Explosion at the chlorine electrolyser of a water treatment plant

An explosion occurred at a tank of the chlorine electrolyser of a water treatment plant
The plant products were sodium hypochlorite as a disinfectant for water treatments. The production process used was electrolysis of NaCl brines, which has hydrogen as by-products and has to be vented.
Perhaps, the cause of this event was the erroneous closure of the hydrogen vent line, forcing the hydrogen gas into the liquid holding tank where it accumulated.
During a maintenance stop, plant workers had drained the tank to within a few inches, to be able to repair a leak. They lowered then an electric pump into the tank to remove the remaining liquid. When the pump was switched on, the tank exploded.

The INITIATING cause was the ignition of the hydrogen present in the NaCl tank.

According to the investigation of the company CHEMAXX, the presence of hydrogen in the tank was probably due to the failure of or wrongly operated hydrogen venting system.
The root cause is then probably a wrong procedure (ill-designed, or wrongly implemented).

hydrogen vent, NaOCl storage tank, pump, switch

At the moment of the explosion maintenance works were ongoing on the NaOCl tank.

DESCRIPTION OF THE PROCESS
The water treatment plant used an electrolytic process to generate sodium hypochlorite (NaOCl) from sodium chloride (NaCl).
NaCl + H2O + ENERGY → NaOCl + H2

The main issues investigated by Chemaxx (company that carried out investigation) focused on the mechanisms and rates by which hydrogen gas is generated and subsequently accumulated in the holding tank. The need for active venting, warning signs and incidents of a similar nature were also reviewed.

The use of liquid sodium hypochlorite (NaOCl) instead of the more classic gaseous chlorine (CL2) had the advantage of handling a liquid, which is generally safer. The disadvantage was the production of hydrogen as a by-product of electrolysis. The hydrogen had then to be safely handled, for example by venting it to the atmosphere before the storage of the sodium hypochlorite in a tank.

The company that carried out investigation (Chemaxx) identified the need to guarantee active venting hydrogen. They required also warning signs and the implementation of an alarm system able to detect failure of the venting system.