At 9:14 pm, fire broke out in the furnace of one of the 2 carbon sulphide (CS2) unit’s production lines at a chemical plant. A technician on his rounds detected the blaze, which was brought under control by in-house first responders. Property damage was relatively small and limited to the heat insulation. Liquid sulphur had been spraying for 4 weeks prior as a result of opening the relief valve, given a pressure rise on the line due to condenser fouling. All valve discharge was supposed to be collected by a pipe network hooked up to a flare. The spraying of sulphur in the unit was due to the absence of a plug on the valve unloading cross-section, with this plug nowhere to be found on the premises (flawed maintenance procedure?). Various remote areas in the unit had not been adequately cleaned after the incident; the plant operator emphasised the hypotheses of zones made inaccessible for cleaning afterwards and self-ignition of sulphur in contact with a hotspot on the unit furnace. Moreover, the inspection checklist was deficient for the purpose of valve reassembly with respect to both the subcontractor and in-house maintenance staff. After 2 other incidents that night (see ARIA nos. 25247 and 25248), procedures were modified and corrective actions subsequently adopted: a thorough cleaning of all production lines before restart, and recall of guidelines for keeping installations in a perfect state of cleanliness.