[AR] Re: Double seal arrangement
- From: Ben Brockert <wikkit@xxxxxxxxx>
- To: "arocket@xxxxxxxxxxxxx" <arocket@xxxxxxxxxxxxx>
- Date: Sat, 1 Feb 2014 19:05:06 -0700
Neat project. Are you using the same kind of U-seals as in the tank?
If so, it seems like you could just use two or three of them without
the vents or the pressurization. Normally you don't want to have
multiple seals without a vent in between, because you can build up
pressure in there and fail the seal, but those U-shaped seals are more
like check valves than o-rings.
If you plugged the vent hole do you think it would still leak out the stem?
On Sat, Feb 1, 2014 at 6:51 PM, Robert Watzlavick <rocket@xxxxxxxxxxxxxx> wrote:
> I'm doing some cold LN2 tests this weekend of a new semi-custom cryo ball
> valve but it's leaking more than I expected. I'm considering changing it to
> a double-seal arrangement with a pressurized cavity but I wanted to see if
> anybody has advice or experience in this area. I took the ball and seats
> out of a 3/8 inch Swagelok 62-series ball valve and built an all-new
> aluminum body, end caps, and new stem. I found that most of the torque
> required to turn a ball valve came from the stem packing so I wanted to use
> some PTFE spring energized shaft seals to seal at cryo temperatures with
> lower torque. I've had good luck with these seals in static cryo
> applications but this is the first time I've tried them in dynamic use. The
> valve is actuated by a Dynamixel AX-12A servo in a direct drive arrangement.
> The valve and servo together weigh about 0.35 lb.
>
> I don't have any drawings but arrangement of the stem seal is as follows:
> On the shaft starting above the ball, there is a 1/2 x 3/8 inch PTFE sleeve
> 1/4 inch long (so the shaft doesn't rub against the valve body) and above
> that is the spring energized seal. Then there is a miniature 1/2 x 1/4 inch
> bearing to keep it all concentric. Initial tests yesterday with the valve
> just hooked up to the dewar (~25 psi) showed some leakage when the ball vent
> was pointing upstream. When the valve was open, there was a pretty good
> drip of LN2 coming out the end where the bearing is (it's an open bearing).
> The good news is that when the vent was pointing downstream, there were no
> leaks so the body seals are working great. The Swagelok 62 seal design is
> spring loaded so it seemed like a good choice.
>
> Last night, I modified the valve to add a second seal: Above the ball,
> there is now a spring energized seal, a 1/2 x 3/8 inch PTFE sleeve 0.15 inch
> long with vent holes in it, then another spring energized seal. I drilled a
> hole in the side of the valve body (in between the seals) and put a #10-32
> port so I could see how much it was leaking. With an upstream pressure of
> 200 psi and the valve open, there was a pretty good stream of LN2 squirting
> out the vent hole. Since it's confined to the vent, I could just vent it
> overboard but there has to be a better way. Then I had the thought, I could
> flip around the bottom seal and pressurize the cavity between the seals
> (essentially a pressurized double-seal as I've found out). I could either
> use the same helium supply that is feeding the tank or I could use the
> coiled LOX->GOX heat exchanger that I'm using for the igniter. Either one
> would be much warmer than the direct cryo liquid inside the valve. I would
> think the GOX would be better as to not introduce N2 bubbles into the LOX
> stream for the real rocket.
>
> I think part of the problem is that the shaft at the top end has a bearing
> but at the bottom end, there is just the PTFE sleeve (and it shrinks) so the
> stem moves around a bit while it turns and causes the seal to leak. I
> suppose I could put another bearing down closer to the ball so the shaft
> turns completely concentric.
>
> Anybody have any suggestions on the double-seal arrangement? No matter
> which pressure source I use, it should reduce the delta P across the first
> seal to essentially zero.
>
> -Bob
>
Other related posts:
