Failure analysis of high pressure flange in a reactor outlet

A cracked gasket ring and a flange belonging to a ring type joint (RTJ) of hydrocracker unit-2 were sent to the laboratory for investigation. Examination revealed that the failure was by sulphide stress corrosion cracking (SCC). Analysis suggests that most probably, the SCC was promoted in the components because of formation of polythionic acid in the system in presence of O2 and/or H2O. Compositional analysis showed that the ring and the flange were fabricated from SS 347 and SS 321 steels respectively. The materials were found to conform to specification and they were used in annealed condition. No metallurgical abnormalities were found responsible for the failure. A detailed analysis of the failure is presented in this report, and a few recommendations suggested