Valve positioner (also known as: pneumatic valve positioner) is the main control valve accessories, usually used in conjunction with the pneumatic control valve, which accepts the regulator output signal, and then use its output signal to control the pneumatic control valve, when the control valve After the action, the displacement of the valve stem is fed back to the valve positioner through the mechanical device, and the position of the valve position is transmitted to the host system by the electric signal. Valve positioners are among the most important accessories for regulating valves in a wide range of control applications. Especially for a particular application, if you want to choose the most suitable valve positioner, then you should consider the following factors: 1) Whether the valve positioner can achieve "split (Split_ranging)" to achieve "split" is easy , Convenient? Having a "split" function means that the positioner only responds to a range of input signals (eg 4 to 12 mA or 0.02 to 0.06 MPaG). Therefore, if we can "split", then according to actual needs, only one input signal has been achieved to control two or more control valve. 2) Is it easy and convenient to calibrate the zero and span of the positioner? Is it possible to adjust the zero and span without opening the lid? It is worth noting, however, that in order to avoid incorrect (or illegal) Operation, this can be adjusted at any time need to be banned. 3) What is the stability of the zero position and span of the positioner? If the zero point and span are easily drifted with changes in temperature, vibration, time or input pressure, then the positioner needs to be recalibrated frequently To ensure that the control valve trip action accurate. 4) Accuracy of valve positioner Under ideal conditions, corresponding to a certain input signal, the TrimParts (including ball / valve core, valve stem, valve seat, etc.) of the control valve should be accurately positioned each time it is required Regardless of the direction of travel or how much load the trim valve's inner part is subjected to. 5) What is the air quality required by the positioner? As only a handful of air supply units are capable of supplying air meeting the ISA standard (ISA standard F7.3 for air quality in instrumentation), for pneumatic (or Electrical - gas) valve positioner, if you want to withstand the test of the real environment, it must be able to withstand a certain amount of dust, water vapor and oil. 6) Is calibration of zero and span both interdependent or independent? The interaction of zero and span requires more time for zero and span calibration because the operator must repeatedly adjust these two parameters , In order to gradually achieve the exact set. 7) Does the valve positioner have a "by-pass" that allows the input signal to act directly on the regulator? This "by-pass" can sometimes simplify or eliminate calibration of the actuator assembly settings such as: Seat Component Setting "and" Spring Block Load Setting "- This is because in many cases the pneumatic output signal of some of the pneumatic regulators exactly matches the" Standoff Component Setting "of the actuator without any need for Then set (in fact, in this case, the valve positioner can save unnecessary .Of course, if you choose, you can also use the valve positioner "bypass" so that the pneumatic regulator pneumatic output signal directly In the control valve). In addition, the presence of "by-pass" can sometimes allow limited on-line calibration or repair and maintenance of the positioner (ie bypassing the positioner for normal operation of the regulator without forcing the regulator off-line ).