Implicit safety margins in the ASME code acceptance criteria for flaws in austenitic piping

The Main Committee of the ASME Code recently approved evaluation procedures and acceptance criteria for flaws in austenitic piping. These revisions will appear in the Winter 1983 Addenda to Section XI of the ASME Code. The allowable flaw sizes were specified in IWB-364-1 as a function of applied stress and loading category (e.g., normal and upset, emergency and faulted conditions). The allowable flaw sizes were intended to provide a minimum safety margin of 2.77 for normal and upset conditions and 1.39 for emergency and faulted conditions. Recently, concerns have been expressed on whether the Code allowable flaw sizes do indeed provide the intended safety margins. The purpose of this is to demonstrate the technical basis for the acceptance criteria and confirm the Code margins by considering selected examples