Control valve body type selection introduction

Control valves, also known as regulating valves, are essential actuators that receive control signals from the control unit to regulate fluid flow through power operation. A typical regulating valve consists of an actuator and a valve body. Depending on the type of power used, control valves can be classified into three main categories: pneumatic, electric, and hydraulic. Pneumatic control valves use compressed air as their power source, electric control valves rely on electrical energy, and electro-hydraulic valves operate using pressure from a liquid medium such as oil. Additionally, there are specialized types like solenoid valves, electronic valves, intelligent valves, and fieldbus-enabled valves, each designed for specific applications. When selecting the valve body, several factors must be considered. Common types include single-seat, double-seat, angle, diaphragm, small-flow, tee, eccentric rotary, butterfly, sleeve, and ball valves. The choice depends on the following aspects: 1. **Flow Characteristics**: The shape of the valve plug should be selected based on the desired flow characteristics and unbalanced forces. 2. **Abrasion Resistance**: If the fluid contains abrasive particles, the internal materials of the valve should be hard and durable. 3. **Corrosion Resistance**: For corrosive media, it's best to choose a simple and robust valve structure. 4. **Temperature and Pressure**: When dealing with high temperature or pressure changes, the valve body and seat materials should remain stable under varying conditions. 5. **Prevention of Flash and Cavitation**: These phenomena occur only in liquid media and can cause vibration, noise, and reduced valve life. Therefore, it’s important to select a valve that minimizes these effects. Choosing the right actuator is crucial for proper valve operation. The actuator must provide sufficient output force to ensure tight sealing and full valve opening. For double-acting actuators (pneumatic, hydraulic, or electric), there is no return spring, so the force is independent of the direction of movement. The key is to determine the maximum force and torque required. For single-acting pneumatic actuators, the output force is dependent on the valve position, and the force balance across the valve opening must be carefully considered. The type of actuator should be chosen based on process requirements and environmental conditions. In explosion-proof areas, pneumatic actuators are preferred. From an energy-saving perspective, electric actuators are more efficient. For high-precision control, hydraulic actuators are often the best choice, especially in applications like turbine speed control in power plants or temperature regulation in catalytic reactors. The action mode of the control valve is determined when using a pneumatic actuator. It is defined by the combination of the actuator's positive or negative action and the valve's positive or negative action. There are four possible combinations: direct-acting (air-to-close), reverse-acting (air-to-open), and others. The selection of the action mode should consider three main factors: process safety, media properties, and the need to minimize economic loss while ensuring product quality.

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