For a super-critical thermal power plant,the main steam pipe is designed with ASTM A335 Grade P91 material. The pipe size selected is about 14", but for the high pressure requirement, Schedule 160 pipe wall is not adequate. So we have specified a controlled inside diameter pipe of 356mm ID and a minimum wall thickness of 66mm, without any negative tolerance.

The tolerance on pipe diameter, wall thickness and weight are controlled by ASTM A335 and A999. We have contacted US pipe fabricators and they have told us for controlled ID pipes with minimum wall thickness with zero negative tolerance, the positive tolerance on wall thickness may be as high as +28% on the specified minimum wall. This is far in excess of (-)12.5% minimum wall over nominal wall of an outside diameter controlled ASME B36.10 standard steel pipes, for which we always divide the Code calcated minimum wall thickness by 0.875 to arrive at nominal wall and use that in stress analysis.

ASME B31.1, mandatory Appendix-D calculates all flexibility factors and SIF on the basis of nominal dimensions - diameter and wall thickness. So how do we specify diameter and wall thickness for this controlled ID, minimum wall pipe in Caesar-II correctly?