Hiroyasu Torii

Sealing Performance Evaluation of Pipe Flange Connection Under Elevated Temperatures

Since a lot of pipe flange connections are exposed to elevated temperature during operation, it is important to evaluate the sealing performance of the connections under operational temperature. In this study, the sealing performance of pipe flange connection was experimentally evaluated by measuring the amount of gas leakage at room temperature (RT), 100 °C and 200 °C in addition to internal pressure. Non-asbestos graphite gaskets were used in the experiment. The nominal size of the pipe flange used during the experimental study was 3 inch. The eight bolts and nuts were tightened according to ASME PCC-1 and the Japanese method (HPIS). During assembly, the axial bolt force was individually measured with strain gauges attached to each bolt. The scatter in the axial bolt force during the assembling process was examined and compared between the ASME and the HPIS. In addition, the gasket contact stress distribution of each assembly process was calculated by finite element method under elevated temperature and internal pressure. In the FE study, experimentally measured physical properties such as elastic modulus and thermal expansion were used. ηhe amount of leakage was estimated from the contact gasket stress distribution obtained by FE analysis. The estimated axial bolt forces were compared with those of experimental ones.Copyright

Sealing performance evaluation of pipe flange connection with spiral wound gasket under cyclic thermal condition

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 which result from the lack of the materials properties of gaskets at elevated temperature. The authors have been evaluating the effects of the material properties of spiral wound gaskets (SWG) and the sealing performance of the pipe flange connections at elevated temperature with numerical and experimental analysis. The gasket stress induced by the mismatch of the thermal expansion between the gasket and flange/bolt increased as increasing the temperature in the case that the thermal expansion coefficient of the gasket was larger than that of bolt/flange material and the sealing performance of the pipe flange connection was improved. However, the thermal expansion coefficient declines at temperatures higher than 210 °C. In addition, the binder compositions of SWG vaporize and the mechanical properties slightly change at high temperature such as higher than 200°C. Therefore, the sealing performance of pipe flange connection with SWG might decline more than 200°C. The sealing performance and long-term durability of pipe flange connection with SWG at elevated temperature were evaluated in this study. The long term leakage test with heat cycle revealed that the sealing performance of pipe flange connection didn’t show any degradation for 21 days. The compression tests for the new, the thermal aged and the used gaskets were done to investigate the effect of material characteristics on the long term sealing performance. In addition to the leakage test and the compression tests, the thermal characteristics of SWG were evaluated by the Thermogravimetry/Differential Thermal Analyzer (TG/DTA) measurement. The measurement showed the weight loss of SWG more than 130°C. According to those experimental results and FE analysis, the effect of temperature on the sealing performance was discussed.Copyright

Effects of Scatter in Axial Bolt Force on the Sealing Performance of Pipe Flange Connections at Elevated Temperature

Since scatter in axial bolt force exists in assembling pipe flange connections, it is important to evaluate the effect of scatter in axial bolt force on the sealing performance of pipe flange connection. The FE and experimental analyses were done to evaluate the effect of scatter in axial bolt force on the sealing performance of pipe flange connections with 3″ (NPS 3″ ) and 20″ (NPS 20″ ) nominal diameters. The axial bolt force was measured with strain gauges mounted on the shank of each bolt. The two kinds of torque controlled assembly procedures, ASME PCC-1 procedure and JIS 2251 procedure, were evaluated as an assembling procedure and compared the scatter in axial bolt force with that of the uniform axial bolt force condition. The results revealed that the significant difference was not observed between ASME PCC-1 and JIS 2251 procedures. The scatter in axial bolt force were not changed at elevated temperature, however, the axial bolt force increased when increasing the temperature. Therefore, the tightness of the flanged connection is improved with increasing the temperature up to 200 °C. The assembly efficiency was evaluated with the experimental results of 20″ nominal diameter leakage tests and FE analyses. In addition to the scatter in the assembly axial bolt force, the flange rotation affected the axial bolt force when internally pressurized. As a result, the tightness parameter, Tp, was slightly lower than that evaluated with the assembly efficiency, η = 0.85, defined in ASME PCC-1 procedure. The results are discussed and a modified assembly efficiency is proposed.Copyright

Evaluation of Sealing Performance of Pipe Flange Connection Subjected to External Bending Moments

The sealing performance of pipe flange connection subjected to an external bending moment was evaluated with the FEM and the experiments. The experimental leakage test using water revealed that the bending moment had an important effect on the sealing performance. The FE analyses suggested that the contact gasket stress, which was a function of the bolt preload, determines the leakage. The changes in contact gasket stress at tension side and compression side when the external bending moment applied were not symmetrical. The reduction in the contact gasket stress of tension side was larger than that of compression side due to the non-linear stress-strain behavior of the gasket. In addition, the hub stress of the flange when external bending moment applied, was evaluated from FE result and the discussion for optimizing the flange design subjected to external bending moment was done in this paper.Copyright

Effect of Material Properties of Gasket on the Sealing Performance of Pipe Flange Connections at Elevated Temperature

The sealing performance of the pipe flange connection depends on the compressive gasket stress. Therefore, the unexpected leakage can be avoided by estimating the change in compressive gasket stress during operation. In order to make such estimation, the material properties of the gasket are required as well as those of bolt and flange material. When the pipe flange connection is used under heated condition, the thermal expansion coefficient shows an important role. For example, if the thermal expansion coefficient of gasket is larger than that of pipe flange material, the gasket stress is expected to be increased by increasing the temperature and the sealing performance of pipe flange connection is improved. However, a gasket tends to be softening at high temperature. Beside, the creep phenomenon of gasket induces the decrease of gasket stress. Thus, the behavior of the gasket is complicated and it should be investigated to make a reliable design.In this paper, the high temperature properties, including thermal expansion and stress relaxation property of spiral wound gasket with expanded graphite are examined by compression tests at elevated temperature. The stress analysis with high temperature material properties obtained from the compression tests also conducted to estimate the sealing performance of pipe flange connection. As a result, the pipe flange connection with spiral wound gasket is expected to keep the superior sealing performance at 300 C for long time.Copyright

Sealing Performance and FE Analysis of Pipe Flange Connection With Gasket Under Cyclic Thermal Condition

Pipe flange connections with gaskets have been widely used in mechanical structures such as nuclear plants, petroleum refining plants and chemical plants and so on. Those connections are usually exposed to elevated temperature with internal pressure. Thus some investigations on the mechanical behavior of pipe flange connections with gaskets under thermal condition such as the sealing performance, the contact stress at the interfaces and a variation in the axial bolt force, have been performed. However, it is important to investigate the sealing performance of pipe connections under long term thermal exposure for using them safely. In this study, the effects of cyclic thermal condition on the sealing performance and mechanical behavior in pipe flange connection will be discussed based on the experimental leakage tests of pipe flange connections with gasket and FEM analyses.© 2011 ASME

The Stress Analysis and the Sealing Performance Evaluation of the Pipe Flange Connection at Elevated Temperature

The authors have been evaluating the effects of the material properties of spiral wound gaskets (SWG) and the sealing performance of the pipe flange connections at elevated temperature with numerical and experimental analysis. The contact gasket stress increased by the mismatch of the thermal expansion between the gasket and flange/bolt as increasing the temperature and the sealing performance of the pipe flange connection was improved. The authors have also evaluated the effect of scatter in axial bolt force and revealed the assembly efficiency is not as high as expected in case of large nominal diameter pipe flange connection. Following the previous studies, the sealing performance of pipe flange connection and the assembly efficiency with SWG at elevated temperature were further evaluated with the experimental leakage test and 3-D FE analysis in this study. In order to make a precise evaluation, the experimentally obtained material properties of SWG were used in the analysis. The uniform axial bolt force tests and the scattered axial bolt force test were done to compare the sealing performance and the assembly efficiency evaluated by the ratio of the tightness parameter of each test at elevated temperature. Although the leak rate improved as increasing the temperature, the assembly efficiency decreased. The leakage based assembly efficiency was proposed according to the experimental results and numerical analyses.Copyright

Effect of external bending moment on the sealing performance of pipe flange connection

Since an external bending moment affects the sealing performance of pipe flange connection, it is important to investigate this effect. This paper analyzed the contact gasket stress distribution of pipe flange connections and evaluates the effect of external bending moment on the sealing performance from the viewpoint of changes in contact gasket stress. The study includes the FE analyses and the experimental leakage tests. The FE analyses suggested the large decrease of contact gasket stress at tension side and small increase at compression side. The difference in change in contact gasket stress was caused by the non-linear hysteresis characteristics of stress-displacement curve of gasket. The FE analyses also suggested that the loading order, internal pressure and external bending moment, also affected the sealing performance due to the non-linear deformation characteristic of the gasket. The sealing performance when the external bending moment applied prior to the internal pressure was degraded more than when the internal pressure was applied prior to the external bending moment. The experimental leakage tests using helium (He) gas were analyzed by the finite element method and discussed. This paper also evaluated the stress distribution in the pipe flange under external bending moment. The results suggested that the hub stress dominated the flange structure and the most important factor in designing the flange.Copyright

Effects of Scatter in Bolt Preload on the Sealing Performance of Pipe Flange Connections Under Internal Pressure (Case Where the Nominal Diameter of Pipe Flange Connection is 20

It has been well known that a scatter in axial bolt forces in pipe flange connections tightened by the torque control method is substantial. In practice, pipe flange connections with the large nominal diameter tightened by the torque control method have been used in chemical industry, and so on. In our study, the characteristics of the connections with smaller nominal diameter (less than 8″ ) have been shown. So, in an optimum design of pipe flange connections with gaskets, it is necessary to understand the characteristics of the pipe flange connections with large nominal diameter under internal pressure and the contact gasket stress distributions due to the scatter in axial bolt forces in the connections tightened by the torque control method. In this paper, the experimental and FE analyses were done to evaluate the effect of scatter in axial bolt force on the gasket stress distribution and the sealing performance of pipe flange connections with 20″ nominal diameter. Two types of torque controlled assembling procedures, that is, ASME PCC-1 and JIS B 2251 procedures, are evaluated as an assembling procedures and an effect of scatter in the axial bolt forces obtained from the above methods is examined. Then, the effect of scatter in the axial bolt forces on the sealing performance of the connection is evaluated. The contact stress distribution in the larger pipe flange connection with 20″ nominal diameter under internal pressure is analyzed using FEM by taking into account a non-linearity and a hysteresis. The measured scatter in the axial bolt forces is applied in the FEM calculations.Copyright

Stress Characteristics of Large Diameter Pipe Flange Connection With Non-Asbestos Gasket Under Elevated Temperature

Since the flange rotation caused by internal pressure affects the sealing performance of pipe flange connections more by increasing the pipe nominal diameter, it is important to evaluate the sealing performance of larger nominal diameter pipe flange connection. However, most experiments were conducted with small-scale model due to the difficulty of large-scale experiment and numerical analyses containing uncertainties because of the lack of material models of gaskets. In addition, a sealing performance evaluation at high temperature is more difficult due to the thermal effect though the thermal condition is not avoidable in practical use. In this study, the 20” diameter pipe flange connection was used as a large nominal diameter pipe flange connection to analyze the stresses in the connection. In the FE analysis, the experimentally obtained material properties were used and the effect of diameter on the sealing performance was evaluated. The analysis revealed the thermal effect and the effect of flange rotation on the sealing performance. Moreover, the effect of material properties on the contact stress and hub stress of the pipe flange connections was evaluated. The experimentally obtained result was discussed with the FE result in this study.Copyright