Pollution
Humain
Environnement
Economique

A fire broke out at the top of the flare stack of a Seveso-classified chemical plant at around 2:45 a.m. during a violent thunderstorm. The flare stack is used to discharge hydrogen (highly flammable gas) produced by the restarting of chlorine electrolysis cells. The fire was ignited although steam and nitrogen had specifically been injected to prevent it from occurring. The plant’s internal emergency plan was implemented. At around 3:10 a.m., the site’s firefighters cooled the flare stack while the technicians turned off the electrolysis cells to cut off the hydrogen feeding the flames. The electrolysis cells had been restarted following a power outage that occurred at 1:20 a.m. at the start of the storm. The fire was extinguished at 4:57 a.m., the internal emergency plan was lifted at 5:05 a.m., and the electrolysis unit was restarted at 5:10 a.m. As the minimum ignition energy (MIE) of the oxygen-hydrogen mixture was very low, a bolt of lightning to the flare stack was sufficient to start a fire. Steam and nitrogen are injected in order to dilute the hydrogen stream and raise the MIE. However, this does not altogether eliminate the risk of ignition.

A similar accident that occurred five years earlier had led to:

  •  A velocity seal being fitted in the flare stack to prevent flashback into the unit in the event of negative pressure.
  •  The diameter of the top of the flare stack being reduced to increase the flow of the stream.
  •  The second stack being raised to reduce the effect of thermal flows on the adjacent chlorine cooling tower.