Quote:
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.
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First of all any rigorous discussion on fluid dynamics that mentions water hammer and air in the same sentence gets points off. But this is CD and not rigorous. :0
Taking the second reference first. The "water hammer" reference was discounted by the NSTB. The accepted conclusion was aluminum has a fatigue life & the plane exceeded it by the high number of take off & landings relative to the relatively short flight miles of the air frame. See the Dehavilland Comet failures for another classic example of the fatigue of aluminum and stress risers which BTW effectively handed the commercial airplane market to the United States.
First example second. You should really read the article. Water hammer requires a quick closing valve on the exit of the fluid column. The pressure spike comes from the rapid deceleration of the fluid column. The spike is increased by the weight (directly related to the density) & the velocity of the column. It is decreased by the compressibility of the elements involved. So when you shoot a tee shirt, you might see actually see a vacuum in the barrel when the tee shirt exits the tube. Hard to predict what happens upstream of the valve without the specifics. I expect we see a vacuum spike in the accumulator of our cannon. it is a relatively small volume & we precharge it to 20-120 psi & fully discharge it on each shot. The cannon will constantly shoot a 70 yard field goal.
The reason why you get a large number of "PVC" works for me is that the pressure involved is well within the working pressure of PVC pipe. Failure is not a regular occurrence. The issue comes from the severity of the failure mode & the documentation from both the pipe manufacture & OSHA saying don't do it.