On-Stream Catalyst Replacement Reactors in the Atmospheric Residuum Desulfurization
Unit (ARDS) are provided with 4? dia. X sch. 160 TP321 stainless steel (SS) piping for
draining the reactor inventory during shutdowns. A total of three on-stream failures of
these drain lines caused unit shutdowns and Safety, Health & Environment (SHE) hazards .
Failures were in the form of extensive multi branched cracks in the parent metal/weld/heat
affected zone (HAZ). Failure was the result of exposure to high concentrations of aqueous
chlorides, and stress concentrations due to poor weld quality and mismatch of thickness
between 90o elbows and pipe. High pressures (stress) in the system, high chloride
concentration in the stagnant section of pipe, and low temperatures in the stagnant section of
the pipe aggravated the situation, which resulted in typical Chloride Stress Corrosion
Cracking (SCC) failures.
Keywords: Stress Corrosion Cracking, Austenitic Stainless Steel, Transgranular Cracking, ARDS,
OCR, Weld Joint Mismatch, High Pressure, High Temperature