(33c) ADVANCES IN PSA VESSEL LIFE MANAGEMENT

Pressure Swing Adsorbers (PSA) are pressure equipment in cyclic service that are used at hydrogen plants for the purification of hydrogen. Due to the inherent nature of cyclic service the PSA vessels have a limiting fatigue life and are typically designed for 20 years or less. In many cases, towards the end of the original design life or during mid-life (50-60% of design life), operators will elect to conduct a life extension study. Usually, the focus is on evaluating the areas of high cyclic stress where fatigue cracks could plausibly grow to the point of fracturing the pressure boundary. The quantitative portion of this life extension study includes a Level 3 Fitness for Service assessment and Finite Element Analysis (FEA). The key outputs from the assessment, minimum reliably detected planar flaw sizes and estimated time intervals for cyclic crack growth to failure, are used to help develop or refine an inspection strategy. Often online inspections are conducted, however inspection during a turnaround or planned outage is an option.

It is not uncommon for PSA vessels to develop cracks during their lifetime. Several locations have been identified as critical, including internal weld attachment points and longitudinal seams welds. This paper is concerned with the analysis of the longitudinal seam welds. There are several factors associated with the fatigue life of PSA vessels; some of these are straightforward such as pressure cycle range, cycles per year, and design pressure - others are not. One of the more nuanced considerations is local stress concentrations associated with shell geometric anomalies, especially at the longitudinal seam welds. Non-ideal geometries such as shell ovality, hi-lo misalignment, and weld peaking are common considerations. To address this, field measurements are typically conducted to measure actual dimensions, which can then be used as a basis for creating the PSA vessel geometry in finite element models. Geometric anomalies and their deleterious effects tend to be more pronounced in vessels fabricated before modern best practices were adopted. A second consideration is fatigue crack growth associated with pressure cycles in a hydrogen environment, which can seriously affect the fatigue life. Laboratory material testing research focused on cyclic crack growth and fracture continues for this specialized topic.

The work described in this paper highlights how it is possible to operate PSAs beyond the original design life by implementing an inspection program that is based, in part, on a combination of laser scanning, other field measurement techniques, finite element analysis, and fracture mechanics. This paper will cover initial considerations before starting a life extension study through final implementation.