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

Vibration and Noise in Pumps 433
field. Typical examples of this type of turbulence include flow around
obstructions or past deadwater regions (i.e., a closed bypass line) or by
bi-directional flow. The shearing action produces vortices, or eddies that
are converted to pressure perturbations at the pipe wall that may result in
localized vibration excitation of the piping or pump components. The
acoustic natural response modes of the piping system and the location of
the turbulence has a strong influence on the frequency and amplitude of
this vortex shedding. Experimental measurements have shown that vortex
flow is more severe when a system acoustic resonance coincides with
the generation frequency of the source. The vortices produce broad-band
turbulent energy centered around a frequency that can be determined
with a dimensionless Strouhal number (S n ) from 0.2 to 0.5, where
where
f = vortex frequency, Hz
S n = Strouhal number, dimensionless (0.2 to 0.5)
V = flow velocity in the pipe, ft/sec
D = a characteristic dimension of the obstruction, ft
For flow past tubes, D is the tube diameter, and for excitation by flow
past a branch pipe, D is the inside diameter of the branch pipe. The basic
Strouhal equation is further defined in Table 18-1, items 2A and 2B. As
an example, flow at 100 ft/sec past a 6-inch diameter stub line would
produce broad-band turbulence at frequencies from 40 to 100 Hz. If the
stub were acoustically resonant to a frequency in that range, large pulsation
amplitudes could result.
Pressure regulators or flow control valves may produce noise associated
with both turbulence and flow separation. These valves, when operating
with a severe pressure drop, have high-flow velocities which generate
significant turbulence. Although the generated noise spectrum is very
broad-band, it is characteristically centered around a frequency corresponding
to a Strouhal number of approximately 0.2.
Pulsations. Pumping systems produce dynamic pressure variations or
pulsations through normal pumping action. Common sources of pulsations
occur from mechanisms within the pump. The pulsation amplitudes
in a centrifugal pump are generated by the turbulent energy that depends
upon the clearance space between impeller vane tips and the stationary
diffuser or volute lips, the installed clearances of seal, wear rings and the
symmetry of the pump rotor and case. Because these dimensions are not
accurately known, predicting the pulsation amplitudes is difficult. Even
identical pumps often have different pressure pulsation amplitudes.