Pumps, Motors, and Mechanical Equipment
Core concepts
Understand pump curves, system curves, and the operating point; the difference between centrifugal and positive-displacement pumps; affinity laws (flow ∝ speed, head ∝ speed², power ∝ speed³); NPSH available versus required and the risk of cavitation; and the relationship among flow, head, efficiency, and power. Know motor basics-nameplate data, service factor, voltage/phase, overload protection, VFDs-and mechanical components such as seals, bearings, couplings, and check valves.
Traps to watch
Affinity-law direction is frequently tested: doubling speed doubles flow but quadruples head and roughly octuples power. Cavitation (from insufficient NPSHa, high temperature, or a restricted suction) causes noise, vibration, and impeller damage-do not confuse it with air binding or water hammer. Remember that a pump operates where the pump curve meets the system curve; throttling a discharge valve moves the operating point up the pump curve (lower flow, higher head). Efficiency peaks at one flow-best efficiency point (BEP)-and running far from BEP stresses equipment.
Worked example / rule of thumb
Power rule of thumb: water horsepower (whp) = (gpm × head in ft) ÷ 3960; brake horsepower = whp ÷ efficiency. Example: 500 gpm at 100 ft = 50,000 ÷ 3960 ≈ 12.6 whp; at 70% efficiency, bhp ≈ 18 hp. Affinity check: raising speed 10% raises flow 10%, head ~21%, power ~33%.
Exam strategy
Compute power with the 3960 constant and always divide by efficiency; check affinity exponents before choosing.