Pumps, Motors, and Mechanical Equipment
Pumps and their behavior
Know the main pump types: centrifugal (most common for water; flow varies with head), positive displacement (constant flow regardless of pressure, used for chemical feed), submersible, and vertical turbine. Understand the pump curve and system curve: the operating point is where they intersect. Learn NPSH (net positive suction head) and cavitation - the formation and collapse of vapor bubbles that causes noise, vibration, and impeller damage. Affinity laws describe how flow, head, and power change with impeller speed or diameter.
Concepts to master
- Motor basics: horsepower, efficiency, power factor, and the difference between service factor and rated load.
- Variable frequency drives (VFDs) for flow control.
- Seals, bearings, and lubrication; vibration and temperature as failure indicators.
- Valves: check valves, gate valves, and air-release valves.
- Preventive maintenance schedules and recordkeeping.
Traps
A frequent trap: assuming a centrifugal pump's flow is constant; it varies with system head. Another: confusing cavitation (suction-side vapor bubbles) with air entrainment. Also watch for questions implying a VFD changes the pump curve; it changes the operating speed, shifting the curve.
Rule of thumb
If a pump is noisy, vibrating, and losing capacity with a low suction head, suspect cavitation - check suction conditions and NPSH margin before replacing the impeller.