ALWAYS REFER TO WHMIS/ MSDS SHEETS TO GET INFORMATION ON THE DRUM(S) WHERE HOLDING.
CSA Standard W117.2-94, Safety in Welding, Cutting, and Allied Processes.
The National Fire Protection Association (NFPA) Standard 51B “Fire Prevention in the Use of Cutting and Welding Processes” serves as the basis for the fire codes and many fire prevention practices adopted by industry.
The small fuel tanks and other containers that have been used to store flammable or combustible liquids, gases, or solids pose a special problem for shipyards, garages and shops that are asked to repair them. Their size normally precludes entry for inspection and cleaning, and liquid or solid residues that can release flammable vapors are probably present. Obviously, these must be removed before welding, cutting or other hot work commences that could cause a fire or explosion.
Before attempting to clean or purge such a tank or container of fumes: Disconnect or remove all sources of ignition from the vicinity; Reduce any pressure in the tank or container to atmospheric; Disconnect, plug or block off all piping, traps, standpipes, etc.; and Then drain and safely dispose of all liquid or solid residues removed from the tank.
Next, remove all flammable vapors and solid residue by completely filling and draining the tank or container with water several times. Fill the tank completely; make sure you do not leave any vapor pockets. You may also gas free a tank or container and maintain a ‘safe for hot work’ atmosphere by purging with air and then maintaining the air flow for continuous ventilation. You have to be careful when purging with air because there probably will be a short period at the beginning when an explosive mixture of air and gas will form before a safe atmosphere is obtained. You can minimize the hazard of an explosive air/gas mixture forming when purging with air if you first purge with an inert gas (such as carbon dioxide or nitrogen), then maintain air flow for continuous ventilation.
Finally, you can also displace the oxygen in a tank or container to a point where combustion will not take place by filling it with an inert gas such as carbon dioxide or nitrogen. However, you must be very familiar with the limitations and characteristics of the inert gas being used and you have to maintain the oxygen content at substantially zero during the entire hot work process.
After cleaning, purging, and ventilation of any fuel tank or container, you must test for flammable vapors with a combustible gas indicator prior to hot work. (If you are purging the tank or container by flooding with an inert gas, then you also must test for oxygen.) The person conducting the test should be trained to use the instrument and to make the checks recommended by the manufacturer.