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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4.1. Design There are mainly two different designs, a drum-type rotor which is more often used for reaction-type blading and a disc-type rotor which typically uses action-type blading. Both are shown on Figure 11. The drum-type rotor is a solid rotor in which the blade slot bottoms are machined. Consequently, the diameter and hence its thermal inertia is larger compared to a disc-type rotor. The critical location in a drum-type rotor is typically the first stage blade slot bottom which also represents a larger stress concentration factor compared to the fillet radius of the first stage disc in a disc-type rotor. In order to reduce the stress concentration, the first stage slot bottom has an asymmetric shape with a larger rounding at the more solid side of the slot bottom (Figure 9). Sometimes, an inlet groove is used to relax the stress at the first stage slot bottom (Figure 6), and in that case a symmetrically shaped slot bottom is still used. The blade itself can have an impact on the thermal stress levels in the slot bottom as the clearance in between the blade and the rotor can get consumed during a start. A larger diameter and stress concentration increase the lifetime consumption of the rotor when similar operational gradients would be imposed.

Figure 11: Drum-type rotor with reaction-type blading where blade slot bottoms are machined in the rotor body (left); Disc-type rotor with action-type blading where blades are fixed to discs (right)

Besides the geometry, the rotor material plays a key role as well. Historically, most of the high-pressure and intermediate-pressure steam turbine rotors were made of 1%CrMoV material. Research over the last decades has enabled the development of rotor steels of higher strength, typically referred to as COST E, F and FB2 materials. These 10%Cr materials have a higher strength at high temperatures, both for what concerns creep and fatigue. Consequently, when imposing similar operational gradients, the risk for thermo-mechanical fatigue cracking is strongly reduced compared to 1%Cr materials. 4.2. Operations The operational profile of the power plant has an important impact on the resulting lifetime consumption. Within the ENGIE fleet, primary creep due to mechanical loading at nominal conditions is typically less damaging compared to creep-fatigue damage caused by cyclic operation. Hence, base load units with few starts are typically not at risk for cracking. Many units however have accumulated near 1000 starts, while some are even exceeding 2000 starts. This clearly has an impact on the risk for cracking. Some units have also implemented some flexibility improvements over the last decade, thereby increasing the start-up gradients to reduce the start-up costs. The price to pay is an accelerated lifetime consumption which needs to be considered in the long-term maintenance planning. An example is shown in Figure 7 where the increase in stress levels due to flexibility improvements is clearly visible. 4.3. Maintenance In some cases, crack detection is possible by borescope inspection during a short stop of the unit without having to disassemble the unit. For case study 1, borescope inspection is also performed on a regular basis to check the reappearance of a crack. For other turbines, inspection is only possible during a major overhaul when the unit is fully disassembled. For drum-type rotors, inspection at the first stage slot bottom requires not only removal, but also replacement of the first stage blades as they get damaged during the removal process. To resolve this issue, ENGIE