Sulfide Stress Cracking of column overhead pipe to flange fitting joints in a petroleum industry

Abstract

The light hydrocarbon containment leakage in a petroleum industry overhead piping have caused severe fire explosion. This paper reports Sulfide Stress Cracking (SSC) in an overhead piping of splitter unit which separates out lighter components from heavier hydrocarbon. The failed overhead piping has been analyzed to determine the root cause of fire incident. The piping metallurgy was SS316L and chemistry of failed fitting components & its weld have been complied with NACE specification. Initially fittings were evaluated for intergranular corrosion to determine as-received material condition and confirmed defect free material. However, metallographic evaluation clearly evidenced transgranular brittle mode of cracking while hardness was higher than NACE specification (22HRC). Furthermore, virgin pipe joint was compared with damaged pipe joint for microstructural assessment. Moreover, actual service conditions were evaluated to obtain pH and H2S concentration in overhead condensed boot water. Microstructural factors have been discussed in detail as well. The results revealed that cold deformed austenitic structure attributed to SSC and undesired manufacturing steps were employed. Further, the root cause of an accident investigation also revealed failed components were not according to NACE specified metallurgy for sour service conditions.