On the grand stage of industrial production, control valves play an extremely important role. They act like precise flow “regulators,” adjusting the valve opening via an actuator according to the signals sent from the controller, thereby achieving precise control of fluid flow. This control function ensures that various parameters in the production process remain stably within the required process ranges, facilitating process automation. Control valves are not only an indispensable part of an automated control system, but their correct selection and use directly affect the quality of the entire control system and can even impact the quality of the production output. However, many automated control systems fail to operate normally due to improper selection of control valves. Therefore, correctly choosing the right control valve is crucial for every automation control technician.
Control valves are commonly used regulatory devices in industrial production. Their core function is to change the valve opening through an actuator based on the control signal issued by the controller, thereby achieving precise fluid flow control. This control capability ensures that production parameters remain stable within the required ranges, facilitating automation. Control valves hold the following critical importance in automated control systems:
Key to Control Quality: The performance of a control valve directly impacts the quality of an automatic control system. An improperly selected valve can result in reduced control accuracy or even failure to achieve precise control.
Guarantee of Product Quality: Proper use of a control valve ensures the stability of process parameters, thereby maintaining consistent product quality. Any issues with the control valve may lead to product quality fluctuations or even waste.
System Operational Stability: The stability and reliability of a control valve directly affect the operational stability of the entire production system. Frequent valve failures can cause production interruptions, increasing maintenance costs and downtime.
Selecting a control valve is a complex process that requires consideration of multiple factors. The main points for valve selection are as follows.
Control valves come in a wide variety of body types, including straight-through single-seat, straight-through double-seat, angle, diaphragm, low-flow, three-way, eccentric rotary, butterfly, sleeve, and ball valves. When selecting a valve body type, the following factors must be considered:
Flow Characteristics and Unbalanced Force: Different valve body types exhibit varying performance in flow control and force balance. For instance, straight-through single-seat valves are suitable for low flow, low pressure drop, and low leakage scenarios. They feature simple structure, good sealing performance, but relatively large unbalanced forces, which may require a more powerful actuator. Straight-through double-seat valves, on the other hand, are suitable for high flow and high pressure drop conditions. They feature two plugs and seats, partially balancing the forces, though the leakage is relatively higher.
Abrasiveness: When the fluid medium contains highly abrasive particles, the internal valve materials must be hard. For example, in slurry transport, the fluid contains many solid particles that can wear down internal valve components. Choosing valve components made of wear-resistant materials, such as hard alloys or ceramics for plugs and seats, can significantly extend the valve’s service life.
Corrosiveness: If the medium is corrosive, it is advisable to choose a valve with a simple structure. Simpler valves are easier to maintain and replace, and they reduce corrosion “dead zones.” For example, diaphragm valves, which control flow via the elastic deformation of a diaphragm, are suitable for highly corrosive media like acidic or alkaline solutions.
Temperature and Pressure: When the medium exhibits high and fluctuating temperature or pressure, the valve’s plug and seat materials should be minimally affected by these changes. In high-temperature, high-pressure steam systems, materials such as high-temperature alloy steels ensure that the valve operates reliably under extreme conditions.
Flash and Cavitation: Flashing and cavitation are common problems in liquid media passing through control valves. Flashing occurs when a liquid rapidly vaporizes due to pressure reduction, forming bubbles. Cavitation refers to bubble collapse in high-pressure regions, generating shock waves. Both phenomena can cause valve vibration, increased noise, and damage. Valve selection should aim to minimize these issues, either by choosing valves with anti-cavitation design or adjusting operating parameters to keep fluid pressure changes within safe limits.
For a control valve to operate properly, the actuator must provide sufficient output force to ensure proper valve sealing and movement. The actuator is the power source for the valve, responding to the controller’s signals to drive valve opening or closing. When selecting an actuator, the following factors must be considered:
For double-acting pneumatic, hydraulic, or electric actuators, there is generally no return spring. The applied force is independent of the operating direction. The key in selection is understanding the maximum output force and the torque of the motor. Large industrial systems often require high force; in such cases, an actuator with high output force should be selected.
For single-acting pneumatic actuators, the output force varies with the valve opening, affecting the valve’s motion characteristics. It is essential to balance forces across the valve’s full stroke. Single-acting pneumatic actuators are widely used in small control systems due to their simple structure and low cost.
Explosion-proof Requirements: In hazardous environments, pneumatic actuators are preferred because they use compressed air and pose no electrical spark risk.
Energy-saving Considerations: Electric actuators are generally more energy-efficient and provide precise motor control.
Control Accuracy: Hydraulic actuators offer high output force and control precision, suitable for applications requiring very fine adjustment, such as turbine speed control in power plants or temperature control in refinery reactors.
The action mode is mainly considered when using pneumatic actuators, determined by the combination of actuator action and valve action. There are four combinations: direct-direct (air-to-close), direct-reverse (air-to-open), reverse-direct (air-to-open), and reverse-reverse (air-to-close). The action mode can be categorized as either air-to-open or air-to-close. Selection should consider:
Process Requirements: Choose the action mode based on the process. For example, in systems handling toxic gas, air-to-close valves are preferred, so that the valve closes automatically in case of air supply failure, preventing leaks.
Medium Characteristics: For systems requiring continuous operation, such as steam heating systems, air-to-open valves are preferred to maintain flow if air supply fails.
Product Quality and Economic Losses: Choose an action mode that minimizes product quality fluctuations and economic losses in critical applications.
The structural form of a control valve should consider process conditions (temperature, pressure, flow), medium properties (viscosity, corrosiveness, toxicity), control system requirements (rangeability, leakage, noise), and cavitation prevention. Commonly used valves include single-seat, double-seat, sleeve, and butterfly valves. Specific selections:
Single-seat valves: Suitable for low flow, low pressure drop, and low leakage; simple, economical, and reliable.
Double-seat valves: Suitable for high flow, high pressure drop, and less strict leakage requirements.
Sleeve valves: Suitable for high-pressure drop and vibrating media; complex but excellent for cavitation mitigation.
Flow characteristics define the relationship between valve opening and flow rate. Common characteristics include:
Equal Percentage: Flow rate increases exponentially with valve opening; suitable for a wide range of flows, providing smooth and sensitive control. Widely used in engineering.
Linear: Flow rate increases proportionally with valve opening; suitable for low-pressure variation applications but can cause overshoot or oscillations in small flow conditions.
Parabolic: Flow rate increases with the square root of valve opening; improves linear characteristic performance at small openings. V-ball valves often use this characteristic.
Quick-opening: Flow reaches maximum quickly with small opening; ideal for fast on-off applications.
Flow characteristics directly affect control quality and system stability, so proper selection is critical.
Valve sizing commonly uses the C-value method (flow coefficient method):
Determine the valve type and flow characteristic based on process conditions and control requirements.
Determine the C-value calculation method and formula for the chosen valve type and flow characteristic.
Calculate the maximum flow Cmax by substituting values into the formula.
Choose the rated Cv value closest to 1.2 × Cmax from standard tables; the corresponding valve diameter is selected.
Verify noise and valve opening: maximum flow generally ≤ 85% open; minimum flow ≥ 20% open. Adjust Cv if needed until satisfactory results are obtained.
Material selection covers the valve body/cover and internal components (stem, plug, seat):
Body and Cover: Act as a pressure vessel; must withstand temperature, pressure, and corrosion. For high-temperature, high-pressure steam, carbon steel or alloy steel is typical.
Internal Components: Responsible for throttling; must resist corrosion and erosion. Corrosive media may require stainless steel or hard alloy materials to enhance durability.
Two key principles govern material selection: safety and reliability (resistance to temperature extremes, pressure, cavitation, and corrosion) and performance, lifespan, and cost-effectiveness.
Control valves are critical components in industrial automation systems. Their proper selection affects production stability and product quality. Selecting a control valve requires comprehensive consideration of valve body type, actuator, action mode, structure, flow characteristic, sizing, and material. Body type selection should account for flow characteristics, medium properties, temperature and pressure variations, and flashing/cavitation. Actuators must meet output force, explosion-proof, energy-saving, and control precision requirements. Action mode should align with process needs, medium properties, and economic considerations. Structure selection must integrate process conditions, medium properties, system requirements, and cavitation prevention. Flow characteristics directly impact control quality and stability. Sizing follows the C-value method, and materials must ensure safety and durability while balancing performance and cost. Scientific and rational selection ensures stable operation, high control accuracy, extended equipment life, and efficient automation, providing a solid foundation for production stability and product quality.