Page 159 - Centrifugal Pumps Design and Application

Vertical Pumps 137

crating parameters and the installation, including the rigidity of the sup-
porting structure and attached piping, have a strong influence. The latter
may cause vibration from sources such as hydraulic resonance in piping,
turbulence at the pump intake, cavitation problems, low flow recircula-
tion, and structural resonance in the pump/driver assembly.
The availability of data collectors and matching computer hardware
and software has greatly facilitated collecting and analyzing vibration
data. The establishment of vibration signatures is not only a means of
verifying the satisfactory condition of a new installation, but can also
serve as the basis for scheduling pump maintenance.
Measurement of axial and lateral vibration on the pump and driver is
measured either as absolute movement on driver bearing housings or as
relative movement between the shaft and the bearing housing or pump
structure. Torsional vibration is seldom a problem in vertical pumps be-
cause the exciting force generated by the rotating impeller vanes passing
the stationary bowl vanes is small. However, for applications where right
angle gears and engines are used, exciting forces can be generated that
may cause damaging torsional vibration. When these types of drivers are
used, an analysis for torsional critical frequencies should be performed at
the design stage. The computer models for performing these analyses are
quite accurate and give good results. This subject is discussed in more
detail in Chapter 18.
Figure 9-16 shows the desired locations for taking vibration measure-
ments. The axial reading is taken as an absolute measurement directly on
the motor thrust bearing housing. The lateral readings on the motor and
discharge head are also taken as absolute measurements and should be
taken in line with, and at right angle to, the discharge nozzle. Measure-
ments on the shaft should be taken as relative measurements, 90° apart,
just above the stuffing box.
Absolute vibration measurements are taken with velocity transducers
or accelerometers. Accelerometers should either be permanently at-
tached or attached with a magnetic base, while velocity transducers can
be handheld. Velocity transducers and accelerometers are directional and
must be installed with the base perpendicular to the desired direction of
measurement. Relative vibration measurements are taken with proximity
transducers. It should be noted that proximity transducers, due to their
working principle, are sensitive to shaft material properties as well as
surface finishes.
Because of the wide varieties and sizes of vertical pumps in use, the
issue of acceptable vibration levels becomes rather complicated. How-
ever, both the Hydraulic Institute and the American Petroleum Institute
have published acceptance criteria, specifically applicable to pumps,
covering both overall vibration levels and filtered vibration, i.e., read-