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

Vibration and Noise in Pumps 441
natural frequencies. Consequently, pulsation amplitudes are usually affected
by changes in the operating conditions or fluid composition.
Pulsations are commonly initiated by flow turbulence at changes in
flow cross section, at restrictions (orifices, valves, etc.) or at the pump
impeller. When the frequency of this turbulent energy excites one of the
acoustic resonances of the piping system, and if the pump is situated near
a velocity maximum in the resonant piping system, then high amplitude,
self-sustaining pulsations can result. Pulsations can be minimized by
moving the pump to a velocity minimum, but often a new pulsation frequency
will be generated such that the pump is again situated near a velocity
maximum for a higher mode oscillation. With proper care and detailed
analysis of the relative strength of the various pulsation resonance
modes of the piping, piping designs can be developed to avoid these
strong resonances. Controlling piping stub lengths (i.e., in by-pass piping)
so that their quarter-wave stub resonances are far removed from the
Strouhal excitation frequencies, will also help in minimizing the potential
for resonant pulsations.
This type of instability is more probable at low flows because acoustic
damping that is generated by flow friction effects is greater at higher
flow rates.
Design/Manufacturing
Dynamic response of the pump components to normal exciting forces
within its operating frequency range can result in problems from excessive
maintenance to catastrophic failure. Improper manufacture or assembly
can cause unbalance resulting in damaging vibrations if the speed
approaches natural frequencies of the rotor system. A rotordynamic audit
[18] of the pump rotor design is crucial in avoiding speed-related vibrations.
The following section discusses the factors that are important in controlling
the rotordynamic responses of a centrifugal pump.
Rotordynamic Analysis
A lateral critical speed is defined by API [7] as the speed at which a
peak vibrational response occurs. At the critical speed, the rotor is more
sensitive to unbalance than at any other speed. The critical speeds of a
pump should be avoided to maintain acceptable vibration amplitudes.
This section discusses the techniques involved in calculating the lateral
critical speeds of centrifugal pumps.