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Pressure Venting in 100% Water System

  1. Oct 19, 2013 #1
    Hi all, new to these forums, was hoping someone could explain a hypothetical scenario which I can't seem to wrap my head around.

    Suppose there is a large diameter pipe containing flowing water at 10atm, there's no air whatsoever in the pipe, just water. The pipe diverges into two smaller diameter pipes (the diameters aren't important, just similar cross sectional area to the single pipe) for some time and then the two pipes reconverge into one as before.

    Now say one of the small diameter pipes can be isolated from the other at both ends, by means of two gate valves with perfect seals. When the pipe is isolated, the water is sitting stationary at approximately 10atm (correct?). If the isolated pipe were to be opened to atmosphere, by means of a small high-point venting valve, what physically happens to the system? Obviously the pressure will drop, but at 10atm the water compression is negligible, so how is the loss in pressure realized (I'm assuming water won't be shooting out of the vent)? If there were a pressure gauge in the isolated line, would it shoot from 10atm to 1atm the moment the vent is opened, or would it be somewhat gradual, depending on the size of the vent?

    Similarly, if the vent is closed and the isolated line is then deisolated, will the pressure jump back to 10atm immediately, because there's still no air in the system? If only one of the isolation valves is opened (to equalize pressure without causing flow in the isolated line), will there be any disruption to the flow in the other line (the one which was already flowing)?

    Maybe this is too idealized, in practice would dissolved gases in the water result in any perceivable ramping up/down of pressure?

    If you've made it this far and can provide what I'm assuming should be a straight-forward simple answer (or an overly complicated one), I'd really appreciate it!
  2. jcsd
  3. Oct 20, 2013 #2


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    Ideally; there would be no loss of water and there would be an instant change of pressure. That is the beauty of ideal hydraulics.

    In reality, dissolved gas would come out of solution when the pressure was released. Those micro sized bubbles would seed, grow and remain for some time on the walls of the pipe and cause an equal volume of water to be gradually displaced.

    The elastic pipe will have expanded in diameter under pressure. When pressure is released water will be expelled as the pipe reduces in diameter.
  4. Oct 29, 2013 #3
    Thanks Baluncore. This question spawned from some recent relevant field work. The large pressure increase of a very large volume system from a very small fill point was indeed sharp, but not instantaneous.
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