Leon wrote:
"krw" wrote in message
...
....
... There is no air in there at all, even without a gas cap the
vapor pressure of the gas will push any out.

Obviousely clueless.

Not quite, no...not _exactly_ "none" wrt to air but the high vapor
pressure (very low flash point) of gasoline _is_ the key. To save
having to dig up the flammability and explosive mixture data, the most
cogent explanation I found on the web quickly was at a site discussing
the Flight 800 explosion that went on to compare the situation w/ jet A
and an airline tank and the fuel pump in automobile tank and gasoline.
From that site--

Cars contain fuel pumps and wiring inside the fuel tanks - why
don't they blow up more often?

Gasoline tank vapor spaces are almost never flammable while Jet A
tanks in airplanes will always pass through a flammable regime during
normal flight operations.

The ullage of gasoline fuel tanks in automobiles is almost always too
rich to be flammable except at very low temperatures. This is due to the
much lower flash point (about -40 C) of gasoline in comparison to Jet A.
The vapor space in a partially-filled gasoline tank does not become
flammable until the temperature has dropped below about 10F and a
serious hazard will exist below 0 F down to about -40 F for a typical
gasoline (Reid Vapor Pressure (RVP) of 9.5 psi, flammability limits
between 1.4 and 7 % by volume). [See W.F. Marshall and G. A. Schoonveld,
SAE Transactions, Vol. 99, No. 4, 594-617, 1990]

For this reason, the probability that a fuel tank containing liquid
gasoline has a flammable vapor space is extremely small in most climates
except in the artic regions. The exception to this is when the tank is
removed for servicing and the fuel is drained from the tank. Since
gasoline has such a high vapor pressure, the tank can have a flammable
vapor space even if there is no liquid fuel visible inside that tank.
For example, the complete vaporization of about 1-2 tablespoons of
gasoline will result in a flammable mixture inside a 15-gallon capacity
automobile tank. This is the reason why welding on or near "empty" gas
tanks is extremely hazardous and thorough purging of the tank with
steam, carbon dioxide, nitrogen, or other inert gas is required before
repair work is started.

The in-tank fuel pumps in modern fuel-injected vehicles are designed
to run submerged in fuel, as are the fuel pumps in aircraft, and the
fuel circulates through the pump, including the sparking
brush-commutator system. Unless the gasoline tank is almost
completely emptied of fuel, this will not pose a hazard since there
will be only fuel and not a fuel-air mixture within the pump. So in
order for an in-tank fuel pump to pose an ignition hazard, the tank
must be "run dry" under very cold conditions so that flammable vapor
surrounds the commutator.

In short, the answer is that the high vapor pressure of gasoline does
create an atmosphere that is too rich for explosion even when the tank
is uncapped if there is any liquid fuel at all in the tank at anything
above very cold ambient temperature.

I will agree it _is_ something of a mental twist that the very
volatility of gasoline is in fact the property that mitigates the fire
hazard suppressant from the fuel pump when the tank is low (although I
presume the pumps are mounted in a well or at least at the lowest point
in the tanks in order to keep them submerged until absolutely all fuel
that can be picked up has been. I suppose one could even raise the
inlet pipe a little to ensure that; don't know if they do or not.)

hth....

-dpb

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