Quote:
Originally Posted by Cody Carey
When originally designing our T-shirt cannon, We looked at the pressure ratings listed in the Engineering toolbox.( http://www.engineeringtoolbox.com/pv...res-d_796.html )
It clearly lists that the maximum allowable operating pressure for 3" schedule 40 (which is what we used) is 158 Psi, and the minimum bursting pressure is 840 psi. Considering that that is 38 psi above what the exhaust valve on the pump we were using is set to, combined with the fact that we had the tank encased in a plexiglass shield and we wouldn't be operating it in temperatures above 140F (thank God)... I'd say we were perfectly within the confines of what would be considered "safe".
As for wrapping the barrel and/or tank in Duct tape, and placing it in a larger section of pipe, that would not only make it a little safer, it would virtually eliminate any safety hazard at all. No shrapnel means no major injuries... and the chances of the larger pipe bursting (if it is sealed) are minimal as well, because the largetr pipe would be exposed to a much smaller pressure PSI than the little one was.
As for the reasoning behind FIRST having the teams limited to 60 PSI, I doubt it has to do with the bursting pressures of PVC,but can more likely be attributed to them being overly stringent with their safety regulations... but as there have been no major pneumatic related injuries that I know of thus far in the program, I'd be an idiot to argue with their reasoning.
I'd say that as long as you have the tank well guarded from any shock, and you don't abuse the thing, you're safe to use PVC in a 120 PSI system. |
Even though these ratings are printed, for what type of application are they defining? Liquid or Gas?
Liquid does not compress very well, so the chance of PVC exploding with a liquid is very slim. However, gas does compress well, and therefore, the chance of the PVC exploding is much greater. You must also think about the size of the molecules and how they interact with the structural integrity of the PVC pipe. Liquid molecules in general are much larger than gas molecules, so liquid can be contained much easier.
Here is an example of what I'm getting to:
You have 2 balloons, fill one with water and one with gas. Wait a week and look at the size of both balloons. I bet the one with liquid is much larger than the one with gas. Also try popping both balloons. The one with air will be much more violent. This same concept applies to PVC pipe.
We've had our own accident with PVC exploding in a kids face, and he could have been killed very easily. He was well in the 120PSI zone (around 40PSI) when a coupling split in two pieces and made the whole tank shoot off like a rocket into the ceiling.
Also, PVC tanks do weaken over time from expanding and contracting. Water lines do the same, but most water lines remain compressed all the time, so the process is much slower. With an air cannon, it goes from 0PSI to xPSI every time it is shot.
There have been many horror stories of PVC exploding and people saying not to do it:
http://www.osha.gov/dts/hib/hib_data/hib19880520.html
http://www.youtube.com/watch?v=AQo81nqx-W8
http://www.cnczone.com/forums/archiv...p/t-35978.html