Types and Functions of Steam Turbine Valves

1. Types of turbine valve control

As the prime mover of large-scale high-speed operation of steam turbines, it is one of the main equipment in thermal power plants today. It is used to drag the generator and convert mechanical energy into electrical energy for users to use. The steam turbine has the characteristics of large volume and fast rotation. When it changes from the static state of normal temperature and pressure to the high temperature, high pressure and high-speed operation, the regulating valve of the steam turbine play a key role in stabilizing the rotating speed and controlling the load. Only by controlling the stability, rapidity and accuracy of the valve can the steam turbine run safely, stably and efficiently.

The Digital electro-hydraulic control (DEH) system, the most basic control system of the steam turbine at present, is the best system to control valves. In this system, there are two ways to control the valves: single valve control mode and multi-valve control mode, which can be switched without disturbance.

Single-valve control refers to the way that each high-pressure regulating valve simultaneously enters steam when the high-pressure cylinder of the steam turbine enters steam, that is to say, the instruction and the opening degree of each high-pressure regulating valve are the same.

Multi-valve control refers to the way that a single high-pressure regulating valve is used to gradually enter the steam when the high-pressure cylinder of the steam turbine enters the steam. That is to say, the instruction and opening of each high-pressure regulating valve are different, and the opening of each high-pressure regulating valve is output according to the instruction corresponding to its own flow curve.
　　
2. Function of the valve control

In the DEH control system, the opening instruction of the regulating valve is actually output by valve control, and the signal received by valve control is the system's request for the total steam flow into the steam turbine, that is, the flow given value signal generated in the speed control loop and load control loop of DEH system is converted into the opening instruction signal of each valve through valve control. This given signal is output to the valve control card (servo card), subtracted from the actual valve position signal of the valve position sensor (LVDT), and amplified by the servo amplifier to control the servo valve to reach the required opening. Therefore, the valve control is actually a software dynamic function generator, and its main tasks are: (1) When the unit is operating in single-valve regulation or multi-valve regulation mode, the valve control determines the opening of each regulating valve through the valve flow curve according to the given flow signal provided by DEH system, and outputs analog signals; (2) Ensure that the power of the unit will always remain unchanged during the process of switching between single-valve regulation and multi-valve regulation; (3) In the process of valve switching, if the flow request changes, the on-going valve switching will be stopped to meet the flow requirements of the unit first, and then the valve switching will continue; (4) Ensure that DEH system can smoothly switch from manual mode to automatic mode.
　　
3. New strategy of the valve control

3.1 Comparison of single valve and multi-valve

For the generator set operating at constant pressure, the adjustment of the steam turbine is mainly for the speed and power of the steam turbine, and the method to adjust the speed and power is to change the steam intake of the steam turbine. The steam intake of a steam turbine is realized by changing the opening number and opening degree of steam turbine regulating valves, that is, changing the steam intake area of steam turbine valves. Therefore, it can be produced according to the different steam inlet areas.

Single valve control and multi-valve control of steam turbine are introduced. Under the single valve control mode, each regulating valve acts together, and the opening of the valve is the same so that the steam inlet of the turbine is uniform, and the cylinder and rotor are heated relatively evenly. When the load of the steam turbine changes, the steam temperature change after the regulating stage is very small, and the generated thermal stress is relatively small, which makes the operational flexibility of the unit better, and it is suitable for starting and changing the load of the unit. However, when the single valve mode operates at a low load, the throttling loss is large, the regulating efficiency is low, and the unit economy is relatively poor.

Under the multi-valve control mode, each regulating valve acts in a planned way according to a certain sequence to change the steam inlet area of the turbine. In low-load operation, only one (or two) valves have throttling loss, and the other valves are fully opened or closed, so the regulating efficiency is high and the unit economy is good. However, in this mode, the steam inlet of the steam turbine is uneven, and the heating of the cylinder and rotor is uneven. In addition, when the load is changed, the steam temperature after the regulating stage changes greatly, the thermal stress is large, and the operation flexibility is poor.

In order to solve the contradiction between the flexibility and economy of the unit operation, in the process of starting, increasing the speed and changing the load, it is hoped that single valve control will be adopted to make the unit heat evenly, increase the speed and change the load flexibly, and when the unit is brought to a certain load, it is hoped that multi-valve control will be adopted to improve the efficiency of the unit. Therefore, it is necessary to apply the valve control function of DEH to switch the valve without disturbance, so that the turbine can run most effectively. Each control and regulating valve of a steam turbine has its own servo card, servo valve and oil motive, which makes the valve controlled by DEH more flexible and stable, and it is also convenient to ensure the operation of the unit when a single valve fails, and it is convenient to replace the servo card and servo valve online.

3.2 Ways to improve unit efficiency

Compared with the traditional regulating system, the DEH control system can control each valve independently. In order to reduce the throttling loss of the unit and improve the economy of the unit, it can be improved through the following aspects.

(1) Increasing the steam distribution valve point refers to the PowerPoint where there is no throttling loss in the valve inlet, and the steam turbine has the highest operating efficiency in this PowerPoint. Single-valve control is only when the steam turbine generates electricity at full capacity. Since all regulating valves are open, there is only one valve point in the steam turbine, so there is basically no throttling loss. However, in other loads, the regulating valves are not fully opened, so there is a large throttling loss, which reduces the efficiency. Multi-valve control means that each regulating valve supplies steam to the unit separately, and opens each regulating valve in a certain order according to the demand of the load. Taking four high-pressure steam turbines as an example, first, the steam turbine will open No.1 and No.2 valves, and when a certain load is brought, No.3 valve will slowly open when No.1 and No.2 valves are basically fully opened, while No.4 valve is still closed at this time, which results in multiple steam distribution valve points, resulting in throttling losses at these points. The more valve points, the smaller the throttling loss of the turbine, and the higher the overall efficiency of the turbine in the whole load range.

(2) Reorganization of valve group Because the number of regulating valves of a steam turbine is certain, in order to get more valve points under a limited number of valves, you can get more valve points by changing the number of nozzles corresponding to the valves. In the process of gradually increasing the load, the valves with relatively few nozzles should be opened first, and then other valves should be opened according to the increasing load, thus reducing the throttling loss of the steam turbine at low load, thus enabling the steam turbine to obtain relatively more valve points and improving the efficiency of the unit at different loads.

(3) Reducing the overlap. Because the operation of the steam turbine requires that the total flow rate has good linearity, and the valve flow rate of the steam turbine has a certain saturation characteristic, this requires the steam turbine to open the second valve before the linearity of the flow rate characteristic of the previous regulating valve becomes worse, thus resulting in the overlap of the valves. As the flow of each regulating valve has a certain relationship and influences each other, in order to ensure the overall flow linearity of the turbine, the stability of the dynamic regulation of the unit and the response speed of the unit during primary frequency modulation, it is required that the valves have a certain degree of overlap. Usually, many power plants require strict load assessment of the units, so they usually adjust the overlapping degree of the units to a large extent, so that the operation of the units will be relatively stable, and the fluctuation of the loads will be small or almost absent. As a result, the throttling loss of the unit becomes larger, the regulating efficiency decreases, and the overall economy also decreases. After research, properly reducing the valve overlap will only affect the local flow characteristics. Although it will make the linearity of the unit near the overlap point worse, it will not have a big impact on the key characteristics of the unit such as load rejection, and it will help reduce the throttling loss of the unit and improve the economic benefits of the unit.

3.3 New strategies to improve unit efficiency

According to the idea of valve group reorganization, the valve control in the DEH control system can realize the undisturbed switching between single valve control and sequential valve control, so that the efficiency of any load section of the steam turbine can reach the highest when running at constant pressure, thus improving the economy of variable load operation of the unit. However, the valve characteristics of different units are different, and the corresponding nozzle number and valve opening sequence are different. 

The valve opening sequence is that GV1 and GV2 are opened at the same time, then GV3 is opened, and finally, GV4 is opened. According to the field situation, when the valve is switched, the vibration of 2 watts and 3 watts reaches above the alarm value. After analysis and recalculation of the main engine plant, the valve opening sequence was changed to GV1 and GV2 first, then GV4 and finally GV3. When switching, the vibration of the unit did not reach the alarm value. It can be inferred that when the unit switches valves, the opening sequence of valves is very important, that is to say, the quality of valve control is one of the key factors that determine the normal operation of the unit. The key to valve control lies in whether the valve switching process is undisturbed switching, which requires the program compiled by the DEH system to be perfect. In general, the undisturbed switching of the control system can be realized by external integration. If the power loop is put into operation, the PID regulator acts as an external integrator. When the power of the generator changes due to the change of the regulating mode of the regulating valve, the change of PID output will be automatically compensated according to the actual situation, thus realizing the undisturbed switching; If the power circuit is not put into operation, the influence on the power of the unit in the process of valve switching depends on the accuracy of valve flow characteristic curve calculation.
　　
4. Conclusion

Nowadays, most power plants demand to improve the economy and the generating efficiency of units. The valve control of the DEH system can reduce the throttling loss of units. Using different valve control methods in different load sections, giving full play to the superiority of DEH computer control that can control each valve independently, can meet the load requirements of the unit under different working conditions, and thus regulate the load of the unit safely, stably and quickly, which has high practical application value.