At a chemical plant, leak occurred at 5.20 pm while a tank car containing 48 tonnes of ammonia (NH3) was being unloaded. Twenty-seven tonnes of NH3 were released over 37 minutes until the operator closed the tank’s bottom valve by releasing the turnbuckle using a metal shim that had been thrown roughly fifteen meters away.
The internal contingency plan and special intervention plan were put into action (safety perimeter:). 5 km 8,000 to 10,000 persons concerned), vehicles equipped with loudspeakers directed the population of 2 neighbouring communities to confine themselves indoors. A school and a supermarket were confined, and then evacuated after the cloud had dissipated. For 4 hours, 80 firemen intervened with 25 vehicles, ambulances and a mobile chemical response unit. The internal contingency plan was called off at 8.40 pm. The accident caused one child to experience respiratory difficulties, and who was hospitalised for 2 hours, and roughly fifteen people experienced discomfort. The ammonia odour was nevertheless detectable up to 8 km from the release point.
The accident was caused by the automatic disconnection of the unloading arm and the blockage of the 2 series-mounted safety valves in the open position: an unexplained failure of the anti-impact safety device (detection pedal) caused the transfer arm to disconnect, the turnbuckle actuating the tank car’s bottom valve remained blocked (due to mechanical and automatic control problems) and the valve mounted on the transfer arm (tank car side) remained partially open as it was blocked by a foreign body (the origin of the bolt blocking the valve is unknown).
Following the accident, the corrective measures taken concentrated on the installation of positive safety type pneumatic turnbuckles, the limitation of arm disconnection scenarios, the replacement of disconnection pedals by rocker type stop cleats preventing all risk of accidental impact and the installation of a filter on the end of the unloading arm to protect the valves from the introduction of foreign objects…