Life Management of PSA Vessels – Part 3: Life Management Plan and Inspection Strategy
This is the third and final part of the blog trilogy relating to life management of pressure swing absorber (PSA) vessels. In this article, we will explain how to develop a life management plan and an inspection strategy based on fatigue analysis.
In Part 1, we explored the significance of measuring longitudinal weld peaking in PSA vessels and its impact on stress and fatigue life. Accurate measurements using methods like laser scanning or manual templates were discussed, highlighting how weld anomalies can dramatically reduce fatigue performance.
In Part 2, the focus shifted to Non-Destructive Testing (NDT) techniques for identifying critical defects, especially in weld seams. We emphasised the importance of high-quality inspections, the correlation between detectable defect sizes and remaining life, and the necessity of advanced techniques like ultrasonic testing to ensure reliable measurements are used to establish inspection intervals.
Fatigue Assessment Summary
As outlined previously, the fatigue assessment is carried out in two phases:
- Fatigue screening analysis employing the ASME VIII Div 2 assessment of welded joint approach. This analysis is used to evaluate the risk for when (or if) a fatigue crack would initiate. In this assessment we determine the design and the mean fatigue life. This is then compared to the current pressure cycles the vessel has experienced to calculate the percentage of consumed life.
- Fatigue crack growth analysis determining the number of cycles from a reference flaw to the final “critical” crack size. This crack is the maximum crack that is assumed an inspector could miss during an inspection (95% confidence level). This analysis is used to establish the inspection interval.
Once the fatigue analysis has been completed, the future inspection strategy and life management plan can be developed. While we know there can be significant differences between the stress level for each vessel’s longitudinal seam welds due to the weld peaking and/or misalignment, we would typically assume the nozzles for all vessels have the same stress level. This means we would capture the stress distribution at critical locations—including the inlet and outlet nozzles and the skirt-to-shell weld—using finite element analysis based on vessel drawings. The stress distribution then informs the fatigue analysis of these locations.
Risk Ranking
We start by identifying the risk level for each vessel.
Figure 1 shows an example of the results from a screening analysis of 6 PSA vessels. Following Table 1, it can be seen that two vessels (A and D) have consumed the calculated design life but are still 50% below the mean life. This indicates these two vessels are at medium risk, while the other vessels remain at low risk of initiating fatigue cracking. The vessel with least consumed fatigue life (Vessel E) will reach the end of design life in 5 years, at which point all vessels will be at medium risk of developing fatigue cracking.
Inspection Intervals
The inspection intervals are determined based on the time it takes for an initial assumed reference flaw to grow to become unacceptable in accordance with a Level 2 failure assessment diagram approach (“critical” flaw size). For the initial reference flaw, see Part 2 of the blog. For the current assessment, we assume a reference flaw of 1.5 mm in height and 10 mm in length on the ID of the longitudinal seam welds, while for the nozzle welds a 2 x 15 mm initial crack size was used. The skirt-to-shell welds have been omitted as the stresses are very low.
The diagram shown in Figure 2 represents the time for the reference flaw to reach the critical flaw size.
Life Management Plan
For the life management plan, we will focus our attention on the longitudinal seam welds being the critical locations; however, the nozzles could follow the same strategy. Considering the last inspection, which in our case was an ultrasonic inspection from the OD, the inspection strategy is established based on half-life from the calculated life as provided in Figure 2. The recommendations for future inspection are provided in Table 2.
It should be noted if defects are detected in the welds, the crack growth analysis and detectable defect limit should be employed to revise future inspections.
Once inspection commences, it is recommended to inspect 100% of long-seam welds and to spot-check circumferential seams as well.
Conclusion
The inspection strategy and risk ranking should be based on the consumed fatigue life from fatigue screening, while inspection intervals should follow the calculated fatigue crack growth analysis. While the crack growth analysis is based on conservative crack growth parameters suitable for welds in a hydrogen environment, it is recommended to apply a half-life inspection frequency approach.
Ensure the longevity of your PSA vessels with a tailored life management plan. Contact Becht today to discuss your inspection strategy.
