International Journal of Engineering and Techniques - Volume 1 Issue 6, Nov – Dec 2015 RESEARCH ARTICLE
OPEN ACCESS
Analysis of Leakage in Bolted-Flanged Joints with Gasket and Under Thermal Condition, a Critical Review Wasim B. Patel*1, Pundlik N. Patil2, Raghunath Y. Patil3, Prasad P. Patil4 1(
PG Scholar,Department of Mechanical Engineering, North Maharashtra University, Jalgaon. Email: wpwasimpatel@gmail.com) 2,3,4 (Assi. Prof. Department of Mechanical Engineering, North Maharashtra University, Jalgaon. Email: pnp09@rediffmail.com)
Abstract: Bolted flange joints are widely used in engineering structure. The evolution of leakage is studied using detailed contact finite element analysis. The distribution of stress at the gasket is analyzed using a contact condition based on slide-line elements using ABAQUS, a commercial finite element code. Slide-line elements also take into account pressure penetration as contact that is lost between flange and gasket. Results are presented for a particular flange, a raised face flange sealed by a mild steel gasket. Although a lot of pipe flange connections are exposed to elevated temperature during long-term plant operation, a sealing performance of the pipe flange connections at elevated temperature is not well understood because of the experimental difficulty and the analytical problems due to the lack of the materials properties of gaskets at elevated temperature. The authors have been evaluating the effect of the material properties of spiral wound gaskets (SWG) and the sealing performance of the pipe flange connections at elevated temperature Keywords — Flanged joints Bolted Joint, Stress Analysis, Sealing Performance, Gasketed Joint, Thermal Conduction Condition
I. INTRODUCTION In a flanged joint, it is always required to select a suitable gasket depending on the kind of fluid, the pressure and the temperature. However, even though suitable gaskets are chosen, their sealing performance usually deteriorates over the years. Therefore, maintenance to repair leaks and/or replace gaskets is required. Bolted joints are very often the weak link between pressure vessels as they are very prone to leakage. This is especially true when these joints are subjected to high temperature. A comprehensive survey _1_ conducted in 1985 showed that high operational temperature and thermal transients are one of the major sources of flanged joint failure. In the piping industry, there are several problems that continue to receive a considerable amount of research attention, particularly in the area of bolted-flanged joint design. Two problems that are critical in bolted flanged joint design are strength of the joint and leakage. The first problem has been studied since
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the 1920s for metallic joints with a general consensus on the available solution well established [3]. The second problem has been studied for almost an equally long period yet leakage analysis continues to be the subject of much study, as evident by the number of articles published in the past quarter century [4],[5],[2]. Pipe flange connections with gaskets in chemical plants, electric power plants and other plants are usually exposed to internal pressure under elevated temperature environment. Even a plant operated at lower temperature like room temperature, pipe flange connections are exposed to a change in ambient temperature. In many studies the sealing performance of the pipe flange connections has been investigated and evaluated by analyzing the contact stress at the interfaces, a variation of the axial bolt force and creep behavior of gaskets [6],[7],[1]. These studies evaluated the sealing performance of the pipe flange connections under elevated temperature and
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the difficulty of detail evaluation of the connections due to the uncertain behavior of the gasket under thermal conditions was revealed.
Sr. No. 1 2 3 4 5 6 7 8 9
Characteristics
Value
Nominal flange size (mm) Gasket material type Gasket width (mm) Inside diameter of the gasket Effective gasket width (mm) Gasket factor m Gasket yield factor, y (Pa) Number of bolts Size of bolts
300 Soft flat mild steel 15 315 6.9 5.5 124,100 16 M48
Flange description A. The flange geometry and dimensions used in the analysis were extracted from [8],[2] and are shown in fig. 1. The characteristics of the gasket and number of bolts are also from [8], all of which are TABLE 1 BOLT AND GASKET CHARACTEISTICS summarized in table 1. The flange is constructed of steel with Young’s modulus of 200 GPa and II. FINITE ELEMENT MODEL Poisson’s ratio of 0.3. the ASME code assumes We used PATRAN [2] to create the finite flange failure when the material yields. the stresselement mesh depicted in Fig. 3, and ABAQUS strain diagram of the gasket is assumed to be [10],[2] to perform the analysis. Second order trilinear as shown in fig. 2 [9],[2]. axisymmetric elements, CAX8 are used throughout the mesh of the flange and gasket. A. Gasket Contact
Contact between gasket and flange face has been modeled using ISL22A elements on the gasket and a sideline that is attached to the flange face. Also, since the gasket is not rigidly attached to the flange, it can be blown out by the internal pressure (this can happen in cases where softer gaskets are used and flange faces are very smooth). To model this, we had used a standard Coulomb friction model. We assume a coefficient of static friction of 0.8, a very rough surface. B. Bolt Holes
Fig. 1 Flange geometry and dimensions.
Fig. 2 Gasket stress-strain diagram.
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The material of flange is not homogeneous because of the presence of the bolt holes. This is handled by smearing the material properties used in the bolt hole area of the mesh. That is, using material properties of weaker material in the bolt hole area. Guidelines can be found in the ASME code for determining effective material properties for perforated plates. For the model given here, the effective material properties are calculated using an elasticity moduli reduction factor. This factor is equal to one minus the ratio of the volume of the bolt holes to the volume swept by the bolt diameter along the entire circumference of the flange along the bolt circle diameter. Hence, the reduction factor is 0.6. The effective in-plane moduli of elasticity are obtained by multiplying the reduction factor times the flange modulus. The in-plane Poison’s ration is left unchanged. The modulus in the hoop direction should be very small and the hoop
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Poison’s ration should be zero. The effective shear modulus is computed from its respective modulus of elasticity and Poison’s ratio. These lead to the following material properties for the bolt hole area: Er = Ez = 120 GPa, Eθ = 0.12 MPa, νrz = 0.3, νzθ = νrθ = 0, Grz = 46.1 GPa, and Grθ = Gzθ = 0.06 MPa. These Poison’s ratios and elasticity moduli are specified for the bolt hole part of the mesh. The material properties for the gasket and the rest of the flange are shown in Table 1[2].
the reason why we solved the problem using an axisymmetric model. = πGy =pB2/4 + p(G2 – B2 )/4 + 2πbGmp Am = max (Wm1 / Sb , Wm2/Sa ) W = ½ (Ab + Am ) Sa
(1) (2) (3) (4)
C. Boundary Conditions
We specify axisymmetric boundary conditions on the symmetric plane of the gasket. That is, the axial displacement in the middle of the gasket is zero (Fig. 3). D. Bolt Load
The bolt load computed using Eq. (4) is smeared over the bolt hole upper surface as a normal pressure, as shown in Fig. 3. These are the load applied at the beginning of the analysis, the seating condition. In this case, there could be no contact loss during the loading and the reacting gasket pressure should be larger than the minimum effective seating pressure, y. E. Internal Pressure
The internal pressure loading can be divided into three loads: (1) the internal pressure, which acts on the internal surface of the vessel; (2) the hydrostatic end load, which is the membrane stress acting far from the joint in the pipe due to the internal pressure and is computed using the first term of Eq. (2); (3) the penetrating pressure as the contact between the flange face and the gasket is lost. The PPENn sub-option of the distributed load option is used to simulate pressure penetration between surfaces in contact. This fluid pressure shall penetrate into the mating surface interface until some area of the surfaces is reached where the contact area pressure between the abutting surfaces exceeds the fluid pressure, cutting off further penetration. The pressure penetration loads start from the inside of the vessel, left side in Fig. 5, and penetrate between the surfaces continuously from this side. The pressure penetration path can be specified in ABAQUS. The pressure penetration option in ABAQUS is only supported in plane stress and axisymmetric elements, not 3-D, which is
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Fig. 3 Axisymmetric finite element mesh.
III.
EXPERIMENTAL METHOD
A. Leakage Test
This paper also study about the leakages of flanged joints. The leakage tests using pipe flange connection were also conducted to evaluate the sealing performance of the SWG. The pipe flange and the bolts both are made of stainless steel SUS304 (Japan Industrial Standard, JIS). The SWG
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sealing element is made of non-asbestos filler. The inner and outer rings are also made of the stainless steel SUS304 (JIS). The nominal diameter of bolts used is M20 (JIS) and the size of the gasket taken is ASME/ANSI Class 600 [11]. Figure 4 gives a schematic diagram of the experimental setup [1] taken for study. Two pipe flanges including a gasket were clamped by the torque control method with the target torque T. The axial bolt forces are measured by two calibrated strain gauges attached on the shank of the bolts in the tightening process. The internal pressure was applied by using Helium gas, and the magnitude of the internal pressure was measured by the pressure transducer. The amount of gas leakage evaluated by the change in the internal pressure over some time interval. The pipe flange was heated by using an electric heater shown in Fig. 4. The outputs of set up were recorded by the analysing recorder through dynamic amplifiers. The tightening procedure, JIS B2251 [12], was applied to tighten the pipe flanges in the experiments. The target torque T (=k･Ff･d) was determined from the torque coefficient "k", the target initial clamping bolt force (bolt preload) Ff and the bolt nominal diameter "d". The torque coefficient "k" was previously obtained in tightening test using one bolt. The lubricating oil was applied at the contact surfaces in the experiments, and the torque coefficient "k" was obtained as k=0.1465. After the target initial clamping bolt force Ff was determined, the values obtained by R.O.T.T. (Room Temperature Operational Tightness Test) were used to obtain the new gasket constants [13],[14]. The target initial clamping bolt force Ff determined for a given tightness parameter Tp under an internal pressure P by the PVRC procedure, and the assembly efficiency that was 0.85 [15]. In addition, the leakage tests with heat and inner pressure cycle condition were conducted and the change of the sealing performances was evaluated.
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Fig. 4 Schematic of experimental setup for helium gas leakage test
IV.
RESULTS AND DISCUSSIONS
A. Material Properties of Gasket
Figure 5 shows the stress-strain curves of new SWG measured in compression test at each temperature. Stress-strain curve shifted to the higher strain side as increasing the temperature. In other words, the result showed that the deformation resistance, that is the modulus of stress-strain curve, became lower as increasing the temperature[1]. Figure 6 shows the stress-strain curve measured in cyclic compression test at room temperature. Figure 6 suggested that the hardening effect was observed as increasing the cyclic number. Figure 7 shows the result of stress-strain curves of new, aged and used SWG. The elastic moduli were calculated from the slope of stress-strain curves in 35-45MPa. The elastic modulus of the aged SWG in the unloading process was a lower compared with that of the new SWG. On the other hand, the elastic modules of the aged SWG and the new SWG in the loading process didn’t show a significant difference. It is suggested that the decrease of the gasket stress in the new SWG is larger than that of aged SWG and the new SWG is more sensitive in inner pressure loading. Figure 8 shows the thermal expansion coefficient evaluated [16],[1]. The gasket expanded as increasing the temperature and the thermal expansion coefficient evaluated from the slope of the curve changed at 210℃. The thermal expansion coefficient was 218.1x10-6/℃ up to 210℃ and it decreased 29.0x10-6/℃ more than 210℃.
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those properties on the sealing performance of bolted flange connections under thermal condition and internal pressure were analyzed. The method presented in this paper can be used to study leakage behavior and validate the ASME code formulation for other gasket materials and joint configurations. The stress-strain curve of SWG shifted to the high strain side while becoming a high temperature. The deformation resistances of the SWG increased as increasing the cyclic load/unload. In conclusion, the ASME code method for designing bolted joints, although not theoretically exact, is sufficiently accurate for most practical purposes, and is simpler to implement than the finite element formulation.
Fig. 5 Stress-strain curves of new SWG
Fig. 6 Stress-strain curves of cyclic compression test of new SWG at room temperature
ACKNOWLEDGMENT I acknowledge the support of Shri Gulabrao Deokar College of Engineering, Jalgaon. Central college liberary facility providing support. I also like to express my deep and sincere gratitude to my Project guide Prof. P. N. Patil and Head of Mechanical Engineering Department Prof. R. Y. Patil without their support it could not happen. REFERENCES 1.
Yuya Omiya *1, Toshiyuki Sawa. “ Sealing Performance Evaluation of Pipe Flange Connection with Spiral Wound Gasket under Cyclic Thermal Condition.” International journal of Mechanic Systems Engineering, Volume 3, 102-110
2.
Hector Estrada “ Analysis of Leakage in BoltedFlanged Joints Using Contact Finite Element Analysis.” journal of Mechanics Engineering and Automation, Volume 5, 135-142
3.
Waters, E. O., Wesstrom, D. B., Rossheim, D. B., and Williams, F. S. 1937. “Formulas for Stresses in Bolted Flanged Connections.” Transactions of the ASME 59: 161-9.
4.
Bertini, L., Beghini, M., Santus, C., and Mariotti, G. 2009. “Metal to Metal Flanges Leakage Analysis.” Presented at the ASME 2009 Pressure Vessels and Piping Division Conference, Prague, Czech Republic.
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Estrada, H., and Parsons, I. D. 1999. “Strength and Leakage Finite Element Analysis of a Fiber Reinforced Plastic Stub Flange Joint.” International Journal of Pressure Vessels and Piping 76: 543-50.
Fig. 7 Stress-strain curves of new, aged and used SWG
Fig. 8 The result of TG/DTA of a filler material in SWG
V. CONCLUSIONS The material and thermal properties of SWG were experimentally evaluated and the effect of
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7.
Sawa T., Takagi Y. and Torii H., “Sealing Performance Evaluation of Pipe Flange Connection Under Elevated Temperatures”, Proceedings of ASME PVP2007/Creep 8 Conference, 2007 Abid M., “Determination of safe operating conditions for gasketed flange joint under combined internal pressure and temperature: A finite element approach”, Vol. 83, 6, pp. 433- 441, 2006
8.
Nishioka, K., Morita, Y., and Kawashima, H. 1979. “Strength of Integral Pipe Flanges (No. 2 Gasket Seating Stress and the Influence of Number of Bolts).” Bulletin of the JSME 22 (174): 1705-11.
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Flinn, R. A., and Trojan, P. K. 1990. Engineering Materials and Their Apparitions. 4th ed. Boston: Houghton Mifflin Company.
10. Hibbitt, Karlsson & Sorensen, Inc., ABAQUS User’s Manual.
13. Payne, J.R., Bazergui, A. and Leon, G.F., “A New Look at Gasket Factors,” Proc. 10th International Conference on Fluid Sealing, BHRA, H1, pp. 345363, 1984 14. Bickford J., Gaskets and Gasketed Joints, Marcel Dekker Inc, New York, 1998 15. Leon, G.F. and Payne, J.R., “An Overview of the PVRC Research Program on Bolted Flanged Connections,” Proc, 6th International Conference on Pressure Vessel Technology, Vessel Technology, 1, pp. 217, 1989 16. Takagi Y., Torii H., Sawa T. and Kawasaki N., “Stress Analysis Sealing Performance Evaluation of Pipe Flange Connection At Elevated Temperature”, Proceedings of ASME PVP2008 Conference, 2008S. M. Metev and V.P. Veiko, Laser Assisted Microtechnology, 2nd ed.,R. M. Osgood, Jr., Ed. Berlin, Germany: Springer-Verlag, 1998.
11. ASME/ANSI B16.5, Section Ⅷ, Division Ⅷ, 1988 12. JIS B2251, “Bolt Tightening Guidelines for Pressure Boundary Flanged Joint Assembly”, 2008
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