Specific speed, metric units

-Impeller shrouds

Impeller shrouds

Radial-vane area

FIGURE 4.9 Specific speeds of various types of centrifugal pumps.

Radial-vane area

Impeller hub Axis of

Funnels-vane area Mixed-flow area Axial-flow area ro'a'lon propellers

FIGURE 4.9 Specific speeds of various types of centrifugal pumps.

of about 13 meters and an rpm of 990. These values substituted into Equation (4.43), after converting the rpm of 990 rpm to radians per second, produces an Ns of 1.73. A number of calculations similar to this one need to be done on other characteristics curves in order to produce Figure 4.9. In other words, this figure has been obtained under conditions of best operating efficiencies. Therefore, specifying pumps using specific speeds as the criterion and using figures such as Figure 4.9 ensures that the pump selected operates at the best operating efficiency. From this discussion, it can be gleaned that specific speed could have gotten its name from the fact that its value is specific to the operating conditions at the best operating efficiency.

Example 4.6 A designer wanted to recommend the use of an axial-flow pump to move wastewater to an elevation of 30 m above a sump. Overall friction losses of the system and the velocity at the discharge side are estimated to be 20 m and 1.30 m/s, respectively. The operating drive is to be 1,200 rpm. Suction friction losses are 1.03 m; the diameter of the suction and discharge lines are 250 and 225 mm, respectively. The vertical distance from the sump pool level to the pump centerline is 2 m. Is the designer recommending the right pump? Design the pump yourself.



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