In the machine room of a food processing plant, ammonia (NH3) was released into the atmosphere for a few seconds during a planned maintenance intervention on the compressor of a refrigeration installation.

The small quantity of gas emitted was however sufficient to trigger the 2nd level of an alarm whose measurement cell displayed an offset of the “0” point (120 ppm instead of 0) and for which a sensor was positioned 1 m above the intervention zone; 1,300 ppm (i.e. 0.13%) were detected at 9:16 am, for a concentration of less than 600 ppm in the remainder of the room, while the other detectors were not activated.

The coalescent cartridges were scheduled for replacement following an incident of excess oil consumption. The corresponding intervention, which began at 8:15 am, involved 2 site employees and a subcontracted refrigeration specialist. The installation was commissioned, then the compressor was vacuum racked at 8:30 without, however, removing any of the lubricating oil present prior to opening the circuit, by withdrawing the first bolts beginning at 9 am. Shortly after observing that the check valve on the suction side was defective, the technicians separated the flanges in order to extract the valve when at 9:15 the short-lived release of NH3 occurred, activating the alarm subsequent to “degassing” of the lubricating oil. All 3 workers at the scene exited the premises.

The alarm and circuit-breakers were reset at 9:17. The technicians resumed their intervention at 9:20.

No bodily injury or property damage was reported. The detector supplier recalibrated the measurement chain in order to eliminate the observed offset.