Pumps, Motors, and Hydraulics
The pump only knows its curve
A centrifugal pump operates where its pump curve crosses the system curve. Change either one - a throttled valve, a fouled main, a new subdivision, a worn impeller - and the operating point moves. Questions favor that idea, plus the affinity laws (flow varies with speed, head with speed squared, power with speed cubed) and series versus parallel: pumps in series add head at the same flow, pumps in parallel add flow at the same head.
Own these terms: static head, friction head, total dynamic head, shutoff head, best efficiency point, NPSH available versus required, cavitation (noise, vibration, pitting, falling flow), and wire-to-water efficiency. Motors are matched to brake horsepower with a service factor; check nameplate voltage, phase, full-load amps, and rotation before startup. Variable-frequency drives change speed and therefore slide the operating point along the system curve.
Traps: using suction pressure as total head; forgetting that throttling a discharge valve changes the system curve, not the pump curve; assuming a bigger motor fixes cavitation; and mixing head (feet) with pressure (psi).
Rule of thumb: brake horsepower = (gpm × head in feet × specific gravity) ÷ (3,960 × efficiency). For 500 gpm at 100 ft and 70% efficiency: (500 × 100) ÷ (3,960 × 0.70) = 50,000 ÷ 2,772 ≈ 18.0 bhp. A 20 hp motor with a 1.15 service factor is a reasonable match; a 15 hp motor is not.