NFPA 20 Fire Pumps and Water Supply
Pump performance
NFPA 20 fire pumps are rated at a flow and head, such as 1,000 gpm at 100 psi. The pump must deliver at least 150% of rated flow at not less than 65% of rated head, and shutoff head must not exceed 140% of rated head. Electric motors are sized for the pump’s maximum brake horsepower, including the 150% flow point. Controllers, relief valves, and suction conditions are part of the listed assembly.
Water supply
A flow test gives static, residual, and pitot pressure. Use Q^1.85 to estimate available pressure at other flows. The pump must be selected so its curve stays above the system demand curve with a margin, and the suction supply must not go negative.
Worked example
A flow test shows 80 psi static and 60 psi residual at 1,000 gpm. At 1,500 gpm, friction loss rises by (1,500/1,000)^1.85 = 2.12. The 20 psi drop becomes 42.4 psi, so estimated residual = 80 − 42.4 = 37.6 psi. A pump demanded at 1,500 gpm must have its required pressure below 37.6 psi after accounting for suction and elevation; otherwise the supply is inadequate.
Traps
Using rated flow instead of 150% flow; confusing churn with shutoff; sizing a motor only at rated BHP; ignoring suction-side friction; and treating a pump as a replacement for an inadequate water main.