Technical Basis for Alternate Successive Inspection Requirements for Vessels and Piping Welds as Prescribed in Code Cases N-526 and N-735

Flaws that are detected during in-service inspections of Class 1 and 2 components (vessels and piping) are first compared to the acceptance standards in ASME Code Section XI, Subsection IWB-3500. When the acceptance standards are met, no further action is required and the component is returned to service. If the acceptance standards are not met, IWB-3600 allows an analytical evaluation to determine the suitability for continued operation. When the provisions of IWB-3600 are used to return a flawed component to service, the Code provides augmented requirements for successive and additional examination of the flawed location. These inspections are believed to be useful for cases where the flaw is service-induced, particularly for surface flaws. However, the usefulness of these augmented examinations for subsurface flaws, which have been clearly shown to be fabrication related, is very questionable. Many of the inspections for these subsurface flaws have not shown any growth and none has resulted in pressure boundary leakage. Section XI currently provides surface proximity rules for discriminating surface from subsurface indications, based on elastic calculations of the stress intensity factor, K_I. However, it is believed that these rules may not be sufficiently conservative for granting of an exemption from the requirement for successive examinations, because of the possibility of yielding of the ligament between the flaw and the inside surface. Code Case N-526 provides alternate more conservative proximity criteria for distinguishing subsurface from surface defects in Class 1 and 2 vessels, while Code Case N-735 provides similar criteria for Class 1 and 2 piping welds, in order to eliminate the need for the successive inspection requirements in these components. The technical basis for the criteria in both Code Cases has its foundation on an approach that determines how close a flaw can be to a free surface before the stress in the remaining ligament exceeds the material yield or flow stress, which would potentially put it at risk for rupture of the remaining ligament.