Pumps, Valves, Instrumentation, and Maintenance
Moving water and trusting the numbers
Know the centrifugal pump: flow follows the pump curve, and the actual operating point is where the pump curve crosses the system curve. Throttling the discharge moves the operating point; throttling the suction invites cavitation - the gravelly noise, vibration, and lost capacity that occur when NPSH available drops below NPSH required. Learn the affinity laws: flow varies with speed, head with speed squared, power with speed cubed. Recognize positive-displacement pumps (metering, diaphragm, progressive cavity) for chemical feed, along with suction lift, priming, packing versus mechanical seals, and standby units.
Valves: gate (isolation, not throttling), globe (throttling), butterfly (compact isolation/throttling), plug and ball, check (prevents backflow and water hammer), air/vacuum release, pressure-reducing and altitude valves.
Instrumentation: magnetic, venturi/orifice, and propeller flow meters; bubbler, ultrasonic, float, and pressure-transducer level sensors; nephelometric turbidimeters; pH meters calibrated with fresh pH 4, 7, and 10 buffers; and chlorine or fluoride analyzers verified against grab samples.
Preventive maintenance is examinable: lubrication schedules, exercising valves, run-hour meters, vibration and seal-leak checks, spare parts inventory, and documented work orders.
Worked example: a pump at 1,750 rpm delivers 500 gpm at 100 ft of head. At 1,400 rpm the speed ratio is 0.8, so flow = 500 × 0.8 = 400 gpm, head = 100 × 0.8² = 64 ft, and power falls to 0.8³ = 51% of the original.
Traps: closing a suction valve to control flow; expecting a centrifugal pump to gain head by suction throttling; calibrating a pH meter against a single buffer.