Eight kilos of chlorine leaked out of a 50 l cylinder at a municipal water chlorination plant. The leak started at 8:00 a.m. An employee noticed the leak (alarm & visual observation) and gave the alert but did not attempt to stop it.

The leak, which started in the courtyard of the plant, created a cloud of chlorine gas dozens of metres in size. The cloud then spread to a nearby street where a school and a municipal swimming pool are located. Emergency services set up a 50-m safety cordon. The local residents stayed indoors and traffic in the area was halted for several hours. Fire-fighters wearing sealed suits closed the valve and stopped the leak around 10:30 a.m. Eight people (two workers and six local residents) were treated for exposure to the toxic gas by emergency services. The municipality issued a press release.

The leaky cylinder had been connected the previous day with its valve open but without any flow. Chlorine was supplied by a second cylinder.

The water-treatment facility’s coupling became loose. Two 49-kg cylinders of chlorine are connected by a goose neck to a chlorinator. One cylinder immediately takes over when the first one is empty. Three other cylinders are stored as spares. The cylinders are pressure equipment items. The chlorinator and the downstream pipe leading to the water circuit operate at negative pressure. The goose neck operates at positive pressure but is not subject to the Pressure Equipment Directive. When the chlorinator is not injecting doses of chlorine, it acts as a shut-off valve on the chlorine exhaust circuit.

An external assessment requested by the operator found a weakness in the coupling . A connection test confirmed a significant risk of the female coupling being incorrectly positioned onto the male end of the valve. The goose neck, the assembly’s weak point, generated shear forces on the lead seals, causing the chlorinator to overhang. For an unknown reason — most likely caused by forces generated by the assembly — the bayonet connection between the goose neck and the chlorinator failed at the seals. The bayonet connection was disassembled and the seal was changed each time the cylinder between the connection and the goose neck was changed. However, the seal between the goose neck and the chlorinator had not been changed; both remained in place.

The supplier removed the cylinder from service so that it would be ready for its periodic inspection and attributed the loosening of the coupling to fouling of the female portion with grease and yellowish deposits of residual chlorine that limited the free movement of the retaining bars. It was also found that the adapter/bar assembly had seized, preventing the spring retracting and the bars being placed in the low position. The quick coupling’s accidental disconnection was caused by incorrect fitting of the system due to zero servicing of the female coupling which had been in service for one year.

This type of assembly is not compliant with the French ministerial order of 17 December 2008 (rubrique 1138). The inspection authorities for classified facilities recommended assessing the assembly’s safety level. Both female couplings were replaced. The operator revised its internal procedures and plans to find a new supplier.