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Quick question (pipe rating/pressure)

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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Quick question (pipe rating/pressure)

Unread postAuthor: inonickname » Sun Jun 27, 2010 3:46 am

Just looking for an opinion. I've ran it through a burst calculator and it should hold, but I've messed up with the calculator before and wanted to be certain.

So;

24.85mm OD
18.85mm ID
3mm Walls
2014 T-6 Aluminum

Do you believe this will hold at least 800 psi (aka; unregged CO2) or higher? Endcaps will be attached VIA bolts.

I'm asking as I got 4 meters of hydraulic pipe today, and plan to put it to use (it's very hard, straight and smooth, and it will hold in excess of 3000 psi). I decided a co-axial would be a decent start. The pilot valve will probably be of my own construction. Pressurized to 400 psi by a shockpump until I get some source of high pressure air.

Is it workable, or do I need to go invest in some stainless tube?
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Unread postAuthor: SpudBlaster15 » Sun Jun 27, 2010 4:12 am

So long as you can attach the end caps effectively, that pipe should have no problem with unregulated CO2.
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Unread postAuthor: inonickname » Sun Jun 27, 2010 4:38 am

That was what I was thinking.

For the piston, I was thinking of something different. A stepped piston with two diameters. One fits inside the chamber, the other inside the barrel. The section in the chamber is standard for a co-ax piston. The section inside the barrel has an o-ring to act as a seal.

Thoughts? I was thinking of this to improve reliability, as my last co-ax suffered reliability issues with the piston.
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Unread postAuthor: SpudBlaster15 » Sun Jun 27, 2010 4:59 am

A stepped piston with an o-ring that sits against the inner surface of the barrel might seal more effectively than a traditional setup, but I suspect you'd have issues with re-seating the piston after the valve had been triggered.

I've never had reliability issues with standard piston designs, whether the valve was built into a T fitting or a coaxial setup. What kind of specific problems did you experience with your previous launcher?
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Unread postAuthor: D_Hall » Sun Jun 27, 2010 1:31 pm

Didn't even look at your pipe dimensions but I will say this... If you're going with unregulated CO2 you need to plan for much higher pressures than 800 psi. That's nominal pressure at about room temperature. What if you want to take your gun out on a nice summer day? It's going to be hotter and the pressures are going to be higher.

Planning on unregged CO2? You should be planning for more like 1500-2000 psi.
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Unread postAuthor: Technician1002 » Sun Jun 27, 2010 4:33 pm

inonickname wrote:For the piston, I was thinking of something different. A stepped piston with two diameters. One fits inside the chamber, the other inside the barrel. The section in the chamber is standard for a co-ax piston. The section inside the barrel has an o-ring to act as a seal.

Thoughts? I was thinking of this to improve reliability, as my last co-ax suffered reliability issues with the piston.


I tried that. In a nutshell, you will need a new o ring on the piston for each shot. Due to the way o rings seal, gas pushes the ring against one side of the groove and up against the wall of the cylinder it rides inside. As your cork comes out of the barrel, the ring in a vain attempt to keep a seal on the barrel will blow out of the groove and off the end of the piston. The ring joins the projectile downrange. Been there and done that with an early design.

Depending on the ring size, this can happen as low as 20 PSI. For a 1 inch barrel, it happened at about 50 PSI and above.

It took me a while of pumping to figure out that the piston would no longer seal for the second shot without the o ring. I launched a few rings before I figured out why they were lost.

I have a long list of failures before my successful builds. Don't be discouraged and learn from others mistakes so you don't have to repeat them.

The piston below does not come out of the barrel. It opens ports in the barrel. This was tried to keep the rings in place. The small ports I used were severely choking the flow. It was a ring of 1/8 inch holes.
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Attachments
Saber Valve.JPG
A shorter piston in an earlier launcher launched the o ring on a regular basis. This longer piston used ports like my QDV to retain the ring. The ports were too small and the piston too heavy so performance sux.
Last edited by Technician1002 on Sun Jun 27, 2010 9:29 pm, edited 2 times in total.
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Unread postAuthor: inonickname » Sun Jun 27, 2010 8:19 pm

D_Hall wrote:Didn't even look at your pipe dimensions but I will say this... If you're going with unregulated CO2 you need to plan for much higher pressures than 800 psi. That's nominal pressure at about room temperature. What if you want to take your gun out on a nice summer day? It's going to be hotter and the pressures are going to be higher.

Planning on unregged CO2? You should be planning for more like 1500-2000 psi.


Of course a fair safety factor is a given. I build a lot of my stuff to nearly a factor of 10. Another alternative to CO2 I'm looking into is to purchase or machine a small bore high pressure pump to reach similar pressures.

Thanks Tech, just the information I was looking for (killed that plan unfortunately, conventional it is)

Thanks guys
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