In a chemical plant, sulfur began burning from the flare-stack of a carbon sulfide unit (CS2) at 2.45 am. CS2 is obtained through a reaction of natural gas and sulfur at high temperature. The two CS2 production lines had been placed on hot stand-by between 8 pm and 9 pm; one owing to the shut-down of the site’s methyl mercaptan workshop (M.S.H.), and the other following an incident a few hours earlier (sulfur fire, see No. 25246). The gas alarm was sounded. The unit’s flood network brought the incident under control in just 10 minutes.

The external emergency services, alerted by the guards, remained on site while the internal contingency plan was in force from 3.30 am to 6.30 pm. The flare-stack remains “lit” by 3 methane-fed pilots and is backed up by a propane tank. The flare-stack is designed to burn off various residual gaseous effluents: excess hydrogen sulfide not consumed by the sulfuric unit’s furnace, gas released from the line valves due to accidental overpressures and decompression gasses from the CS2 unit when opened, controlled by the line’s emergency shut-down or by the general emergency shut-down, on/off safety valves of the reaction section. The accident occurred when the installations were not in operation, and resulted from the progressive and sequential filling of the various tanks of the CS2 unit caused by the opening of valves and the continued supply of new sulfur via the recycled sulfur line (a pump had remained in operation, valves open; the closure of these valves is not required during hot stand-by of the CS2 production lines, and a non-hermetic valve on the sulfur recycling line…). The damaged electric wiring of the heating system and the control circuits controlling the flare-stack’s gas flow rate were repaired.

Another incident happened on this same unit just a few hours later (see No. 25248). The operator undertook the following measures: refurbishing of the valve and its systematic overhaul during programmed shut-downs every 24 months, modification of the automatic control systems of certain valves (positions, level slaving) or ensuring the overall shutdown of the installations, protection that is both reinforced and allows for better maintenance of the critical parts of wire troughs around furnaces…