Wow For a second I thought Brad was back. Sent from my iPhone
> On Jan 2, 2019, at 5:34 PM, å... .... <[email protected]> wrote: > > > > > After combining FM Global's orifice equation[1] (which is a modified form of > Pitot's 1732 equation ) > with the Bernoulli equation (presented in ~ 1738) > (apparently those two guys were doing a lot of studying too, Steve, back > in the day) > one rearranges and solves for P, pitot through substitution of velocity. > > P,pitot = (P1 – P2)* (2/ρ)*( π/{4*a*C})^2 eq (1a) > > While not a perfect reflection of reality (no friction or turbulence > considered), if we assume > P1 = P,static [Pa] > P2 = P,residual [Pa] > C is the hydrant discharge coefficient that FM states varies from 0.6 -> > 0.9 [2] > and the constant term (2/ρ)*( π/{4*a*C})^2 is 1.24 and is dimensionless > found by using a density, ρ, for sweet water, C=0.9 and 'a' = 0.035 > which is converted from FM's units of {bar,L,min} to SI. > > The equation (1a) suggests that, yup, there are conditions where P, pitot > will be larger than P,resdual, even with flat ground. > Those conditions become easier to create when the hydrant discharge > coefficient gets smaller (when there is more non-uniform water discharge) or > when the pressure drop is larger (created either by larger water > discharge or a very small cross-sectional gravity-source of water or a pump > with a really steep performance curve). > The small diameter water source is impractical with money constraints, > and not many listed fire pumps have extremely steep performance-curves. > > Equation (1a) suggests: > a). with an inset hydrant outlet (C = 0.6) and a P,static of 5 bar (75 > psi), the pitot pressure matches the residual pressure IF the residual > pressure drop is about 27%. > b). with a smooth well-rounded hydrant outlet (C=0.9) and the same > P,static, the pitot pressure matches the residual pressure if the residual > pressure drop is 45%. > > I have seldom got anywhere near pressure drops that large, it would take a > large opening or openings. > ICYMI, pipe diameter falls out of the set of independent variables (it > doesn't influence the results)... at least in theory. > NFPA 291 recommends a 25% pressure drop, and AWWA recommends at least 10 psi. > Even at the 25% pressure drop recommended by NFPA 291, theory suggests we > will not see P,pitot closely approach P,residual unless the hydrant discharge > is very uneven. > > The remaining question I have is... "why NFPA 291 does not openly endorse > P,residual and P,pitot at the same hydrant?". It could be another case of, > "we have always done it > that way." Given the previously mentioned seldom fully-accounted for > parameters in water flow testing, it seems that the turbulence issue at the > proposed single-measurement > hydrant would be an issue of minor consequence. Australia tests pitot and > residual pressure at the same hydrant, but maybe that is because their water > curls in the other direction. > > > [1]. FM Global Data Sheet 3-0, pp. 47, equation 1 > [2]. FM Global, op. cit., pp. 52, Table 4, row 3 > > > Scot Deal > Excelsior Risk & Fire Engineering > gms: +420 606 872 129 (GMT + 1) > > >> On Thu, Jan 3, 2019 at 12:15 AM Steve Leyton <[email protected]> >> wrote: > >> I will leave the science part to Scot and Cecil; my college thesis was >> titled, “Our Friend the Beaver”. And that was in architecture school … >> >> >> >> You’re right about the basic theorem, hence my question/assumption about the >> large diameter main feeding the test hydrants. It’s not uncommon to have >> extremely generous flows with low static and residual pressures if the main >> is 12”, 16” and above. Bernoulli indeed … >> >> >> >> SML >> >> >> >> >> >> From: Sprinklerforum [mailto:[email protected]] >> On Behalf Of Skyler Bilbo >> Sent: Wednesday, January 02, 2019 11:20 AM >> To: [email protected] >> Subject: Re: Pitot Pressure Above Residual Pressure >> >> >> >> 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
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