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Piston Travel Distance in a Tee Valve

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 Travel Distance in a Tee Valve

Unread postAuthor: Velocity » Tue Jul 03, 2012 11:12 pm

I've been thinking about piston travel distance lately. The old rule is that the piston should travel back one quarter of the diameter of the exit for maximum efficiency. This can be easily derived; allow r=radius of the exit port, d=diameter of the exit port, L=piston travel:

Flow through area around the barrel port = flow in the barrel
2*pi*r*L = pi*r^2
d*L = 0.5*d^2
L = d/2

Now this clearly applies in an ideal situation, where the flow around the barrel port is not restricted. I imagine a situation where the piston housing and chamber are combined into a spherical chamber surrounding the barrel port, allowing this unrestricted flow. However, these conditions are likely closely approximated in a coaxial setup.

I believe that this all changes for a tee valve. The following refers to a barrel-sealing valve, though can be generalized to a chamber-sealing valve as well. A tee valve likely does not allow for this unrestricted flow around the barrel port, given that the chamber is attached to one side. If the exit port and piston diameter are close (which is favored for better performance), the flow into the barrel port coming from the side OPPOSITE to the chamber side would not be full flow, for the tee housing prevents this from occurring. I understand that some gas would certainly enter the barrel port from this "bad" side of the tee valve, but I can't help but feel as though it would be less that the optimum amount.

With this in mind, I've been thinking that the ideal piston travel for a tee valve is probably closer to between 3d/8 and d/2, given that the effective area for the flow is reduced. Does anyone has any input/experience on this topic?
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Unread postAuthor: Technician1002 » Tue Jul 03, 2012 11:35 pm

I have also rejected the 1/4 D rule because it applies to liquids. When dealing with gas, the first restriction induces a pressure drop along with gas expansion. The expanded gas at lower pressure is then expected to enter the barrel without encountering a second flow restriction, pressure loss, and gas expansion.

As a result of just the above, I generally aim at having the piston open 1/3D minimum or ideally about 1/2D for best flow. This is in a coaxial design with no major upstream flow restrictions. I want as little pressure drop prior to the flow entering the barrel as possible.

For a piston in a T (which I have never built yet) I would consider 1/2D minimum and hope for 2/3D to 1D for best flow.

Just my 2 cents on the subject. I could be entirely wrong. Any fluid dynamics engineering types here to weigh in?
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Unread postAuthor: jackssmirkingrevenge » Wed Jul 04, 2012 12:12 am

1) good to see you're still around :)

2) http://www.spudfiles.com/forums/qualify ... 11354.html
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Unread postAuthor: Technician1002 » Wed Jul 04, 2012 12:24 am

I have a new toy taking up my time. It includes a way to haul all my cannons and a 40 Amp generator to run a larger compressor. :D I'm taking care of a few minor problems on it as it is 18 years old and needs some work.

Planning a trip later this summer.

I had to fix the electric step, a leak on the front windshield, and upgraded the TV to a flatscreen to double as a monitor for a backup camera I'm installing. Added an inverter to run the monitor/tv. No need to crank up a 4.8KW generator to run a 40 watt TV. :D
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Last edited by Technician1002 on Wed Jul 04, 2012 12:32 am, edited 1 time in total.
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Unread postAuthor: jackssmirkingrevenge » Wed Jul 04, 2012 12:30 am

If it doesn't look like this by the time you're done with it the gentlemen of the board will be very disappointed :D

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Unread postAuthor: Technician1002 » Wed Jul 04, 2012 12:34 am

Due to a spouse who has a vote, most of the cosmetics will remain stock. It's what is in the basement that counts. :D Wifi is going in too. Added a 20 Amp outlet in the generator compartment for use with a larger compressor or to feed the house in an outage. I'll get photos of the improvements later. Future plans include ~ 1KW solar electric. Can't buy trip gas and the solar in the same year, so it is on hold. :(

Think Urban Assualt Vehicle from Stripes.. :D

Enough thread hijack for now. Back to valve CV.
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Re: Piston Travel Distance in a Tee Valve

Unread postAuthor: velocity3x » Wed Jul 04, 2012 9:59 am

For a tee valve, I only design piston retract to be = or > than the piston diameter so the piston is completely retracted and flow is not impaired. In testing it produces a lot of power. I've run them in a Flow Works simulation and the performance is substantial.
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Re: Piston Travel Distance in a Tee Valve

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Unread postAuthor: sgort87 » Wed Jul 04, 2012 10:36 am

Piston diameter shouldn't have anything to do with flow restriction (just opening time). It's the port diameter that's relevant.

This topic is actually addressing something I've been wondering for years and haven't bothered to test. I thought the same, so I use about 1/3 the port diameter for my valves. I really wonder what the optimum will be.

I suppose this may heavily depend on barrel length and pilot valve flow. Since the larger pilot volume will open the valve slower, full diameter stroke will make the biggest difference for long barrels. Shorter barrels may be better off with 1/4-1/3 D stroke to keep pilot volume down for fast opening.

Who has answers? TELL ME YOUR SECRETS! :shock:
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Unread postAuthor: velocity3x » Wed Jul 04, 2012 10:49 am

sgort87 wrote:I suppose this may heavily depend on barrel length and pilot valve flow. Since the larger pilot volume will open the valve slower, full diameter stroke will make the biggest difference for long barrels.


I think you've defined it very well. I don't think there is a "one size fits all" solution regarding the piston retract. I should have said that my design(s) are high pressure, large bore, long barrel, heavy projectile and large/ fast pilot valve.
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Unread postAuthor: Velocity » Wed Jul 04, 2012 11:27 am

Thanks for all the responses.

I realize that there may be an easy way to begin testing this subject. If the piston is constructed from threaded rod, the length of the piston can be changed by moving the location of the sealing face. By moving the sealing face forward or backward, it would have the effect of decreasing or increasing the piston travel distance, respectively. Velocity figures could be obtained for each travel distance ranging from d/4 to something like d or greater. Each test would need to be standardized of course, with the same pressure and projectile used.

The outcome of the experiment would rely on a number of different factors, including:
- projectile weight
- barrel length
- chamber:barrel ratio
- piston:port ratio
- valve geometry
- selected pressure, etc.

This would be quite simple (and cheap) to carry out on a small scale, like with a 3/4" porting valve for paintballs...alas, if I only had a chronograph...
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Unread postAuthor: POLAND_SPUD » Wed Jul 04, 2012 11:54 am

Due to a spouse who has a vote, most of the cosmetics will remain stock.
Paint the barrels pink - she'll say they look cute :D
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