Explosion in a chlorine production plant

The event occurred at the chlorine drying towers of a mercury amalgam cell chlorine plant. The towers, made of PVC, exploded suddenly and violently.
At the origin of the event was a failure of the electrical power system, which caused the activation of a circuit breaker. The mercury pumps stopped, and the steel bottom plates in the cells became exposed. The alarm did not activate, because the alarm system was powered by the same AC power line which was been interrupted. Meanwhile, the brine supply to the cells continued, because powered DC power which was not interrupted.
Under these circumstances, hydrogen generated at the steel plate cathode and oxygen at the anode with a mole ratio of 4:1, formed an explosive gas mixture which passed to the chlorine drying towers.

Three towers were destroyed, except for the bottom structures which were made of steel. Window glass in the plant was broken within 50 m. One operator was killed and twelve seriously injured. They were all engaged in the repair work on one of the towers.

INITIATING cause was power failure with consequent stop of the mercury pumps. This triggered an accidental production of hydrogen which entered the chlorine flow up to the towers.

The most likely IGNITION SOURCE was a discharge spark from an electrostatic charge caused by dripping of sulphuric acid in the towers (the acid was used to dry the wet chlorine gas). A static potential of -5 kV was measured.

An inadequate plant safety design was the ROOT CAUSE.

chlorine drying towers

All started with a failure of the power system, which activated a circuit breaker

The most important and most generic lesson is that the critical safety devices, such as alarms on process deviations and malfunctioning, should be made able to function independently from these malfunctioning. This is valid also for the monitoring and control of safety-critical process parameters. This could imply to set up and independent or back-up power system for such devices.

A conclusion of the post-incident study regarded the need to avoid as far as possible ignition sources in the towers. They should be constructed of an acid-proof and conducting material in the future, so that they can be held at the earth potential together with sulphuric acid to prevent electrostatic charges building up. This would avoid or at least reduce the probability of ignition of flammable gases formed before in the electrolytic cells.

These plant improvements were suggested as result of the post-incident study [Tabata, 1986]:
(1) Install a current breaker of sufficient capacity to deal with an abnormal load.
(2) The AC power supply for control, measuring and alarm system should be taken from separate sources other than that for the mercury pumps of the DC power supply. The control, measuring and alarm systems must be capable of operating during a plant failure. A emergency power supply for this purpose should be considered.
(3) The continuous hydrogen analysers should be installed on the chlorine outlets of each cell to enable it to monitor any change of hydrogen concentration within as short a time as possible.
(4) Install an interlock system between the DC power supply to the cells and the AC power supply to the mercury pumps, and so also between the DC power supply and the continuous hydrogen analysers, so that an interruption of the power supply or an abnormal sudden increase of hydrogen concentration would shut down the entire plant.
(5) As for the shape of the towers, each chlorine gas outlet should be placed at the highest point of a tower, and not at its side wall. A conical, not a cylindrical, shape is desirable for the top part of the tower to eliminate any dead pockets where hydrogen gas could accumulate.
(6) An acidproof and conductive material should be adopted for the construction of the towers, so that the tower structure can be held at the earth potential, with sulphuric acid, to eliminate completely the electrostatically charging phenomena.