[AR] Re: LOX Test Stand Integration

  • From: Henry Vanderbilt <hvanderbilt@xxxxxxxxxxxxxx>
  • To: arocket@xxxxxxxxxxxxx
  • Date: Fri, 4 Nov 2016 09:02:48 -0700

On 11/4/2016 5:52 AM, Graham Sortino (Redacted sender gnsortino for DMARC) wrote:

Ball Valve Vent*
“In the upstream face of the ball, centered in the port when the valve
is closed. It also needs to be well deburred and fillleted or it will
tear up the seal.”<<
Just to clarify does this mean the hole is drilled in the actual ball? I
had thought from reading other’s posts that the parts were first removed
the valve and then a hole was drilled in the valve body? If possible, a
picture would be very helpful.

The problem being solved is this: When you close a cryo ball valve, the cylindrical passage through the ball still contains a slug of cryo fluid. Once closed, that cryo slug is trapped inside the ball by the valve body sides and the ball seals. If the cryo fluid slug is left there as the system warms, overpressures will result. Bang! as the seals, valve body, or both let go...

The standard solution to this is indeed a hole drilled through the when-closed upstream center of the ball, so the ball-passage contents are no longer trapped, but rather can feed back into the upstream cryo feed system, thus presumably being handled in the same manner as the rest of the feed system's contents.

One major reason why this is traditional is the way the ball seals work. Given a high-pressure and low-pressure end of the valve (and if not, why do you need a valve?) the ball will be pressed more firmly into the seal on the low-pressure end. The seal on the high-pressure end thus may seal less reliably. Since venting the ball passage also vents the valve-body between the seals, venting the ball to the downstream (low-pressure) end risks a steady leak past the upstream seal, into the valve body, into the ball passage, and out the ball vent. (Venting the valve body itself would present the same problem.)


*
Vendors*
You can google cryogenic ball valve and find suppliers. Expect to
spend around $1k for a specifically cryogenic ball valve with a
pneumatic actuator.<<
Thanks & understood. I was thinking of using a servo actuated valve and
building the housing for this myself. That is what I did for my non-cryo
valves and it worked well although they were plug style valves rather
than ball.

It should not be surprising that companies who have found success with non-cryo
valves in cryo use haven't shared all the details, since it does
represent hard fought experimental knowledge.<<
Thanks for explaining I hadn’t considered this but of course I
understand the rationale. I was hoping to using something like a
Swagelok valve which I’ve used in the past and like them quite a bit but
since safety is more of a concern than cost or weight I wouldn’t want to
do this unless others had at least a reasonable amount of success here.
I thought maybe some of the academic teams or their mentors who are on
the list might be able to provide some suggestions since its less of a
trade secret.

For your test stand, find a surplus 1/2 or 3/4 inch Worcester
cryogenic ball valve and add a rotary or linear actuator to it.
Thanks Bob, I think I saw someone post about Worcester valves before and
mentioning they were good but quite pricey. I will keep an eye out.

Two things to keep an eye on here: LOX-compatible materials - brass, aluminum, or the proper stainless types - and cryo-rated *LOX-compatible* seals. Generally these will be some flavor of PTFE. You want to be very careful about seal LOX-compatibility in using non-cryo valves. It would be a safe guess that successful use of non-cryo valves in cryo apps involves both making sure the valve materials are cryo-capable and replacing the seals with cryo seals.


*Krytox*
Don't lubricate valves at all. Krytox is LOX compatible, meaning it
won't explode, but it'll still freeze. PTFE is solid lubricant, in a
ball valve there's not really any significant metal on metal movement.<<
I see your point but I’m a bit confused by this because there was a post
from Carl on Thursday, April 23, 2015 12:21 PM regarding the team he
mentors where he says “We are using a COTS 3-piece ball valve… To modify
it for use as a cryo-valve we disassemble it and clean it, drill a vent
hole in the ball to vent out trapped LOX, and re-lube it with a light
coating of Krytox.”

I can testify that Krytox does in fact freeze solid well before cryo temps. Speculative: The "light coating" in this instance ends up being a thin solid film under cryo conditions, thin enough and/or strategically enough placed that it doesn't immobilize the valve. Non-speculative: They are not including near enough details here to safely duplicate their application.

Henry



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