When loading a sphere in a fertilizer and nitrogen products plant, ammonia (NH3) was released into the air for 1 hour via the chimney of a vent on a non-condensables collection tank.

The foreman noticed frost on a connection pipe and raised the alarm. The NH3 sensors on the property’s boundary indicated a peak of 25 ppm to the North of the site and 40 ppm to the South. No other consequences were noted.

The refrigeration circuit used to maintain pressure in the spheres included a refrigeration unit that liquefied the gaseous NH3 from the storage spheres, and a tank which received the liquefied NH3, which was directly connected to the spheres and whose gaseous phase was connected to a second non-condensables collection tank in the workshop.

The accident occurred after a technical incident that occurred the day before on the ammonia pumping station which led to switch over of the installation to safety mode by shutting down the pumps and closing the safety valves. The technician then reset the shutdown sequence and restarted the pumps, but left the safety valves closed, including the safety valve fitted on the connection pipe between the liquefied NH3 tank and the spheres.

The next day, when ammonia unloading started, the refrigeration circuit was activated, causing the liquid NH3 tank to fill. Due to the pressure increase in the tank, the automatic valve (opening control at 15 bar) fitted on the pipe connecting the 2 tanks opened causing ammonia to be discharged via the vent of the second tank.

The accident’s cause analysis highlighted an operating error during a transient phase and a rapid alarm transmission fault on the NH3 sensors on the site’s boundary. Consequently, several corrective actions were taken:

• drafting of a guideline on the actions to take in the event of switch over of the ammonia pumping station to safety mode,
• additional training for technicians,
• transmission of the NH3 detection alarms in the control room.