The allowable stress for 2.25Cr IMo 0.25V steel per ASME Section VIII Div.2 2007 edition (hereinafter called New Div2) becomes higher than that per old Div.2. Therefore the thickness of flange can be thinner than that per old Div.2. The internal fluid leakage is main concern as well as stress level especially for large size flanges per New Div.2. In this paper the analysis method using axi-symmetric model are investigated and the numerical analyses using axi-symmetric models for two types of flange design are performed in order to compare the flange rotation, gasket contact stress and stress level of flange and bolts. The contact stress of New Div.2 flange was lower than that of Old Div.2 flange totally. However the maximum gasket contact stress of New Div.2 flange which occurred at outer edge of the gasket is almost same as that of Old Div.2. Therefore it is expected that the sealing performance of flange connections per both Old Div.2 and New Div.2 is almost same.
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