A Comparison of Design by Analysis Techniques for Evaluating Nozzle-to-Shell Junctions per ASME Section VIII Division 2

Part 5 of ASME Section VIII Division 2 offers several design by analysis (DBA) techniques for evaluating pressure retaining equipment for Code compliance using detailed computational stress analysis results. These procedures can be used to check components for protection against multiple failure modes, including plastic collapse, local failure, buckling, and cyclic loading. Furthermore, these procedures provide guidance for establishing consistent loading conditions, selecting material properties, developing post-processing techniques, and comparing analysis results to the appropriate acceptance criteria for a given failure mode. In particular, this study investigates the use of these methods for evaluating nozzle-to-shell junctions subjected to internal pressure and nozzle end loads. Specifically, elastic stress analysis, limit load analysis, and elastic-plastic stress analysis are utilized to check for protection against plastic collapse, and computational results for a given load case are compared. Additionally, the twice elastic slope method for evaluating protection against plastic collapse is utilized as an alternate failure criterion to supplement elastic-plastic analysis results. The goal of these comparisons is to highlight the difference between elastic stress checks and the non-linear analysis methodologies outlined in ASME Section VIII Division 2; particularly, the conservatism associated with employing the elastic stress criterion for nozzle end loads compared to limit load and elastic-plastic analysis methodologies is discussed. Finally, commentary on the applicability of performing the Code-mandated check for protection against ratcheting for vessels that do not operate in cyclic service is provided. The intent of this paper is to provide a broad comparison of the available DBA techniques for evaluating the acceptability of nozzle-to-shell junctions subjected to different types of loading for protection against plastic collapse. Predicted deformations and stresses are quantified for each technique using linear and non-linear, three-dimensional finite element analysis (FEA) methodologies.