Allowable local thickness of wall-thinned straight pipes in ASME Code Case N-597-2

摘要

The wall-thinning problem has become a hot issue in the assessment of the structural integrity of pipes and pipe items. A criterion is necessary to determine the possibility of continuing use for the wall-thinned pipes detected by non-destructive inspection. The American Society of Mechanical Engineers (ASME) Boiler & Pressure (B&PV) Vessel Section XI, Div. 1, Code Case N-597-2 [Requirements for analytical evaluation of pipe wall thinning, 2003] provides the criterion. But the code case has several limitations and difficulties for application and sometimes gives non-consistent allowable local thickness values. So works are in progress to modify and extend the code. For that purpose, it is also necessary to understand fully the technical bases of the code case. In Code Case N-597-2, the allowable local thickness of a thinned straight pipe is given through three different methods. Because of different technical bases, each method gives different thickness values and sometimes gives contradictory values. So, in this paper, attempts were made in order to propose a unified rule for the allowable local thickness and in order to remove or relax the restrictions on the application of the code case. For this purpose, elastic stress analyses were made using the finite element method and the stress results were examined. Based on the obtained bending stress results, a very simple procedure was proposed to obtain the consistent allowable local thickness for the thinned straight pipes.