Evaluation of the API 579-1/ASME FFS-1 Equations for Equivalent Membrane and Bending Stress in Thick Wall Applications

Abstract

Abstract For polynomial and arbitrary through-wall stress distributions, the API 579-1/ASME FFS-1 2021 Fitness for Service (API 579) standard provides equations for calculating equivalent static membrane and bending stresses. The membrane and bending stress values are then used in various stress intensity factor and reference stress calculations to evaluate crack-like flaws. In the 2021 edition, the equivalent static membrane and bending stress procedures are covered by equations 9B.1 to 9B.5 and 9C.1 to 9C.5. These equations were independently evaluated in this work for circumferential flaw configurations in cylinders and spheres. It was determined that the current equations produce reasonably accurate results for cylinders and spheres with large internal radius to thickness ratios (Ri/t). However, for thick-wall applications where Ri/t ≈ 5 or less, new equations are proposed to improve the accuracy. The main features of the new equations include representing the stress distribution as a function of the radial position and implementing integration in a cylindrical coordinate system. At large Ri/t ratios, the results from the new equations are practically the same as results associated with the current API 579 approach. The new equations are recommended as improvements for fitness for service assessments that involve circumferential flaws in thick-walled cylinders and spheres.