Understanding Valve Positioners: Types and Functions

Valve positioners are essential components in the automation of control systems, particularly in industries that deal with pneumatic and hydraulic systems. These devices are designed to ensure a valve is precisely positioned to meet specific requirements. In this article, we will delve into the working mechanism of valve positioners, their types, and their functions.

What is a Valve Positioner?

A valve positioner is a device that assists in the precise positioning of a valve by adjusting the air pressure on the actuator. It acts as a middleman, receiving signals from a process controller, processing these signals, and then delivering the necessary adjustments to the actuator to move the valve to the desired position. This intermediary step ensures that the valve accurately reflects the intent of the process controller, providing better control and reliability.

Function of Valve Positioners

Valve positioners are typically mounted on the yolk or top casing of pneumatic actuators for linear control valves, or near the end of the shaft for rotary control valves. They are mechanically linked to the valve stem or shaft, allowing for direct feedback and control. The positioner works by receiving a signal from the process controller, then transmitting this signal to the actuator through a feedback linkage. The linkage mechanism confirms the movement of the valve and reports back to the positioner, ensuring the valve is correctly positioned.

Types of Valve Positioners

There are four main types of valve positioners, classified based on the method they use to drive the actuator: pneumatic, electronic, electro-pneumatic, and digital. Each type has distinct advantages and is suited to different applications.

1. Pneumatic Positioners

Pneumatic positioners use air to drive the actuator. They are intrinsically safe, providing a large amount of force to close valves. There are two primary subtypes: single-acting and double-acting. Single-acting pneumatics send and exhaust air from one side of the valve actuator, while double-acting pneumatics do the same from both sides.

2. Electronic Positioners

Electronic valve positioners use electrical signals to control the valve. These can be further divided into three subtypes based on the type of voltage: single-phase alternating current (AC), three-phase AC, and direct current (DC).

3. Electro-Pneumatic Positioners

Electro-pneumatic positioners combine both electric and pneumatic signals. They convert current control signals into equivalent pneumatic signals, making them versatile and adaptable for various industrial applications.

4. Digital Positioners (Smart Positioners)

Digital positioners, also known as smart positioners, use microprocessors to position the valve actuator. They are highly accurate and use less air compared to analog positioners. Additionally, they enable online digital diagnostics, allowing for real-time monitoring and maintenance.

Conclusion

Valve positioners are critical components in the automation of control systems, ensuring precise and reliable operation. Whether you need an intrinsically safe pneumatic positioner or a digitally advanced smart positioner, understanding the different types and their functions is key to selecting the right device for your application.