In the power plant of a slaughterhouse, a technician was purging a low-pressure cylinder in a refrigeration plant. At the end of the operation, he was surprised by an ammonia leak. He tried to actuate the balance valve, but it was blocked. Given the extent of the leak, he decided to don protective equipment before continuing his intervention. While he hesitated on which equipment to use, the second ammonia detection threshold was triggered. The electricity was switched off, and the shipping dock’s alarm went off. The technician was joined by his co-workers. A two-man team donned breathing equipment and entered the power plant. They were able to spray down the opaque cloud of ammonia and close the valve. During this time, production was stopped and the buildings were evacuated.

A specialised company was brought in to check the refrigeration installation, which was put back into operation one hour after the leak. The amount of ammonia was estimated at 92 kg. The concentration of ammonia in the plant room gradually degreased and eventually reached 4 ppm twenty-four hours after the incident.

Several factors contributed to the initial leak:

• the purge was not identified as a risky operation. The operations and the precautions to be taken were not formalised by a procedure;
• although the technician had received ammoniac risk training in 2001, no refresher training had been offered since;
• no personal protective equipment adapted to the cylinder purge operation was available, requiring the technician to move away during the operation. This faulty practice had been identified, although was not corrected.

The initial leak was aggravated by:

• the failure of the balance valve when the first attempt was made to close it. The valve was more than 10 years old. A pasty mixture of oil and ammonia had prevented it from being closed. The maintenance performed on this equipment was insufficient. Several malfunctions had been reported, however. The valve had not been replaced, as doing so would have required that the installation be shut down;
• the late closure of the balance valve. Several elements contributed to this: the absence of any disconnection means outside the ammonia cloud, protective equipment remotely located and the absence of leak management instructions.

The plant operator took several corrective measures following the event:

• upgrading and annual inspection of personal protective equipment and its accessibility;
• replacement of the faulty valve and reinforcement of the refrigeration installation’s maintenance contract;
• shut-off valve added upstream of the ammonia system;
• identification of operations at risk and formalisation of their operating procedure;
• enhanced qualification training for the technicians handling the ammonia installations.