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

432 Centrifugal Pumps: Design and Application
Figure 18-4. Noise spectra of cavitation in centrifugal pump.
times revolutions per second) and multiples of it predominated. Cavita
tion noise of this type usually produces very high frequency noise, best
described as "crackling."
Flashing is particularly common in hot water systems (feedwater pump
systems) when the hot, pressurized water experiences a decrease in pressure
through a restriction (i.e., flow control valve). This reduction of
pressure allows the liquid to suddenly vaporize, or flash, which results in
a noise similar to cavitation. To avoid flashing after a restriction, sufficient
back pressure should be provided. Alternately, the restriction could
be located at the end of the line so that the flashing energy can dissipate
into a larger volume.
Flow Turbulence. Pump generated dynamic pressure sources include
turbulence (vortices or wakes) produced in the clearance space between
impeller vane tips and the stationary diffuser or volute lips. Dynamic
pressure fluctuations or pulsations produced in this manner can cause impeller
vibrations or can result in shaft vibrations as the pressure pulses
impinge on the impeller.
Flow past an obstruction or restriction in the piping may produce turbulence
or flow-induced pulsations [2]. These pulsations may produce
both noise and vibration over a wide-frequency band. The frequencies
are related to the flow velocity and geometry of the obstruction. These
pulsations may cause a resonant interaction with other parts of the acoustic
piping system.
Most of these unstable flow patterns are produced by shearing at the
boundary between a high-velocity and low-velocity region in a fluid