Project: sc090025/R1

Figure 3.6: Typical axial and mixed flow arrangements 

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Figure 3.7: Vertical direct drive, suspended and canister pump type installations may be 
axial or mixed flow pumps 
Figure 3.8: Typical vertical axial flow pump installation 
3.4 Centrifugal flow pumps 
Centrifugal, or radial, flow pumps are generally used across a wide range of applications of 
head and flow. In the context of flood defence, the older installations tend to be of the horizontal 
large diameter type, often dual inlet. These rotate at relatively slow speeds to match the low 
head requirements. As the impellers are raised above the incoming water level, they will 
normally require a priming system to operate prior to operation. 

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Figure 3.9: Large radial dual flow installation in flood defence application (top casing 
removed) 
Figure 3.10: Typical centrifugal impeller configurations 
Modern types of centrifugal pumps may be configured in a multitude of ways and can be 
coupled so that the discharge from one pump feeds the intake of a second or third stage, 
increasing the delivery head. In this way it is possible to construct centrifugal pumps which can 
deliver heads of hundreds of metres. 
Centrifugal pumps tend to be less tolerant of solid material entering the water stream, although 
lower delivery head pumps suffer less from this problem. It is common to find that in waste 
water transfer applications, centrifugal pumps are designed to be tolerant of solid material at 
considerable expense to efficiency.
Generally, modern centrifugal pumps operate at higher speeds than the other types of pump 
especially when higher pressures are required; speeds typically range between 1,400 rpm and 
3,000 rpm. Matching the pump operating speeds with the application is crucial to maximising 
pump efficiency, as high operating speeds usually require more power and need to be 
controlled in order to avoid losses. 
Modern centrifugal pumps are commonly used for sewage transfer where large solids may be 
present. Unlike screw type pumps, the centrifugal pump does not readily lend itself to passing 
large solids. Many pumps of this nature are necessarily very open, with large open spaces 
within the pump bowl and with generous openings within the impeller.
This is particularly a problem with smaller sewage pumps where the ratio of open space to 
impeller within the pump bowl is higher (solids tend to be of a similar size regardless of flow 
when dealing with sewage). 

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