Pollution
Humain
Environnement
Economique

At 2:25 p.m., an explosion followed by a fire occurred on a pipe of a potline at a chemical manufacturing plant. Flames shot out of the end of the pipe, which discharges to a 75 m³ open-air tank containing electrolysis effluent (water, hydrochloric acid, sodium hydroxide, sodium). Workers sprayed the tank’s bottom with fire hoses. At around 2:30 p.m., firefighters mounted a platform and cooled the equipment from above. Upon seeing the flames reignite when spraying was reduced, the operator halted production at around 3:00 p.m., putting an end to the fire. The firefighters checked the piping over the tank with a thermal imaging camera. They left at around 4:00 p.m.

The epoxy tank and three plastic pipes were burnt. The potline was shut down for just over 24 hours.

The event was caused by the overflow pipe of the potline’s hydrogen non-return flow device (condensate). This pipe has a vent and was connected to the non-return flow device’s hydrogen exhaust stack when it should have been separate. Hydrogen was therefore continuously present in the pipe discharging to the open-air effluent tank. Excess hydrogen pressure in the network allowed gas to escape to the liquid effluent tank. At the same time, maintenance was being performed on the nearby hydrochloric acid synthesis furnace. Either a spark generated by this maintenance or faulty electrical equipment caused the hydrogen cloud to detonate and then ignite.

The ATEX zone was not identified during this maintenance work because hydrogen should not normally have been present in the area. The facility’s design did not match the risk analysis diagram used (HAZOP). There was an ‘as-built’ version, but apparently was not provided.

The operator took the following actions in order to restart the facilities the following day:

  • retrofitted the facilities so that the non-return flow device’s overflow pipe is vented separately,
  • checked the electrical equipment in the area,
  • checked the history of changes to the facilities,
  • checked the risk analyses (HAZOP) of the hydrogen, chlorine and hydrochloric acid circuits,
  • checked the unit’s hydrogen detection system,
  • identified the pipework.

The site has seen various other accidents since 2015 (ARIA 48354, 48177).