At 10:15 am, a 1 t ammonia (NH3) leak from a gaseous refrigerant occurred at a site producing nitrogen to supply a chemical platform. The body of a valve originally made of cast iron (in 1978) broke during discharge (50 mm) of a compressor started up 2 hours earlier after a one day shutdown to regulate production flows. The installation was placed in secure mode (gas detectors activated); however, a defective check valve downstream of the compressors led a technician to intervene for 10 minutes wearing a self-breathing apparatus yet with good visibility (ventilation + door opened by the adiabatic flash). The internal emergency plan was triggered; 3 employees were sent to the infirmary for 1 hour of observation. The NH3 cloud blew towards an uninhabited zone and seemed not to be noticed away from the site.

During a preventive maintenance operation in April 2003, the manufacturer had changed the valve stuffing box and recommended its replacement given its condition. As part of a repair job deemed temporary, a sealed casing was installed by a specialised firm in October on a leaky valve.

An emergency order made any installation restart contingent upon submitting a report on circumstances surrounding the accident, in addition to technical and organisational proposals for lowering the probability of recurrence. Measures were adopted over the subsequent days, featuring:

• preventive replacement of the same type of valve by a steel valve on a 2nd compressor;
• installation of 2 one-way check valves at the NH3 tank inlet and a heat exchanger;
• backup human monitoring while awaiting technical modifications for faster facility isolation in the event of a leak;
• permanent presence of water curtains and/or hoses to bring down toxic clouds;
• drafting of a procedure to guarantee extreme safety when injecting NH3 and restarting the installations, along with specific documentation to record malfunctions likely to arise during restart steps.

Automatic safety valves with emergency shutoff in detecting leaks were installed over the next few months, plus temporary refrigerating sets running on an HFC-type refrigerant. All these modifications complied with the internal safety management manual, and the safety report scheduled for updating within 2 months addressed all these elements. By the end of January, the site operator renounced NH3 in favour of a chlorofluorinated refrigerant.