During the recommissioning phase of a refinery after a long shutdown, while on patrol at around 6 am, a technician detected a gas smell around a fixed roof atmospheric tank (slops tank) containing waste water from a 50 m³ settling tank treating effluents loaded with hydrocarbons from the debutaniser (atmospheric distillation columns). He took an explosimeter measurement which was negative but placed warning signs 45 m from the tank prohibiting the passage of vehicles nearby. At 7 am, a subcontracted welder stopped his vehicle in front of the warning signs and left his engine running when an explosion occurred. A technician who witnessed the event raised the alarm and the worker, who received 3rd degree burns, was taken to hospital. The water inlet on the tank was closed and its contents sent for combustion by the torch. Flames were coming out of the vents and the roof had to be sprayed extensively. At 10 am, the authorities organised a crisis unit, preventively closed the RD 37 minor road situated 400 m away and requested the confinement of schools and a hospital centre located less than 1 km away. Nitrogen was injected via the roof of the settling tank after 12 pm, which extinguished the fire within 20 minutes, and then the tank was emptied until 10:20 pm. The intervention involved around thirty fire-fighters (internal + external).

The enquiry showed that on the day before the accident, the by-pass valve of the valve controlling the passage of waste water between the tank for the debutanisers and the settling tank had been leaking; the passage of hydrocarbons had caused the pressure to rise in the settling tank and a high pressure safety mechanism caused a safety solenoid valve to close upstream of the control valve and its by-pass. The subsequent pressure drop in the settling tank caused this solenoid valve to reopen and the leak to resume. This cycle was repeated hundreds of times during the afternoon of the day before the accident, which was revealed by the examination of the operating parameter records. At around 8:30 pm, the safety solenoid valve failed due to overheating, it moved back towards the closed position (fail-safe position) but remained blocked in the intermediate position. The report in the control room displayed a closed position because the automated controller only recorded the closure command and not the actual position of the valve; the leak from the by-pass valve to the settling tank resumed at a continuous flow rate of 5 m³/h, whereas the recovery pump from the settling tank to the tank was only extracting 2.5 m³/h. In the settling tank that operates by separating water and hydrocarbons, the hydrocarbons that had accumulated in the hydrocarbon compartment overflowed at the beginning of the morning into the waste water compartment, and then were sent to the storage tank for 2 hours, i.e. a volume between 2 and 7 m³ of hydrocarbons was transferred. They evaporated through the tank’s vents and formed an explosive cloud that ignited on contact with the car engine (hot spot).

At 6 am, a technician had detected that the solenoid valve was blocked, but despite being reported in the control room, it had not been followed up. The display scale for the pressure in the settling tank on the unit’s control screens was too small, making it difficult to detect the overpressure cycle. The start-up of another unit at 7:30 pm on the day before had monopolised the attention of the operating technicians and shift manager who had not checked the ongoing pressure alarms in the settling tank. After 8:30 pm the pressure alarms became recurrent, triggered a banner alarm on the synoptic display panel of the priority equipment and activated a horn. For over 15 hours, the shift technician, also in charge of 3 other synoptic display panels, acknowledged these alarms without attempting to understand their consequences on the process. There were no instructions concerning the ranking of alarms and how to prioritise their handling.

The operator is reviewing the activation of the safety solenoid valve (activation according to the water/hydrocarbon level) and redefining the safety actions associated to the transition curves for the unit’s operating parameters.