Pollution
Humain
Environnement
Economique

During a shift change at around 5 am in a prepared foods plant, a forklift truck operator detected an ammonia (NH3) smell and sounded the alarm; 55 employees were evacuated, of whom 4 complained of headaches with 2 being hospitalised for observation. Plant technicians placed the facilities in safe operating mode at 5:30 am. Fire-fighters called to the scene recorded concentrations of 150 mg NH3 and ventilated the workshops. Only traces of NH3 could be detected 4 hours later. No consequences were perceptible beyond the site boundary. A subcontracted refrigeration specialist defrosted and inspected the refrigeration installations, tightened a leaking bolt and restarted the unit between 10 and 11 am. The accident could be traced to 2 causes: a loose plug and a loss of electrical power supply a few hours earlier. According to the plant operator, pipe vibrations had loosened the plug, yet the seal stayed intact as long as the equipment continued running at a 0.5-bar pressure drop. The shutdown of operations, which undoubtedly occurred between midnight and 1 am upon tripping a circuit-breaker, led to a loss of this pressure drawdown in favour of rebalancing pressure at 3 or 4 bar in the circuits (service pressure: 11 bar), followed by the leak. Not only failing to satisfy safety requirements or maintain thermal conditions in the facility’s cold storage rooms, but the untimely shutdown of operations was not detected in real time: did the recording of defaults and circuit-breaker tripping information not get relayed to the control room, or was it that the 500-ppm NH3 detection threshold was not reached or that the detector was poorly positioned? An expert forwarded several recommendations: examine the positioning of both the existing NH3 detectors; design a method of preventing air circulation in the attic so as to avoid interference with NH3 leak detection; locate the manual and automatic shutoff valves (indications and references on drawings); and install a device indicating the facility status (on/off) at all times. Several procedures were also outlined: periodic de-icing in order to streamline valve handling, level reading and, more generally, to limit constraints tied to ice mass build-up on the plant’s systems; completion of periodic tests to verify effective operations of the NH3 safety detection chain; monitoring equipment accessibility (for de-icing); periodic valve opening/closing; and verification of leak detection. A fluorinated refrigerant leak would subsequently occur on 19th October, 2005 at this same plant (ARIA 31364).