A rupture disk opened at 5 pm on the installation for treating stack effluents of a chemical plant; a gas with a high content of hydrogen sulphide (H2S) / mercaptans (RSH) was released into the atmosphere.

The facility’s IOP was activated and a crisis centre set up in order to disseminate information to all individuals potentially exposed. One of the units and a neighbouring industrial site were confined for 25 min. Two of the eight detectors placed in the environment measured an H2S concentration of 18 ppm for 4 min. Given that plant technicians had shut down the production facilities before the incident, the amount of H2S emitted was limited to approx. 30 kg. The IOP was lifted at 6:05 pm.

During the accident, the stack effluent processing unit received gas from only one unit (the one emitting the least H2S). Following the detection of a leak in the upstream compressor seal of the processing furnace (thermal oxidiser), the technician switched to backup mode and directed the gas flow, as well as the flow of gases leaving the oxidiser, where treatment was to be finalised using a potash washing column. To accomplish this, the technicians closed the valve fitted to the discharge chimney and tried, though unsuccessfully, to open the valve placed upstream of the backup column. With both valves closed, pressure increased inside the furnace, leading to rupture of the safety disk in less than 5 min.

Several corrective measures were adopted after the incident: installation of automatic controls between the two valves, reinforcement of controls on the valve that had remained blocked during the incident, etc.