11  Hydraulic aspects of centrifugal pumps

11.1  Specific speed

 

Centrifugal pumps are traditionally divided into three basic types: radial flow, mixed flow and axial flow. The names indicate the direction of flow of liquid through the impeller. The term specific speed (ns) can be used as an aid to the classification of the pump type and is indicative of the shape of the impeller and the pump hydraulic characteristics (Figs 4.45 and 4.46), which are taken from Steponoff [19].

Pump efficiency versus specific speed and pump size

 

Effect of specific speed on head power and efficiency

The expression involves the three fundamental factors in pump design — head, flow and speed. All geometrically similar pumps have the same specific speed which is calculated in non-dimensional units from the following formula:

 

There is no firm demarcation between the various pump types. Each merges into the next, so that for a given specific speed there can be two pumps of differing construction. However, specific speed can be defined as falling into the following categories:

Type of pump Specific speed range (ISO notation)
Radial flow 0.2 to 1.8
Mixed flow 1.8 to 3.0
Axial flow 2.8 to 8.0

It is evident from Fig 4.45 that to optimise the hydraulic selection on an efficiency basis, a specific speed around 0.8 is necessary. For high-head large flow pumps, optimum efficiency when using low speed prime movers necessitates a large number of stages.

By comparison, efficiency optimisation could be achieved by using single-stage high speed machines. Both these extreme options, however, can have their own potential disadvantages. The high speed machine can lead to excessive impeller tip speeds and consequential erosion problems, while the slow speed arrangement results in a flexible shaft design unable to cater for transient operational requirements. Thus the pump hydraulic design is usually a compromise to take account of all relevant factors, including mechanical limitations and suction head considerations.

The effect of moving away from best efficiency can be seen in Fig 4.46. A specific speed below optimum reduces the peak level of efficiency, but there is a consequential improvement in the spread of higher efficiency. This can be of use when plant is required to operate frequently over a large flow range.

Moving to high specific speeds is seen to produce a steeper head/flow characteristic. This can be advantageous when operating pumps in parallel as it minimises the risk of unstable operation. One disadvantage of the high specific speed designs is the shape of the power/flow characteristic which can be highest at closed valve. This results in excessive motor ratings to cater for start-up requirements.

 

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