The following table summarises the differences between centrifugal and positive displacement pumps. So, in applications where accurate dosing is required, a positive displacement pump is preferred. Additionally, if the feed pressure is variable, a centrifugal pump produces a variable flow a positive displacement pump is insensitive to changing pressures and will provide a constant output. Centrifugal pumps are therefore not suited to any application where the supply is intermittent. In such cases, the lower speed of a positive displacement pump is preferred.Ī further limitation is that, unlike a positive displacement pump, a centrifugal pump cannot provide suction when dry: it must initially be primed with the pumped fluid. Positive displacement pumps often operate at considerably lower speeds and are less prone to these problems.Īny pumped medium that is sensitive to shearing (the separation of emulsions, slurries or biological liquids) can also be damaged by the high speed of a centrifugal pump’s impeller. Slurries such as mud, or high viscosity oils can cause excessive wear and overheating leading to damage and premature failures. In general, centrifugal pumps are therefore suited to low pressure, high capacity, pumping applications of liquids with viscosities between 0.1 and 200 cP.

With high viscosity feeds, centrifugal pumps become increasingly inefficient: there is greater resistance and a higher pressure is needed to maintain a specific flow rate. The efficient operation of a centrifugal pump relies on the constant, high speed rotation of its impeller. What are the limitations of a centrifugal pump? However, there are a number of applications for which positive displacement pumps are preferred. In some chemical plants, 90% of the pumps in use will be centrifugal 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. There are two main families of pumps: centrifugal and positive displacement pumps. What are the main features of a centrifugal pump? The asymmetry of the volute design can result in greater wear on the impeller and drive shaft. 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. Diffuser designs can be tailored for specific applications and can therefore be more efficient. In this case, the fluid pressure increases as fluid is expelled between a set of stationary vanes surrounding the impeller (Figure 3). The same basic principle applies to diffuser designs. This design causes the fluid pressure to increase towards the outlet (Figure 2). In a volute casing, the impeller is offset, effectively creating a curved funnel with an increasing cross-sectional area towards the pump outlet. The purpose in both designs is to translate the fluid flow into a controlled discharge at pressure. There are two basic designs of pump casing: volute and diffuser. The rotational motion of the impeller accelerates the fluid out through the impeller vanes into the pump casing. 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). For fluids with entrained solids, an open or semi-open impeller (backed by a single disc) is preferred (Figure 1).įluid enters the impeller at its axis (the ‘eye’) and exits along the circumference between the vanes. These are normally sandwiched between two discs (an enclosed impeller). The impeller is the key component of a centrifugal pump. The pump casing is specially designed to constrict the fluid from the pump inlet, direct it into the impeller and then slow and control the fluid before discharge. The action of the impeller increases the fluid’s velocity and pressure and also directs it towards the pump outlet. Fluid enters the rapidly rotating impeller along its axis and is cast out by centrifugal force along its circumference through the impeller’s vane tips. Useful information on centrifugal pumps What is a centrifugal pump?Ī centrifugal pump is a mechanical device designed to move a fluid by means of the transfer of rotational energy from one or more driven rotors, called impellers.

Pumping out high viscosity liquids