Pumps, Compressors, Valves, and Fluid Handling
What the exam tests
This domain tests pump and compressor types, affinity laws, NPSH and cavitation, system curves, valve selection, steam traps, water hammer, and safe operating practice. The exam favors applied reasoning over catalog recall.
Core rules
- Centrifugal pump flow varies with speed; head varies with speed squared; power varies with speed cubed, for geometrically similar conditions.
- NPSH available must exceed NPSH required. If it does not, cavitation causes noise, vibration, and impeller damage.
- Gate valves isolate, globe valves throttle, check valves prevent backflow, and relief valves protect equipment. Do not use a gate valve as a throttling control valve.
- Positive-displacement pumps must not run against a closed discharge; centrifugal pumps may start at shutoff but only briefly.
Worked example
A centrifugal pump delivers 100 gpm at 80 ft head and 10 hp at 1,800 rpm. Speed is reduced to 1,200 rpm, ratio 0.667. New flow = 100 x 0.667 = 66.7 gpm. New head = 80 x 0.667 squared = 35.6 ft. New power = 10 x 0.667 cubed = 2.96 hp. The reasoning: affinity laws scale flow, head, and power by different exponents, so power falls fastest.
Traps
Do not confuse NPSH available with NPSH required; do not throttle a positive-displacement pump on suction; do not assume a check valve is a control valve; and do not ignore water hammer caused by fast valve closure.