Some Features Relating to the Occurrence of Low-Frequency Vibration in Large Steam Turbines and Methods for Removing It

The article addresses general matters concerned with the occurrence of low-frequency vibration (LFV) in turbine units. It is pointed out that, despite the level of knowledge that has been achieved in regard to LFV, it still arises from time to time in power plant turbine units. Along with LFV caused by aero- and hydrodynamic excitation, LFV can also bear a subharmonic pattern. It is emphasized that the measures taken to remove LFV depend on the LFV occurrence origin. The article presents LFV occurrence and removal examples, including those relating to the use of honeycomb seals in high-pressure cylinders. With honeycomb seals, decreased sizes of channels and an increased channel component of overshroud forces caused by aerodynamic excitation are typically observed. It is pointed out that, in some cases that involve rotor rubbing against the stator, a multicomponent LFV with subharmonic and self-oscillation components is observed. It is shown that the regulatory documents do not contain criteria for estimating a multicomponent vibration in the low-frequency band. It is pointed out that multicomponent LFV can be a diagnostic indicator pointing to rubbing of the rotor against the babbit or seals. Recommendations on removing LFV of various origins are suggested. A diagnostic table that helps determine factors causing the LFV and that produces recommendations on increasing the turbine units operational reliability is given. It is stated for the first time that the turbine thrust bearing can behave as a source of oil excitation. It is also noted that the conditions under which a self-oscillation type LFV occur and its suppression methods should differ from the methods for suppressing self-excited LFV of a subharmonic nature.