PSI - Issue 13

Marko Katinić et al. / Procedia Structural Integrity 13 (2018) 2040 – 2047 Author name / Structural Integrity Procedia 00 (2018) 000–000

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The Goodman diagram is shown in Fig. 8. The moving blade material is martensitic stainless steel Z10CD13 according to AFNOR. As can be seen from diagram, the safety factor for all considered blade elements is between 3 and 4. Minimum safety factor belongs to the blade root and it is about 3. However, the reduction of fatigue endurance limit due to the influence of stress concentration (for instance in the blade root), surface quality and corrosion has not been considered here. This means that the actual safety factor is lower than estimated in the below diagram.

Fig. 8. Goodman diagram for the blade elements performed of Z10CD13 - Petrokemija d.d. (2009)

4. Modification of the 5 th turbine stage The above analysis has determined that the root cause of blades failure is corrosion fatigue. In order to extend the useful fatigue life of the blades, certain modifications of the 5 th turbine stage were performed. First, the stator and rotor blades were redesigned. The length of the blades was reduced and the number of blades decreased, so that the flow area remained unchanged. In this way, the steady stress at the blade roots are reduced by 9,5 %. Also, the existence of resonant interferences in the operating speed range is avoided (Fig. 9). Secondly, the material of the moving blades was changed. Instead of the steel Z10CD13 (according to AFNOR), highly heat-resistant martensitic steel Z12CNDV12 was used to produce moving blades. The Table 2 gives the values of tensile strength and fatigue limit for both types of materials. It is clear that the tensile strength and fatigue limit of the new material is higher by 29 %.

Table 2. Material data – Petrokemija d.d. (2009) Z10CD13

Z12CNDV12

Tensile strength at 100 °C, MPa Fatigue limit at 100 °C, MPa

655 327

846 423

The Goodman diagram for the moving blade elements of the 5 th redesigned turbine stage is shown in Fig. 10. As can be seen, the safety factor values for blade elements of the modified stage is higher than safety factor values for blade elements of the old design (Fig. 10). Minimum safety factor belongs to the blade root and it is about 5. It means that minimum safety factor of the modified stage is significantly higher than minimum safety factor of the old design stage. Estimated increase in safety factor is about 65 %.