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.
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.