Control valve knowledge

Control valves, also known as regulating valves, are essential components in automated control systems. They function as actuators that manipulate fluid flow by responding to control signals from a regulating unit. A typical control valve consists of an actuator and a valve body. Depending on the power source used by the actuator, control valves can be categorized into three main types: pneumatic, electric, and hydraulic. Pneumatic valves use compressed air, electric valves rely on electrical power, and electro-hydraulic valves operate using pressurized liquid, such as oil. Additionally, there are specialized types like solenoid valves, electronic, intelligent, and fieldbus-enabled valves, each suited for specific applications. When selecting the body type of a control valve, several factors must be considered. Common valve body types include single-seat, double-seat, angle, diaphragm, small-flow, tee, eccentric-rotary, butterfly, sleeve, and ball valves. The choice depends on: 1. **Flow Characteristics**: The shape of the valve plug should match the desired flow pattern and account for unbalanced forces. 2. **Abrasion Resistance**: In applications involving abrasive media, the internal materials of the valve should be durable and resistant to wear. 3. **Corrosion Resistance**: For corrosive media, it is advisable to choose valves with simpler designs to minimize damage. 4. **Temperature and Pressure Conditions**: In high-temperature or high-pressure environments, materials that remain stable under varying conditions should be selected. 5. **Prevention of Flash and Cavitation**: These phenomena occur only in liquids and can cause vibration, noise, and reduced valve life. Therefore, it's important to select valves designed to mitigate these effects. The selection of an actuator is equally critical. It must provide sufficient force to ensure proper sealing and valve operation. Double-acting actuators (pneumatic, hydraulic, or electric) typically do not require a return spring, and their output force is independent of direction. Thus, the key is determining the maximum force and torque required. Single-acting pneumatic actuators, however, depend on the valve’s position, requiring careful consideration of force balance during operation. Choosing the right actuator also depends on process requirements and environmental conditions. For explosion-proof environments, pneumatic actuators are preferred. From an energy-saving perspective, electric actuators are often more efficient. For high-precision applications, such as turbine speed control in power plants or temperature regulation in catalytic reactors, hydraulic actuators may be the best option. Finally, the mode of action of the control valve—whether it opens or closes when air or power is applied—is determined by the combination of the actuator’s and valve’s actions. There are four common configurations: direct-acting (air-to-close), reverse-acting (air-to-open), and others. This choice is influenced by process safety, media properties, and the need to minimize economic losses while ensuring product quality.

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