At around 11:30○am, 50○kg of ammonia (NH3) gas at 10○bar began leaking from a tunnel freezer that was being commissioned at a frozen foods plant (test phase of a new unit). Employees at the plant stopped the leak. The plant’s 120 employees were evacuated.○The site was shut down and its power supplies were shut off.

Production was shut down for three hours and the new tunnel freezer was only commissioned one month later. Four of the 19 people who were sickened by the fumes were taken to hospital for observation.

Cause of the accident

According to the operator, a piece of metal blocking a discharge solenoid valve on the tunnel freezer caused excess pressure in one of the defrosting stations to build up and open a valve.

The commissioning tests of the hot-gas defrosting system were carried out using ammonia, but the system’s operation had not been previously checked using compressed air or any other gas. In addition, the outlets of the pressure vessel’s valves had not yet been connected to an external line. If they had, the ammonia fumes would not have been released indoors and the workers would not have been sickened.

Compliance with pressure vessel regulations

A commissioning declaration had not been filed for pressure vessels on the new tunnel freezer. The opinion of a notified body was sought to:

• analyse the accident scenario and ensure that the equipment could be safely be recommissioned ;
• check whether the tunnel freezer complies with pressure vessel regulations (constructional details).

Measures taken

The operator took the following actions:

• replacement of the defective equipment (solenoid valve) ;
• test of the other solenoid valves with compressed air to ensure that there was no similar failure on the system.
• connection of the safety valves to an external line ;
• checking the tightness of the pipe flanges ;
• modification of the tunnel freezer’s program so that the ammonia supply is automatically shut off if the system pressure rises (even before the valves actuate) ;
• leak test of the system with a progressive pressure increase using compressed air (or nitrogen) to limit the risk in the event of a new leak.

The authorities suggested to the prefect that he issue a formal notification order that all the pressure vessels at the plant be brought into compliance with regulations.