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Alert: Welding on Containers

  • ISSN: 1195-5228
  • Issued: March 1995
  • Content last reviewed: January 2011

Hazard Summary:

In two recent incidents, explosions during welding on fuel tanks killed three workers. In the first, two workers welding a 150-gallon diesel fuel tank were fatally injured. In the second, a 500-gallon gasoline tank exploded during welding, killing a worker. These accidents would not have happened if proper cleaning, purging and testing procedures had been followed.

After the first incident, it was discovered that the diesel fuel in the tank was contaminated with gasoline. As little as two per cent of gasoline in diesel fuel can create an explosive mixture in a closed container, with the flashpoint (the temperature at which a spark or other ignition source will cause an explosion) falling below the temperature inside the container.

Required Precautions:

For any hot work such as welding or cutting on a container that may have contained flammable or combustible material, the following minimum precautions must be taken:

  • The container's internal layout must be determined to make sure that fittings such as baffles will not interfere with cleaning or purging.
  • The container must be drained and cleaned using appropriate methods.
  • To determine whether draining and cleaning has made the container safe, its interior must be tested with a combustible gas detector both before hot work begins and periodically during the work.

However, some containers cannot be drained and cleaned well enough to make them safe. Such containers may be made safe by purging and inerting with an inert gas, but only if these precautions are taken:

  • Recognized procedures and proper equipment must be used.
  • The oxygen level inside the container must be monitored with an oxygen analyser and maintained at essentially zero for the duration of the work.
  • Workers must be made aware of the limitations of the inerting process.

Never assume a container is clean or safe. Make sure that it is made safe and that its safety is verified by testing before any hot work begins. Not following this rule is likely to kill you!

The Regulations for Industrial Establishments (R.R.O. 1990, Reg. 851, s. 78) require that where repairs or alterations are to be made on a drum, tank, pipeline or other container, it must be drained and cleaned or otherwise made free from any explosive, flammable or harmful substance.

See Engineering Data Sheet 4-14 "Welding and Other Hot Work on Containers", available from ServiceOntario Publications, for more information. For assistance call the Ministry of Labour Health & Safety Contact Centre.

This Ministry of Labour Alert has no legal effect and does not constitute and is not a substitute for legal advice. If you require specific assistance with respect to the interpretation of a legislative provision and its potential application to you please contact your legal counsel.

Remember that while complying with occupational health and safety laws, you are also required to comply with applicable environmental laws.

Please photocopy Ministry of Labour Alerts, distribute them widely and post them where people will see them.