What do you need to know about control valve positioners?
So far, we’ve talked about control valves as basically body and actuator. Depending on the application, you may need some accessories to beef it up. Today, let’s review one of the most important, valve positioners.
What’s a valve positioner?
Valve positioners translate signals from the control system to send the right pressure to an actuator. In a nutshell, it tells the actuator to move the valve to a certain position based on the input data.
Valve positioners come in different types. Some use a pneumatic input signal to send pressure for opening or closing the valve. Some use analog signals to do the same thing. And now, we have smart positioners changing analog to digital to control your valves. Let’s take a closer look at these valve positioners.
This type uses the flapper/nozzle system. We have an article that discusses this working principle here. It requires compressed air to make everything work correctly and uses a signal of 3-15 pounds per square inch to receive the data from the controller.
The input tells the bellows to expand or compress, depending on the signal. The bellows moves the flapper assembly and the beam that measures the valve stem feedback through the cam.
In all this movement, the flapper will move closer or further from the nozzle. Thus the relay will increase or decrease the air output to the valve actuator. When the valve stem moves, it sends feedback through the beam. Then the air output will decrease or increase again, to position the valve properly based on the input signal.
The working principle for this one resembles the pneumatic positioner. But rather than a pneumatic signal, you get an analog signal, usually 4-20 milliamps.
However, the signal works differently internally. It creates an electromotive force (EMF) based on the electrical signal applied to a coil. Basically, this force moves the flapper and increases or decreases the air to the valve actuator. You also have feedback from the valve stem and positioner to decrease or increase the output to the valve actuator if needed.
Smart valve positioners bring a lot of benefits to the end user. They provide more flexibility for setup, plus they make it easier to get diagnostics. And these diagnostics can increase the life of your valve, reducing maintenance and unscheduled downtime.
You have different types of working principles that vary by vendor. So you can find Hall-effect positioners, devices with potentiometers, and more. Best of all, these smart positioners use loop power and offer a handful of field protocols for you to choose from.