Wall thinning acceptance criteria for degraded carbon steel piping systems using FAD methodology

摘要

As part of the structural integrity assessment for Savannah River Site (SRS) piping systems, an acceptance criteria methodology for minimum pipe wall thickness has been developed for carbon steel piping. If a measured pipe thickness during inspection cannot meet the 87.5% of the nominal wall thickness specified in the ASME Code Case N-480, the acceptance criteria must be invoked. For a particular pipe, the larger of the two minimum thickness values obtained from the code stress check and the CEGB-R6 Failure Assessment Diagram (FAD) methodology is the minimum wall thickness for the acceptance criteria. The code stress check is based on the ASME/ANSI B31.1 Code, ASME Code Case N-480, and the SRS reactor restart criteria. The pipe wall thickness is calculated from the code equations and the applied loads. In fracture analysis, three types of axial and circumferential flaws are assumed to exist in the pipes based on the weld defects found in service history. For each flaw configuration, the stress intensity factors and the limit load solutions are calculated. These quantities are input to FAD to solve for the corresponding wall thickness required for the pipe to sustain the postulated flaws and to meet ASME safety margins under the applied loads.