Considerations for Selecting the Optimum Bolt Assembly Stresses for Piping Flanges

Abstract

In order to minimize the likelihood of leakage from flanged piping joints, it is a good practice to maximize the initial bolt assembly stress. Present bolting guidelines (ASME PCC-1 [1]) use a standard percent of bolt yield to set the assembly stress level. This approach does not allow for the difference in strength between standard pipe flange sizes, differences in material yield strengths (carbon steel versus stainless steel), raised face (RF) versus ring type joint (RTJ) flange configurations and the actual gasket stress achieved across all flange sizes and classes. Since there is no assessment of stresses, such an approach may cause failure of joint components. In addition, because the standard percentage of bolt yield technique does not look at gasket stress, it is prone to gasket leakage due to low stress or gasket destruction due to over-compression for some joints. In addition, some joints may require bolt loads well in excess of the standard value to develop an acceptable gasket stress level in order to prevent leakage. This paper examines an alternative approach, based on the actual gasket and flange stresses. The approach examines the minimum and maximum gasket stress levels to determine what bolt stress range is acceptable and then looks at the flange stresses and flange deformation issues to ensure that the flange will not be permanently damaged, while maximizing the specified bolt load. The practical application of this method is in the development of standard bolt assembly stress (or torque) tables for standard pipe flanges using a given gasket type.