Document: NRC Regulatory Guide
Document ID: a4944d20-ef6e-480b-a45d-14425bedd2e5
Document Type: regulatory_guide
Title: ASME Code Cases Not Approved for Use (Rev. 8)
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML2219/ML22196A065.pdf
Revision Date: 2023-05
Chapter: 
Section ID: RG-1.193
CFR Part: 
CFR Title: 

Content:
on and certification program able to be used as an alternative to ASME code Section XI Section IWA-2300. 1/17E N-806-1 Evaluation of Metal Loss in Class 2 and 3 Metallic Piping Buried in a Back-Filled Trench, Section XI, Division 1 The NRC staff has concerns regarding (1) the factors used in Equation 1 of the code case, (2), how the allowable time for examination is calculated, and (3) the safety factor used in Appendix A of the code case, (4) loading considered in Appendix A of the Code Case, and (5) soil stiffness used in Appendix A of the Code Case. 4/15E DG-1408, Page 18 CODE CASE NUMBER TABLE 2 UNACCEPTABLE SECTION XI CODE CASES SUMMARY DATE OR SUPPLEMENT/ EDITION (1) Equation 1 of the code case contains factors to account for uncertainty in predicted rate of metal loss. The code case does not specify the factors. It is not clear how the factors will be derived or obtained. In other ASME code cases, a factor of 2 is applied to the measured corrosion rate to derive the projected corrosion rate for the pipe in question. If the measured corrosion rate cannot be obtained for the pipe in question, in certain NRC-approved relief requests, a factor of 4 is applied to the measured corrosion rate taken from a different pipe at the plant as a substitute to derive the projected corrosion rate for the pipe in question. (2) Paragraph 3.2 states that “The time interval, Tallow, from the start of the evaluation period, for the wall thickness of the metal loss region under analytical evaluation to reach the minimum allowable wall thickness in accordance with the acceptance criteria of this Case shall be determined.” Code Case N-806-1 needs to provide a specific instruction or equation to show how Tallow is calculated. (3) The NRC staff finds that the equations used in Appendix A of the code case for pipe ovality, through wall bending, soil and surface pressure, compressive stress in side walls, buckling due to external pressure, buoyancy and flotation are the same as used