At 1:00 a.m., an explosion occurred in a sublimation chamber at a plant specialising in the manufacture of phytosanitary products. The 560 kg door was blown out, and a fire broke out due to sulphur being blown out of the chamber. The operator on duty deployed a fire hose station to fight the flames. Two other operators were called in to assist and alerted the site director. The extinguishing water was contained inside the building.

The pressure compensation tank associated with the chamber involved exploded. Part of the roof and the door of a sublimation chamber next to the one involved were also damaged. The solid sulphur production process was stopped.

The sublimation chamber, heated to 50 °C, is designed to condense sulphur vapours. These vapours are obtained by evaporating liquid sulphur in an electric furnace and then used to manufacture solid sulphur or flower of sulphur. The oxygen content in the chamber must remain below 8% to prevent any reaction between the sulphur and the oxygen present in the air. Nitrogen inerting by a programmable, automated controller is therefore carried out, and the oxygen level is continuously monitored by an oxygen meter. The operator must not initiate production until the oxygen level is well below 8% during the inerting process.

A few days before the accident, the operator had stopped the solid sulphur manufacturing cycle after identifying a malfunction in the process. The liquid sulphur pressure sensing probe in the furnace displayed an abnormal value, and this probe had been replaced and checked before the furnace was put back into operation. Shortly before the explosion, a loss of communication had been noted on the flow meter. The furnace was being abnormally filled with liquid sulphur and experienced a loss of display of the actual level (out of range of the level probe).

The operator issued several hypotheses for the explosion:

• oxygen present at a level above 8%, which, in the presence of sulphur dust, would have brought the mixture into its explosive range;
• the presence of pollutants in the raw material led to the explosive range despite compliance with the oxygen content below 8% (this hypothesis was rejected).

An untimely presence of oxygen could be related to the following:

• a defect on the breathing tank (unlikely);
• a leak in the chamber (probable);
• a valve malfunction (unlikely).

The Classified Installations Inspectorate noted that the oxygen meter’s inspection frequency had not been respected. Moreover, during the last production cycle before the explosion, the operator on duty had not completed the control sheet to trace the verification actions (oxygen meter cleaning, inerting monitoring, start of heating, etc.).

Before restart, an emergency measures order requires that the integrity and compliance of potentially damaged equipment be verified. An accident report was compiled, including causal analysis and measures to prevent a similar event from happening again. The site’s hazard study was updated based on feedback from the event.

An explosion had already occurred at the site in 2013 (ARIA 44468).