Comparison of BS 7910 and API 579-1/ASME FFS-1 solutions with regards to leak-before-break

Abstract

One of the ways to aid the decision whether or not to live with defects in pressurised component is through the demonstration of Leak-Before-Break (LBB). In this paper, three of the main solutions to carry out the LBB assessment, namely Stress Intensity Factor (SIF), Reference Stress (RS) and Crack Opening Area (COA) have been evaluated and compared for both BS 7910 and API 579/ASME FFS-1 standards. Differences with respect to the choice of solutions and boundary conditions are illustrated and discussed. Same applied loads and material properties have been used when applying each procedure. Different geometries for potential pressurised components which are of interest with regards to LBB have been considered for each solution. Focus is made on cylinders where axially and circumferentially oriented through-wall and surface cracks were analysed. While SIF solutions produce similar results for both standard, reference stress solutions show higher differences in results. However, in LBB assessments it is the reference stress solution which is more relevant, since most LBB assessments pre-suppose the material to be ductile. Here there are significant differences between the different assumptions. In terms of COA, solutions are not given at the same location however they seems to agree well within the common range of applicability. Differences in the assessment route between the standards is also discussed. Experimental data from literature has been also been compared to the different standard predictions, to illustrate the accuracy of the solutions for axially oriented surface cracks. Aptitude of solutions to predict the boundary between leak and break is discussed, in relation to how this shows the level of conservatism.