Why You Should Use Control Valve Positioners?

A valve positioner is a device used to enhance valve control by ensuring precise positioning. In a basic setup, the process controller sends a signal directly to the actuator to move the valve, but this can be slow and prone to offset, limiting accuracy. In applications needing frequent adjustments or high precision, a valve positioner helps achieve fast and accurate responses.

The positioner acts as an interface between the process controller and the actuator, monitoring the valve stem to detect its exact position. It receives an input signal from the controller and sends an output signal to the actuator, ensuring precise valve movement.

Basics of Control Valve Positioners

A valve positioner is a device that adjusts the air pressure applied to a control valve actuator to align the valve stem’s position precisely with the controller’s output signal. This helps maintain accurate valve positioning, especially when dealing with variable process conditions.

Positioners are typically mounted on the side-yoke or top casing of a linear control valve’s pneumatic actuator, or near the end-of-shaft for rotary valves. They are mechanically linked to the valve stem or shaft, allowing them to compare the actual valve position with the position set by the controller. When the controller sends a signal to adjust the valve, the positioner’s feedback linkage senses and confirms the valve’s new position and the magnitude of movement.

Positioners are essential for maintaining continuous control across the entire valve stroke, enabling the valve’s position to closely match the control signal. However, in applications with fast process feedback proportional to valve position, positioners may not be necessary, as the controller can directly adjust the actuator’s force to control valve movement.

Purpose of a Positioner in a Control Valve

A control valve’s positioner is used to precisely regulate the valve’s position in order to achieve a predetermined point for a process variable, such as flow, temperature, or pressure. Positioners ensure the valve responds accurately to the control signal, achieving faster and more precise adjustments than direct control alone.

The benefits of positioners, the basic concepts of the three primary types of positioners, and the alternatives provided by manufacturers will all be covered below.

How Does a Positioner Work?

In order to ensure accurate valve positioning in response to a control signal, normally from a process controller, a valve positioner is a device installed on the actuator of a control valve. It maintains precise control over the position of the valve by varying the hydraulic or air pressure that powers the actuator. 

The positioner ensures that the valve moves exactly to the position dictated by the controller’s signal, accounting for any resistance, friction, or external forces acting on the valve.

Click here for Working of Pneumatic Valve Positioner

Step-by-Step Functionality of a Positioner

Installation and Setup

• Linear Valves: On linear control valves, the positioner is generally installed on the yoke or top casing of the actuator, and it monitors the stem’s linear travel.
• Rotary Valves: For rotary control valves, the positioner is mounted on top of or alongside the actuator and is aligned with the valve and actuator stems to measure the degree of rotation.

Receiving the Control Signal

• The process controller continuously monitors the process variable (e.g., temperature, pressure, flow) and compares it to the desired set point.
• If a deviation occurs, the controller sends an input signal to the positioner. This signal may be pneumatic, electrical, or digital, depending on the type of positioner.

Comparing and Adjusting Position

• The positioner compares the actual valve position with the desired position indicated by the control signal.
• It detects the difference (known as an error) between the current valve position and the desired position.
• The positioner then adjusts its output to the actuator, which either increases or decreases the force applied to move the valve.

Feedback Mechanism

• Positioners have a feedback mechanism, which may be mechanical or electronic, that constantly tracks the position of the valve stem or shaft.
• This feedback allows the positioner to confirm that the valve is moving as expected and to make further adjustments if necessary.
• As the valve moves, the positioner receives updated position feedback and continues to adjust until the valve reaches the exact position specified by the controll

Balancing the Valve Position

• Once the valve reaches the desired position, the positioner maintains a balanced pressure on the actuator, keeping the valve steady at that position.
• If any further deviations occur, the positioner will respond promptly to correct the position again, ensuring continuous alignment with the control signal.

Click here for Basics of Control Valve Positioners

Types of Positioners

There are three primary types of positioners:

1. Pneumatic Valve Positioner: Operates purely with pneumatic signals to control the actuator.
2. Electro-Pneumatic (EP) Valve Positioner: Receives an electrical signal from the controller and converts it to a pneumatic signal for the actuator.
3. Digital Valve Positioner: Uses digital communication for precise control, often with advanced diagnostics and remote monitoring capabilities.

What is a Pneumatic Valve Positioner?

• A pneumatic valve positioner is a device used to control the position of a control valve actuator by receiving and interpreting a pneumatic signal from a process controller. 
• The controller sends a low-pressure air signal (typically in a range of 3-15 psi or 6-30 psi), representing the desired valve position based on the process variable (e.g., temperature, flow, pressure). 
• The pneumatic positioner adjusts the air pressure to the actuator to ensure that the valve opens or closes to the exact position specified by the controller signal.

How a Pneumatic Valve Positioner Works

Receiving the Input Signal

• The pneumatic positioner is connected to a controller, such as a Distributed Control System (DCS) or Programmable Logic Controller (PLC), that sends a variable air pressure signal through the I to P convertor.
• The input signal, typically in a standard range (e.g., 3-15 psi), indicates the desired valve position based on the current process conditions.

Comparing Desired Position with Actual Position

• The positioner is mechanically linked to the valve stem or shaft, which enables it to continuously monitor the valve’s actual position.
• Inside the positioner, a comparison occurs between the current valve position and the position specified by the input signal. If there is any discrepancy between the two, the positioner interprets this as an error that needs correction.

Adjusting Air Supply to the Actuator

• To correct the valve position, the positioner adjusts the air pressure supplied to the actuator.
• By increasing or decreasing the air pressure, the positioner drives the actuator in the correct direction (either open or close) until the valve reaches the position that corresponds to the input signal from the controller.

Feedback Loop and Fine Control

• Pneumatic valve positioners have a feedback loop that monitors the valve’s movement as the actuator responds.
• The feedback mechanism directs the positioner to adjust the air pressure incrementally as the valve approaches the target position, ensuring it stabilizes at the desired setting.
• This ensures that the valve’s position remains stable and accurate, closely matching the input signal.

Refer the below link for the Step by Step Calibration of Control Valve Positioner

What is an Electro-Pneumatic (EP) Positioner?

• An Electro-Pneumatic (EP) Positioner, also known as an analog positioner, is a device used to control the position of a pneumatic valve actuator by converting an electrical control signal into a pneumatic signal. 
• The positioner receives an electric input (typically 4-20 mA or 0-10 VDC) from a process controller, such as a Distributed Control System (DCS) or Programmable Logic Controller (PLC), and translates this signal into a proportional pneumatic output. 
• This pneumatic signal then directs the actuator to position the valve accurately in response to the control signal.

How an Electro-Pneumatic (EP) Positioner Works ?

Receiving the Electrical Signal:

• The EP positioner receives a continuous electrical signal, often in the 4-20 mA or 0-10 VDC range, which reflects the desired valve position based on the process conditions (e.g., flow, pressure, or temperature).
• Since many modern control systems operate with electronic signals, EP positioners connect the gap between electronic controllers and pneumatic actuators.

Converting Electric Signal to Pneumatic Output:

• Inside the EP positioner, the electrical signal is converted into a corresponding pneumatic signal. 
• This conversion is typically achieved through an I/P (current-to-pressure) converter within the positioner.
• The pneumatic output is then fine-tuned to meet the actuator’s requirements, delivering the exact air pressure needed to position the valve accurately.

Driving the Valve Actuator:

• The EP positioner sends this pneumatic signal to the actuator, which responds by adjusting the valve position according to the control signal’s instructions.
• The positioner continuously monitors the valve position and fine-tunes the pneumatic output to keep the valve in the exact position ordered by the control signal.

Refer the below link for the Calibration Procedure of Control valve 

What is a Digital Valve Positioner?

• A Digital Valve Positioner is an advanced device used to control a pneumatic valve actuator by receiving a digital or analog signal from a process controller. Similar to electro-pneumatic (EP) positioners, it converts an electrical control signal into a pneumatic signal to drive the actuator. 
• However, digital valve positioners, also known as “smart positioners,” incorporate a microprocessor that enables more sophisticated control and monitoring capabilities.

How a Digital Valve Positioner Works

Receiving the Control Signal

• The positioner receives an electric control signal, typically in the form of a 4-20 mA signal or digital protocols such as HART, PROFIBUS, or Foundation Fieldbus.
• These digital signals enable two-way communication, allowing the controller to not only instruct the positioner but also receive real-time feedback on valve performance and diagnostic data.

Signal Conversion and Valve Position Control

• Instead of relying on traditional mechanical components like beams, cams, or flappers (as in EP positioners), the digital positioner uses a microprocessor to interpret the control signal.
• The microprocessor accurately converts the input into a pneumatic signal, sending precise air pressure adjustments to the actuator. This results in more reliable and accurate valve positioning, even under complex process conditions.

Advanced Diagnostics and Monitoring

• The digital positioner continuously gathers data on valve position, actuator performance, and other parameters, enabling it to perform diagnostics and alert users to potential issues, such as wear, leakage, or performance degradation.
• This data can be communicated back to the controller, allowing for predictive maintenance and reducing unexpected downtime.

Enhanced User Capabilities

• Digital positioners often feature intuitive programming and calibration, usually with simple interfaces such as buttons and displays for easy setup and manual operation.
• Many models, such as the Siemens PS2, offer low air consumption, using air only when adjustments are needed. This design helps lower operational costs associated with air generation, maintenance, and material use.

Example: Siemens PS2 Digital Positioner

The Siemens PS2 is a widely used digital valve positioner known for its versatility and efficiency. Key features include:

• Flexible Operation: The PS2 can be operated either manually or electronically, adapting to various process requirements.
• Simple Calibration: Equipped with a user-friendly interface, it requires only a few button presses for setup and calibration.
• Communication Protocols and Enclosure Options: The PS2 supports multiple communication protocols (e.g., HART, PROFIBUS) and can be housed in enclosures that suit various environmental needs.
• Low Air Consumption: It minimizes air use, saving on costs associated with air supply and maintenance.

Click here for more about Smart control Valve Positioners

What is Electric Valve Controller ?

• The Electric Valve Controller is an advanced solution that combines the positioning capabilities of a valve positioner with additional control functions specifically designed for the oil and gas industry. 
• It is compact, user-friendly, and enables remote monitoring and precise control of valve operations, enhancing production efficiency and accuracy. 
• The controller can manage various control tasks, such as back pressure control, suction control, pressure differential control, and hi-low gap control, to ensure optimal system performance. 
• With its simple 3-button interface, operators can easily set up and make adjustments in the field. 
• Additionally, the Electric Valve Controller supports remote monitoring and control, allowing operators to oversee valve performance and make adjustments without being on-site. 
• This reduces manual intervention, improves safety, and increases operational flexibility, particularly in remote or hazardous areas common in oil and gas production.

Reasons to Install  a Valve Positioners for Control Valves

More Accurate Control

• Valve positioners provide precise control by continuously monitoring and adjusting the valve’s exact position. 
• This ensures better control, particularly in the lower part of the valve stroke, where inaccuracies are more common.

Faster Control Response

• Positioners enable control valves to react more quickly to changes in process variables, reducing the time the system operates outside the desired setpoint, which is crucial for maintaining stability and productivity.

Consistent Valve Position

• Even when differential pressures across the  control valve vary, positioners maintain a stable valve position. 
• This helps minimize instability in control loops caused by fluctuations in pressure.

More Flexible Configurations and Functions

• Positioners allow greater flexibility in system design, enabling the separation of the controller and valve. 
• They support the use of both diaphragm and piston-controlled actuators and offer the ability to switch between direct and reverse control action or change flow characteristics.

Reduced Friction Effects

• By minimizing friction in the valve stem packing, positioners reduce hysteresis and deadband, which are factors that can reduce system productivity and accuracy.

Minimizing Friction

• Positioners effectively mitigate the effects of friction in valve stem packing, which helps eliminate issues like hysteresis, especially with high-temperature packing materials like graphite.

Increased Usage of 4-20 mA Signals

• Positioners support I/P (current-to-pressure) converters directly on the valve, making it easier to integrate with 4-20 mA electronic signals for better control.

Improved Accuracy

• Positioners enhance the resolution and accuracy of valve movement by compensating for variations in the actuator’s spring rate, which can fluctuate by as much as ±8%.

Increased Response Time

• By improving the responsiveness of control valves to process changes, positioners allow for faster loading and venting, which is particularly valuable in dynamic systems.

Compensates for Flow-Induced Reactions

• Positioners negate the internal force imbalances that occur when there are higher pressure drops, ensuring more stable operation and better control over the valve.

Increased Flexibility

• Positioners enable the use of piston actuators with high instrument air supply pressures, offering more options for various applications.

Selection of Non-Matching Actuator Bench Set Ranges

• Positioners allow for the selection of actuator benchset ranges that don’t match the signal input directly, facilitating more complex system setups like multiple ratioing.

Enables Split-Ranging

• Positioners allow for the use of one controller for two valves, which enables split-ranging. 
• This can be useful for applications requiring two control ranges, such as 4-12 mA and 12-20 mA.

Click here for Control Valve Hunting due to Valve Positioner: Troubleshooting

Characterization Cams

• Positioners facilitate the integration of characterization cams, allowing for adjustments from the rotary control valve’s inherent characteristics to a desired formed characteristic for better flow control.

Overcoming Seating Friction

• Positioners help overcome seating friction effects, particularly at valve travel below 10%, which is important for accurate throttling near the “closed” position in rotary control valves.

Test your Knowledge with our Quiz on Control Valve Positioners

Since control valve positioners are an important component of the control system in many different industries, it is essential that instrumentation professionals understand them. You can improve your understanding of control valve positioners and be ready for interviews by using the Multiple Choice Questions (MCQs) with explanations and answers found at the following web pages.

Click here for Control Valves in Process Industries: A Collection of In-Depth Articles