How Much Vibration Will Cause Fatigue Damage to Components?

For various types of steam turbine generator units, the structural shapes of dynamic and static components are complex and changeable, and their amplitude (deformation) is not a single value function of dynamic stress.

Therefore, it is obviously unscientific to make a conclusion on fatigue of these components only based on the vibration amplitude.

However, the following experiences can be obtained from the statistical summary of a large number of unit operation experiences.

1. Meaning of excessive vibration

As far as the bearing amplitude of the steam turbine generator is concerned, there are three meanings as follows:

1. More than 50 μm and less than 120 μm.

It is common on-site that the unit vibrates excessively at the rated speed.

2. More than 120 μm and less than 300 μm.

For the unit with a rated speed of 3000r/min, the vibration cannot be operated for a long time under the working speed.

Some units start and stop frequently when passing the critical speed of the rotor.

3. More than 300 μm.

If the vibration is low-frequency oil film oscillation and subharmonic resonance, which occurs on the generator bearing, it will not cause obvious damage in a short time.

However, if it is a fundamental frequency vibration, it will cause serious malignant accidents at the rated speed, such as major shaft bending and shafting damage accidents;

If it occurs at the critical speed of the high-pressure rotor of the turbine, it will also cause the shaft bending accident;

If it occurs at the critical speed of the generator rotor with low speed, even though there is no obvious damage to the generator, the unbalance of the generator rotor will cause a significant increase in the vibration of the turbine high-pressure rotor at the critical speed, causing and aggravating the shaft rubbing, and inducing the shaft bending accident.

The above three kinds of vibrations are summarized in terms of excessive vibration, strong vibration and large vibration respectively.

As far as the vibration causing component fatigue is concerned, it obviously only includes excessive vibration and strong vibration, because large vibration cannot exist in the long-term operation of the unit.

If this vibration is the fundamental frequency, even if it occurs for a short time, a major vibration accident will occur before component fatigue occurs.

How Much Vibration Will Cause Fatigue Damage to Components? 1

2. Relation between amplitude value and component fatigue

According to the statistics of a large number of unit operation experiences for a long time, the component fatigue caused by vibration is directly related to the amplitude value, and the rules are as follows:

1. For any component of the unit, regardless of its shape and structure, for the unit with working speed of 3000 r/min, when the vibration frequency is less than or equal to the fundamental frequency, and the maximum amplitude of bearings or components in three directions is less than 120 um, these components will not suffer fatigue damage in long-term operation.

2. For parts with high stiffness, such as bearing pedestal, when the maximum amplitude in one direction exceeds 150 um, fatigue damage will occur at the joints with other parts during long-term operation, such as bearing pedestal fixing screws, secondary grouting, etc.

3. For some parts with low stiffness, such as the generator mirror end cover where the pipe is not directly connected to the bearing seat, when the vibration in one direction exceeds 250 um, fatigue damage will occur to the connection between these parts and the parts with high stiffness after the long-term operation, such as the connection between the pipe and the bearing seat.

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