NPT Valves in High Vacuum Range
NPT Valves in High Vacuum Range
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?
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?
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Re: NPT Valves in High Vacuum Range
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
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
- Richard Hull
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Re: NPT Valves in High Vacuum Range
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
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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Re: NPT Valves in High Vacuum Range
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.
> 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.
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- Chris Bradley
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Re: NPT Valves in High Vacuum Range
> 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.
> 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.
Re: NPT Valves in High Vacuum Range
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
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
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Re: NPT Valves in High Vacuum Range
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
Tyler
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Re: NPT Valves in High Vacuum Range
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.
> 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.
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Re: NPT Valves in High Vacuum Range
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
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