Explosion at a chlorine production plant

An explosion occurred at a chlorine production plant during the restart of the installation.
An hydraulic syphon had probably been blocked by fire extinguishing products following a previous fire on the hydrogen exhaust system. While restarting the chlorine production unit, the hydrogen pressure started to increase, allowing hydrogen to permeate through the electrolyser membrane into the chlorine compartment.
The chlorine /hydrogen mixture then entered the chlorine drying unit before exploding. The explosion was probably initiated by a discharge of static electricity or UV radiation.
The internal emergency plan was started. The were not victim. The desiccation towers were destroyed and the chlorine manifold damaged.

The INITIATING cause was the clogging of the hydrogen venting pipe, so that an increased hydrogen pressure allowed it to permeate into the chlorine cell.
The increase in the chlorine’s hydrogen content had been detected by an on-line analyser 15 minutes prior to the explosion, but apparently it did not trigger any countermeasure.
The root cause were ill-designed prevention measures and safety procedures.

hydralic syphon,
hydrogen venting sytsem,
diaphragm,
drying tower

A fire had occurred on the hydrogen exhaust system, which required the use of fire extinguishing products. These had blocked the hydraulic guard on the hydrogen exhaust.

Fire fighting means were not appropriate and monitoring procedures were not adapted to hydrogen overpressure or pollution of chlorine with hydrogen.

The accident demonstrated that the preventive measure for hydrogen fire were inappropriate or insufficient: in particularly the (i) detection of overpressure in the hydrogen manifold, (ii) the analysis of hydrogen in the chlorine compartment, (iii) inappropriate operating procedures mitigating these events.

The following modifications were implemented:
(1) Elimination of the possibility of pressure build-up in the H2 manifold,
(2) Reinforcement of hydrogen overpressure detection systems
(3) Reinforcement of the detection of hydrogen in the chlorine,
(4) Adaptation of the operating procedures for these cases.