Development of Code-Specific RSFa for Use in API 579-1/ASME FFS-1 Assessments

Abstract

Abstract For metal loss assessments in the 2021 edition of API 579-1/ASME FFS-1 (hereinafter “API 579”), the Remaining Strength Factor (RSF) is “utilized to define the acceptability of a component for continued service.” The RSF is defined as the ratio of the plastic collapse load of the damaged component divided by that of the undamaged component. For metal loss assessments, the RSF is a function of the corroded geometry, as evidenced in the equations in Part 5 of API 579 and as documented in WRC 505. If the calculated RSF is less than an allowable RSF (RSFa), then the MAWP of the component must be reduced by the ratio of RSF/RSFa; otherwise, the component is acceptable at the undamaged MAWP. The recommended value for RSFa in API 579 is 0.90. The 2007 edition of API 579 included Table 2.3 which provided recommended RSFa values based on the construction code, although the value of RSFa was listed as 0.90 for all codes in the table; the inclusion of this table (which was subsequently removed in the 2016 edition) was ostensibly for the purpose of eventually providing different values for different construction codes. To provide different values of RSFa, a permissible stress value distinct from the construction codes is needed, such that the RSFa for each construction code could be adjusted to provide a constant margin against burst pressure. This approach does not eliminate the possibility of including penalizing factors for certain construction details and/or codes which have less-rigorous inspection requirements or for similar reasons. In a prior ASME PVP paper (PVP2022-84710), a generic equation was introduced to allow for quantification of the likelihood of ductile failure as a function of applied stress based on the semi-log assumption in API 581 (i.e., in the section related to pressure relief devices). Leveraging this approach, this study introduces an equation for a code-specific RSFa which allows for a constant margin against burst pressure for different construction codes. Comparison of the proposed RSFa definition across different construction codes is illustrated with an example.