figure 14(a) shows the principal stress variation inthe axial direction at the hub flange fillet using FEA.During bolt tightening closer to the bottom and toplocations, stresses at side locations are observed

reduced and vice versa. Stress is maximum at alocation close to the bolt being tightened and mini-mum along the location at an angle of 908. Tighteningbolts in star pattern (sequence 2), maximum stressfluctuation is observed during tightening of firstfour bolts (1, 5, 3, 7) in the respective pass. Duringtightening the remaining four bolts (2, 6, 4, 8), stressvariation is reduced. Maximum scatter is observedduring bolt 5 tightening for the first four passes andduring bolt 6 tightening for the last pass. For

Fig. 14 Principal axial stress variation at hub flange fillet during Bolt up: (a) FEA (b) experimental

Fig. 13 Contact stress variation during bolt up (a) in pass 1, (b) at the end of each pass



sequence 1, almost same stress variation pattern butwith higher magnitude during each pass is observed.Although higher stresses are achieved at the last passbut still a substantial stress variation is concluded.Maximum axial stress of 245–255 MPa at flange sideand top locations during last pass shows yielding.FE results are found in good agreement with theexperimental results in Fig. 14(b), however, maxi-mum experimental stress is observed less than theyield strength of the flange material. It might be dueto the greater clamp load loss, i.e. 85–90 per cent[1, 2] due to the thread friction in the experiments,

whereas in FEA 80 per cent load loss due to the fric-tion is assumed.

3.5.2 Stress variation at the hub centre

Figure 15(a), shows almost the same axial stress vari-ation pattern as for hub-flange fillet but with com-paratively less values during bolt up. Relaxation isobserved during bolt 4 tightening. Each time, forthe next first higher torque values, the stress is maxi-mum close to the bolt and minimum at an angle of908. Tightening the first two bolts in all the passes

Fig. 15 Principal axial stress variation at hub centre during bolt up: (a) FEA (b) experimental

Fig. 16 After last pass: (a) principal stress in flange, (b) axial stress in bolt, (c) gasket stress



relaxed stresses at the sides. Maximum scatter isobserved during bolt 5 tightening. Comparing FEAand experimental results in (Fig. 15(b)) almost similartrend is observed, with more abrupt changes in theexperimental result along sides locations indicatingcompressive stresses during tightening bolt 5 inpass 3 and pass 4. It may be due to the reason of par-tial constraining the flange in the axial direction,which is neglected in FEA.Axial principal stress variation in the flange, axial

stress in the bolt, and gasket contact stress at the com-pletion of all the passes are shown in Figs 16(a) to (c).

4 CONCLUSIONS

From detailed comparative FEA and experimentalstudy following results are concluded.

1. FE model is verified with the experimental resultsand is found in good agreement.

2. The joint integrity and sealing performance isvery much dependent on the material propertiesof the gasket used in the joint. Using spiral woundgasket, which is a non-linear gasket, relativelylarge amount of flange rotation is observed dueto the gasket’s flexibility.

3. Bolt relaxation is obvious due to higher elasticinteractions in the non-linear gasketed joint andflange rotation due to the gap between theflanges.

4. Bolt scatter is prominent in all the bolts during allthe passes. The difference between the maximumand minimum bolt stress is substantial and pre-load variation and scatter are observed toincrease remarkably with increasing torqueduring each pass.

5. Bending behaviour of each bolt is different andunique from the others and bolt bendingincreases with increasing torque and directionsalong which bolt bends are different for differentbolts.

6. A substantial stress variations along hub flangefillet and hub centre of joints with non-linear gas-kets is observed, which increased with each pass.

7. Yielding is observed to initiate at hub flange filletduring the last pass even at a bolt preload of only25–30 per cent of the bolt yield.

8. Gasket undergoes uneven compression circum-ferentially. A substantial gasket stress variationsis concluded during bolt up, which under operat-ing conditions can be worst resulting in substan-tial leakages.

9. Owing to the gasket flexibility, rotation, non-uniform compression, and physical damage (per-manent deformation), a spiral wound gasketedjoint experiences large bolt scatter, bolt bending

and relaxation and much greater individual boltpreload variation with bolt up.

10. Bolt scatter, bolt bending, and bolt relaxation areconcluded the main factors affecting the joint’sperformance.

11. To control these the use of proper bolt tighteningsequence, number of passes is concludedimportant.

12. Summarizing, a dynamic mode in a gasketedjoint is concluded, which is the main reason forits failure.

REFERENCES

1 Abid, M. and Nash, D. H. Joint relaxation behavior ofgasketed bolted flanged pipe joint during assembly.In 2nd WSEAS International Conference on AppliedAnd Theoretical Mechanics (MECHANICS’06), 2006,pp. 319–325.

2 Abid, M. Experimental and Analytical studies of conven-tional (gasketed) and unconventional (non gasketed)flanged pipe joints (with special emphasis on the engin-eering of ‘joint strength’ and ‘sealing’). PhD Thesis, 2000.

3 Abid, M. and Nash, D. H. Bolt bending behavior in abolted flanged pipe joint: a comparative study. InASME International PVP Conference, 2006, pp. 1–9.

4 Fukuoka, T. and Takaki, T. Finite element simulation ofbolt-up process of pipe flange connections. J. Press.Vessel Technol., 2001, 123, 282–287.

5 Fukuoka, T. and Takaki, T. Finite element simulation ofbolt-up process of pipe flange connections with spiralwound gasket. J. Press. Vessel Technol., 2003, 125,371–377.

6 Abid, M. and Baseer, U. 3-D nonlinear finite elementanalysis of gasketed flanged joint under combinedinternal pressure and different temperatures. J. Eng.Mech. ASCE, 2006, 133(2), 1–8.

7 ANSYS Inc., ANSYS elements manual, 7th edition, 2006.8 ASME Boiler and Pressure Vessel Code, Section VIII,

Part D, American Society of Mech. Eng., 1998,New York, USA.

9 Nagata, S., Shoji, Y., and Sawa, T. A simplifiedmodelingof gasket stress–strain curve for fem analysis in boltedflange joint design. In ASME International PVP Confer-ence 2002, 2002, vol. 433, pp. 53–58.

10 European Sealing Association. Guidelines for safe sealusage – flanges and gaskets. Report no. ESA/FSA 009/98, 1998, pp. 1–40.

BIBLIOGRAPHY

Takaki, T. and Fukuoka, T. Methodical guideline forbolt-up operation of pipe flange connections (a caseusing sheet gasket and spiral wound gasket). In ASMEInternational PVP Conference 2003, 2003, vol. 457,pp. 23–30.



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