Pollution
Humain
Environnement
Economique

At around 8:00 p.m., an explosion occurred on the filter used to collect dust from the ventilation system of a sulphur-based fungicide manufacturing unit of a plant where phytosanitary products are made. The dust filter’s explosion vents opened. Alarms went off in the control room. The unit’s technicians left the building. They saw flames coming from the filter’s vents. The internal emergency plan was implemented at 8:45 p.m. A worker was slightly injured and taken to hospital, but he was quickly discharged. Firefighters contained the fire at around 11:00 p.m. The internal emergency plan was lifted at 11:15 p.m. The operator had the building monitored overnight.

The unit was dismantled for investigation, cleaning and repair. According to the operator, the resulting losses amounted to €4 million. The unit was restarted in April.

Analysis of the causes

Due to the very unfavourable weather conditions (thunderstorm and strong winds), the facility was shut down at 7:50 p.m. (just a few minutes before the accident) in accordance with prevailing procedure.

The explosion detection system indicated that two events occurred within two seconds of one another:

  • a small explosion in the cyclone-separator area, where air in the atomisation tower is treated (drying);
  • then an explosion, followed by a fire, in the dust filter on the fourth floor. This filter is connected, via the ventilation system, to the airlock located under the cyclone separators.

An assessment found that an explosion had occurred in the airlock under the cyclone separators. The explosion was sparked by the combined presence of oxygen (the airlock was being filled when the accident occurred), an explosive concentration of dust (particularly fine suspended dust during the filling phase), and ignition energy. The most likely cause of this ignition energy is the presence of an electrostatic ignition source. The relatively high temperature of the fungicide during the unit’s shutdown phase unit (up to 104 °C, compared to 70 °C during production) may have made the fungicide more sensitive to such a discharge. In addition, the low air humidity during the shutdown phase may have generated increased static electricity. The theory that the fungicide self-ignited was excluded because the temperatures required for layers to self-ignite are much higher than those reached when the event occurred (220–250 °C vs 104 °C).

The explosion in the airlock may have:

  • spread to the cyclone separators above the airlock, igniting the fungicide in the bottoms of the cyclone separators;
  • sent an ember from the exploding airlock to its vent, which then reached the dust filter via the ventilation ducts. Because the airlock was in the process of being filled, the vent valves connected to the ventilation system were open. The dust filter contained an explosive concentration of dust. The ember from the airlock caused the filter to explode and then burn.

Feedback and measures taken

Following this accident and before restarting the unit, the operator:

  • improved the temperature monitoring system at the atomisation tower’s outlet, particularly during shutdown phases;
  • replaced the polymer filter vents by metal vents (which have a more accurate activation pressure than polymer vents).

Further improvements are planned in the following months:

  • a system to inject water into the filter from a distance will be installed to mitigate the consequences of another explosion (human consequences by distancing the intervention area, and material consequences by improving the responsiveness of the extinguishing system);
  • studies on:
    • the possibility of using additional temperature monitoring systems (at the outlets of the cyclone separators and filter) to detect hazardous temperatures early;
    • the feasibility of a smouldering fire detection system to detect combustion as soon as possible;
    • the benefits of using additional automatic safety controls (emergency stops) if an explosion is detected in the dust filter or the filter’s vents open;
    • technical solutions to reduce the risk of embers passing between the airlock and the bottoms of the cyclone separators and between the airlock and the dust filter;
    • the possibility of reducing the amount of fungicide in the airlock (to reduce the likelihood of electrostatic discharge).