Pollution
Humain
Environnement
Economique

The facility has 2 liquid ammonia storage bins containing 6,500 t of ammonia at the time of the incident. These cryogenic tanks are maintained at a pressure of less than 50 mbar thanks to a re-liquefaction system composed of 2 stages of compression and cooling by sea water exchangers. The gas is liquefied and re-injected into the storage bins after relaxation. Between these 2 stages, a separator ensures the protection of the compressors of the 2nd stage in case of liquid ammonia or oil drive.

At around 7:00 p.m., the operator of a company that manufactures fertilisers and nitrogenous chemicals halted its production of ammonia and set up an emergency response unit after the compressors on the ammonia storage refrigeration system shut down one after the other. The site did not have compressor redundancy. This meant that the gaseous ammonia might not be cooled, its pressure might rise in the circuit and it might be vented to atmosphere by the relief valves. Until the compressors could be repaired and in order to limit the impact of an ammonia release, the operator implemented:

  • the possibility of sending the gas to a burner;
  • a movable flare stack ;
  • a tank-based ammonia absorption system;
  • equipment to spray the valves.

The inspection authorities for classified facilities conditioned the restarting of the facilities on an emergency measures order. The operator could restart 15 days later provided he prove that the equipment was in proper working order following the ingress of water, that the compression capacity was sufficient, and that flaring was continued.

The successive failure of several compressors was due to cooling water, which migrated into a first stage compressor that had a perforated cylinder head bottom (occasional damage that may have been caused by a casting defect). The water then migrated into the separator, triggering a high-level alarm. The technicians had set up a purge system, but the flow rate was insufficient to drain off the amount of water entering through the perforation. The water then infiltrated as far as the second compression stage, causing the compressors to shut down one after the other.

The operator has since taken the following measures:

  • increased the diameter of the separator purge and added a recovery tank;
  • introduced a procedure for safely shutting down the shops if a high-level alarm occurs on the separator;
  • revised the preventive maintenance programme for the compressors with checks of the cylinder head bottom thickness;
  • purchased a new compressor to serve as a backup;
  • looked into implementing a passive backup to provide compression if the compressors fail.