At around 2:00 p.m., a crack was discovered on a pipe at a refinery’s sulphur production unit. The unit was immediately cooled with nitrogen. Just before the crack was found, the temperature of the pipe had suddenly risen to 535 °C for a design temperature of 190 °C. The sulphur unit was in the process of being shut down. The furnace upstream of the pipe was burning natural gas in order to neutralise the trace amounts of sulphur remaining in the circuit (operating fluid charged with hydrogen sulphide). An excess of burning oxygen caused an exothermic reaction with the residual sulphur in the unit. The crack formed downstream of the measurement sensor flange. The unit was being shut down for scheduled maintenance (the process started at 9:27 a.m.).The gas carried in the pipe consisted of carbon dioxide, nitrogen, oxygen and trace amounts of hydrogen sulphide.

The cracked pipe was inspected and the damaged section was replaced. The assessment found that the crack was caused by localised corrosion on the pipe’s inner surface, most likely induced by condensation. This condensation may have formed during a period of downtime conducive to the introduction of moisture and the presence of oxygen. Hydrogen-induced fatigue and damage were ruled out and no metallurgical changes in the materials due to increases in the pipe’s temperature were found. The inspection plan was revised. The procedure for shutting down the unit was revised to better control the air flow rate to the furnace.