At around 9:30 am, the headspace of a 2.3 t tank used for hot-process liquid wax production ignited at a detergent, soap, and cleaning products production plant. The flash fire blew off the tank’s manhole, releasing 1-m-high flames. The technician in the workshop closed the manhole cover and smothered the fire. The power supply was cut off. An emergency stop was activated to cut off the supply of white-spirit, which was being poured into the tank.

The workshop’s fire detection system did not work. The hose between the tank and the suction system melted from the heat.

The headspace ignited at the end of the filling of the white spirit, which was being poured on the liquid wax mixture. The explosion blew off the tank’s manhole, which was not locked. The manholes were left unlocked as part of a measure taken following feedback from a previous accident that occurred on a nearby tank in July 2015 (ARIA 49415).

According to the expertise carried out, the main cause of the ignition was an electrostatic discharge through a vapour cloud. A 30 cm explosive atmosphere had formed above the liquid during filling; the filling pipe was too close to the liquid surface. Electrostatic discharges built up because the white spirit had been splash-filled too quickly and at too high of a stirring speed. The accident was also caused by human error, as the technician had failed to follow the instruction to open the extraction system completely (the valve was opened only 70% of the way).

In accordance with the expert’s recommendations, the operator implemented the following measures to reduce the risk of creating explosive atmospheres and their ignition:

• installation of an extraction system at the manhole, which is closed if the extraction system fails;
• manhole equipment with a “return to closed position” system;
• prohibition of access near the manhole during filling
• modification of solvent inlets inside the tanks: rain filling replaced by a filling by licking the walls;
• reduction of the filling speed;
• limitation of the tank’s liquid high level to less than 50 cm from the manhole’s bottom;
• control of the mixing speed;
• checking the electrical potential.

In the medium term, the operator also plans to:

• the installation of a CO2 inerting/extinguishing system on the tanks;
• the continuous measurement the LEL at the manhole’s bottom using a control;
• the installation of a high level sensor in the tank, with servo-control.