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Practical Solutions to Machinery and Maintenance Vibration Problems

Chapter 9, Fluid Flow

Section 1, Cavitation

On a centrifugal pump, cavitation is caused by a lack of Net Positive Suction Head (NPSH). It can also be induced by turbulent (non-laminar) flow conditions caused by sharp pipe bends immediately upstream of the pump suction. To prevent turbulence induced cavitation, it is recommended that a minimum of 6-10 pipe diameters of straight pipe be installed immediately upstream of the pump suction.

The source of the cavitation induced vibration is the tiny implosion caused by pockets of steam condensing into considerably smaller droplets of water (or other liquid). Except for the volume of the tiny droplets, the space the steam occupied becomes a vacuum. This causes the liquid droplets to explode into a burst of steam followed by another condensation, implosion-explosion, etc. Though small in size, the continuous implosions and explosions impinge on the pump casing and impeller. These random impacts are usually located at the suction side of the pump, and the amplitudes of vibration do not usually exceed alert values unless resonance is involved (when peaks may reach 0.6-0.7 in/sec). Although the amplitudes are usually relatively small, cavitation is extremely destructive and causes erosion of the various components. It can also cause axial surging which, if excessive, will damage both bearings and seals. The spectra produced have a very wide frequency range and low amplitude. There are usually no sharp peaks, but instead, peaks that look like haystacks.

Cavitation produces very erratic and varied frequencies. Therefore, if a FFT analyzer is used, its effect on the vibration spectrum can be reduced by increasing the number of averages. Synchronous time averaging will also remove the cavitation-related portions from the spectrum.

The highest amplitudes are most often found in the suction area of the pump and are often highest in the axial direction. One frequency that does not usually show up in cases of cavitation alone is vane passing frequency or number of vanes x rpm.

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