Page 56 - Centrifugal Pumps Design and Application

Impeller Design 39

The vane development and layout for the back shroud is done in the
same manner, taking into account the required hub vane angle. This pro-
cedure should result in Point 10 reaching the estimated hub vane diame-
!
ter. If this diameter is missed by more than /4-in., the hub vane angle
should be changed and the layout repeated.
Complete the vane layout by adding vane thickness (Figure 3-11) and
indicating a slight underfile at the vane OD and a thinning at the suction,
This underfile will produce higher head and improved efficiency. The
vane spacing location of the second vane on the end view will be deter-
mined by the number of vanes.
At this point it is necessary to check the ratio of the vane area A v to the
suction eye area A e. Referring to Figure 3-12, this ratio should be 0.4 to
0,6. The area A v is the area which is shaded on the plan view. If the area
ratio falls outside the limits shown, it will be necessary to change the plan
view profile and/or vane thickness.
Pumps in the higher specific speed range (N s 1,000-5,000) have suc-
tion velocity triangles that dictate different suction vane angularity at the
impeller hub and impeller eye. This difference in angularity restricts
vane removal from the core box, requiring a segmented core and more
costly patterns and casting. Hydraulically, it is very necessary, and we
must learn to live with it. On low specific speeds however, (N s 500-
1000) a straight vane is acceptable and will not jeopardize hydraulic per-
formance,
Design Suggestions
The following points should be kept in mind when designing an impel-
ler.
• Standardize the relationship between the number of vanes and dis-
charge vane angle. The relationship shown in Figure 3-13 is suggested.
Number and angularity of vanes greatly affect H-Q pump perfor-
mance. Standardization as shown will lead to more accurate perfor
mance prediction.
• Figure 3-14 shows the effect of the number of impeller vanes in the
same casing, on H-Q performance. Please note that with less vanes, the
lower is the BEP head and efficiency and steeper is the shape of H-Q
curve.
» It is possible to maintain same BEP, Q, and efficiency by increasing b 2
where number of vanes and angularity are reduced (Figure 3-15). For
best efficiency, the velocity ratio between volute and impeller periph-
eral must be maintained. Similarly for the same impeller diameter,
vane number and discharge angle BEP will change with change in b 2
(Figure 3-16).
• Avoid using even number of vanes in double volute pumps.