A little after midnight, an extremely flammable gas leak occurred in the ammonium manufacturing unit of a chemical plant manufacturing nitrogen fertilisers. The gas, mainly, consisting of hydrogen, escaped from a valve. Released at 220 bar and 120°C, it ignited spontaneously and formed a flame jet towards the pipe transporting it. Heated by the flame jet, the pressure increased in the pipe; it then opened over 1 m on its upper side and released the gas which exploded.
The alert was raised and the police activated an emergency plan; traffic was stopped on the roads close to the plant and residents were asked to stay in their homes.
The remaining gas burned at the rupture point for 2 hours. The leaks were plugged and the fire was brought under control at 2.45 am. The site was secured.
In all, 2 employees suffered from bruises and minor cuts, and were treated on site. The plant’s activity was interrupted for the enquiry.
The accident was caused by a gas leak from a valve joint (in open position and with a maximum flow rate). There were no seals between the 2 touching metal parts. The internal metal surfaces of the valve were of insufficient quality in relation to the specifications. The valve, changed in 2002, was not considered as a critical safety element, even though the process had been identified as being able to generate a major risk. The choice of sub-contractor at the time of the maintenance was made based on the price alone. No transfer of information occurred between the operator and the sub-contractor at the time of the operation and no work acceptance was carried out (in particular no check of the valve bolting).
The operator revised its procedures, improved the management of the maintenance and sub-contractors (qualifications, information, monitoring of operations, etc.). It also revised the emergency response conditions (staff trained and in sufficient numbers, even during periods of reduced activity).