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Q.  What is specific speed in relation to centrifugal pumps?

A. Specific speed is a dimensionless index which numerically describes the geometry of a pump's impeller and classifies the pump as to its type. The equation for specific speed (designated Ns ) is:

 $N_{s}=\frac{N\cdot \sqrt{Q}}{H^{0.75}}$

Where:    N = pump rotational speed (rpm)

Q = pump capacity at BEP, full diameter (gpm)

H = pump head per stage at BEP, full diameter (ft)

The most common types of centrifugal pumps develop head by redirecting the liquid flow radially away from the centerline of the shaft.  These are referred to as radial flow pumps, and typically have low specific speeds of about 2000 or less.  Common characteristics of radial flow pumps are relatively low flow and high head.  At the other end of the spectrum are axial flow pumps which develop head through axial forces.  These are also referred to as propeller pumps, and typically have high specific speeds of about 8000 or more.  Common characteristics of axial flow pumps are high flow and low head.  In between these two types of pumps are the mixed flow pumps which are neither pure radial flow nor pure axial flow.  Instead, they are some combination of the two, and include Francis vane impellers.  The specific speeds for mixed flow pumps range in between 2000 and 8000.

Another characteristic of a pump's specific speed is its ability to predict the shape and slope of the flow vs. head and flow vs. brake horsepower curves.  On a normalized scale, low specific speed pumps are going to have the flattest flow vs. head curves, and shutoff heads which are often no more than 20% higher than the head at BEP.  Mixed flow pumps have steeper flow vs. head curves with shutoff heads around 60% higher than the head at BEP.  And high specific speed pumps have the steepest flow vs. head curves with the shutoff head around 200% higher than the head at BEP.  The brake horsepower curve shapes are also affected by specific speed.  Radial flow pumps require an increasing brake horsepower with increasing flow rate, and the maximum bhp is typically at the maximum flow.  Mixed flow pumps also show increasing brake horsepower with flow rate, although the curves are typically flatter than those of the radial flow pumps.  Axial flow pumps on the other hand require a decreasing amount of brake horsepower as the flow rate increases, with the maximum bhp typically at the minimum flow rate.  For this reason, axial flow pumps are typically not run at low flows because of the higher horsepower requirement.

Information for this article and a wealth of additional information can be found in the following resource:

1. Volk, M., Pump Characteristics and Applications, 2nd Edition, CRC Press, 2005.

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