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For anyone contemplating a propane conversion on their on-the-road vehicle, only permanently-mounted ASME motor fuel tanks should be considered. Many propane conversion web sites and discussions on internet forums recommend the use of DOT lift truck fuel tanks. These are generally illegal and dangerous for use in on-road vehicles.
The off-roaders like the DOT tanks because they are portable and they can change out a fuel tank on the trail if they run out. Fork lift tanks are available in 20, 33.5, and 43.5 lb sizes. Liquid propane's specific volume is 8.547 ft³/lb which corresponds to 4.24 lb/gallon (US) 5.1 lb/gallon (Imperial). Therefore, this corresponds to capacities of 4.72, 7.90, and 10.26 US gallons. However, an ASME motor fuel tank's much greater capacity virtually eliminates the possibility of running out fuel from a single day of off-roading.
Manchester Tank (a manufacturer of both ASME and DOT tanks) does not recommend that their DOT lift truck cylinders be used for over-the-road engine fuel use as they were not designed for that purpose.
CSA B149.5 (Installation Code for Propane Fuel Systems and Tanks on Highway Vehicles) prohibits the use of DOT tanks in on-road vehicles. Although NFPA 58 (Liquefied Petroleum Gas Code) permits the use of these tanks in on-road vehicles, other requirements of NFPA 58 (free to view on-line) make the use of these tanks non-compliant. The reasons that DOT tanks are dangerous for use in motor vehicles that:
- DOT tank brackets are not certified to withstand collisions.
- DOT tank venting from safety relief valves is difficult to make compliant with NFPA 58.
- The appurtenances (fittings) on DOT tanks can NOT be enclosed in a vapour box for vehicle interior installations.
Plain and simple: ASME tanks are designed for use in motor vehicles and DOT tanks are designed for lift trucks. If you have an off-road vehicle and decide to use a DOT tank, do NOT mount it inside the vehicle. A spark from a switch (from opening the door, etc) can set off an explosion from a leaking fitting or from the blow-off of the safety relief valve. You must have a vapour box around the appurtenances of tank mounted inside a vehicle.