Everything You Need To Know About A Control Valve Positioner

Positioners change valve actuators’ positions in response to control signals. A positioner like this can be used in various control applications because of its accuracy.

When it comes to linear control valves, valve positioners are installed on the yolk or top casing or closer to the shaft when it comes to rotary control valves. There are two types of positioners: those attached to valve stems or those attached to valve shafts. By doing so, the controller can compare the valve position with the desired position. A valve positioner provides feedback when a control signal differs from the actuator’s work, allowing the actuator to be moved until it reaches the correct position.

What is a Control valve positioner?

Control valve positioners are motion-control devices used to position a valve by raising or lowering the air load pressure on the actuator so that the valve stem or shaft matches the set point from the control system. 

What Does A Positioner work?

It is common for linear control valves to have valve positioners installed on their yolks or top casings. Generally, rotary valve positioners are placed either on top or side of the valve and actuator stems. The type of actuator used determines the installation.

When the positioner is attached to the actuators, it may measure the stem travel or degree of rotation. This also alters the position of the valve as needed based on the instrument controller’s input signal.

The instrument controller provides the positioner with an electrical or pneumatic signal when the process variable deviates from the desired set point. This changes the pneumatic output to the actuator, causing the valve to open or close as needed. This will continue until the process variable reaches the specified set point.

Valve Positioner Types 

Valve positioners come in a variety of configurations. Depending on the positioner, air or electricity is used to move the actuator. Let’s go over some of the more common solutions.

Pneumatic

Pneumatic positioners are pneumatic signal receivers. The positioner then sends the necessary air pressure to the valve actuator, causing the valve to move to the required position. Pneumatic positioners are safe and may apply significant force to seal a valve.

Electric

Electric valve positioners accept electric signals (often 4-20 mA). They serve the same purpose as pneumatic positioners, but their input signal is electrical rather than air pressure. There are three forms of electric actuation: single-phase, three-phase, and direct current (DC) voltage.

Electro-pneumatic 

An analog signal, typically 4-20 milliamps, is sent to electro-pneumatic positioners via a controller. According to the electrical signal given to a coil, the signal generates an electromotive force. This force moves the flapper, increasing or decreasing the air delivered to the valve actuator. When necessary, input from the valve stem and positioner is provided to reduce or increase the output to the valve actuator.

Digital 

A microprocessor is used in digital or “smart” positioners to position the valve actuator and monitor and store data. As a result, they are pretty accurate. They provide several advantages to the user. They utilise less air than analogue positioners and offer more configuration flexibility, as well as online digital diagnostics, which extends the valve’s life and reduces maintenance and unexpected downtime. The best part is that these intelligent positioners use loop power and have a variety of field protocols to select from.