Pumps, Compressors, and Auxiliary Equipment: Selection, Curves, and Operation
What this domain tests
This domain tests centrifugal and positive-displacement pumps, air and gas compressors, fans, and the auxiliaries that support them. Questions cover pump curves versus system curves, NPSH and cavitation, affinity laws, compressor types and controls, and the operating limits of each machine.
Core rules and concepts
- Centrifugal pumps move liquid by centrifugal force; the operating point is where the pump curve meets the system curve. Throttling moves the system curve, not the pump curve.
- NPSH: available NPSH must exceed required NPSH to avoid cavitation. NPSHa depends on suction pressure, liquid vapour pressure, and suction losses.
- Affinity laws for a fixed impeller: flow varies with speed, head with speed squared, power with speed cubed.
- Positive-displacement pumps deliver essentially constant flow against varying pressure and must have a relief valve on the discharge.
- Compressors: reciprocating for high pressure and low flow, rotary screw for continuous medium flow, centrifugal for large flows. Screw compressors use slide valves or speed control for capacity; reciprocating units use unloading or clearance pockets.
- Fans follow the same affinity laws as pumps; system effect and damper position change the operating point.
Worked example
A centrifugal pump delivers 100 m3/h at 40 m head and 15 kW at 1450 rpm. What happens if speed is raised to 1740 rpm?
Reasoning: speed ratio = 1740/1450 = 1.2. Flow = 100 x 1.2 = 120 m3/h. Head = 40 x 1.2 squared = 57.6 m. Power = 15 x 1.2 cubed = 25.9 kW. The motor, originally sized near 15 kW, is now overloaded; this is why speed increases require checking motor rating and NPSH margin, since NPSHr rises with flow.
Traps that cost marks
- Applying affinity laws across a change in impeller diameter without correcting for the small-diameter efficiency drop.
- Assuming throttling a centrifugal pump reduces power proportionally; it does not, because power follows the operating point.
- Omitting a relief valve on a positive-displacement pump discharge.
- Confusing NPSHa (system) with NPSHr (pump); cavitation occurs when NPSHa falls below NPSHr.