Page 118 - Centrifugal Pumps Design and Application

NPSH 99

Figure 8-17. Influence of plate inserts on NPSHR.

It can be assumed that this improvement is influenced by reduced sepa-
ration, improved flow stability and streamlining, a progressive increase
in velocity, and the resulting reduced turbulence. It must also be accepted
that any disturbance in the approach to the impeller can cause unequal
distribution of flow rates into the two impeller eyes at different locations,
These diversions from the correct angle of attack at the leading edge of
the blades produce a corresponding head loss.
The test confirms that the cavitation characteristics of a good impeller
design can be impaired by poor suction nozzle design. This is particu-
larly true with double-entry impeller pumps where the complex nozzle
geometry can adversely affect KI. A well designed suction nozzle has a
gradual decrease in area from nozzle to impeller, allowing a progressive
increase in velocity. Area distribution guidelines are shown in Figure
7-4. The range suggested permits impellers of different suction specific
speeds to be used with a common suction.
Using the actual suction nozzle area ratio at Section A-B gives a rea-
sonable means of estimating K } (Figure 8-18). For the same incidence
angle a, K 2 has a higher value at capacities above design. For estimating
K 2 use Figure 8-19.
influence of Liquid

The boiling of the liquid in the process of cavitation is a thermal pro-
cess and is dependent on the liquid properties, pressure, temperature, la-
tent heat of vaporization, and specific heat. To make this boiling possi-
ble, the latent heat of vaporization must be derived from the liquid flow.