Finite element analysis is carried out to clarify the effects of temperature change on the bolt load and gasket load of the bolted flange connections with ring type joint gasket so called RTJ. The RTJ type bolted flange connections are usually utilized for such severe operating conditions as high pressure and/or elevated temperature. It is important for the RTJ type flange connections to be clarified its behavior under internal pressure and thermal load. In this paper, finite element analysis is performed for the piping flange connection of 12 inch class 900 RTJ subjected to the combinations of internal pressure 10 MPa and/or uniform temperature 450°C. Three material combinations are also studied such as both low alloy flange and bolt, stainless steel flange and low alloy bolt and both stainless steel flange and bolt. Elastic analysis and elastic-plastic analysis are both computed and compared. From the analysis results, it is observed that the elastic analysis and elastic-plastic one give completely different behaviors especially in the bolt load change under the loading and the unloading conditions. The difference in coefficients of thermal expansion is essential on the bolt load change when the different materials are used for the flanges and the bolts. As a conclusion, in order to establish a simplified calculation of the RTJ type flange connection for its optimal design, it is necessary to consider the elastic-plastic material behavior at the contact between the gasket and the groove. It is also required to incorporate the difference of thermal expansion and stiffness between flange and bolt with temperature dependency.

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