Single Acting vs Double Acting Positioners: Differences & Applications

Brian Craig

August 23, 2023

In the world of industrial automation, where precision and efficiency are paramount, positioners play a crucial role in ensuring that machinery and equipment perform their designated tasks with utmost accuracy. Positioners are devices designed to control the position of a mechanical element, typically a valve or an actuator, in response to a control signal. In essence, they bridge the gap between the digital control signals used in automation systems and the physical movement of industrial machinery.

By precisely adjusting the position of valves or actuators, positioners ensure that processes run smoothly, consistently, and with the desired level of accuracy. In this article, we delve into the realm of positioners, with a particular focus on the battle between single acting and double acting positioners.

Single Acting vs. Double Acting Positioners

What Is a Valve Positioner?

A valve positioner is a control device that ensures a control valve reaches and maintains the exact position commanded by the control system. It receives an input signal from a PLC, DCS, or process controller and adjusts the actuator using pneumatic or electro-pneumatic signals to accurately position the valve.

Valve positioners play a critical role in improving control accuracy, response time, and process stability. They are widely used in industries such as oil and gas, chemical processing, power generation, water treatment, pharmaceuticals, and food & beverage, where precise valve control directly impacts product quality, safety, and operational efficiency.

Importance of Positioners in Industrial Applications

Imagine a scenario in a petrochemical plant where the flow of volatile chemicals must be precisely controlled to ensure safe and efficient operations. A minor error in valve positioning could lead to catastrophic consequences. This is where positioners step in. From oil refineries to pharmaceutical production lines, from power plants to food processing facilities, positioners are omnipresent behind the scenes, orchestrating the dance of industrial machinery.

How Does a Valve Positioner Work?

A valve positioner continuously compares the desired valve position received from the control system with the actual valve position. If a difference exists, the positioner automatically adjusts the air pressure supplied to the actuator until the valve reaches the required position.

The operating sequence typically follows these steps:

The control system sends an input signal to the positioner.
The positioner compares the signal with the valve's actual position.
Pneumatic output pressure is adjusted accordingly.
The actuator moves the valve stem.
Feedback confirms the correct valve position has been achieved.

This closed-loop operation improves positioning accuracy, minimizes overshoot, and helps maintain stable process control even under changing operating conditions.

Quick Comparison: Single Acting vs Double Acting Positioners

Feature	Single Acting Positioner	Double Acting Positioner
Air Supply	One pneumatic output	Two pneumatic outputs
Spring Return	Required	Not required
Fail-Safe Operation	Yes	External fail-safe required
Positioning Accuracy	High	Very High
Air Consumption	Lower	Higher
Response Speed	Moderate	Faster
Typical Applications	Standard process control	High-performance control valves
Maintenance	Lower	Slightly higher

This comparison provides a quick overview before exploring each positioner type in more detail.

Overview of Single Acting and Double Acting Positioners

Before we dive into the pros and cons of each type, let's briefly understand the fundamental differences between single acting and double acting positioners.

Single Acting Positioners: These positioners use a single pneumatic signal to control the position of the actuator or valve. When the control signal changes, the positioner adjusts the position accordingly. Single acting positioners are often known for their simplicity and cost-effectiveness.
Double Acting Positioners: In contrast, double acting positioners utilize both the positive and negative pneumatic signals to control the actuator's position. This enhanced control mechanism allows for finer adjustments and improved responsiveness, making them suitable for applications that demand high precision.

Understanding Single Acting Positioners

Single acting positioner involves a pneumatic control signal that corresponds to the desired position of the actuator. This signal is compared with the actual position of the actuator, and the positioner's internal mechanism adjusts the air pressure to the actuator accordingly, driving it to the desired position. The positioner maintains this balance by regulating the air pressure in response to any deviations between the desired and actual positions.

A. Components of a single acting positioner typically include:

Input Signal Interface: This is where the control signal from the automation system is received.
Pneumatic Control Unit: The heart of the positioner, which manages the airflow and pressure adjustments.
Feedback Mechanism: Sensors that provide information about the current position of the actuator.
Output Connection: This is where the positioner interfaces with the actuator or valve it's controlling.

B. Pros of Single Acting Positioners

Simplicity and Cost-effectiveness: Single acting positioners are known for their straightforward design. Their simplicity not only translates to lower initial costs but also contributes to ease of installation and maintenance.
Lower Air Consumption: With only one pneumatic signal involved, single acting positioners generally consume less air compared to their double acting counterparts. This can lead to energy savings in the long run.
Reliability in Certain Applications: Single acting positioners are well-suited for applications where high precision isn't critical. In scenarios where a minor variation in positioning won't significantly impact the process outcome, these positioners can provide reliable control.

C. Cons of Single Acting Positioners

Limited Control Accuracy: Due to the reliance on a single pneumatic signal, single acting positioners may struggle to achieve the same level of precision as double acting positioners.
Reduced Responsiveness: Single acting positioners might exhibit slightly slower response times compared to their double acting counterparts, which can be a concern in situations requiring rapid adjustments.
Incompatibility with Complex Systems: As processes and control systems become more intricate, the limitations of single acting positioners become apparent. They may struggle to keep up with the demands of highly complex industrial setups.

Exploring Double Acting Positioners

In a double acting positioning system, two pneumatic signals are used to regulate the movement of the actuator. These signals work in tandem to drive the actuator to the desired position, while the positioner constantly adjusts the air pressure to maintain this alignment.

A. Operational Mechanism and Key Components

Control Signal Input: The automation system sends dual pneumatic signals, one positive and one negative, to dictate the desired position of the actuator.
Pressure Regulating Unit: This component meticulously manages the air pressure sent to the actuator, ensuring smooth movement and precise positioning.
Dual Feedback Mechanism: Feedback sensors relay information about the actuator's position, allowing the positioner to make instantaneous adjustments.
Output Interface: The positioner interfaces with the actuator or valve to control its movement.

B. Pros of Double Acting Positioners

Enhanced Control Accuracy: The dual pneumatic signals enable double acting positioners to achieve exceptional precision in positioning, making them ideal for applications that demand high-level accuracy.
Improved Response Time: The ability to control both positive and negative signals results in faster response times compared to single acting positioners. This attribute is crucial in time-sensitive processes.
Compatibility with Complex Control Systems: Double acting positioners shine in intricate industrial environments where systems involve numerous interconnected components. They are more adaptable to complex control architectures.

C. Cons of Double Acting Positioners

Higher Air Consumption: The utilization of two pneumatic signals increases air consumption, which can impact energy efficiency and operating costs over time.
Increased Complexity and Cost: The added sophistication of double acting positioners comes with a higher upfront cost and increased complexity in terms of installation and maintenance.
Potential Reliability Concerns in Certain Situations: While generally reliable, double acting positioners might encounter reliability challenges in specific conditions due to their more intricate design. In applications with stringent reliability requirements, careful consideration is essential.

Selecting the Right Positioner for Your Application

When it comes to selecting between single acting and double acting positioners, a thoughtful decision can significantly impact the efficiency, precision, and overall success of your industrial application. To guide your choice, consider the following factors:

Application Requirements:

Single Acting: If your application doesn't demand extreme precision and minor variations in positioning won't affect the outcome, a single acting positioner might suffice.
Double Acting: For processes where precision is paramount and even minor errors can lead to significant repercussions, a double acting positioner is better equipped to meet stringent requirements.

Control Precision Needed:

Single Acting: If your operation can tolerate some degree of positioning inaccuracy, single acting positioners could provide a cost-effective solution.
Double Acting: When precision is non-negotiable, double acting positioners offer the necessary accuracy to maintain consistent control.

System Complexity and Integration:

Single Acting: In less complex systems, single acting positioners are straightforward to integrate and maintain.
Double Acting: For intricate systems with numerous interdependencies, double acting positioners provide the adaptability required for seamless integration.

Operational Costs and Budget:

Single Acting: With lower upfront costs and reduced air consumption, single acting positioners might fit more modest budgets.
Double Acting: While the initial investment is higher, the potential for improved efficiency and reduced errors can justify the expense over time.

Common Valve Positioner Problems and Maintenance Tips

Proper maintenance helps valve positioners deliver accurate and reliable performance throughout their service life. Over time, environmental conditions, air quality, and mechanical wear can affect positioning accuracy.

Common issues include:

Low or unstable instrument air pressure
Air leakage in pneumatic tubing
Incorrect calibration
Sticky valve stem movement
Worn feedback mechanisms
Blocked air filters
Loose pneumatic or electrical connections

Routine inspection, periodic calibration, clean instrument air, and manufacturer-recommended preventive maintenance help reduce unexpected failures and improve long-term control valve performance.

Smart Valve Positioners and HART Communication

Modern digital valve positioners offer advanced diagnostic capabilities that extend beyond basic valve positioning. Many smart positioners support communication protocols such as HART, allowing engineers to remotely configure devices, monitor valve performance, perform diagnostics, and identify maintenance issues without interrupting normal operation.

Smart valve positioners are commonly used in automated process plants where predictive maintenance, asset management, and improved control performance are important operational requirements.

Single Acting vs Double Acting Positioners in Hazardous Areas

Both single acting and double acting positioners are widely used in hazardous industrial environments, provided they are installed with equipment certified for the classified area. In applications involving flammable gases, vapors, or combustible dust, engineers should ensure that the selected positioner complies with applicable standards such as ATEX, IECEx, or Class I Division 1/Division 2 requirements.

The choice between single acting and double acting positioners in hazardous areas depends on the actuator design, fail-safe requirements, process safety objectives, and the level of control accuracy required by the application.

Based on your evaluation, select the positioner type that best aligns with your application's needs and priorities. By carefully considering these factors and following a systematic decision-making process. At The Transmitter Shop, we offers one-stop solution for all your control valve requirements. Among the many well-known brands, Fisher is one of the leading manufacturers of valve positioners. We have a wide range of valve positioners / controllers manufactured by Fisher. If you have any questions, please contact us.

Frequently Asked Questions (FAQ's)

What is the difference between a valve actuator and a valve positioner?

A valve actuator generates the mechanical force required to open or close a valve, while a valve positioner controls the actuator to ensure the valve reaches the exact position requested by the control system. The actuator provides movement, whereas the positioner improves positioning accuracy and overall process control.

Can a single acting positioner be used with a double acting actuator?

No. A single acting positioner is designed for actuators with a spring-return mechanism, while a double acting actuator requires pneumatic pressure on both sides of the actuator. For proper operation and reliable performance, the positioner must match the actuator type.

Which valve positioner is better for fail-safe applications?

Single acting positioners are generally preferred for fail-safe applications because the spring-return actuator automatically moves the valve to a predefined safe position if the air supply is lost. Double acting systems typically require additional accessories to provide similar fail-safe functionality.

Do digital valve positioners support HART communication?

Yes. Many modern digital valve positioners support HART communication, allowing engineers to configure devices, monitor diagnostics, verify calibration, and perform predictive maintenance without interrupting the standard 4–20 mA control signal.

How do you calibrate a valve positioner after installation?

Valve positioner calibration typically involves verifying the input signal, adjusting the zero and span settings, confirming valve travel, and checking feedback accuracy. Many digital positioners also offer automatic calibration routines that simplify commissioning and improve positioning precision.

Can valve positioners be installed in hazardous areas?

Yes. Valve positioners can be installed in hazardous locations when they are certified for the appropriate area classification, such as ATEX, IECEx, or Class I Division 1/Division 2. Proper installation and compliance with local safety standards are essential.

What causes valve positioners to hunt or oscillate?

Valve hunting or oscillation can result from incorrect tuning, unstable instrument air pressure, excessive actuator friction, improper feedback adjustment, or an oversized control valve. Routine maintenance and proper calibration help maintain stable valve performance.

How do I choose between a single acting and double acting positioner?

The selection depends on factors such as process safety, required positioning accuracy, actuator type, fail-safe requirements, available air supply, and overall control objectives. Single acting positioners are typically used with spring-return actuators, while double acting positioners are preferred for applications requiring precise bidirectional control.

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