-- Just a small amount of flammable substance left inside an enclosed vessel could become a potentially deadly situation when exposed to spark or flame such as those encounter during metal cutting process.

What Had Happened

A technician was injured when he was trying to cut open the top of a 200-liter empty metal drum with a flame-cutting tool.

As soon as a hole was burnt through the top of the metal drum by the cutting tool, a stream of hot air shot out. The drum started to shake and then bounced up to above eye level, accompanied by a loud “bang” sound. The technician was found lying on the ground, injured.

Investigation revealed that the empty drum had been used for holding toluene, which is a highly flammable and toxic substance.  It was obvious that the tank had not been cleaned thoroughly and sufficient amount of flammable vapor was left inside the drum, which was ignited by the spark generated by the cutting tool.  The end plates of the drum were found bulged out afterwards.  It was indeed very fortunate that the drum did not explode. Otherwise, more serious injury and damages would have occurred.

Lesson Learned

If a welding torch is used on a tank or drum containing even a small quantity of flammable material that left over-- like a teaspoon of petrol in a 45-gallon drum -- the drum can explode violently. Similarly, for toluene, as little as 10 ml of residue liquid is sufficient to vaporized to create an explosive situation in a 200-liter vessel.  People have been killed or seriously injured by such explosions.

Reuse of “used chemical containers” should be avoided as far as possible. If it is unavoidable, they must be handled with extreme care. The history of the containers must be known and extreme cautions must be taken to ensure that they are completely free from residual flammable or toxic substances.

Even after adequate cleaning, safer methods such as cold cutting should be used as far as possible to modify a chemical drum. If it is inevitable that hotwork have to be conducted on these containers, adequate precautionary measures have to be taken, such as filling the container with water or an inert gas, such as nitrogen.