Page 122 - Centrifugal Pumps Design and Application

NPSH 103

Step 3: Determine KI*

From Figure 8-9:

From Figure 8-19, as 1,800 GPM is less than BEP:

Step 4: Calculate NPSHRfrom Equation 8-2.

From Figure 8-20,

Figures 8-21, 8-22, and 8-23 show examples of calculating NPSH A,

Suction Piping

As described earlier, NPSHR is influenced by suction nozzle design.
Similarly, poor suction piping can adversely affect NPSHR and pump
performance. Double-suction first-stage impellers are particularly vul-
nerable to a nonuniform approach of the liquid. If elbows are located
close to the pump, they should be oriented to provide equal distribution
of flow into both eyes of the impeller. An elbow parallel to the pump
shaft directly before the pump is conducive to spiral flow and unbalanced
flow distribution into the two eyes. Unequal flow distribution can result
in excessive vibration, high axial thrust loads, noise, and cavitation.

Effect of Viscosity

Although the influence of viscosity is predictable on other hydraulic
characteristics, particularly head, capacity, and efficiency, little general
information is available to indicate the effect on NPSHR. From experi-
ence we know that up to 2,000 SSU we are safe in using water NPSHR