At around 5.30 in the morning, during the preventive maintenance of a filter on the refrigeration installations of a Viennese pastry establishment, an ammonia (NH3) leak on the LP feeder circuit of a deep-freezer led to the evacuation of the 59 employees present. The SMUR (French emergency medical services) treated 8 people affected by headaches, one of whom was hospitalised as a precaution. A chemical emergency squad took measurements which were found to be negative. The three shutdown production lines resumed their activities at 12:30 pm.

The technicians closed a valve upstream of the filter, drained the pipe, degassed and removed the filter, before taking it to the workshop for cleaning. During their absence, the seal on the valve expanded with the ambient temperature of the attic spaces resulting in a leakage of 10 l of liquid NH3. The triggering of a detection system (threshold 1 at 70 ppm) switched on the extraction fans. Technicians equipped with protective masks and gloves retightened the valve and stopped the leak at around 6 am. The NH3 spilled in the retention tank under the valve stations then triggered threshold 2 (100 ppm/max. measurement of 237 ppm of NH3) as it evaporated, resulting in evacuation of the plant. After the cleaning of the affected area using water, a proportion of the ammoniac solution spilled into the waste-water network connected to the treatment plant and released NH3 which rose up into the work areas through the unprimed drain traps. The staff had resumed their activities at around 7 am following atmospheric checks in the buildings and authorisation given by the technicians, and the first person complained of breathing difficulties at around 8:30 am.

The accident highlighted a lack of staff training in particular on the response to an NH3 leak. After being informed indirectly and belatedly, the inspection authorities for classified facilities drew up a report establishing the facts. The leak could have been prevented if the valve had been sufficiently tightened and if the filter had been replaced right away. The guidelines concerning the maintenance of filters have been improved: work carried out outside of production periods, modification of the procedure (immediate replacement of the filter, the old filter being cleaned and reused for the next operation), and acquisition of a portable device for measuring NH3 concentrations in the air. Finally, the staff are now better informed about the risks relating to NH3, which, in particular, will prevent any panic in the event of NH3 odours.