A brief discussion on the working principle and use of valve positioners

If you take a walk around the chemical plant workshop, you will definitely see some pipes equipped with round-headed valves, which are regulating valves.

Pneumatic diaphragm regulating valve

You can know some information about the regulating valve from its name. The key word “regulation” is that its adjustment range can be adjusted arbitrarily between 0 and 100%.

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

Valve positioners include intelligent positioners and mechanical positioners. Today we are discussing the latter mechanical positioner, which is the same as the positioner shown in the picture.


Working principle of mechanical pneumatic valve positioner


Valve positioner structural diagram

The picture basically explains the components of the mechanical pneumatic valve positioner one by one. The next step is to see how it works?

The air source comes from the compressed air of the air compressor station. There is an air filter pressure reducing valve in front of the air source inlet of the valve positioner for purification of compressed air. The air source from the outlet of the pressure reducing valve enters from the valve positioner. The amount of air entering the membrane head of the valve is determined according to the output signal of the controller.

The electrical signal output by the controller is 4~20mA, and the pneumatic signal is 20Kpa~100Kpa. The conversion from electrical signal to pneumatic signal is done through an electrical converter.

When the electrical signal output by the controller is converted into a corresponding gas signal, the converted gas signal is then acted on the bellows. Lever 2 moves around the fulcrum, and the lower section of lever 2 moves to the right and approaches the nozzle. The back pressure of the nozzle increases, and after being amplified by the pneumatic amplifier (the component with a less than symbol in the picture), part of the air source is sent to the air chamber of the pneumatic diaphragm. The valve stem carries the valve core downward and automatically gradually opens the valve. get smaller. At this time, the feedback rod (the swing rod in the picture) connected to the valve stem moves downward around the fulcrum, causing the front end of the shaft to move downward. The eccentric cam connected to it rotates counterclockwise, and the roller rotates clockwise and moves to the left. Stretch the feedback spring. Since the lower section of the feedback spring stretches the lever 2 and moves to the left, it will reach a force balance with the signal pressure acting on the bellows, so the valve is fixed at a certain position and does not move.

Through the above introduction, you should have a certain understanding of the mechanical valve positioner. When you have the opportunity, it is best to disassemble it once while operating it, and deepen the position of each part of the positioner and the name of each part. Therefore, the brief discussion of mechanical valves comes to an end. Next, we will expand the knowledge to gain a deeper understanding of regulating valves.


knowledge expansion

Knowledge expansion one


The pneumatic diaphragm regulating valve in the picture is an air-closed type. Some people ask, why?

First, look at the air inlet direction of the aerodynamic diaphragm, which is a positive effect.

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

Pneumatic diaphragm air chamber ventilation source, the diaphragm presses down the six springs covered by the diaphragm, thereby pushing the valve stem to move downward. The valve stem is connected to the valve core, and the valve core is installed forward, so the air source is the valve Move to the off position. Therefore, it is called an air-to-close valve. Fault open means that when the air supply is interrupted due to construction or 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 shut-off valve?

How to use it is considered from the perspective of safety. This is a necessary condition for choosing whether to turn the air on or off.

For example: the steam drum, one of the core devices of the boiler, and a regulating valve used in the water supply system must be air-closed. Why? For example, if the gas source or power supply is suddenly interrupted, the furnace is still burning violently and 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 drum will be burned out in 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 lead to a shutdown of the furnace. Accidents happen. Therefore, to avoid dry burning or even furnace shutdown accidents, a gas shut-off valve must be used. Although the energy is interrupted and the regulating valve is in the fully open position, water is continuously fed into the steam drum, but it will not cause dry money in the steam drum. There is still time to deal with the regulating valve failure and the furnace will not be shut down directly to deal with it.

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


Knowledge Expansion 2


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

The regulating valve in the figure is positive acting. The eccentric cam has two sides AB, A represents the front side and B represents the side. At this time, the A side is facing outward, and turning the B side outward is a reaction. Therefore, changing the A direction in the picture to the B direction is a reaction mechanical valve positioner.

The actual picture in the picture is a positive-acting valve positioner, and the controller output signal is 4-20mA. When 4mA, the corresponding air signal is 20Kpa, and the regulating valve is fully open. When 20mA, the corresponding air signal is 100Kpa, and the regulating valve is fully closed.

Mechanical valve positioners have advantages and disadvantages

Advantages: precise control.

Disadvantages: Due to pneumatic control, if the position signal is to be fed back to the central control room, an additional electrical conversion device is required.



Knowledge expansion three


Matters related to daily breakdowns.

Failures during the production process are normal and are part of the production process. But in order to maintain quality, safety, and quantity, problems must be dealt with in a timely manner. This is the value of staying in the company. Therefore, we will briefly discuss several fault phenomena encountered:

1. The output of the valve positioner is like a turtle.

Do not open the front cover of the valve positioner; listen to the sound to see if the air source pipe is cracked and causing leakage. This can be judged with the naked eye. And listen to whether there is any leakage sound from the input air chamber.

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 square valve and check the diaphragm.

2. The output of the valve positioner is boring

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

2. Check whether the signal line wiring is correct (problems that arise 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 condition of the electromagnetic component coil

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

Then, a signal is input, but the output pressure does not change, there is output but it does not reach the maximum value, etc. These faults are also encountered in daily faults and will not be discussed here.



Knowledge expansion four


Regulating valve stroke adjustment

During the production process, using the regulating valve for a long time will lead to inaccurate stroke. Generally speaking, there is always a large error when trying to open a certain position.

The stroke is 0-100%, select the maximum point for adjustment, which are 0, 25, 50, 75, and 100, all expressed as percentages. Especially for mechanical valve positioners, when adjusting, it is necessary to know the positions of the two manual components inside the positioner, namely the adjustment zero position and the adjustment span.

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

Step 1: At the zero adjustment point, the control room or signal generator gives 4mA. The regulating valve should be fully closed. If it cannot be fully closed, perform zero adjustment. After the zero adjustment is completed, directly adjust the 50% point, and adjust the span accordingly. At the same time, note that the feedback rod and the valve stem should be in a vertical state. After the adjustment is completed, adjust the 100% point. After the adjustment is completed, adjust repeatedly from the five points between 0-100% until the opening is accurate.

Conclusion; from mechanical positioner to intelligent positioner. From a scientific and technological perspective, 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 skills and learn skills, a mechanical positioner is the best, especially for new instrument personnel. To put it bluntly, the intelligent locator can understand a few words in the manual and just move your fingers. It will automatically adjust everything from adjusting the zero point to adjusting the range. Just wait for it to finish playing and clean up the scene. Just leave. For the mechanical type, many parts need to be disassembled, repaired and reinstalled by yourself. This will definitely improve your hands-on ability and make you more impressed with its internal structure.

Regardless of whether it is intelligent or non-intelligent, it plays a dominant role in the entire automated production process. Once it “strike”, there is no way to adjust and automated control is meaningless.


Post time: Aug-31-2023