Occasionally one does get a much higher residual pressure in the flatlands but that's due to additional pumps coming on line as flagged by Cecil.
The methodology identified by NFPA is for a very simple system and has not yet evolved to address the part of the world where simple does not apply. I believe the discharge Pv (as read by your pitot gauge) is the same as the Pn as read at the other outlet minus losses imposed by squirting water through an orifice. The Pn is acting on the walls of the pipe (as well as the orifice of the sprinkler or hydrant) which is perpendicular to the flow of the water (so that doesn’t belong in your definition of Pv). Think of Pv as the energy tied up by the actual movement of the water. The energy tied in by the direction of flow can not push water out through an orifice attached to the wall of the pipe. One last thought, except for tanks feeding ONLY your system, static does not mean ZERO flow. Your static reading is actual a residual reading that reflects the OTHER demands on the system. Your residential reading is just the additional demand YOU put on the system. Roland Roland Huggins, PE - Senior VP Engineering American Fire Sprinkler Assn. Dallas, TX http://www.firesprinkler.org <http://www.firesprinkler.org/> Fire Sprinklers Saves Lives > On Jan 2, 2019, at 11:19 AM, Skyler Bilbo <[email protected]> wrote: > > 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] > <mailto:[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 > > > > > > _______________________________________________ > Sprinklerforum mailing list > [email protected] > http://lists.firesprinkler.org/listinfo.cgi/sprinklerforum-firesprinkler.org
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