Principles and Components

A centrifugal pump adds energy to a liquid by accelerating it outward through a rotating impeller, then converting that velocity into pressure in the volute or diffuser. It does not create suction: atmospheric or system pressure pushes liquid into the eye of the impeller, which is why suction conditions determine whether the pump works at all.

Main components

• Impeller — open, semi-open or closed. Closed impellers are most efficient; open impellers tolerate solids better.
• Casing — volute or diffuser type, converting velocity to pressure.
• Shaft and bearings — carrying radial and axial loads.
• Mechanical seal or packing — containing the process fluid at the shaft penetration.
• Wear rings — a sacrificial close clearance between impeller and casing, limiting internal recirculation.

Head, not pressure

A centrifugal pump develops a given head — a height of liquid column — regardless of fluid density. The same pump at the same speed produces the same head in water and in a light hydrocarbon, but the resulting pressure differs because pressure equals head multiplied by density. This is why a pump tested on water can behave unexpectedly when it sees the actual product, and why specific gravity must always be stated.

Affinity laws

For a change in speed, flow varies directly with speed, head with the square of speed, and absorbed power with the cube. A ten percent speed reduction therefore cuts power by roughly a quarter, which is the basis for variable speed drives being so effective on pumps.