Pumps, Compressors, and Auxiliary Equipment
Pumps, Compressors, and Auxiliary Equipment
This domain tests centrifugal and positive-displacement pumps, compressors, receivers, traps, filters, and the operating limits that protect them. Expect NPSH, cavitation, affinity laws, intercooling, and safety-valve questions.
Core rules
- Centrifugal pumps do not self-prime; trapped air causes loss of suction.
- NPSH available must exceed NPSH required to avoid cavitation.
- Throttling a centrifugal pump discharge reduces flow; throttling suction risks cavitation.
- Positive-displacement pumps and compressors need a relief device on the discharge.
- Compressor intercooling reduces power and moisture; receivers store air and damp pulsation.
Worked example
A centrifugal pump runs at 1,750 rpm and delivers 200 gpm at 100 ft head. If speed rises to 3,500 rpm, affinity laws give flow proportional to speed: 200 x (3,500/1,750) = 400 gpm. Head varies as speed squared: 100 x 2^2 = 400 ft. Power varies as speed cubed: 8 times, subject to efficiency. Reason: doubling speed doubles flow but greatly increases head and power demand.
Traps
Do not run a centrifugal pump dry; do not use suction throttling to control flow; do not omit a relief valve on a positive-displacement compressor; and do not assume a lower discharge pressure always means lower power-check the curve and system.