A leak on a pressurised fire water supply network flooded the gallery of accelerator pumps handling the water from the hydrocyclone of rolling mills at a steel plant. These pumps switched off and could not be restarted after an initial pumping had been performed using mobile equipment connected to the plant’s treatment basins. To perform the necessary repairs, the plant operator decided to confine all pumped water to: the site’s industrial water network, the storm basin associated with the lift station for this water, and a channel for discharging the site’s non-polluted water into the natural environment after building a slag dam inside this outlet. Despite such an obstacle and the installation of a floating boom, hydrocarbon-laden water (0.03 mg/l) still flowed into the ORNE River, forcing pumping to be halted.

The consequences of this fire network leak were exacerbated by the malfunction of a check valve installed on the pipe connecting the hydrocyclone to the site’s industrial water treatment basins. The gravity flow of this water had caused persistent flooding of the gallery despite the mobile pumping resources implemented: between 5,000 and 7,000 m³ had to be subsequently removed. The gallery was only dewatered entirely once this pipeline had been plugged by a cofferdam.

Following the accident, the operator approved a number of measures, namely:

• replace the defective check valve; assess the integrity of fire network pipes;
• conduct studies to protect the electrical cabinets and cut off the channel during such incidents;
• clean the plant’s sewers and recycle the rinsing water;
• eliminate the polluted slag through a certified stream;
• treat the water still held in the channel.

Upon Inspection Authorities’ request, the operator was required to inspect all critical check valves ensuring the protection of both individuals (e.g. drinking water network) and the environment.