Applying fundamentals, one can see that a propane tank can produce conditions approximating an "overfilled" condition even without disabling the OPD.

The ODP device maintains an 80% fill capacity at a some standard temperature. The 80% fill capacity is by volume, not mass. However, at constant volume (thinking in terms of Thermodynamics), the pressure will be a function of the mass and temperature. The mass will be a function of the density of the propane assuming a relatively a relatively large vapor space volume to absorb liquid bulk expansion for a non-ideal gas. Thus, if the cylinder is filled with relatively low temperature propane, the temperature being relative to the standard temperature used to calculate the 80% fill capacity, there will be more mass in the tank relative to the standard temperature. Therefore, as the propane in the tank warms up to the standard temperature, the pressure and volume of liquid will increase.

At the same time, the vapor space fill volume will decrease. When this volume decreases, then the pressure inside the tank goes from behaving like an Ideal Gas (subject to the Ideal Gas Law) and behaves more like a non-ideal gas where small increases in temperature can create much larger increases in volume and pressure in a non-linear manner.

Douglas J. Carpenter, MScFPE, CFEI, PE, FSFPE
Vice President & Principal Engineer
Combustion Science & Engineering, Inc.
8940 Old Annapolis Road, Suite L
Columbia, MD 21045
(410) 884-3266
(410) 884-3267 (fax)
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