An hexane vapour cloud explosion (VCE) occurred at 00:47 a.m. during the start-up of an extractor in a factory manufacturing raw materials for the food industry and farmers (soja flakes).
The start-up had been initiated 3 hours earlier (9:30 pm on the 14/07) after the completion of maintenance works and inspection by the plant operator. The toaster reached proper operating temperatures around 21:45, allowing the admittance of flakes through the inlet screw conveyor. After a shift change around 23:00, the technicians noticed a temperature drop and increased the heating. They heard the safety flap valve releasing hexane and steam into the extractor building, but it was not considered abnormal during start-up
At 23:25, the system partly stopped automatically due to overloading. However, the steam and hexane flows were still in function. High hexane concentrations were detected in the extractor building. The plant operator, however, was of the opinion that the steam and hexane flows were stopped according to the operation procedure.
Ten minutes later, the flake outlet conveyor was restarted and the operating staff began to search for the hexane release point but without success. The hexane concentration in the building reached a level which forced the staff out and activated the installation’s sprinkler system.
The operating staff left the facility at 00:15 and called for internal assistance (including chief plant operator). They stopped a bus passing by ; the bus driver informed the Traffic Control Centre, which in turn alarmed the Police and the Fire Brigade.
The Chief Plant Operator (CPO) arrived on site at 00:35 and located the leak to the safety flap. The CPO and the Fire Inspector agreed on cutting off the power supply in order to stop the leak. The power plant engineer rejected to stop the loaded transformers due to the inherent risk of possible sparks. However, for unknown reasons, he disconnected a third unloaded transformer. That action most probably ignited the hexane cloud and an explosion occurred 30 seconds later, injuring 27 persons (staff members, firemen and police officers) and destroying the 6 buildings of the plant. Windows were broken up to 1.5 km. 15 t of hexane were involved in the explosion.
The leakage of flammable gas was due to the incorrect operation of the safety flap valve caused by the failure of the interlocking system to cut-off the steam and hexane flows to the toaster when one of the extractor outlet conveyors was stopped.
The plant has not been rebuilt. An expert group made recommendations for the prevention of similar accidents :
1- the release of gas from the safety flap must occur in a safe location;
2- the design of the extractor building and its ventilation system should be improved, together with safety and interlock systems, gas detection and fire protection systems;
3- the safety management should be improved.
