How does a valve positioner work? Locator failures and solutions

There are intelligent positioners and mechanical positioners for valve positioners, and today we are talking about the latter mechanical positioner, which is the same as the positioner shown in the picture
How does a valve positioner work? Locator failures and solutions

How does a valve positioner work? Locator failures and solutions

Schematic diagram of valve positioner structure

In the figure, the components of the mechanical pneumatic valve positioner are basically explained one by one, and the next thing is to see how it works?

The air source comes from the compressed air of the air compressor station, and there is also an air filtration pressure reducing valve in front of the air source inlet of the valve positioner for the purification of compressed air. The air source from the outlet of the pressure reducing valve enters from the valve positioner, and the amount of air entering the membrane head of the valve is determined by the output signal of the controller.

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

When the electrical signal output by the controller is converted into the corresponding gas signal, the converted gas signal is then applied to the bellows. The lever 2 moves around the fulcrum, and the lower section of the lever 2 moves to the right near the nozzle.

The back pressure of the nozzle increases, and after the pneumatic amplifier is amplified (the part with a smaller symbol in the picture), a part of the air supply is fed into the air chamber of the pneumatic membrane, and the valve stem with the spool is automatically and gradually reduced in opening.

At this time, the feedback rod connected to the stem (the pendulum rod in the figure) moves downward around the fulcrum, so that the front end of the shaft moves downward, and the eccentric cam connected to it rotates counterclockwise, and the roller rotates clockwise to the left, thereby stretching the feedback spring. Because the lower section of feedback spring stretch lever 2 moves to the left, it will reach force balance with the signal pressure acting on the bellows at this moment, so the valve is fixed in a certain position and does not move.

Through the above introduction, you should have a certain understanding of the mechanical valve positioner, and then operate it when you have the opportunity, so that you can disassemble it once and deepen the position of each part of the positioner and the name of each part. Therefore, the discussion of mechanical valves has come to an end, and the next step is to expand the knowledge and let the control valve have a deeper understanding.

Knowledge Extension One

The pneumatic diaphragm control valve in the picture belongs to the gas-closed type, and some people ask, why?

, look at the direction of the air intake of the pneumatic film, it is a positive effect.

Second, look at the installation direction of the spool, which is positive.

Pneumatic diaphragm air chamber ventilation source, the diaphragm presses down on the six spring springs covered by the diaphragm, thereby pushing the valve stem to move downward, 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 moving to the closed position. Therefore, it is called an air-shut-off valve. When the gas 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.

How to use the gas shut-off valve?

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

For example, one of the core devices of the boiler, the steam drum, a regulating valve for the water supply system must be gas-closed. Why? For example, if the gas supply or power supply is suddenly interrupted, the furnace is still burning violently, constantly heating the water in the steam drum. If the air is used to open the control valve, the energy is interrupted, the valve will be closed, and the steam drum will be burned dry (dry burning) in minutes without moisture, which is very dangerous, and it is impossible to deal with the failure of the control valve in a short time, which will lead to furnace shutdown accidents.

Therefore, to avoid dry burning or even furnace shutdown accidents, it is necessary to use the gas shut-off valve. Although the energy is interrupted, the control valve is located in the fully open position, and the water is constantly fed to the steam drum, but it will not cause the steam drum to run out of money, and there is still time to deal with the failure of the control valve without directly stopping the furnace to deal with it.

Through the above examples, we should now have a preliminary understanding of how to choose the air opening control valve and the air closing control valve!

This little knowledge is about the change of the positive and negative effects of the locator.

The control valve in the picture is positive. The eccentric cam has two sides, A for the front and B for the side. In this case, the A side is facing outward, and turning the B side outward is the reaction. Therefore, changing the A direction to the B direction in the diagram is a reactionary mechanical valve positioner.

The physical picture in the picture is a positive-acting valve positioner, and the controller outputs a signal of 4-20mA. When 4mA, the corresponding gas signal is 20Kpa, and the regulating valve is fully open. When 20mA, the corresponding gas signal is 100Kpa, and the regulating valve is fully closed.

Mechanical valve positioners have both advantages and disadvantages:

Pros: Control***.

Disadvantages: Due to pneumatic control, if you want to feed back the position signal to the central control room, you need to do an additional electrical conversion device.

Matters and solutions to the day-to-day failure of the locator:

Failure in the production process is normal, it is part of the production process. However, in order to ensure quality, safety and quantity, we must deal with problems in a timely manner, which is the value of staying in the company. Therefore, we will briefly talk about several fault phenomena encountered:

First, the output of the valve positioner is like turtle speed

Do not open the front cover of the valve positioner; Listen to the sound to see if the air source tube is cracked and leaked, which can be judged by the naked eye. As well as whether there is a leakage in the input air chamber by listening.

Open the front cover of the valve positioner:1. Whether the throttle hole is blocked2. Check the position of the bezel3. Check the spring force of the feedback spring

4. Disassemble the square device and check the diaphragm.

Second, the output of the valve positioner is not strong

1. See whether the air source pressure is within the specified range and whether the feedback rod has fallen off, which is a simple step.

2. Check whether the signal line is wired correctly (the problems that occur in the later stage 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, the input signal, but the output pressure does not change, and the output is not up to the highest value, etc., these faults are also daily failures will be encountered here.

Stroke adjustment of the control valve

In the production process, the control valve will be used for too long and the stroke will be inaccurate, and there is always a large error when you want to open to a certain position.

The stroke is 0-100%, and the largest point is selected for adjustment, which is 0, 25, 50, 75, and 100, all of which are expressed as percentages. Especially for mechanical valve positioners, it is necessary to know where the positions of the two manual components inside the positioner, the adjustment zero position and the adjustment span, are clearly located during adjustment.

If we take the air opening control valve as an example, adjust it.

At the zero adjustment point, the control room or signal generator gives 4mA, and the regulating valve should be fully closed, and the zero adjustment should be carried out if it is not fully closed. After the zero adjustment is completed, the point of directly adjusting the 50% is completed, and the span is adjusted with the difference, and at the same time, pay attention to the feedback lever and the valve stem should be in a vertical state. After adjustment, adjust 100% of this point, and then adjust it repeatedly from five points between 0-100% until the opening is ***.

From mechanical positioners to smart positioners. From the perspective of science and technology, the rapid development of science and technology has reduced the labor intensity of front-line maintenance personnel. Personally, I think that if you want to exercise your hands-on ability and learn technology, mechanical positioners are the best ones, especially for new instrument personnel. To put it bluntly, the intelligent locator will understand a few words on the manual, just move your fingers, what to adjust the zero point to adjust the range, adjust the stroke and automatically adjust, as long as you are stupidly waiting for it to finish playing by yourself, it will sort out the scene and leave. For the mechanical type, many parts have to be dismantled and repaired and installed by themselves, which can improve the hands-on ability and have a better impression of its internal structure.

Whether it is intelligent or non-intelligent, it is in a dominant position for the entire automated production process, once the "strike", there is no way to adjust, and automatic control is meaningless.

How does a valve positioner work? Locator failures and solutions

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