At around 5 pm, high levels of hydrocarbons were found on the site of a chemical plant. Protective measures were taken by the staff; a portion of the site traffic was stopped and the neighbouring company was alerted. In conjunction with these steps, local network sensors for monitoring air quality recorded an increase in the concentration of benzene at the hospital measuring stations (up to 1,081 µg/m³).

At 6:18 pm, the cause of these emissions was identified: malfunction of the heating device for a storage tank containing 205 t of benzene (capacity: 1,930 t) initiated a rise in temperature and boiling of the benzene followed by emissions into the atmosphere through the tank safety valve. Isolating the device stopped the emissions flow; containment measures were lifted at 8 pm and the operator issued a press release and estimated that 4.4 t of benzene had been released into the atmosphere within 7.5 hours.

The overheating of the benzene was due to the failure of the temperature sensor directly controlling the steam valve that feeds the heating system. The measures from this sensor not being recorded nor transmited to the control room, the tank is equipped with another independant temperature sensor which measures are transmited to teh control room. Storage monitoring recordings showed that the emissions started around 11:30 am (upon reaching a temperature level of 76°C and a pressure of 18 mbar, corresponding to the valve set point). They also show that the temperatute inside the tank had been exceeding 40°C for several days. The high temperature alarm fitted to the tank, set at 40°C, had probably been switched off without setting a new alarm threshold.

The Classified Facilities Inspectorate noted that the discharge was only detected from recordings of benzene concentration in the air and not by taking into account the tank’s monitoring parameters, despite several hours of alarm warnings in the control room. The Inspectorate also noted the absence of procedures or instructions defining both alarm thresholds for the various reservoir operating parameters and measures to be taken should the limits be exceeded.

Several corrective measures were implemented: installation of an alarm on the pressure sensor of similar tanks; reconfiguration of the high temperature alarm to eliminate its accessibility to plant technicians; replacement of the steam control valve with a flow of 4 t/hour at 4 bar by a valve with a maximum flow of 250 kg/h that was better suited to the need to keep the tank heated at 15°C.