The incineration of flammable organic effluents (cyclohexane and heavy tars) from a chemical plant was underway since 5:30 a.m. when the feed rate of the line supplying the oven became unstable, causing the burning operation to stop at 11:25 a.m. Unsuccessful attempts were made in the control room to restart the burner. As such, at 11:30 a.m., the pipeline operator shut down the pump supplying the line from the effluent tank. An operation’s technician was dispatched to the installation to restart it manually. Everything appeared normal (noise, pressure), and the installation restarted after the third attempt at 11:33 a.m., but stopped 1 minute later. The technician smelled the organic effluent and detected a leak in a pipe elbow. He alerted the control room and moved closer to examine the leak when it ignited with a flash, and then exploded. The technician and another operator fought the fire with portable fire extinguishers, although were unable to bring the burning leak under control. A 100-litre fire extinguisher, available in the installation, was used to bring the fire under control while the valves were closed to isolate the supply line, and the furnace was shut down. The site’s in-house fire-fighters arrived at the scene of the accident and got control of the second fire with fire hoses. Once alerted, the emergency services arrived on the scene but did not have to intervene. The operator sent out a press release as the noise produced by the explosion had been heard by the residents of the neighbouring village. The alert was lifted at 12:15 p.m.

Two days before the accident, an exceptional operation to incinerate deperoxidised organic waste had meant that the tank, storing the organic effluents to be burned, was isolated from the oven, and the oven’s supply line was connected to a container holding these wastes. The tank’s recirculation system had thus been cut off. On the morning of the accident, the operators switched the boiler’s supply back over to the tank, while forgetting to open the valve on the recirculation system. When the supply of the oven by the tank was restarted, the line gradually increased in pressure due to a choked filter (never cleaned) and the inability to release pressure to the fixed tank because the recirculation valve was closed. It was increasingly difficult for the feed pump to maintain the nominal pressure, causing the oven’s burner to switch off. The supply line was rinsed with water the day before, when the container was empty, but the operators had neglected to empty the rinse water. Leading up to morning of the accident, this piping system (steam-heated during winter due to the risk of freezing) had remained full of water for 29 hours, resulting in the thermal expansion of the water which weakened the line’s joints due to the excess pressure. The pressure surges generated in the line by the successive attempts to restart the pump caused a seal to rupture at the flange of an elbow, then the leakage of flammable organic effluent which vaporised in the hot atmosphere of the incineration unit (t° > 35 °C), ignited and then exploded (UVCE) in contact with the boiler’s casing (t° > 250 °C).

Hydrostatic testing of the recirculation line showed that there was a 2nd faulty flange seal. There were no formalised operating procedures or checklists for switching the oven’s supply loop between the tank (normal mode used most of the time) and a container (used more rarely, once a month). During the changeover sequence, the unit’s operators discuss any actions to be performed. Given that this information is only communicated orally, there is an increased risk that an operator will forget one of the 5 valves involved in the sequence. Furthermore, the operator had not studied the accident scenario because it was not likely to cause a major accident or effects beyond the site’s property limit, which led to an underestimation of the risk of operating the pump at excess pressure and thermal expansion in the supply line.

The operator replaced all of the line’s seals and all the wiring, valves and instrumentation damaged by thermal effects (representing 9 days of work), locked the tank’s recirculation valve in the open position, installed a high-pressure safety valve to stop the feed pump and eliminated line’s steam heating system, which is not required for the effluent to be burned. In a second phase, the operator removed a maximum number of flanges on the line and added anti-splash shields on the remaining flanges. The power of the feed pump was reduced as it was initially overpowered, and the filter was removed (of no use). Finally, exceptional injections of deperoxidised organic waste into the incinerator via the organic effluent line were stopped.