Re: T-shirt Launcher
 

No, but we've only used the current barrels at three games, perhaps 100 cycles per barrel including testing. I'm not going to wait to have a problem to fix an unsafe condition. ::safety::

Re: T-shirt Launcher
 

PVC also deteriorates with exposure to UV radiation. This may not be a problem immediately, but years down the road this will become a concern.
The cement used to bond PVC is also not rated for air pressure.

Re: T-shirt Launcher
 

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.

Re: T-shirt Launcher
 

The 'gray' PVC is 'UV stabilized', but don't forget that simply painting PVC protects it from most UV.

Re: T-shirt Launcher
 

I agree with you that using PVC for the barrel is fine because the barrel is not storing pressure. I am more concerned about people using PVC as the pressure storage tank.

Re: T-shirt Launcher
 

Water hammer produces such high pressures because the water is incompressible and massive, so stopping it suddenly requires a large, quickly applied force - hence a hammer. Air is light and compressible. In fact, small vials of air are used as water hammer suppressors. Bottom line is, that I don't think that the problem with air in PVC is really water hammer.

I'm still planning to replace those PVC barrels.

Re: T-shirt Launcher
 

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.

I have no idea who's right but you did put some science in which validates your opinion a bit. I'm a starting to wonder if this "no pvc under air pressure" Is like "always wear an anti static strap while building pcs". I don't know who to trust but I'm going to look and try to find some metal pipe big enough. If I can't find a free pipe pvc it is. There will be testing and the pvc barrel will be a wooden box so if it did break it would be contained?

I've played with pvc making gymnastics equipment and have seen it break under stress. But I can't find any videos of someone getting injured from pvc bursting at 60psi

Re: T-shirt Launcher
 

Placing the PVC in a wooden box will do nothing to contain an explosion, it will just create more shrapnel. Do not use PVC as an air tank. Only use air pressure rated tanks and fittings. And as Daniel sad above, the "water hammer" explanation is questionable. Every PVC pipe manufacturer, and several regulatory agencies do not recommend or prohibit using PVC for pressurized gas. See warnings W-3, W-4, and W-5 in this document. W-5 specifically warns against compressed air guns:

Compressed air is dangerous. Even at relatively low pressures, there is a lot of energy involved. If you use PVC for compressed air situations, you are putting yourself, and everyone around it at risk. If you can't afford proper materials now, wait until you can.

Re: T-shirt Launcher
 

What did 254 use for their barrels and fittings?

Re: T-shirt Launcher
 

Aluminum barrels, and pressure rated tubing and fittings.
http://team254.com/category/frc/shockwave-build/

Re: T-shirt Launcher
 

254 used aluminum barrels.

Edit: Ninja'd.

Re: T-shirt Launcher
 

well..... we cant afford anything 254 does

Re: T-shirt Launcher
 

Your team is located in the same town as Northrop Grumman, I think. Have you ever worked with them as a sponsor? Perhaps they could fund this project, or provide you with materials. They certainly have aluminum tube stock sitting around that they could donate.

Re: T-shirt Launcher
 

As for valves, we used this: Asco 8353C035. It is a high flow air valve designed for back pulsing filters.

As to the PCV. Pressure is pressure. But PVC (the kind they use in for pipe) is brittle. When liquids decompress, they don't expand like gases do, so liquids don't throw shrapnel the same way air does when the pipe shatters. The density of air is so small & the distances involved are small so you do not get significant pressure spikes from "water hammer"

The PVC is not going to randomly burst under normal conditions, with air or water. But if it does, maybe it gets bumped in the wrong way, something hits it, etc, it has a bad failure mode. Well documented. Big legal liability. Especially for any mentors on the team that are registered professional engineers. Makes the cost for other material seem small.