Acoustic
Pulsation Problems in Compressors and Pumps
J. C. Wachel, Dara Childs Lecture Series, Texas
A&M University, November 1992 .
It is well recognized in industry that a majority of vibration problems
are induced by internal fluid pulsations. There is sometimes a tendency
to attribute these pulsations solely to the action of the pumps
or compressors within the piping system, and to direct remedial
steps either toward modifying the pump internals or providing more
effective pipe support. While each of these approaches is valid
and useful, an alternate or complementary approach lies in modifying
the pulsation response of the piping. When pulsation frequencies
from a reciprocating pump, for example, coincide with a mechanical
vibration resonance of piping, the process of mismatching the driving
force from the normal vibration response modes is quite effective.
In addition, pulsation resonant frequencies within the piping can
also amplify pulsation effects to the point that mechanical fatigue
failures occur in the piping or pump components. Thus, successful
treatment of resulting vibration problems can often be achieved
merely by mismatching these pulsation resonances from major pump
frequencies, and by providing acoustic filters to attenuate pulsation
amplitudes over a wide range of frequencies. The acoustic filter
approach is particularly applicable to variable speed pump installations
where operational flexibility dictates operation over a range of
speeds and pulsation frequencies.
Similar pulsation
problems exist with centrifugal machinery, but influence of the
piping is often even more important in defining both pulsation
frequencies and amplitudes. Pulsation problems with reciprocating
pumps can now be adequately dealt with, and reliable techniques
for predicting and solving such problems have been previously
described in the technical literature. Therefore, this paper will
deal chiefly with centrifugal pump and compressor pulsation problems.
By far, the
most difficult problem in analytically describing plant dynamics
problems lies in predicting driving forces from the internal flow
or pulsation forces which excite the vibrations. These pulsation
driving forces are a function of both the pulsation response of
the piping system and the source strength and frequencies of the
pulsation generating mechanisms.
|
