[AR] Re: Self-pressurization of LOX tank
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Tue, 13 Sep 2016 14:50:19 -0400 (EDT)
On Tue, 13 Sep 2016, Robert Watzlavick wrote:
While sitting there, the tank pressure increased about 70 psi per minute
and after 5 minutes, it reached 370 psi which was right around the
regulator setpoint of 390 psi. ... what will happen if the test gets
delayed and I need to vent the tank? Is there danger of a BLEVE due to
the rapid pressure drop?
Fast answer: no.
A BLEVE is what you'd get if you don't vent the tank. :-) Say you've got
a bad weld and a fitting breaks off at 500psi. The 500psi GOX expands out
the hole. If there was only gas in the tank, this would cause the
pressure in the tank to drop. But instead, there's LOX in equilibrium
with 500psi GOX. So some of it boils, and the GOX pressure stays at
500psi as more and more expands out of the hole. This is why BLEVEs are
special -- there is far more gas in there than you'd think from the size
of the tank, and it's all coming out. A small pressurized tank mostly
full of boiling liquid contains the same energy as a *much* larger tank
holding gas only.
With controlled venting, what you'll initially see is that the pressure
*doesn't* rapidly drop -- the liquid is boiling to keep the pressure up.
Keep venting, and the boiling will gradually chill the liquid enough to
reduce its vapor pressure, and so the pressure will start to drop. No
problem, just expect slightly slower results.
The reason why it's only slightly slower is that unless you've been
stirring the liquid vigorously, it's not *all* in equilibrium with the
ullage gas. It'll be stratified, with the warmest liquid at the top and
cooler liquid farther down, and only a thin surface layer will actually be
at equilibrium with the gas. (This happens even in non-self-pressurized
cryo systems; it's common for the liquid temperature at the engine to rise
noticeably right at the end of the burn.)
Is there any difference between allowing the tank to be pressurized with
helium vs. boiled off GOX?
Self-pressurization is generally not a good idea, because stratification
makes it difficult to control the pressure well -- for example, sloshing
in flight may mix the upper layers enough to suddenly reduce your tank
pressure! -- and you may get two-phase flow toward the end. Warmer LOX is
also noticeably less dense, which reduces tank capacity and messes up
gauging and mixture-ratio control.
Henry
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