Abstract
Pressure cycles in a hydrogen environment can seriously affect the fatigue life of pressure swing absorbers (PSAs). PSAs are typically designed for a 20 to 30-year life and by the time the intended design life is approaching, many operators are challenged with making decisions as to whether they need to purchase new vessels or explore whether or not life extension is a viable option.
In many cases, towards the end of design life or during midlife (50–60% of design life), operators elect to conduct a life extension study that includes a Level 3 Fitness for Service assessment and Finite Element Analysis (FEA) to evaluate the areas of high stress where fatigue cracks might initiate. The outcome from the FEA is used to develop an inspection strategy. In most cases, it will show that the areas of high stress are around the inlet and outlet nozzles. However, most PSA vessels are well fabricated with nozzles or manways having integral reinforcement and blended nozzle welds, making these areas less susceptible to crack initiation.
In the authors’ experience, the real integrity concern is weld peaking in the long seams and/or weld misalignment, and this will not be highlighted by an FEA made from design drawings. Therefore, this paper examines the effect on weld peaking at the longitudinal seams on the fatigue life. However, other potential crack locations are at internal attachment welds, for example, where the inlet screen is welded to the bottom head and these should in general also be considered when inspection strategies and integrity management plans are developed for PSA vessels.
The work described in this paper highlights how it is possible to operate PSAs beyond design life by developing inspection plan based on a fracture mechanics approach and basing the underlying FEA stress analysis on laser scan data to capture the level of weld peaking in the long seam welds.
