An Update on Selecting the Optimum Bolt Assembly Stress 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]) outline the use of a percent of bolt yield across all flange sizes and classes to set the assembly stress level. These guidelines do indicate that aspects such as component strength and gasket stress should be considered, however the most common application of the approach is to use a standard percentage of bolt yield across all flange sizes and classes. This approach does allow for adjustment for differences in material yield strengths (carbon steel versus stainless steel) and raised face (RF) versus ring type joint (RTJ) flange configurations. It does not, however, adjust for the difference in strength between standard pipe flange sizes nor the actual gasket stress achieved across all flange sizes and classes. Since there is no assessment of flange strength, 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 is a continuation of the paper presented during PVP 2006 in Vancouver (Brown [2]), which examined the variables that must be considered and drew some preliminary conclusions regarding the use of flange stress limits in determining the maximum allowable bolt load for a given flange size. Subsequent to writing that paper, further investigation found that the code calculated flange stresses are a poor indicator of the maximum acceptable bolt load. The most practical measure of this load is obtained by using elastic-plastic finite element analysis (FEA) to determine the point of gross plastic deformation of the flange. This paper details the maximum bolt load limit results of elastic-plastic FEA on most sizes of standard ASME weld neck flange sizes. 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. In addition, a new code equation and additional limits are developed, by comparison to the elastic-plastic FEA results, which allow the determination of the maximum assembly bolt load for non-standard weld-neck flanges and standard weld-neck flanges with different bores, materials or gaskets than used in the elastic-plastic FEA presented in this paper.