Turbine
and Compressor Vibrations J. C. Wachel, CEP,
Vol. 15, Technical Manual published by American Institute of
Chemical Engineers, New York, 1973, pp. 69-76 .
The use of high speed turbine and compressor strings in chemical
process plants has increased interest in vibration monitoring and
control since the outage of any piece of equipment can cause considerable
downtime of the process. With downtime sometimes costing up to $100,000
a day, the economic impact can be overwhelming.
The increased
complexity of processes many times prevents the complete simulation
and testing of the dynamic vibratory characteristics of turbines
and compressors in the shop acceptance tests, since it is usually
not possible to simulate all design parameters such as pressures,
temperatures, process gas, etc., in a manufacturer’s facility.
In many cases, the unit startup is the first opportunity to check
out extrapolated design features. Many troubles occur during startup
because of this. Increased surveillance during plant startups
is, therefore, a necessity.
This article
will present vibration data measured on a steam turbine and syn
gas compressor rated at 10,600 rev/min. No problems arose during
the shop tests of these machines; however, when installed, several
types of vibration problems occurred during startup and operation.
Vibrations were caused by: 1) unbalance, 2) misalignment, 3) mechanical
resonances of attached structural components, 4) running near
critical speeds, 5) sleeves that shifted during operation, 6)
instabilities which excited subharmonic frequencies, and 7) intermittent
faulty operation of governor control and overspeed trip. The symptoms
and dynamic characteristics of vibrations, such as those caused
by unbalance, misalignment, mechanical resonances, and critical
speeds are well known, and rather simple frequency analysis techniques
are sufficient to define them. However, detailed spectral information
and analyses of destructive instabilities that occurred in the
compressor have not been previously reported. This information
is valuable since it shows that the use of tilted pad bearings
is not sufficient to prevent subharmonic instabilities at the
shaft criticals. Additional detailed spectral information is also
presented for the first time on simultaneous subharmonic excitation
of the two lowest shaft criticals on the turbine. Of prime importance
is a time history of an overspeed failure which was actually recorded.
This data shows how quickly failures can occur and indicates the
need for additional safeguards in speed control circuitry.
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