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

Vibration and Noise in Pumps 423
mechanisms and common noise conduction paths by which noise can be
transmitted. If noise itself is the major concern, it can be controlled by
acoustic enclosures or other treatment [1, 2|.
Mechanical Noise Sources
Sources of Pump Noise
Common mechanical sources that may produce noise include vibrating
pump components or surfaces because of the pressure variations that are
generated in the liquid or air. Impeller or seal rubs, defective or damaged
bearings, vibrating pipe walls, and unbalanced rotors are examples of
mechanical sources.
In centrifugal machines, improper installation of couplings often
causes mechanical noise at twice pump speed (misalignment). If pump
speed is near or passes through the lateral critical speed, noise can be
generated by high vibrations resulting from imbalance or by the rubbing
of bearings, seals, or impellers. If rubbing occurs, it may be characterized
by a high-pitched squeal. Windage noises may be generated by motor
fans, shaft keys, and coupling bolts. Damaged rolling element bearings
produce high-frequency noise [3] related to the bearing geometry
and speed,
Liquid Noise Sources
These are pressure fluctuations produced directly by liquid motion.
Liquid noise can be produced by vortex formation in high-velocity flow
(turbulence), pulsations, cavitation, flashing, water hammer, flow separation,
and impeller interaction with the pump cutwater. The resulting
pressure pulsations and flow modulations may produce either a discrete
or broad-band frequency component. If the generated frequencies excite
any part of the structure including the piping or the pump into mechanical
vibration, then noise may be radiated into the environment. Four
types of pulsation sources occur commonly in centrifugal pumps [2]:
• Discrete-frequency components generated by the pump impeller such
as vane passing frequency and multiples.
• Flow-induced pulsation caused by turbulence such as flow past restrictions
and side branches in the piping system.
• Broad-band turbulent energy resulting from high flow velocities.
• Intermittent bursts of broad-band energy caused by cavitation, flashing.,
and water hammer.