Pumps, Motors, Instrumentation, and Process Control
Core concepts
Centrifugal pumps move water; positive-displacement pumps (progressive cavity, diaphragm, peristaltic) suit sludge and chemical feed. Know the pump curve, system curve, operating point, NPSH, cavitation (noise, vibration, pitting, flow loss), and affinity laws: flow varies with speed, head with speed squared, power with speed cubed.
Motors: horsepower, service factor, full-load amps, insulation class, VFD operation, and the difference between running overload and starting current. Know that a VFD reduces energy at part load per affinity laws.
Instrumentation: level (ultrasonic, pressure, float), flow (magnetic, ultrasonic, weir), pH and ORP (requires calibration and temperature compensation), DO probes, and turbidity meters. Control modes: manual, on/off, PID (proportional, integral, derivative). Integral eliminates offset; too much causes oscillation; derivative anticipates. SCADA and alarms should be tested.
Traps
Questions confuse power affinity (cubed) with head (squared). Another trap: a pump running but not delivering is often a closed valve, air lock, or worn impeller - not always a motor issue. Don't confuse pH probe drift with true process change; check calibration and reference electrode. Watch for PID tuning questions where the answer is "reduce integral" for cycling.
Worked example / rule of thumb
A pump at 1,750 rpm draws 40 hp. At 1,400 rpm, power ≈ 40 × (1400/1750)³ ≈ 40 × 0.512 ≈ 20.5 hp. Rule of thumb: a small speed reduction yields a large energy saving, which is why VFDs dominate part-load pumping.