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PVC Burst Pressures

Post about things you have launched or thought about launching. Also post about various materials used for building cannons. No posts about explosive projectiles!
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Unread postAuthor: dongfang » Fri Aug 03, 2007 2:53 pm

Ok, to get an end to the air/water argument, I suggest:

Make the pipes a little longer, a couple of feet.

In test, have the pipes stand vertically, suspended a little above the ground, and half filled with water. Mark the upper (air) half of the pipe with one pen color, and the the water part with a different color. Run the pipes through a tightly fitting hole in a big, fat piece of plywood (of course you should stick them through the hole before gluing on the last end cap). The plywood should help to stop failures from propagating from one end to the other.

Ignore the fact that the pressure is a tiny bit higher (because of weight of water) at the bottom - I hope we can all agree that it's insignificant.

Inflate with your shock pump or CO_2 as originally, through valve in upper end cap (or lower - you decide).

Make a statistics: How many failed with the air part seemingly more intact than the water part, and how many were the other way round?

I know what I would expect from a sufficiently large number of samples, but noname I think your experiment is a great idea, and I hope you will have the time and money to blow a lot of pipe. I'd happily contribute all my end caps and pipe scrap, but it's metric and I'm rather far away to send it...

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Soren
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Unread postAuthor: noname » Fri Aug 03, 2007 3:01 pm

To all those who are confused by me being a defect detector ( :tard: ), I can't see microscopic imperfections. Apparently, I was incorrect when I thought you would think of discoloration, cracking, chipping, scratches, and anything like that.

Dongfang, thank you for thinking along the lines of which I hoped others would think, as there was nothing accomplished in 14 posts. I appreciate your cooperation and that you would volunteer to help.
Europe is quite a ways.....
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Unread postAuthor: dongfang » Fri Aug 03, 2007 3:38 pm

To all those who are confused by me being a defect detector


Is that the same as a defector? Nooooo stay :wink:
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Unread postAuthor: Gepard » Fri Aug 03, 2007 4:29 pm

noname wrote:<snip>I've put over 600 psi WATER pressure in PVC; did it blow? No. But when the same stuff blows at 140 psi AIR, you still think there's absolutely nothing different between the two?
<snip>


Another thread where the physics textbook is rewritten......

Michael
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Unread postAuthor: MrCrowley » Fri Aug 03, 2007 4:45 pm

When part of my chamber blew up on me with nothing wrong with the 260psi rated PVC that blew at 100psi, the only reason I could think of for it going boom was rapid filling and firing the gun.
I would've fired the gun probably 30 times in an hour and a half each to 100psi.
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Unread postAuthor: clide » Fri Aug 03, 2007 4:48 pm

I must agree with everyone else. Physically there is nothing different between 600 psi of water and 600 psi of air.

Why is PVC rated at 1/3 of its bursting pressure? The same reason you had one pipe hold 600 psi and the other fail at 140 psi. All pipe is not made equally, there will be defects in the manufacturing process, mishandling in the distribution process, and various other problems that must be accounted for by a factor of safety. This factor of safety depends on many things like the frequency of defects, ease of mishandling, statistics from real life testing, and cost of failure. A rating doesn't guarantee that it will hold that much pressure, but it represents a high certainty that it will hold that pressure.

Why is PVC not rated and not recommended for compressed air? The cost of failure is too high since PVC fractures and becomes a projectile when in bursts with air in it, it can not be rated to a reasonable pressure with enough certainty to override the cost of failure.

I too have had a piece of PVC fail at well below the stress that I would expect it to take. I am sure there was a defect in the pipe, and I couldn't see it before or after the failure, but it's the only explanation that makes sense.
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Unread postAuthor: noname » Fri Aug 03, 2007 6:52 pm

OK, I'll test 3 different pieces of pipe for each size, from different manufacturers.
There is a reason to why I limit the pressure of my PVC guns to 90 psi, but have shot a steel gun at (supposedly) 800+ psi. Maybe, just maybe, this test will allow me to bring my launchers to a higher pressure. I might also do it with some ABS pipe as well, do to the better failure characteristics ABS has.
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Unread postAuthor: MisterSteve124 » Fri Aug 03, 2007 9:49 pm

By reading what you said you plan to do you won't be putting water pressure in the pipe it will still be air. If you fill it with water and pump it with a bike pump it will still be air pressure not water. And it doesn't matter if its water or air. If its measured in psi one psi of water is 1 pound per square inch and 1 psi of air is one pound per sqaure inch it doesn't matter.
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