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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3. Case studies of cracked steam turbine rotors 3.1. Case study 1

This unit was the first one within ENGIE that showed cracks due to thermo-mechanical fatigue cracking. It concerns a Combined Cycle Gas Turbine (CCGT) unit with 2 gas turbines feeding a single steam turbine. The cracks were detected after inspection following a trip of the unit due to excessive vibrations. As shown on Figure 5, the vibration amplitudes significantly increased in less than one day. Inspection following the forced shutdown revealed multiple cracks in the HIP ST rotor. While the cracks on the IP side were limited in depth, a crack nearly half-way through the rotor was found in the inlet groove of the HP side (Figure 6). By then, the unit had accumulated 1280 starts and only 20k operating hours.

Figure 5: Important increase in the bearing vibration amplitudes of the steam turbine rotor due to rotor cracking

A root cause analysis including a detailed finite element calculation and lifetime assessment over the full operational history of the plant (2006-2019), pointed towards thermo-mechanical fatigue as the root cause along with a malfunctioning of the RSE, failing to protect the rotor from excessive thermal stress.

Figure 6: A crack half-way through the rotor was found in the HP inlet groove

An important increase in the stress levels at the HP inlet groove was found after the implementation of flexibility improvements which had accelerated the failure (Figure 7). The increase in vibration amplitudes over the last days before the trip was attributed to an increased crack propagation rate as, given the significant crack depth, bending stresses started to propagate the crack as well.