Considerations of Stricter TOFD Acceptance Criteria for Severe Refinery Services

ASME Code Case 2235 and the new adoption of this Code Case into ASME Code Section VIII, Div. 2 has acceptance criteria which were based on predicted flaws and fracture mechanics modeled after the nuclear industry. These criteria were developed in response to a query about how to apply the acceptance criteria of ASME Article 4, Appendix 12 when using non-amplitude based ultrasonic methods such as phased array and time-of-flight diffraction. Since the traditional acceptance criteria were based on amplitude, they could not be applied for these alternative methods. The Code committees responded with a flaw size acceptance criteria based on the fracture mechanics properties of the materials and service conditions found in the nuclear industry. This was a major improvement and added a technical basis for the criteria lacking in past standards, however, there are no limitations or qualifications in CC 2235 on its applicability to other materials or service conditions. This is a concern for some oil refining and other plant services, especially those leading to embrittlement, various types of hydrogen-induced damage or high cycle fatigue cracking. The CC 2235 criteria were also not adequate for the recent major industry problem with reheat cracking in 2 1/4 Cr-1 Mo-V reactor fabrication. This paper summarizes the basis for CC 2235, describes the concerns with applying the acceptance criteria without consideration of material or service conditions, and suggests how to approach the issue from a better informed perspective and in some cases, establish stricter maximum flaw sizes.