T-shirt Launcher

[Sperkowsky]

Originally Posted by hrench

Okay, I have quite a few people disagreeing with me saying that the failure mode for PVC isn't water hammer, but no one presenting evidence. If you simply read the wikipedia entry on water hammer, you'll see that is it common with air too. With water it isn't explosive as it is with air because water isn't compressible. Even the engineers studying this airplane crash attributed it to water hammer.

And it is true that in a water-system you can introduce compressible trapped-air to prevent water hammer because you've introduced a 'spring' and suddenly you have a second-order differential equation-- that system won't be able to find 'reflection' to 'hammer' the end into a pressure spike.

And I've seen some other people saying that the barrel will "fail catastrophically" but it's not clear to me how even that can result in shrapnel from the back end of the PVC when the barrel-end of the system is OPEN. And again, I point out, I've done this and I've seen PVC fail using it for air pressure on a test system for train brakes. I didn't shrapnel, it just blew at a joint. I still maintain that the barrel won't introduce shrapnel, even in a catastrophic failure. It will blow a joint or split.

Also, when I blew up that accumulator tank that I was using, it was while quickly opening then CLOSING 1-inch dia piloted valves. I maintain you have to close a valve to get water hammer and with these systems, you never close a valve under high flow.

If I'm wrong...and I sometimes am...provide me with a logical explanation of why and how PVC--rated to usually 100+psi for water--fails with air.

Wow this has become a pretty big debate can't wait to see some evidence from both sides.

Ive seen pvc crack under pressure (tried to make long paralletes out of pvc and when I did handstand on it it cracked) thd crack was super clean and smooth.

But putting flex on it and air pressure through it are different. I kinda want to see a video of someone's pvc cannon exploding or an experience of that. If there's nothing I'll still try not to use pvc but won't go crazy if we need to.