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A pump with specified pump flow and head, what kind of pump should we use ? If it is centrifugal type, how do we know if it is single stage pump or multi-stage pump ?

To many engineers who are familiar with pump, you may have the quick answer to it. The following chart may be used of quick guideline for a pump type selection.

SI unit :


FPS unit :

EXAM 1.
For a pump flow of 30 m3/h (~132 gpm) and pump head of 50m, a single stage centrifugal pump would be sufficient base on above chart.

Let make a checking, the specific speed (Ns) and centrifugal pump efficiency estimated based on curve presented in "Estimate Pump Efficiency base on Specific Speed (Ns)" is about 727 and 62%. The specific speed (Ns) is more than minimum recommended Ns of 500 (for flow more than 100 gpm), single stage centrifugal pump still consider acceptable although the pump efficiency is only average.

EXAM 2.
For a pump flow of 30 m3/h (~132 gpm) and pump head of 300m, a multi-stage centrifugal pump would be required base on above chart.

Lets check why not a single stage pump. The specific speed (Ns) calculated based on equation presented in "Estimate Pump Efficiency base on Specific Speed (Ns)" is about 189 which is far below 500 (for flow more than 100 gpm). A single stage pump is not recommended. Assuming the pump head is equally splited between stage, 4 stages centrifugal pumps is required in order to meet Ns more than 500. With 4 stage centrifugal pump, the calculated Ns is about 537 and pump efficiency is about 57%. This 4 stage centrifugal pump configuration is acceptable.


EXAM 3.
For a pump flow of 10 m3/h (~44 gpm) and pump head of 300m, a multi-stage centrifugal pump might be considered base on above chart. However...

With single stage pump, the specific speed (Ns) and efficiency estimated based on equation presented in "Estimate Pump Efficiency base on Specific Speed (Ns)" is about 109 and 10% for flow less than 100 gpm. Obviously single stage pump is not recommended with such low efficiency.

Assuming the pump head is equally splited between stage, 8 stages centrifugal pumps would lead to Ns of 520 and pump efficiency is about 46%. With such a low efficiency and many stages of pump (high capital cost), one may consider a reciprocating pump i.e gear pump in this service.

Above only a quick selection method and checking criteria based on 2 factors i.e specific speed and pump efficiency, there may be other factor affecting the selection and will not be discussed here.

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posted by Webworm, 2:48 PM

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