From: Mike Lorrey (mlorrey@datamann.com)
Date: Wed May 29 2002 - 15:02:51 MDT
spike66 wrote:
>
> Charlie Stross wrote:
>
> >> I understood that they are pressurized but not to 1 atmosphere.
> >> The pressure in the tank would be lower at cruise altitude than
> >> on the deck. spike
> >>
> > Pedantic point: the cabin isn't pressurized to 1 atmosphere, either!
> > Ever wondered why you need to swallow while the airliner's
> > descending?
> >
> No Charlie, I didn't wonder, I realize the cabin pressure
> is below 1 atmosphere in flight. What puzzles me is that a
> fuel tank could explode if the pressure is below 1 atmosphere
> and way rich in fuel, even given an ignition source. Kerosine
> is much less explosive than gasoline, and we all know
> what happens when we have insufficient compression in
> one of the cylinders in a car engine, this with an
> ideal fuel-air mixture. Mike what is the typical pressure
> in a jet's tank at altitude?
Oh, now you are asking the really technical questions. I know that air
cabins start pressurization at 8000 feet, and either maintain that level
of pressure, or taper it up to 12k proportionately as you reach maximum
altitude (generally 40-45k feet). With a fuel tank, you are dealing with
two factors: a) pressure of the tank air when sealed on the ground, and
b) the volume taken up by fuel and how this volume changes as fuel is
consumed. I think it's safe to assume that a fuel tank will remain
pressurized to this range, and if the fuel level doesn't change much, it
will retain the O2 concentration in that volume it had on the ground,
since any flow of tank air will be in the outward direction that early
in the flight, not inward. There will also be O2 dissolved in the fuel,
and as pressure drops, fuel will evaporate at a lower temperature, just
as water does, thus increasing the vaporization within the tank.
In the case of TWA Flight 800, the fuel tank that exploded was nearly
empty on takeoff, and over 120 degrees F, allowing the gaseous vapor
mixture in the tank to reach a very potent degree of fuel/air mixture.
Lighting off gas vapors is far easier than a cooler, more inert liquid
(try playing with torching flour in a barrel with and without blowing
air in to blow flour into the air)
Comparing a fuel tank to an internal combustion motor is really
inappropriate, primarily because the engine block is solid metal, either
iron or aluminum with a steel sleeve in the combustion chamber, while a
fuel tank's wall thickness can be measured in thousandths of inches,
plus the wall thickness in proportion to the volume of the fuel tank is
way smaller than any IC engine would have, so consequently it takes a
far lower explosive shock to rupture it.
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