At around 10:00 a.m., thermal oil at a temperature of 275 °C began leaking from the nozzle of a sensor during a maintenance operation. A sprinkler bulb broke, spraying the area. To prevent the oil igniting (flash point: 210 °C), the firefighters sprayed a blanket of foam. The mixture of water, emulsifier, and oil flowed into the building’s bunding and was treated by the wastewater treatment plant.

An estimated 5 m³ of thermal oil spilt into a drip tray (blowdown tank) and onto the floor. Production was shut down for six hours after the event.

Immediate causes of the accident: removal of a sensor, unperformed draining, and leak on a tank

The leak was caused by human intervention during maintenance of a pump. The mechanics had removed a temperature sensor without checking the pressure of the fluid in the system. Moreover, the system should have been drained before the mechanics started work. 

The massive spill of hot oil caused a weld on the blowdown tank — which usually is only filled with cold oil — to spring a leak.

Root causes

Regarding the fact that the system had not been drained before work began, the inspection authorities for classified facilities noted that no specific pump maintenance procedure was in place. Furthermore, the drain valves are not easily identifiable (absence of identification numbers or other markings).

The supply of hot oil instead of cold oil can be explained by a valve that remained open after a maintenance operation the previous month. The inspection authorities for classified facilities stressed a number of areas for improvement:

• the valve lock-out procedure for maintenance work deals with lock-out but not release from lock-out ;
• identifying the open/closed positions of the valves is not easy (quarter-turn valves cannot be used on the system) ;
• the absence of markings (valve numbers) complicates matters.

Measures taken

After the event, the inspection authorities for classified facilities asked the operator to:

• repair the blowdown tank;
• identify the valves in the boiler room and indicate their normal positions;
• write a procedure for draining the pumps on the thermal oil system and identify sensitive operations;
• supplement the valve lock-out procedure with release from lock-out.