Pollution
Humain
Environnement
Economique

Whilst on patrol at a solvent production unit, a night technician detected a burning hydrogen leak in the unit’s gutter. The unit’s emergency response team brought the fire under control within minutes. Technicians cut off the flow of hydrogen by closing a valve. The hydrogen network was purged with nitrogen.

Of the three reactors at the unit, only one was operating when the incident occurred. The gaseous effluents (mainly hydrogen) from each of the reactors were collected via a network of vents and discharged to atmosphere via a set of hydraulic valves.

On the day of the incident, a section of the shop had been completely emptied for maintenance and inspection. The hydrogen exiting from the operating reactor flowed through this section, which had fewer obstructions (all the equipment had been purged and some valves had been left open) than the hydraulic valve set in the vent system. On leaving the section, the hydrogen escaped to atmosphere through the gutter, which was not free of impurities. The hydrogen (minimum ignition energy of 0.017 mJ) carried solid particles, which rubbed against the walls of the pipe and generated an electrostatic discharge that ignited the hydrogen. The gutter had been drained the day before, but the solid particles carried by the hydrogen could not be removed by the water.

The risk analysis did not foresee a return of hydrogen to a via through the venting system. After this accident, the UIC (French Chemical Industries Association) issued the following guidelines:

  • When multiple sections are connected to the same equipment, all issues that could occur on either side of the equipment should be identified along with their consequences; In particular, situations where operational states are steady on one side but transient on the other (cased of equipment stops and starts) must be studied.
  • When such an incident scenario is identified, the unit’s operating instructions must be modified to include the operation of closing the corresponding valves.