A Case Study for Justifying an Attachment Weld and Pad for a Seal Membrane on Pressure Vessels

Abstract Seal membranes are effective as the pressure-retaining components between the body of a ferritic vessel body and its end closures. To provide suitable attachment points, stainless steel attachment pads may be built-up using layers of weld-deposited material. This paper presents a case study where the initial defect tolerance assessments of an attachment pad and the attachment weld found reserve factors below target against credible postulated defects caused by the welding processes. To demonstrate the acceptability of the design, additional evidence was presented under four themes as per safety justification process in the UK: good design and manufacture; functional testing; failure analysis and forewarning of failure. A key focus was demonstrating that the postulated defects would not grow into the ferritic vessel or end closure. This was achieved by demonstrating that either the crack propagation direction was away from the ferritic component or that the postulated defect would not grow sufficiently to reach it by the end of the component’s life. In addition, a deep hole drilling residual stress measurement technique was used in laboratory testing of a representative section of the attachment pad. This demonstrated that the level of weld residual stress used in the analysis could be significantly reduced. Additional assurance is provided by defence-in-depth considerations. These include a manufacturing assurance inspection, an in-service inspection regime, the inspection of the welded pad of a vessel removed from service, considerations of the consequences of a breach in the sealing region, hydrostatic testing, and the likelihood of the postulated defects.