Technical Basis for Extension of Section XI Appendix C Pipe Flaw Evaluation Procedures to Pipe Diameters Less Than NPS 4

Abstract

A lower limit restriction on nominal pipe size (NPS) was originally set as NPS 4 (DN 100) in ASME Section XI, Appendix C pipe flaw evaluation procedures (1983 Edition). Pipe less than NPS 4 that contained flaws could not be evaluated to the code and had to be repaired or a relief request submitted. The primary basis for this analysis restriction was that stress analysis information for small diameter lines required for flaw evaluation would be limited for a detailed flaw evaluation. The plant owner would opt to repair such flawed pipe rather than perform an evaluation. However, current evaluation procedures are generally applicability to smaller pipe sizes. Because ASME Appendix C procedures are referenced in code cases that cover small diameter pipe, and because small diameter pipe can be difficult to repair due to cost and radiation exposure, a proposed revision to ASME Section XI has extended Appendix C procedures to pipe sizes down to NPS 1 (DN 25). This improvement is now in the 2013 Edition of Section XI and gives the plant owner the option for evaluation for such situations.