There are a lot of different types of valve positioners out there. How do you know if you need a pneumatic, electric or electro-pneumatic positioner? In this post, we'll cover the basics of valve positioners in general, as well as an overview of the different valve positioner types.
A valve positioner is a device that adjusts the valve actuator’s position based on a control signal. These positioners are best used in control applications because of their precision.
Valve positioners are usually mounted on the yolk or top casing of a pneumatic actuator for linear control valves, or near the end of the shaft for rotary control valves. For either configuration, the positioner is connected mechanically to the valve stem or valve shaft. This allows for the valve’s position to be compared with the position requested by the controller. When a control signal differs from the valve actuator’s position, the valve positioner sends the necessary feedback to move the actuator until the correct position is reached.
We came up with 10 Reasons to Install a Valve Positioner so you can discover ways that a positioner could improve your control process. Here is a quick summary of reasons to use a valve positioner on your rotary or linear control valves:
- Ability to have fine control over a precise process.
- Increase the speed of response to change in process, allowing for faster loading and venting.
- Allow for split ranging (one controller for two valves).
- Overcome seating friction in rotary valves.
- Allow for increased usage of 4 – 20mA electronic signal.
- Minimize valve stem packing friction effects and the resulting hysteresis. This is particularly important for high temperature packing materials like graphite.
- Negate flow-induced reactions to higher pressure drops and compensates for internal force imbalances.
- Allow the use of characteristic cams in rotary valves.
There are a few different types of valve positioners available. Depending on the type of positioner, it either uses air or electricity to move the actuator. Let’s discuss some of the popular options.
Pneumatic positioners receive pneumatic signals (usually 3-15 psig). The positioner then supplies the valve actuator with the correct air pressure to move the valve to the required position. Pneumatic positioners are intrinsically safe and can provide a large amount of force to close a valve.
Electric
Electric valve positioners receive electric (usually 4-20 mA) signals. They perform the same function as pneumatic positioners do, but use electricity instead of air pressure as an input signal. There are three electric actuation types: single-phase and three-phase alternating current (AC), and direct current (DC) voltage.
Electro-Pneumatic
Electro-pneumatic valve positioners convert current control signals to equivalent pneumatic signals. It uses a mix of both electricity and air, as implied by the name.
Digital
Digital or “smart” positioning devices use a microprocessor to position the valve actuator while monitoring and recording data. They function very similarly to an analog-type positioner, except the electronic signal conversion is digital rather than analog. Smart positioners are very accurate, use less air than analog positioners, and allow for online digital diagnostics. For more information, read our blog post about Reducing Air Loss with Smart Valve Positioners.
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