Shortly before 01:00 a.m., an ammonia leak occurred on the packing of a feed pump supplying product to the production workshops of a solid ammonium nitrate fertilizer production facility. Ammonia passed into the packing’s barrier fluid (methanol) causing a dripping overflow from the methanol reservoir. As the overflow from the reservoir was in line with an optical fibre cable on the leak detection system of a nearby ammonia pipe, the leak had automatically triggered the shutdown of the ammonia supply for the entire plant by shutting down the pump and closing the valves. The operators were able to secure the production installations. The compressor used to reliquefy the gaseous ammonia in the storage tanks had stopped, increasing the pressure in a cryogenic storage tank filled to 60% with ammonia, normally at 26 mbar. Two high (50 mbar) and very high (54 mbar) pressure alarms were triggered. The safety valve, set at 70 mbar, opened at 1:47 a.m. The operators in the control room started another reliquefaction compressor at 2:40 a.m. in an attempt to reduce the pressure in the tank and close the valve.

The opening of the valve for 50 minutes resulted in a release of 176 kg of ammonia. The plant’s internal contingency plan was not initiated. The sensors located within the tank’s retention area had not detected any ammonia. According to the operator, the meteorological conditions on the day of the incident were favourable for the high-level dispersion of ammonia. Based on the dispersion model, and as the emission point was at a height of 28 m, the operator determined that the cloud’s low point was at 13 m, which explains why no odour was detected at ground level. The threshold for reversible effects had reached a height of 22 m outside the site’s boundaries.

The checklist of actions to be taken to secure the facilities in the event of a shutdown did not include verification to ensure that the reliquefaction compressors were back into operation. Moreover, the high and very high-pressure alarms on the ammonia tank had not been identified as priorities.

The operator implemented the following corrective actions following the event:

• repositioning of the ammonia line’s ammonia detector (fibre optic),
• improved monitoring of pump packing,
• modification of the shutdown checklist to ensure proper operation of the compression system,
• classification of high and very high-pressure alarms in the tanks as priority alarms,
• raising the awareness of production teams regarding the impact of safety sequences on ammonia storage.