Quote:
Originally Posted by hrench
We've built a t-shirt cannon, and I'm going to be really unpopular here and mention something that the non-PVC'ers haven't said.
PVC is rated for high enough pressure--it just isn't rated for air pressure. The reason is because of water-hammer--go figure--that it's fine to use PVC with water and not with air.
"Water hammer" if you didn't know, is a pressure spike that occurs when you close a valve quickly on moving flow, which can cause instantaneous spikes of pressure as high as 400psi or more. Water hammer--with air--is what explodes PVC pipes. I used to design railroad pneumatics, so yes, I've exploded PVC pipes before. It's loud and dangerous and I don't like loud and dangerous. Please forgive me, we've most all done stupid stuff before.
But think about this...If I use a sprinkler valve that's plumbed with steel NPT fittings--I have all metal in the 'firing' section of the system.
I only need to use PVC for the barrels. The barrels CAN NEVER see a spike caused by water hammer, because you cannot shut off the air you've put in them. If the barrels were to fail--even catastrophically, the worst thing that could happen is that it shoots a t-shirt. One end of the barrel is open to the world. Even if they developed a huge crack in them, the air would still exit the open end with the t-shirt.
People have been making potato guns and t-shirt cannons with PVC barrels for many years. Do you know how much a three-inch ID piece of steel or copper costs? Or how much it weighs? Just to be safe, we never run more than 60 psi, but our 'gun' is four years old and still not broken. Also, the design of a sprinkler valve--which is piloted--prevents it from being closed quickly. Another built-in protection from spikes.
We built a working 2-PVC-barrel t-shirt cannon and the shooting parts of it cost less than $50.
The air-bubble, the Thomas compressor and the kit-frame all came from 'inventory' in the shop.
I think all of the no-PVC'ers here are going to prevent many teams from building t-shirt guns.
|
This is not correct.
Water hammer spikes pressure as high as it does because it is not compressible. And while Air hammer will occur, the pressure spike is much smaller.
The issue, again, with PVC and gasses is the
FAILURE MODE. A barrel will see at least enough pressure to move the ammunition, if not significantly more pressure. Should the barrel fail, it will fail catastrophically and explosively. Please, DO NOT USE PVC for ANY gas pressure.
and just because some people have made potato guns out of PVC and pressurized air, does not make it right or safe. Remember, these t-shirt cannons are, by their very nature, designed to be in front of the public where a catastrophic failure could cause injury to people near by. Think of this before you design your cannon.