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High-Pressure Help/Questions

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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High-Pressure Help/Questions

Unread postAuthor: Tantum » Mon May 24, 2010 8:35 pm

I'm planning to build a high pressure (in the 300 to 400 PSI range) pneumatic that fires 1/4" steel shot sling shot weights. Sorry if this is the wrong section. I know this is with 1/4" ammo, but it seems that the BB gun section is filled with BBMG/Vortex/expensively wasteful full auto set-ups. I felt this section was more appropriate. Let me know if this was the right choice.

Anyway, I've run into a few hiccups regarding valves rated to pressures that high...

1. I haven't been able to find any half-way affordable 1/4" QEV's rated for over ~150 PSI. Any ideas on this?

2. I'm having the same problem with finding a high pressure pilot (I was going to go for blow gun in this case). All the high-pressure blow guns I've seen were well over 100 dollars.

3. What pipe and pipe fittings should I use that can safely hold this kind of pressure? Steel, brass, aluminum, etc?

Just a few notes that may be useful:
-its not semi auto, probably just a manually operated bolt with a hopper
-pressurizing will be done with a shock pump

Thanks for any and all help!
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Unread postAuthor: Col. Kurtz » Mon May 24, 2010 8:48 pm

300-400 is huge for 1/4. PVC's fail point is somewhere near 400, to give you an idea. I don't know about the metals, but at that pressure, you're going to get a lot of condensation, and possibly even some frost on the valves. Frankly, it's a little overkill unless your going to be launching something at least 3-4 in in diameter.

One more thing, what's a shock pump? I'm a little new to this.
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Unread postAuthor: jor2daje » Mon May 24, 2010 8:55 pm

Id build the piston valve myself, its pretty easy and a good learing expirience. As for triggers people have used standard blowguns up to about 400 psi, but I would recommend a ball valve trigger its safer at those pressures. You could build it out of brass or steel fittings, they would easily hold in small diameters.
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Unread postAuthor: btrettel » Mon May 24, 2010 9:19 pm

1) Generally, QEVs can be run at pressures higher than what they are rated for. I don't suggest using anything beyond it's recommended limits, but it's an option.

2) Same applies for other valves. Generally, the smaller the valve, the higher pressure it can handle.

3) Most steel pipe you can get at a hardware or home improvement store can take 500 psi or more. I don't remember the precise name of the spec. for steel pipe, but I'm certain the pressure ratings for it are listed here: http://www.engineeringtoolbox.com/pipes ... -t_40.html

You can find what specs. a pipe or fitting satisfies on McMaster-Carr.

If you know the specification(s) satisfied, try a Google search to see what you can find.
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Last edited by btrettel on Mon May 24, 2010 9:24 pm, edited 1 time in total.
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Unread postAuthor: Gaderelguitarist » Mon May 24, 2010 9:24 pm

I know that someone here has taken some QEVs up to 800 psi. So you should be fine with one rated for 150 at 300 psi. Generally, though, the concern with this is the diaphragm failing.

Either be risky and use a regular blowgun, or get a pressure washer handle.

Brass is ideal, as it is mainly used for the containment of compressed matter, but steel would be fine, as would aluminum. Just make sure that the pipes you're working with aren't too thin. As you're working with a small caliber, I would suggest a mingling of steel and brass, as those are readily available at most chain hardware stores.


Does that help?
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Unread postAuthor: MrCrowley » Mon May 24, 2010 9:42 pm

Frankly, it's a little overkill unless your going to be launching something at least 3-4 in in diameter.

Why would high pressure be overkill? It's a much easier way of getting high performance. More often than not, it is increasing chamber volume, not pressure, that is pointless.

300-400 is huge for 1/4. PVC's fail point is somewhere near 400

300-400PSI is not huge for 1/4" at all. Small diameter PVC is rated for about that but bursting pressure seems to be far higher. Regardless, I doubt he'd use PVC anyway since he didn't even mention it.
I think you'll find that high pressure is suitable for small bore cannons, not the other way round.

As for the topic at hand;
1) Like others have stated, most metal QEVs probably can take over 500PSI even though they're rated for 150PSI. Most likely, only the diaphragm/piston would fail/leak anyway.
2) Like above, blowguns can take the pressure even though they're rated for 90PSI. Worst thing that will happen is they'll leak.
3) Galv iron, black iron, steel, aluminium, copper etc pretty much any of those pipes will take the pressure. The iron fittings at Home Depot definitely can. If you find some random tube that is metal, as long as it has a decent (2-3mm or so) wall thickness, you should be fine.

I imagine you'll only need about 10 cubic inches of chamber volume at that pressure, max. Everything seems to be pretty straight forward otherwise. Check out some of the high pressure cannons already posted for ideas.
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Unread postAuthor: c11man » Mon May 24, 2010 10:21 pm

i agree with Mrcrowly above, a qev can take the pressure, i use my 1/2inch up to 750 and the 3/4iinch up to 600. i use the iron fititngs up to the 600psi, same with brass.

@Col. Kurtz
you seem to have it backwards, small diamater launchers typicly are high pressure to accieve high velocities. my 1/4inch gun is 600 psi inorder to shoot the 1/3inch bearings at a high velocity. while my 4inch piston cannon uses only 100psi to get high velocites
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Unread postAuthor: velocity3x » Mon May 24, 2010 10:30 pm

Col. Kurtz wrote:it's a little overkill unless your going to be launching something at least 3-4 in in diameter.


High pressure is your friend. No such thing as high pressure being overkill. High pressure + unnecessarily high volume is overkill.
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Unread postAuthor: Lockednloaded » Mon May 24, 2010 11:23 pm

2x the volume = about 2x the velocity, but 2x the pressure = about 4x the velocity. I would use some galvanized pipe and fittings if I were you. Get yourself a pipe wrench and some teflon tape and you will have an 800 psi capable chamber.
Col. Kurtz wrote:One more thing, what's a shock pump? I'm a little new to this.

They are small diameter pumps used to inflate bike shocks, but we use them to achieve high pressure in small chambered cannons
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Unread postAuthor: jhalek90 » Tue May 25, 2010 1:57 pm

My ebay 1/4 QEV has been to 600psi a few times.. and still works like the day i got it.

It is rated to 150psi.

Also, a good blowgun will handle that high of pressures... but a 1/4 ball valve will also pilot the QEV very very well.
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Unread postAuthor: Tantum » Wed May 26, 2010 1:06 am

Thanks a ton for the help, guys. This pretty much perfectly answers all my questions and concerns. If anything else pops up I'll post it here.
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