Explosion in an electrolyser producing chlorine

An explosion took place in the electrolysis room of a plant producing chlorine and its derivatives. An auxiliary outgoing circuit breaker opened, due to their ageing, to the overload of the auxiliary transformer and to high temperature. In absence of power, the mercury and brine pumps and the chlorine compressors stopped.
Under normal operating conditions, the circuit breakers of the three transformers supplying the current to the auxiliary systems of three electrolysers should all open (trip) automatically when the coil power supply is interrupted.

In this case, however, one of the circuit breakers did not trip and remained closed, supplying 20 kA power to and one electrolysis room. The explosion occurred when the auxiliary transformer was finally closed after several attempts, 15 minutes later. The chlorine main collector upstream of the compressor was destroyed, 15 electrolytic cells were seriously damaged, but no casualty was reported.

Since the malfunctioning circuit breaker was found in good condition and had been in operation for 2 years, the most probable explanation was the presence of a strong magnetic field generated by the nearby 50 kA conductors, which could have either excited the coil or induced a residual magnetisation of the breaker core.

A plausible cause for explosion was the formation hydrogen by the acidic attackat the bottom of the cells and production of sodium and potassium hypochlorite. The still running chlorine compressor extracted the hydrogen and reacted with the chlorine remaining in the pipe.

As the operator did not realise that the circuit breaker was not closed, he did not prohibit the reactivation of all auxiliary circuit breakers, as he should have done.

electrolyser, circuit breaker

The circuit breaker which did not operate was in good state and only two years old.

The common chlorine collector and various compressors were destroyed. The electrolyser unit where the explosion took place was heavily damaged. No injuries among the personnel.

The mitigating strategy worked as planned. The flammable gas detectors alarmed the operative room and the plant was shut down, the reactor purged with nitrogen, and the affected reactor components were cooled by the fire brigade. The fire was extinguished by 5 minutes.

(1) Instructions were amended,
(2) Safety devices were installed to prevent magnetisation of the components of the circuit breakers,
(3) Provisions were made to reduce the power load of the circuit breaker of the auxiliary components.