Assessment of Inspection Resolution Acceptance Criteria for Local and General Metal Loss in API579-ASME/1 Code by Means of Finite Element Method

Abstract Life management programs are implemented at aging nuclear facilities to determine the degradation mechanisms and support life extensions until future planned outages. Microbiologically accelerated corrosion can cause severe thinning in the shell side of heat exchangers and piping exposed to lake water. Manual or automated ultrasonic inspections are typically used to measure the thickness profile, where the inspection resolution varies significantly depending on the inspection method. API 579 / ASME FFS-1 is used for fitness-for-service (FFS) assessment of the degraded equipment, where the spacing distance Ls for thickness readings should be selected to allow for an accurate characterization of the thickness profile. In this paper, the impact of the resolution of the thickness profile (i.e., Ls) on the outcome of Level 3 FFS assessment of a pressurized vessel is investigated by finite element analysis. An algorithm is developed to map the measured raw thicknesses into the cylindrical shell with separation distances between inspection data points varying from 2.0 mm to 25.0 mm. The rules of Annex 2D in API 579-1/ASME FFS-1 for Level 3 FFS assessment using the elastic plastic analysis method are utilized to confirm the remaining life of the vessel, where protection against plastic collapse, local failure, and buckling is assessed. Results showed large variation in the expected remaining life of the vessel depending on the resolution of the inspection data.