Sunday, November 11, 2007

Valves, Valves,Valves and Valves...

We started this project last year looking for light weight actuators. We assumed that we could just use off the shelf ball valves.

When we sit down and evaluate the response time needed and the hysterisis of ball valves then we realize that in the small sizes we need ball valves are problematic. There is slop between the ball and the stem and rotationally the ball valve is not very linear. We have also had leak and wear problems with our ball valves. So given the actuator we like we are now trying to find a better valve to go under it. Today we manually tested the following valves with 200 PSI liquid nitrogen:

Large O2 rated solenoid. (Cryogenic solenoids are larger and heaver)
Failed stuck open.

Buttery fly valve we were using as a throttling vale as the only valve.
Failed Leaking, very very non-linear.

SwageLoc Plug valve with 4 different kinds of alternate o-rings including
$70.00 of Kalrez o-rings.
All rotating seals failed at Ln2 Temperatures.

Small Brass Ball Valve.
Froze Solid and Jammed.

Small SS Ball valve.
Worked, sealed and had signiifcantly more hysteresis at LN2 temps than at room temperature.

For my next bout of insanity Paul builds a valve....

This is my "Simple" custom valve idea. I build a small linear valve in a copper plumbing tee. Making cryogenic seals work is hard, so I cheat. I add some heat to the end of the thin wall stainless to keep the seal end warm. The force necessary to operate this valve is within the range of the small fast robot servos I've been using. the valve is operated by pulling the stainless rod in and out. The Cage pins are there to make the ball stay in place. I'm concerned that the flow will make the ball chatter. If the seals give me trouble I can go to an electroformed bellows.
From Servometer
This makes me wonder if I should try a stock off the shelf.... bellows valve


Carl Tedesco said...

Have you thought about getting some Worcester short stemmed cryo-valves. Not seeing the failure modes in person I can only speculate, but I think your leakage problems could be due to thermal expansion problems not accounted for in non-cryo rated valves. Also, I noticed in the NASA valve guides that all their cryo ball& butterfly valves were bearing mounted (BTW the Worcesters are not), which would better control the ball-to-seal loads. It looks to me like XCOR uses them in their rocket plane (it looks like they machine AN ports for them).

--- Carl T.

Jon Goff said...

So the main problems you had with the butterfly approach were that the flow characteristics were very nonlinear, and that the butterfly had a hard time sealing against pressure at cryogenic temperatures? Or were there other problems. Back when you were doing a ball valve in series with a butterfly valve, what kind of response times were you seeing for your system?

Just curious,


Paul Breed said...

A butterfly in series with the ball valve was very fast, In one of our throttled engine tests we were seeing valve command to chamber pressure changes of just less than 50msec.


gate valves said...

hy there., i appreciate the info. very helpful and educational. great blog.

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