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The valve positioner is one of the important accessories and components of pneumatic control valves, serving to position the valve. Pneumatic control valves are widely used in various control applications, and valve positioners, as the main components of these control valves, play a decisive role in determining their positioning. It’s not just about using a valve positioner effectively – it’s also important to choose the right one! Changhui Instruments shares with you the various factors to consider when selecting valve positioners; this is also the fundamental reason why it’s better to choose the right valve positioner from the start. How to choose a suitable valve positioner? 1. Is a split-range function required? Can the valve actuator achieve “split-ranging”? Is it easy and convenient to achieve “staged travel”? The \"range segmentation\" function means that the valve positioner responds only to a certain range of the input signal (e.g., 4-12 mA or 0.02-0.06 MPaG). Therefore, if it is possible to divide the process into stages, it is possible to use just one input signal to control two or more control valves in sequence, depending on actual needs. 2. Is it easy and quick to adjust the zero point and range of the locator? Is it possible to adjust the zero point and range without opening the box cover? It is worth noting for instrument technicians that sometimes, in order to avoid incorrect (or illegal) operations, such arbitrary methods of adjustment need to be prohibited. 3. What is the stability of the zero point and range of the locator? If the zero point and range are prone to drifting due to changes in temperature, vibration, time, or input pressure, then the valve positioner needs to be recalibrated frequently to ensure accurate operation of the control valve. 4. What is the accuracy of the valve positioner? Under ideal operating conditions, the precision of a valve positioner ensures that, for a given input signal, the internal components of the control valve (trim parts, including the ball/valve element, stem, seat, etc.) are always positioned accurately at the desired location, regardless of the direction of movement or the load applied to these internal components. 5. What are the requirements of the valve positioner regarding air quality? Since only a very few air supply units can provide air that meets the requirements of the ISA standards (the standards for air quality used in instruments: ISA Standard F7.3), pneumatic (or electro-pneumatic) valve positioners must be able to withstand a certain amount of dust, moisture, and oil contamination if they are to function properly in real-world conditions. 6. Calibration of zero point and range: Are the calibration of the zero point and range interrelated or independent of each other? If they are interrelated, then adjusting these two parameters will require more time, as the technician must make repeated adjustments to gradually achieve the desired accuracy. 7. Does the valve positioner have a “bypass”? This “bypass” can sometimes simplify or eliminate the need to verify the settings of the actuator’s assembly, such as the “support component settings” and “spring seat load settings” of the actuator. This is because, in many cases, the pneumatic output signal from certain pneumatic regulators matches exactly the “support component settings” of the actuator, making it unnecessary to adjust them further (in fact, in such cases, a valve positioner can be completely omitted). Of course, if it is selected, the “bypass” of the valve positioner can also be used to allow the pneumatic output signal from the pneumatic regulator to act directly on the control valve. Furthermore, having a \"bypass\" can sometimes allow for limited adjustment or maintenance of the valve positioner while it is still in operation (that is, by using the bypass of the valve positioner, the control valve can continue to function normally without the need to take it offline). 8. Is the function of the valve positioner fast? The greater the airflow, the more frequently the valve positioner compares the input signal with the actual valve position, and adjusts its output based on the difference between them. If the valve actuator responds quickly to such deviations, then the amount of air flowing per unit time is high); the control system responds more rapidly to changes in the set point and load – which means that the system’s error (lag) is smaller and the control quality is better. 9. What are the frequency characteristics of the valve positioner? What are the frequency characteristics (or frequency response – that is, the system’s steady-state response to sinusoidal inputs) of a valve positioner? Generally speaking, the higher the frequency characteristic (i.e., the greater the sensitivity to frequency response), the better the control performance. It must be noted, however, that the frequency characteristics should be determined using stable experimental methods rather than theoretical ones, and when evaluating these frequency characteristics, the valve positioner and actuator should be considered together. 10. The supply air pressure for valve positioners: For example, the maximum rated supply air pressure for some valve positioners is specified as 501 b/in2 (i.e., 50 psi; 1 psi = 0.070 kgf/cm2 ≈ 6.865 kPa). If the rated operating pressure of the actuator is higher than 501 b/in2, then the valve positioner becomes a limiting factor for the driving force generated by the actuator. 11. The positioning resolution of the valve positioner has a significant impact on the control quality of the regulation system. Higher resolution means that the valve can be positioned closer to the ideal value, which helps to minimize fluctuations caused by excessive adjustment of the valve. This, in turn, enables limitation of the periodic variations in the regulated quantity. 12. Forward and reverse action of valve positioners: Is it possible to switch between forward and reverse action? Is such switching easy to accomplish? Sometimes this function is necessary. For example, to change a mode of \"signal increase – valve closed\" to a mode of \"signal increase – valve open\", the reverse action switching function of the valve positioner can be used. 13. What is the level of complexity in the operation and maintenance of valve positioners? It is well known that the more components there are, the more complex the internal structure becomes; this requires more technical training for maintenance personnel, as well as a larger inventory of spare parts. 14. Air consumption of valve positioners. The parameter of steady-state air consumption of valve positioners is crucial for certain industrial installations, and it may constitute a limiting factor. 15. The feedback linkage mechanism of the valve positioner: The feedback linkage mechanism of a valve positioner must be able to accurately reflect the position of the valve spool. 16. Material of the valve positioner: The valve positioner must be durable, capable of withstanding environmental influences and corrosion, and easy to install and connect. Common faults and their causes of valve positioners – Changhui Instruments shares information on the frequent fault occurrences in valve positioners as well as the reasons behind them, for reference by instrument technicians. It is hoped that this will help everyone resolve valve positioner issues more quickly. Fault symptom 1: The output pressure of the downstroke valve positioner changes slowly. Possible causes: ① The air inlet ball valve of the amplifier is seated too deeply. ②The amplifier consumes a large amount of gas. ③The intake ball valve of the amplifier is contaminated, reducing the flow area. ④The ratio of the diameter of the constant-throttle orifice to the nozzle diameter is less than the specified value (technical requirement value). ⑤The fit between the nozzle and the baffle is poor. ⑥There is slight friction between the armature and the coil former. Fault symptom 2: Poor linearity of the valve positioner. Possible causes: ① Incorrect selection of the feedback cam or spring. ②The feedback mechanism is not installed properly. ③The feedback cam or spring is not installed properly. ④The nozzle or baffle is contaminated. ⑤ There is friction between the spool in the slide valve amplifier and its contact surfaces. ⑥There is a slight leakage in the back pressure. ⑦Its positive electrode is improperly installed. ⑧There is sticking in the feedback link surface control valve. Fault symptom 3: The valve positioner receives a signal input, but there is no output pressure signal. Possible causes: ① In electric/pneumatic positioners, there are foreign objects between the armature and the coil frame. ②The constant throttle orifice is blocked. ③The nozzle baffle is misaligned or damaged. ④The diaphragm in the amplifier (metal diaphragm or rubber diaphragm) is damaged. ⑤The connection of the seven channels is incorrect (including the amplifier). ⑥The positive and negative terminals of the input signal wire for the flow control electrical valve actuator are reversed. ⑦The diode in the input terminal box of the locator is open-circuited or has poor connections. ⑧The pressure of the air supply is not within the required range. ⑨The gas consumption of the amplifier is significantly higher than the specified value. ⑩The installation polarity of the magnet in electric/gas valve positioners is different. ⑪The amplifier preload force is too high. ⑫The spool in the slide valve amplifier is stuck by foreign objects. ⑬“The “Manual/Automatic” switch position is incorrect (not in the manual or automatic mode). ⑭The input signal of the electric/pneumatic valve positioner is short-circuited. ⑮The balance spring is not properly installed or adjusted. Fault symptom 4: No input signal, but the valve positioner has output pressure. Possible causes: ① The nozzle is blocked. ②Recently, the ball valve of the amplifier has become contaminated, resulting in failure to lock properly or damage to the sealing surface. ③The diameter ratio of the constant orifice to the nozzle diameter is greater than the rated value. ④There is a problem with the connection of the various air circuit boards in the amplifier. ⑤Deformation of the amplifier’s metal diaphragm or improper installation causes the valve stem to push open the ball valve (in the case of amplifiers with an adjustable preload). ⑥The baffle has covered the position of the nozzle. Fault symptom 5: Insufficient actuator travel (the output pressure of the valve positioner does not reach its maximum value). Possible causes: ① The contact position between the feedback rod and the connector of the actuator’s push rod is incorrect. ②The magnetic field strength generated by the permanent magnet is lower than the rated value. ③The initial positions of the baffle and nozzle were poorly selected. ④The initial position of the feedback cam was selected improperly. ⑤The main lever balance spring is improperly installed. These experiences cannot cover everything or list all the possible causes of failures in valve positioners; I hope you will also contribute your insights by leaving comments below to improve this article. Source: Digital display instruments http://yunrun.com.cn/product/