At around 7 a.m., a technician detected a seal leak at the level of a drainage-line flange and valve at the bottom of a tank at a flammable liquid storage facility. The terminal’s operations team and safety team stopped the leak. The leaky seals were replaced. A preventive foam blanket was put in place.

42m³ of flammable liquid were discharged into the retention basin which was isolated. 7t of liquids (mixture of water and foam compound) were recovered, the remainder having evaporated.

The weather conditions of the preceding days, including a long cold spell, had caused freezing of the water present at the level of the drainage line at the bottom of the tank and rupture of the flange seal upon thawing. The presence of water is due to the fact that the product intrinsically contains a fraction of water and the tank shape is single-slope. This slow leak was not detected quickly despite the presence of a gas detector in the retention basin. The operator checked that this detector was operating correctly and that it was reported to the supervision centre (25% threshold), on the telephone of the shift manager, and that a local alarm was triggered (50% threshold). It attributed the failure of this gas detector to be actuated to the fact that the leak occurred on a small section (1 inch), with a very volatile product and a detector positioning in the retention system at the level of a sump. As regards the failure to detect liquid in the basin, the level switch was located at a low point. Due to the significant evaporation, there was no liquid at the level of this low point.

Following the event, the operator took the following actions:

• preventive inspection of the drains of tanks of the same design;
• installation of protection from cold on the tanks’ drainage lines;
• design study of a drain valve for tanks of single-slope shape;
• installation of tank level leak detection on closed valves.