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If you walk around a chemical plant workshop, you will surely see valves with round heads installed on some pipes; these are control valves. Working principle of mechanical pneumatic valve positioners: The diagram explains each component of a mechanical pneumatic valve positioner in detail. Next, we will take a look at how it functions The air source is compressed air from the air compressor station; there is also an air filter and pressure reducing valve located before the air inlet of the valve positioner, which is used to purify the compressed air. The air supply coming from the outlet of the pressure reducing valve enters through the valve positioner; the amount of air that reaches the valve’s die is determined by the output signal from the controller. The electrical signal output by the controller is 4–20 mA, while the pneumatic signal ranges from 20 KPa to 100 KPa; the conversion from an electrical signal to a pneumatic signal is carried out through an electrical converter. When the electrical signal output by the controller is converted into a corresponding pneumatic signal, this transformed pneumatic signal is then applied to the bellows. Lever 2 moves around the fulcrum, with its lower section moving to the right and approaching the nozzle. As the back pressure at the nozzle increases, it is amplified by the pneumatic amplifier (the component with a minus sign in the diagram), and a portion of the air supply is sent to the air chamber of the pneumatic diaphragm. As a result, the valve stem, along with the valve core, moves downward, gradually reducing the opening degree of the valve. At this point, the feedback rod connected to the valve stem (the swing rod in the diagram) moves downward around its pivot point, causing the front end of the shaft to move downward. The eccentric cam connected to it rotates counterclockwise, while the roller rotates clockwise and moves to the left, thereby stretching the feedback spring. As the feedback spring pulls the lower section of the lever 2 to the left, it reaches a force balance with the signal pressure acting on the bellows, and as a result the valve remains fixed in a certain position and does not move. Through the above introduction, one should have a basic understanding of mechanical valve positioners. If there is the opportunity, it is best to actually disassemble one of them to better understand the location of each component as well as the name of each part. Thus, this brief overview of mechanical valves comes to an end; next, we will expand on this knowledge to gain a deeper understanding of control valves. The pneumatic diaphragm control valve shown in the diagram is of the air-shut type; some people have asked why First, check the air intake direction of the pneumatic diaphragm; it is of positive action. Second, check the installation direction of the valve core; it should be in direct action mode. The pneumatic diaphragm chamber serves as the air supply source; the diaphragm presses down on the six springs it covers, thereby pushing the valve stem downward. The valve stem is connected to the valve core, which is installed in the correct position, so that when air is supplied, the valve moves toward the closed position. Therefore, it is called a gas shut-off valve. The term \"faulty open\" refers to the situation where the gas supply is interrupted due to construction work or corrosion of the gas pipes; under the force of the spring, the valve returns to its original position, ending up in a fully open state. How to use a gas shut-off valve? The method of use is determined from a safety perspective; this is a necessary condition for deciding whether to choose an air-on or air-off setting. For example ； One of the core components of a boiler is the drum; a control valve used in the feedwater system must be of the air-operated type. Why? For example, if the gas or power supply is suddenly interrupted, the furnace continues to burn intensely, keeping heating the water in the steam drum. If a gas-controlled regulating valve is used, and the power supply is interrupted, the valve will close, and the steam drum will dry out within minutes due to the lack of water supply – this is extremely dangerous. It is not possible to fix the problem with the regulating valve in a short time, which can lead to a shutdown of the boiler. Therefore, to prevent dry burning or even shutdown accidents, gas shut-off valves must be used. Although there is an energy outage and the control valve is in the fully open position, allowing water to keep flowing into the drum, this will not cause the drum to run dry. There is still time to address the issue with the control valve, so it’s not necessary to shut down the furnace immediately to fix it. Through the examples above, one should now have a preliminary understanding of how to choose between gas-open control valves and gas-close control valves! Matters related to everyday faults. It’s normal for failures to occur during the production process; it’s part of what happens in manufacturing. But to ensure quality, safety, and quantity, it is necessary to address issues promptly – and that is the value of staying at the company. Therefore, let’s briefly discuss several common fault phenomena: First, the output of the valve positioner is extremely slow. Do not open the valve positioner front cover ； Listen to the sound and check whether the gas supply pipe is cracked and causing a leak, which can be determined visually. And listen for any sound indicating leaks in the intake chamber. Open the front cover of the valve positioner ； 1. Check whether the constant-throttling orifice is blocked. 2. Check the position of the baffle. 3. Check the elasticity of the feedback spring. 4. Disassemble the device to inspect the diaphragm. II. Weak output from the valve positioner 1. Check whether the air supply pressure is within the specified range and whether the feedback rod has come loose; these are the simplest steps to take. 2. Check whether the signal wire connections are correct (problems that arise later on are generally ignored). 3. Verify that nothing is stuck between the coil and the armature. 4. Check whether the alignment between the nozzle and the baffle is proper. 5. Inspect the condition of the coil in the electromagnetic assembly. 6. Check whether the adjustment position of the balance spring is appropriate. Subsequently, input a signal; if the output pressure does not change, or if the output does not reach its maximum value, these are also common faults that occur in daily use, so no further explanation is needed here.