Page 109 - Centrifugal Pumps Design and Application

90 Centrifugal Pumps: Design and Application

evidence that cavitation is present under test conditions. They do caution,
however, that where it is important to establish normal operation with an
appreciable margin over the minimum required NPSH, values as low as
1 % should be used. They fiirther caution, that the pump should be oper-
ated above the break-away sigma if noise and vibration are to be avoided.

influence of Suction Specific Speed (N ss)
When evaluating NPSHR one should always refer to the suction spe-
cific speed (N ss) and not to sigma, whose value depends on the head de-
veloped. Thus the same impeller when tested before and after a diameter
trim will show different values of sigma yet will behave identically under
cavitation. In comparing cavitation performance and predicting NPSHR,
we prefer to use the suction specific speed parameter. This can be applied
in the same manner as sigma yet has more significance as it relates only
to the inlet conditions and is essentially independent of discharge geome-
tries and pump specific speed.
Figure 8-6 shows suction velocity triangles for N Ss from 7,000 to
16,000. It graphically shows that as suction specific speed increases, nor-
mal vane entrance angle becomes flatter, C MJ relative to shaft becomes
smaller, and peripheral velocity at impeller eye becomes greater. The ra-
tio of CMI to LJ t for higher suction specific speed, is very small and
chances for cavitation are greatly increased. This cavitation would ap-
pear for the following reason: To move liquid from one point to another,
a change of pressure gradient must take place. But as CMI (velocity) is
2
reduced, velocity head, V /2g, is also reduced. When it becomes lower
than the head required to overcome impeller entrance losses, the liquid
will backflow creating cavitation and pump damage.
Figure 8-7 represents a test of a four-inch pump with eight different
suction specific speed impellers. Best efficiency point of all impellers is
the same. Impeller profile is also the same, but impeller eye geometry is
different for each suction specific speed. Some tests on different types of
pumps might show different results, but the trend should be the same.
This test shows the stable cavitation-free window for seven suction spe-
cific speeds and could be used as a guide for pump selection.

High Speed Pumps

The prediction methods and NPSH testing just described under moder-
ate speed pumps are valid and well substantiated for low- to moderate-
speed pumps. It is our opinion, however, that neither method will ensure