The leak occurred at the hydro-isomerisation unit of a refinery. Following the observation of a pressure drop between two heat exchangers, controls were carried out on site. A noise was heard, but no gas detection was activated on the unit. A hydrogen leak was detected but the source was not identified. Following the detection of hydrogen, the access to the unit was forbidden and emergency services and the fire department alerted. The unit was decompressed using the safety flare. When the pressure was sufficiently reduced, two operators looked for the origin of the leak.

The accident originated in the isomerisation unit and propagated to the benzene saturation unit. The accident source was a mechanical rupture of one of the two compressors used to transfer hydrogen mixtures to the isomerisation unit . The rupture caused the release of a gas mixture containing hydrogen, which ignited by mixing with air, forming a jet fire. The jet fire headed was directed towards the adjacent benzene saturation unit, hitting a pressure vessel located 14 m from the release point (the jet flame could have been 30 m long in absence of obstacle).