Fatigue and Fitness for Service evaluations were performed on pressure swing adsorption (PSA) vessels that were in service at a waste facility – but had not been performed as required by the ASME Boiler & Pressure Vessel Code.
Since the vessels had been in service and sustained an undetermined amount of fatigue damage, a fitness-for-service evaluation was conducted. The fitness-for-service evaluation of the vessels was performed in accordance with API 579-1/ASME FFS-1, “Fitness For Service” and included: (i) an on-site inspection of the vessels to determine their current condition, (ii) measurements of the thickness of the vessels’ shell and heads, (iii) collection of the operating history and intended future operation of the vessels, and (iv) a fatigue evaluation.
The methodology and acceptance criteria for performing fatigue evaluations in accordance with API 579-1/ASME FFS-1 are identical to Section VIII, Division 2 of the ASME Boiler & Pressure Vessel Code. The vessels were evaluated for pressure cycling. Temperature changes in the vessels’ shell and heads due to thermal cycling were determined to insignificant.
Finite element analysis determined stress distribution in the vessels and their components. The results from the FEA were used to calculate fatigue design life for the vessels. The fatigue calculations included metallurgical notch effects and stress concentrations.
Fatigue life was calculated using the design fatigue life curves contained in both the ASME Boiler & Pressure Vessel Code and API 579-1/ASME FFS-1. These curves include factors of safety on both the magnitude of alternating stress and the number of cycles.
The results determined that vessel remediation was required. Remediation options included: 1) additional weld inspection, 2) grinding the toe of fillet welds, or 3) replacing the vessels.
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