PSI - Issue 57

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

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An extensive repair program was set up together with the OEM, resulting in a weld repair of the HP inlet groove region. The feasibility of the weld repair was confirmed by mock-up testing, along with a series of non-destructive and destructive material testing. The rotor was unavailable for a full year, but is again operational since then, albeit with a reduced remaining lifetime expectation. Some mitigation actions have been implemented to reduce the risk for renewed cracking: • the inlet groove geometry was optimized in order to reduce the stress concentration • the operational start-up gradients have been reduced to lower the stress levels • borescope inspection is performed on a regular basis to detect the onset of cracking • the RSE was found malfunctioning (wrong rotor temperature initialization) and was corrected.

Figure 7: Stress levels obtained with 1D model at the HP groove over the full operational history show an important increase due to flexibility improvements

3.2. Case study 2

The second unit concerns a more recent example of a drum-type steam turbine rotor with cracks at different locations on the HP rotor as shown on Figure 8. The crack depths were less than 10mm. The unit had accumulated 1200 starts and 100k operating hours.

Figure 8: Shallow cracks at three different locations were found on the HP steam turbine rotor

Based on the geometry, ENGIE Laborelec recommended to inspect the first stage slot bottom which required removal of the first stage blade. A crack of 5mm depth was found. Indication of the location and image of the crack are shown on Figure 9. The root cause analysis pointed towards thermo-mechanical fatigue due to cyclic operation, although a missing drain causing a quenching event after an overhaul accelerated the lifetime consumption as evidenced by surface cracks found on the location of crack 2 (Figure 8) following this event. As for case study 1, the RSE was found to be malfunctioning and hence not properly protecting the rotor against excessive thermal stresses.