At around 6:15 a.m. on a Sunday, a technician making rounds in a company specialising in the manufacture of antioxidants discovered acid vapours from a leak on a storage tank. While it was being dismantled, the tank had been placed in an open-air retention area. It contained 2 t of sulphur bichloride (SCl2, corrosive liquid) and 5 t of propylene carbonate, a non-flammable solvent designed to dissolve the 3 tonnes of solid deposits in the bottom of the tank. The roundsman raised the alarm, and water curtains were set up to lower the gas emissions and avoid any emanation into the air. A call for help was made at around 7:45 a.m. following an unsuccessful attempt to repair the leak. The 11 employees present at the site are evacuated, and the internal contingency plan was initiated at 8:34 a.m.

The fire brigade sealed off the three manholes. The leak, caused by a 3 cm horizontal crack at the bottom of the tank, was temporarily plugged with a chemical dressing applied to a Teflon strip (the strip prevents the dressing from being damaged by the product). A tarpaulin was placed on the side of the building, in the open air. Upon contact with air, sulphur bichloride releases hydrochloric acid. The firefighters conducted atmospheric measurements which turned out to be negative, although nearby residents smelt an odour and were invited to stay indoors. For safety reasons, the nearby major road was closed to traffic.

The suppression water was treated in the site’s wastewater treatment plant. The POI was lifted at about 11 p.m., and the site gradually resumed operations until the following day. A specialised company arrived at around 6 p.m. to pump the products that remained in the bottom of the tank and in the retention area: 14.5 t of products were thus placed in drums and transported to an approved destruction site. The tank was decontaminated, and the water from the potentially acidic water curtains was conveyed to the site’s treatment plant.

The leaky tank was a 38 m³ horizontal steel cylinder located in a 40 m³ retention area designed to hold the leaked product. Although being 20 years old, it had been regularly monitored under ‘pressurised equipment’ directives (maximum working pressure: 2 bar, test pressure: 5 bar). It had not been in operation for 8 months and was disconnected from all systems (thus no longer considered as pressurised equipment), although its thicknesses were still monitored.

Two hypotheses were considered:

  • a weakness of the material: a few days before being decommissioned, a leak had already occurred in the same area along the weld of a curved bottom exposed to wind and bad weather;
  • the material had been weakened by a chemical sludge that had been accidentally placed in the tank by a supplier 5 years before. The tank had to be cleaned up at that time.