Pollution
Humain
Environnement
Economique

In a workshop producing carbon disulphide (CS2: toxic and highly flammable) at a chemical plant manufacturing sulphurous products, a technician added soda tablets into a 1-m³ sodic water preparation tank and then opened the hot condensate inlet valve to execute a mix when fire broke out at 6 pm. Technicians sounded the alarm at 6:06 and manually tripped the automatic extinction system. The site’s alarm siren was activated and around 6:15, in-house volunteer fire-fighters set up a water curtain outside the workshop (the CS2 combustion was producing toxic smoke) and finished extinguishing the tank using 3 powder extinguishers. The internal emergency plan was in effect for 1 hour; this outbreak was successfully dealt with by 6:20. No victims or environmental degradation were reported. The extinction water was collected in a fire basin. Electric cable openings were damaged, but the workshop was able to reopen 24 hours later. The operator informed neighbouring municipalities and issued a press release.

The operator undertook an investigation and determined that a check valve had been leaking on the tank outlet pipe. This equipment had been replaced a few days earlier during a down period dedicated to installation maintenance, as technicians had suspected the presence of CS2 at the tank bottom (due to a yellowish colour). Pressure in the pipe feeding CS2 storage from the synthesis column was thus greater than the pressure occupying the pipe feeding the column from the sodic water preparation tank. This then led to a CS2 discharge into the tank ahead of the specific operation. Heavier than water, CS2 settled at the bottom of the tank under residual water. Upon mixing sodic water, the technician sought to start the process with a clean tank and drained the residual water by bleeding the tank bottom despite the instruction not to completely empty the tank; the liquid CS2 wound up losing its water cover. When the condensates reached 105°C in the tank they caused the CS2 vaporization (Teb of 80°C at ambient pressure) and their ignition in contact with a hot spot (stirrer motor?). According to the shop operator, some 10 litres of CS2 had ignited. An assessment of the leaky check valve revealed that it did not correspond to the model designed for this pipe (flap valve used for sodic water pipes with a nominal diameter of 50 to 150, while a piston valve and spring had been specified for diameters of 15 to 40).

Several measures were adopted, including:

  • permanent instruction banning complete tank drainage ;
  • replacement of the non-compliant valve and inspection of the 65 valves installed in the CS2 workshop: no other non-compliance was found ;
  • modification of the operating procedure: no simultaneous filling of CS2 storage during the procedure, isolation of the tank outlet pipe when idle or during condensate filling, verification of the absence of CS2 (colour, olfactory detection threshold less than 0.1 ppm) when the tank bottom is bled ;
  • installation of a direct rule cutting off CS2 supply in the tank outlet pipe (closure of the automated tank bottom valve and shutdown of the suction pump) upon detection of a reverse CS2 flow originating from storage (dual flow meter).