Pumps, Motors, and Equipment Maintenance
Pumps and motors
Centrifugal pumps dominate water supply; positive-displacement pumps suit chemical feed. Know how to read a pump curve (flow vs head, efficiency, BHP) and where the operating point sits - the intersection of the pump curve and system curve. Affinity laws: flow varies with speed, head with speed squared, power with speed cubed. Cavitation (noise, vibration, pitting) comes from insufficient NPSH available; fixes include lowering suction lift, larger suction pipe, or reducing temperature.
Maintenance
Preventive maintenance: vibration analysis, lubrication, seal inspection, alignment, amp/voltage checks, and exercising valves. Record baseline amperage; a rising amp draw can mean wear, blockage, or misalignment. On three-phase motors, check voltage imbalance (should be low) because imbalance causes overheating.
Traps
- Using gpm instead of gpd in horsepower calculations.
- Forgetting specific gravity (water ≈ 1.0).
- Mixing up brake horsepower and motor nameplate horsepower.
- Assuming a VFD changes head and flow linearly.
Worked example
Water horsepower: WHP = Q × H × SG ÷ 3960. For 500 gpm at 100 ft: 500 × 100 ÷ 3960 = 12.6 whp. Brake horsepower = WHP ÷ efficiency; at 70%: 12.6 ÷ 0.70 = 18.0 bhp. Choose a motor at or above that with service factor.
Rule of thumb
If flow doubles by speed, head quadruples and power roughly octuples - a small speed change is a big energy change.