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Piston Valve questions - Chamber Sealing VS Barrel Sealing

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Piston Valve questions - Chamber Sealing VS Barrel Sealing

Unread postAuthor: carl1864 » Mon Nov 05, 2007 9:33 pm

So I've been reading alot about piston valves and am about to build my first one, but I have about 3 questions.

1.)I'm wondering if there are any advantages with one over the other? Which one performs better and lasts longer?

Let me know if my theories are right...

The barrel seating valve seems not only easier to build, but also less prone to failure. A chamber sealing valve has 2 places it can fail (not counting the piston itself), the rubber sealing face, or the O ring on the pilot chamber. Wheras, if an o-ring was to leak a little bit on a barrel seating valve it wouldn't matter as much right? There must be some advantage to the chamber sealing valve though right?

2) What is the best size hole to make for your equalization hole? I don't want to make it too big or too small.

3) How safe is it to connect your combustion chamber to your piston valve with a threaded fitting? I figure it probably is safe being the supah valves being sold seem to have threaded fittings. I plan to use threads for interchangeable barrels and as long as its safe, would like to use one for the combustion chamber as well so if my piston valve ever fails I can replace just the valve without chopping off the chamber.





The following is something I typed as far as my first question, but it turned out so complex I'm not sure if anyone can understand it, so I just put it at the bottom, you can read it and try to answer, or skip it.

I'm also wondering if the It also seems like perhaps depending on the size of your T that your building it in, can make a difference as far as which works best? Depending on which gives you the most pressure pushing back against the pilot chamber to shove the piston back faster? I figure, The area of a 2" pipe is 3.14, 3" is 7.07, and 4" is 12.56.
So if you were using a 3 inch T, with 2 inch pipe, a barrel sealing piston might be better. Since the surface area of the surrounding area of the barrel (7.07 - 3.14 = 3.93) is larger than the surface area of the barrel itself 3.14. However with a 4 inch T with 3 inch pipe, perhaps a chamber seating piston is better since the surface area of the chamber (7.07) is greater than the surface area of the surrounding part (5.49). Or am I putting way too much thought into something I won't be able to physically see a difference in?

Thanks.
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Unread postAuthor: octane89 » Mon Nov 05, 2007 9:37 pm

1- Barrel Sealing>Coaxil>Chamber Sealing
2-1.5mm
3-People do use threaded PVC fittings, but they hold less pressure than other pressure rated parts, hence they leak more.
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Unread postAuthor: Zen/// » Mon Nov 05, 2007 9:40 pm

A 1.5mm circle or slit?
Diameter or radius?
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Unread postAuthor: spudgunnerwryyyyy » Mon Nov 05, 2007 9:42 pm

If you build a barrel sealing valve, it doesn't need o-rings. and if the no orins no equalization hole is necesary. Talk to Mr. Crowley, he is the master at teacher noobs to build simple barrel sealers. Helped me a bunch.

You can put o-rings on a barrel sealer and it increases performance like 200%.
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Unread postAuthor: DYI » Mon Nov 05, 2007 9:44 pm

You can put o-rings on a barrel sealer and it increases performance like 200%

That's BS. If it increased performance by 200%, it wouldn't be worth building barrel sealers without O-rings.

I agree that MrCrowley is good at explaining piston valves, you should ask him.
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Unread postAuthor: octane89 » Mon Nov 05, 2007 9:44 pm

1.5 diameter hole.

Here's Mr.Crowley's how to-
http://www.spudfiles.com/forums/how-to- ... 10381.html
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Unread postAuthor: LikimysCrotchus5 » Mon Nov 05, 2007 9:46 pm

Same with me, and spugunnerwyyy helped me also. I just recently finished my first barrel sealing piston valve.
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Unread postAuthor: spudgunnerwryyyyy » Mon Nov 05, 2007 9:53 pm

Yeah its just a coaxial in a tee basically. If you think of it that way it is really. Simple.

thanks for the credit likmycrotch, no ones ever thanked me for helping with spudguns before
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Unread postAuthor: LikimysCrotchus5 » Mon Nov 05, 2007 10:15 pm

Hey we all help each other now, we just forget sometimes to give the credit to the right person. 8) :roll:
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Re: Piston Valve questions - Chamber Sealing VS Barrel Seali

Unread postAuthor: clide » Mon Nov 05, 2007 11:53 pm

carl1864 wrote:Or am I putting way too much thought into something I won't be able to physically see a difference in?


In a way, most of the time it won't matter much which type you chose except for the shape of the gun you want. Sometimes it can make a performance difference, but rarely is it significant.

As for the equalization hole. If your not using o-rings then you don't need one. If you are using o-rings then it is worth it to put a check valve in the piston in my opinion. With a check valve it will fill as fast as you can put air into it, but it will have the performance of having no equalization hole at all.

Most of my valves have a threaded chamber and barrel port, so yes that is fine.
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Unread postAuthor: Hotwired » Mon Nov 05, 2007 11:55 pm

octane89 wrote:1- Barrel Sealing>Coaxil>Chamber Sealing


If you check out the cross section of a barrel sealer and a coaxile, you'll notice the barrel sealer is exactly a coaxile but with a 90* to the chamber.

If anything I'd say a coaxile has better flow.

Incidentally one thing that no one else has mentioned is that piston valves can be slimmer than barrel sealers (if you're interested in that king of thing) as they don't need the doughnut space around the barrel end to force back the piston.
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