Explosion in a chlorine production plant

During chlorine-alkali electrolysis an accident occurred in which chlorine was released due to a chlorine-hydrogen explosion.
The person charged with the control of the measurements was informed by an acoustic signal that a short circuit had occurred in one of the cells. This occurrence is not exceptional and is generally caused by a slight pressure fluctuation in the hydrogen system, which is passed back through the sloping connection between the hydrogen and the chlorine sections.

Searching the cause of the increased hydrogen pressure, the controller found out that the hydrogen compressor, which removed all hydrogen from the plant, had stopped. The controller tried to restart the compressor, which succeeded in the first instance. But short time afterwards, the aggregate shut down again, due probably to the low-pressure blocking system caused the break down of the aggregate.

The crew observed at that time that a considerable release of hydrogen occurred from the small pipes of the amalgam decomposers. The reason of it was the insufficient purge of hydrogen over roof passing through a hydraulic guard foreseen for such cases. This hydraulic guard has the function to transport hydrogen without danger over the roof. On the day of the accident, however, the function of the safety devices was limited by a non-negligible dirt deposition had formed inside, which increased the counter pressure. This situation was not recognised. Hydrogen reached the cell-space through the mercury pump, which transfers the regenerated mercury again in the electrolytic cell. This fast and massive intrusion of hydrogen caused a local formation of an explosive mixture. The first heavy explosion destroyed a part of the cells as well as the chlorine collection pipe passing under the explosion point.

The plant was shut down by emergency shut down by the crew immediately after the explosion, but the ongoing decomposition of the amalgam did not stopped the production of hydrogen. Following that, another heavy explosion occurred, which destroyed the externally passing chlorine collection pipe.

According to the source, the electrolyser was working as expected, and there was no sign of any malfunctioning of the safety functions.
Therefore the INITIATING CAUSE, which was a material malfunctioning, could be also the root cause. However, again according to the source of the incident, the malfunctioning of the hydraulic guard (which is a safety device) remained unnoticed because a significant amount of dirt had accumulated. This points at a lack of maintenance and/or at a shortcoming of the guidelines.

hydraulic guards, mervury pump, electrolytic cell

The electrolysis of chloro-alkali was performed, as it had been for the previous days, near to the technical capacity. The plant produced 15.6 tonnes chlorine/h and was in normal conditions. There were no evident deviations from the safe operating mode. At the time of the disturbance (accident, failure, breakdown) the shift was manned with a regular crew of 9 persons.

Three firefighters and one employee suffered upper respiratory tract irritation after their chlorine gas masks punctured. The affected employee was also in shock for several days.
Large material loss occurred.

After the first explosion the operation team activated the emergency shutdown of the electrolyser system. However, this did not stopped the production of and a second explosion.

The electrolyser system was technically improved, (i) to avoid the risk of a block of the drainage due to dreck, (ii) to allow for continuous visual inspection to control the process.