Despite its name, pump specific speed (NS) does not have much to do with the actual speed of a pump. While the rpm is included in the formula, the calculation’s main purpose is to establish the impeller’s shape and contour and to link this to other pump parameters—such as head rise to shutoff or the maximum achievable theoretical efficiency. The values of specific speed can change drastically, depending on the point on the head-flow curve at which it is calculated.
A video experiment illustrates how to conduct pump testing and how to produce a head-flow curve. Visit www.pumpingmachinery.com/pump_school/pump_school.htm, and view PVA #6. Note that this video does not address at which flow point (Q) a pump’s specific speed is calculated.
In the PVA #6 video, one example illustrates how drastically the values of specific speed can change depending on which point on a head-flow curve it is calculated. The raw data were taken as discharge and suction pressures at several flow points, and the pump head was calculated from these data. For simplicity, the velocity head components and gauge elevation adjustments were not considered because the correction would be insignificant for this test.
The formula for specific speed (see Figure 1) indicates that high values come from high-flow pumps and low head, and low specific speed values result from high-head pumps with low flow (see Figure 2). Depending on the specific speed value in the test, a different type of pump would be expected. A purely radial-flow impeller (Type 1) would be expected if the specific speed were 433. A mixed-flow impeller type (typically used for vertical turbine pumps) would be expected if the specific speed were 1,480.
To avoid confusion, the convention is to use the flow and head within the specific speed formula at the best efficiency point (BEP) flow. Producing a head-flow curve in field testing is relatively simple—following essentially the same process outlined in the PVA #6 video—but determining the BEP is a more involved process.
Measurements of power and additional calculations for pump efficiency are necessary to determine where a true peak efficiency flow is. These additional calculations will be covered in upcoming videos. In the meantime, I would like to hear your comments and questions, which you can send me through the Pump School website.