In a petroleum refinery, during in a stand-by condition in hydrodesulfurization unit, 10 m³ of gasoil penetrated the catalytic cracking unit due to a not-tight gate valve. Gasoil burned when the catalytic cracking plant re-started. The combustion is stopped by blowing water vapour into the unit. The products of combustion are discharged through a chimney and form an important opaque cloud (gasoil and soot) that worried the population. The catalytic cracking plant is shut-down. Police and fire brigade are alerted. No people are injured during the accident.

Insufficient process analysis and plant design or human error may be at origin of the accident.