Supplementary Technical Basis for ASME Section XI Code Case N-597-2

Abstract

Section XI of the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code provides rules and requirements for maintaining pressure boundary integrity of components, piping, and equipment during the life of a nuclear power plant. Code Case N-597-2 of Section XI, Requirements for Analytical Evaluation of Pipe Wall Thinning, provides evaluation procedures and acceptance criteria to justify continued operation of Class 1, 2 and 3 piping items subjected to wall thinning by a mechanism such as flow-accelerated corrosion. The acceptance criteria ensure that margins equivalent to those of the ASME B&PV Code are maintained. The technical basis for Code Case N-597-2 was previously presented at the 1999 ASME Pressure Vessels and Piping Conference. Since then, the ASME Section XI Working Group on Pipe Flaw Evaluation has identified the need for further explanation of the technical basis for the Code Case, such as the procedures for evaluation of wall thickness less than the Construction Code Design Pressure-based minimum allowable wall thickness, tmin. This paper provides an additional description of the Code Case technical basis and validation against experimental and historic wall thinning events.