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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accurate stresses and strains the mesh is refined in the area of transition radii. After performing a sensitivity analysis, the final mesh consists of 2976361 elements and 4231073 nodes. The axial force, which is a maximum of 300 kN, was converted into uniform pressure on the active surface of the thrust collar, which defined the loading of the model. The distribution of von Mises stresses is shown in the Fig. 3. The equivalent stress in the root of the transition radius is about 272 MPa, which is less than the material's yield strength of 550 MPa. In other words, assuming uniform static loading of the thrust collar, fracture is not expected to occur.

Fig. 3. Distribution of von Mises stresses obtained using FEM analysis.

However, in the case of cyclic loading and due to the notch effect at the radius, a crack could initiate at the root of the notch and propagate until it reaches its critical size, leading to rapid brittle fracture. Cyclic loading of the thrust collar can occur due to the non-uniform pressure distribution on the active surface of the collar in combination with the rotation of the turbine rotor. Upon reviewing the manually recorded turbine operation data, it was observed that the absolute expansion displacement of the turbine casing, prior to the incident described, was approximately 1 mm less than the normal value. Axial thermal expansion of the turbine casing is accommodated by the front bearing housing, which is designed to slide along the surface of the base plate. A lower measured absolute expansion indicates that the casing did not elongate sufficiently due to difficulty in sliding the bearing housing. Specifically, increased friction between the sliding surfaces limited the axial elongation of the casing, causing the bearing housing to tilt slightly in the vertical plane and partially lift at the rear. As a result, the rotor thrust collar rested unevenly on the thrust bearing pads, leading to non-uniform collar loading. Since the rotor is rotating, this uneven loading is cyclical, which can result in high-cycle fatigue failure. However, fatigue analysis of the thrust collar could not be performed because the S-N curve for steel 21CrMoV5 11 was not available, and the exact pressure distribution across the collar surface was not known. 4. Rotor and turbine repair The damaged thrust collar was removed from the shaft through machining. The shaft was machined to a final diameter of 185 mm over a length of 152 mm, measured from the shaft front end. A new thrust collar was then fitted onto this shaft journal, creating a shrink fit connection as can be seen in Fig. 4 (taken from internal technical documentation Petrokemija d.d. Kutina). The connection is further secured with a nut. The shrink fit connection between the thrust collar and the shaft must provide strong resistance to the longitudinal movement of the connected parts. Pressure acts on the contact surfaces of the collar and shaft, generating the frictional force needed to transmit the axial force of the turbine rotor. The maximum axial force on the thrust collar, which results from steam expansion through the turbine, is 300 kN. The selected minimum and maximum values for the diametrical interference in the shrink fit connection are 0.165 mm and 0.25 mm, respectively. The actual interference achieved was 0.2 mm. The pressure on the contact surface of the thrust collar-shaft connection is calculated using the following expression: