Page 51 - Centrifugal Pumps 2E

impeller Design 37

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(2) , as final results will not be accurate. On the impeller suction side
however, divide the inlet flow and eye area by 2 for each side of the
impeller centerline.
Impeller Layout

Development of Impeller Profile (Plan View)

Lay out the impeller width b 2 at full diameter (Figure 3-9). Begin to
develop the hub and shroud profiles by expanding this width by approxi-
mately 5° on each side of the vertical center line toward the suction eye.
Complete these profiles by ending in such a way as to produce the re-
quired eye area. The area change from eye to discharge should be grad-
ual. To minimize axial and radial thrust, make the impeller as symmetri-
cal as possible about its vertical centerline.

Development of Impeller End View

Draw a circle to the full impeller diameter. Divide the circle into sev-
eral even-degree segments. In this case 15° segments have been chosen
(Points 1-10). The smaller the segments, the more accurate the vane lay-
out will be,
Impeller Inlet Angles

Inlet angles are established from a layout of the velocity triangle,
which shows the various component velocities of flow, entering the im-
peller (Figure 3-10). The vector connecting U t and C Mi represents the
angle of flow 6. Vane angle B ( is drawn to intersect P SJ and should be
greater than 6 to allow for recirculated flow and nonuniform velocity.
For reasonable design the prerotation angle should not exceed 30° and
for optimum NPSH it is recommended that P Si = 1.05 to 1.2 times C M1 ,

Development of Impeller Vane

Begin the vane development by drawing a line equal to the discharge
angle. On the end view estimate and locate the distance a r. Transmit a r to
discharge angle line on the vane development establishing the distance a.
Transmit a to the front shroud on the impeller profile (plan view). Mea-
sure R 2, then scribe R 2 on the end view, locating Point 2. Check to see if
the estimated a r was approximately correct, making a new estimate and
repeating the process if necessary. Continue this same procedure to the
minimum impeller cut diameter, in this case Point 4. Complete the vane
development for the shroud contour by drawing several lines equal to the