Pumps at sea: centrifugal, screw, and eccentric screw pumps

A ship is filled with pumps — for cooling water fuel oil ballast water and cargo handling. But all pumps are not alike. As a ship fitter you must be able to overhaul centrifugal pumps screw-spindle pumps and eccentric screw pumps and be able to choose a pump based on optimised pump curves taking into account the medium's viscosity and fluid chemistry. This requires understanding how the three principles differ from each other.

§The centrifugal pump — the workhorse for thin liquids

The centrifugal pump uses a rotating wheel (an impeller) to fling the liquid outward with centrifugal force thereby converting rotational energy into pressure and flow. It is well-suited to large quantities of thin liquid such as coolant or seawater but it is less efficient at lifting a medium at low speeds and it produces a pulsating flow compared to displacement pumps.

§The screw spindle pump — gentle and smooth

A screw pump consists of two or more spindles that rotate in a common pump housing. It builds pressure slowly and linearly works with very little pulsation has low noise level and transports the fluid gently. Screw pumps are self-priming and can therefore handle fluids that release air or gas — properties that make it suitable for example for oil and fuel under pressure.

§The eccentric screw pump—powerful at low speeds

The eccentric screw pump has a single-screw rotor that rotates eccentrically inside a double-screw stator. This creates a cavity on the intake side where the medium is drawn in and transported smoothly toward the pressure side without pulsation. Even at low speeds it can lift a medium high—a clear advantage over the centrifugal pump—and it is well-suited for thick, viscous, or contaminated media.

• 01Centrifugal pump — large quantities of thin liquid pulsating flow
• 02Screw spindle pump — self-priming, smooth pressure, gentle on the medium (e.g. oil)
• 03Eccentric screw pump—even flow without pulsation, strong at low speeds, handles thick media

§Selection of pump according to pump curve, viscosity and fluid chemistry

When a pump is to be selected or replaced, you look at the pump curve—the relationship between flow and back pressure for that specific pump—and compare it with the system's requirements. Add to that the medium's viscosity: thick oil requires a different pump type than seawater. And finally the fluid chemistry: if the medium is aggressive toward certain materials, the pump's wetted parts must be in a material that can withstand it, regardless of whether the pump is in alloyed or non-alloyed form.

Being able to overhaul these pump types is in practice about knowing their construction well enough to safely disassemble them, check wear limits on rotor, stator or spindle, and reassemble them according to the manual's tolerances — because a pump that isn't assembled correctly quickly loses its performance.