The Characteristics and Uses of Centrifugal Pumps

If you were to place a sugar cube or some similar object near the centre of a turntable and set the latter to rotate at 45 revolutions per minute, the item will, almost certainly, remain where you placed it. However, if you then gradually increase the speed of rotation, at some point, the object would begin migrating towards the edge of the turntable. The rotational force involved is identical to that responsible for the action of the centrifugal pumps commonly used in various domestic and industrial applications. In these devices, the specially designed vanes of a rapidly rotating impeller serve to generate the outward force, which then acts on a continuous stream of liquid rather than a single, solid object.

This principle has other applications. Among the most notable of these is the laboratory centrifuge. This device provides a means to separate the cellular components of blood from the plasma or an alternative to filtration for separating precipitated solids from liquids in the pharmaceutical industry. By contrast, centrifugal pumps are the tool of choice for close to half of all industrial fluid transfer operations. The rotary action of the impeller draws fluid into the housing along its axis. Next, the impeller’s curved vanes act to direct the incoming liquid outwards against the walls of the pump casing, where it then proceeds to exit via the appropriate outlet.

When used to handle clear fluids, the impeller is often enclosed between a pair of discs. However, when backed by just a single disc or left open, the impeller is more effective in centrifugal pumps that may be required to transfer liquids containing significant quantities of suspended solids. In each case, the rotary action is provided by a driveshaft connected to an electric motor or internal combustion engine that typically operates at speeds of between 500 and 5 000 revolutions per minute.

While the operating principle remains the same and still depends on the rotation of a closed, semi-closed or open impeller, the casings that serve to house the working components of a centrifugal pump are of two fundamentally different designs. These are known as volute and diffuser casings. The volute casing has an asymmetric appearance due to the offset positioning of the impeller. The liquid is drawn through an axial input pipe as usual but then discharges into a funnel-shaped structure that curves around the impeller mounting. The funnel-like design of the casing results in a gradual increase in pressure as the discharged fluid approaches the outlet.

By contrast, a diffuser casing is circular, and the impeller is mounted centrally along the axis. The action of these centrifugal pumps also increases the fluid pressure, but they employ a ring of stationary vanes mounted in the casing to do so. One might reasonably ask why it should be necessary to have two different designs. The simple answer is that each has performance characteristics that are best suited to particular applications. The volume casing will be the more effective choice when pumping viscous fluids or liquids that contain a high concentration of suspended solids. In such cases, the stationary vanes in the diffuser casing would tend to obstruct the flow.

On the downside, wear and tear on the driveshaft and impellers of centrifugal pumps with a volute casing tends to be greater due to the asymmetric design. In most circumstances, the diffuser casing improves a pump’s efficiency and can also be tailored to perform specific tasks, thus also making it the more versatile option.

In practice, modern pumping equipment falls into one of two main families based on its operating principle. The positive displacement types have no impellers. Instead, they have a piston, a plunger, or some equivalent mechanism that first draws the fluid in and then discharges it. The centrifugal pumps operate at high speed and, consequently, are more suitable for high-capacity pumping operations involving water, aqueous solutions, chemical solvents, and other low viscosity fluids. Because the positive displacement models operate at lower speeds, they are the preferred option for the transfer of highly viscous fluids such as coal slurries, emulsions and thick oils where high pressure is more important than high flow rates.

Whatever the task, if it involves pumping equipment, Prochem Chemical Pump Manufacturers have an appropriate positive displacement or centrifugal pump or can design one to meet your specific requirements.

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