On Thu, 6 Jun 2002, David Weinshenker wrote:
> Hmmm.... IIRC, the liquid density of cryo gases such as nitrogen and
> oxygen is about 1000 times their gas density at 1 atm. ... so if LN2
> were sealed in a full tank and then allowed to warm to ambient, the
> tank would then contain gaseous nitrogen at about 1000 atm. (15000 psi).
Alternatively, if the tank starts out 1/3 full you get circa 5000psi.
(Well, those pressures are high enough that you'd want to use something
more accurate than the ideal-gas law to calculate the results, but that's
an unimportant detail unless/until you start making serious plans.)
> Of course, in order to use this process as a safe source of high-
> pressure gas, one would need valves and tankage that would withstand
> this pressure with a reasonable safety factor (over the entire temperature
> range in question - which would require materials not subject to low-temp
> embrittlement)!
Avoiding low-temperature embrittlement is *almost* a matter of one simple
rule:
Don't use carbon steel.
Low-temperature embrittlement is actually not that common in metals; it's
a serious engineering issue only because carbon steel sees so much use.
There *are* other things which get brittle at low temperatures, which is
why I say "almost", and you definitely want to use only materials that are
verified to be safe, but avoiding carbon steel is most of the problem. Of
course, almost anything else that's reasonably strong will cost more...
Low-temperature strength is, in fact, not a big issue. Almost all metals
are stronger cold, so if it'll hold that pressure at ambient, it'll hold
it at lower temperatures. This is why insulated rocket cryo tanks tend to
have the insulation on the outside, to exploit the strength increase.
(It's also why they often put helium tanks inside the LOX tank.)
(Brittleness comes about because the yield strength and the ultimate
strength don't necessarily increase at the same rate. If yield strength
increases faster than ultimate strength, and eventually passes it, you
have something that breaks before it yields, i.e. suddenly it's brittle.)
Overall, though, this *is* a complication, best avoided if you can.
Henry Spencer
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