The 20 PSI limit is actually based on the federal Clean Water Act and some states have also adopted as law or regulation. I know NJ has it as regulation.
Dave Herbert Retired from NJ Fire guy in the Sunshine State Guest Sent from my iPhone > On May 14, 2014, at 9:53, "Mark A. Sornsin, P.E." <[email protected]> > wrote: > > Craig, it has been my understanding that the 20psi number represents the > lowest residual pressure fire departments 'generally' want to see when > drawing from a municipal supply - to avoid the potential for collapsing mains > at some higher elevation than where the water is being drawn. There are > exceptions such as in flat areas like Fargo where they can safely drop below > 20 psi in most circumstances. So the flow at 20psi generally represents the > maximum flow rate they can safely achieve at that point on the system. > > It is a 'fictional' number since in most cases they are using extrapolation > to get to it. On the other hand, it's not all that fictional if your test > from which the extrapolation is run produced a 25% pressure drop from the > static. That drop helps define an accurate 'slope' on the N^1.85 graph. > Determining the flow at 20 psi does not warrant multiple significant digits - > certainly fewer than in sprinkler design (which should use a couple at most). > The extrapolation won't be that far off from reality. > > Mark A. Sornsin, P.E. | Karges-Faulconbridge, Inc. | Fire Protection Engineer > | Fargo, ND | direct: 701.552.9905 | mobile: 701.371.5759 | > http://www.kfiengineers.com > > -----Original Message----- > From: [email protected] [mailto:[email protected]] > Sent: Wednesday, May 14, 2014 8:42 AM > To: Mark A. Sornsin, P.E.; [email protected] > Subject: RE: Can a hydrant flow from the pumper connection be used for the > hydraulic calculations ? > > And there's another point of discussion, how about that extrapolation at 20 > psi. > > What is the value of that number in reality? > > I've seen 20 psi flow rates that we knew without any doubt were unachievable > by the municipal water supply yet when trying to convince an owner that the > water supply would not meet his system demand and the site would require a > pump and tank, they would quickly point to the 20 psi flow rate and say > "there, see we've got lots of water!" It just confuses the uninformed. > > I've also seen the additional data point of how much flow you get when at 0 > psi. I'm still trying to get my head around that one. Maybe someone can > explain it. > > So why put these fictional numbers on a test report? > > > > Craig L. Prahl, CET > Fire Protection Group Lead > CH2MHILL > Lockwood Greene > 1500 International Drive > Spartanburg, SC 29303 > Direct - 864.599.4102 > Fax - 864.599.8439 > CH2MHILL Extension 74102 > [email protected] > > > > -----Original Message----- > From: Sprinklerforum [mailto:[email protected]] > On Behalf Of Mark A. Sornsin, P.E. > Sent: Wednesday, May 14, 2014 9:34 AM > To: [email protected] > Subject: RE: Can a hydrant flow from the pumper connection be used for the > hydraulic calculations ? > > Steve - you seem to be off on a tangent and did not understand the question. > Its o.k. - that happens at your age. > > I agree with Travis. I have done several water flow tests using the 4-1/2" > port. Apply the proper coefficients and life is good. Of course, flowing > that port can have logistical problems due to the large amount of water > and/or damage cause by the discharge - so be careful, plan accordingly, or > find an alternative approach if necessary. > > But he raises another peeve of mine. Further to Ron's comments, why do we > have to reduce residual pressure to the point of being at least 25% lower > than the static? If my sprinkler design anticipates 700 gpm for sprinklers > and hose combines, and I run a flow test that produces 1,000 gpm with only a > 1 psi drop in pressure, why would I attempt to open more ports or hydrants? > > In this example we have a flat curve. We're going to need to flow a huge > amount of water relative to my design demand to achieve the prescribed 25% > drop. To what benefit would we do such a thing? The only reason I can imagine > is "because our contract/AHJ says so" - which a crappy reason, but may be > legit, sadly. > > My personal belief is that 291 requires a certain drop in pressure because > they want better accuracy for when they extrapolate to a residual pressure of > 20 psi. I.e., the flow test is for fire departments to evaluate available > firefighting water for their purposes - not necessarily for sprinkler design > purposes. Will this method give more accurate result for sprinkler design as > well? Yes - but too often it is a level of accuracy not needed (as in my > example above) and is not worth the effort expended. > > > > Mark A. Sornsin, P.E. | Karges-Faulconbridge, Inc. | Fire Protection Engineer > | Fargo, ND | direct: 701.552.9905 | mobile: 701.371.5759 | > http://www.kfiengineers.com > > -----Original Message----- > From: Sprinklerforum [mailto:[email protected]] > On Behalf Of Travis Mack > Sent: Tuesday, May 13, 2014 4:12 PM > To: [email protected] > Subject: Re: Can a hydrant flow from the pumper connection be used for the > hydraulic calculations ? > > Are you asking if you can do a flow test from the pumper connection? If so, > then sure you can. You just need to modify the coefficient based on your > pitot. > > How are you inflating the gpm? If you measure 1200 gpm from the 2½" and you > measure 2000 gpm from the 4.5", how is that inflating flow? You should > typically have a lower residual pressure when flowing the pumper connection > than the 2½" connection. > > If you are looking to 291 for direction, are you getting the required 25% > drop in static and residual from your 2½" flow? If not, what do you do? You > are required to open up additional hydrants to get the flow. > So, to get additional flow, you can also flow from the pumper connection. > > Maybe I am misunderstanding your question. > > Travis Mack, SET > MFP Design, LLC > > _______________________________________________ > Sprinklerforum mailing list > [email protected] > http://lists.firesprinkler.org/listinfo.cgi/sprinklerforum-firesprinkler.org > > ______________________________________________________________________ > This email has been scanned by the Symantec Email Security.cloud service. > For more information please visit http://www.symanteccloud.com > ______________________________________________________________________ > _______________________________________________ > 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
