In a refinery, the water contained in storage tanks (gasoline and diesel) was being periodically channelled into a sump and then pumped to a drip basin prior to treatment. During a manual drainage step, 20 m³ of liquid and vaporised hydrocarbons overflowed the vents on a basin with a floating cover. A puddle formed at the foot of the basin along with an inflammable cloud that drifted 60 m out to the adjacent road.

An explosion (of the UVCE type) occurred, most likely initiated by the crossing of 2 trucks. Two other explosions ensued. The backfire caused the puddle to ignite first followed by the drip basin and ancillary equipment. The internal emergency plan was activated; the site’s responders arrived, installations were shut down as an emergency measure and stationary cooling systems were turned on. External public fire-fighters, notified by the on-call agent, showed up 15 min later. The police blocked road traffic and evacuated, as a precaution, all neighbouring residences and offices for 1 hour. The incident was under control within 90 minutes.

Both truck drivers sustained burns (resulting in 7 and 15 days’ work leave). The drip basin, water distribution pipes and a vehicle parked near the basin were damaged; window panes on a building 100 m away were shattered. The operator appraised property damage at 8.5 million euros.

This accident was due to corrosion of the internal heating coil, which caused a vapour leak inside the tank, where temperature rose from 20° to 60°C. The hydrocarbons forming the supernatant layer were partially vaporised, thus causing both the vent protection to open as a result of the pressure surge and flow on the roof. The duration of the fire, coupled with the absence of basin instrumentation, suggested that the quantity of hydrocarbons contained therein at the time of the accident had exceeded the design quantity by a wide margin.

The accident highlighted the inadequacies of risk analysis during the design, modification and maintenance phases of the basin. Basin overfilling and overheating had not been anticipated and were not detected by any instruments (temperature and level sensors were missing). Addition of the heating coil was not accompanied by updated maintenance procedures (corrosion control, coil replacement).

The operator had planned on adopting written procedures for drainage and installation of an automatic temperature control system on the basin with a heating shutdown threshold. The operator also sought to move certain facilities further from the road.

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