At 6:45 p.m., the visual and audible alarm for boron trifluoride (BF3), a toxic and corrosive gas, was triggered on the electrochemical detectors in the control room and the resins workshop of a petrochemicals plant. A leak was found on the BF3 supply station. After being shut down for three weeks, the workshop’s polymerisation section had been restarted with a supply of BF3 at 8:00 a.m. At 6:50 p.m., the emergency stop was activated, isolating the spheres, and the BF3 station’s deluge sprinkler system was turned on. Shortly before 7:00 p.m., the building housing the workshop’s control room was confined and the roads were cut off to traffic. Despite the spheres being isolated, the site’s employees saw that gas was still flowing out. Technicians wearing SCBAs went to the rack to isolate the circuit. At 7:15 p.m. they closed a bleed valve on a gauge manifold, which was partially open. The leak of BF3 stopped one minute later.

The operator estimated that 15 kg of BF3 had escaped into the air. Five people were sickened and taken to hospital for testing. The workshop and its power supplies were shut down for four days. Before restarting the workshop, all its electrochemical BF3 detectors that were exposed during the event and the seal on the leaky bleed valve were checked. The bleed valves on the BF3 station’s manifolds were found to be clogged.

The leak was caused by a manifold valve that had been incorrectly closed after a 120 bar nitrogen leak test had been performed on the BF3 feed system the day before the workshop was restarted. A plug of BF3 residue ensured the circuit’s tightness for a few hours.

The operator reported that the instructions for checking the tightness of the BF3 station did not specify the position the manifold valve should be in at the end of the test. In addition, there was no way to check the valve’s tightness and no data on the valve’s position was relayed back to the control room. The operator said that the automatic control systems would subsequently be modified to allow even faster activation of the safety sequences and that the bleed valves on the manifolds now feature a double closure system (valve plus seal).