AnsweredAssumed Answered

Flow distribution in a combined system and PRV

Question asked by najafafzal on Sep 7, 2020
Latest reply on Sep 15, 2020 by najafafzal

A question that has always flummoxed me is the use of Pressure reducing valves before individual zone control valve assemblies below 12.1 bar (175psi). I am going to quote an arbitrary example below so that my peers reading this can understand easily.
Let's say the sprinklers at the ZCV in the basement of a combined system office building with a single riser requires 350 gpm @ 70 psi but the maximum demand from the system is 500 gpm @ 165 psi. The pump room is on the same floor as the basement and the line losses are negligible. That would mean that if there is no pressure reducing valve in the zone control valve assembly, the sprinklers would discharge around 537 gpm (assuming a constant k factor at the ZCV) if the pump could discharge more than 500 gpm. That means that the system is over discharging than the required/calculated amount. Is that okay? 
This leads me to another question. NFPA suggests that if the building is fully sprinklered in a combined system, the higher demand between the sprinkler system or the standpipe system would be used. But if a landing valve is opened at the same floor, the flow would obviously be divided. How would the flow be divided then? The sprinklers won't get the required demand or if they do get that, the landing valve won't get 250 gpm. 
Would really appreciate if anyone could help me with this problem that I am stuck with.
Thank you! 

Outcomes