Page 143 - Improving Machinery Reliability

Machinery Reliability Audits arid Reviews 115

from shaker tests or by modal analysis methods. The modal analysis technique uses a
two-channel analyzer and an impact hammer and accelerometer to determine the nat-
ural frequencies and mode shapes. For example, the natural frequencies and mode
shapes of a centrifugal impeller were measured using modal analysis techniques
(Figure 3-27). When these frequencies were compared to values determined from a
shaker study, good correlation was obtained. The mode shape for the two-diameter
mode is given in Figure 3-28.
An interference diagram for this impeller is given in Figure 3-29. Note that poten-
tial excitation mechanisms include vane passage frequency (1 5X) and two times
vane passage frequency (3OX).
It is sometimes impossible to completely avoid all interferences over a wide speed
range, since there are so many natural frequencies. For most systems, in order for a
failure to QCCU~, several things usually occur together. First, there must be a mechan-
ical natural frequency. Second, there must be a definite excitation frequency, such as
vane passing or diffuser vane frequency. Third, there must be some acoustical reso-
nant frequency that amplifies the energy generated; and fourth, there must be the
appropriate phase relationship that causes the pulsation to cause a shaking force on
the impeller or blade. The best way to avoid such problems is to avoid coincidence
of the resonances with the excitation mechanisms.

Figure 3-27. Natural frequencies of centrifugal impeller.