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This post was last edited by 鎴戠編鎴戜换鎬р樅 on 2020-9-8 at 16:26. Hi, this is the Industrial Control Assistant. Every day, I share a little piece of knowledge about industrial control, so you won’t get lost on your journey toward automation. Today, we’re sharing some useful information on valve positioners. Learning the correct methods for installing valves not only ensures their proper operation but also reduces wear on the valves and lowers production costs. Next, let’s take a look at what’s interesting about valve positioners. Valve positioners enable the automation of the production process. Actuators can be classified into three categories based on their energy or driving source: pneumatic, electric, and hydraulic, with pneumatic control valves and electric control valves being the most common types. Electric actuators are driven by a power source, offering advantages such as speed and ease of centralized control, but they have a complex structure and poor fire and explosion resistance. A hydraulic actuator uses hydraulic principles to drive the actuating mechanism; it possesses high thrust and is suitable for applications with heavy loads, but its auxiliary equipment is large and bulky. Pneumatic actuators use compressed air as an energy source, and they offer advantages such as simple structure, reliable operation, easy maintenance, stable performance, low cost, as well as fire and explosion resistance. The pneumatic actuator of a valve positioner is also known as a pneumatic control valve. The actuator serves as the driving mechanism of the control valve; it generates a corresponding thrust based on the magnitude of the control air pressure signal, causing the push rod to move accordingly and thus driving the valve element into action. The valve (valve body component) is the regulating mechanism of a control valve; its internal chamber comes into direct contact with the process medium. Driven by the actuator, the valve element changes the flow area between the valve element and the valve seat, thereby altering the resistance to fluid flow through the valve and achieving the purpose of adjusting the flow rate of the medium. Among various control valves, although the actuators differ, their valves are universal. Depending on different application requirements, there are various types of control valve structures, such as straight-through single-seat, straight-through double-seat, angle type, three-way valves, butterfly valves, diaphragm valves, cage valves, cam-flexible valves, ball valves, etc. Installation of the valve: 1. Fix the valve positioner to the mounting plate by opening the upper housing of the valve positioner and using three M5*20 screws to connect it to the mounting plate through the three holes in the housing. 2. Connect the feedback component of the valve positioner to the actuator, loosen the stem nut on the control valve, and insert the support plate in the feedback component between the indicator and the connection nut, then tighten it. 3. Fix the mounting plate with the valve positioner to the actuator bracket at the two screw holes using M10*15 screws in the desired positions. 4. Insert the connecting pin on the feedback connection plate of the feedback component into the dry opening slot of the valve positioner cam feedback. 5. All types of actuators should ensure that the cam feedback rod is in a horizontal position when the valve position is at 50%. 6. After adjusting all the parts that are to be connected, secure them with locks. After the valve positioner is installed, it needs to be tuned. First, connect the air supply properly and set the pressure to an appropriate level. Manually operate the nozzle damper, check whether the pressure gauge on the positioner indicates that it is in operation, listen for any abnormal noises from the valve positioner, and also verify that the valve can open and close properly. Connect the analog power signal, set it to 4MA, adjust the zero point; once the zero point is properly adjusted, increase it to 8MA. At this point, observe the valve opening degree. If everything is normal, proceed in the same manner; if the standard isn’t met, manually adjust the range. If the range is fine, then gradually increase the signal level, paying attention to the hysteresis. Based on on-site assessment, the backlash should preferably not exceed 3%. Single-acting pneumatic actuators for valve positioners usually adopt a diaphragm design. With this design approach, the spring used can reduce the load on the valve seat as well as withstand the full closing pressure. Typical double-acting pneumatic actuators use a piston design. This design approach differs from the film-type design; there is no need to limit the supply pressure, and the full supply pressure can be used to achieve a higher closing pressure. For piston-type designs, the higher the pressure, the better the stability and control sensitivity. To achieve full load with a tightly sealed valve seat, it is necessary to calibrate the valve so that the closing elements (such as the plug, diaphragm, valve disc, etc.) are accurately positioned on the valve seat, in order to maintain the designed leakage rate and prevent the sealing surfaces from corroding. Well, that’s all for today’s content. If you have any questions about automation tips, feel free to leave a message at any time. New updates will be posted regularly, so please stay tuned.