How to Choose Between On Off and Modulating Butterfly Valves

On-off butterfly valves are mainly used to fully open, fully close, or isolate a pipeline. Modulating butterfly valves adjust the valve opening in response to a control signal to regulate flow, pressure, or temperature.

The main difference between the two usually lies not in the butterfly valve body itself, but in the control duty and actuator configuration. In some applications, the same butterfly valve can be fitted with either an on-off or a modulating actuator. 

However, for modulating service, the valve size, flow characteristics, maximum differential pressure, seat material, actuator duty cycle, and positioning accuracy must also be considered.

On-Off vs. Modulating Butterfly Valves: What’s the Difference?

How Does On-Off Butterfly Valve Control Work?

On-off butterfly valves are mainly used to open or close a pipeline.

When the actuator receives an open or close signal, it rotates the valve stem and disc. Once the disc reaches the fully open or fully closed position, the limit switches provide valve status feedback to the control system.

Electric On-Off Butterfly Valves

Electric on-off butterfly valves are typically controlled by open/close signals or dry-contact inputs. When selecting an actuator, confirm the supply voltage, output torque, opening and closing time, and feedback requirements.

Pneumatic On-Off Butterfly Valves

Pneumatic on-off butterfly valves usually consist of a pneumatic actuator and a solenoid valve. The solenoid valve controls the compressed air supplied to the actuator, causing the valve to open or close.

If the valve must automatically return to a predefined position when the air supply is lost, a spring-return pneumatic actuator can be used.

On-off butterfly valves are commonly used for:

• Pipeline isolation
• Pump suction and discharge lines
• Tank inlet and drain lines
• Equipment bypass lines
• Water distribution network switching
• Batch processing systems

Some on-off actuators can temporarily stop at an intermediate position. However, they are not designed for frequent, stable adjustment of the valve opening. If continuous control of flow, pressure, or temperature is required, a modulating butterfly valve should be used.

How Does Modulating Butterfly Valve Control Work?

Modulating butterfly valves adjust the valve opening according to system requirements.

For example, when the system flow rate is too high, the actuator reduces the valve opening. When the flow rate is too low, the actuator increases the opening. This helps maintain flow, pressure, or temperature within the specified range.

Electric Modulating Butterfly Valves

Electric modulating butterfly valves typically use electric actuators with integrated positioning control modules. Common control signals include 4–20 mA and 0–10 V.

Pneumatic Modulating Butterfly Valves

Pneumatic modulating butterfly valves usually consist of a pneumatic actuator and a valve positioner. The positioner receives the control signal and regulates the air supplied to the actuator, thereby controlling the valve opening.

Modulating butterfly valves are commonly used in:

• HVAC chilled-water systems
• Cooling-water systems
• Heat exchangers
• Water treatment systems
• Industrial process water systems

Can a Butterfly Valve Be Used for Modulating Flow Control?

Butterfly valves can be used for flow control, but the valve size and actuator must be selected correctly.

The disc opening does not directly indicate the flow rate. For example, a valve that is 50% open does not necessarily provide 50% of the maximum flow. The actual flow rate also depends on the flow coefficient (Cv), the differential pressure across the valve, and the resistance of the piping system.

Valve size is also important. If a butterfly valve is oversized, it may operate at a small opening for long periods under normal conditions. This reduces the effective control range and may lead to unstable control, noise, vibration, or localized erosion.

Therefore, a modulating butterfly valve should not be selected based on pipe size alone. The design flow rate, maximum differential pressure, minimum flow rate, and system control requirements should also be considered.

How to Choose Between On-Off and Modulating Control

Use the following five checks to select the appropriate control method and actuator configuration.

1. Define the Valve Function

• Choose on-off control for opening, closing, or isolating a pipeline.
• Choose modulating control when flow, pressure, temperature, or liquid level must be continuously regulated.

2. Confirm the Control Signal

• On-off actuator: Receives open and close commands and normally provides fully open and fully closed feedback.
• Modulating actuator: Typically receives a 4–20 mA or 0–10 V signal and may provide continuous position feedback.

Confirm compatibility with the PLC, BMS, or DCS before selecting the actuator.

3. Check the Operating Frequency

Modulating actuators reposition the valve frequently, so their operating requirements are generally higher.

• Electric actuators: Check the duty cycle, maximum starts per hour, and thermal protection.
• Pneumatic actuators: Check the air pressure, air quality, and air consumption.

4. Verify the Actuator Torque

Do not select an actuator based on valve size alone. Consider:

• Maximum differential pressure
• Seat material
• Process medium
• Operating temperature
• Actual valve operating torque

5. Determine the Fail-Safe Position

Decide how the valve should respond if power, air supply, or the control signal is lost:

• Fail closed
• Fail open
• Fail in place

The final position should be selected according to the system’s safety requirements.

About the Author

Tina Wei

Sales Manager · Tianjin Union Valve Co., Ltd

More than 8 years of experience in the valve sales industry. Have extensive qualifications and experience.