First noticed at 3 pm outside the premises of a plant specialised in meat-based breaded food products, an ammonia (NH3) smell was apparently stemming from a room housing refrigeration installations. Quick verification revealed no leak or malfunction; pressure in the high-pressure circuit was a bit high yet below the activation threshold. The smell, which had dissipated to eventually disappear at 3:15 pm, was observed in the workshops near lines 1 and 5. A test paper detected the presence of NH3 at the exhaust of roof valves. With the height of valve pipes and the wind direction, a discharge occurred near the air inlets of the central units serving both of these lines. Plant activities were suspended and employees evacuated as a precautionary measure. After being notified, the subcontractor responsible for the installations spent an hour onsite after 4 pm; he decoupled the valve / exhaust collector pipe connections and in looking for trace NH3 detected some on one of the valve outlet connections for a compressor. A high pressure switch test indicated that the valve had opened at 15 bar, 1 sec prior to compressor shutdown. The pressure switch threshold controlling compressor shutoff was lowered to 13 bar. The valve that had stayed open after activation was replaced.

This incident was due to a combination of 3 events: high outside temperature, a production increase that also caused a high demand for cold air, and pressure switch / valve thresholds set too close to one another (i.e. a pressure ‘peak’ in the high-pressure circuit activated the valve before the pressure switch). The NH3 release was evaluated at 65 kg of the 5,900 kg on hand in the installations. The unsuitable valve calibration adjustment might have occurred in 2004 when a detailed inspection of pressurised facilities led to valve modifications. The subcontractor agreed to attend the subsequent detailed site inspection scheduled for June 2005 regarding use of NH3. The plant was closed from 3 to 5 pm. External fire-fighters were not alerted. No victims were reported and no discomfort noticed outside the site boundary. In June, the operator modified the valve outlet line and produced a training film on NH3 risks for plant staff.