Explosion at a chlorine electrolyser

This incident occurred at a chlorine production plant.
A number of minor explosions occurred in the interrupter pot on a chlorine cell of the electrolyser. No injuries or damage were reported.
No previous explosion had occurred, and this was considered an indication that hydrogen concentration within the pot was normally below flammable limits.
The ignition was believed to have resulted from the detachment of the dispersing tray in the pot, the resultant lack of a controlled break in the conductive sodium hydroxide stream leading to the release of sufficient electrical energy to ignite the flammable gas mixture. However, it was also thought that other conditions could provide sufficient energy to ignite hydrogen and air mixtures, and that elimination of the possibility of electrical ignition would be very difficult.

The INITIATING CAUSE was the formation of an explosive mixture containing hydrogen in the electrolyser cell, with successive ignition and explosion.

The source did not identify the underlying reasons for the presence of hydrogen more than the lower flammability limit. Nevertheless, the return of experience revealed the absence of diagnostics able to provide information on the status of the parameters process and to identify deviation from the normal operative conditions. In this sense, at least one component of the ROOT CAUSE is the lack of proper safety design.

chlorine electrolyser, cell

DESCRIPTION OF THE FACILITY
The event description provided does not specify which type of chlorine electrolysis technology was in place. It is highly probably that this was an electrolyser based on the Carsten-Keller process, using a mercury cathode. In this unit, the production of chlorine happens in the electrolytic cell, while sodium hydroxide and hydrogen (as byproduct) are formed in the amalgama cell (see for example https://vula.uct.ac.za/access/content/group/9eafe770-4c41-4742-a414-0df36366abe6/Chem%20Ind%20Resource%20Pack/html/learner-sheets/C/LS_C10.pdf).

This was the first time that an explosion had happened in the unit. Nevertheless, an explosive mixture of hydrogen and air could not be excluded in the interrupter pot of a chlorine cell, simultaneously with an ignition source. To understand the phenomena playing role in this incident, plant operators should gather lesson learnt on explosion occurred by other chlorine producers with this type of interrupter pot.

Moreover, a better process control system should be in place, to understand the operative parameters and to act timely to prevents reoccurrence. This includes:
1. Pressure gauge in the interrupter pots.
2. Hydrogen concentration detector in the interrupter pots.

To prevent reoccurrence, the presence of flammable mixtures in the pot must be avoided.
The following possibilities were identified:
1. Prevent hydrogen concentration from reaching the lower explosive limit by having excess induced air into the pots.
2. Prevent a flammable mixture by providing an inert atmosphere.
3. Prevent a flammable mixture by keeping the hydrogen concentration above the upper flammable limit by modifying the seal caps on the interrupter pots.