Abstract: Valve positioner selection is good or bad, will directly affect the performance and quality control valve and regulating system. So how to choose the correct and reasonable positioner in the control area is particularly important. Keywords: Valve Positioner Selection Guide The valve positioner is one of the most important accessories for regulating valves in a wide range of control applications. Especially for a particular application, consider the following factors if you are looking for the most suitable (or best) positioner: 1) Whether the positioner can achieve the SPLIT- ranging "? Is it easy and convenient to realize the "split"? Having a "split" function means the positioner responds only 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) Zero and span adjustment is easy and convenient? Is not to open the lid can be completed zero and span adjustment? However, it is worth noting that sometimes it is forbidden to adjust this method to avoid incorrect (or illegal) operations. 3) What is the stability of zero and span? If the zero and span are easily drifted with changes in temperature, vibration, time or input pressure, the positioner needs to be recalibrated frequently to ensure that the governor valve travels accurately. 4) What is the accuracy of the valve positioner? Ideally, Trim Parts (including spools, stems, seats, etc.) should be positioned exactly where they are required for each input, regardless of the direction of travel or Regulating valve with the internal parts of the load. 5) valve positioner on the air quality requirements? Since only a handful of air supply units are capable of supplying air meeting the ISA standard (Standards for Air Quality of Instrumentation: ISA Standard F7.3), therefore, for aerodynamic or electro-pneumatic valve positioners, Live the test of the real environment, we must be able to withstand a certain amount of dust, water vapor and oil. 6) Zero and span of the calibration of the two are mutually influential or independent? If they affect each other, it takes more time to calibrate the zero point and range, because the operator must repeatedly adjust these two parameters in order to achieve accurate settings step by step. 7) Does the valve positioner have a service "bypass" that allows the input signal to act directly on the regulator? This "by-pass" can sometimes simplify or eliminate calibration of Actuator Settings such as: actuator's Benchset settings and Seat Load settings This is because in many cases the pneumatic output signal of some pneumatic regulators exactly matches the actuator's "bearing assembly settings" and does not need to be set again (in fact, in this case Of course, if you choose, then you can also use the valve positioner "bypass" so that the pneumatic regulator pneumatic output signal directly on 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 ). 8) The role of valve positioner is fast? The greater the Airflow (the positioner continuously compares the input signal to the valve position and adjusts its own output based on the deviation between them.) If the positioner responds quickly to this deviation, the air per unit of time The amount of flow is large), the faster the control system responds to Setpoint and load changes - which means the smaller the system error (hysteresis), the better the control quality. 9) The frequency response (Frequency Response - ie G (jω)), what is the steady-state response of the system to the sinusoidal input? In general, the higher the frequency characteristic (ie, the more sensitive the frequency response High), the better the control performance, but care must be taken that the frequency characteristics should be determined using the Consistent Test Methods rather than the theoretical methods, and the valve positioner and actuator should be incorporated in the evaluation of the measured frequency characteristics Consider 10) What is the maximum rated supply pressure of the positioner? For example, some valve positioners have a maximum rated supply pressure of only 501b / in² (ie, 50psi, lpsi = 0.07kgf / cm²≈6.865kpa). If the actuator is rated for operating pressures above 501b / in², Then the valve positioner has become the driving force of the implementing agencies output constraints. 11) When the valve and valve positioner assembly combination, their positioning resolution (Positioning Resolution)? This has a very significant effect on the control quality of the control system because the higher the resolution the closer the positioning of the control valve is to the ideal value and the change of the fluctuation due to the Overshooting of the control valve can be strangled to finally reach The purpose of limiting the amount of adjustment cycle changes. 12) positive and negative valve positioner conversion is feasible? Is conversion easy? Sometimes this function is necessary. For example, to change the mode of "signal increase-valve off" to "signal increase-valve on", the positioner's positive and negative function can be used. 13) What is the complexity of the internal operation and maintenance of the positioner? As we all know, the more parts, the more complicated the internal operation structure, the more training (maintenance) personnel will be trained and the more stock spare parts will be available. 14) What is the Steady-state Air Consumption of the positioner? For some plant installations, this parameter is critical and may be a limiting factor. Of course, other factors should also be considered when evaluating and selecting valve positioners. For example, the feedback linkage of the positioner should reflect the position of the valve core. In addition, the positioner must be durable, anti-environmental protection and anti-corrosion, and the installation and connection are easy and convenient.
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