NFPA 20 limits the suction pressure from a suction tank to -3 psi @ 150% pump capacity. With your 7.5 psi friction loss at that flow, you would need 4.5 psi from the tank which correlates to 10.4 ft of water (p=0.433h). This effectively makes the bottom of your tank at 10.4 ft and only the water stored above that level can be used for determining the capacity of the tank.
There are also different sizing requirements for break tanks based on a minimum capacity of 150% pump rated flow @ 15 minutes and separate refill rate requirements based on if the tank supplies over/under a 30 min sprinkler demand. This will be discussed on Monday’s Tech Update, so be on the lookout. On Fri, Dec 4, 2020 at 4:57 PM Steven Jenkins via Sprinklerforum < [email protected]> wrote: > We have an existing horizontal split case fire pump and ground level water > storage tank. The storage in the protected building is changing, and the > amount of water needed for fire protection is going up. > We are changing the tank to a “break tank”, but even then there is not > much water left in the tank after the required supply duration, only a few > feet left in the bottom of the tank. > The pump is right next to the tank, so not much loss due to friction in > the connecting pipe. > With the tank full, before the pump is turned on, the gauge on the suction > side reads about 15 psi, due to the height of water in the tank. When the > pump is running at 150%, this drops to about 7.5 psi. > At the end of the required duration I will only have about 1.5 psi of head > from the height of water in the tank. So if I lose another 7.5 with the > pump on, the gauge would read -6 psi. > Is this a problem for compliance with 2019 NFPA 20, or since the pump is > already running does this make the negative 6 psi gauge reading OK? > 2019 NFPA 20 4.16.3.2 > Thanks, > J > _______________________________________________ > Sprinklerforum mailing list > [email protected] > > http://lists.firesprinkler.org/listinfo.cgi/sprinklerforum-firesprinkler.org > -- Kevin Hall, P.E., MSFPE Coordinator, Engineering and Technical Services American Fire Sprinkler Association _______________________________________________ Sprinklerforum mailing list [email protected] http://lists.firesprinkler.org/listinfo.cgi/sprinklerforum-firesprinkler.org
