Pumps, Motors, Meters, and Instrumentation
The operating point rules everything
A centrifugal pump runs where its pump curve crosses the system curve. Throttle or add head and flow moves left on the curve; open up and flow moves right, drawing more power and risking cavitation. The affinity laws scale a known curve: flow varies with speed, head with speed squared, and power with speed cubed - the reason a VFD saves so much energy.
Cavitation (sounds like gravel, flow and pressure drop, impeller pitting) occurs when available NPSH falls below required NPSH. Fixes include lowering the pump, raising suction level, reducing suction lift or losses, and cooling the water. Pumps in series add head; in parallel add flow.
Water horsepower: WHP = (gpm × head in feet) ÷ 3,960. Brake horsepower = WHP ÷ pump efficiency; motor size also accounts for motor efficiency and service factor. Electrical knowledge expected: voltage, current, power factor, three-phase power, overloads, and proper rotation.
Meters: positive displacement, venturi, turbine, electromagnetic, ultrasonic, and propeller types. Magnetic meters need conductive water; venturis need straight pipe runs. Accuracy is stated as a percentage of actual or full flow, and meters should be tested and calibrated against a known volume. Instrumentation uses 4–20 mA loops (4 mA = zero, 20 mA = span; below 4 mA signals a fault), plus turbidimeters, pH probes, and chlorine analyzers that must be calibrated and cleaned on schedule. Always verify a reading with a grab sample before acting.
Traps
- Doubling speed to double flow - power actually rises eightfold.
- Confusing NPSH available with NPSH required.
- Trusting an uncalibrated online analyzer over a bench test.
Worked example
1,000 gpm against 100 ft of head: WHP = (1,000 × 100) ÷ 3,960 ≈ 25 hp. At 70% pump efficiency, brake hp ≈ 25 ÷ 0.70 ≈ 36 hp, so select at least a 40 hp motor.