Revamps 2022 Issue

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

Annette Karstensen Becht Inc.

C an 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 manage- ment and 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 deci- sions 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 pos- sible to potentially extend life by developing safe inspec- tion 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 con- duct 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 misalign- ment, and this 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 loca- tions have also been known to develop fatigue cracks in the past and need to be considered when developing an inspection plan for these vessels. Local or global peaking API 579¹ recommends measuring the peaking using a tem- plate across the seam weld. This will result in measuring the deformation locally across the longitudinal seam weld to obtain the local peaking. However, in Becht’s experience, the deformation often seen in the PSA can vary between local and global peaking, and calculating the associated stress will yield significantly different results. Local peaking is well captured by using a template across the seam weld; however, ‘global peaking’ or out-of-roundness will not be captured by this method. The issue with peaking or out-of-roundness is the inherent bending stress that occurs due to the force misalignment. This bending stress needs to be consid- ered in addition to the membrane stress when complet- ing an integrity assessment. API 579 provides analytical closed-form solutions to calculate the bending compo- nent as a function of the peaking height based on local

Shell with imperfections

2 θ p

Shell with imperfections

Shell without imperfections

(a) Local peaking - cylinder and sphere

(b) Global peaking - cylindrical shells only

Figure 1 Local and global peaking as per API 579

Revamps 2022