In a fungicide plant, the safety system on a spray tower (dryer) used in the manufacturing of wettable sulphur shut down the tower at 8:20 pm subsequent to a strong rise in pressure detected at the level of the spray nozzles. The installation shutdown was normal operating procedure when plugging the nozzles.

The shift change was scheduled 40 min later; to avoid having the night crew perform a restart at the beginning of their shift, the technicians decided to quickly perform the task themselves by “short-circuiting” some of the steps. As opposed to official procedure, air heating was not shut off and the fluidised bed was neither drained nor verified prior to restart. After unclogging the spray nozzle, the installation could restart around 8:40 pm.

Since the hot air continued to circulate within the fluidised bed, the bed temperature rose from 50°C (operating temperature) to 87°C, which was the injected air temperature. At this latter temperature, the product began to decompose, denature and normally should not have been packaged. Moreover, the sulphur became more combustible. Upon starting up the spraying operation, the inflow of colder product caused a thermal shock that proved sufficient to trigger spontaneous ignition of the product along with an explosion inside the spraying tower.

The safety devices responded appropriately and the pressure surge vents opened at 8:59 pm. Neighbours hearing the explosion were the first to sound the alarm. Upon the arrival of fire-fighters at 9:02, the fire had already been brought under control by the new shift. The rescue team still extinguished another fire that had broken out on a parked vehicle in the municipal pound 5 meters from the spraying tower.

A 200-kg sulphur cloud escaped via the tower and caused discomfort to 6 employees. Recordings taken in neighbouring dwellings did not reveal any risk of heightened toxicity.

The consequences of this accident were limited yet still demonstrated a number of technical, organisational and human malfunctions:

• the spraying tower assembly should have been automatically placed into total security mode subsequent to a system interruption, without requiring any human intervention: automatic shutoff of the air heating valves and drainage of the fluidised bed;
• installation restart should not have been possible as long as the controls set forth in the procedure had not been executed (via sensors on the visit hatches, whose opening had also been stipulated in the procedure) in order to avoid shunts.

The operator had also been asked to revise the site’s internal emergency plan and procedures since the telephone numbers were no longer valid, and the individual assigned to oversee implementation and manage the internal emergency plan had not been explicitly appointed. Moreover, the emergency messages to be disseminated to neighbours were not yet ready. In the meantime, the installati