Fire in the hydrogen washing tower of a chlorine plant

An explosion occurred inside the hydrogen washing tower of a chlorine production plant.
The explosion was preceded by an abrupt reduction in the electrolysis flow due to a rectifier malfunction. The fact that the electrolyser was working at low power caused a significant increase in the oxygen level in the hydrogen gas.
For the temperature prevailing in the cubicle, the hydrogen gas contained an excessive amount of diluting water vapour which, however, was absent in the top section of the washing tower, allowing for the production of flammable oxygen-hydrogen mixture. The explosive gas ignited and broke the burst disk. The discharged hydrogen ignited, causing a minor leakage of hydrochloric acid in the hydrochloric acid top container above it and the related pipes.

The electrolysis flow in the unit was stopped by the emergency shut-off switch two minutes after the explosion, at which point nitrogen flushing to dilute the gas was also started. Burning hydrogen gas was discharged from the rupture disc opening until the connection to the main hydrogen pipe was closed.

The INITIATING CAUSE was the formation of a hydrogen-oxygen mixture in the upper part of the washing tower, with escalation outside it due to the opening of the burst disc and the release of ignited hydrogen.

Although the burst disc functioned as designed and avoided over-pressurisation of the tower, the event still highlighted a certain lack of additional safety measures which could have avoided escalation up to explosion, such as automatic shutdown at too low power, or gas detection. The ROOT CAUSE could be identified by two factors: shortcoming in operative procedure and in safety design, this latter triggered probably by less than optimal risk assessment.

burst disks, hydrogen washing tower

The electrolyser was functioning at low power, due to the malfunctioning of the rectifier.

DESCRIPTION OF THE PROCESS
A hydrogen washing tower in a chlorine production plant is a piece of equipment used to cool, dehumidify, and clean the hydrogen gas produced during the electrolysis of brine, ensuring high purity and safety before it is used or stored. This tower circulates an alkaline, water-based solution to remove moisture, caustic soda and other impurities like chlorine from the hydrogen stream.

Oxygen can enter the hydrogen flow due to inadequate seals or to membrane leakage or crossover. In this latter case, oxygen produced as a side reaction at the anode can cross over into the cathode side and travel with the hydrogen to the washing tower.
Membrane can become damaged due to overpressure or ageing. Membrane cross-over may occur during transients such as start-up, shutdown, low-load operation or rapid load changes. The two last cases were present during this event, due to the malfunctioning of the rectifier and the consequent strong reduction of the power to the electrolyser.

The oxygen which entered the hydrogen flow travelled with it up to the hydrogen washing tower. By the progressive elimination of the other impurities and the reduction of water steam towards the top of the tower, the relative concentrations of hydrogen and oxygen increased, up to the moment that the lower flammability limit of oxygen in hydrogen was reached (5% vol.). The over-pressure caused by the explosion of the mixture in confined space (ignition source not reported), the pressure relief device (burst disks) worked as designed, avoiding over-pressurisation of the tower and its catastrophic failure.

Thus, mitigating measures in place worked as expected; nevertheless, some additional safeguards would have further reduced escalation and damage.
Continuous assessment of the hydrogen quality in the washing tower and detection of oxygen concentration in the hydrogen header of the electrolyser would detect as early as a possible the development of a hazardous situation and trigger alarms and automatic actions in case of trespassing of safety thresholds. These detectors need to be quick and reliable, possibly thanks to redundancy. Moreover, interlocks would be able to avoid operation of the electrolysers outside the allowed operative ranges.