That Container Only Looks Empty

Containers that have held flammable or combustible liquids can remain explosive even after the liquid has been removed. The liquid in the container is replaced by air which mixes with the hazardous vapors. This combination can be explosively ignited by a spark or heat. In fact, these containers are normally more explosive than a full container.

How many times have you seen a 5 gallon pail or a 55 gallon drum being used as a welding or grinding stand? This is very dangerous. Any sparks produced could ignite the vapors. Also, the torch flame, heating the container, could ignite the vapors within the drum. The auto-ignition temperature of many flammable vapors is far below the melting point of steel. Some auto-ignition points are as low as 450 degrees.

Never attempt to do any hot work on a container unless it has been thoroughly and properly cleaned.

Steam, followed by a caustic soda wash and fresh water rinse, is the best method.
Dry with warm circulating air.
Inspect the inside for cleanliness. If it is not clean, the procedure will need to be repeated.
Use a combustible gas indicator to test the container for the presence of flammable vapors immediately before beginning the hot work. Never assume the container is clean enough. Be certain!
As an added precaution, fill as much of the container as possible with water. Inert the remaining space with nitrogen or carbon dioxide.
Complete these steps only in a well ventilated space. Once all this has been done, the hot work can be performed with a reasonable degree of safety. The key is a clean container that has been verified to be free of vapors. There is no other safe way.

Don't forget, vapors can travel to a source of ignition that is located well away from the work area. For this reason, always replace the cap on any container. With the cap off there is a greater likelihood of product vapors escaping into the air where they could be ignited, or where a source of ignition could be accidentally introduced into the opening.