A valve positioner is a device mounted on the actuator that exerts or reduces air pressure as necessary to make sure the valve achieves the correct position.
When there is no positioner, the control signal goes directly to the actuator. When a
positioner is installed, it intercepts this signal and then outputs a different signal to the actuator.
Positioners allow tighter control over the process variable by increasing the speed and accuracy of the actuator response. Because a positioner’s job is to make sure the valve is in the right place, it also helps in overcoming factors that affect control valve performance, such as friction, as well as problems like non-linearity and deadband. Positioners can also amplify or reverse an input signal as needed.
Essentially, a positioner ensures that the final control element in a loop exerts optimal control.
There are three types of positioners:
Pneumatic positioners. These devices receives a pneumatic (air) signal from the controller and output a pneumatic signal to the actuator.
Analog, or electro-pneumatic, positioners. Here, the input signal is electrical, rather than pneumatic.
Digital, or smart, positioners. These positioners also receive an electrical signal, but it’s digital as opposed to analog.
Digital positioners came on the scene about 20 years ago, but they only really started gaining popularity recently as automation has started to take off in plants and along pipelines.
The main reason digital positioners are popular is that they can do much more than just control the position of the valve. The newest positioners on the market can also collect data about the valve to automatically alert users about how the valve and its assembly are performing, and even aid in diagnostics and maintenance.
Since they have fewer mechanical moving parts, digital positioners last longer than their traditional pneumatic and analog counterparts. Plus, they don’t bleed any air while the valve is at rest, which reduces energy consumption.
Auto tune the positioner in minutes, as opposed to the hours analog positioners can take
Get diagnostic information about the health and performance of your control valves
Identify potential problems before they happen
Integrate the
positioner with other control systems so all of your data is in one place
What this all boils down to is higher valve productivity, decreased downtime, and overall better plant performance.
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