FLAW SIZE ACCEPTANCE LIMITS FOR A STAINLESS STEEL PRESSURE VESSEL

The ASME Boiler and Pressure Vessel Code Section XI provides flaw size acceptance standards for ferritic steel pressure vessels. Section XI Table IWB-3510-1 presents allowable flaw size limits in terms of flaw depth, length and vessel thickness. These flaw size limits are based on linear elastic fracture mechanics calculations that assume a brittle fracture failure mode. As yet, no allowable flaw size standards are provided in Section XI for stainless steel reactor or non-reactor pressure vessels. This paper presents allowable flaw size limits for a stainless steel pressure vessel. These limits were based on elastic plastic fracture mechanics analyses that considered limit load and ductile tearing failure modes. Although the flaw acceptance levels were developed for a specific stainless steel vessel, insights gained from this work may be useful in a general methodology for ASME Code purposes. Tabulated flaw size acceptance levels, for several aspect ratios and inspection intervals, are presented for the axial shell welds. Results show the axial seam welds were the most flaw sensitive of the various welds analyzed. The acceptable flaw sizes were limited by the ductile tearing failure mode.