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Barrel PSI vs chamber PSI!

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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Barrel PSI vs chamber PSI!

Unread postAuthor: boyntonstu » Wed Sep 09, 2009 7:33 pm

Here is an axample :

You are shooting marbles through a CPVC barrel rated at 400 PSI could you use 600 PSI pressure in the steel/brass/copper chamber?

IOW How much actual pressure does the barrel see with a loose fitting projectile?
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Unread postAuthor: Technician1002 » Wed Sep 09, 2009 7:50 pm

Have you tried to use the Barrel pressure graph in GGDT?

Pressure will vary depending on the valve flow, projectile mass, projectile size, friction, etc.

In most I have modeled, the high pressure is short lived and only while the projectile is starting to accelerate.
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Unread postAuthor: twizi » Wed Sep 09, 2009 7:53 pm

it should not there only like half a second of presure yes it shock presure but its expanting that why u can use a high presure in a bbmg there not much presure in the chamber or in the barrel
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Unread postAuthor: btrettel » Wed Sep 09, 2009 7:59 pm

Technician's right about how to find the peak barrel pressure. I'll tell you some other things worth knowing.

For extremely short duration loads, materials generally are stronger. The pressure rating usually has a safety factor on it too, so you should be able to exceed the pressure rating without significant issues.

With that being said, the material might fatigue quickly from the short period of high stress. Fatigue refers to failure from a periodic load below the load required to cause failure. The reason for this is simple: Each load causes a small amount of damage even if it's below the breaking point and this adds up. It's worth noting that in a materials class I took the professor noted that fatigue is the top cause of failure, so it's not something to take lightly.

I'd look into a metal barrel myself. But I doubt you'll run into issues, at least immediately. Don't take any of this as an endorsement of exceeding the pressure rating though... they exist for a reason and the engineers who choose them know it better than I do.
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Unread postAuthor: Hotwired » Wed Sep 09, 2009 8:06 pm

Worst case scenario is the projectile jams in the barrel at the breech and you get a shock loading of the barrel and the fitting holding it to the valve.

I've had a copper barrel launch off my cannon before because of that, admittedly it was mostly due to a poorly tightened compression fitting but still, use a crap enough barrel or connection (in relation to pressure and volume used) and and it might rupture in the case of a blockage.
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Unread postAuthor: psycix » Thu Sep 10, 2009 5:27 am

If the projectile jams airtight, you could get unlucky.
If you are using loose and smooth ammo, it can be safe-ish.
To increase safety, use a slower opening valve, so that the 600psi isnt dumped into the barrel while the projectile isnt moving yet.
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Unread postAuthor: Technician1002 » Thu Sep 10, 2009 12:31 pm

Hotwired wrote:Worst case scenario is the projectile jams in the barrel at the breech and you get a shock loading of the barrel and the fitting holding it to the valve.

I've had a copper barrel launch off my cannon before because of that, admittedly it was mostly due to a poorly tightened compression fitting but still, use a crap enough barrel or connection (in relation to pressure and volume used) and and it might rupture in the case of a blockage.


Most of these all result in the energy direction downrange. Very seldom does the resulting destruction release significant energy in parts toward the operator. If you are shooting at close range there could be ricochet, so an uncluttered area in front is a good idea.

The worst case is a barrel burst at the chamber that would propagate a crack back into the reducer of the chamber causing a chamber to grenade in the operator's hands.

Next worst is fragments from a barrel burst next to the chamber embedding in the operator's hand where he is holding the barrel.
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Unread postAuthor: ramses » Thu Sep 10, 2009 5:57 pm

However, a crack propagation probably won't happen if you use a metal chamber, although copper may "tear" if you are unlucky.
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