Turbine main valve-1

Function of main valve

In order to meet the needs of operation and maintenance of hydropower station units, in the diversion system of hydropower station and the flow passage system of hydraulic turbine, corresponding gates or valves shall be installed at different positions to control the flow, such as the service gate and maintenance gate at the inlet, the maintenance gate at the outlet of draft tube and the valve in front of the turbine volute, etc. The valve installed in front of the turbine volute is called the main valve.

The main valve usually only has two working conditions of full opening and full closing, and it is not allowed to adjust the flow through partial opening, so as to avoid excessive hydraulic loss and vibration caused by affecting the flow stability. The main valve is not allowed to open under the condition of dynamic water.

1. Function of main valve

1) provide safe working conditions for maintenance of water turbine generator unit. When the penstock adopts combined water supply or group water supply, a water inlet valve shall be installed in front of each unit, so that when one unit is under maintenance, closing the water inlet valve of this unit will not affect the normal operation of other units.

2) reduce the water leakage of the unit during shutdown and shorten the time required for the unit during restart. After the turbine generator unit is shut down, the guide vane of the turbine cannot be completely closed, and there is a large amount of water leakage. Especially after a period of operation, cavitation and wear occur at the guide vane gap, which increases the water leakage. According to statistics, the leakage of general guide vane is 2% ~ 3% of the maximum flow of hydraulic turbine, which can reach 5% seriously, resulting in a large loss of water energy. Some small units with high water head can not be shut down even because of excessive water leakage. Therefore, when the unit is shut down for a long time, closing its main valve can greatly reduce the water leakage of the unit.

After the short-time shutdown of the water turbine generator unit, in order to ensure the speed and flexibility of the unit operation when the unit is restarted, the water inlet gate is generally not closed to avoid discharging the water in the penstock, because the water needs to be refilled before the next startup, which not only prolongs the starting time of the unit, but also makes the unit unable to maintain the standby state at any time. Therefore, the installation of the main valve has more obvious effect on the hydropower station with high head and long penstock.

3) prevent the expansion of runaway accident of water turbine generator unit. When the failure of the governing system of the hydrogenerator unit leads to the failure of the water guide mechanism, the emergency closing of the main valve can quickly cut off the water flow to prevent the runaway time of the hydrogenerator unit from exceeding the allowable value and avoid the expansion of the accident.

2. Conditions for installation of main valve

It can be seen from the above analysis that the main valve must be installed for the water turbine generator set. However, due to the high price of the main valve, and the installation of the main valve will increase the civil construction investment and equipment installation and operation costs of the hydropower station. Therefore, the installation of the main valve should generally meet the following three conditions:

1) for hydropower stations with combined or grouped water supply mode, the penstock shall be equipped with main valve in front of each turbine.

2) the main valve should be installed before the water turbine with the water head higher than 120m. Because of the high water head, the pressure channel is often long, the water filling time is long, and the water leakage of the guide vane of the turbine is large. In fact, some high head hydropower stations can also be equipped with two valves, one is installed at the beginning of the penstock, the other is installed in front of the water turbine, respectively as the protection facilities of the penstock and the unit.

3) the main valve shall not be installed in front of the water turbine with the water head lower than 120m, but the quick gate shall be installed at the water inlet. However, there are exceptions. For example, in some medium and low head power stations on muddy river, the intake gate is easy to wear and tear, so it can not guarantee to cut off the flow quickly, so the main valve is also installed.