Following a hydraulic test on a 6,000-litre tank in a service station, diesel fuel was released through the discharge outlet. Roughly 3,300 litres of diesel fuel spilt into the unloading area. The company in charge of equipment maintenance close the valve and lock out the discharge outlet. After being locked, a few litres of diesel fuel came out through the tank vents. The tank was then isolated, and the pump’s power supply cable was disconnected downstream of the circuit breaker.
Limited consequences
Measurements were performed with a photoionization detector 10 days later. VOC values ranged from 0 to 125 ppm. Soil samples determined that only localized pollution had occurred. The majority of the product was recovered by the separator and the overflow tank, and the separator was cleaned. The soil is not excavated given the low vulnerability and insensitivity of the environment around the site and the presence of piping in the spill area. Piezometers were used to ensure that the pollution was contained within the site’s boundaries.
A tagout error identified as the origin of pollution
Upon completion of the hydraulic test, the technician in charge of the maintenance operation tagged out all the valves. He had not noticed that one of them had been locked out during operations required to bring the storage tank into compliance. Its closure prevented the tank from filling up every time a technician used a nozzle. The tank was equipped with submersible pumps on the discharge side, so it started to fill up. A first high-level alarm was triggered in the accounting office, but it went unnoticed. Once the tank was completely full, it began overflowing through the discharge outlet. The diesel fuel then made its way to the hydrocarbon separator. When hydrocarbons were detected in the separator, the system was closed off. The high-level alarm in the separator, located on the low-voltage master distribution panel, was triggered but went unnoticed. Once the separator was full, the diesel fuel began to flow into the overflow tank and then into the unloading area. It was at this point that the maintenance staff noticed the release. They initially closed the unloading valve, which caused the diesel fuel to exit through the vents. They then re-opened the unloading valve until they were able to locate the valve that would stop the tank from being filled.
The technician in charge of maintenance did not conduct the required inspections following the hydraulic test. His tagout error highlights a deficiency in training. In addition, the information provided regarding the reason for the lockout was also called into question.
Organisational measures to prevent another accident from occurring
Following the accident, the subcontractor announced plans to enhance technician training programmes and to improve lockout/tagout instructions and procedures. The alarms were moved to the cash registers, where the staff is always present.
