PSI - Issue 82

Marko Katinić et al. / Procedia Structural Integrity 82 (2026) 220 – 226 M. Katinić, P. Konjatić/ Structural Integrity Procedia 00 (2026) 000–000

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coated with white metal. The thrust collar, which is part of the turbine rotor, rests on tilting pads and transmits the axial load to the stationary elements of the turbine. The thrust collar is either an integral part of the rotor shaft or is shrink fitted onto the rotor shaft. A steam turbine can experience various malfunctions during its operation. Rotor failure is one of the statistically most common failures because it is under the influence of serious loads. Some rotor failures can cause catastrophic damage to the entire turbine and thus cause long unplanned downtimes (Barella et al., 2011; Trebuňa et al., 2017; Yadavar et al., 2016; Vinod et al., 2020; Katinić et al., 2019). Fatigue caused by cyclic load change is the most common cause of rotor failure. The result of fatigue is the initation and propagation of a crack or multiple cracks. Ultimately the development of a crack or cracks can lead to fracture of either the shaft or rotor disc or blade and catastrophic failure of the turbine. Fatigue, one of the primary mechanisms responsible for the failure of steam turbine rotors, occurs as a result of the material being subjected to typically high-cycle stresses that do not exceed its tensile strength. Subjecting a material to cyclic stresses can, over time, locally degrade the material and lead to the formation of microcracks. With each new stress cycle, the microcracks grow, coalesce, and eventually become macrocracks. When a macrocrack reaches its critical size, a sudden and rapid brittle fracture occurs. The role of fatigue in steam turbine failures has been investigated in several studies (Qiqi et al. (2023), Segure et al. (2017), Mazura et al. (2008), Weija et al. (2020) and Ramírez et al. (2023)). Studies have shown that fatigue can significantly reduce the life of critical turbine components such as the rotor. The importance of precise engineering analysis and prediction of the material behavior of turbine components under the action of cyclic stresses was emphasized, in order to undertake preventive and corrective maintenance activities in time. Accumulation of damage in materials due to long-term effects of cyclic stresses combined with other loads and phenomena such as material creep make components such as turbine rotors susceptible to failure. Specialized powerful computer software that performs finite element analysis (FEA) is used to model and simulate the behavior of mechanical systems under various loading conditions. FEA is an important tool that is also used to investigate the fatigue phenomena that occur in systems subjected to cyclic stresses. Fatigue life calculations for critical steam turbine components use FEA. FEA is often used to investigate and identify the causes of failures such as turbine rotor fractures. Engineers are enabled to use computer software to simulate the behavior of turbine components subjected to mechanical and thermal loads (Katinić et al. (2023)). In this way, potential weak points can be detected, failure locations predicted, and the time to failure can be predicted. If the turbine is properly designed and regularly maintained, fracture of the rotor thrust collar should not occur. In the available literature and open access sources, no cases of fracture of the rotor thrust collar have been recorded. In this paper, a case of thrust collar fracture that occurred in the distant past and has not yet been published in a scientific journal is analyzed. The rotor of a 35 MW single-case steam turbine generator experienced a thrust collar fracture during operation in 1998. The broken collar led to an extended turbine shutdown which resulted in power supply disruptions. The researchers conducted a thorough investigation to determine the fracture origin and establish preventive measures for future occurrences.

Nomenclature p d

contact pressure in a shrink-fitted joint (MPa) diametrical interference value (mm)

δ E r 1 r 2 r 3

Young's modulus (MPa) inner radius of the shaft (mm)

outer/inner radius of the shaft/collar hub (mm) outer radius of the collar hub (mm)

axial force (N)

F a

connection length (mm)

l

friction factor (-)

μ