Examining the benefits of Screw Centrifugal pumps by Malcolm Walker - Product Manager Centrifugal Pumps, AxFlow
In any pumping application, it is always necessary to match the pump type to the application in order to achieve the desired result. One aspect of this selection process is matching the impeller type to the nature of the fluid being pumped. Where liquids with a large solid content of a delicate nature are concerned then free passage through the pump housing and a gentle pumping action are essential. One particular centrifugal pump fits this particular bill perfectly and does so across a wide spectrum of applications in food processing and beverage production; and that is the screw centrifugal pump.
The Technology Although centrifugal screw pumps work on the same principle as any other centrifugal pump, the impeller’s relative length, which results in a more gradual increase in the impeller’s diameter along with the design of the inlet blade profile, means that media entering the pump undertakes a long slow turn around the long radius, low angled vane. The hydraulic gradient within the pump generates a gradual pressure and slow change from axial to radial flow in a smooth process with little shear. As a result the screw centrifugal pump offers several important benefits to the food production engineer. The low shear pumping action enables them to handle delicate products; the wide fluid channel within the impeller allows the free passage of solids up to 120 mm in diameter while the gradual pressure increase results in a very low Net Positive Suction Head requirement (NSPHr).
The impellers come in different profiles each designed for the nature of the fluid being pumped. Open impellers are most typically employed for handling abrasives, coarse suspended solids, fibrous materials and viscous fluids, whereas closed impellers are associated with highly abrasive media or the gentle movement of sensitive media(Fig.2).
Applications
Fig. 1 Hidrostal Blue End suction pump Furthermore, the internal components experience less wear than many alternative pump types contributing to lower maintenance costs and a longer working life. The efficiency of the screw impeller pump (Fig.1) is significantly higher than other types of pumps used for similar applications. The pump head curves are continuous and steep, making it costefficient to adjust the pump operation to the specific pumping requirement and in particular speed- controlled operation.
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Originally designed for use in the water and waste water industry where they have proved excellent at handling various types of solid-ladened sewage and the safe transfer of fresh water containing fish, screw centrifugal pumps have found numerous applications within the food and beverage industry where delicate solids are an issue. Hidrostal’s F-Type pump is a good example of where the screw impeller pump is used extensively in the food industry. This version uses a shrouded single spiral vane impeller that is axially extended for smoother operation. Because of their ability to pump liquids with a high solid content delicately F- Type pumps can be found in brewing where they are commonly used in malt production for carrying grain in water without any damage to the germ, as well as for mashing, lautering, wort circulation and trubbing. The transfer of fresh produce such as fruit, vegetables and
Fig.2 Impeller principles chart
fish is another area where Hidrostal screw centrifugal pumps have proved very successful, particularly as they offer the versatility to operate within closed pipe transfer systems as opposed to being restricted to conveyors, elevators or containers. The enclosed pipework system has the benefit of being more hygienic and cleaner as it is protected from any potential harmful elements in the immediate environment. Operational costs are also reduced with enclosed pipeworks as maintenance is far easier than with conveyors and belts. There is also health and safety advantages as there are fewer exposed moving parts. Sensitive handling of raw ingredients is not all that the F-Type pump offers users. The pump is ideally suited to the energy efficient circulation and removal of wash water used in the cleaning of raw ingredients and applications where there is limited NPSH available.
Summary The screw centrifugal pump provides yet another example of a technology finding applications in sectors for which it wasn’t originally developed and one where the higher initial investment is more than offset by the economics of its longer term reliability and aptitude to a good job.