Pumps, Motors, Maintenance, and Instrumentation
Pumps, curves, and the operating point
A centrifugal pump's performance is its curve: head falls as flow rises. The system curve rises with flow. Where they cross is the operating point - the flow the pump actually delivers. Changing the system (throttling a valve, fouling a filter) moves the operating point along the pump curve; changing speed moves the curve itself.
Affinity laws and cavitation
For a fixed impeller: flow ∝ speed, head ∝ speed², power ∝ speed³; trimming the impeller follows similar rules with diameter. NPSH protects against cavitation: if NPSH available falls below NPSH required (high suction lift, warm water, clogged strainer, throttled suction valve), the pump cavitates - noise, vibration, pitting, and lost capacity.
Motors and maintenance
Brake horsepower: BHP = (gpm × head, ft × specific gravity) ÷ (3960 × efficiency). Check nameplate voltage, amperage, service factor, and insulation class. Preventive maintenance means vibration analysis, alignment, lubrication, seal and packing inspection, and infrared scans of electrical connections. Lock out and tag out before work.
Instrumentation
Know 4–20 mA loops (4 mA = zero, 20 mA = span), two-point calibration, and the difference between magnetic meters (need conductive liquid), ultrasonic (need a clean sound path), and differential-pressure meters. Test SCADA alarms; a level sensor that fails high can overflow a wet well while one that fails low can dry-run a pump.
Traps
Affinity-law questions that scale power linearly, and cavitation questions blamed on the discharge side instead of suction.
Worked example
500 gpm at 100 ft, specific gravity 1.0, 70% efficiency: BHP = (500 × 100) ÷ (3960 × 0.70) = 50,000 ÷ 2,772 ≈ 18 hp.