In many control applications, the valve positioner is one of the important accessories of the regulating valve. Especially for a specific application, if you want to select an applicable (or excellent) valve positioner, you should pay attention to the following factors:
1. Can the valve positioner achieve "split_ranging"? Is it easy and convenient to realize "split process"? The "split range" function means that the valve positioner only responds to a certain range of input signal (such as 4 ~ 12mA or 0.02 ~ 0.06mpag). Therefore, if you can "split range", you can control two or more regulating valves successively with only one input signal contact according to the actual needs.
2. Is the adjustment of zero point and range easy and convenient? Can the zero point and range be adjusted without opening the box cover? However, it should be noted that sometimes in order to avoid incorrect (or illegal) operation, this random adjustment method needs to be prohibited.
3. How stable are the zero and span? If the zero point and range are easy to drift with the changes of temperature, vibration, time or input pressure, the valve positioner needs to be readjusted frequently to ensure the accurate stroke action of the regulating valve.
4. How accurate is the valve positioner? Ideally, corresponding to an input signal, trim parts of the regulating valve (including valve core, valve stem, valve seat, etc.) should be accurately positioned at the required position every time, regardless of the direction of travel or how much load the trim of the regulating valve bears.
5. What are the air quality requirements of the valve positioner? Since only a few air supply devices can provide air that meets the provisions of ISA standard (standards related to air quality for instruments: isa standard f7.3), for pneumatic (or electro-pneumatic) valve positioners, if they want to withstand the test of real environment, they must be able to withstand a certain amount of dust, water vapor and oil stain.