Control valve positioner types: how they work and when to use which one
Control valves have three main parts: body, actuator, and positioner. This article explains the various control valve positioner types. We will go into how they work, as well as their applications, advantages, and disadvantages.
What is a control valve positioner?
Control valve positioners translates 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, allowing remote, digital control of this mechanical device.
Control valve positioner types
Control valve positioners come in different types. Pneumatic control valve positioners use a pneumatic input signal to send pressure for opening or closing the valve. Others use analog signals to do the same thing. Smart control valve positioners can even change analog signals to digital to control your valves.
Pneumatic control valve positioner
Pneumatic control valve positioners use the flapper/nozzle system. They require compressed air to make everything work correctly and use 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 move 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.
Electro-pneumatic control valve positioner
The working principle of the electro-pneumatic control valve positioner resembles that of the pneumatic control valve positioner. But rather than a pneumatic signal, you get an analog signal, usually 4-20 milliamps.
The signal creates an electromotive force (EMF) based on the electrical signal applied to a coil. 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 control valve positioner
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.
To know more about control valves, you can read our article on how control valves work
Control valve positioner indicator
You’ll see control valve positioner indicators most often at on/off valves, sending feedback on the valve position to the controller. When you remotely open and close the valve, you need that feedback to make sure the valve responds to your commands.
Vendors sell a variety of valve positioner indicators, using different principles to convert mechanical movement into an electrical signal. And most valve indicators offer local visualization, so you can see if the valve is closed or open.
You also have many ways to collect the data sent from the valve indicators, from analog to digital communication. For a long time, DeviceNet held the top spot, offering simple structure and communication for this type of signal.
Today, the boom of wireless communication has helped a lot in monitoring valves. You can find vendors offering valve indicators with wireless communication built-in. They also offer quick update times and reduce human errors, making them much more beneficial.
Plus, they eliminate the need for messy restrictive cables. Of course, you can use a cable solution if it suits you better, but you should probably check the other solutions first. Best of all, some smart valve indicators will monitor the valve strokes and provide you with advanced diagnostics.
To know more about control valves, you can get in touch with our engineers!