Pumps, Motors, and Hydraulics
Pumps, Motors, and Hydraulics tests whether you can match a pump to a system, calculate head and power, and recognize cavitation and inefficiency. Expect questions on pump curves, system curves, NPSH, affinity laws, series and parallel operation, and motor protection.
Core rules and concepts
- The operating point is where the pump curve crosses the system curve. Changing valve position or demand moves the system curve.
- Total head includes static lift, friction loss, and velocity head. Pressure is converted to head using 1 psi = 2.31 ft of water.
- Water horsepower = (gpm × head in ft) ÷ 3,960. Brake horsepower = water horsepower ÷ pump efficiency.
- NPSH available must exceed NPSH required. Low NPSH causes cavitation: noise, vibration, pitting, and reduced flow.
- Affinity laws: flow varies with speed, head with speed squared, power with speed cubed.
- Pumps in series add head; pumps in parallel add flow at the same head. Motors need correct voltage, overload protection, and ventilation.
Worked example
A pump delivers 500 gpm at 120 ft of head with 75% efficiency. Water horsepower = (500 × 120) ÷ 3,960 = 15.15 hp. Brake horsepower = 15.15 ÷ 0.75 = 20.2 hp. The motor must be sized above the brake horsepower with a reasonable service factor, and the calculation uses total head, not just the pressure gauge reading.
Traps
Using pressure instead of head; sizing a motor exactly at brake horsepower; confusing NPSH available with required; running a pump far off its curve; assuming parallel pumps double head; ignoring affinity-law effects when changing speed.