Pumps, Compressors, and Auxiliary Equipment
What this domain tests
Pumps, Compressors, and Auxiliary Equipment checks that you can match a pump type to a duty, explain cavitation, apply the affinity laws, and describe the auxiliary machines that keep a plant running: deaerators, fans, sootblowers, air compressors, and their dryers and receivers.
Core rules
- Centrifugal pumps: flow varies with head; control by discharge throttling, bypass, or variable speed. Positive displacement pumps deliver nearly constant flow and must have a relief valve on the discharge.
- Cavitation: vapour bubbles form and collapse, giving noise, vibration, and pitting. Prevent it with adequate NPSH available above NPSH required, a short large suction line, cooler water, and throttling the discharge, never the suction.
- Affinity laws at constant impeller diameter: flow varies with speed, head with speed squared, power with speed cubed.
- Compressors: reciprocating units use staged cylinders and intercoolers; rotary screw units are oil-flooded; both need an aftercooler, receiver, dryer, and trap.
- Head and pressure: psi = ft of head / 2.31, so 100 ft of water is about 43.3 psi.
Worked example
A pump delivers 200 gpm at 80 ft of head drawing 6 bhp at 1750 rpm. The drive is changed to 3500 rpm. By the affinity laws: flow = 200 x 2 = 400 gpm; head = 80 x 4 = 320 ft; power = 6 x 8 = 48 bhp. The motor must be upsized accordingly, and the higher head may push the duty point off the curve, so the actual result must be checked against the manufacturer's data.
Traps that cost marks
- Throttling the suction to reduce flow, which causes cavitation.
- Dead-heading a positive displacement pump without a relief valve.
- Ignoring NPSH when pumping hot deaerator water.
- Mixing head in feet with pressure in psi.