During operation, the electric pump's feedwater control valve regulates the normal water level in the steam drum at low unit loads. When the load reaches 40%, the system automatically switches to the steam-driven pump for operation. However, under high-load conditions, if the steam-driven pump trips, the electric pump must be able to quickly supply water during startup, ensuring stable steam drum water levels and pressure. Therefore, the feedwater control valve is required to exhibit excellent regulating characteristics, specifically a linear control characteristic. (If an equal-percentage characteristic were used instead, the valve would open 2–3 seconds more slowly during electric pump startup, leading to unstable steam drum water levels and forcing operators to manually adjust the system. Additionally, this would place extremely high demands on the performance of the hydraulic coupling.) The flow capacity of the feedwater control valve must also meet the required feedwater flow rate at 50% of the unit's load.

In thermal power plant piping systems, control valves are widely used to regulate the flow and pressure of process media. These valves operate under high parameters and are typically managed via a DCS during operation. When operating conditions change, it is crucial for each system to maintain high stability and coordination. Therefore, analyzing operational modes and their effects—along with considering flow characteristics and process parameters—is essential for providing valuable guidance in selecting the right control valves for power plant applications.

2 Principle of Adjustment

The control valve in a pipeline acts as a throttling element with variable resistance. As the medium flows through it, it must overcome this resistance, consuming a certain amount of energy in the process. This results in losses of fluid velocity and head, ultimately enabling precise control of flow rate and pressure. Key parameters that reflect the structural and operational characteristics of the control valve include the flow coefficient Kv, flow characteristics, nominal pressure, nominal diameter, and valve opening degree. Among these, the flow characteristics directly determine the valve's regulating performance and play a critical role in ensuring the stable operation of the entire system.

According to the relationship between valve relative flow and relative opening, media flow can be categorized 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 exhibit significant relative flow changes at small openings, making them highly sensitive but difficult to control precisely. In contrast, at larger openings, their relative flow variation decreases, leading to slower response times. On the other hand, valves featuring equal-percentage flow characteristics have a low amplification factor at small openings, ensuring smooth and gradual regulation, while at higher openings, the amplification factor increases, enabling quick and effective adjustments. The parabolic flow characteristic falls somewhere between linear and equal-percentage flow behaviors. Finally, fast-opening valves deliver substantial flow even at minimal openings; as the valve opens further, the flow rapidly approaches its maximum value, after which additional increases in opening result in only minor changes in flow rate.

3 Options to Choose From

3.1 Condensate High- and Low-Load Control Valves

The condensate low-load regulating valve maintains the deaerator water level before unit startup and is capable of meeting the unit’s 40% load requirement. When the opening of the condensate low-load regulating valve reaches 87.5%, the condensate high-load regulating valve begins to open, joining the system in regulating the condensate flow. However, if the condensate high-load regulating valve opens to less than 10% at a 50% unit load, it becomes difficult to maintain proper control of the condenser hotwell water level. During shutdown, the high-load regulating valve must remain tightly closed. Additionally, the high-load valve’s flow capacity should account for potential issues such as failure of the low-load regulating valve, condensate recirculation malfunctions, and system leaks. Depending on the operating mode, the flow characteristics of the condensate low-load regulating valve should be linear, while those of the condensate high-load regulating valve should follow an equal-percentage curve.

3.2 Water Supply Control Valve

During operation, the electric pump's feedwater control valve regulates the normal water level in the steam drum at low机组 loads. When the load reaches 40%, the system automatically switches to the steam-driven pump for operation. However, under high-load conditions, if the steam-driven pump trips, the electric pump must be able to quickly supply water during startup, ensuring stable steam drum water levels and pressure. Therefore, the feedwater control valve is required to exhibit excellent regulating characteristics, specifically a linear control characteristic. (If an equal-percentage characteristic were used instead, the valve would open 2–3 seconds more slowly during electric pump startup, leading to unstable steam drum water levels and forcing operators to manually adjust the system. Additionally, this would place extremely high demands on the performance of the hydraulic coupling.) The flow capacity of the feedwater control valve must also meet the required feedwater flow rate at 50% of the unit's load.

3.3 Steam Pump Recirculation Control Valve

The steam pump recirculation flow setpoint is set at 25% of the steam pump’s rated flow. When the steam pump flow rate is between 25% and 30% of the rated flow, the steam pump recirculation control valve opens to 25%. However, if the steam pump flow exceeds 25% of the rated flow, the recirculation valve closes completely. Conversely, when the steam pump flow drops below 25% of the rated flow, the recirculation valve adjusts its opening from 25% up to 100%. Between 0% and 25%, the valve features a quick-opening characteristic with linear control behavior, preventing the valve components from being eroded due to excessive wear at very low opening positions.

3.4 Heater Control Valve

For some units, the emergency drain control valves are designed with fast-opening flow characteristics. When the heater water level becomes too high or the normal drain control valve fails, the emergency drain valve fully opens, causing the heater to operate without any water level—resulting in poor efficiency, severe steam-and-water erosion, and significant vibration in the internal piping system. Therefore, it is recommended that the emergency drain control valve be selected with a linear characteristic. This design allows the valve to quickly open in response to heater leaks, ensuring adequate flow while also enabling precise estimation of the leakage rate based on the valve’s opening degree. Additionally, when the normal drain control valve malfunctions, the emergency drain valve can step in to maintain proper drainage, safeguarding the heater’s operation. Typically, the emergency drain valve’s flow capacity is 1.5 to 3 times greater than that of the normal drain control valve.

3.5 Shaft Seal Control Valve

During startup and at 50% load, the turbine unit is supplied with auxiliary steam to the shaft seal system. The shaft seal system includes a steam supply control valve, a pressure relief valve, and a desuperheating water control valve, among others. When the unit load exceeds 50%, the shaft seal steam supply control valve closes, as the steam leakage from the main steam valve stem now sufficiently meets the shaft seal requirements. At this point, the system relies on the shaft seal pressure relief valve for fine-tuning, enabling the shaft seal system to operate in a self-sealing mode. Since the main unit's shaft seal system directly impacts operational safety—excessive shaft seal pressure can lead to water ingress into the lubrication oil system, while insufficient pressure may result in poor vacuum levels—it’s crucial to maintain stable conditions. To prevent unnecessary shutdowns caused by excessive pressure fluctuations or valve malfunctions during operation, it is recommended to select an equal-percentage flow characteristic for the shaft seal control valve and to incorporate manual operation capability. For reference, Table 1 provides examples of how regulating valves are applied in the piping systems of thermal power plants.

Table 1: Application of Each Control Valve in the System

4 Conclusion

Based on the operating mode and system principles, appropriately selecting the flow characteristics of control valves can enhance their regulation performance, reduce media energy losses, extend valve life, ease the workload of operators, minimize environmental noise, improve plant efficiency, and ensure the safe and stable operation of the unit.