Most of the time diesel tanks do not need to be zoned but as with any of these things there are exceptions. It must be remembered that ultimately this will require a component person to make an assessment of the individual installation in accordance with AS/NZS 60079.10.1.
Why aren’t diesel tanks normally classified as giving rise to a hazardous area? The answer is the flashpoint of the diesel. The flashpoint of a liquid is the temperature at which a particular organic compound gives off sufficient vapour to ignite in air. Most grades of diesel have a flashpoint above 60°C which according to AS/1940 makes them combustible liquids and not flammable liquids. This means at normal atmospheric conditions they should not give off enough vapour to create an explosive atmosphere.
So when can a diesel tank or diesel fuel create a hazardous area:
1: If the bulk mass of the fuel is being heated by means other than solar radiation, or is involved in any process that may reduce the flash point below 60°C then hazardous areas need to be assessed,
2: The vapour space in a diesel tank that has previously contained flammable liquids or shares a vent line with other tanks with lower flash point fuels needs to be considered for hazardous area classification due to the potential for vapour from other sources,
3: If diesel is sprayed and could form a mist the combustible liquids could ignite even at temperatures below its flashpoint.
“This following information represents and opinion only of a general nature, the specifics of each individual situation must be taken into account with reference to the relevant Legislation, Codes of practice and Australian standards. Professional advice should be sort if there is any doubt.”