"how do hydrants are pressurised with 7 bar all the time even when the jokey pump is off"

We all know that water cannot be compressed, if compression is not possible, how can we maintain the pressure without an external force. we know that jockey pump is provided to maintain the pressure at 7 bar when ever it reduces, but when it stops the force or pressure exerted over the water body is also stopped, the water comes to it normal condition. (e.g. take a sringe fill it with water, exert pressure over the water by pushing the piston, at the other end u can feel the pressure, as soon as u release the piston there is no pressure exerted over the water, now you will not feel the pressure at the other end). Is there any compressed air provision attached to the hydrant system to maintain the pressure @ 7bar, if so please explain it.

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Permalink Reply by Ernie Scheinert on February 20, 2011 at 10:52am

Just wanted to point out that you have answered your own question here!! In this example you mention the COMPRESSOR, which is required to get the volume of air required to fill the tire. Ironically, that is what your pumps are doing with the water. Without the compressor applying mechanical force to the air, you would not be able to get any air into the tire either. Both applications require mechanical means for them to work as well as CHECK VALVES to contain them in their respective containers. Of course, your other problem here is the fact that you keep trying to compare a gas to a liquid which have completely different physical properties anyway.