In a fertiliser plant, the release of white smoke (ammonium nitrate fog according to the operator) occurred at around 5:20 pm during start-up of the hot ammonium nitrate solution production unit. This smoke plume was visible from outside the site and moved towards the SEINE River and Canteleu Forest before dissipating by around 6 pm. Despite a slightly foul odour, no complaints from the neighbourhood were recorded. No pollution measures had been implemented by the operator downwind of the discharge point. The IOP was not activated. The unit was shut down due to a recently installed ON/OFF valve.
An analysis of the incident revealed two malfunctions on the nitric acid supply circuit equipping the saturator, when filled with water as required by the restart procedure:
- incorrect information from the flow meter, indicating the absence of nitric acid flow at the saturator inlet;
- the valve held open at 50% on the nitric acid supply circuit.
Filling and overflowing of the nitric acid saturator caused a release of acid fumes, which was then exacerbated by the depressurisation that occurred during tank drainage.
Flow meter failure was caused by opening its measuring loop following removal for maintenance. Under these conditions, the measurement indicator remained blocked at 0; the alarm signal resulting from this defect and reported in the control room was probably acknowledged before restarting the unit.
The control valve of the nitric acid supply was adjusted from the control room so as to be held in the closed position, yet clogging of the vent (by crystallised ammonium nitrate) ensured that the valve control kept it in a half-open position, allowing for acid transfer. Given that no information on the actual state of the valve was brought to the attention of the control room, the technician could not have detected the fault.
Various measures were adopted in order to improve installation safety, namely:
- preparation of an operating procedure regarding unit start-up that emphasises the need to take into account the active alarms;
- control of measuring loop continuity of the active sensors during their return to service;
- systematic testing of main valves, which is important prior to restart after an extended shutdown.
