A leak in an ammonia (NH3) refrigeration system occurred at 6:15 pm inside an insulated utility room in the production zone of a food processing plant (bread, sandwiches). The NH3 released engulfed the site.

In lowering the refrigerant (composed of 30% glycol / 70% water) used to cool production rooms to -8°C, this unit comprised: 4 compressors (3 screw compressors and 1 piston-activated as a backup), a low-pressure tank containing 700 kg of NH3, a condenser circulating both the cooled water by 2 cooling towers and the high-pressure NH3, plus a plate evaporator. The 3 screw compressors were lubricated by 400 litres of oil circulating, via a pump installed on the oil / NH3 separator, to the 3 compressors. A refrigerant kept the oil at a temperature below 85°C. A controller ran the entire set-up.

At 6:35 pm, a remote monitoring company reported a code 3 (NH3 alarm) to a maintenance technician working the 12:30-9 pm shift; upon entering the machine room, this employee observed activation of a local visual alarm after crossing the 2nd detection threshold on 2 NH3 sensors. In also smelling NH3 outside the room, he notified the on-call team manager, who arriving at 7:20 pm wearing a cartridge mask saw that the refrigerated unit had been automatically turned off and promptly shut down the unit with the emergency switch on the building’s outer wall. Failing to identify the leak source, he informed the site’s Head of Safety and alerted local fire-fighters. Besides 9 maintenance personnel remaining on-site, 57 employees left their stations, assembled in the site cafeteria and were sent home.

During an initial reconnaissance mission around 8 pm, despite feeling ill due to the NH3 permeating their gear, 2 fire-fighters in a self-breathing apparatus: detected a major gas leak under the backup compressor’s oil separator, closed a valve under this separator without stopping the leak, and left the room with an installation map posted on a wall. Response efforts continued with the help of a maintenance technician and preliminary identification of the valves to actuate. At 9:15, fire-fighters wearing diving suits isolated the compressor by closing valves. NH3 recordings in the air confirmed stoppage of the leak. By 10:15 pm, facility hygiene personnel were able to clean the production lines. Fire-fighters left the site at 10:30 pm after starting up 2 of the 3 screw compressors. Production resumed at 5 am.

This leak stemmed from a crack in the upper part of a float in the piston compressor’s oil separator. An external refrigeration specialist repaired the installations. The operator noted the good working order of site alarms: gas detection controlling room ventilation, alarm and cut-off of electricity entering the room, and remote transmission. Inspectors of classified facilities took issue however with reliance on remote transmission after crossing the 2nd threshold, given the absence of a local sound alarm when surpassing the 1st threshold. Managing these alarms would be addressed in a subsequent study.