Thermal Power Cases

Event: A steam turbine unit at a thermal power plant started on September 24. On September 28, shaft vibration at Bearing No. 1 suddenly increased and began fluctuating, triggering an abnormal alarm. Site personnel requested diagnosis and recommendations.

Shaft Vibration Trend at Measurement Point 1X

Shaft Vibration Trend at Measurement Point 1Y

Diagnostic conclusion:：
The bearing vibration trend increased significantly. Multiple spectra showed that when vibration at turbine measurement points 1X and 1Y rose, the dominant energy was at 50 Hz, primarily the 1× rotational component. Components at 0.5×, 2×, 3× and 4× were also present at lower amplitudes. The shaft orbit was approximately elliptical; the 1× phase varied with vibration amplitude; the vibration trend fluctuated without a clear pattern; and bearing housing vibration showed no significant change. The root cause was therefore assessed as abnormal changes in the clearance between the rotor and bearing pads.

Recommendations：
1. Focus on all potential rotor–stator clearance locations, including steam seals at each stage, oil deflectors and diaphragm seals,
looking for fresh polished rub marks, metal powder or other signs of wear.
2. Inspect the corresponding rotor surfaces.
3. Check bearing clearances according to the applicable procedures to rule out oil-film problems.

Site verification：
A detailed inspection found slight deposits and minor deformation in stationary parts of the steam seals and oil deflectors, resulting in insufficient local rotor–stator clearance. After correction, the unit was restarted and connected to the grid. Shaft vibration at Bearing No. 1 stabilized within the normal operating range, irregular fluctuations disappeared completely, and 1×/harmonic vibration energy and phase changes returned to stable levels. Equipment operation returned to good condition.