Pumps, Compressors, and Mechanical Drive Equipment
Domain 5: Pumps, Compressors, and Mechanical Drive Equipment
This domain tests selection and troubleshooting of centrifugal and positive-displacement pumps, compressors, fans, gear drives, and couplings.
Core rules
- A centrifugal pump operates where its head-capacity curve meets the system curve. Shutoff head is at zero flow; runout is at maximum flow.
- Cavitation occurs when NPSH available falls below NPSH required. It causes noise, vibration, and erosion, and is not cured by throttling suction.
- Affinity laws: flow varies with speed, head with speed squared, power with speed cubed, for the same impeller diameter.
- Positive-displacement pumps and compressors need relief protection; centrifugal compressors can surge at low flow.
- Alignment, lubrication, and baseplate condition determine coupling and bearing life.
Worked example
A pump takes suction from an open tank at 33.9 ft atmospheric head. Suction lift is 10 ft, suction friction loss is 3 ft, and water vapor pressure at the pumping temperature is 1 ft. NPSH available = 33.9 - 10 - 3 - 1 = 19.9 ft. If the pump requires 22 ft NPSH, it will cavitate. Raising the tank, reducing lift, or increasing suction pipe size raises NPSHa.
Traps
Using discharge pressure as total head; ignoring vapor pressure; applying affinity laws across impeller trims incorrectly; and diagnosing cavitation as an air leak.