A 2 to 3 cu.m. leak of gaseous ammonia (NH3) occurred inside a vegetable transformation/canning facility.

A refrigeration installation containing 9 tonnes of refrigerant was undergoing compliance modifications according to the conclusions of a safety report, in particular to reduce the quantity of NH3 released during an eventual accidental discharge and to limit irreversible effects around the site boundary. The day before, at 9 am, as a prerequisite to the prescribed works, the installation’s 2 to 3 cu.m. medium pressure tank was drained of the liquid NH3 it had contained after being isolated both upstream and downstream: before the compressors, on the high liquid pressure supply, heading towards the low-pressure bottle and on the deep freezer condensate return. Once this drainage step had been completed, the residual gaseous NH3 pressure of the drum was around 4 bar by the end of the day.

During the night, an electrical resistance that was maintaining oil fluidity in the separator downstream of the medium pressure tank was left on in order to vaporize the liquid NH3 remaining in the canister, allowing for the subsequent drainage of this oil. The gaseous NH3 gradually rose in both temperature and pressure, until reaching 11 bar by the following morning, which equalled the opening pressure for the safety valve on top of the separator. Irritated by an NH3 odour, a neighbour residing 150 m from the installation notified the plant around 8 am and then the local gendarmerie. Personnel arrived onsite at 8:10 am, a certified employee wearing a self-breathing apparatus was able to switch the circuit onto the emergency valve; the NH3 leak was stopped and pressure dropped back to 9 bar. The bottle was degassed towards the low-pressure drum at 4 bar and then connected to a vacuum pump to return the effective pressure to 0. The electrical resistance was turned off. Resistance operations had caused a pressure increase not only in the NH3 contained in the separator, but undoubtedly in the ammonia contained in the medium pressure drum as well, as the connection between the two had not been shut. The alert was over before the onsite arrival of fire-fighters and of the Inspection authorities for classified facilities around 9:15 am. The operator modified its procedures; the oil canisters were drained at the same time as NH3 capacities were racked, with this operation still being considered exceptional.