SSC failures in a jacketed reactor at an olefins unit: Cobalt deposits and S contamination

Abstract

The purpose of this study is to analyze the root causes of two failures of similar sections of a thickwalled olefin reactor at a petrochemical plant. The failed sections had to be taken out of servicedue to through-cracks in the neutral line of upper curves. The mechanisms of damage and possiblemitigation methods are investigated. The experimental analyses included chemical, microstructural and mechanical characterization of base material, visual and SEM inspection of samplesfrom the cracked areas, and chemical characterization of inner deposits. Branched, intergranularand mostly axially oriented cracks initiated in the inner surface of the elbows, in contact with thefluid process, under metallic Co deposits which obturated leaks. Elbow materials showed nomicrostructural degradation. The main observation was the presence of sulfur in deposits on theinner surface and inside cracks. The components were affected by an SSC (sulfide stress cracking)mechanism. The elbows are particularly susceptible to the phenomenon, due to manufacturingresidual stresses and operating temperatures. Checking the existence of other cracks in criticalareas of the reactor and replacement of cracked components were the logical recommendations,with some extra care to ensure specified yield strength in the lower limit and ensuring residualstress relief after manufacture of elbows. Most relevant for the operator was defining the origin ofS in the fluid: a testing scheme was applied to determine when and how S was introduced to theprocess current during service, with inconclusive results.