Explosion in the chlorine line of a sodium hydroxide electrolysis unit

The incident occurred at a caustic soda electrolysis plant.
A chlorine suction pump failed on a sodium hydroxide electrolysis unit. The arrest of the pump caused the stop of the electrolysis current.
Recovery operations were carried out including nitrogen purging of hydrogen piping and air purging of chlorine piping before starting to add chlorine into the electrolysis vessel. A sudden explosion occurred about 40 minutes later at a chlorine chilling tower and chlorine transfer pipeline.

The cause of the incident was a leak through the valve on the chlorine pipeline. This valve had been correctly closed after the injection of air, but it was leaking. Since the fume removing equipment continued to operate, it created a negative pressure in the chlorine compartment. Therefore, residual hydrogen gas in the cathode cell went into the chlorine pipeline, forming a hydrogen-air explosive gas mixture.

The INITIATING CAUSE was the failure of an isolation on the chlorine compartment, which allowed hydrogen to access the chlorine compartment crating a hydrogen-air explosive mixture.
This incident occurred during the recovery work followed a non-hydrogen related emergency.
A contributing factor has been breaking of one of the exchange membranes, due to pressure fluctuations during the shutdown. This was due to the fact the discharge capacity of the water-sealed relief valve was insufficient, causing the pressure inside the electrolysis cell to fluctuate rapidly.

Despite the operational steps undertaken, material failures and lack of diagnostics allowed the occurrence of the explosion. The ROOT CAUSE can be attributed to shortcoming in performing the risk assessment and in designing the related safe procedures.

hydrogen pipeline, chlorine pipeline, electrolytic cell,

A recovery action was ongoing, caused by the arrest of a pump.