Using a tool which generates heat, like an angle grinder, to remove the top of a 200 litre drum may be dangerous, particularly if the drum has previously contained a flammable liquid. The drum is an enclosed space, and the application of heat from a cutting tool on the outside of the drum, will potentially vaporise any flammable residue inside and provide a source of ignition the resulting air/fuel mixture. So warn the Australian Transport Safety Bureau in its report of the death of an oiler aboard the Cape Darnley on 8 July 2010.
In this case the drum had previously contained Mobilgard 412, a mineral oil, and had not been cleaned or rinsed out. Later in was filled with up to 70 litres of waste water and carbon residue from an exhaust gas boiler clean. sealed and kept in the engine room for seven days. During that time oil/carbon residues separated from the water and floated on top of it, producing an explosive vapour.
When the angle grinder was used to remove the top of the drum the vapour ignited resulting in the fatal explosion.
The most immediate lessons are that closed drum should be opened, rinsed thoroughly and left with the cap off to ventilate and, preferably use something like a cold chisel rather than an angle grinder.
A second lesson, of course, is to be aware of the hazard and ensure that an appropriate job safety analysis is done.