Common Types of Centrifugal Pumps and their Operation

A common technique for separating suspended solids from a liquid is centrifugation. Rotating a test tube containing a fluid suspension, like blood, at high speed will force the contents to the bottom. The densest particles are red cells and form the bottommost layer with leucocytes, then platelets and clear plasma above. The same principle is employed by centrifugal pumps, although, in this case, the goal is not separation but propulsion.

Many industrial and commercial applications involve the use of pipelines to transport or circulate liquids. However, these are merely metal or plastic tubes of varying lengths and diameters. Therefore, some mechanical force is essential to initiate and maintain the movement of liquids through a pipe. Where gravity alone is insufficient to sustain the action, the energy must be supplied by a suitable mechanical device. While not necessarily the best solution in every case, a centrifugal pump will frequently be the tool of choice.

Centrifugal Pumps

Leveraging rotational energy is just one of several principles employed in pumping equipment. However, the simple design and high efficiency of devices that utilise this principle have made them a popular all-round choice with many end-users. A component similar to a domestic fan or a ship’s propellor is at the heart of these devices and responsible for their action. In practice, it is known as an impeller, and the purpose of its specially shaped blades is to convert the rotational force generated by centrifugal pumps into hydrodynamic energy.

Water or other liquids, such as chemical solutions and light oils, are drawn into the device via an inlet pipe. As the impeller rotates, the fluid is forced forward or outwards onto the casing walls by the said rotational force. Sustained by the continued impeller action, the liquid is propelled to an exit pipe, guided, in part, by the shape of the casing. Increasing the impeller speed will increase the flow rate. Also, if a nozzle should impede the flow, it will cause an increase in fluid pressure. This ease of operation is another reason why centrifugal pumps are such a popular choice.

These devices are available in various forms according to operational factors such as the nature and viscosity of the fluid to be pumped, the pressure and temperature involved and whether they must operate submerged or not. Multistage units with two or more impellers can provide increased power when faced with a high head of pressure. Some manufacturers may even offer bespoke designs for specialised use. However, standard models are available, utilising flow patterns and casing designs to deliver various operational characteristics. Each has its pros and cons.

In radial flow centrifugal pumps, the fluid is drawn in along the axis of the impeller. It is then directed against the housing and discharged in a direction lateral to the impeller’s rotation. Radial flow units are suitable for numerous applications and offer the advantage of generating a free fluid flow without any pulsation. On the downside, this design lacks the suction power of the alternatives.

In axial models, the flow pattern is reversed. Water enters laterally along the housing walls and is later discharged in direct line with the axis of the centrally located impeller. This design is perfect when a high flow rate is necessary. Also, these centrifugal pumps will operate under a wide range of conditions. However, discharge pressures are low. Thus, axial models are not the best choice in applications that require lifting fluids by suction. Alternatively, mixed flow units employ an intermediary principle in which axial flow changes to radial in mid-operation. However, one must balance the simplicity of operation in mixed flow units against the larger space requirement when installing them.

As mentioned earlier, casing designs also influence performance. Turbine models are compact, reliable, have high head capabilities, and are suitable for handling gas-liquid mixtures. However, they have a low flow rate, are not easily adjustable and are best for handling liquids free of entrained solids. By contrast, centrifugal pumps with a volute type casing have no notable disadvantages, are low-priced and need no diffuser.

It is always crucial to select the right tool, whatever the job. However, this is especially true when that task involves pumping liquids. Prochem is an industry leader in the design and manufacture of pumping equipment. Few companies in South Africa are better equipped, qualified or experienced to provide expert guidance and deliver world-class standard or bespoke products, including an extensive range of centrifugal pumps.

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