pic: T Shirt Shooting Robot

[Chris is me]

The reason why everyone has been fighting this is because if there is a crack, a hole, or terrible gluing job (like only half an end cap for ex.) the air will expand rapidly upon that initial failure.

Partially. The other fault is that it is nearly impossible to tell when PVC is on the brink of failure. That's the main thing.

[gallo26]

Normally with PVC you will hear the air leak, normally with anything with air pressure. The tank would have to be instantly filled up to pressure to cause it to explode. But if no one notices the air leak, and you continue to fill up (though it WOULD be slower) you can and probably will experience the failure.

[Chris is me]

Originally Posted by gallo26

Normally with PVC you will hear the air leak, normally with anything with air pressure. The tank would have to be instantly filled up to pressure to cause it to explode.

Any time pressure is released, an air tank is instantly being applied pressure on all surfaces equal to the air pressure inside the reservoir. So this instant fill up... happens every time the gun is fired.

But if no one notices the air leak, and you continue to fill up (though it WOULD be slower) you can and probably will experience the failure.

Except PVC doesn't have a "leak" warning. It fails instantly and catastrophically.

[gallo26]

Originally Posted by Chris is me

Except PVC doesn't have a "leak" warning. It fails instantly and catastrophically.

A leak warning would be hearing the noise. I'm almost positive that you will hear it, and we have on past tanks. You hear it on robots with pneumatics or anything with leaks. Its the exact same thing if there's a leak in an inner tube or a track ball. You can even notice the pressure drop while testing at a low pressure (we watch our gauges and test before going up to full pressure). So I don't believe that is correct. But I do believe that the instant failure is correct.

[EricH]

Originally Posted by gallo26

A leak warning would be hearing the noise. I'm almost positive that you will hear it, and we have on past tanks. You hear it on robots with pneumatics or anything with leaks. Its the exact same thing if there's a leak in an inner tube or a track ball.

Wait... Let me get this straight. You have never heard a leak in PVC. You have heard it on pneumatics or "anything [air-filled] with leaks". (The edit is because you don't exactly hear a water leak in the same way--or a steam leak.)

Am I correct?

Because if I am, you probably don't know what an air leak in PVC sounds like. It'll likely sound similar, yes... But it won't be the same. You see, an FRC robot's pneumatic system has two types of leaks. One is from a punctured tube, the other is from a poorly sealed valve. A bike tire or trackball sounds about the same as a puncture tube; a poorly-sealed valve will be hard to hear due to the slow nature of the leak. And in fact, I'd rather you had that than a leak in the PVC itself.

What a pneumatic tube, a bike tire, and a trackball have in common is that they are "soft" plastics--thin-walled, flexible, and somewhat "bang-resistant"--that is, they won't make a big mess if they do burst. There are still FIRSTers who can tell you of the doublers in 2004, which, if mistreated, could go all at once on a slight hole. Being flexible, they can bend a bit out of the way. PVC pipe is not nearly as flexible, especially at the size you're using. And if you do develop a leak, you've probably got about 1 shot to realize you do before something happens. You might not hear it, you might not notice a pressure drop, but something's going to give and you hope it isn't catastrophic. That's why we're pushing you guys to switch to a safer material, such as metal--PVC isn't exactly "bang-resistant". (Metal isn't either, but it can sure take more pressure than PVC.)

[gallo26]

You can hear the leak of the escaping air, and if you submerge it under water you see the bubbles. Our PVC has leaked before, and you can hear the air escaping. and I know that PVC with running water you don't hear it, but you do see the water that it leaks. We had a rather bad gluing job done on an older version of our cannons and you can clearly hear the leak as well as see the pressure drop. So sorry, but i don't think your correct.... because i do know what it sounds like, and I have seen them in water

[Daniel_LaFleur]

Originally Posted by gallo26

You can hear the leak of the escaping air, and if you submerge it under water you see the bubbles. Our PVC has leaked before, and you can hear the air escaping. and I know that PVC with running water you don't hear it, but you do see the water that it leaks. We had a rather bad gluing job done on an older version of our cannons and you can clearly hear the leak as well as see the pressure drop. So sorry, but i don't think your correct.... because i do know what it sounds like, and I have seen them in water

*sigh* ... so much misinformation...


PVC leaks cannot be heard as they will cause the PVC to fail violently. The leaks that you describe are leaks at the bonds or joints, not in the PVC itself.

Lets consider PVCs pressure rating for a moment. PVC is only pressure rated for liquids (and for a very good reason):
Water is incompressable, much like a steel block ... if you push down on a steel block with 50 PSI it will not compress and if you release that force instantly the steel block will not move.
Air, on the other, hand is compressable, like a 50 pound spring. Apply 50 psi to it and the sprin compresses. release that force instantly and the spring not only goes back to it's original shape but rockets off of the surface it was on.

Now on to PVCs material properties:
PVC is rigid polymer. This allows it to flex while maintaining it's strength, however once it is flexed too far it will crack and the PVC pieces will then (violently) attempt to return to their original shape. PVC is extruded which means that it's wall thickness is not consistant. Also each time it is flexed it gets reduced in strength (each pressurization cycle flexes the PVC).

If the PVC pipe had pressurized liquid inside then when the PVC pipe fails all of the pressure within the pipe is released but does not violently explode because it is like the steel block and was not compressed.

If the PVC pipe had pressurized air inside then when the PVC pipe fails the air will violently rush out the failed spot which will continue to apply pressure to the now failing spot causing it to fail further. Pieces of PVC at this point can be ripped off and carried (or shot) by the escaping air.

In short, a failure of the PVC (not a bond or joint leak) will not be 'heard' until it violently fails. There will be little to no warning. The PVC is not rated for pressurized gas, and the shock loads that a cannon put on PVC are far beyond what it is designed to hold.

Now I'm not saying that yours WILL fail, but considering that you will expose the public to any failure you have, I would suggest that you take every precaution possible to ensure the publics safety. FIRST (and the FIRST teams) do not need the political ramifications from a publicity stunt gone bad.

P.S. @ Krunch dude. The chart you referenced is out of ASTM D1785 which specifically states that PVC is for liquids only and that those numbers do not reflect gas pressurization.Read Abstract here.

[robself705]

Here is our t-shirt cannon

http://www.youtube.com/watch?v=xKCwbomY82c

[gallo26]

Originally Posted by Chris is me

Any time pressure is released, an air tank is instantly being applied pressure on all surfaces equal to the air pressure inside the reservoir. So this instant fill up... happens every time the gun is fired

I'm also confused about that... What do you mean exactly? Because if the air is released, shouldn't the pressure on the surfaces inside the reservoir be just a few psi (or whatever is left over. usually 0-5psi)? And how is it instantly filling up in the gun is firing? that's an instant release... I think I'm confused here

[Chris is me]

Originally Posted by gallo26

I'm also confused about that... What do you mean exactly? Because if the air is released, shouldn't the pressure on the surfaces inside the reservoir be just a few psi (or whatever is left over. usually 0-5psi)? And how is it instantly filling up in the gun is firing? that's an instant release... I think I'm confused here

Pressure extered by a fluid is exerted evenly on all surfaces of its container. So when the valve is released and 120 psi of air are instantaneously in the pipe leading to the t-shirt, all of the PVC is experiencing 120 psi of force.

[gallo26]

Originally Posted by Chris is me

Pressure extered by a fluid is exerted evenly on all surfaces of its container. So when the valve is released and 120 psi of air are instantaneously in the pipe leading to the t-shirt, all of the PVC is experiencing 120 psi of force.

Ohhh ok, so your talking about the barrel then? I gotcha! that makes sense... thanks!

[EricH]

Originally Posted by gallo26

I'm also confused about that... What do you mean exactly? Because if the air is released, shouldn't the pressure on the surfaces inside the reservoir be just a few psi (or whatever is left over. usually 0-5psi)? And how is it instantly filling up in the gun is firing? that's an instant release... I think I'm confused here

It's physics and such. Let's use some everyday examples...

You always have 14 psi pressing down on you, due to atmospheric pressure. But now, you decide to have some fun. You go into a room that is totally sealed, with an air compressor hose coming down to a smaller chamber that seals to your skin. You have gauges and things of that nature all over the room. The room gauges read zero to start--they've been calibrated so 14 psi is zero. You pressurize the smaller chamber to some higher pressure (say, 50 psi as the gauges measure). Now, something jerks the small chamber away from your skin. Instantly, all the pressure is released from that chamber into the other chamber, and air being air, it fills the entire space as fast as it can. The gauges now measure more air pressure than zero (how much more depends on the size of the room and the size of the small chamber). Instantly.

But you've rigged this room with a small air cannon, and it's a small room, with an insane amount of pressure in the small chamber. Suddenly, the small chamber breaks, and all that air finds the cannon, which is loaded. The ammo moves under the pressure, which at the instant the chamber broke, was applied throughout the room, and shoots out, releasing all the pressure. The room returns to the normal air pressure of 14 psi, the gauges read zero, and you now have to reload and try again.

Does this help?

[gallo26]

Yes sir! I wasn't sure what part he was talking about, tank or barrel. Thank You

[Pierce]

I know this thread is pretty much dead. None the less, I am the current president of Krunch 79 and we are still using the T-shirt cannon, completely unaltered. Just to get a gauge for how much we use this, we fire it off about 20 times per barrel each football game. So every week for a 3-4 month season for 2 years now. We have not had any problems with this robot, no leaks, noises, or explosions. I do understand the risks and effects of using weak PVC, but due to time and cost constraints we are holding off on the much need repairs until mid 2016. Please do not continue to educate me on the dangers of PVC as I am well aware off the dangers of using PVC. I also do not want any comments on why we waited 6 or so years to replace the PVC as I said we had time and money constraints. Just looking out for my fellow Krunch members, Gallo!

[EricH]

I only have a few comments left.

1) I hope nobody gets hurt; the change to metal cannot come too quickly for me.

2) Again, do not assume that water and air pressures will behave the same in the same material.

3) MURPHY'S LAW. Also note that this is a case where nothing can go wrong, so invariably it will go wrong all at once when it does. Also read "The Wonderful One-Hoss Shay" by Oliver Wendell Holmes, Sr., a poem about something that was constructed with no weak points.

4) The cover also can't go on fast enough for me. Again, safety. Remember the fan covering debates we had this year?

5) Usually when a teenager is 100% positive about something, especially if it involves something dangerous, I head for somewhere safer.