Control Valves - An Overview

Control Valve Functionality:

A control valve regulates fluid flow within a process system by using a positionner and an actuator, allowing for precise control based on an external control signal.

Key Components:

• Control Valve: The valve itself, which can be a globe, ball, or butterfly valve, that adjusts fluid flow based on its opening position.
• Actuator: Converts the control signal into mechanical movement to set the valve’s position.
• Pneumatic Actuator: Uses compressed air to move a diaphragm or piston linked to the valve stem.
• Positioner: Ensures accurate valve positioning by receiving feedback from the valve and making corrective adjustments to the actuator.

Working Principle:

• Control Signal Input: A control system (e.g., PLC, DCS) sends a control signal (4-20 mA) to the positionner.

- 4 mA (or 3 psi): Valve fully closed (0% open).

- 20 mA (or 15 psi): Valve fully open (100% open).

- Intermediate values set the valve to specific opening percentages (e.g., 12 mA = 50% open).

• Positionner Functionality: The positionner compares the control signal with the actual valve position (from a feedback sensor). If there is a discrepancy, it adjusts the actuator to achieve the correct position, often using PID control for smooth and precise positioning.
• Actuator Movement: The positioner’s commands move the actuator, which adjusts the valve stem and thereby the valve plug or disc:

*For Pneumatic Actuators: Air pressure moves a diaphragm or piston, adjusting the stem.

• Feedback Control Loop: The control system continuously monitors the process variable (e.g., flow rate or pressure). If deviations occur, a new control signal is sent to the positioner, which readjusts the actuator and valve to maintain the desired set point.

Summary:

The control valve system operates in a closed-loop manner, using a positionner for accurate valve positioning based on control signals. The actuator translates this signal into mechanical movement, adjusting the valve’s opening to regulate fluid flow. This coordinated operation ensures optimal performance in industrial processes by maintaining the target operational parameters.

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