Pump specific speed Specific speed

low-specific speed radial flow impellers develop hydraulic head principally through centrifugal force. pumps of higher specific speeds develop head partly centrifugal force , partly axial force. axial flow or propeller pump specific speed of 10,000 or greater generates head exclusively through axial forces. radial impellers low flow/high head designs whereas axial flow impellers high flow/low head designs. in theory, discharge of purely centrifugal machine (pump, turbine, fan, etc.)is tangential rotation of impeller whereas purely axial-flow machine s discharge parallel axis of rotation. there machines exhibit combination of both properties , referred mixed-flow machines.

centrifugal pump impellers have specific speed values ranging 500 10,000 (english units), radial flow pumps @ 500-4000, mixed flow @ 2000-8000 , axial flow pumps @ 7000-20,000. values of specific speed less 500 associated positive displacement pumps.

as specific speed increases, ratio of impeller outlet diameter inlet or eye diameter decreases. ratio becomes 1.0 true axial flow impeller.

n

s


=



n


q




(
h

)

3

/

4







{\displaystyle n_{s}={\frac {n{\sqrt {q}}}{(h)^{3/4}}}}

where:

n

s




{\displaystyle n_{s}}

specific speed (dimensionless)




n


{\displaystyle n}

pump rotational speed (rpm)




q


{\displaystyle q}

flowrate (l/s) @ point of best efficiency




h


{\displaystyle h}

total head (m) per stage @ point of best efficiency

note units used affect specific speed value in above equation , consistent units should used comparisons. pump specific speed can calculated using british gallons or using metric units (m/s or l/s , metres head), changing values listed above.

the following equation gives dimensionless specific speed.




n

s


=



n


q




(
h

)

3

/

4







{\displaystyle n_{s}={\frac {n{\sqrt {q}}}{(h)^{3/4}}}}