Life or Death of the PSA
Weld peaking or out-of-roundness affecting PSAs is evaluated, highlighting the value of conducting peaking measurements to determine a PSA’s inherent stress concentration. Below is an excerpt from the article.
This article was published in the Revamps Issue of PTQ 2022
Can the life of a pressure swing absorber (PSA) be extended beyond its intended design life, and how do you manage the PSA’s ‘midlife crisis’? PSAs are typically designed for a 20 to 30-year life with pressure cycling in a hydrogen environment. What can possibly go wrong? Quite a lot! But with appropriate integrity management and an inspection strategy in place, a lot can go right too, and it is not uncommon that these vessels can survive well in advance of the original design life.
By the time the intended design life is approaching, many operators are challenged with having to make decisions regarding life extension or purchasing new vessels. While it is known that some PSA vessels do crack, the work described in this context highlights how it is possible to potentially extend life by developing safe inspection intervals based on a fracture mechanics approach.
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 Becht’s experience, the real integrity concern is weld peaking in the long seams and/or weld misalignment – which will not be highlighted by an FEA made from design drawings. Other potential crack locations are at internal attachment welds, for example, where the inlet screen is welded to the bottom head. These locations have also been known to develop fatigue cracks in the past and need to be considered when developing an inspection plan for these vessels.
A life assessment of a PSA vessel has been carried out to analyze the effect of peaking measurements with the intent of developing a life management plan that helps the future safe operation of the PSA vessels and minimizes the need for internal inspection. The full study can be read in the Revamps issue. Based on the outcome of the analysis covered in the study, the recommended steps for the life management and inspection strategy are suggested as follows:
- At half design life, review fabrication tolerances for peaking, misalignment, and plate tolerances. Review construction records to determine if the vessels have been subjected to rolling after fabrication, and conduct peaking measurement, preferably by laser scanning, on the weld seams (including circumferential welds).
- Determine the stress associated with peaking and other anomalies using stress analysis, including the stress associated with the inlet/outlet nozzle and manway.
- Use an appropriate fatigue assessment method to determine the consumed life fraction based on the stress profile.
- Conduct advanced ultrasonic inspections on welds (longitudinal and circumferential) and conduct external surface inspections on the nozzle and manway welds.
- As the consumed life fraction approaches 80% (Step 3), conduct a fracture mechanics-based fatigue analysis based on the stress profile from the laser scan and the FEA, and crack-growth parameters that include the effect of hydrogen charging. Use the half-life (i.e. remaining crack growth life divided by two) to determine the future inspection frequency.
- If cracks are discovered during the inspection (Step 4), employ a fracture mechanics approach to estimate remaining fatigue life and apply a half-life inspection strategy to determine when vessels should be taken out of service for repair.
If you would like more information on PSA’s design life assessment, feel free to contact Becht.
