Gas control - choice of valve types for min leakrate?

How to get to vacuum, what the classes are, and what is needed for what job.

Gas control - choice of valve types for min leakrate?

Postby chrismb » Sun May 15, 2011 5:41 pm

Just a quickie [perhaps?]: I have rigged up three different gas bottles to my one regulator, so that I can choose whichever at a tap turn. All is fine and it is a good way to pressure test the regulator before putting expensive gas into the HP side, but that the ball valves right next to the regulator High side tends to leak a little. By leak, I mean ~ 0.05 scc/day/atm. So if there is 10 atm in the line to the regulator and the regulator has ~nothing in it, there is a ~0.5scc per day leakage into it.

I'm not so bothered about the leak rate itself, but it only has about [I estimate] 10 cc in the high side, so if I only have, say, 2 atm left in it and 10 in the line then after 4 days it will be 10% diluted with whichever gas was last in the line.

I can
1) add one or two more ball valves of the same time, such that I can vent high pressure into the space between the valves and it will leak out in both directions, mitigating dilution, or

2) I can buy a different valve, but should I try another ball type, or a needle valve, or other?

3) I could try adding a touch of thick silicone grease to the surface of the balls in the valves - i think it would actually be a good approach for this kind of leak rate, but would this help or hinder the ball's work?

4) I could just drop the multi-line approach and have one gas per regulator, of course!! ;) This will necessitate some gas loss each time I swap the lines at the chamber end.
chrismb
 
Posts: 620
Joined: Thu Aug 05, 2010 6:32 pm

Re: Gas control - choice of valve types for min leakrate?

Postby Jerry » Sun May 15, 2011 8:50 pm

You must have one regulator per bottle unless they are all the same gas.
Jerry
 
Posts: 561
Joined: Sun Jul 18, 2010 12:07 am
Location: Beaverton, OR

Re: Gas control - choice of valve types for min leakrate?

Postby chrismb » Mon May 16, 2011 1:05 am

Why?
chrismb
 
Posts: 620
Joined: Thu Aug 05, 2010 6:32 pm

Re: Gas control - choice of valve types for min leakrate?

Postby Doug Coulter » Mon May 16, 2011 8:38 am

You'll have more issues of cross contamination and leaks with things on the HP side -- there's more volume of gas there per CC of actual plumbing because it's at a higher pressure. I get quite a number of fusor runs off the gas in the pipe between the regulator and tank -- the 1" long one. If you need a multi gas system, probably best to have a separate regulator for each, and a separate feed line to the tank for each. That latter in my systems gets pumped to vacuum when not flowing gas. Obviously if you're dealing with non-inert gases you don't want to mix them at HP.

The kind of pressure drop you have on the HP end to ATM will just push grease aside. I'm curious where you found ball valves rated for 2k psi anyway -- you might be doing something fairly unsafe. The big SS ones I see listed only go to 600 psi and most gas tanks are filled to roughly 4x that pressure. Boom! I presume your valves are leaking out around the packing on the actuation shaft?
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
User avatar
Doug Coulter
 
Posts: 3023
Joined: Wed Jul 14, 2010 7:05 pm
Location: Floyd county, VA, USA

Re: Gas control - choice of valve types for min leakrate?

Postby chrismb » Mon May 16, 2011 12:08 pm

OK, so sometimes I try to keep a post short and miss details, other times I write too much!

The missing detail that I think you've both picked up is that I do not vent the whole pressure of the gas bottle to the regulator. I understand why Jerry made the comment, now.

I am only bleeding in 5 to 10 bar at a time, of whichever gas, so that I can use that quantity up and then decide what my next experiment [and next gas] is going to be. The High side is around 8 to 10 cc, so that represents, say, 100 scc or so. There is 12 cc in the line and the fixture to the chamber. So at 2 sccm, say, that's an hour of steady operation.

I have a fine [fixed] flow needle setup for control which gives ~1 sccm with a differential pressure of 1 atm, so if I want 1 sccm I feed in 1 atm into the chamber fixture. The regulator maintains 1 atm in that part, so I get a steady 1 sccm. I can adjust with a resolution of under 0.1 sccm in this way and it doesn't vary by more than 0.01 sccm during operation (around the 1 micron range of chamber pressure). Usually I run higher flow rates because my outgassing rate from the stuff in the chamber is around 0.05 sccm at 1 micron [depends how long outgassed beforehand, of course] so I have to try to swamp that with a good flow rate which I do in the 2 sccm range. I can flow accurately down to below 0.1 sccm, but then there is as much outgassing as there is gas going in!

So the regulator is not so much 'attached' to gas bottles, it is one sub 10 bar flow controller in a network of flow paths. I'll do some pics when I get on it, but photos wouldn't explain that I only feed in drips of gas.

So, the question du jour remains - is there a valve type/construction which minimises leakage [in the ~10 bar range].
chrismb
 
Posts: 620
Joined: Thu Aug 05, 2010 6:32 pm

Re: Gas control - choice of valve types for min leakrate?

Postby johnf » Mon May 16, 2011 5:08 pm

Since pis are worth a thousand words
this is how we do it all swagelock product
gas bottles to gas selector switch then on to pressure reduction reg then leak valve then through braided bellows to ion source

IMG_0530.jpg


IMG_0531.jpg


IMG_0533.jpg
johnf
 
Posts: 381
Joined: Sun Aug 08, 2010 3:51 pm
Location: Wellington New Zealand


Return to Vacuum Technique

Who is online

Users browsing this forum: No registered users and 1 guest

cron