please could u tell me that in calculating the back surge , do we need to consider the closing time of guide vanes or just we need to know the closing time of MIV in case of a francis turbine.
The penstock determines the wicket gate closing rate, the pressure rise in the penstock is relative to the length, head, velocity, and water temperature. A long penstock usually requires a surge tank immediately upstream of the powerhouse.
If there is no penstock to speak of then the trashracks being loaded with debri, i.e. close the gates too fast and the surge will tip over the trashracks if they are heavy with debri.
If the trashracks are anchored well and no penstock, then any mechanical over-speed limit and turbine-generator rotating parts WR2 determines gate closing rate.
Usually maximum speed (2 times synchronous speed ) is reached within 5 seconds after the generator breaker opens. The gate angle at the time of the fault determines the maximums reached, worst case of full gate opening, as I recall a Francis turbine does limit the flow and the speed.
You should also consider the back-up (head gate) for the wicket gates, as it limits overall damage if a wicket gate or two breaks a link. If there is a long penstock, none or a small surge tank at the powerhouse, it will spin for a while as the water drains from the penstock, the penstock vent size and potential icing "capping" the penstock vent can also lead to collapse of the penstock.
If there is a bifurcated penstock feeding multiple turbines, another case similar to a long penstock, best to have a surge tank.
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Guide vanes in francis turbines.
Re: Guide vanes in francis turbines.
