1 overview

Regulating valves are widely used in the pipeline system of thermal power plants to regulate the flow and pressure of the medium. Their working parameters are high, and DCS operation is used in operation.When changing operating conditions, each system is required to have high stability and coordination. Therefore, the analysis of the operating mode and function, according to the flow characteristics and operating condition parameters, has important reference value for the selection of power station control valves.

2 Adjustment principle

The regulating valve is equivalent to a throttle with variable resistance in the pipeline. When the medium passes through, it must overcome the resistance and consume a certain amount of energy, resulting in the loss of fluid speed and pressure, thereby achieving the purpose of regulating fluid flow and pressure.The parameters that reflect the structural characteristics and working characteristics of the regulating valve are flow coefficient Kv, flow characteristics, nominal pressure, nominal diameter and disc opening.Among them, the flow characteristics determine its adjustment performance and play a key role in the stable operation of the entire system.

According to the relationship between the relative flow rate of the valve and the relative opening, the medium flow can be divided into linear flow characteristics, equal percentage flow characteristics, parabolic flow characteristics and quick-opening flow characteristics (Figure 1).

Figure 1 Flow characteristic curve

Valves with linear flow characteristics have a large relative change in flow rate when they are opened, high sensitivity, and are not easy to control, while when they are opened, the relative change value of flow rate is small and the adjustment is slow.The equal percentage flow characteristic is that when the opening is small, the magnification coefficient of the regulating valve is small, and the adjustment is stable and gentle. When the opening is large, the magnification coefficient is large, and the adjustment is sensitive and effective.The parabolic flow characteristics are between linear and equal percentage flow characteristics.The fast-opening flow characteristic has a larger flow rate when the opening is small. With the increase of the opening, the flow rate quickly reaches the maximum. After that, the opening is increased, and the flow rate changes very little.

3 selection

3.1 Condensate high and low load regulating valve

The condensate low-load regulating valve maintains the water level of the deaerator before the unit starts, and can meet 40% of the unit load.When the opening of the condensate low-load regulating valve reaches 87.5%, the condensate high-load regulating valve is opened to participate in the regulation of the condensate system.When the unit is 50% loaded, if the opening of the condensate high-load regulating valve is less than 10%, the water level of the condenser hot well is difficult to control.During the shutdown process, the high-load regulating valve is required to be tight.At the same time, the flow capacity of the high-load regulating valve should also take into account the problems of low-load regulating valve failure, condensate recirculation failure and system leakage.According to the operation mode, the flow characteristics of the condensate low-load regulating valve should be linear, and the flow characteristics of the condensate high-load regulating valve should be equal percentages.

3.2 Water supply control valve

During operation, the water supply control valve of the electric pump adjusts the normal water level of the steam drum when the unit is under low load, and when it reaches 40% of the load, it is cut to the operation of the steam pump.However, in a high-load state, if the steam pump is tripped, the electric pump should be able to quickly pump water during the start-up process to stabilize the water level and pressure of the steam drum. Therefore, it is required that the water supply control valve should have good adjustment characteristics and linear adjustment characteristics should be adopted (If equal percentage characteristics are used, the valve opens slowly during the start-up of the electric pump for 2~3s, resulting in the water level of the steam drum cannot be stabilized, and the operation is forced to be adjusted manually. At the same time, the performance requirements of the hydraulic coupling are extremely high).The flow rate of the water supply control valve should be able to meet the flow rate of the water supply at 50% of the load of the unit.

3.3 Steam pump recirculation control valve

The set point of the steam pump recirculation flow rate is 25% of the rated flow rate of the steam pump.When the flow rate of the steam pump is 25% to 30% of the rated flow rate, the opening of the steam pump recirculation control valve is 25%, and when the flow rate of the steam pump is greater than 25% of the rated flow rate, the steam pump recirculation control valve is closed.When the flow rate of the steam pump is less than 25% of the rated flow rate, the opening of the steam pump recirculation control valve is 25% to 100%.At 0~25%, it is fast opening, and the adjustment characteristics are linear, which avoids the adjustment of the steam pump recirculation control valve at a small opening, causing the valve internals to scour.

3.4 Heater control valve

For some accidents, the trap control valve is designed to have a quick-opening flow characteristic. When the water level of the heater is too high or the normal trap control valve fails, the accident trap control valve is fully opened, causing the heater to operate without water level, poor economy, serious soda scouring, and large vibration of the internal pipe system.Therefore, it is recommended that the accident trap control valve be adjusted linearly. When the heater leaks, it can be opened quickly to meet the flow requirements, and the amount of leakage can be judged according to the opening of the accident trap control valve. When the normal trap control valve fails, the accident trap control valve can participate in the adjustment to ensure the normal water trapping of the heater.Usually, the flow capacity of the trap control valve is 1.5 to 3 times that of the normal trap control valve.

 3.5 Shaft seal regulating valve

Under the start-up and 50% load state of the unit, the auxiliary steam is supplied to the shaft seal system.The shaft seal system has a steam supply control valve, a pressure drain valve and a temperature-reducing water control valve.When the load of the unit is greater than 50% of the load, the steam supply control valve of the shaft seal is closed, and the steam leakage volume of the main valve stem can already meet the needs of the shaft seal. The pressure drain valve of the shaft seal is used for adjustment, and the shaft seal system performs self-sealing operation.Because the host shaft seal system affects the safety of the host, the pressure of the shaft seal is too high, which can easily cause water to enter the lubricating oil system, and the pressure of the shaft seal is too small, resulting in low vacuum in the unit.In order to prevent excessive pressure fluctuations of the shaft seal during operation and unnecessary downtime caused by valve failure, it is recommended to choose equal percentages for the flow characteristics of the shaft seal regulating valve and increase the manual operation function.The application of regulating valves in the pipeline system of thermal power plants is shown in Table 1.

Table 1 Application of each control valve in the system

4 Conclusion

According to the operation mode and system principle, reasonable selection of the flow characteristics of the regulating valve can make the regulating valve have good regulating performance, reduce the energy loss of the medium, extend the life of the valve, reduce the labor intensity of operators, reduce environmental noise, improve the economy of the unit, and ensure the safe and stable operation of the unit.