How does a marine centrifugal pump work?

The centrifugal pump is the workhorse of the ship: seawater and freshwater cooling, ballast, bilge, fire main, general service and much tanker cargo work are all done by centrifugal machines because they are simple, compact, smooth and tolerant. The working principle takes one paragraph. Liquid enters axially at the centre, the eye, of a rotating impeller. The spinning vanes accelerate it outward, so it leaves the impeller rim at high velocity. The surrounding casing, either a spiral volute of increasing area or a ring of diffuser vanes, then slows the flow down, and that reduction in velocity converts the liquid's kinetic energy into pressure. The liquid leaves the discharge branch at a pressure set mainly by impeller diameter and speed.

Several practical characteristics follow from the principle, and they are what an engineer actually uses:

• It cannot pump air. The impeller must be flooded, so a pump above its suction level must be primed by an ejector or vacuum pump, a flooded suction, or a foot valve holding the line full.
• Head and flow trade off along the pump curve. Throttling the discharge valve moves the operating point to lower flow; this is the normal way to regulate a centrifugal pump, and starting against a nearly shut discharge reduces the starting load on the motor.
• It can run briefly against a closed discharge without dangerous overpressure, unlike a positive-displacement pump, but the trapped liquid churns and heats, so minutes only, never hours.
• Flow varies with speed, head with speed squared and power with speed cubed, which is why variable-speed drives save so much energy on cooling pumps.

Mechanically the pump is a shaft, an impeller with wear rings that control internal leakage, a mechanical seal or packed gland where the shaft leaves the casing, and bearings. Routine attention means checking seal leakage, unusual noise or vibration (a crackling gravel sound means cavitation), bearing temperature, and discharge pressure against the known good figure for that duty. Falling capacity usually means worn wear rings, a fouled impeller or air ingress on the suction side. Understand the curve, keep it primed and vented, and a centrifugal pump will run for years between overhauls.