Roland, I'm back to being confused about this. I know the definition I gave for velocity pressure, Pv, was exactly wrong, and I should have used the word parallel instead of perpendicular, but I do understand the concept.
The more that I think about it, the more that I think I was right originally. Let's use the real test I was given: Static = 37 PSI, Residual = 29 PSI and the Pitot reading was 34 PSI, giving a flow of 980 GPM, which I will round to 1,000 GPM. The residual pressure was 29 PSI, and I believe this is Pn (please correct me if I'm wrong). The information wasn't given, but I believe it was on an 8" main, so there would be 0.24 PSI of velocity pressure down at the hydrant tee where the water was flowing by the test hydrant towards the flow hydrant. If we ignore pressure loss between the hydrants for a minute, this means the total pressure, Ptot, at/between them is 29.24 PSI. There is no way for Pv at the flow hydrant (which is our pitot reading) to exceed this total pressure of 29.24 PSI. This town is very flat, has one water tower, and one supply, and I'm pretty confident that the supply did not affect the results. They don't have any large users, and this test was about a block or two away from the water tower. I don't know all of the main sizing in the town, but I suspect that there are bottlenecks and I still suspect that they need to try the test backwards. Can anyone confirm that if your pitot reading is higher than your residual pressure, you should try to run the test backwards (reverse the flow and test hydrant)? Am I thinking about this correctly yet? The last time I had this situation, the flow test was much better, to the point it didn't really matter. This time, because of the relatively low pressures, it's going to make a real difference. I need every psi/gpm that I can get, but I don't want to use an incorrect test to get it. "Everything Should Be Made as Simple as Possible, But Not Simpler" - Einstein... Where is he when you need him? Thanks, Skyler Bilbo On Wed, Jan 2, 2019 at 7:04 PM Bruce Verhei <[email protected]> wrote: > Per Roland’s comment, I always tried to get new people to use ‘background’ > or ‘normal consumption’ flow. > > And to kill mythology that around 0700 or 1800-2000 being highest normal > flows. Highest background flow, by far, was 0100-0300, May-September. > Commercial landscape irrigation is biggest flows, at least in Puget Sound. > > Best. > > Bruce Verhei > > On Jan 2, 2019, at 16:30, Roland Huggins <[email protected]> > wrote: > > 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 > > > 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]> > 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 > > > _______________________________________________ > Sprinklerforum mailing list > [email protected] > > http://lists.firesprinkler.org/listinfo.cgi/sprinklerforum-firesprinkler.org > > _______________________________________________ > Sprinklerforum mailing list > [email protected] > > http://lists.firesprinkler.org/listinfo.cgi/sprinklerforum-firesprinkler.org >
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