Revision of Proximity Rules for Subsurface Transforming to Surface Flaws for Class 2 Ferritic Vessels

There is a rule in Acceptance Standards of Class 1 and 2 ferritic vessels in the ASME B&PV Code Section XI describing how subsurface flaws sufficiently near a component surface are to be transformed to surface flaws. This is the flaw-to-surface proximity rule for the subsurface flaws. If the subsurface flaw is located sufficiently close to the component surface, the subsurface flaw may not be acceptable to the standard, but may become acceptable after transforming it to a surface flaw following the current flaw proximity rules. There is considered to be a problem with the flaw proximity and transformation rules, and the region of the acceptance standards where this occurs is referred in this paper as a “discontinuity” in the subsurface-to-surface flaw transformation rule in the current Acceptance Standard for Class 1 and 2 ferritic vessels. Authors had developed a solution for the discontinuity region and proposed the proximity rule for Class 1 vessels at the ASME PVP 2007 conference. The revised Acceptance Standards will be issued in the 2008 Addenda of ASME Code Section XI. However, Acceptance Standards for Class 2 vessels still contain this discontinuity of the proximity rule. In addition, allowable depths in the Acceptance Standards for Class 2 ferritic vessels are too large for thin wall thickness vessels. After the discussion at the ASME Code Working Group on Flaw Evaluation, the proposed proximity rule and allowable flaw depths were revised. This paper describes the revision of new flaw-to-surface proximity rule to minimizing the change of description to the current allowable subsurface flaws of the Class 2 ferritic vessels for the Acceptance Standards.