For different types of steam turbine generating units, the shapes of the dynamic and static components are complex and variable, and their deformation (amplitude) cannot be described by a simple dynamic stress function.
Consequently, it is incorrect to determine the fatigue of these components based on vibration amplitude alone.
Despite this, some insights can be gained from statistical analysis of a large number of unit operational experiences.
1. Meaning of excessive vibration
With regard to the bearing amplitude of the steam turbine generator, there are three meanings, as follows:
1. More than 50 μm and less than 120 μm.
It is common locally for the unit to vibrate excessively at rated speed.
2. More than 120 μm and less than 300 μm.
For units with a rated speed of 3,000 RPM, prolonged operation at working speed may cause excessive vibration.
Some units may experience frequent starts and stops when exceeding critical rotor speed.
3. More than 300μm.
If the vibration is caused by low-frequency oil film oscillation and subharmonic resonance in the generator bearing, it may not cause significant damage in the short term.
However, if it is a fundamental frequency vibration, it can cause serious and harmful accidents at the rated speed, such as significant shaft bending and shaft damage.
If vibration occurs at the critical speed of the turbine's high-pressure rotor, it may also result in shaft bending accidents.
Furthermore, if vibration occurs at the critical speed of the low-speed generator rotor, even if there is no visible damage to the generator, the unbalanced rotor can lead to increased vibration of the high-pressure rotor at the critical speed. , causing and exacerbating shaft friction and ultimately inducing a shaft bending accident.
These three types of vibrations are generally called excessive vibration, strong vibration, and large vibration, respectively.
In terms of component fatigue caused by vibration, only excessive vibrations and strong vibrations are relevant, since large vibrations cannot occur during prolonged operation of the unit.
If the vibration is at the fundamental frequency, even if it is only present for a short period of time, it can result in a serious vibrational accident before component fatigue occurs.
2. Relationship between amplitude value and component fatigue
According to statistics collected from extensive experience operating units over a long period of time, component fatigue due to vibration is directly related to the amplitude value. The following rules have been established:
1. For any unit component, regardless of shape or structure, when operating at a speed of 3000 r/min, if the vibration frequency is less than or equal to the fundamental frequency and the maximum amplitude of the bearings or components in three directions is less at 120 micrometers, these components will not suffer fatigue damage during long-term operation.
2. For parts with high rigidity such as bearing pedestals, if the maximum amplitude in one direction exceeds 150 micrometers, fatigue damage will occur at joints with other parts during long-term operation, such as bearing pedestal fixing screws and secondary grouting.
3. For parts with low rigidity, such as the end cover of a generator mirror where the tube is not directly connected to the bearing seat, if vibration in one direction exceeds 250 micrometers, fatigue damage will occur at the connection between these parts and parts. with high rigidity after long-term operation, such as the connection between the tube and the bearing seat.
