On the Required Distance to Major Structural Discontinuity From Local Metal Loss Area

Abstract API 579-1/ASME FFS-1 Fitness for Service Part 5 local metal loss Level 2 assessment requires a sufficient distance between local metal loss area and neighboring major structural discontinuity based on S.Timoshenko’s model [1] [2]. The model assumed a concentrated force and moments on a cylindrical shell. A sufficient distance to settle down the bending moment from the point was evaluated. Occasionally, a flaw is observed around a major structural discontinuity, and the required distance limits the application of Lv.2 assessment. While the model would be well applicable for piping nozzles and supporting attachments that takes external force and moment, some of those limiting components, such as stiffeners, vessel heads and manway nozzles in kind, are not taking any external forces and moments. They might affect to the local metal loss area in a different manner than piping nozzles and such. In 2023 ASME PVP, S.Bouse et al. [2] discussed the effect of ellipsoidal head and hemisphere head in their paper ; PVP 2023-106371 Re-Evaluating the Minimum Required Distance Between Local Thin Area In Cylinder and an Ellipsoidal Head. The conclusion was that ellipsoidal heads was beneficial for local metal loss assessment based on FEM model data. The outcome is incorporated in 2021 version of API579-1/ASME FFS-1 Fitness-For-Service, table 4.2 [3]. To discuss which type of “Major Structural Discontinuity” would be beneficial for local metal loss assessments one step further, it would be important to consider the significance of Folias factor, Mt, and surface correction factor, Ms, of current API579-1/ASME FFS-1 Part 5 assessment. As discussed in our previous paper [4], authors reorganized the significance and derivation of Mt and Ms. Based on the model of Ms, the pressure load working on remaining ligament underneath flaw was equal to flow stress level and the surplus pressure load was traveling to side ligaments. The idea of model indicated RSF would be increased according to the increase of side ligament cross section. If this was true, stiffening ring, manway nozzles and flanges would be a good candidate to exempt from current “Distance to Major Structural Discontinuity”, Lmsd, requirement [3] in addition to ellipsoidal head. Because those components simply exist on cylindrical shells provides extra cross section without any significant external forces and moments on cylindrical shell unlike piping nozzles. In this paper, using FEM models of stiffening rings and ellipsoidal heads as example cases, an interaction mechanism between “Major Structural Discontinuity” and flaw will be discussed considering the concept of Ms model. This would contribute to enhance the “distance to major structural discontinuity” criteria discussion.