Inside a refinery’s catalytic cracking unit, an explosion occurred on a layer of gas at ground level. Following a level drop in the decoupling drum of the gas washing process, the manual valve for allowing water to enter opened. The water level rose in the drum due to a deficiency in the drum’s regulation control chain before the manual valve could be closed. To accelerate drainage, the drum’s regulation valve bypass towards the flare was opened even though the bleed valve of this same drum had remained open without supervision. The butane contained in the reflux drum then freely entered the flare decoupling drum and ultimately reached the sewer, forming a layer of gas at ground level. A flash was quickly triggered and the technician, caught in the explosion, was killed.

The accident led to a temporary shutdown of refinery installations. The origin of this flash may have been the lamp used by the technician, given that the accident happened at night. The shock of the valve square driver on a piece of machinery or ignition of the gas cloud on the slurry reboiler (reputed as the catalytic cracking unit’s heaviest liquid effluent) at 325°C, adjacent to the drum and sewer, are two othe r possible causes. The C4 distillate cut had contained a high concentration of trans-butene-2, whose self-ignition temperature lies below 320°C.