At a refinery, a low-pressure separator exploded inside a hydrocracking unit undergoing a restart procedure. The explosion, of a charge equivalent to 90 kg of TNT, was heard and noticed up to 30 km away. The gases and inflammable liquids discharged (1 tonne of hydrogen, 400 tonnes of hydrocarbon) ignited and fuelled a fire that would cover 35,000 m². The human toll amounted to 1 fatality (a technician) and 7 injured. The missile-like effects of the explosion had threatened the safety of 200 people. Property damage was extensive, estimated at €7 million inside the site and another €7,000 outside (broken window panes, damaged roofs, etc.).

The accident occurred while the hydrocracking unit was being reset during a restart procedure subsequent to a routine shutdown. The inappropriate manual opening of a valve between the high-pressure separator (155 bar) and the low-pressure separator (9 bar) caused liquids to be transferred towards the low-pressure separator, whose relief valves had been designed solely to account for the fire risk, making these valves inadequate for the purpose of releasing excess pressure in the low-pressure separator (10 m long, 3-m diameter, 200 kg), which exploded at an estimated pressure of 50 bar. The suspected valve did not close automatically because the level alarm set very low inside the high-pressure separator had been disconnected during a modification carried out in the unit several years prior; moreover, site technicians doubted the reliability of the main level indicator, preferring instead to refer to the backup detector recording table, and it was the misreading of this recording that led them to believe that the level inside the high-pressure separator was normal.

After the accident, several remedial measures were adopted, namely: reconstruction of the unit with correctly-designed relief valves on the low-pressure separator and improved instrumentation; implementation of procedures intended to drain the liquid present in the high-pressure separator during shutdowns; establishment of a system for monitoring flaws and maintenance needs on the equipment and appropriate personnel training; and a more frequent and stringent auditing programme.