NPT Valves in High Vacuum Range

Every fusor and fusion system seems to need a vacuum. This area is for detailed discussion of vacuum systems, materials, gauging, etc. related to fusor or fusion research.
Post Reply
terryphi
Posts: 21
Joined: Fri Nov 09, 2012 6:08 pm
Real name:

NPT Valves in High Vacuum Range

Post by terryphi » Fri Feb 15, 2013 2:42 am

Hey,

does anyone know how well NPT valves perform in the high vacuum regime? Would it be okay if I used a NPT valve between my gas supply and vacuum boundary?

John Futter
Posts: 1625
Joined: Thu Apr 22, 2004 2:29 am
Real name: John Futter
Contact:

Re: NPT Valves in High Vacuum Range

Post by John Futter » Fri Feb 15, 2013 6:56 am

Terry
NPT valves
What sort
In my book NPT stands for national pipe thread
this covers all valves from water taps, the plastic taps you screw into a plastic barrel all the way through to bellows sealed valves with NPT threads on them


so in general to answer your question:
no, maybe, yes

User avatar
Richard Hull
Moderator
Posts: 12290
Joined: Fri Jun 15, 2001 1:44 pm
Real name: Richard Hull

Re: NPT Valves in High Vacuum Range

Post by Richard Hull » Fri Feb 15, 2013 3:17 pm

I have found that large, brass bodied ball valves with NPT threads are sufficient as foreline valves. Some need to have their packing grease removed or changed to vacuum rated grease. For a demo fusor in my demo fusor II they worked fine. I would not try to use them in a major fusion effort, however and never as a throttle valve to a fusor. That work demands a formal vacuum rated valve.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

User avatar
Rich Feldman
Posts: 1321
Joined: Mon Dec 21, 2009 11:59 pm
Real name: Rich Feldman
Location: Santa Clara County, CA, USA

Re: NPT Valves in High Vacuum Range

Post by Rich Feldman » Fri Feb 15, 2013 5:52 pm

Terry Price wrote:
> does anyone know how well NPT valves perform in the high vacuum regime? Would it be okay if I used a NPT valve between my gas supply and vacuum boundary?

I'm going to address Terry's second question, which has little to do with high vacuum. The answer is a qualified YES, depending on where in the gas path. To begin with, the on/off valve on top of high pressure gas cylinder generally has NPT threaded connection to the cylinder; the other end depends on type of gas.

You could certainly use ordinary, small (1/8 NPT?) valves downstream of the regulator, but what for? Their flow adjustment range will be too coarse for fusor-type flows, unless operated from extremely low regulated pressure, or you find a fine needle valve with NPT fittings. Then yes, on the vacuum side of valve you need to be sure the thread sealing material won't be leaking or outgassing.

If the valve is going between pressure regulator and a traditional flow-restricting orifice or capillary, then hardware store valve could be used for on/off control. Off with a slight leak might as well be On. If valve stem is on inlet side, beware of flammable gas leaking out through the packing gland. If valve stem is on vacuum side, beware of air leaking in when valve is Off. [edit] I think none of those is likely to be a problem with valves like the one pictured here.
Attachments
valve.JPG
valve.JPG (7.61 KiB) Viewed 4226 times
All models are wrong; some models are useful. -- George Box

User avatar
Chris Bradley
Posts: 2930
Joined: Fri May 02, 2008 11:05 am
Real name:

Re: NPT Valves in High Vacuum Range

Post by Chris Bradley » Fri Feb 15, 2013 8:19 pm

> Terry Price wrote:
> does anyone know how well NPT valves perform in the high vacuum regime?

Along the lines of RF's reply, what is it that you want them to actually do? NPT is just the fitting. No 'one' valve is a good idea between gas supply and vacuum chamber, whatever its fitting.

terryphi
Posts: 21
Joined: Fri Nov 09, 2012 6:08 pm
Real name:

Re: NPT Valves in High Vacuum Range

Post by terryphi » Fri Feb 15, 2013 9:28 pm

The desigin I'm picturing is as follows:
a DN40-CF to 1/4" NPT adapter on the end of a DN40 CF Tee fitting. This Tee fitting is attached to the vacuum boundary of my fusor (IE the inside of it is the inside of my fusor.) I would screw in a 1/4inch brass globe valve to this fitting and attach a nipple on the other end of the globe valve. To inject gas into the fusor I would open this valve.

My question is: would air leak through the screw threading of the NPT fitting into my fusor?

Regards,

-TP

Tyler Christensen
Site Admin
Posts: 551
Joined: Tue Feb 24, 2009 2:08 am
Real name:

Re: NPT Valves in High Vacuum Range

Post by Tyler Christensen » Fri Feb 15, 2013 9:31 pm

Air shouldn't leak if you properly use teflon tape. If you don't or improperly use it, then yes air will leak through the threads. It's unlikely you'll ever get a UHV seal with NPT fittings and teflon tape, but for fusors you can do fine. I made extensive use of NPT in my fusor.

Tyler

User avatar
Chris Bradley
Posts: 2930
Joined: Fri May 02, 2008 11:05 am
Real name:

Re: NPT Valves in High Vacuum Range

Post by Chris Bradley » Fri Feb 15, 2013 10:48 pm

Terry Price wrote:
> My question is: would air leak through the screw threading of the NPT fitting into my fusor?
An NPT connection can be made vacuum tight, but it can be a struggle to make it so. Aggressive tightening is usually counter-productive, and so is too much teflon tape or other sealants. But similarly, modest tightening and sealant may not be enough and you try again. There are few guarantees with NPT but it is not impossible.

Bear in mind there are various flavours of NPT; NPS, NPTE and not forgetting NPTF which would probably be the better choice for vacuum attempts.

However, my principal answer would be that it hardly matters, because you will flood the chamber instantly when you try to let gas into a vacuum vessel with a ball valve.

It may be useful for you to review my FAQ on how to flow deuterium. A regulating orifice to the chamber and a pressure regulator from the gas supply is the way to go.

Dan Tibbets
Posts: 578
Joined: Thu Apr 17, 2008 5:29 am
Real name:

Re: NPT Valves in High Vacuum Range

Post by Dan Tibbets » Wed Feb 20, 2013 12:30 pm

I have used multiple NPT hardware needle valves between pump and chamber, vacuum gauge, side port for gas introduction,etc. Most seem to work fairly well. I have also recently used hardware store ball valves recently for their larger orifices. With these I have recently achieved vacuums of ~ 15 Micros and less than 0.1 Micron per second leak rates in a small Demo Fusor made from a pipe T. and only a foreline pump. This may be good enough (?) for a neutron producing Fusor (with the addition of a diffusion pump). The largest issue seems to be sealing the threads. I have heard that Teflon thread sealers are not good. Also, other thread sealer compounds from the hardware store have been problematic. Sealing with epoxy seems to be the best solution I have used. The epoxy can be 'broken' through the local application of heat if revisions are needed. After separation the epoxy can be cleaned off with organic solvents such as acetone on a rag.

Adequately controlled introduction of deuterium gas may be a different matter. I have speculated that several needle valves (like the one pictured earlier in the thread) in series might work for a low pressure source like deuterium gas from electrolysis that has been stored in a small reservoir, at least as a pulsed source.

Dan Tibbets

Post Reply