Pumps, Motors, and Pumping Station Operation
Core concepts
Know the pump curve and the system curve and that the operating point is their intersection. Understand centrifugal pump behavior: flow varies inversely with head, efficiency peaks at the best efficiency point (BEP), and running far from BEP causes cavitation or vibration. Grasp NPSH - net positive suction head available must exceed NPSH required to avoid cavitation. Understand affinity laws: flow ∝ speed, head ∝ speed², power ∝ speed³. For motors, know horsepower, efficiency, and power factor relationships and why a motor is sized above the pump's brake horsepower. Understand pump station operation: variable-frequency drives, cycling, wet-well level control, and the difference between constant-speed and variable-speed control.
Traps to watch
Affinity-law questions are common - candidates forget power changes with the cube of speed. Another trap: confusing brake horsepower (at the pump shaft) with motor input power (higher, because of motor efficiency). Also watch NPSH questions that mix up available vs. required. Cavitation symptoms (noise, vibration, pitting, reduced flow) are frequently tested.
Worked example
A pump delivers 500 gpm at 100 ft head drawing 20 hp. Double the speed: new flow ≈ 1000 gpm, new head ≈ 100 × 2² = 400 ft, new power ≈ 20 × 2³ = 160 hp. The motor would need substantial upsizing - a classic trap answer.
Rule of thumb
Affinity laws: flow ×N, head ×N², power ×N³. Always check NPSH available > NPSH required with margin.