We performed structural and fatigue / fracture analysis on the main and upstream shafts in a hydroelectric plant. Inspection of the main shaft had detected linear indications at the outer surface of the shaft. The circumferential cracks initiated at pits propagated into the wall of the hollow shaft. The UT examination was performed with transducers located at the inner surface located in the up-stream end section of the hollow shaft. The highest bending stresses occur in the midsection of the straight part of the shaft.
Elastic Fracture Mechanics Analyses indicated that the existing cracks could propagate to the unacceptable sizes within weeks. The turbine was shut down, and plans were made to refurbish the shaft and provide local repairs of the cracks to enable restarting the turbines and continue their safe operation for at least one year.
Due to the depth of cracks on the main shaft, a uniform 1.20 inches from the entire shaft length was removed from the diameter. An initial material removal depth in the upstream shaft and flange region was set at approximately 0.20 inches, with additional material removal up 0.40 inches to fully eliminate existing cracks and insure that severely fatigue damaged material just beyond the crack was removed.
A finite element model of the main shaft, upstream shaft, and connecting bolts was created from the drawings and sketches provided to determine the overall effect on the upstream flange and shaft when additional material was removed to eliminate existing cracks.
Fatigue analysis was performed to determine the original and remaining life expectancy of the upstream and main shafts. Since a uniform amount of material was removed on the main shaft, hand calculations were performed to determine the stresses and resulting fatigue life. Stresses obtained from the finite element analysis were used to accurately determine the total range of cyclic stresses, which were used to calculate the fatigue life in this area. Because of the complexities of precisely calculating the local stresses in the flange region of the upstream shaft, a finite element analysis was created to more accurately determine the total range of cyclic stresses, which were used to calculate the fatigue life in this area.
O’Donnell Consulting performs vibration, fatigue and fracture analysis for clients in industries including manufacturing, energy and petrochemical.
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