Proposal of inspection and evaluation rules of the Rules on Fitness-for-Service to eliminate the circumferential crack angle limitation of class 1 piping

Abstract

The Rules on Fitness-For-Service (the Rules on FFS) of the Japan Society of Mechanical Engineers (JSME) limits the circumferential crack angle to 60 degrees or less for Class-1 piping. This limitation was based on a deterministic evaluation because of the concern about fracture that would occur if the measurement error of the crack size was taken into consideration. However, ASME Section XI, which is a standard similar to the Rules on FFS, has no limitation on the circumferential crack angle, and the crack angle limitation is unique to the Rules on FFS. After the first edition of the Rules on FFS was published in 2000, some research on crack size measurement were performed. The number of inspections according to the measurement error that satisfy the target fracture frequency regardless of the crack angle can be evaluated by using probabilistic evaluation. In this study, the number of measurements of the crack size required to satisfy the target fracture frequency is determined based on the thickness of pipes, crack size measurement error, the fracture frequency according to the number of crack size measurements, and the conditional core damage probability due to pipe fracture. Based on this result, the requirement for inspection and evaluation of the Rules on FFS was proposed.