PSI - Issue 57

Malik Spahic et al. / Procedia Structural Integrity 57 (2024) 833–847 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

847

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Table 2: Comparison of the state of the rotor before and after machining. Without machining, there is a high probability that the rotor will have a crack before the next overhaul in 2030. Forecast 2030 with machining Forecast 2030 without machining Lifetime consumption (in %) - HP 1 st row of Rotor C 70 128 6. Conclusion This article has presented a thermo-mechanical crack risk assessment from an end user’s perspective, adopting a fleet risk screening involving many different aspects of design, operations, maintenance and protection. Following the high-level fleet screening, a more detailed risk assessment per unit was performed for the units at risk based on a lifetime calculation using a combination of 2D finite element modelling and a more simplified yet calibrated 1D model enabling calculation over the full operational history. The latter can reveal flexibility improvements, but also issues during the start/shutdown procedure, which consume a lot of lifetime and that would otherwise be left unnoticed. Based on the calculated lifetime consumption, a couple of risk mitigation actions were described which have already been implemented within the ENGIE fleet. Their main goal is to avoid unexpected cracking and associated downtime of the steam turbine rotor. As a steam turbine rotor is only disassembled once every 5 to 10 years, the lifetime study and potential actions within the major overhaul should be well prepared in advance. Besides inspection, the most promising one is to perform surface machining before the onset of cracking which resets the lifetime consumption to a large extent. Other actions have been presented that can be taken outside of the major overhaul period. This involves reducing the operational gradients and also reviewing the Rotor Stress Evaluator, as it is often found to be malfunctioning and not properly protecting the rotor from excessive thermal stress. Especially older steam turbine units were not designed for the demanding flexible operation they are currently facing. Newer units are typically made of higher strength 10%Cr material steels which outperform the 1%Cr steels in terms of creep and fatigue, thereby reducing the risk for rotor cracking. To conclude, Laborelec experts have developed numerous methods and approaches to prevent and mitigate the apparition of crack on the rotor’s surface. These methods target in priority the existing old 1%Cr units that accumulated a large number of starts and that were not designed to such flexible operation. In some occasions, cracks have already appeared after only 400 starts emphasizing the need for risk mitigation actions for these rotors. Clearly, the design of the new steam rotors needs to take into account the flexibility requirements, in order to reduce needed

repair and down time. Acknowledgements We would like to thank Engie Laborelec for funding the work presented here. References

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