Fitness For Service Evaluation Research Articles

Fitness for service assessment of a pressure vessel subjected to fire damage in a refinery unit

Abstract The fire-damaged pressure vessel reactor, evaluated in this study, was a cylindrical vessel with design conditions of 460 psig (3.17 MPa) at 413 °C. The fitness for service (FFS) of the fire-damaged pressure vessel was assessed according to API 579-1/ASME FFS-1. First, the maximum temperature during the fire accident was determined based on deterioration of outer protective layer. Then, the possible damage was examined by hardness test, in-situ field metallography and metallographic replicas. Grain growth and spheroidized pearlite were observed in the base metal at the inner surface of the vessel. Outer surface of the vessel showed ferritic microstructure with smaller grain size as compared to the inner surface. Decarburizing and carbide formation were also visible at the outer surface. The lower hardness of the base metal at both sides of the vessel was compared to the standard value and related to the microstructure evolution during high temperature/fire exposure condition. The results of the FFS evaluation indicated that the vessel is not suitable for the current design conditions, and therefore a new maximum allowable working pressure was determined.

Fitness for Service Evaluation of Ring Joint Groove Cracking

Cracking of ring joint style flanges has been a recurring problem in the petrochemical industry, particularly in high-pressure hydrogen processing vessels. The cracking of the ring joint groove region is an inherent problem with the design. The ring groove is subjected to high stresses from the wedging action of the gasket and the ring groove radii are not substantial enough to effectively reduce the stress concentrating effect. A fitness for service assessment was conducted for a hydrogen processing vessel containing cracks in the ring groove radius region of a ring joint style flange. The flange was forged 2-1/4Cr-1Mo material with a Type 347 SS overlay. Results of an elastic-plastic numerical fracture mechanics assessment showed that the driving force for crack propagation was high for a very localized region near the ring groove radius. However, the driving force decreased significantly for deeper cracks as the crack tip became removed from the very localized high stress region of the ring groove. The assessment also showed that the highest stresses occurred during the bolt-up operation. Metallurgical tests were performed on a small sample removed from the flange. The chemistry, grain size, microstructure, and hardness of the material indicated that the probability that this material had very low resistance to hydrogen-assisted crack growth during downtime was quite small and that the resistance to crack advance during service was good. Therefore, the combined results of the fracture assessment and metallurgical testing were used to justify continued operation without repair of the cracks present in the flange. [S0094-9930(00)01901-6]