Gasketed Joint's Relaxation Behaviour During ... - Springer Link

Gasketed Joint’s Relaxation Behaviour During Assembly Using Different Gaskets: A Comparative Study Muhammad Abid and Saad Hussain GIK Institute of Engineering Sciences and Technology, Topi, Pakistan Abstract Gasketed bolted flange pipe joints are always found prone to leakage during operating conditions. Therefore performance of a gasketed flange joint is very much dependent on the proper joint assembly with proper gasket, proper gasket seating stress and proper preloading in the bolts of a joint. For a gasketed flange joint, the two main concerns are the joint strength and the sealing capability. To investigate these, a detailed comparative three dimensional nonlinear finite element analysis of a gasketed joint is carried out using gasket as a solid plate and as spiral wound. Bolt scatter, bolt bending and bolt relaxation are concluded the main factors affecting the joint’s performance. In addition, the importance of proper bolt tightening sequence, number of passes on joint performance are also presented. Summarizing, a dynamic mode in a gasketed joint is concluded, which is the main reason for its failure. Keywords: Bolt, relaxation, gasketed, joint, dynamic, tightening, sequence, solid, gasket

1.

Introduction

Gasketed pipe flange joints are widely used in industry to connect pipe to pipe or pipe to equipment. These are used in a wide variety of different applications from water supply to high pressure and high temperature applications. In gasketed pipe joints, problem of bolt scatter, sealing and joint relaxation is observed and it is difficult to achieve uniform bolt stress during joint assembly as dynamic mode-ofload governs in the gasketed joint [1,2]; hence resulting in poor sealing and joint strength. Some experimental and numerical investigations [3-4] are performed to estimate bolt preload scatter due to the elastic interaction in the process of successive bolt tightening. These investigations are limited to the linear elastic material modeling. In addition, these do not consider bolt bending behaviour, flange rotation and flange stress variation. A detailed experimental studies are performed by Abid [1,2] to highlight bolt-bending behaviour, flange stress variation and flange rotation with special emphasis on joint strength and sealing capabilities. In the present study; a detailed three dimensional nonlinear finite element analysis of a gasketed joint is carried out using gasket configurations as a

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M. Abid and S. Hussain

solid plate. This is considered as gasket sealing portion is considered to be completely compressed at the seating stress load applied during joint tightening. Bolt scatter, bolt bending and bolt relaxation are concluded the main factors affecting the joint’s performance. In addition, importance of proper bolt tightening sequence, number of passes influence of elastic and elasto-plastic material modeling on joint performance are also presented. Summarizing, a dynamic mode in a gasketed joint is concluded, which is the main reason for its failure. A Flange joint of four-inch 900# class is used in the present study.

2. 2.1

Finite Element Analysis Modeling

Abid et al [5-6] investigated joint strength and sealing capability under combined loading for an axi-symmetric 3-D model where the preload of each bolt was thesame using a solid plate gasket. An angular portion (22.5q rotation of main profile or 1/16th part) of flange was modeled with a bolt hole at required position and then reflected symmetrically to complete 360 degree model. Gasket is modeled by rotating an area pattern about y-axis through 360 degrees in 16 numbers of volumes; it is possible to model half gasket with respect to thickness due to symmetry of geometry and loading conditions. Bolt is modeled by rotating an area pattern about axis defined by key points through 360 degrees in 4 numbers of volumes and then remaining 7 bolts are generated by virtue of symmetry in z-axis; the objective pipe flange connection is tightened by eight bolts. Half portion of bolt was modeled due to plane symmetry of bolt. Only a small portion of pipe is modeled to reduce computational time. The resulted flanged joint model is shown in Figure 1.1a. Commercial FEA software ANSYS [7] is used during the analysis. A four inch 900# class, ANSI flange joint is selected for this study. 2.2

Element Selection

Eight-nodded structural SOLID45 lower order isoperimetric element is used for modeling of flange, bolt, solid gasket and pipe. Three-dimensional ‘surface-tosurface’ CONTA174 contact elements, in combination with TARGE170 target elements are used between the flange face and gasket, bolt shank and flange hole, the top of the flange and the bottom of the bolt, to simulate contact distribution. No friction was employed between any of the surfaces, since the forces normal to the contact surfaces would be far greater than the shear forces, therefore, this is a reasonable assumption. 2.3

Meshing

Before volume mesh generation area mesh is created on one side of the flange, bolt and solid plate gasket by specified number of divisions and space ratio for each line. Hub-flange fillet and raised face areas of flange are fine meshed due to high

Gasketed Joint’s Relaxation Behavior During Assembly Using Different Gaskets

93

stress concentration. The areas of bolt head which makes contact with flange top is meshed with small size elements for fine mesh. Unmeshed volume of flange is then filled with elements by sweeping the mesh from adjacent area through the volume. Complete 360-degree flange model mesh is then generated from the angular portion of flange by symmetry reflection for 3-D finite elements as shown in Figure 1b. For bolt and solid plate gasket volumetric mesh is generated by sweeping the mesh from an adjacent area through the volume [Figure 1c, 1d]. In order to simulate non-linear gasket in ANSYS, INTER195 interface element are defined and are generated by meshing gasket volume ensuring one element through thickness with correct node numbering., Stress-strain curve is input to characterize the through thickness response for gasket material parameters. 3-D mesh of spiral wound gasket is shown in Figure 1f.

(a)

(b)

(c)

(d) Figure 1. (a) Full Gasketed Flanged Joint (360 Degree); Volumetric Mesh (b) Flange (c) Bolt (d) Solid plate gasket, (e) Sprial wound gasket

2.3.1 Material Properties Allowable stresses and material properties for flange, pipe, and bolt and symmetry plate [8] are given in Table 1. An elastoplastic material model is used consists 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 gradient of this section is the Young’s Modulus of Elasticity. The second section which functions beyond the yield stress, and models the behavior of the plastic material, has a gradient of the plastic tangent modulus, which for this study was 10% of the Young’s Modulus of Elasticity previously [2]. Both the elastic and aelasto-plastic material models are used for comparative joint’s relxataion behaviour study. Contact pairs are generated between flange, bolt head, gasket, nolt shank and bolt hole. Table 1. Material properties Parts

As per code

E (MPa)

υ

Flange/ Pipe

ASTM A350 LF2

173058

0.3

Allowable Stress (MPa) 248.2 (2/3rd σy)

Bolt Gasket (SPG)

ASTM SA193 B7 ASTM A182

168922 164095

0.3 0.3

206.8 (2/3rd σy)

723.9

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2.4

Boundary Conditions

The flange and the gasket are free to move in the axial and the radial direction, providing flange rotation and the exact behaviour of stress in flange, bolt, and gasket. Symmetry conditions are applied to gasket lower portion. Bolts are constrained in radial and tangential direction by taking UX, UZ equal to zero on the neutral axis line of bolt. An axial displacement is applied on the bottom area of the bolt shank to get required pre-stress [Figure 2a]. 2.4.1 Bolt Preloading To ensure a proper pre-load on the joint, the sequence in which bolts are tightened during a pass has a considerable importance in flange joint tightening as the joint relaxation mostly depends upon this factor. Bolts are tightened as per sequence-1 during the first four passes and as per sequence-2 during the last pass. In the present work, following two sequences are used x x

Sequence-1: 1,5,3,7,2,6,4 and 8 [2] [Figure 2b] Sequence-2: 1,2,3,4,5,6,7 and 8 [2] [Figure 2c]

Bolts are tightened one by one with the torque control method [4] i.e. each bolt is tightened to a target stress for a given pass. In the experimental work [2], author tightened the joint in increments of torque 210, 310, 400 and 505 Nm as per sequence-1. Finally, all the bolts were tightened again to 505 Nm in one pass round as per sequence-2 to achieve uniform preload values. Target torques is converted to the bolt preloads for each pass. In simplified form, for lubricated fasteners the relationship of bolt preload achieved against a given torque with 0.2 as Factor of load loss due to friction is calculated as per [9]. Average bolt stress is then calculated by dividing the bolt preload by the nominal area of bolt shank, the joint is tightened to the target stress for each pass calculated as above. For this purpose an optimization routine is developed and used in manner that each time UY is applied on the bolt, the resulting stress on mid node of bolt shank (close to the strain gauge location) is compared with the target stress and in case of difference the UY is incremented and comparison is done again. Similarly the UY is incremented till it reaches an optimum value for which the target stress in bolt is achieved. Table 2 shows the bolt preloads and target stress calculated above against the applied torques.

(a)

(b)

(c)

Figure 2. (a) Boundary Conditions; Bolt tightening (b) Sequence-1 (c) Sequence-2


95

The magnitude of the axial displacement, UY applied to the bottom area of the bolt shank to pre stress each bolt to the target stress, is given in Table 3. Maximum displacement applied is to achieve 30% of the yield of the bolt, although this is considered very low but it avoids gasket crushing, based on this, maximum recommended applied torque by the gasket suppliers is 505 Nm [2]. Table 2. Target Stress calculated for each pass Bolt B1 B5 B3 B7 B2 B6 B4 B8

P1 SPG 0.105 0.095 0.079 0.069 0.103 0.094 0.090 0.088

P1 SWG 0.176 0.083 0.224 0.070 0.265 0.080 0.222 0.170

P2 SPG 0.138 0.138 0.128 0.123 0.146 0.142 0.138 0.136

P2 SWG 0.270 0.170 0.324 0.158 0.366 0.157 0.333 0.278

P3 SPG 0.172 0.170 0.161 0.156 0.179 0.175 0.169 0.168

P3 SWG 0.367 0.247 0.440 0.230 0.475 0.221 0.455 0.275

P4 SPG 0.224 0.222 0.206 0.204 0.224 0.221 0.213 0.212

P4 SWG 0.445 0.315 0.560 0.283 0.582 0.290 0.540 0.450

P5 SPG 0.224 0.225 0.220 0.220 0.220 0.222 0.223 0.222

P5 SWG 0.406 0.443 0.610 0.450 0.605 0.410 0.500 0.540

Table 3. Magnitude of UY for each pass Torque (Nm) 210 310 400 505

3. 3.1

Bolt preload (KN) 37 54 70 89

Target Stress-SPG (MPa) 57 86 112 145

Results and Discussions Bolt Scatter and Relaxation

The comparative variation of axial bolt stress of bolt-1 during first pass tightening is shown in Figure 3a for the two gaskets. For joint using solid plate gasket, bolt-1 which is the first bolt to be tightened to the target stress (57MPa) is observed at higher stress (66MPa) at the end of the pass indicating 13 % rise in the preload. Bolt-1 for spiral wound and compressed asbestos gasket experiences 90% and 65 % preload relaxation at the pass completion. The preload relaxation when neighbouring bolts are tightened is remarkable for the non-linear gaskets as compared to the solid plate gasket. It is concluded due to the flexible nature of nonlinear gaskets causing the bolt to relax during bolt up. Tightening bolts at 90 degree, i.e. bolt-3 and 7 causes to relax bolt-1 in case of non-linear gaskets, however for solid plate gasket, bolt-1 preload is observed to increase with tightening the bolts at 90 degree. It is thus obvious that joint using non-linear gaskets experiences higher elastic interactions due to the gasket compression and permanent deformation in the axial direction causing the bolt to relax. The increase in the bolt-1 preload with bolt-5 tightening is observed greater for the non-linear

96


gaskets than the solid plate gasket. In case of non-linear gaskets, relatively large amount of flange rotation is observed due to gasket’s flexibility when a bolt is being tightened, causing the other end of flange to open and increase preload of bolt at that end. Whereas, solid plate gaskets, being stiffer enough resists compression and thus constraints the flange to open. Figure 3b shows the bolt scatter obtained at the end of each pass using solid plate and spiral wound gasket. From the results, it is obvious that the scatter obtained when using non-linear gaskets is much greater than the solid plate gasket. It is concluded due to the highly non-linear behaviour of SWG gaskets in loading (compression) and unloading conditions. Focusing on the stress-strain diagram for spiral wound gaskets the modulus of elasticity for the bolt up (loading) stage is different from the decompression (unloading) stage. During unloading, the spiral wound gasket yields permanent deformation even at very low stresses. With increasing torque, gasket seating is accelerated, the contact surface of gasket with the flange, is seated non-uniformly with thickness variation along circumference, i.e. uneven compressed, this results in increase in the bolt scatter, relaxation and bolt preload variations with bolt up. 175

SPG SWG

40 20

150 125 100 75 50 25 0

Bolt up Se que nce

(a)

B8

B4

B6

B2

B7

B3

B5

B1

0

(a

(b

B1 B5 B3 B7 B2 B6 B4 B8 B1 B5 B3 B7 B2 B6 B4 B8

60

Stress (MPa)

Stress (MPa)

80

Bolt up Se que nce

(b)

Figure 3. (a) Comparative preload variation of bolt-1 (Pass-1) as per Tightening Sequence1, (b) Bolt Scatter at completion of each pass: (a) SPG (b) SWG

The bolt stress variations obtained at the completion of each pass for the joint using different gaskets is shown in Figure 4. The joint with solid plate gasket experiences almost uniform bolt stress and shows almost same variations at all passes. All bolts preloads are found scattered for the joint using SWG and the difference between the maximum and the minimum bolt stress is substational and preload variations are observed to increase remarkably with each pass. Bolts 1,3,5 & 7 are observed to relax during the first four passes. Although preload variations are reduced but difference remains substational even at the last pass. It is observed that the joint using SWG experiences greater preload difference with remarkable increase in scatter pass wise as compared to the SPG

Stress (MPa)


150

(a

97

(b

100 50

5

4

3

2

1

5

4

3

2

1

0 Pass Num ber

Figure 4. Variations of Bolt Stress at completion of each Pass: (a) SPG (b) SWG

3.2

Bolt Bending Behaviour

Almost similar bolt bending behaviour is observed for all bolts when using a solid plate gasket [Figure 5]. Tensile stress in all the bolts is observed at all bolt locations during the bolt up. The difference in axial stresses between the side nodes is negligible for all the bolts, indicating slight sidewise bolt bending. However the difference in axial stress between the inner/outer nodes is obvious. Inner nodes for all bolts remains in maximum tension while the outer nodes in minimum tension. Bending behaviour of each bolt is different and unique from the others for a joint with SWG. Bolt-1, which is tightened first, B-1/1 (node on inner side) is in tension and B-1/2 (node on outer side) is in compression, but as the torque increase, difference in axial stress between these two locations is increased and B1/2 still show compressive stress, identifying increase in bolt bending. Difference in axial stresses between bolt nodes at side locations is also noticed, which means that bolt-1 not only bends outward but also sidewise. For bolt-5, difference among inner and outer nodes is little, however B-5/4 is found in compression and B-5/3 in tension, indicating bolt-5 bending is mainly sidewise. With bolt up, bolt-3 inner node goes in tension and outer node in compression, the difference increases rapidly with increasing torque, smaller difference among side nodes is noted. For bolt-7, sidewise bending is noted, as B-7/3 is tension and B-7/4 in compression, the difference between inner/outer nodes is noted negligible. Difference between inner and outer nodes for bolt 2 is remarkable and is increased rapidly with each pass, the difference between side nodes is smaller as compared to the inner/outer nodes difference. For bolt, B-6/4 is in tension and B-6/3 in compression, the difference between side nodes is observed greater than the inner/outer nodes, bending is maximum at pass 4 and is minimized at last pass. Bolt-4 shows tensile stresses at all the bolt locations for all the passes, noticeable difference between inner and outer as well as side nodes with rapid increase with increasing torque, indicating bending in both the directions, Bolt-8 is observed also with tensile stresses at all the locations, with almost equal difference between inner/outer and side nodes with bending maximum at pass 5. For all the models, the bolt bending is increased with the increase in torque and is observed maximum during the last pass.

98


B1

B5

B3

B7

B2

B6

B4

B8

220

(a)

Stress (MPa)

170 120 70 20

1 2 3 4 5

1 2 3 4 5

1 2 3 4 5

1 2 3 4 5

1 2 3 4 5

1 2 3 4 5

1 2 3 4 5

1 2 3 4 5

1 2 3 4 5

1 2 3 4 5

1 2 3 4 5

-30

Pass No. 220

(b

Stress (MPa)

170 120 70 20

1 2 3 4 5

1 2 3 4 5

1 2 3 4 5

1 2 3 4 5

1 2 3 4 5

-30

Pass No. B1/1

B1/2

B1/3

B1/4

B5/1

B5/2

B5/3

B5/4

B3/1

B3/2

B3/3

B3/4

B7/1

B7/2

B7/3

B7/4

B2/1

B2/2

B2/3

B2/4

B6/1

B6/2

B6/3

B6/4

B4/1

B4/2

B4/3

B4/4

B8/1

B8/2

B8/3

B8/4

Figure 5. Individual Bolt Bending Behaviour: (a) SPG (b) SWG

3.3

Contact Gasket Stress Distributions

A joint using the solid plate gasket experiences relatively uniform and higher contact stresses as compared to the joints with non-linear gaskets. For the same pass and same torque levels, the contact stresses for solid plate gasket are observed greater than the non-linear gaskets. It is concluded due to the greater stiffness of solid plate gaskets than the non-linear gaskets. Contact stress variations for a solid plate gasket are found maximum for the 1st pass which decrease with each pass and are minimum for the last pass [Figure 6].


99

Stres s (MPa

0 -20

(a)

-40 -60 -80 -100

(b)

-120

0

45

90 135 180 225 270 315 0 45 90 135 180 225 27 0 315 Circum ferential coordinate

Figure 6. Contact Stress Distributions at the end of each Pass: (a) SPG (b) SWG

4.

Conclusions

From the analysis, following conclusions are made: 1.

The joint integrity and sealing performance is very much dependent on the material properties of the gasket used in the joint. Behaviour of a joint using non-linear gaskets is observed totally different from the joint’s behaviour using a solid plate gasket 2. In the case of non-linear gasket, relatively large amount of flange rotation is observed due to the gasket’s flexibility. 3. Bolt relaxation is obvious due to higher elastic interactions in the nonlinear gasketed joint. All bolts preloads are found scattered for the joint using SWG and the difference between the maximum and the minimum bolt stress is substational and preload variations are observed to increase remarkably with each pass. The joint with solid plate gasket experiences almost uniform bolt stress and shows almost same variations at all passes. 4. Bolt bending behaviour is very much affected by the type of gasket used in a joint. With solid plate, bending behaviour of all the bolts is almost similar. Similar bolt bending behaviour for the first and last four bolts is observed when using compressed asbestos sheet gasket. For a joint using spiral wound gasket, bending behaviour of each bolt is different.

5.

References

[1] M.Abid, D.H.Nash, (2006), Joint Relaxation Behavior of Gasketed Bolted Flanged Pipe Joint During Assembly. 2nd WSEAS International Conference on Applied And Theoretical Mechanics (MECHANICS’06) pp 319-325 [2] Abid, M. (2000), Experimental and Analytical studies of conventional (gasketed) and unconventional (non gasketed) flanged pipe joints (with special emphasis on the engineering of ‘joint strength’ and ‘sealing’). PhD Thesis 2000.

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[3] Toshimichi Fukuoka, Tomohiro Takaki, (2001), Finite Element Simulation of Bolt-up Process of Pipe Flange Connections. Journal of Pressure Vessel Technology, Vol. 123, pp.282-287 [4] Toshimichi Fukuoka, Tomohiro Takaki, (2003), Finite Element Simulation of Bolt-up Process of Pipe Flange Connections With Spiral Wound Gasket. Journal of Pressure Vessel Technology, Vol. 125, pp.371-377 [5] M.Abid, Baseer Ullah, (2006), 3-D Nonlinear Finite Element Analysis of Gasketed Flanged Joint under Combined Internal Pressure and Different Temperatures. Journal of Engineering Mechanics by ASCE, Vol. 133/2, pp. 1-8 [6] M.Abid, D.H.Nash, (2006), Bolt Bending Behavior in a Bolted Flanged Pipe Joint: A Comparative Study. ASME International PVP conference, 2006, pp.1-9 [7] ANSYS Inc., (2004), ANSYS Elements Manual, Seventh Edition. [8] ASME Boiler and Pressure Vessel Code, Section VIII, American Society of Mech. Eng., New York, USA. [9] European Sealing Association, (1998). Guidelines for safe seal usage - Flanges and Gaskets. Report No. ESA/FSA 009/98, pp. 1–40.