Inside a fungicides manufacturing plant, an explosion followed by fire occurred around 5:20 pm within a spraying tower (dryer) used to produce wettable sulphur, 47 minutes after restart of the sprayer subsequent to a maintenance intervention.
The safety system featured a temperature and pressure detection with redundancies at all sprayer levels, along with an isolation system for the machinery by means of injecting a (powder) retardant agent within a few milliseconds, in addition to 4 vents sized at 50 mb. This system blocked all spreading of the explosion and ensured complete shutdown and inerting of the installation with nitrogen in order to cool the equipment and limit SO2 dispersion. Nonetheless, the explosion still opened the vented parts of the tower, which had contained 400 kg of sulphur. The wind then pushed a sulphur dioxide (SO2) cloud towards a residential district. The personnel closed the vents and drowned the installation, thereby stopping the cloud from further forming. Local emergency services confined the 80 children in a nearby school as well as district residents for 15 minutes. They installed 2 water curtain nozzles in order to prevent smoke from rising and around 7:10 pm, using a foam nozzle, extinguished a fire that had spread into the heat insulation of the sprayer and suspended ceilings in the adjacent workshop (spreading by grit penetrating via roof interstices). The fire-fighters finished their mission around 9 pm.
As an initial analysis and given the state of some sections of the heat insulation, a microscopic leak had formed on the sprayer wall at the level of hot air injection under the fluidisation bottom, which in turn caused a gradual combustion of sulphur dust contained in the insulation and generated a flashpoint on the wall. This situation triggered the spontaneous ignition of dust in suspension above the fluidised bed, leading to an explosion of the product in suspension within the sprayer column. Before the restart of its installations, the site operator will:
- disassemble the fluidised bed to locate the potential leak;
- analyse the recordings of various sensors, especially pressure sensors, to determine the origin of this pressure surge;
- verify that stroke limiters on the pressure surge vents had in fact been operable;
- assess the possibility of an automatic closing of these doors following an incident (to limit the release of SO2 while avoiding the need for human intervention in a high-risk situation);
- identify “dust nests” inside the workshops capable of fanning the flames of a fire;
- clean or eliminate the suspended ceilings.
The restart of installations was subject to a preliminary inspection by the classified facilities inspectorate in order to ensure that all safety mechanisms were once again operational and/or had been replaced.
