Centrifugal Pumps Design and Application 2nd ed - Val S. Lobanoff, Robert R. Ross (Butterworth-Heinemann, 1992)

96 Centrifugal Pumps: Design and Application
Figure 8-12. Influence of wear ring clearance on NPSHR.
Influence of Suction Nozzle
KI is largely influenced by the pump suction nozzle approaching the
impeller eye. To confirm values of KI, a pump was modified by installing
area reducing steel plates in the suction approach as shown in Figures
8-13, 8-14, 8-15, and 8-16. The plates were first formed in wood conforming
to existing suction nozzle core box shape, then manufactured in
steel and welded in place. NPSHR tests were conducted before and after
the modification in accordance with the standards of the Hydraulic Institute,
using 3% head loss as the criterion. The results, shown in Figure 8-
17, illustrate the influence of nozzle geometry approaching the impeller
and lead to the observations now described.
The test pump was of the double-inlet split-case type. This complex
suction passage from pozzle to impeller eye, makes it difficult to determine
analytically the expected losses. It is documented that a design of
this type, which changes the direction of flow at least four times and
moreover splits the flow into two separate channels, can have appreciable
influence on the energy loss—sometimes as high as 2% to 3% of the
effective head of one impeller. The improvements experienced in the test
are difficult to confirm analytically. However, it is obvious that the reduced
energy loss could not result from velocity change alone.