ASME Section III Treatment of Stress Distribution in Cylindrical Vessels With Symmetric Thin-Walled Discontinuity

In equipment design, the designer often attempts to minimize cost and maximize performance while meeting the design specification requirements. In the Power Industry, for example, a significant portion of the design requirements are user defined and the rest are dictated by Codes and Standards. In most applications such as tanks, valves, and pumps, the designers are familiar with the technical issues and are able to meet them. In some cases, however, balancing the two requirements (i.e., the user defined requirements and the ASME Codes and Standards) is complex. An example of this challenge can be found in the design and analysis of a pressurized cylindrical vessel that allows the user to impose a “weak-link” section that under certain operating scenarios can cause the vessel to rupture or fail. In such a case, the designers have to address the discontinuity that is imposed on the structure while still meeting the ASME required stress criteria for structural integrity. The term “weak-link” is used herein to identify the subcomponent or subsystem which is most likely to fail under a postulated operating condition. The loading condition can be either internal or external. In internal loading, the loading is generally an overpressure excursion. In the external case, the load can be seismically induced or an applied actuator load for operation. This paper presents a case study of internally induced loading condition in an ASME Section III cylindrical pressurized vessel where the cylinder is required to fail or rupture only when it is impacted by an actuating load. In this case, the ASME Section III requirements for pressure integrity are combined with the failure mechanism of a symmetrical discontinuity in the design and analysis to ensure that the operational intent of the valve is met. The challenge for this design is two fold; (a) ensure that the device maintains structural integrity without any leakage during normal operation, and (b) guarantee vessel rupture and relieve pressure predictably. This paper presents the ASME treatment of such a design using finite element analysis (FEA).