Explosion of a chlorine distillate tank of a chemical plant

The pressure vessel in a chlorine separation system exploded after the corrosion of the steel process equipment allowed chlorine and hydrogen to mix and form a volatile mixture.
The chlorine receiving tank ruptured violently into five pieces (three large ones and two smaller) causing considerable damage to nearby equipment. The appearance of the pieces confirmed that a the tank had failed by a rapid brittle fracture. The large size of the pieces and the degree of external damage implied a vapour-phase explosion rather than a more energetic liquid-explosion. No organic material traces were detected so that hydrogen became the only possible responsible of explosion.
A possible source was found in the corrosion of the steel packing below the feed point of the distillation column. It was not possible to identify a credible ignition source as in most previous chlorine-hydrogen explosions.

The INITIATING CAUSE was the generation and ignition of an explosive atmosphere in the chlorine tank.

The composition of this explosive atmosphere could only be guessed. From the analysis of the explosion debris, a vapour phase explosion appeared more plausible than a liquid one.
By exclusion, the culprit could be found in the generation of hydrogen from corrosion of steel. if this was the case, lack of inspection could be the ROOT CAUSE.

chlorine tank, chlorine distillation column