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Now the main valve disc of the large unit is very large, and the main steam pressure is very high. When the door disc is opened, the pressure difference between the front and back of the valve is very large, which requires a large lifting force of the oil motor to open, so the size of the oil motor needs to be designed very large. Therefore, the main steam valve is provided with a pre-opening valve structure, the lifting force of the opening of the main valve will be reduced, so that the control device structure is compact.

In addition, when the metal temperature of the steam chamber is lower than the saturation temperature corresponding to the existing main valve inlet pressure, continue to use the pilot valve of the main steam valve (pre-opening valve) control, while continuing to heat the steam chamber, the purpose of doing so is to prevent the pressure of the steam chamber from rising after the switch of the valve, which is easy to cause condensation in the steam chamber and increase the thermal stress. Therefore, the steam temperature should be equal to or greater than the minimum temperature of the main valve inlet, until the metal temperature of the steam chamber reaches saturation temperature and then switch to the control valve (GV).

A through hole is drilled on the valve regulator of the high pressure main steam valve, and the end of the valve stem passes through the hole. The pre-opening valve is placed at the end of the valve stem, the pre-opening valve is connected to the valve stem, when the valve stem action to open the main valve, the pre-opening valve first opens into a small amount of steam, so that the pressure difference before and after the main valve is reduced, at the same time after preheating the main valve pipeline and high pressure regulating valve shell pre-opening valve role is in the main valve has not acted, the pre-opening valve action, after opening, There will be a small flow of steam through the pre-opening spool to reach the main valve, to reduce the pressure difference before and after the main valve, conducive to the smooth opening of the main valve, at the same time, because the diameter of the pre-opening valve is much smaller than the main valve, it is convenient to use small flow of steam on the turbine and speed up.

The pre-start valve is located in the main steam valve spool, the pre-start valve spool is directly connected to the main steam valve stem, the main steam valve oil motor begins to act, the first to open the pre-start valve, when the pre-start valve is fully opened, the main steam valve is gradually opened under the drive of the valve stem. From the structure of the pre-opening valve, after the automatic closing device (automatic main steam valve) action, the pre-opening valve is opened first, and the main steam valve is opened only after the pre-opening valve is fully opened.

When the main steam valve is opened, it reduces the pressure difference on both sides of the main steam valve core and reduces the opening torque of the main steam valve.

But the pre-opening valve plays a more important role in the process of steam turbine flushing: it controls the speed in the process of steam turbine flushing. In the whole process of steam turbine shutdown, speed up and valve switching, the steam turbine adopts the main steam valve control (realized in DEH). At this time, the high regulator door is fully open, and the speed of the steam turbine is controlled by the main valve. At this time, the control of the speed of the steam turbine is actually the pre-opening valve.

Because the steam turbine in the rush, speed up to 3000 RPM (idling), the steam required is the most small, and the high pressure main steam valve spool section is much larger than the high profile valve (that is, the main steam valve flow capacity is much larger), at this time if the main steam valve is directly used for speed up and accurate control of the turbine speed, it is difficult to achieve. Therefore, steam turbine design will be the steam turbine impulse, speed up to 3000 RPM this stage function cleverly integrated with the design of pre-opening valve. Because of the small cross-sectional area of the pre-opening valve spool (i.e. small flow capacity), steam turbine speed control can be easily and smoothly achieved by this small valve.

In the design of high pressure main steam valve, the flow capacity when the pre-opening valve is fully opened is just designed to raise the steam turbine speed to about 3000 RPM (related to the steam intake parameters when starting up). Therefore, in the process of steam turbine flushing, the main steam valve is actually closed, and the steam turbine speed is controlled by the pre-opening valve. Because the steam turbine with load is controlled by the valve, it is necessary to switch the valve before the load of the steam turbine (the main steam valve is fully open, and the steam intake of the steam turbine is controlled by the valve), the switch is generally carried out at 2900, at this time, the pre-opening valve has been basically fully open, the pressure difference before and after the main steam valve is small, as long as click "valve switch" on the DEH disk, You can easily switch the valve work.