Centrifugal Pump

The impeller is the key component of a centrifugal pump. It consists of a series of curved vanes. These are normally sandwiched between two discs (an enclosed impeller). For fluids with entrained solids, an open or semi-open impeller (backed by a single disc) is preferred. Fluid enters the impeller at its axis (the ‘eye’) and exits along the circumference between the vanes. The impeller, on the opposite side to the eye, is connected through a drive shaft to a motor and rotated at high speed (typically 500-5000rpm). The rotational motion of the impeller accelerates the fluid out through the impeller vanes into the pump casing. There are two basic designs of pump casing: volute and diffuser. The purpose in both designs is to translate the fluid flow into a controlled discharge at pressure. In a volute casing, the impeller is offset, effectively creating a curved funnel with an increasing cross-sectional area towards the pump outlet. This design causes the fluid pressure to increase towards the outlet. The same basic principle applies to diffuser designs. In this case, the fluid pressure increases as fluid is expelled between a set of stationary vanes surrounding the impeller. Diffuser designs can be tailored for specific applications and can therefore be more efficient. Volute cases are better suited to applications involving entrained solids or high viscosity fluids when it is advantageous to avoid the added constrictions of diffuser vanes. The asymmetry of the volute design can result in greater wear on the impeller and drive shaft. There are two main families of pumps: centrifugal and positive displacement pumps. In comparison to the latter, centrifugal pumps are usually specified for higher flows and for pumping lower viscosity liquids, down to 0.1 cP. In some chemical plants, 90% of the pumps in use will be centrifugal pumps. However, there are a number of applications for which positive displacement pumps are preferred.

This type of impeller generally has 5 to 7 blades, the minimum size of which is 125 mm and the maximum size 550 mm. The blades have a simple backwards curve with a radial movement in the pumps, typically with a low number (high heads and low volume flows), while they can have a double backwards curve with semi-axial movement in the pumps with a high number (low heads and high volume flows). The blades are completely closed between the hub disc and the crown disc. The efficiency of this type of impeller varies from 0.6 for the smallest impellers to 0.83 for the largest impellers. This type of impeller is suitable for clean liquids or those containing light impurities. This type of impeller has a reduced number of blades that varies from 3 to 4, the minimum size of which is 270 mm and the maximum size 450 mm. The blades generally have a double backwards curve and a predominantly radial movement and are completely closed between the hub disc and the crown disc. The hub is angled much further backwards compared to the closed impellers described above. These impellers are comparatively much less efficient since the liquid is not guided as well due to the limited number of blades and the hub which is angled much further backwards. However, these devices enable the creation of internal channels with a large opening through which waste water, containing suspended solids that are also sizeable, can pass through. This type of impeller is fitted with 9 blades with a simple backwards curve with radial development. No crown disc is fitted to this impeller, whereas the hub disc, seen in sections, has a "spoon" movement; the hub is created in order to provide the impeller inside the casing with a position that is angled much further backwards. This impeller is comparatively less efficient since the liquid is not guided like it is in other types of impeller. This is a new impeller for use with dense and highly viscose liquids, or with solutions containing large amounts of dry residue. The impeller is fitted with two or four blades (across the whole range of sizes) depending on the viscosity or pastiness of the liquid being pumped. Two of these blades, arranged symmetrically as regards the hub, are fitted with an appendix that stretches from the hub to the suction nozzle with a spiral movement. This type of impeller has no crown disc, for which the seal of the liquid between each vane is secured by the extremely reduced clearance between the blades and the wear plate for the casing. This impeller is more efficient than channel and vortex impellers and has better suction characteristics.