Stable, safe and full, "is the general goal of hydropower units, but also the strong advantages of hydropower units - low cost, high efficiency of the concrete manifestation of most of China's hydropower stations for mixed-flow hydro-generating units, due to Francis Turbine structure Simple, wide application of a wide range of head, manufacturing technology is more mature, mixed flow models have occupied the dominant position of hydropower units. Francis turbine has a strong vitality, but there are problems that can not be ignored, unit vibration and blade cracks is the current Francis turbine Stable operation of the focus.Especially some of the hydropower station turbine vibration and cracks have seriously affected the unit's stable operation and economic benefits of the stability of the turbine operation, especially the flow of Francis Turbine stability has become the discussion of the turbine hot topic.

Analysis of Main Causes of Stability of Francis Turbine

2.1 Hydraulic reasons

Hydroelectric generating units due to hydraulic factors caused by vibration mainly in the following cases. First, the turbine in the non-design conditions to run the vibration. When the turbine is in the no-load, low head, small load, high head, overload and high flow of the operating state, in addition to the pressure of the fluid load, but also bear the pressure pulsation and hydraulic disturbances caused by the dynamic load. Second, the pressure vortex generated by the vortex belt in the draft tube. Since the Francis turbine is a fixed-blade hydraulic machine, when the turbine is deviated from the optimal operating conditions, since the rotor outlet water flow has a rotational circumferential velocity, a large vacuum appears in the area near the tail pipe axis below the runner, When the vacuum reaches the fixed value after the cavity cavitation cavitation, the vortex is a spiral around the tail pipe axis of the unstable rotation, will cause the unit vibration, and the tail pipe has a considerable damage. Third, the turbine rotation and fixed part of the gap caused by uneven pressure pulsation. Due to the uneven rotation of the turbine and the fixed part of the gap so that the center of the wheel away from the center of the unit, the wheel around the circumference of the fixed and rotating maze ring gap with the rotation of the wheel is constantly changing, the gap between the water pressure also changes along the wheel The integral of the hydraulic pressure, there is a role in the runner on the transverse water pressure of the force, the force is always forced to further shift the center of the wheel center, exacerbate vibration. Four vortex caused by vibration. When the flow around the flow of leaves, from the exit side of the flow, it will produce a vortex at the exit side, from the front and back of the blade alternately appear to form the impact of alternating leaves. When the natural frequency of the blade and the frequency of the same hit, it will resonate. Five is the hydraulic imbalance caused by pressure pulsation. When the water flow into the runner is axially symmetrical, an unbalanced lateral force occurs, causing the wheel to vibrate.

2.2 wheel design and manufacturing reasons

With the increase of the capacity of the unit, the unit size is relatively increased, due to the design and manufacture of the unit equipment caused by reduced stiffness, the inherent frequency decreases, so that large units more easily induced resonance or vibration, resulting in the unit due to hydraulic interference more prone to Water resonance. From the results of a large number of mixed-flow unit operation, most of the cracks occur in the leaves of the leaves and the crown of the welding and the leaves of the water and the next ring welding. Analysis of the reasons for the following points: First, the leaves and the crown, the next part of the welding parts, as a result of the "T" type of welding, space geometry is the location of sudden changes in the stress concentration area, but also the lowest fatigue strength The Second, the rotor blade and the crown and the lower ring compared to the larger size and cross-sectional size is relatively small; and the crown, the lower ring welded into the wheel after the minimum stiffness in the run to withstand alternating load prone to fatigue crack. Third, the leaves of the water side of the relative side of the water, the intensity is poor, under the action of alternating load is most prone to cracks. Fourth, in the process of welding wheel due to welding process and improper heat treatment and other factors, resulting in slag, pores, stress concentration and other defects in the alternating load will reduce the fatigue strength and service life.

2.3 the reasons for the operation of the turbine

At present, most of the mixed-flow runners with cracks at home and abroad have typical fatigue crack characteristics. This is due to the characteristics of the short start and stop time of the hydro-generator set. In the grid, the system is mainly responsible for the peaking and frequency modulation of the system. The long-term operation in the rotating standby state is the long-term operation in the unstable area, The effect of pressure pulsation greatly increases the chance of fatigue crack in the runner.