Proposal of acceptance criterion for plastic collapse assessment rule on local metal loss

Abstract

There are several Fitness-For-Service (FFS) standards with evaluation rules in terms of plastic collapse for pressure equipment possessing a local metal loss simultaneously subjected to internal pressure and bending moment. The authors have already reported the results of a comparative study of FFS rules, including the remaining strength factor approach in API 579-1/ASME FFS-1 and the p–M (pressure and moment) diagram method, which pointed out that there could be significant differences in allowable flaw sizes. In this paper, acceptance criterion for plastic collapse assessment on local metal loss at high temperatures is proposed with taking into account the effect of decreasing yield strength of material at high temperatures, such as 350°C. The allowable flaw depth at high temperatures derived from API 579-1/ASME FFS-1 is larger than that obtained by the p–M diagram method. However, it is verified by finite element analysis that the allowable flaw size of the p–M diagram method is set on the stress state of general yielding near a local metal loss area if safety factor is not considered and it is possible to evade ratcheting due to cyclic bending moment in service such as that caused by earthquake if the proposed acceptance criterion is used.