Resolve 60 MW LMZ Steam Turbine Trip Issue During Cooling Water Pump Changeover

Abstract

Steam turbine reliability depends upon the combined stability of the steam path, condenser system, lubrication system, rotor-bearing dynamics, and protection system. A trip of a 60 MW condensing steam turbine occurred during a routine cooling water (CW) pump changeover operation. Initial investigation identified condenser vacuum may deterioration following temporary reduction in cooling water flow. However, detailed analysis of turbine vibration trends revealed a delayed increase in rear bearing vibrationreaching 4.5 mm/s approximately three minutes after before unit trip. Subsequent inspection during outage revealed excessive top oil clearance in the rear journal bearing and a 0.46 mm shim installed at the bearing parting plane. The investigation concluded that the condenser disturbance acted as an initiating event, while degraded bearing dynamic characteristics amplified the rotor response, leading to turbine protection operation.

Introduction

This case study investigates a 60 MW LMZ K-60-9.3 steam turbine trip caused by excessive vibration in the rear journal bearing during a routine condenser cooling water (CW) pump transfer operation. Although condenser disturbances are often associated with vacuum loss, the investigation revealed that the trip was primarily caused by an underlying mechanical weakness in the bearing system that was exposed by a temporary process disturbance.

Under normal operation, the turbine was running at approximately 58 MW with stable condenser vacuum and normal vibration levels. During the transfer of cooling duty from operating CW pumps to a standby pump, rear bearing vibration increased to 4.5 mm/s after a delay of about three minutes. The delayed response indicated that the vibration was not directly caused by pump switching but resulted from secondary dynamic effects.

Inspection of Bearing No. 2 revealed significant mechanical damage, including babbitt material dislodgement, rubbing marks, dents, and journal scoring. Measurements showed that the top oil clearance was considerably higher than the design value, indicating excessive bearing clearance. A 0.46 mm shim was also found on the bearing parting plane, which altered bearing geometry and increased effective clearance. Additional observations included improper RTD positioning and an oversized bearing lock.

The study explains that journal bearings support the rotor through a pressurized oil film that provides stiffness and damping. Excessive bearing clearance reduces these stabilizing properties, making the rotor more sensitive to load variations, thermal distortion, and pressure fluctuations. The presence of the shim further reduced oil film stiffness, caused uneven load distribution, and lowered the rotor-bearing system's stability margin.

Root cause analysis identified:

• Immediate Cause: Temporary reduction in cooling water flow during pump transfer.
• Mechanical Cause: Excessive rear bearing clearance.
• Latent Cause: Shim-induced alteration of bearing geometry.
• Fundamental Root Cause: Reduced rotor-bearing dynamic stability due to degraded bearing condition.

The three-minute delay before vibration escalation was attributed to gradual thermal effects, changes in oil film characteristics, rotor centerline movement, and eventual crossing of the bearing stability threshold. As condenser pressure and thermal loading increased, the weakened bearing could no longer maintain stable rotor support, resulting in rapid vibration growth.

Corrective actions included replacing the damaged bearing with a refurbished spare, restoring proper oil clearance, polishing the journal surface, and machining the bearing lock to the correct dimensions.

The study concludes that the turbine trip was not caused solely by the condenser disturbance but by the interaction between a temporary process upset and a pre-existing bearing defect. It highlights the importance of maintaining proper bearing geometry and clearances to ensure rotor stability and prevent failures such as bearing wiping, journal scoring, oil whip, rotor rubbing, and turbine trips.

Conclusion

The cooling water pump changeover initiated resulting rotor dynamic loading acted upon a rear bearing already operating with excessive clearance and modified geometry due to a 0.46 mm parting-plane shim. The degraded bearing condition amplified rotor motion, causing vibration to increase to 4.5 mm/s approximately three minutes after the condenser disturbance. The event demonstrates how latent mechanical weaknesses can transform a manageable process upset into a turbine trip.

References

[1] LMZ manual [2] Thermo-Mechanical Effect on Rotating Machinery ASME Journal [3] Field Diagnostic Reports available with us [4] 4.OEM Technical Advisories on Thermal Distortion Management

Copyright

Copyright © 2026 Bijay Kumar. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.