Individual Steam Turbine Had Unique Thermal Memory and Cooldown Pattern

Abstract

Introduction

The catenary design of turbine rotors is critical for smooth operation, especially during hot trip conditions. Deviations from the original catenary value can occur during installation due to bearing pedestal grouting errors, known as seal bore error. Correcting this deviation during final rotor alignment is essential for optimal bearing oil flow and pressure during barring gear operation. If misaligned, higher oil flow and pressure are required, increasing the risk of rotor jamming post hot trip due to thermal changes affecting oil properties.

Key Incidents and Lessons Learned:

• Incident 1 (Feb 2024, Mahan Aluminium): Rotor jammed after a hot trip during barring gear operation due to rotor sagging from oil temperature below 45°C. The rotor resumed operation only after cooling to about 339°C. Lesson: Similar turbines behaved differently despite being the same model.

• Incident 2 (May 2024): Rotor jammed after manual barring gear engagement without monitoring oil temperature. The motor was damaged, and a fatal injury occurred due to mechanical failure. Lesson: Operations team lacked awareness of the critical impact of lubrication oil temperature on rotor behavior.

Thermal Memory in Steam Turbines:

• Definition: Thermal memory refers to residual thermal stresses and distortions in turbine parts from past operational cycles, affecting expansion, bowing, and stress patterns.

• Uniqueness: Each turbine has a unique thermal memory influenced by aging, material fatigue, manufacturing differences, and site-specific conditions like cooling water temperature and load cycles.

Factors Influencing Cool-Down Patterns:

• Variations in material thickness (rotor vs. casing cooling rates)

• Heat transfer dynamics between rotor and casing

• Heat sinks in shaft bearings

• Ambient environmental conditions

• Previous operational load and soak times

• Rotor thermal coefficients

Parameters to Monitor for Thermal Memory & Cool-Down:

• Axial and radial shaft expansion

• Casing temperature and shrinking patterns

• Bearing temperature and oil wedge consistency

• Rotor bow and vibration levels

• Axial thermal gradient along the rotor length

Conclusion

Each turbine possesses a unique thermal memory that significantly influences its cool-down patterns following operation or emergency shutdowns. Ignoring these individual thermal characteristics can lead to severe mechanical issues such as misalignment, rotor jamming, and ultimately mechanical failure. To mitigate these risks, it is mandatory to perform detailed rotor thermal profiling after every emergency trip for each machine. This analysis should focus on developing plant-specific thermal cool-down curves, complementing or refining the original equipment manufacturer (OEM) curves provided during installation. These tailored curves allow for more accurate monitoring and preventive maintenance.

References

[1] BHEL KN-30 OEM manual [2] Thermo-Mechanical Effect on Rotating Machinery ASME Journal [3] Field Diagnostic Reports available with us [4] OEM Technical Advisories on Thermal Distortion Management

Copyright

Copyright © 2025 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.