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the correct gpm and psi to select a pump

the correct gpm and psi to select a pump

(OP)
Hello.

I have some doubts about a design that i'm doing for the university.

I am designing a mall, which has a floor level and four more levels upwards. The design has sprinkler system and hoses.

I used ordinary risk 1, and for the most remote design area i have 225 gpm. (0.15 gpm/ft2 and 1500 ft2).

I have problems with the pressure lost, not in the calculation of piping, fittings etc. i have problems with the interpretation of the nfpa standars, because the nfpa 13 says that i need a minumum of 65 psi of residual pressure in the hose, pressure that i have to guarantee, by the other side, the pressure lost from the last sprinklers to the pump, (adding all the other sprinklers and water volumes of the design area) is 50 psi (just in exaplme).

The thing is that the pressure lost in the most remote demand of water of all the building is smaller that the pressure required for the last hose, so what i did was, with the 50 psi from the sprinklers to the pump, add 65 psi more (and 100gpm more), asuming that the both system are going to be activated at the same time. which giveme something like 325 gpm vs 115 psi (more or less, is an example). is that correct?

the nfpa 13 also says, to add 250 gpm for internal or external hydrant connections. so, what? i just add to my design area water flow 250gpm more and just calculate the pump with 475gpm vs 40psi? i dont understand this part of the design.

thank you

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