SUPPLEMENTARY TECHNICAL BASIS FOR ASME SECTION XI CODE CASE N-597-2

Abstract

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, t min . This paper provides an additional description of the Code Case technical basis and validation against experimental and historic wall thinning events. NOMENCLATURE a = depth of an axial flaw A = reinforcement area required for a Class 1 pipe under internal pressure in accordance with rules in Section III of the ASME B&PV Code A i = predicted inside area of the cross-section of the pipe A o = total cross-sectional area of the pipe based on nominal outside diameter A p = predicted metal cross-sectional area of the pipe A rein = reinforcement area required for a Class 2 or 3 pipe under internal pressure in accordance with rules in Section III of the ASME B&PV Code B = parameter used to calculate maximum allowable length of an axial flaw in ANSI/ASME B31G d = distance from the center of a local thinned area defining the limits of reinforcement for Class 2 and 3 piping in accordance with the Construction Code D o = nominal outside diameter of the piping item f = stress range reduction factor for cyclic conditions for Class 2 and 3 piping i = stress intensification factor for Class 2 and 3 piping i 0 = stress intensification factor based on the design-basis geometry of the piping item k = constant used to describe the assumed linear increase in stress intensification factor i L = maximum extent of a local thinned area with t p < t nom L A = distance used to define limits of reinforcement for Class 1 piping in accordance with rules in Section III of the ASME B&PV Code L ax = maximum allowable length of an axial flaw from ANSI/ASME B31G L m = maximum extent of a local thinned area with t p < t min L m(a) = axial extent of a local thinned area with t p < t min L m(t) = transverse (circumferential) extent of a local thinned area with t p < t min M b = bending moment n = number of load cycles N = number of allowable load cycles N' = number of allowable load cycles corresponding to an assumed linear increase in stress intensification factor i N 0 = number of allowable load cycles based on the as-installed geometry of the piping item P = Design Pressure R = mean radius of the piping item based on nominal outside radius and nominal wall thickness R min = mean radius of the piping item based on nominal outside radius and t min s = stress range due to cyclic loading s 0 = stress range due to cyclic loading based on the design basis geometry of the piping item 1 Copyright © 2006 by ASME = predicted distribution of wall thickness at the end of the evaluation period t p,min = minimum predicted wall thickness at the end of the evaluation period y = factor required by the applicable piping Construction Code in the calculation of t min , and is equal to 0.4 Z min = predicted minimum section modulus of the thinned section of pipe δ = nominal distance between the center of the pipe and the neutral axis of the thinned pipe section σ