At around 7:10 pm, a supervisor at a chemical plant detected a leak that was forming a vapour cloud on a rack above storage tanks filled with hydrochloric acid 33%, a corrosive and irritant. He issued the gas alert from the control room. A few litres of acid flowed into the bund before being neutralised. Two technicians turned off the recirculation pump and closed the storage tanks’ valves. The leak, which started on a pressure transmitter, was stopped. The alert was cancelled at 7:22 p.m. As the event was unfolding, an emergency response unit was set up and stakeholders (town halls, prefecture, train station and the plant’s partners) were immediately alerted by phone. A press release was written.

An assessment revealed that one flange bolt was missing and another one was loose. The leak on the flanges was caused by thermally induced creep of the seal on the pipe clamps between two fixed points. These phenomena irreversibly distorted the seal, allowing the acid to leak out of the pipe. This expansion also loosened the bolts seen during the assessment.

After the incident, the top clamp was removed and replaced by a longitudinal guide (tightened U-clamp) to limit horizontal movements created by potential external stresses and vibrations of the pipe. The other flanges in the area were checked, but no equivalent assemblies or problems were found.