randompkguy wrote:So what happens when it bursts with water as opposed to air? I'm guessing that water decelerates faster than air, so the particles of pvc move slower?
Very simple really. Water, and other liquids, are essentially incompressible. Air is very compressible. To put it another way, you really can't store much energy in compressed water. You can store a lot of energy in compressed air.
As an example, consider a 100in<sup>3</sup> chamber filled with water. To pressurize it to 10 ATM you would only need to pump a very small volume of additional water into the chamber. Ballpark figure is 0.05in<sup>3</sup> of water into 100in<sup>3</sup> gives a 10 ATM pressure rise. (Assuming the chamber itself does not expand.) That means you added just 0.05% of the original volume to the chamber.
Now consider the same chamber filled with air. To get the pressure of the chamber up to 10 ATM you need to pump nine times as much air into the chamber as there was to begin with. You have added 900% of the original volume to the chamber.
Now consider what happens when the two chambers fail.
The water filled chamber cracks, 0.05in<sup>3</sup> of water leaks out, the pressure drops to atmospheric, the crack stops growing, the chamber sits there and drips water.
The air filled chamber cracks, a little air leaks out, the pressure drops very little since there is 9X more air in the chamber than there should be. The crack propagates, eventually the crack breaks off a hunk of the pipe. The air is still expanding, the air continues expanding until it has grown to 10 times larger than the chamber. During that expansion the pieces of plastic are being accelerated by significant force. Even after the pressure has dropped to 1 ATM the pieces are moving at high velocity.
So, failure of a chamber filled with a liquid is usually a pretty mild occurence. Failure of an air filled chamber results in the release of much more energy.
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The failure pressure of a chamber is independent of what the chamber is filled with. Doesn't matter if the chamber is filled with air, water, or watermelons.
Then why do the PVC pipe manufacturers say there pipe is not rated for compressed gases? That implies that the pressure rating is different for gases versus liquids.
This brings up what Joannardway pointed out on another thread, the failure pressure is not the same as the pressure rating. The failure pressure of a piece of pipe is the same regardless of whether it contains air or water.
The pressure rating is different for air and water because the
pressure rating takes into account the
consequence (and liability) of the failure. If a pipe pressurized to 200 PSI with water fails it leaks water on the floor. Probably a fairly low liability issue for the pipe manufacturer.
If a pipe pressurized to 200 PSI with air fails something is going to get damaged. If there are people nearby they may get hurt. The liability for that is huge. Therefore, if the pipe makers did quote pressure ratings for compressed gases they would be much lower than for liquids. Not because their is a difference in the failure rate but because there is a
difference in the potential for damage and the resulting liability.
Just keep your pressures reasonable and make good solvent welds, and you should have nothing to worry about.