Fundamental Consideration of Allowable Flaw Lengths Given by the ASME Code Section XI Using Flat Plates Under Tensile Loading

Abstract When flaws are detected in power plants, the flaws are evaluated for their impact on the integrity of the components. There are three conditions imposed on the flawed components to ascertain safe operation. The first condition is that the applied stress should be less than the allowable stress. The second condition is the allowable flaw depth to prevent coolant leakage from the pressurized pipes. The ASME Code Section XI provides that the allowable flaw depth should be less than 75% of pipe wall thickness, even if the first condition is satisfied. The third condition is the allowable flaw length to prevent guillotine break for circumferential flaw, or to prevent split fracture for axial flaw. The allowable flaw length is the length at which the through-wall flawed piping fails due to the applied stress. Therefore, the current allowable lengths are irrespective of flaw depths. However, the failure stress for shallow flaw is higher than that for through-wall flaw, as well as the elongation for shallow flaw is longer than that for through-wall flaw of a plate subjected to tensile loading. Besides, when the length for the shallow flaw is greater than the allowable length determined by through-wall flaw, the shallow flaw is not acceptable. The current third condition of the allowable flaw length is inconsistent in that it does not allow for shallow and long flaws that are not actually detrimental for piping integrity. This paper examines the characteristics of flaw lengths using a flat plate model with surface flaws, and proposes a new methodology for determining the allowable flaw lengths.