For a hydropower station with a long pressurized water diversion channel, the acceleration time constant of the water flow will tend to be quite large. Of course, the pressure regulating chamber can be used to reduce the load value, and the unit rate rise value and the water hammer pressure rise value value during the load rejection. The contradiction between. The setting of the surge chamber also changes the operating conditions when the load is increased.
However, the construction of the surge chamber has a large investment, a long construction period, and sometimes even natural conditions are particularly beneficial, and it is economically more advantageous to install it than to build a pressure regulating air. The three-valve group, also known as the air-displacement valve or the pressure-reducing valve group, is generally installed near the vortex of the Francis turbine with a high head. From the perspective of the valve itself, it is a two-valve group that connects the pressure pipes.
The role of the three-valve group is to prevent excessive water hammer pressure, that is, to reduce water hammer pressure. How does the three-valve group prevent excessive water hammer pressure from occurring?
When the turbine vane is quickly closed, the three-valve group controlled by the same governor will automatically open at the same time, releasing a part of the water flow. In this way, the rate of change in flow in the pressure line is very small and it is not affected by the rapid closure of the turbine guide vanes. This avoids excessively high water hammer pressure.
In normal operation, when the unit load produces a small change, the three-valve group will remain closed, that is, the three-valve group will not operate. In this case, the water hammer pressure generated is not large, and it is felt in the design of the pressure arm that it can withstand such water hammer pressure. A large amount of discarded water will be added regardless of the tube action of the ferrule type joint when the small load changes. The three-valve group does not work when the negative sign is added. Therefore, the valve is always designed to remain closed when the pressure is reduced. Therefore, the valve has no limiting effect on the pressure drop occurring in the pressure pipe. The closing time of the three-valve group is generally 20 to 30 (or even 60) seconds.
In summary: the role of the three-valve group is to reduce the rate of change of flow in the pressure pipe during load rejection, thereby preventing excessive water hammer pressure; the action of the three-valve group is: quick opening, slow closing; small load changing ice The three-valve group should not be operated; the three-valve group does not work when the load is increased.