Control Valve Selection Guide: Types, Actuation, Sizing, and How to Choose

What Is a Control Valve?

A control valve is a power-operated device that modulates fluid flow in response to a signal from a controller. Specifically, Unlike isolation valves (gate, ball) that only open or close, control valves position the valve trim at any point between fully open and fully closed to maintain process variables — flow rate, pressure, temperature, or level — at set points. This guide covers essential control valve selection information.

Control valves are the final control element in any process control loop. Together with a sensor, transmitter, and controller, they form the closed-loop system that keeps industrial processes running within specification.

Key Components of a Control Valve

• Body — The pressure-containing housing that connects to the piping
• Trim — Internal components (seat, disc/plug, stem, cage) that control flow
• Actuator — Pneumatic, electric, or hydraulic device that positions the trim
• Positioner — Device that compares the control signal to the valve position and adjusts the actuator accordingly
• Accessories — I/P converter, limit switches, solenoid valves, air sets, volume boosters

Main Types of Control Valves

1. Globe Control Valve (Linear Motion)

The most common control valve type. Notably, The plug moves linearly against the seat to modulate flow. Available in single-seat, double-seat, cage-guided, and balanced plug designs.

Best for: Precise throttling, high pressure drop, cavitating or flashing service, wide rangeability.

2. Rotary Control Valve (Quarter-Turn)

Rotary control valves use a rotating element (ball, disc, or plug) to modulate flow. For example, They offer higher capacity and lower cost than globe valves for certain applications.

3. Diaphragm Control Valve

Uses a flexible diaphragm and weir body design. The diaphragm isolates the fluid from the bonnet and actuator, making it ideal for corrosive, toxic, or sterile fluids.

Best for: Corrosive chemicals, pharmaceutical, food & beverage, water treatment.

4. Control Valve vs. Isolation Valve

Actuator Selection

Pneumatic Actuators

Most common in process industries. Fast response, low cost, fail-safe (spring-return). Types: diaphragm (linear), piston (linear/rotary), rack & pinion (quarter-turn).

Electric Actuators

No air supply required. Slower response but precise positioning. Ideal for remote locations, clean environments, and where instrument air is unavailable.

Hydraulic Actuators

Highest thrust capacity. Ultimately, Used for very large valves, high-pressure service, and where rapid response with high force is needed. Common in pipelines and hydroelectric plants.

Control Valve Flow Characteristics

The flow characteristic describes how flow changes relative to valve stem position. This affects loop stability and control range.

• Linear — Flow is proportional to stem position. Best for level control and constant-pressure systems.
• Equal Percentage — Equal changes in stem position produce equal percentage changes in flow. Best for pressure control, high ΔP systems, and when most of the system pressure drop is in the piping.
• Quick Opening — Large flow increase at small openings. Used for on-off service and relief valves.

Control Valve Sizing Fundamentals

Proper sizing prevents cavitation, noise, and premature wear. Key parameters:

• Cv (Flow Coefficient) — Number of US gallons per minute of 60°F water that flow through the valve with a 1 psi pressure drop
• Pressure Drop (ΔP) — Difference between upstream and downstream pressure
• Choked Flow — Maximum flow condition where downstream pressure can no longer increase flow
• FL (Pressure Recovery Factor) — Accounts for valve geometry’s effect on pressure recovery
• Rangeability — Ratio of maximum to minimum controllable flow (typically 30:1 to 100:1 for globe valves)

Control Valve Material Selection

8-Step Control Valve Selection Method

1. Define service conditions — Fluid, temperature, pressure, flow rate (normal/max/min), viscosity, specific gravity
2. Determine valve type — Globe (precise control, high ΔP) or rotary (high capacity, lower cost)
3. Calculate Cv — Use standard sizing equations per IEC 60534-2-1 or ISA-75.01
4. Check for cavitation / flashing — Compare ΔP to allowable pressure drop. Use anti-cavitation trim if needed
5. Select flow characteristic — Linear for level control; equal % for pressure and flow control
6. Choose body and trim materials — Based on temperature, pressure, corrosion resistance, and erosion
7. Size the actuator — Account for required thrust, process pressure forces, packing friction, and safety factor
8. Specify accessories — Positioner, I/P converter, solenoid valve, limit switches, air filter regulator

Common Control Valve Applications by Industry