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Working principle and use of valve positioner

  • Author:Cowinns
  • Source:Cowinns
  • Release on:2020-07-24
Working principle and use of valve positioner

Pneumatic control valve

The control valve can know some information from its name, and the adjustment range of the keyword adjustment is between 0 and 100%.

Careful friends should find that there is a device hanging under the head of each regulating valve. Those familiar with it must know that this is the heart of the regulating valve, the valve positioner. Through this device, the amount of air entering the head (pneumatic film) can be adjusted. Precisely control the position of the valve.

The positioner intelligent positioner and mechanical positioners, today's discussion is the latter mechanical positioners, shown in the picture locator same.

Working principle

The air source comes from the compressed air of the air compressor station, and there is an air filter pressure reducing valve in front of the air source inlet of the valve positioner to purify the compressed air. The gas source from the outlet of the pressure reducing valve enters from the valve positioner. As for how much gas enters the membrane head of the valve, it is determined according to the output signal of the controller.

The electrical signal output by the controller is 4-20mA, the pneumatic signal is 20Kpa-100Kpa, and the electrical signal to the pneumatic signal is carried out through an electrical converter.

When the electrical signal output by the controller is converted into a corresponding air signal, the converted air signal is then applied to the bellows. The lever 2 moves around the fulcrum, and the lower part of the lever 2 moves to the right close to the nozzle. The back pressure of the nozzle increases. After being amplified by the pneumatic amplifier (the part with the symbol less than in the figure), a part of the air source is sent to the air chamber of the pneumatic membrane, and the valve stem takes the valve core downward to automatically gradually open the valve Become smaller. At this time, the feedback rod connected to the valve stem (the swing rod in the figure) moves down around the fulcrum, so that the front end of the shaft moves down, the eccentric cam connected to it rotates counterclockwise, and the roller rotates clockwise to move to the left, thus Stretch the feedback spring. Since the lower part of the feedback spring tension lever 2 moves to the left, it will be in force balance with the signal pressure acting on the bellows, so the valve is fixed at a certain position and does not move.

Knowledge expansion 1

Pneumatic diaphragm air chamber ventilation source, the diaphragm presses down the six spring springs covered by the diaphragm, thereby pushing the valve stem downwards. The valve stem is connected with the valve core, and the valve core is installed in the forward direction, so the air source is the valve Move to the off position. Therefore, it is called a gas shutoff valve. Failure to open means that when the air supply is interrupted due to construction or the corrosion of the air pipe, the valve is reset under the reaction force of the spring, and the valve is in the fully open position again.

How to use the air shutoff valve?

How to use it is considered from the perspective of safety, which is a necessary condition for selecting air on or off.

For example; one of the core devices of the boiler is a steam drum, and a High pressure pneumatic diaphragm control valve used in the water supply system must be air-closed. why? For example, if the gas supply or power supply is suddenly interrupted, the furnace is still burning violently, continuously heating the water in the drum. If the gas is used to open the regulating valve and the energy is interrupted, the valve will be closed, and the steam drum will be boiled off within minutes without water (dry burning). This is very dangerous. It is impossible to deal with the regulating valve failure in a short time, which will cause the boiler to stop. The accident happened. Therefore, to avoid dry burning or even shutdown accidents, air shutoff valves must be used. Although the energy is interrupted and the regulating valve is in the fully open position, the steam drum is continuously filled with water, but it will not cause the steam drum to dry up. There is still time to deal with the failure of the regulating valve and the boiler will not be stopped directly.

Knowledge expansion 2

The output of the valve positioner is like turtle speed.

Do not open the front cover of the valve positioner; listen to the sound and see if the air source tube is cracked and leaked, which can be judged by the naked eye. And input the air chamber to hear whether there is any leakage.

Open the front cover of the valve positioner; 1. Whether the constant orifice is blocked 2. Check the position of the baffle 3. Check the elasticity of the feedback spring 4. Disassemble the Fangda device to check the diaphragm.

The output of the valve positioner is boring

1. Check whether the air pressure is within the specified range and whether the feedback lever has fallen off. This is the simplest step.

2. Check whether the wiring of the signal line is correct (problems that appear later are generally ignored)

3. Is there anything stuck between the coil and the armature?

4. Check whether the matching position of the nozzle and the baffle is appropriate

5. Check the coil condition of the electromagnetic component

6. Check whether the debugging position of the balance spring is reasonable

Subsequently, input signal, but the output pressure does not change, if there is output, the maximum value is not reached, etc. Cowinns supplied Siemens pneumatic control valve to power plant projects. If you are interested in our product please feel free contact with us.