14000

D corrected by Figure 14

in

15.25°

14.26b

D corrected by Figure 15

in

15.60-=

14.93d

= 0.935. Corrected D = = 0.874. Corrected D = 1,953. Corrected D - D' = 1,953. Corrected D - D' =

aD/D = 15.125/16.3125 = 0.927; by Figure 14, corrected D'/D (0.935)(16.3125) = 15.25 in.

bD'/D = 14.000/16.3125 = 0858; by Figure 14, corrected D'/D (0.874)(16.3125) = 14.26 in.

D - D = 16.3125—15.125 = 1.1875; by Figure 15, K = 0.6 at n, = (0.6X1.1875) = 0.7125 and corrected D = 16.3125—0.7125 = 15.60 in.

dD - D = 16.3125—14.000 = 2.3125; by Figure 15, K = 0.6 at n, = (0.6X2.3125) = 1.3875 and corrected D = 16.3125—1.3875 = 14.93 in.

changes in diameter were small, the area of blade removed was rather large for each cut. The second cut eliminated most of the blade overlap.

The characteristics of mixed-flow impellers can be changed by cutting, but very small cuts may produce a significant effect. The impellers of propeller pumps are not usually subject to diameter reduction.

Shaping Blade Tips If the discharge tips of the impeller blades are thick, performance usually can be improved by filing over a sufficient length of blade to produce a long, gradual taper. Chamfering, or rounding, the discharge tips may increase the losses and should never be done. Reducing the impeller diameter frequently increases the tip thickness.

FIGURE 16A through C Diameter reduction of mixed-flow impeller (ft X 0.3048 = m; in X 2.540 = cm; gpm X 0.06309 = l/s; hp X 0.7457 = kW)

overfiling This is shown at B in Figure 17, and the unfiled blade is shown at A. Usually there is little or no increase in the blade spacing d before and dF after filing, so the discharge area is practically unchanged. Experience indicates that any change in the angle b2 due to overfiling usually produces a negligible change in performance.

underfiling This is shown at C in Figure 17. If properly done, underfiling will increase the blade spacing from d to df and hence the discharge area, which lowers the average meridional velocity cm2, at any given flow rate Q. The angle b2 usually is increased slightly. Figure 18A and Eq. 6 show that the head and consequently the power increase at the same flow rate. The maximum efficiency usually is improved and may be moved to a higher flow rate. At the same head, Figure 18B shows that both cm2 and the flow rate will increase. The change both in the area and in Cm2 may increase the flow rate by as much as 10%. Table 7 shows the results of tests before and after underfiling the impeller blades of nine different pumps. in general, they confirm the foregoing predictions based on changes in area and in the velocity triangles.

TABLE 6 Predicted characteristics at different impeller diameters on a mixed-flow pump

Test values

TABLE 6 Predicted characteristics at different impeller diameters on a mixed-flow pump

Test values

D, in

Dm, in

Q, gpm

H, ft

P,hp

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