Centrifugal Pump

What defines this type

A centrifugal pump converts the rotational energy of an impeller into velocity, then into pressure as the fluid slows down through the volute casing. Its output flow is not fixed per revolution the way a gear or screw pump's is — flow and head move against each other along a curve, so the same pump delivers more flow at low head and less flow as the system resists it harder. That makes it well suited to seawater cooling, ballast, fire and general service duties where the fluid is thin and flow needs to vary with demand, and poorly suited to fuel oil or lube oil transfer at low temperature, where a positive displacement pump holds flow steady regardless of viscosity. Most centrifugal pumps also cannot self-prime without a special arrangement, since they rely on the fluid already filling the casing to build pressure.

Main components

• Impeller — open, semi-open or closed design, the rotating element that does the actual work on the fluid.
• Volute casing — converts the impeller's velocity into pressure and directs flow to the discharge.
• Shaft seal — a mechanical seal or gland packing keeping fluid from tracking out along the shaft.
• Bearings — support the shaft and absorb radial and axial loads from the impeller.
• Wear rings — close clearances between impeller and casing that limit internal recirculation as they wear.
• Coupling — direct-coupled to the driving motor in most modern installations.

Selection / sizing

Sizing starts from the system curve: required flow in m3/h and head in metres or bar at that flow, checked against the pump's performance curve near its best efficiency point rather than at either extreme. Net positive suction head available on the installation must exceed what the pump requires with margin, particularly on suction-lift arrangements. Material selection follows the fluid — bronze or cast iron for general service and fresh water, duplex stainless or specially coated cast iron for raw seawater — and essential duties such as cooling, bilge, fire and ballast are fitted in duty/standby pairs so a single pump failure does not stop the function.

Regulations / Class

Class rules require duplication of pumps serving essential functions — cooling water, bilge, ballast and fire — so that one pump out of service for overhaul does not leave the ship without the function. SOLAS Chapter II-2 sets out fire pump capacity and pressure requirements, including the number of fire pumps and their independent power supplies. Bilge and ballast pumping capacity is checked at periodic survey against the figures the vessel was built to.