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What is control valve positioner ?

What is control valve positioner ?

Valve positioners are the devices that are used to put a valve in the right positioner by increasing and decreasing the air load pressure on the actuator. The valve positioner senses the exact position of the valve and communicates to the actuator to drive the valve, instead of the process controller sending a signal directly to the actuator to move the valve.

Function of valve positioner

Valve positioners are usually installed on the yolk or top casing of a pneumatic actuator ( for linear control valves), or near the end of the shaft ( for rotary control valve ). They are connected mechanically to the valve stem or shaft. The mechanical feedback linkage work in a way that the process controller tells the positioner to change position, the feedback linkage reports back to the positioner confirming that a change in the position of the valve stem has occurred and gives a sense of the magnitude of the change in position.

Types of valve positioners

Based on what the positioner uses to drive the actuator, such as air or electricity, there are four basic types of valve positioners: pneumatic, electronic, electro-pneumatic, and digital. Pneumatic positioners send and get pneumatic signals, they are intrinsically safe and can provide a large amount of force to close a valve. Single-acting or three-way pneumatic positioners send air or exhaust air from only one side of a single-acting valve actuator that is opposed by a range spring. Double -acting or four-way pneumatic positioners send and exhaust air from both sides of the actuator. Electric valve positioners send and get electric signals. There are three electric actuation types:single-phase and three-phase alternating current(AC), and direct current (DC) voltage. Electro-pneumatic valve positioners convert the current control signal to equivalent pneumatic signals. They use a mixture of electricity and air. The digital positioner is also calling a smart positioner, it uses a microprocessor to position the valve actuator, monitor, and record data. They are highly accurate, use less air comparing analog positioners, and also allow for online digital diagnostics.

Why you should use a valve positioner in your application?

1. More accurate control: Since valve positioners know the valve’s exact position, they provide more precise control than be achieved by an actuator on its own.
2. Faster control: Positioners help control valves respond faster to changes in the process variable, minimizing the time the system is operating above or below the setpoint, increased speed of response to a change in process, allow faster loading and venting
3. Consistent valve position: Varying differential pressure across the valve can indicate instability in the control loop. A positioner is one solution to help stabilize valve position
4. More flexible configurations and functions: A positioner allows you to put distance between the controller and the control valve.
5. Minimize the effects of friction. Friction in the valve stem packing contributes to both hysteresis and dead band, which reduce productivity, particularly for high-temperature packing material such as graphite
6. Allow distances between the controller and control valve
Allow a wide range of flow variation, such as operate less than 10% travel under

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