Key Control Valves for Power Generation Unit Operation

During the operation of power generation units, the design and function of control valves are critical. These valves affect both the system's stability and the unit's safety and efficiency. Different types of control valves have unique control characteristics and functions under different operating conditions. This article explores the functions, control characteristics, and application requirements of several key control valves. It also offers optimization suggestions based on specific operating modes.

Condensate Low-Load Control Valve

The condensate low-load control valve is mainly used before unit startup to maintain the deaerator water level and meet the operating requirements at 40% load. Its function is to control the flow of condensate at low loads, ensuring the stability of the deaerator water level. When the valve opening reaches 87.5%, the high-load condensate control valve will automatically open. The two valves work together to control the condensate system and keep the condenser water level within the proper range.

Control Characteristics and Applications

Flow Characteristics: The condensate low-load control valve should adopt a linear flow characteristic for precise and stable control.

High-Load Control: At 50% load, if the high-load condensate control valve is open less than 10%, it may cause ineffective control of the condenser's hot well water level. This could threaten unit safety. Therefore, the high-load control valve must maintain an appropriate opening at high loads to control condensate flow.

Shutdown Requirements: During unit shutdown, the high-load control valve must remain tightly closed to prevent system faults or leaks.

Design Suggestion: The flow capacity of the high-load control valve must account for the overall failure of the condensate system. This includes failures in the low-load valve, condensate recirculation issues, and leaks, ensuring system safety and stability.

The condensate low-load control valve uses a linear flow characteristic. The high-load control valve should adopt an equal percentage flow characteristic for precise load adjustment and efficient system operation.

Feedwater Control Valve

The feedwater control valve adjusts the electric pump feedwater system to maintain stable drum water levels during unit operation. At low loads, the valve keeps the drum water level normal. As the unit load increases, the valve switches to feed pump operation. This ensures that water levels and pressure are effectively controlled even at high loads.

Control Characteristics and Applications

Linear Control Characteristics: The feedwater control valve should use a linear flow characteristic. This ensures fast and stable water replenishment when the electric pump starts, preventing excessive drum water level fluctuations.

Load Switching: At 40% load, the feedwater control valve must switch to feed pump operation. If the feed pump trips, the electric pump must start quickly and provide enough water to stabilize the drum water level.

Control Performance Requirements: An equal percentage flow characteristic would slow the valve's opening during electric pump startup (about 2–3 seconds), which may cause the drum water level to become unstable. Therefore, the feedwater control valve must have a high response speed. A linear control characteristic is better suited to this requirement.

The feedwater control valve should meet the feedwater demands at 50% load and have excellent control characteristics to ensure drum water level stability, especially during electric pump startups or feed pump trips.

Pump Recirculation Control Valve

The pump recirculation control valve adjusts the flow rate of the pump to ensure load balance and system stability. When the pump flow is below 25% of the rated flow, the valve opens to adjust the flow to meet system needs.

Control Characteristics and Applications

Control Characteristics: The pump recirculation control valve should use a linear flow characteristic. When the flow is low (0–25%), the valve must open quickly to avoid damage from small openings.

Load Range: When the pump flow is between 25% and 30% of the rated flow, the valve opening will be 25%. If the flow exceeds 25%, the valve will close. If the pump flow is below 25%, the valve opening will adjust within the 25%–100% range.

The pump recirculation control valve must consider load fluctuations and system stability. The linear flow characteristic provides fast response and accurate control, avoiding damage during small opening adjustments.

Heater Control Valve

The heater control valve adjusts the water level in the heater and controls the condensate system. In emergency situations, such as when the normal condensate control valve fails, the emergency condensate control valve will open fully. This ensures the heater operates without water, preventing dangerously high water levels.

Control Characteristics and Applications

Linear Control Characteristics: The emergency condensate control valve should use a linear control characteristic. This allows it to open quickly when a leak occurs in the heater, meeting flow requirements. The linear characteristic helps maintenance personnel detect faults by evaluating the valve opening.

Flow Capacity: The flow capacity of the emergency condensate control valve should be 1.5 to 3 times greater than the normal condensate control valve to provide enough flow in case of condensate system failure.

The emergency condensate control valve should be designed with good control characteristics. It must respond quickly to emergency conditions and replace the normal condensate valve's function if it fails, ensuring safe heater operation.

Shaft Seal Control Valve

The shaft seal control valve adjusts the steam supply to the shaft seal system, ensuring the sealing of the main machine's shaft. When the unit load is below 50%, auxiliary steam is supplied. When the load exceeds 50%, the shaft seal supply valve closes, and steam leakage from the main steam valve rod is used to meet the shaft seal's needs.

Control Characteristics and Applications

Equal Percentage Control Characteristics: Due to the importance of the shaft seal system to the main machine's safety, the shaft seal control valve should use equal percentage flow characteristics. This ensures precise control even with large load fluctuations. Adding a manual operation feature also helps intervene during system abnormalities and prevent shutdown.

Pressure Control: Both high and low shaft seal pressures can cause water ingress into the lubrication system or a reduction in unit vacuum. Precise control of shaft seal pressure is necessary to avoid excessive fluctuations.

The shaft seal control valve should provide stable control during load fluctuations. It should use equal percentage flow characteristics for precise control. Adding a manual operation feature increases system flexibility and emergency response capability.

Conclusion

Control valves play a critical role in various systems of power generation units. From the condensate system to the feedwater system, from pump recirculation to the heater and shaft seal systems, each valve design directly impacts the unit's efficiency and safety. By carefully selecting the flow characteristics and control methods, the stability and flexibility of the system can be significantly improved, reducing the risk of failures. Therefore, rational selection and optimization of control valve functions and control characteristics are key to ensuring efficient and safe unit operation.