Pollution
Humain
Environnement
Economique

Within few minutes, 800 kg of a gaseous mixture of tetrahydrofuran (THF) and 1.5-hexadiene (irritant and flammable substances) were released into the air by a solvent regeneration plant, via a pressure relief valve fitted to an atmospheric distillation column. This column was rapidly shut down. Employees in the industrial area who noticed a solvent smell alerted the firefighters. The firefighters evacuated the 110 employees of a neighbouring plant, including 6 people affected by nausea and discomfort who were hospitalised; they were released from hospital during the day. The other employees evacuated, went home after being examined on site by doctors. Air analyses were carried out in the surrounding areas. The explosivity measurements were negative.

The accident was caused by ice (- 8°C) blocking a valve used for the feeder tank water make-up of the distillation column condenser pumps. With no water supply, the tank gradually emptied due to evaporation causing loss of pump priming. The vapours at the top of the column were no longer condensed, and the temperature and pressure within increased causing the pressure relief valve to open. Unfavourable weather conditions (low and swirling wind and heavy fog) also contributed to poor dispersion of the atmospheric releases. The accident analysis, which had not considered this scenario in the site’s safety report, showed that the water make-up system and the water level in the tanks were not subjected to any particular inspection or report, that the operation of the installations relied on the vigilance of the technicians who continuously checked the main process parameters without any alarms, and that the installation was not equipped with a gas detector for the rapid detection of leaks, etc. The inspection authorities for classified facilities drew up reports establishing the facts.

Before restarting, the operator implemented the following measures: heat tracing and thermal insulation of the valve and the water make-up line section, definition of guidelines regarding the inspection of tank levels, installation of a low temperature alarm on the water feeder pump discharge nozzle, a low level probe on each tank, and a flowmeter with low threshold on the refrigerated water manifolds, etc.