Steve, This was actually very helpful. I was thinking of it wrong. Our pitots measure velocity pressure. The gauge on the test hydrant is measuring normal pressure inside of the pipe, or hydrant. I think I have it sorted, but feel free to correct me. A better explanation is below.
-The normal pressure is the pressure acting on the walls of the pipe, and is what is typically measured with our regular gauges. -The velocity pressure is the pressure acting on anything that is perpendicular to the direction of flow, like one of our pitot gauges (it would be the pressure you would feel pushing you if you tried to stand in front of a flowing hydrant) -The total pressure is both of these things combined. Velocity pressure goes up as you increase the velocity of the water, which can be accomplished by going from a large pipe to a small one (like going from an 8" water main to a 2-1/2" connection on a fire hydrant; 1,000 GPM in an 8" main travels at about 5.96 ft/sec, which equals a velocity pressure of 0.24 psi; 1000 GPM comes out of a 2-1/2" hydrant at about 65 ft/sec *that's why it shoots out so far* with a velocity pressure of about 28.8 psi, which is a pitot pressure of about 35.5 psi, if the opening coefficient is 0.9). This velocity pressure is dependent on the velocity of the water. I was wrong in my original thinking. Hopefully my explanation is useful to others. I don't think the pitot reading should/could ever be larger than the static pressure, however (assuming elevation is the same, no additional water supplies kick on, and no negative gauge pressure possible), due to conservation of energy. The static pressure is the total pressure when no water is flowing, and no matter how much water is flowing after that, no combination of velocity pressure or normal pressure could ever exceed this total pressure. Thanks guys, Skyler Bilbo On Wed, Jan 2, 2019 at 11:33 AM Steve Leyton <[email protected]> wrote: > Pitot measures velocity pressure, residual is atmospheric pressure. > There’s not a fixed correlation between the two values – I’m guessing that > the main supplying the test hydrants is a very large diameter one? > > > > Steve Leyton > > > > >
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