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PVC pressure max

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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PVC pressure max

Unread postAuthor: burgerace69 » Sat Jul 05, 2008 4:18 pm

I need to know what a safe max pressure for pvc is using about a 2" barrel. Also, what would be a typically used pressure (not necessarily safe), what can i actually take it to?
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Unread postAuthor: Hotwired » Sat Jul 05, 2008 4:22 pm

This is one of those "how long is a length of string" questions...

The diameter of the barrel is irrelevant.

You will be given the pressure rating on the pipe itself if there is one or at least a code to look up the rating with.

Normally the wording given where it is sold will tell you the rating.
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Unread postAuthor: burgerace69 » Sat Jul 05, 2008 4:41 pm

right but the sockets are rated differently which was my problem
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Unread postAuthor: SEAKING9006 » Sat Jul 05, 2008 7:04 pm

Look at the ratings. You say the fittings are rated differently, right? Make sure they say NSF-pw. If they say that, they are rated to the same pressure as the corresponding schedule (thickness- I.E. Sch40, Sch80, etc). If they say NSF-DWV, you instantly limit the cannon to 50psi for safety. Any higher, and you risk a catastrophic failure in the fittings. Don't worry about the pipe if it says NSF-DWV, pipe usually says both NSF-DWV and NSF-pw. The only thing you need to worry about is a number on the pipe. It will read XXXXXpsi. That's the pressure rating of the pipe. Harvel Sch40 2" pipe is rated for 280psi max, for example. The rating may vary between brands of pipe.
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Unread postAuthor: MrCrowley » Sat Jul 05, 2008 7:54 pm

If they say NSF-DWV, you instantly limit the cannon to 50psi for safety. Any higher, and you risk a catastrophic failure in the fittings.


No, don't even bother using the fittings to start with. Joints can fail as low was 20psi.

But yeah as long as your pipe or fitting has NSF-PW or a pressure rating such as XXXPSI, it's fine.
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Unread postAuthor: tansit234 » Sat Jul 05, 2008 8:07 pm

Isn't there normally a temperature associated with a given pressure rating for PVC? As I understand it at higher temperatures PVC becomes more elastic and at lower temperatures it becomes more brittle. It's really only the extremes in either case that cause failure but I don't care to test this idea so I never run my cannons terribly cold. I use Charlotte Sch 40 wich is rated to 280 PSI at 75 degrees.
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Unread postAuthor: SEAKING9006 » Sat Jul 05, 2008 8:27 pm

That sounds about right. And yes, PVC becomes pliable at higher temperatures. I used to work PVC with a high-temp heat gun and a toaster oven in my High-School robotics club (We took 1st in the B.E.S.T. regionals and 3rd in nationals that year!). However, that's at temperatures way higher than anything you'll encounter on Earth, unless you like chucking spuds from the top of an active volcano. So the high temp doesn't really matter. But, below around 50 degrees Fahrenheit, stop using your PVC launchers. They will become brittle, and may fail after being dropped. And by fail I mean rupture. Alot.
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Unread postAuthor: noob of noobs » Sat Jul 05, 2008 10:16 pm

Erm, this wouldn't happen to be about your first hybrid, would it? http://www.spudfiles.com/forums/first-hybrid-a-few-questions-t15192.html Because if it is, you really, really shouldn't use PVC unless it's Schedule 80, which is also pretty dodgy.

EDIT: Also, keep in mind that PVC isn't really intended for high pressure air, so if you really want to be anal about it, 0 psi is the "safe" pressure. I mean, we all still use it, but just thought you might like to know. :wink:
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Re: PVC pressure max

Unread postAuthor: starman » Sat Jul 05, 2008 10:28 pm

burgerace69 wrote:I need to know what a safe max pressure for pvc is using about a 2" barrel. Also, what would be a typically used pressure (not necessarily safe), what can i actually take it to?


Typical 2" sched 40 pressure rated pvc pipe has a rating of 280 psi. Assuming everything else in your gun can handle the pressure and you have constructed it is properly, that's a good working number. However, that's a whole lot of psi's to be using with a pvc pneumatic. You should be heading to metal guns by the time you hit that kind of pressure.
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Unread postAuthor: Killjoy » Sat Jul 05, 2008 11:03 pm

I have used 2" sch40 pvc as a barrel for FEAR, and that was taking upwards of 350 PSI. Still that was a bad idea on my part and I was desperate for a barrel. ABS would be a much better choice.
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Unread postAuthor: DYI » Sun Jul 06, 2008 9:08 am

I have used 2" sch40 pvc as a barrel for FEAR, and that was taking upwards of 350 PSI. Still that was a bad idea on my part and I was desperate for a barrel. ABS would be a much better choice.


And I've had ABS fail at 4x oxy/propane. We really don't know what kind of pressure that would be, because no one has done testing on it yet, but the bottom line is, SCH 40 plastic barrels just aren't the greatest idea for hybrids.

That's not to say they can't be used. With my reinforced 1.5" ABS barrel, I've done multiple 20x mixes with heavy projectiles (usually about 2 pounds). The 1" or so of un-strengthened pipe at the very base of the barrel has peeled apart, chipped, and is showing strong signs of melting, but it's all held together. That's 1.5" pipe inside 3" pipe with fiberglass cast into the intervening space, and even by a very conservative estimate, it has a burst pressure of over 4500psi. The real problem is the fiberglass, or whatever casting material you're using - at $20/L, you're a lot better off just buying steel.
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Unread postAuthor: burgerace69 » Sun Jul 06, 2008 2:04 pm

thanks for the extra info :P

Erm, this wouldn't happen to be about your first hybrid, would it? http://www.spudfiles.com/forums/first-h ... -t15192.ht ml Because if it is, you really, really shouldn't use PVC unless it's Schedule 80, which is also pretty dodgy.


:shock: hehe uh yes i actually am starting on a new hybrid project...the problem is that i cant weld...
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Unread postAuthor: daxspudder » Sun Jul 06, 2008 2:34 pm

SEAKING9006 wrote:That sounds about right. And yes, PVC becomes pliable at higher temperatures. I used to work PVC with a high-temp heat gun and a toaster oven in my High-School robotics club (We took 1st in the B.E.S.T. regionals and 3rd in nationals that year!). However, that's at temperatures way higher than anything you'll encounter on Earth, unless you like chucking spuds from the top of an active volcano. So the high temp doesn't really matter. But, below around 50 degrees Fahrenheit, stop using your PVC launchers. They will become brittle, and may fail after being dropped. And by fail I mean rupture. Alot.


One problem. What happens to air when it's compressed? Why do cars with turbos use inter-coolers? What is the number one cause of failure in bike pumps being used above 60psi? Inversely to the rest, how does an air conditioner work? When air is compressed it heats up, ive had a bike pump get above 200f at only 80psi, and like-wise when air expands it cools.

So when using a hybrid, that has a base charge of 59psi(4x) while that wont drastically increase the temperature, when its ignited and compresses and quickly expands it creates a huge thermal change, with a range I would estimate to be roughly anywhere from 400-3000(muzzle flash on a typical gun exceeds 3000 degrees) degrees in less than a second, which can cause extreme brittleness in plastics, even some metals, so while they may not fail immediately or for a few shots, when they do fail it would be safe to say that a more violent reaction can be expected.

Use metal, I would recommend copper, since you can get the kit to "weld" it for just $20 at any hardware store, even walmart, if you need to know how you can search for it, or just PM me for a quick tutorial. Just dont use PVC/ABS/non-metal for a hybrid, at least not without some extreme reinforcements(DYI's barrel).
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Unread postAuthor: DYI » Sun Jul 06, 2008 2:43 pm

So when using a hybrid, that has a base charge of 59psi(4x) while that wont drastically increase the temperature, when its ignited and compresses and quickly expands it creates a huge thermal change, with a range I would estimate to be roughly anywhere from 400-3000(muzzle flash on a typical gun exceeds 3000 degrees) degrees in less than a second, which can cause extreme brittleness in plastics, even some metals, so while they may not fail immediately or for a few shots, when they do fail it would be safe to say that a more violent reaction can be expected.


The gases don't have enough heat capacity to cause any highly significant temperature change in the chamber or barrel material. That's why you can use a PVC combustion launcher generating a theoretical maximum of over 2000 degrees Celcius for years, and thousands of shots, with no failures or damage.
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Unread postAuthor: daxspudder » Sun Jul 06, 2008 2:58 pm

But a hybrid has an exponential amount of heat caused by pressure compared to that of a combustion. And a combustion wont have the same cooling after shot of a pneumatic/hybrid, it is the rapid thermal shift that causes brittleness, i.e. the heating then making cool, rather than heating and allowing to cool.
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