Pump Specific speed

Specific Speed of pump:

The term impeller specific speed is used to classify impellers of various types based on their performance. Specific speed is defined as that speed at which a geometrically similar pump would deliver one unit of flow to one unit of head.

This value tells us something about the type of pump. Is it a radial type pump, which provides high head and low flow or an axial or propeller type pump, which provides high flow but low head

There are varieties of pump designs that are available for any given task. Pump designers have needed a way to compare the efficiency of their designs across a large range of pump model and types. Pump users also would like to know what efficiency could be expected from a particular pump design. For that purpose pump have been tested and compared using a number or criteria called the specific speed (Ns), which helps to do these comparisons.

Equation below gives the value for the pump specific speed;

Specific speed, Ns =  N(rpm) x sq. rt. of flow rate-Q / (Head-H)¾

Specific speed is a dimensionless quantity.

Specific speed is indicative of the shape and characteristics of an impeller. Impeller form and proportions vary with specific speed but not the size. It can be seen that there is a gradual change in the profiles from radial to axial flow configuration. Studies indicate that a pump efficiency at the best efficiency point (BEP) depends mainly on the specific speed, and a pump with specific speed of 1500 is more efficient then the one with specific speed of 1000.

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Pump Specific speed