Fitness for Service Evaluation of Ring Joint Groove Cracking

Abstract

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]