Water Supplies and Fire Pumps
Understand what the water supply can actually deliver
Water-supply questions hinge on distinguishing static pressure, residual pressure, and flow. A hydrant flow test establishes available supply; results are compared with system demand, including elevation and hose-stream allowances. Fire-pump concepts include rated flow, rated head, churn pressure, suction conditions, controller operation, and the 150 percent overload point. NFPA 20 governs pump installation; NFPA 25 covers periodic pump testing. Know that a pump takes suction from the supply and adds pressure; it cannot create flow from a dry source. Watch for jockey-pump cycling, closed suction valves, cavitation, and incorrect gauge calibration.
Traps
Calling static pressure 'available flow'; ignoring residual pressure; mixing up churn and shutoff head; using a pump curve without correcting for suction; assuming automatic transfer switch operation was proven by a manual run; and forgetting that a flow test can be affected by other demands.
Worked example / rule of thumb
A pump is rated 1,000 gpm at 100 psi. At 150 percent of rated flow, or 1,500 gpm, the net pump pressure should generally be at least 65 percent of rated head, about 65 psi, subject to the adopted standard and AHJ. Rule: compare pressure and flow together. A high static reading that collapses under flow points to a supply or piping problem, not a stronger pump.