LOX boils at 90K, LN2 at 77K, so the case is not quite as bad as that.
Using GN2 as pressurant in a system set up for He pressurization won't
work well, mind - plumbing and regulators carefully sized to flow
adequate He will be too small to flow adequate GN2, and tank pressure
will sag as the GN2 flow falls behind. I can attest to that from
personal experience. You need a bigger regulator and larger-diameter
plumbing for N2. But, given the price of He these days, that will
likely pay for itself very quickly.
Someone mentioned a good diffuser on the pressurant inlet as also being
a big help. The reason is, you don't want the pressurant gas jetting
the cryo fluid into a froth and cooling down to near the cryo temp -
this densifies the pressurant causing you to need more of it, it can
lead to more pressurant dissolved in the cryo fluid which may affect
your test results, and it warms the cryo fluid faster. A good diffuser
will allow temperature stratification in the pressurant gas, so only the
layer immediately over the cryo fluid is close to as cold.
Someone mentioned entirely separate press systems for fuel and
oxidizer. On a test stand, this is a very good thing - you aren't
trying to save weight as in a flight system, so the sole benefit is
saving the cost of a second regulator and some plumbing. Meanwhile,
you'll likely be cycling a test stand a LOT more times than a flight
system, so the opportunities for fuel to migrate where it shouldn't
multiply. (A good design assumption is, fuel WILL eventually migrate to
wherever it physically can in a system.) And one bad day with fuel in
the LOX system can negate the minor savings on plumbing by many, many
orders of magnitude.
Remote loading of fuel and LOX isn't essential, in my book. Both are
straightforward industrial procedures, conducted by reasonably trained
techs thousands of times a day around the country. Which is not to say
you shouldn't be rigorously careful about proper procedures and safety
gear - but once you know what you're doing it's not that big a deal.
As someone pointed out, the thing you MUST be prepared to do remotely is
safing the stand from any conceivable combination of failures. Just to
name one, if you've had a no-light make a puddle of lox-fuel jelly out
there (AKA sensitive high explosive) you do NOT want to physically go
near it till the LOX has had a long, long time to all boil off.
Henry
On 4/20/2016 5:28 AM, Ben Brockert wrote:
That is cool. Can you give more details on how to make it work, now
that you're free of the silence of the endless blue? Putting a gas
over a liquid that is significantly below the gas's boiling point has
given bad results in the straightforward attempts I've heard.
On Wed, Apr 20, 2016 at 8:18 PM, David Gregory
<david.c.gregory@xxxxxxxxx> wrote:
I've worked on five different test stands that used nitrogen pressurization for
LOx and only had an issue once. You'll want to pay attention to to
diffusion, but you don't have to use helium.
On Apr 19, 2016, at 11:31 PM, Ben Brockert <wikkit@xxxxxxxxx> wrote:
Hi Kyle,
Sounds like a fun project. Some thoughts:
* If your cryo main valve has a normal pneumatic actuator, it will
typically have a maximum actuator inlet pressure of around 150psi.
That would be an unusually low pressure to do all your engine tests,
so you may want to add another regulator for pneumatic pressure. It
shouldn't be very expensive.
* The vent valves have their own symbol, neither manual or electronic.
What kind of valve are they? Vent absolutely must be remote operated,
and almost always should be normally open (so that if power is lost to
the stand the vents will open). The vent lines themselves out of those
valves need to point in directions that won't spray at people.
* You have to use helium to pressurize LOX, it doesn't matter how much
it costs. You may find it's more cost effective over the long run to
have nitrogen for the fuel and helium for the LOX. It means a little
bit more plumbing but it reduces operating cost and eliminates any
possibility of cross contamination from the fuel side to the LOX side.
* The gauges shown by the engine should probably be transducers.
* It's a fairly minor thing, but the ball on the symbol for a check
valve indicates the inlet end. So two of yours are backwards.
* The helium and possible nitrogen bottles have hand valves on them,
so the hand valve on your high pressure line probably isn't necessary.
High pressure regulators also often have filters on their inlet
already, so you may not have to put that filter in. If the regulators
don't have filters, they're a good idea though.
* Basically every design document says to not relieve pressure from a
line by cracking open a fitting, but... I wouldn't bother with the
vent valve on the high pressure side. Unless the line between the
bottle and the regulator is particularly long, it's not unreasonable
to let that pressure off when unscrewing the CGA fitting from the
bottle. CGA fittings take a big wrench and have a lot of straight
threads, so there's little risk of it coming all the way undone and
whipping around.
* I've used non-cryo rated valves for cryo vent before and gotten away
with it. Needs to have suitable materials (brass or stainless body,
PTFE seats and seals) and it may require experimentation, but it can
be done.
* For remote LOX fill you hook the dewar up to a normally closed cryo
fill valve via a cryo hose (McMaster sells a -8 or -10 one), open the
globe valve on the dewar, and run away. Operate the fill valve
remotely until liquid shoots out the vent. (Have a dip tube on the
vent line inside the tank to ensure some ullage). For remote fuel you
could do a similar setup with an electric fuel pump. I generally
wouldn't bother with either, I prefer to have the LOX dewar and fuel
drum away from the test stand at test time.
* For LOX system drying, the optimum is to not ever put water in the
system after it has been oxidizer cleaned and component level and
assembled. Do liquid nitrogen if you want to do a cold flow before
comitting to LOX, which is a good idea. If you must load water in it,
all ball valves have to be opened half way and blown through with dry
gas as you suggest. One option is to hook up a bottle of nitrogen and
slowly blow the entire bottle through the system. Another is to get an
oilless compressor, put a high quality water and oil filter on its
outlet, and blow clean air through. Any ball valves have to be 45
degrees open though, if full open any water trapped in the cavity
between ball and body will stay there. And operate the ball valve a
dozen or so times with gas flowing.
* You're going to want purges to clear out the lines to the engine
before and after running. Typically they come from above the tank
press valve, through an orifice and a solenoid valve and a check valve
or two, and are injected on the downstream side of the engine run
valve.
What's your plan for propellant tanks?
An important next step is to make another P&ID that goes all the way
to component level, making sure that fitting types sizes are correct
on everything. Then from that you can create a realistic BOM and
figure out how much everything is going to cost.
No mild steel parts anywhere in the LOX system, and if you bolt the
LOX tank or downstream parts to a steel structure they need thermal
isolation.
Ben
On Wed, Apr 20, 2016 at 9:15 AM, Kyle Meeuwsen <meeuwsen.engr@xxxxxxxxx> wrote:
Hello All,
This is my first post and I hope I am doing it correctly...
I am with the Portland State Aerospace Society and we are developing our
first liquid engine test stand. This is a LOX and Ethanol system pressurized
by either Nitrogen or Helium depending on the quote back from Praxair.
Below is a link to the Test Stand PID on Imgur. I am looking for either
approval or to discuss design concerns. Please note that the valve below the
LOX tank is cryo rated with a pneumatic actuator that is pressurized by two
solenoids which are fed from the LOX pressurant line.
The senior design team before me also selected relief valves and solenoids
for venting on the top of the LOX tank that are not cryo rated. I have
concerns that heat transfer from the tank to the LOX might freeze these
components that are attached to the top of the LOX tank. I could attempt a
very shaky heat transfer analysis unless people know right away that these
components will freeze very quickly.
I am also still searching for filters and burst discs, and a way to safely
fill the LOX and Ethanol tank from a great distance away.
Also, any suggestions on how to purge the system of moisture prior to
testing would be greatly appreciated. I have been told to run hot pressurant
gas through the system for an extending period of time but I am currently
not in possession of a heat exchanger or an ample amount of pressurant gas.
https://imgur.com/gallery/wVbtgUz
