I'm going to design a water pipe system onsite fire hydrants for our project site and it requires a booster pump to meet a minimum 20 psi on the pipe system. I'm using waterCAD for analyzing the pipe system. Everytime I run it, it gives me a warning that -20 psi gage pressure on the suction side of the pump and pipe junction upstream or on the side of the pump. The same junction has a positive available pressure, although not enough, when the system is analyzed without the pump. Is the -20 psi gage pressure theoretically caused by the impeller? Is the positive pressure analyzed without the pump (which is gage pressure minus pressure loss due to friction) what I should account for the Net Positive Suction Head Available NPSH (A)? Or Is it the -20 psi gage pressure? Thanks in advance.
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