Pumps, Motors and Mechanical Equipment
Pump behaviour
A centrifugal pump moves water by converting rotational energy into velocity and then pressure. It operates only where its pump curve crosses the system curve, so changing a valve, pipe diameter or wet-well level moves the operating point. For a speed change, the affinity laws apply: flow varies with speed, head with speed squared, and power with speed cubed. Positive-displacement pumps behave differently: flow is nearly constant regardless of pressure, so they need a relief valve and must never be run against a closed discharge.
Cavitation and NPSH
Cavitation occurs when pressure at the impeller eye falls below the liquid’s vapour pressure. Signs include rattling noise, vibration, reduced flow and pitting. Available NPSH must exceed required NPSH with a margin. Other causes of poor performance include air leaks on the suction side, a clogged impeller, wrong rotation and a worn wear ring.
Motors and mechanical components
Check the nameplate for voltage, full-load amps, phase, service factor and insulation class. Confirm rotation before connecting the coupling. Maintain alignment, lubrication and belt tension; watch vibration, temperature and seal leakage. In lift stations, floats or level transducers control lead/lag duty and alternation; check valves and air release valves protect the system.
Traps and a worked example
Common traps: mixing up affinity-law exponents, forgetting specific gravity, confusing head with pressure, and assuming a larger motor fixes a cavitation problem.
Worked example: brake power = (gpm × head in ft × SG) / (3960 × efficiency). For 500 gpm at 40 ft, SG 1.0 and 70% efficiency: (500 × 40) / (3960 × 0.70) = 20,000 / 2,772 = 7.2 hp. At 50% efficiency the same duty needs 10.1 hp, showing why efficiency loss raises energy use.