Work was underway in an industrial cake and pastry making plant to extend the refrigeration facilities. Two subcontracting companies were working on the site: the refrigeration engineer and a metalworking company. An operation began at around 11:30 a.m. to supply ammonia to the new extension. The installation’s ammonia supply line had been cut to install a branch connection. Its disconnection resulted in a leak. Concentrations exceeding 1,000 ppm were measured. The alarm system disconnected the power sources and triggered the signal to evacuate. The 128 people at the site were evacuated. The refrigeration engineer was able to isolate the leak by closing the damaged pipe’s shut-off valve. The premises were ventilated. The liquid ammonia that had spilt onto the floor (1 l) was sprayed down and then absorbed with paper.

Three employees who had inhaled vapours were examined at the hospital. The firefighters conducted control measurements, and the company resumed normal activity at 1:10 p.m. The paper used to mop up the ammonia water was transferred to an authorised disposal facility.

Two operations should have been carried out before the piping was cut:

• closure of the shut-off valve,
• opening of a second nearby valve to purge the oil residues.

Two nearby valves were improperly handled

According to the refrigeration engineer, he had closed the shut-off valve. He then gave instructions for the intervention to an operator of the metalworking company. This operator, however, did not carry out the intervention as initially planned. A second operator of the metalworking company intervened alone. During the intervention to isolate the leak, the shut-off valve was found to be open; the second valve was closed. The refrigeration engineer claims that the second operator of the metalworking company may have confused the two valves.

Several factors contributed to this incident:

• the need to operate two valves, in close proximity to each other, in opposite directions,
• the operator who intervened was not the one initially planned. He had not received his instructions from the works coordinator,
• no provision had been made to avoid the risk of opening the shut-off valve,
• the absence of the refrigeration engineer during the operation is a departure from the operating mode,
• the condition of the shut-off valve was not checked before the intervention.