Micah: The best way to stabilize the pump discharge pressure and give yourself a (nearly) straight line suction pressure curve is to arrange the required secondary water supply as a break tank. Fill the tank with city water (recommend to manifold at least one automatic plus one manual fill; in my neck of the woods we use two automatic and one manual) and base your calculations on the lowest potential water level in the tank. We simply use the pump’s performance curve as a water supply assuming 0 psi suction head. Depending on water column height, your actual discharge pressure variation will be ≤ 5 psi, unless the tank is uniquely configured and multi-story.
The foregoing is my opinion only and does not represent NFPA or the NFPA 14 Technical Committee, nor intended to serve as an interpretation of the standard. Protection Design and Consulting Steve Leyton, President T | 619.255.8964 x 102 | www.protectiondesign.com<http://www.protectiondesign.com/> 2851 Camino Del Rio South | Suite 210 | San Diego, CA 92108 Fire Protection System Design | Consulting | Planning | Training From: Micah Davis <[email protected]> Sent: Monday, September 25, 2023 8:55 AM To: SprinklerForum <[email protected]> Subject: [Sprinklerforum] Standpipe PRV Question Good morning, Forum! I've got a head-scratcher and am interested in the Forum's collective opinion! Important project parameters are as follows. - 42-story high rise with an automatic standpipe - Seismic Design Category - C - IBC 2018 - Fire pump supplied by city water supply - Secondary supply, as required by IBC 403.3.3, is a tank of adequate size - 305 psi @ 1000 gpm pump sized based on the feed from tank - Direct-acting pressure reducing valves on each floor Here is my issue. If I base my PRV settings on city supply, I cannot get 100 psi at the remote valves when calculating tank as supply. If I base my PRV settings on tank supply, I am over 175 psi discharge on the valve when calculating city as supply. If I try to get my settings using the city as static and the tank as residual pressures, there is no PRV setting that works. I have my opinions about what I think should happen here, but I want to see what the Forum thinks. I will not bother the group with what the design team is suggesting I do. Micah Davis Ferguson Fire Design [email protected]<mailto:[email protected]>
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