Pumps, Motors, and Mechanical Equipment
Pump and equipment items test pump-curve literacy, cavitation diagnosis, motor and drive basics, and the arithmetic behind horsepower and the affinity laws.
Pump fundamentals
A centrifugal pump runs where its pump curve crosses the system curve. Shutoff head is at zero flow, runout at maximum flow, and the best efficiency point sits between them. Positive-displacement pumps (diaphragm, piston) suit chemical feed and need a relief valve on the discharge.
Affinity laws: flow varies with speed, head with speed squared, power with speed cubed. Halving speed halves flow, quarters head, and cuts power to one-eighth - the basis of VFD energy savings.
Cavitation occurs when available NPSH falls below required NPSH. Symptoms are noise, vibration, and impeller pitting; causes include excessive suction lift, a clogged strainer, a throttled suction valve, and warm water.
Motors: check voltage imbalance, insulation resistance, alignment, lubrication, and seal or packing condition. VFDs bring harmonics, minimum-speed limits, and bearing-current concerns.
Worked example
A pump delivers 500 gpm at 100 ft of head with 70 percent wire-to-water efficiency. Water horsepower = Q × H ÷ 3,960 = 500 × 100 ÷ 3,960 = 12.6 hp. Brake horsepower = 12.6 ÷ 0.70 = 18.0 hp, so a 20 hp motor is appropriate.
Traps
Multiplying by efficiency instead of dividing. Applying the head law where flow is asked. Ignoring static head when predicting what a speed change achieves. Calling a noisy pump "fine" while it cavitates and destroys its impeller.