Torsional
Vibration Analysis J. C. Wachel/F. R. Szenasi, 22nd
Turbomachinery Symposium, Texas A&M University, September
13-16, 1993.
All rotating machinery systems experience torsional oscillations
to some degree during startup, shutdown, and continuous operation.
Consequently, the torsional response characteristics of rotating
and reciprocating equipment should be analyzed and evaluated to
ensure the system’s reliability. Severe torsional vibrations
often occur with the only indication of a problem being gear noise
or coupling wear. Excessive torsional vibrations can result in gear
wear, gear tooth failures, key failures, shrink fit slippage, and
broken shafts in severe cases.
The torsional
vibration response of rotating machinery components is an important
consideration in defining the operational reliability of a rotary
equipment train. Accurate response prediction requires analysis
techniques which consider all forcing functions in the system
in addition to the mass-elastic properties of the shafts, couplings,
gears, impellers, etc.
The severity
of the torsional oscillations and stresses depends upon the relationship
between the operating speed and excitation frequencies of unsteady
torques and the torsional natural frequencies and mode shapes
of the shaft system (critical speeds). The difference between
these frequencies is referred to as the separation margin. The
magnitude of the stress also depends upon the amplification factor
on resonance (damping) and the stress concentration factors.
The API Codes
for Turbines, Compressors, and Pumps (611, 617, 610) specify that
the torsional modes of the complete unit should be at least 10
percent below any operating speed or at least 10 percent above
the trip speed. In some systems, multiples of operating speed
and blade passing frequencies can excite torsional natural frequencies.
These potential problems should be addressed in purchase specifications
to avoid conflicts. An additional margin of at least five percent
should be allowed for calculation inaccuracy in torsional analyses.
Variable frequency drive motors typically operate from 20 percent
to 100 percent of the motor design speed, making it virtually
impossible to meet the API specification. Other types of drivers
with wide speed ranges may have similar problems. In these cases
where detuning is not practical, more detail analyses with particular
emphasis on accurate stress prediction is essential to determine
reliability.
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