"A chlorine suction pump failed on a sodium hydroxide electrolysis unit.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 which was supposed to be closed after the air purging. Although fume removing equipment continued to operate, residual hydrogen gas in the cathode cell went into the chlorine pipeline, forming a hydrogen-air explosive gas mixture. This mixture was ignited by shock caused by the gas in the chlorine pipeline moving when a dilute salt solution was moved when resuming the operation."