pic: T Shirt Shooting Robot

[Chris is me]

To be fair I've seen quite a bit of PVC pipe used underground but in houses. Probably not what's supposed to be used but it's there. Regardless, a pressurized liquid and a pressurized gas are completely different forces (especially because gas firing tends to shock load), and PVC should never ever be used to hold any kind of pressurized air for any period of time.

[KRUNCH DUDE]

PVC IS used for sewer. It can and will gain pressure while draining, but like stated above it does not compare to air. Draining from the house into the main city sewer system is where PVC is used. Many houses are piped with PVC as well for draining, but not water that comes into the house. The main sewer that provides for the city Liquid pressure is different, clay pipes were used and their being replaced with concrete and a special plastic, which is also PVC, just a REALLY high grade.. With sprinkler and irrigation systems, only when the valves are opened to release will the pressure be at the minimum. Most irrigation systems have a storage tank to help irrigate faster once the valve is opened, and the pump fills water into that tank. While all the valves are closed there is at no point a release for the pressure and the water will pressurize in the PVC. I understand that this is “apples and oranges” but I wanted to make that clear. My irrigation system at home and on my grandparent’s farms up in Delaware use schedule 20 which is extremely thin walled. Now we have slowly replaced that over time. Now we use schedule 40 which can handle up to 200psi or 330psi depending on the diameter of the PVC.

PVC is rated on only straight lengths. Those lengths are rated on their diameter. 220psi is a good range because that is what 4” diameter schedule 40 PVC is rated at. Anything smaller, the psi it can handle will grow. Of course that rating is specifically for liquid, but it can also be used for air. Obviously those cannons will NOT withstand 220psi. Because of the end caps and joints, the stress levels are different. With normal cement the PVC is rated for about… 133psi. heavy duty cement is rated a lot higher. We will be launching approx… 80-100psi MAX. We WILL be shooting at a lot lower PSI, and we will change our barrel of our cannon to a smaller diameter to allow for more distance instead of raising the pressure. 80psi is a LOT better than 120psi. and after gluing the PVC together, we understand that the cement really did its job. We had our guys wearing gloves, because you get burned instantly if it gets on your hands. It even got onto the gloves and they still felt heat. Yes PVC is not the best option for this. But there was a table created to explain the Max Operating pressure of both schedule 40 and 80 PVC along with their Bursting Pressure. That’s the point where it’ll just EXPLODE into a million pieces. Anything above that rated pressure will raise the issue of stress levels on the end caps and reducers/increasers that we attached.

http://www.engineeringtoolbox.com/pv...res-d_796.html

Now I can’t remember if we used Schedule 40 or Schedule 80 because I don’t have the cannons with me now, but we’ll assume that it was Schedule 40, just in case. The max operating pressure is 133psi for 4” diameter PVC. The bursting pressure is something we will never hit with normal competition compressors… So we won’t account for those. The pressure regulators we have on there will only allow up to 100psi. PVC is a very good material for pressure, especially for liquids. 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. Liquids will usually leak and crack the PVC. We and 100% positive that we glued the tanks correctly and there are no existing leaks. The brand new PVC won’t pose a problem in itself because it hasn’t been exposed to the sun for long periods of time and its brand new. The joints are different, because they will be where the PVC will fail if at all. One of our lead mechanical students thought it would be cool to incase the cannons with a cover, like you see on battle ships and tank turrets. We decided that heavy duty lexan would do the trick (since we found a bunch after cleaning our closet) and the tanks will be covered by that, which should deflect the danger of a failure a bit. Its understood that it could still burst through our cover and pose danger, but we’re positive and confident in our design that there won’t be failures, and if at all the safety taken while running the cannons and robot will be an extreme priority. We load the cannons while the tanks are empty so there is no accidental launch. Launches take 10 seconds with a HUGE warning light to signal that it is about to launch. No one will be within touching range of the robot. We will be about 50-60ft away to allow for driver interaction through the old wireless radios. And with the casing around the tanks, a failure will be contained within the robot, extremely damaging the inside of the robot.

So safety has and will always be an important factor with this project and robot. Even though a lot of people are furious with the PVC, we’re confident that no catastrophic injuries will exist from the explosion of PVC, just the usual scratches from FIRST robotics.

[gallo26]

If any of you guys have more comments please PM me personally. I feel that it would better serve its purpose and we can work out the thoughts. Because I know there is a danger with it, and there is a danger with using the machines and tools in our machine shops, and there’s a danger with the robots we build, cars we drive, bridges we cross, everything we use and touch, but I think some of you are ill-informed on what we are actually using and I want to make sure that you know what schedule PVC we’re using, what pressure, what adapters/end caps/reducers, because if someone were to think we’re using Schedule 20, they would think we’re stupid! But I would really appreciate your insights and wisdom for this project.

[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?