I understand the purpose of the 20 psi value. I have seen 20 psi values that in no way would flow. 4000 gpm+ though an 8" water main, or a flow value not achievable by the municipal water supply. My point is there is a danger in posting non-verified values in a flow report. If you want to know what's available at 20 psi then flow the system until it bottoms out at 20 psi then you have factual data. Otherwise when it's needed most that fictional 4000 gpm may only actually be half or less. In the midst of a fire fight, is not the time to find out reality and extrapolation are not the same. Now I realize that most project do not require large flow rates, so it's typically not a big deal. In the industrial world, it's a big deal.
We had a large project, water department did a flow test on a single hydrant butt with less than 1000 gpm flowing, extrapolated the curve out and modeled the water supply to the site which indicated we would have sufficient flow and pressures for our system. When the final pump test was performed the flow rate was not there, not even close, the water department had assured the owner of a "robust" water supply. In actuality, they had "a lot of water" but their modeling was flawed and their test inconclusive due to extrapolation and not actual verification. The owner had to add an above ground suction tank which delayed occupancy of the facility. Facts are facts and extrapolations are nothing but a good guess in many cases. 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 9:53 AM To: [email protected] Subject: RE: Can a hydrant flow from the pumper connection be used for the hydraulic calculations ? 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. 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