The peak over-pressure will not be at the lean flammability limit. It will be just to the lean side of a stoichiometric fuel/air mixture.

The rate of volume expansion due to the change in phase is really not important in this context. It is the expansion of the fuel gas as a result of ignition that is important. Based on temperature, this expansion can be approximated as seven (7) to eight (8) times the initial volume.

This phenomenon can be used to perform a relatively simple calculation of the peak pressure rise from a volume of gas in a fixed and confined volume (e.g. room). Such a calculation can illustrate that the entire room does not have to be filled with a flammable fuel/air mixture to cause an explosion resulting in significant damage (a common misconception or assumption made by many fire investigators). A half inch deep layer of a flammable propane mixture is sufficient to blow out windows at a peak pressure rise of 0.5 PSI.

While I have not done a calculation of your scenario, based on other calculations I have done, a one pound bottle is very likely to to cause an explosion that will dislodge window glass. The issue of a specific distance can be calculated, but the big unknown will be the peak pressure at the time when the glass if released from its mounting as well as the effect of other "vents" that could open prior to dislodging the windshield glass and reduce the peak over-pressure.

The calculation of the distance will provide a required initial velocity that the windshield must be accelerated to in order to reach a distance of 122 feet before gravity pulls it to the ground. This acceleration requires a force that can be related to a pressure. So a reliable model can be developed using a series of linear equations.

The issue of propane being heavier than air is true only under specific referenced conditions (e.g. STP (Standard Temperature and Pressure)). In addition, if the propane gas is part of a mixture, it will change the specific gravity of the propane mixture. Thus, it the temperature of the propane gas is higher than STP, then it may not be "heavier than air." Also, if the propane gas is released under pressure in the form of a velocity jet, the jet will entrain air resulting in a much different specific gravity. This the resulting specific gravity of a propane/air mixture with increased temperature can be calculated. Thus, "it depends" can be applied to the statement that "propane is heavier than air."

Another good example of why the physics and chemistry of fire is important fundamental knowledge that can be applied to practical problems.

Chris Bloom, CJBFireConsultant Wrote:
-------------------------------------------------------
> Gary:
>
> Depending upon the bottle and who filled them
> (commercial or home-filled), they should be about
> 80% filled with liquid when full. A 1-pound
> cylinder (FULL) will have a rough estimate 1 quart
> of liquid propane capacity (1gal=4.24pounds).
> Then depending upon the ref source, the expansion
> rate is anywhere from 270-286:1 liquid to vapor.
> then calculate the lower most flammable limit into
> the equation for the best case (lean) explosion
> mix potential inside. Don't forget that while the
> explosion can kick out windows, walls, etc, there
> should be some flash fire thermal damage to the
> blinds and other furnishings.
>
> If the RV did not have an onboard propane system,
> then there would be no requirement for the LPG
> Detector which would have likely notified them of
> this incident beforehand.
>
> Could the windows have been dislodged due to fire
> suppression using straight streams instead of fog
> stream? that may be another reason for the
> distance issues.
>
> Chris

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)
www.csefire.com