Re: Inert Gas in Fuel Tanks

Date:         18 Nov 96 10:11:15 
From:         "P. Wezeman" <pwezeman@blue.weeg.uiowa.edu>
Organization: The University of Iowa
References:   1 2 3
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On 8 Nov 1996, Bob Standaert wrote:


> Nitrogen gas has a density of 1.25 g/l at 0 C, 1 atmosphere pressure.
> For the liquid, it is 808 g/l at -196 C, for a volume ratio of 646:1.
> If the fuel capacity of a 747 is about 200 kl, you would need 250 kg, or
> 310 l, of lN2, to replace it.  The dewars (vacuum jacketed storage
> vessels) we have hold 200 l of lN2 and weigh about 120 kg empty.  Two
> full tanks on a plane adds up to about 560 kg plus plumbing and
> hardware, and the cost for the liquid would be about $40 at our price.
> (Perhaps the airlines could make up for it by charging the passengers
> extra for the spectacle of the refill -- lots of fog and noise coming
> from the belly of the plane!)
>
> Weight and headaches aside, liquid nitrogen is not without its hazards.
> Some things you need to worry about from a leak:
>
> 1.  Oxygen displacement.  A big leak that evaporated in a confined area
> would asphyxiate any living being within (what is the volume of the
> cargo hold?).
>
> 2.  Cold damage.  Cooling things to -200 C tends to change their
> mechanical properties.
       snip
> 3.  Oxygen condensation.  Surfaces cooled by lN2 can condense O2 from
> the air; lO2 has the nasty property of reacting with any combustible
> material it touches, often explosively.  In the lab, this problem arises
> most commonly when air gets into a liquid-nitrogen cooled vapor trap.
>
   Considering the danger of oxygen condensation should that occur, it
would seem to me that an onboard nitrogen separation system such as that
used on the C-17 would be better for airline use than a cryogenic system
like that on the C-5. Still, I have never heard of a C-5 being in danger
because of the inert gas system so they must have solved the problems
involved. I would like to hear from any aircrew, mechanics or engineers
who have worked with the inert gas systems on either of the two planes
about the pros and cons.
   As I visualize it, the greatest demand on the nitrogen system would be
in the descent stage of a flight, as it had to maintain steadily increasing
pressure in the nearly empty fuel tanks. At a rough estimate, presuming
that a C-17 carried 200,000 pounds of fuel, and was descending as 5,000
feet per minute at sea level with empty tanks, the inert gas system would
have to generate about 1,000 cubic feet of nitrogen per minute to keep
air out. Of course, this would be a transient demand, which might make a
difference in the size of the components required.

                        Peter Wezeman, anti-social Darwinist

                                "Carpe Cyprinidae"