One must puzzle, how specific speed affects pump performance characteristics; and therefore plays an important role in pump selection!
One example on that basis, we must think, that how it affect; when two pumps run with same system requirements, but have different geometrical dimensions, that is size parameters.
A pump with a bigger impeller needs less rotation to satisfy system requirements, less rotation means less specific speed, therefore less NPSHR (Net positive suction head required).
But we have to think on efficiency too, because when specific speed decreases, efficiency also follows similar trend!
Therefore before selecting the operating speed(N) of pump; we have to be very much careful on crosschecking NPSHR and efficiency. NPSHR must be less than NPSHA, and efficiency must not goes down.
If system flow requirement increases, i.e. ‘Q’ increases; therefore it ultimately increases the pump’s specific speed! And consequently, pump needs more NPSH…
…and therefore consequences arise having NPSHR>NPSHA; which are the main causes of cavitation and reduced pump performance, pump runs with less efficiency!
Therefore, it may be suggested to pump the requirement with two or more pump, or by a dual suction pump.
In result the flow rate got halved (in case of two pumps), and specific speed got reduced; decreasing the value of NPSHR!
In case of high speed requirements, i.e. ‘H’…., being ‘H’ in denominator of the equation deciding specific speed; the specific speed increases in multiple and therefore, NPSH required increases!
In such circumstances, multistage pumps plays an important solution….., as head produced per stage of impeller gets divided; specific speed of the impeller reduces in multiple.
Thus in result, the design needs less NPSH than NPSHA.
In conclusion, pump performance parameters plays a big role in pump selection… And the relationship between them can be expressed as pump specific speed.