Following the accidental drainage of a storage tank (n°1) into a recovery tank (n°2) in a PVC manufacturing plant, the staff of a neighbouring department located downwind detected emanations of a pungent-smelling substance of a lacrimatory nature. The operation carried out to neutralise a proportion of the product in the recovery tank (n°2) by introducing 700 l of sodium at 20% caused a sudden release of CO2 and vapours of this substance. The duration of the gaseous emission, of the “gusting” type, lasted for 5 minutes. The quantity of substance (highly toxic by inhalation and flammable) released into the atmosphere was estimated at 2.3 kg, which corresponds to the calculated concentrations of 1.4 ppm at the neighbouring department.

The evacuation alarms for the area in question were activated. Traffic in the plant was preventively halted on the main route between 7 am and 8:30 am. Water samples taken at the basin’s inlet confirmed the absence of accidental discharges of this substance via the liquid effluents. 14 people present on the neighbouring worksites downwind of the incident complained of eye irritations.

The accidental drainage of tank n°1 into tank n°2 via the degassing circuit was caused by 2 separate events: the undetected piercing (level measurement not working) of a condenser on a degassing circuit causing it to be flooded with water, and the incorrect position of a manual valve on the circuit for this toxic substance following inspection operations on a device. The release of vapour during the neutralisation phase was due to the lack of a procedure adapted to the neutralisation of a larger amount of the product in question.

The operator plans to implement several preventive or corrective actions:

• improving the isolation of tank n°1 by subsequently locking a valve before removing it in the short term
• improving the monitoring of tank n° 1 by checking the level for each operation and over the short term by retransmitting the level to the control room
• improving the isolation of this highly toxic liquid in tank n°2 by collecting the vapours for each degassing circuit
• drafting a procedure for the neutralisation operation.

Furthermore, there are plans to provide the staff with training on neutralisation operations.