PSI - Issue 57

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

846 14

To prevent this, pro-active machining can be performed, thereby removing accumulated creep-fatigue damage at and underneath the surface before the onset of cracking. The extent of machining and the locations where it is needed can be determined by the lifetime assessment procedure described in 5.1. Typically, only a few millimeters of surface layer removal is needed to be effective, given the strong stress reduction towards the center of the rotor. This strategy is widely implemented in the ENGIE fleet. An example of surface machining applied to Rotor C (see

Figure 20: Surface machining at the inlet of the HP first stage wheel

Section 5.1) is shown in Figure 20. The geometry before and after machining is visible in Figure 21 along with the stress distribution. A slight increase in stress concentration (12.5% increase) had to be accepted to increase the machinability of the rotor and to avoid impacting the steam glands. Nevertheless, the remaining lifetime is strongly increased as the accumulated creep-fatigue damage is nearly reset to zero by removing the damaged material.

Figure 21: Stress levels increase after machining a) original configuration b) machined configuration

The advantage of machining is illustrated by considering two future scenario’s for Rotor C, one with and without machining. For both cases, the future operational profile is based on the last 2 years of operation. The next major overhaul is foreseen in 2030. The figures from Table 2 shows that machining a high cracking risk area reduced the risk for initiating a crack before the next overhaul in 2030.