Pumps, Compressors, and Rotating Equipment
Core concepts
Expect questions on centrifugal and positive-displacement pumps, their performance curves, and how they interact with system curves. Understand NPSH available versus NPSH required and the causes of cavitation. Know affinity laws and how flow, head, and power change with speed and impeller diameter. For compressors, distinguish reciprocating, rotary screw, and centrifugal types, and understand surge, stall, intercooling, and compression ratio limits. Rotating equipment topics include shaft alignment, balancing, critical speed, vibration analysis, and condition monitoring through oil analysis and thermography.
Traps to watch
A frequent trap is applying affinity laws incorrectly to positive-displacement machines, where flow is nearly independent of head. Another is confusing NPSH available with NPSH required, or forgetting that NPSH available falls as suction lift or suction temperature rises. Compressor questions often hide surge as a low-flow phenomenon rather than a high-flow one. Watch for pump-in-series versus parallel behavior: series adds head, parallel adds flow.
Worked example
A centrifugal pump runs at 1750 rpm delivering 100 m³/h at 40 m head. Doubling speed to 3500 rpm, by affinity laws, gives flow ×2 = 200 m³/h, head ×4 = 160 m, and power ×8. Rule of thumb: flow scales with speed, head with speed squared, power with speed cubed. Always check that the new operating point still sits on the stable part of the curve and that NPSH available remains adequate.