Vibration Signature of Unbalanced Rotors of Centrifugal Pumps

Centrifugal pumps are widely used in many industrial applications. It is necessary
to detect undesired conditions during the operation of it to avoid consequential
damages. Early detection is a vital factor to increase the availability and reliability.
Vibration analysis is an effective and efficient tool in diagnosing rotating
equipment. Vibration signature of unbalanced rotors needs to investigate different
running conditions of the pump. The increase in the vibration level indicates a
probable mechanical failure. Unbalance is one of probable reason of the vibration
in the machines. The unbalanced rotor always produces high vibration level and
excessive loads on the area of the bearing and hence reducing the machine lifetime.
The investigated system monitors a four stages centrifugal pump to expect the
vibration spectrum of the unbalanced rotors. The driving motor runs at 2980 RPM
with rated power 55 kW. This pumping system uses seawater. Comparing the
vibration data of the inspected pump (in normal and faulty conditions) with each
other and vibration limits of ISO10816. The results from this study clarifies the
importance of vibration analysis in detecting the unbalanced rotors. Experimental
results show that primarily 1X shaft running speed with sinusoidal time waveform
can characterize the unbalance in shaft. The time waveform may be pure sine wave
and it may contain noise. To eliminate the noise, the user may use a suitable digital
or analogue filters (i.e. software or hardware). The amplitude of vibration in all
directions for faulty condition is higher than the amplitude value at 1X RPM
frequency in the normal condition. The amplitude at the 1X varies proportional to
the square of the rotational speed. The induced forces due to unbalance on both
bearing of the rotor are always at the same direction where the vibration signal from
them are in phase with eachother