Proportional Control Valve: Actuator Choice, Kvs and Correct Sizing

In heating, cooling, steam and process lines, keeping variables such as flow, pressure and temperature at the desired value is essential for product quality and energy efficiency. This is the job of the proportional control valve. A control valve continuously and gradually changes its flow area (modulation) in response to a pneumatic or electric signal from the controller, precisely adjusting the amount of medium going to the process.

This article covers actuator options, the technical features of control valves and the key criteria for correct sizing. The example values refer to the STEVI series control valves of ARI Armaturen, ORFA’s partner.

How Does a Proportional Control Valve Work?

A proportional control valve essentially consists of the valve body, the trim (plug and seat ring) and the actuator. The controller compares the measured value (for example temperature) with the setpoint and sends a position signal to the actuator. The actuator moves the valve stem accordingly; the gap between plug and seat changes and the flow increases or decreases. Unlike on/off valves, it can hold stable intermediate positions.

Actuator Choice: Pneumatic or Electric?

The actuator, which converts the control signal into mechanical movement, determines the valve’s response speed, positioning accuracy and behaviour on failure.

Pneumatic Actuators

• Principle: Compressed air moves a diaphragm to position the valve stem. With a spring-return design, the valve moves to the safe position (open or closed) defined by the spring when air is lost. The example actuators use a maximum supply pressure of 6 bar.
• Advantage: Fast response, simple design and high closing force. Preferred where fast closing is needed and for high differential pressures.

Electric Actuators

• Advantage: Precise positioning, no need for a compressed air supply and easy integration into automation systems. Position feedback is available with add-on modules such as a potentiometer.
• Extra safety: Models with a fail-safe function that moves the valve to a safe position on power failure are available.

Technical Features (ARI-STEVI Example)

A bellows seal prevents leakage to atmosphere along the stem and is preferred for high-temperature media and applications where emission control matters.

Correct Selection and Sizing

A control valve’s performance depends largely on correct sizing. A valve chosen to match the pipe size is often oversized; it then operates mostly near closed, control becomes unstable and the trim wears faster.

• Kvs value: The water flow (m³/h) the valve passes when fully open at a differential pressure of 1 bar. The required Kv for the process is calculated from flow rate, differential pressure and fluid properties, and a valve with a suitable Kvs is selected.
• Flow characteristic: Equal percentage is commonly preferred where the differential pressure across the valve varies with load (for example heat exchanger temperature control). Linear suits applications where the differential pressure across the valve stays largely constant.
• Noise and cavitation: At high differential pressures, noise level and cavitation risk must also be checked during sizing.

ORFA supports sizing with ARI’s myValve® software. Using process data such as medium, temperature, differential pressure and noise level, the program identifies a suitable control valve. It includes an integrated database of more than 160 media and gives access to ordering information, spare part drawings and data sheets for the selected product.

Frequently Asked Questions

Conclusion

Selected with the right actuator, the right Kvs and a suitable flow characteristic, proportional control valves control process variables stably and precisely. For valves and sizing support matched to your system data, see our Proportional Control Valves page or contact the ORFA technical team.