Assessment Procedure for Multiple Volumetric Flaws (LTAs) in p-M Diagram
Assessment of multiple volumetric flaws (LTAs) is one of the most common problems relating to pressure vessels and piping components. Under the current Fitness for Service (FFS) rules, such as ASME, BS and so on, multiple volumetric flaws are usually recharacterized as an enveloping volumetric flaw (defined as a single larger volumetric flaw) as well as multiple crack-like flaws, following their assessment rules. However, the rules proposed in their codes will not often agree and their justification is unknown. Furthermore, they can provide unrealistic assessment in some cases. In this paper, the interaction between two different sized non-aligned volumetric flaws such as local thin areas (LTAs) is clarified by applying the body force method. Unlike multiple crack-like flaws, the effect of biaxial stresses on the interaction is evident. Based on the interaction which indicates the magnification and shielding effects and reference stress solutions, a new procedure for multiple volumetric flaws is proposed for assessing the flaws in the p-M (pressure-moment) Diagram, which is a simple assessment procedure for vessels with LTAs.