Control Valve Positioner: Function, Types, and Working Principle

Control Valve Positioner: Function, Types, and Working Principle

Control Valve Positioner: Function, Types, and Working Principle
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Introduction

Control valves are integral components in many industrial processes, regulating the flow of fluids, gases, and other materials through pipelines and systems. To ensure precise regulation, a control valve positioner is often added as a key accessory. This device ensures the valve stem or shaft reaches and maintains the required position as dictated by the control system's setpoint. By delivering pressurized air to the valve actuator, the positioner plays a vital role in ensuring accurate, responsive valve control. In this article, we will explore the working principle, types, and applications of valve positioners in control systems.

What is a Positioner in the Control Valve?

A valve positioner is an essential component used in control valves to adjust the position of the valve stem or actuator shaft, ensuring that it aligns accurately with the desired setpoint from the control system. The main function of a valve positioner is to compare the actual position of the valve stem with the required position based on the control signal and apply the necessary pneumatic pressure to the actuator to move the valve stem accordingly.

Positioners are typically pneumatic or analog I/P (current-to-pressure) types and are primarily used for throttling valves that require precise control. The feedback mechanism in positioners ensures that the valve's stem movement aligns with the input signal, overcoming the forces that may affect the stem’s movement, such as friction or differential pressure across the valve plug.

 

Positioner in the Control Valve

The Need for a Positioner in Control Valves

Without a positioner, the relationship between the control signal and valve stem position may not be linear or accurate due to various factors. The two main forces acting on the valve stem are:

Actuating Force

The pressure from the actuator, which is responsible for moving the valve stem.

Spring Force

The spring inside the actuator, which provides resistance to the movement of the stem.

However, additional forces, such as friction from the stem packing and the reaction force caused by differential pressure across the valve plug, can disrupt the ideal relationship between the actuator’s pressure and the valve’s position. These forces lead to deviations between the actual stem position and the commanded position, which can impair the valve’s performance.

To address this issue, the valve positioner serves as a feedback control system, continuously adjusting the air pressure delivered to the actuator to maintain the required stem position despite the interfering forces.

 

Control Valve Positioner

Control Valve Positioner Working Principle

Valve positioners work by continuously monitoring the position of the valve stem and comparing it with the setpoint from the control system. They adjust the air pressure applied to the actuator diaphragm or piston to achieve the desired stem position. This feedback mechanism ensures that the valve responds precisely to changes in the control signal.

The principle of operation can be explained as follows:

Input Signal

The valve positioner receives an input signal, which is typically a 4-20mA electrical signal (for electro-pneumatic positioners) or a 3-15 psi pneumatic signal (for pneumatic-pneumatic positioners) from the controller. The input signal represents the desired stem position or the setpoint.

Feedback

The positioner continuously measures the actual position of the valve stem and compares it with the setpoint signal. The actual position is determined by the valve’s motion, which can be linear or rotary, depending on the type of control valve.

Adjustment

If the actual position does not match the desired position, the positioner adjusts the air pressure sent to the actuator. It increases or decreases the pressure to move the valve stem to the correct position.

Control Loop

The positioner operates as a control system within itself, where the valve’s stem position is the process variable (PV), the input signal from the controller is the setpoint (SP), and the manipulated variable (MV) is the air pressure sent to the actuator.

This feedback loop ensures that the control valve moves accurately in response to the control system’s commands, compensating for external forces that might otherwise affect its operation.

Types of Valve Positioners for Control Valves

There are several types of valve positioners available, each suited to different applications and control systems. The three most common types are:

1. Mechanical Electro-Pneumatic Valve Positioner (E/P Positioner)

An electro-pneumatic positioner is one of the most widely used types in industrial applications. It converts an electrical input signal, typically in the form of 4-20mA, into a pneumatic signal (3-15 psi). The positioner uses this pneumatic signal to control the actuator’s position.

Advantages: Electro-pneumatic positioners provide high accuracy and reliability in controlling valve positions. They are commonly used for applications where precise and repeatable valve control is essential.

2. Mechanical Pneumatic-Pneumatic Valve Positioner (P/P Positioner)

A pneumatic-pneumatic positioner operates solely with pneumatic signals. It typically receives a 3-15 psi pneumatic signal from the controller and adjusts the actuator’s position accordingly by regulating the pneumatic pressure. These positioners do not require an electrical input and are often used in pneumatic systems where electrical components are not feasible.

Advantages: Pneumatic-pneumatic positioners are suitable for environments where electrical control systems are not available or desired, such as in hazardous or explosive atmospheres. They offer a simple design and are highly reliable in basic applications.

3. Digital Positioner (Smart Positioner)

A digital positioner, also known as a smart positioner, offers enhanced features compared to traditional positioners. These devices can communicate digitally with control systems, allowing for more advanced diagnostic and configuration capabilities. Digital positioners are capable of self-calibration, monitoring performance, and providing real-time data on the positioner's health and valve status.

Advantages: Smart positioners offer superior accuracy, flexibility, and diagnostics. They can be easily integrated into modern process control systems, providing more control over the valve’s performance and offering remote monitoring and diagnostics.

How Does a Positioner Work?

The operation of a valve positioner is closely linked to the type of valve and actuator system in use. The positioner is installed either on the yolk or upper casing of a pneumatic actuator for linear control valves or directly on the valve/actuator stem for rotary control valves. The installation location allows the positioner to measure the movement of the valve stem (in linear valves) or the degree of rotation (in rotary valves).

Operation Flow:

1. The process controller sends a signal to the valve positioner based on the desired valve position.

2. The positioner compares the actual position of the valve stem (feedback) with the setpoint signal.

3. If there is a discrepancy, the positioner adjusts the air pressure applied to the actuator diaphragm or piston.

4. The actuator moves the valve stem to the correct position, and the process continues until the desired position is achieved.

The valve positioner continuously fine-tunes the position of the valve stem to ensure that the valve reacts accurately to changes in the setpoint.

Advantages of Using a Valve Positioner

Accuracy and Precision

Valve positioners ensure that the valve stem position matches the control system’s setpoint precisely, even in the presence of external forces.

Improved Performance

By compensating for friction, differential pressure, and other forces, positioners ensure that the valve behaves consistently, providing stable process control.

Enhanced Efficiency

Accurate valve positioning ensures that the control valve operates optimally, reducing energy consumption and enhancing system performance.

Longer Valve Life

Proper valve control reduces wear and tear on the valve components, extending their service life.

Diagnostics and Monitoring: Advanced positioners, especially digital positioners, offer real-time diagnostics, enabling maintenance personnel to monitor valve performance and address issues before they lead to failure.

Conclusion

Control valve positioners are essential components in ensuring accurate and reliable valve operation in industrial control systems. By actively comparing the actual position of the valve stem to the desired position, positioners overcome the various forces that can disrupt valve movement and ensure that the valve responds precisely to the control system’s commands. Whether in pneumatic, electro-pneumatic, or digital forms, valve positioners provide enhanced control, efficiency, and reliability in modern process control applications.

Their role in optimizing performance, ensuring accuracy, and reducing wear makes them indispensable in industries such as chemical processing, oil and gas, and water treatment.