How Does a Control Valve Works ?

Control valve is a process control instrument is used to control the flow, pressure, level and temperature using pipeline fluid flow controlling. Control Valve operates pneumatically and is positioned by a 4–20 mA control signal. control valve operate from PLC, DCS and simple PID controllers

1. Basic Working Principle

A control valve regulates flow, pressure, temperature, or level by changing valve travel according to a controller signal (typically 4–20 mA).

4 mA = min (usually closed) to 20 mA = max (usually open)

2. Pneumatic Operation (step-by-step)

Controller outputs 4–20 mA.
An I/P converter converts current to pneumatic pressure (commonly 3–15 psi or 0.2–1.0 bar).
The pneumatic actuator receives air pressure and moves the valve stem/plug/ball.
The valve opening changes the process flow.

3. Key Components

Positioner — ensures accurate valve position for the given signal; compensates for friction and load.
Actuator types — pneumatic (common), electric, hydraulic.
Valve types — globe valve, butterfly valve, ball valve, diaphragm valve.

4. Control Valve Action (fail-safe)

Air-to-Open (Fail Close): More air = valve opens; on air failure the valve closes.
Air-to-Close (Fail Open): More air = valve closes; on air failure the valve opens.

5. Why Pneumatic Valves?

Fast response, high force, explosion-proof (no electrical sparks), and reliable in harsh environments.

Quick Overview

Controller → 4–20 mA → I/P converter → 3–15 psi → Pneumatic actuator → Valve opens/closes