At around 9:00 p.m., phosgene and chlorobenzene began leaking inside the containment of a chlorinated chemicals plant. The leak was detected by analysers inside the containment. The analysers’ control system automatically directed the air inside the bubble towards the caustic scrubber. A larger leak was detected at 12:20 a.m. Technicians placed the circuit in a safe state by shutting off the pump and isolating the containment, but not before 400 kg of phosgene and 600 kg of chlorobenzene spread inside the containment. There was a noticeable odour near the containment, but the analysers did not detect anything. At 2:00 a.m., the technicians began cleaning the containment by injecting air inside the scrubber. None of the analysers in the surrounding production facilities and labs detected any phosgene despite the fact that the technicians could smell something in the air.
The initial leak was caused by a leak on the pressure sensor of a pump and a leak on a flange seal up line of a flow meter connected to this pump. This seal had been replaced shortly before the accident. It seems that a problem occurred when the new seal was fitted. The bypass valve in the scrubber was also leaking. A portion of the gases to be treated by the scrubber had been directly discharged to the flue-gas stack. Two of the three phosgene analysers in the stack did not detect any phosgene. The 2oo3 voting logic did not allow the technicians to adjust the flow of phosgene sent to the stack. The inspection hatches on the containment’s valves also leaked, making total containment impossible. Before the accident, these valves had been opened for maintenance. However, their seals, ordered from the maintenance department, had not yet been repaired.
After the accident, the operator replaced all its defective equipment and implemented corrective measures. It commissioned an audit of its facility’ safety devices. It instituted an annual leak test of the containment. The closure effectiveness of the valves will also be investigated. The operator revised the gas alert procedures. It created a seal opening checklist for workers performing sealing operations. Outside contractors were reminded of the safety rules to be followed. The rules on verifying the training of outside contractors and sealing operations have been made more stringent. Operating procedures have been defined. The number of people authorised to assess workers was increased.