Page 222 - Centrifugal Pumps Design and Application

1S6 Centrifugal Pumps: Design and Application

It is interesting to note that the shutoff head remains unchanged with DI
variation, so the characteristic curves "hinge" about the shutoff point.
Although a rising curve is generally viewed as being desirable, it should
be noted that many thousands of pumps have operated with satisfaction
through the years despite an unstable curve characteristic.

Pump Noise

Noise generation becomes a subject of increasing concern with the in-
creased power densities associated with sometimes very compact high-
speed pumps. Investigation has shown that noise generation traces to hy-
draulic origins, and more specifically is at blade pass frequency in RE.
pumps. The underlying noise generating mechanism lies with pro-
nounced flow stratification within the impeller, so that flow jets issue
from the pressure side of each blade at the impeller exit. These jets im-
pinge on the geometric anomalies in the discharge vicinity and produce
pressure pulses which set the container walls in motion, which in turn
broadcast airborne noise. Noise also propagates through metallic and
fluid paths making noise control by means of lagging or enclosures ex-
tremely unattractive because the entire pumping system must be treated,
including the pump, driver, base, and piping system.
High-solidity impellers have provided noise reduction typically on the
order of 10 dbA, which viewed in different contexts translates into 90%
sound power reduction but is perceived by the human ear as being half as
loud. Roughly 5 dbA additional noise reduction is available by increasing
the case size relative to the impeller size with virtually no sacrifice in
efficiency. Industrial and governmental noise standards can always be
met or exceeded with high-solidity impellers, whereas their low-solidity
counterparts are sometimes marginal in this regard.
It should be noted that proper volute sizing is a prerequisite to achiev-
ing either the efficiency gains available with volute collectors or the noise
and curve shape advantages associated with high-solidity impellers. Ex-
perience has taught that the cross-sectional area swept by the volute
should be about 15% to 20% greater than the discharge throat area.
The photograph of Figure 11-11 compares low- and high-solidity par-
tial emission pump impellers equipped with inducers. Advantages associ-
ated with the high-solidity impellers will result in this type supplanting in
large measure their low solidity counterparts.

Design Configuration Options

High-speed partial emission pumps are well suited to provide high to
very high heads. Inducers have augmented suction performance so that a