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

474 Centrifugal Pumps: Design and Application
« Direction of impact
» Interface material
• Impact velocity
» Contact time
* Impact location
The objective of the testing should be to identify structural resonant
frequencies that occur within the machinery operating frequency range.
In a complex system, there are many natural frequencies. Consequently,
it may be difficult to identify the individual natural frequencies because
the vibrations can be transmitted between the different elements (pump
rotor, case, and support structure). The impact technique is useful for
separating the structural natural frequencies because each element can be
impacted individually. Individual resonances can generally be identified
by analyzing the vibrational mode shapes and comparing the transfer
function amplitudes.
The transfer function amplitude is an indication of the relative stiffness
of a structure. For example, a rigid structure that is difficult to excite or
resonate would have a low amplitude transfer function (response/force).
A highly responsive structure would have a higher amplitude transfer
function because less force is required to obtain the response. This technique
was used to determine the bearing housing resonance of a pump
that is described in the case histories that follow.
Various types of data presentation formats are in common use and are
described in the case histories. The data format is dependent upon the
type of instrumentation available; however, for convenience of comparison
to criteria or specifications, some formats may be more convenient
than, others.
Troubleshooting
As examples of the types of diagnostic procedures that are used to identify
the causes of some typical pump problems, several field case histories
will be presented.
Bearing Housing Resonance of Feed Charge Pumps. High vibration
amplitudes and seal failures occurred on one of a pair of centrifugal
pumps that are the feed charge pumps for a cat feed hydro-treater plant.
The pumps were driven by electric motors at a speed of approximately
3,587 rpm (59.8 Hz). The pumps have five impellers; the first two impellers
have six vanes and the last three impellers have seven vanes.