Calculation Procedure:
- Determine the upper limit of specific speed
Use the Hydraulic Institute upper specific-speed curve, Fig. 1, for centrifugal pumps or a
similar curve, Fig. 2, for mixed- and axial-flow pumps. Enter Fig. 1 at the bottom at
280-ft (85.3-m) total head, and project vertically upward until the 10-ft (3-m) suction-lift
curve is intersected. From here, project horizontally to the right to read the specific speed
N 3 = 2000. Figure 2 is used in a similar manner.
- Compute the maximum pump capacity
For any centrifugal, mixed- or axial-flow pump, Ns = (gpm)°-^5 (rpm)/H°t-^15 , where H 1 =
total head on the pump, ft of liquid. Solving for the maximum capacity, we get gpm =
(NsH°t-^75 /rpm)^2 = (2000 x 280075 /1750)^2 = 6040 gal/min (381.1 L/s).
- Determine the pump efficiency and impeller shape
Figure 3 shows the general relation between impeller shape, specific speed, pump
capacity, efficiency, and characteristic curves. At Ns = 2000, efficiency = 87 percent.
The impeller, as shown in Fig. 3, is moderately short and has a relatively large dis-
charge area. A cross section of the impeller appears directly under the Ns = 2000 ordi-
nate.
Related Calculations. Use the method given here for any type of pump
whose variables are included in the Hydraulic Institute curves. Figs. 1 and 2, and in sim-
ilar curves available from the same source. Operating specific speed, computed as
above, is sometimes plotted on the performance curve of a centrifugal pump so that the
characteristics of the unit can be better understood. Type specific speed is the operating
Specific speed,, „ N r/min/gal/min
5 - 1-^
FIGURE 2. Upper limits of specific speeds of single-suction mixed-flow and axial-flow
pumps. (Hydraulic Institute.)
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