SUBJECT : All about specific speed 7-3

Specific speed is a term used to describe the geometry (shape) of a pump impeller. People responsible for the selection of the proper pump can use this Specific Speed information to :

Specific speed is defined as "the speed of an ideal pump geometrically similar to the actual pump, which when running at this speed will raise a unit of volume, in a unit of time through a unit of head".

The performance of a centrifugal pump is expressed in terms of pump speed, total head, and required flow. This information is available from the pump manufacturer's published curves. Specific speed is calculated from the following formula, using data from these curves at the pump's best efficiency point (bep.):

N = The speed of the pump in revolutions per minute (rpm.)

Q = The flow rate in liters per second ( for either single or double suction impellers)

H = The total dynamic head in meters

Please refer to the following chart:

Pumps are traditionally divided into three types: radial flow, mixed flow, and axial flow. When you look at the above chart you can see there is a gradual change from the radial flow impeller, which develops pressure principally by the action of centrifugal force, to the axial flow impeller, which develops most of its head by the propelling or lifting action of the vanes on the liquid.

In the specific speed range of approximately 1000 to 6000 double suction impeller are used as frequently as the single suction impellers.

If you substitute other units for flow and head the numerical value of Ns will vary. The speed is always given in revolutions per minute (rpm.). Here is how to alter the Specific Speed number (Ns) if you use other units for capacity and head :

As an example we will make a calculation of Ns in both metric and U.S. units :

If the above results were describing an actual application, we would notice that it was a low specific speed, radial flow pump, meaning It would be a large pump with a low efficiency. Going to 2900 rpm. or higher would increase the Ns to 1000 or more, meaning a smaller pump with a much higher efficiency, but this higher rpm. would have other possible consequences :

The following diagram illustrates the relationship between specific speed and pump efficiency. In general, the efficiency increases as Ns increases.

Specific speed also relates to the shape of the individual pump curve as it describes head, capacity, power consumption and efficiency.

In the above diagram you will note that :

Keep in mind that efficiency and power consumption were calculated at the best efficiency point (bep.) In practice most pumps operate in a throttled condition because the pump was oversized at the time it was purchased. Lower specific speed pumps may have lower efficiency at their bep., but at the same time will have lower power consumption at reduced flow than many of the higher specific speed designs.

The result is that it might prove to be more economical to select a lower specific speed design if the pump has to operate over a broad range of capacity.

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