Causes of cracking in stay vanes of Francis turbines

Conclusions

• 1. The level of static stresses (working σ≤75 MPa and residual after manufacture and concreting σ_res≤70 MPa) is completely allowable

• 2. The natural frequencies of the vanes in water coincide with the frequencies of the Karman vortices for the second group of vanes at a maximum discharge and for the third group at partial discharges (loads 90–100 MW), which in the corresponding zones of operation of the turbine led to the occurrence of self-induced vibrations with respect to the first flexural and torsional modes.

• 3. Modification of the trailing edges of the stay vanes by “dovetailing” is rather effective, and the main cause of cracking in the stay vanes was eliminated by means of it.

• 4. The random character of occurrence of cracks on individual stay vanes is explained by the primary operating regimes of the turbines in unfavorable zones of the operating characteristic and insufficient safety factor with respect to corrosion fatigue strength under dynamic stresses σ_dyn∼17.5 MPa.

• 5. The most effective method of controlling cracking under conditions of the investigated hydrostation is to shift the region of possible self-induced vibrations into the zone of discharges at which the turbines practically do not operate. For this purpose it is recommended to groove the trailing edges of the second and third groups of vanes, having reduced their thickness to the minimum possible. With the indicated grooving the frequency of shedding of vortices from the trailing edges of the third group of stay vanes at a maximum discharge is about 300 Hz and of the second group of vanes about 270 Hz.

  These frequency values were determined with consideration of. the experimental data obtained in investigating flow past model profiles of vanes and their modifications in a water tunnel. In this case the frequency of shedding of vortices coincides with the natural frequency of the vanes at discharges less than those allowed by the operating characteristic.

• 6. To induce residual compressive stresses exceeding the fatigue strength, it is expedient to carry out plastic surface deformation of the places of welding the vanes to the upper and lower collars.