Pollution
Humain
Environnement
Economique

A massive bleach leak in a holding tank

At around midnight, an operator conducting rounds in the storage facilities of a household detergent packaging plant discovered that a holding tank containing two product tanks was full of bleach, and alerted the safety and production managers. As the holding tank was not hermetic, 30 l of product had accumulated in a low point upstream of the stormwater network. The product remaining in the holding tank was pumped into an empty container, while the leak was repaired. A small mobile pump was used to pump the bleach in the stormwater network into an empty container that had been recovered from the waste storage facility. The container, previously containing acid, released chlorine vapours and slightly intoxicated an employee. It was estimated that the holding tank had retained 80 m³ (83.5 t) of 2.6% bleach.

Change in the organisation of operations led to the accident

Four months prior, the operator had eliminated the night shift in charge of transferring products between the storage tanks and the packaging lines. On the night of the accident, the production crew was to fill 5-litre containers of bleach. At around 10 p.m., when the stock in the buffer tank had been exhausted, one of the operators had to go to the building where the stockyard control room was located. From the control desk, he started the transfer pump, then opened the bottom valve on the corresponding tank. Water hammering occurred due to the vacuum created by the pump having been started before the valve was opened. An elbow, downstream from the valve, became detached from its PVC piping. The tank supplied the production line partially, while some of its contents emptied into the holding tank via the leaking elbow until the roundsman discovered the accident 2 hours later.

Design and maintenance problems

The holding tank’s faulty level sensor failed to trigger a visual alarm in the site’s guardhouse. The subsequent investigation revealed that there were no instructions regarding the operation and transfer procedures, and that the production crew was not trained in how to use the transfer control room (previously operated by a specialised crew). Once the transfer was initiated, no one remained in the building where the control room was located in order to remotely monitor the transfer process. The mimic panel for the control room console did not indicate the position of the tank’s bottom valve, which could have alerted the operator to the risk of hammering in the transfer piping. The operation of the transfer pump was not associated with the proper opening of the tank’s bottom valves, nor with the low-level detection in the tanks. Once the leak had been detected, the personnel did not activate the site’s pipe plug in the sewer systems despite the instructions in place.