Pumps, specific speed variations, types

Figure 10-35 shows the schematic of specific-speed variation for different types of pumps. The figure clearly indicates that, as the specific speed increases, the ratio of the impeller outer diameter Di to inlet or eye diameter Do decreases, tending to become unity for pumps of axial-flow type. 

FIG. 10-35 Specific speed variations of different types of pump. 

The majority of centrifugal pumps have performance curves with the aforementioned profiles. Of course, special design pumps have curves with variations. Eor example, positive displacement pumps, multi-stage pumps, regenerative turbine type pumps, and pumps with a high specific speed (Ns) fall outside the norm. But you ll find that the standard pump curve profiles are applicable to about 95% of all pumps in the majority of industrial plants. The important thing is to become familiar with pump curves and know how to interpret the information. 

A set of curves for h, rj, and P as a function of Q are shown in Figure 8.25 from which it is seen that when die pump is operating near optimum conditions, its efficiency remains reasonably constant over a wide range of flowrates. A more general indication of the variation of efficiency with specific speed is shown in Figure 8.26 for different types of centrifugal pumps. The power developed by a pump is proportional to Qghp ... 

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