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Practical Solutions to Machinery and Maintenance Vibration Problems

Chapter 9, Fluid Flow

Section 4, Summary

The following highlights should be considered when analyzing fluid-related vibration problems:

a) Cavitation does not usually show vanepass frequency and is highest on the suction side. The amplitude of the broadband-wide frequency vibration rarely exceeds alert values. Resonance peaks associated with this problem are normally not related to rotational frequencies.

b) Recirculation does usually show vanepass frequency and is strongest on the discharge side. The amplitudes often exceed alert values especially when resonance is involved. The random broadband vibration occurs in the lower frequency ranges.

c) Turbulence usually shows as low amplitude, low frequency range broadband noise with the dominant peaks at sub-rotational frequencies.

d) As with all other vibration problems, history is of paramount importance. Whether the vibrating has been apparent since new or whether it only appears under certain operating conditions, it is information that can be vital to a successful analysis.

e) Analysis tips:

(i) Avoid the use of a logarithmic amplitude scale when analyzing pumps, since the apparent increase in the "noise floor" can be confusing.

(ii) Increasing the number of FFT averages or synchronous time averaging will reduce the effect of flow-related problems which may be useful in distinguishing between a late stage bearing defect and turbulence, cavitation or recirculation.

(iii) The use of "high frequency-type bearing detector" units of measurement can assist in diagnosing some flow-related problems.

 

 

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