PSI - Issue 48

Evgeniia Georgievskaia / Procedia Structural Integrity 48 (2023) 310–317 Georgievskaia / Structural Integrity Procedia 00 (2019) 000 – 000

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The calculation results showed the following:  during nominal HU operation mode LSS cross-section average stresses meet the conditions of strength, but at the same time in some localized zones stresses significantly exceed the permissible values;  maximum static stresses occur in localized zones of butt beam welding to spoke’s central stiffener and to LSS central part: the stress level is 1.5-2 times more than permitted there; maximum dynamic stresses occur in the same zones (see Fig. 4); these combined factors provoke large-scale crack formation within the zone as actual operation continues;  there is an uneven stress distribution in structurally identical spider spokes elements and uneven loading of the fastening elements in the connection of the central part to spider spokes, which is usually not considered during design stage;  when operating HU on low power modes the first LSS axial vibration eigenfrequency (9,9 Hz) is close to the first generator rotor frame longitudinal vibration eigenfrequency (10,6 Hz); it can cause resonance phenomena accompanied by not only generator rotor amplitude vibration increase but also the same increase in LSS; vibration inspections prove this calculation data: increased axial vibration displacement values detected at 20 MW operating mode relative to other operating modes;  LSS has a residual lifetime of about 4.5 years, which is less than the 7-year period between overhauls (as regulated by normative documents), provided that the HU modes remain unchanged. a b Fig. 4. Equivalent stress distribution (intensity) in LSS spoke while in nominal operating mode, MPa. (a) Static stresses, MPa; (b) Relative dynamic stresses, MPa 6. Main crack formation causes in later operating life LSS comprehensive inspection ’s and computational 3D - model’s result analysis allowed to highlight the main causes of fatigue crack formation in LSS welds after long-term operation. All cracks can be conventionally divided into two groups being systematic ones and random ones. Systematic cracks appear due to high local operating stresses. This group should include cracks in the welding area of the butt beam to the spokes and LSS central part (see Fig. 4). Such cracks will occur periodically with the increase of unit running time under normal operating conditions, even with quality repairs being of high quality. The crack formation intensity of this type can be reduced by lowering maximal local stresses, that is, either by reducing the load on the respective LSS zones or by strengthening the structure. The reduction of LSS loads on a running unit can only be achieved by the mode factors, in this case by minimization of the operating time of the HU at about 20 MW when the spider resonant axial vibrations are possible. Reduction of local stresses in the butt beam welding zone by reinforcing the LSS structure can be achieved by welding additional stiffeners which lighten the load on areas of the most stress concentration (more detailed information provided in the following section). Accidental cracks appear due to violation of welding technology (initial welding defects such as caverns or non metallic inclusions, welding residual stress of high level, weld geometry violation etc.). In case of high-quality recovery repairs to mend cracks, the recurrence of cracks of this type is unlikely. In the considered structure, all cracks except those in the welding area of the butt beam to the spokes and the LSS central part are incidental. Notably, structurally different spiders may have other incidental or systematic crack formation areas.