Restraining Torsional Vibration
All rotating equipment power trains found in a power plant have some amount of vibration, usually caused by mechanical unbalance of the rotating system, shaft misalignment, or weakness in the bearing support. Characteristic vibration is also experienced in one or more of four modes: sideways, horizontal, axial, or torsional.
The first three vibration levels are typically measured by either displacement, velocity (the speed of the motion), or acceleration. Torsional modes describe the twisting and untwisting of the power train shaft and are the most difficult vibration problem for which to identify the vibration’s source and solution (Figure 1).
1. Twist equal to strain. The typical method of measuring torsional vibration in a power train shaft is to use strain gauges. Courtesy: GE Energy
Torsional Vibration Is Difficult to Control
Each mode of vibration can be magnified by an exciting force (usually the rotating velocity of the power train or a multiple of that velocity) that resonates with the natural frequency of the support structure, which includes the bearing supports or support structure defect. In power trains with a generator, an exciting force has even been identified as electromagnetic torque caused by a bad air gap.
The torsional resonance frequency of any power train is a strong function of the component dimensions (such as shaft diameter and length) and material properties. Selecting these design parameters also defines the component stiffness, inertia, and other important characteristics important to a well-behaved power train.
Normally, the exciting frequency can’t be changed (it’s a function of the operating frequency of normally 50 Hz or 60 Hz), but the natural response frequency of the structure or power train can be altered by either stiffening (which raises the frequency) or by making it more flexible (which lowers the frequency). Unfortunately, those options are not available for resolving torsional resonance problems; changing shaft stiffness normally requires changing the diameter or length of the shaft.
High-inertia power trains are very susceptible to torsional vibration. High-inertia power trains don’t want to shift their angular position in the bearing, but the oscillating torque placed on the shaft by the torsional vibration does. This rapid flexing of the power train causes high fatigue cyclic stresses that can result in a mechanical failure, many times without warning. Torsional resonance with the oscillating frequency will accelerate the time to failure.
The torsional dynamics of a turbine-generator power train have very little damping as compared to the lateral dynamics. This factor amplifies the impact of a stimulus or exciting force in line with a natural frequency; however, a response will not be seen unless the two are very nearly equal. Torsional stimuli are seen at one and two times the electrical line frequency, as it generally arises from continuous, low-level, harmonic excitation and can also (rarely) be seen as high-level, transient excitation. Both low-level and high-level stimuli can result in catastrophic failure, such as liberating rotating components (for example, last-stage turbine buckets). This component liberation causes very large lateral unbalance that can further damage the turbine-generator set.
New production turbine-generator trains have system dynamics designed with the latest analysis tools to avoid torsional stimulus frequencies. However, in the past this capability was not available and some power trains were designed with torsional natural frequencies very near twice (2x) line frequency. These units may have operated for many years without incident; nevertheless, the risk would be that any modifications to the rotor train could shift a close natural frequency right onto a stimulus frequency.
There are a variety of aftermarket modifications that could significantly affect torsional dynamics, including rotating exciter to static exciter conversion, replacement generator or turbine, replacement rotor or field, replacement buckets, non-OEM replacement components, and many others.
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