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

Vibration and Noise in Pumps 429
other miscellaneous particles which, when trapped inside the bearing,
may cause wear or permanently indent the balls and raceways under the
tremendous stresses generated by the operating load.
Special purpose pumps and large boiler feed pumps commonly have oil
film (hydrodynamic) bearings. The hydrodynamic bearing is superior to
rolling element bearings for high speed or high load application. The hydrodynamic
bearing supports the rotor on a film of oil as it rotates. The
geometry of the hydrodynamic bearing and the oil properties play an important
role in controlling the lateral critical speeds and consequently the
vibrational characteristics of the purnp.
Seals. The fluid dynamics of flow through seals have a dramatic effect
on rotordynamics [9]. Hydrodynamic forces involved may contribute to
the stabilization of rotating machinery or make it unstable. Seals with
large axial flow in the turbulent range, such as in feed water pumps, tend
to produce large stiffness and damping coefficients that are beneficial to
rotor vibrations and stability. Wear of the seals will increase the clearance
and cause greater leakage and possibly change the rotordynamic characteristics
of the seal resulting in increased vibrations.
Hydraulic Effects
Hydraulic effects and pulsations can result in almost any frequency of
vibration of the pump or piping from once per revolution up to the vane
passing frequency and its harmonics. Frequencies below running speed
can be caused by acoustical resonances. Generally, these effects are due
to the impeller passing the discharge diffuser or some other discontinuity
in the case. Any nonsymmetry of the internals of the pump may produce
an uneven pressure distribution that can result in forces applied to the
rotor.
Transients. Starting and stopping pumps with the attendant opening and
closing of valves is a major cause of severe transients in piping systems.
The resulting pressure surge, referred to as water hammer, can apply a
sudden impact force to the pump, its internals, and the piping. Severe
water hammer has caused cracks in concrete structures to which the pipe
was anchored.
Rapid closure of conventional valves used in feedwater lines can cause
severe water hammer. Increasing the closure time of the valve can reduce
the severity of the surge pressure. Analytical methods are available to
evaluate the severity of water hammer in a particular piping configuration
for various closure rates [10].