During a detergent production run inside a chemical plant, an incident occurred at 9:15 pm causing a toxic cloud of nitrous vapours to form. The production shop had 3 tanks of mixture. The first two contained two end products awaiting racking. The manufacturing of a product made from citric acid, sulphuric acid and sulfonyl methane was ongoing. An anti-corrosive organic product was to be injected into the tank. This injection step was performed using a 3-tank supply pipe fitted with pneumatic valves of the “normally open”, automaton-controlled type (i.e. held closed by air pressure).
Due to a pneumatic system malfunction, the first tank’s supply valves stayed open; 10-15 kg of organic product were released into the 3 tonnes of finished mix made from phosphoric and sulphuric acid. An oxidation reaction ensued, releasing intense heat and nitrous vapours.
The manufacturing process technician witnessed the product flow in small quantities into the tank and sounded the alarm. The chain was placed in safe mode and all 7 employees, wearing protective masks, evacuated the workshop. A safety perimeter was established by the local gendarmerie, and a total of 44 fire-fighters in 19 vehicles arrived on the scene, along with 2 chemical emergency squads. The cloud dissipated, and no victims were reported. Nearby residents and the Le Mans-Tours railway line were not adversely affected. All emergency measures were lifted around 11:30 pm. The contaminated product was destroyed at an external site.
The operator subsequently replaced these “normally open” valves by a positive safety model (normally closed valve) on those product lines subject to oxidation risks. The operator would improve the vapour suction system above the tanks. Additional training on chemical risks, targeting ingredients/inputs of the site’s product line, was offered to team leaders and selected manufacturing personnel. The location of the siren sounding the alarm was modified for greater accessibility, and the staff meeting point was placed further from the production site.
