At around 9:45 p.m. in a prepared frozen meal factory, an operation to defrost a deep freezer coil had been underway for 15 minutes when an ammonia leak occurred. The operators present on site detected an odour. The first alert threshold (500 ppm) was reached in the attic area (i.e., 25 times the occupational exposure limit value). A second alert threshold (1,000 ppm) was reached at around 10:30 p.m. At this time, the electricity was shut off. The plant’s internal contingency plan was initiated. The 34 employees present at the site evacuated the building. The alarms were confirmed, and a technician closed the shut-off valves upstream and downstream from the station. The general low-voltage electrical cabinets were switched back on at around 11:00 p.m.

During the incident, three employees experienced a tingling sensation in the mouth and headaches. An estimated 0.2 t of ammonia had been released into the atmosphere.

The leak was located on a pressure gauge on the hot gas station, and the internal weld of the manometer tube had failed. The age of the equipment (20 years old), the temperature differences, and the vibrations of the valve station was the cause of this rupture.

Following the accident, the operator took the following measures:

• check that the shut-off valves of the pressure gauges are open so that no liquid is blocked behind them;
• check the condition of all gauges mounted on A4A valves (such as the one that failed);
• replace manometers more than ten years old and then every ten years thereafter, which involves an addendum to the service contract with the manometer company for technical and metrological verification.