Working principle and fault analysis of pneumatic valve

1 working principle of pneumatic valve

The so-called pneumatic valve, specifically refers to the cylinder as the actuator, with compressed air as the power source to realize the driving of the valve, in order to realize the adjustment of the switch. When the control pipeline receives the control signal from the automatic control system, it will adjust the temperature, pressure, flow and other related parameters. In general, the pneumatic valve has the characteristics of safe and reliable, fast response and simple control. In the process of specific operation, when the signal pressure of 0.02-0.10mpa is input into the regulating chamber, the film will have thrust, and the spring will be compressed during the downward movement of the thrust disc, so as to realize the downward movement of the valve stem, push rod and spool, so as to achieve the purpose of regulating the valve.

2 common faults of pneumatic valve and treatment methods

Through the research and analysis of the fault state of the pneumatic valve, it is known that the possible faults include valve blocking, valve failure, valve leakage and vibration. Therefore, it is necessary to formulate countermeasures according to the actual faults, so as to realize the stable and safe operation of the system and create higher benefits.

2.1 blockage of pneumatic valve

If the action of the valve stem is slow, it means that there may be viscous substances in the valve body, and the blockage of the pneumatic valve mainly occurs in the early stage of the system operation. In this case, the valve or auxiliary line should be opened and closed quickly to flush away the medium blocking the valve. At the same time, the pipe clamp can be used to clamp the valve stem and apply signal pressure to rotate the valve stem with counter force. If the above problems still can not be solved, then the driving power can be increased to move up and down repeatedly. It should be noted that due to the strong professionalism of the above operation, it needs to be completed with the help of professionals when solving and handling this fault.

2.2 valve does not operate

In view of this fault, the first step is to determine whether the air source pressure is normal or not, and find the air source fault on this basis. When the air pressure in the valve is normal, it is necessary to judge whether the amplifier of the electro pneumatic converter has output. When there is no output, there may be two cases: first, the moisture in the air is compressed and placed at the ball valve of the amplifier; second, the constant orifice of the amplifier is blocked. In this regard, it is necessary to clean the air source, remove debris and dredge the constant orifice. If the valve still does not move after the above treatment, it is necessary to remove the valve and carry out a comprehensive inspection.

2.3 the valve is noisy

When the fluid flows through the valve, once the pressure difference between the front and back is obvious, it will cause cavitation. Cavitation is mainly based on the valve core and valve seat, which makes the fluid noise. If the flow capacity value is too large, then it is necessary to reset the control valve, and select the control valve that is suitable and consistent with the flow capacity value, so as to effectively reduce the noise. The following is a detailed introduction and analysis of noise elimination methods: (1) resonance noise elimination method. Only in the state of resonance of the valve can the energy be overlapped, resulting in 100 dB noise. There is a big difference in performance, some strong vibration, some weak vibration, strong vibration noise is not big, weak vibration, noise is big; some vibration and noise are very big. This noise often produces a single tone sound, after eliminating resonance, the noise will also be weakened; (2) eliminating cavitation noise method. As an important part of hydrodynamic noise, cavitation has a more prominent impact on the intensity of noise. During cavitation, the bubble rupture will produce high-speed impact force, and then local strong turbulence will breed, resulting in cavitation noise. This noise will produce rattle, which is very similar to the sound of sand and stone. Eliminating cavitation can effectively reduce noise; (3) using sound-absorbing material method. As a very common sound processing method, attractive material has a very good processing effect. With the help of the attractive material to surround the noise source, the noise can be effectively reduced. Because the noise will spread, and then attract material around which part, will eliminate the noise of this part. The main reason is that this method consumes a lot of money; (4) series muffler method. Series muffler method is more suitable for the noise reduction of aerodynamic noise, this method can eliminate the noise very well. For the valve with high mass flow before and after comparison, it can be clear that this method is very comprehensive, not only the effect is good, but also very cheap. This method can effectively reduce the noise dB, but from the economic point of view, the noise is usually limited to 25 dB.

2.4 valve leakage

According to the research and analysis of the leakage of pneumatic valve, we know that there are the following situations: (1) packing leakage. Due to the plastic deformation of the packing, it will be in full contact with the valve stem under pressure. It should be noted that because the contact between the two is not very uniform, some parts of the contact is tight, some parts of the contact is loose. In this context, under the influence of high pressure, high temperature and strong permeability of the fluid medium, there will be leakage. In this regard, it is suggested that flexible graphite should be used as packing, the fundamental reason is that this material has the characteristics of small friction, good air tightness, and convenient maintenance, which can prolong the service life of the valve; (2) internal leakage of the valve. Because the length of the valve stem does not match with the inside of the valve body, it is easy to have a gap between the valve seat and the valve core, resulting in the leakage of the valve. At the same time, when the valve stem is short, this situation will also occur. In order to solve this problem, it is necessary to adjust the stem to a suitable length; (3) the valve seat and spool are deformed and leaking. The core factor of this situation is that there are problems in the forging stage of production and corrosion occurs. At the same time, when the fluid medium constantly washes the valve, there will also be leakage. In view of this situation, in the treatment stage, when the deformation of valve seat and valve core is light, the sealing finish can be improved by sandpaper grinding to achieve the goal of improving the sealing performance of the valve. When the valve seat and valve core are seriously damaged, the accessories need to be replaced.

2.5 oscillation

There are many reasons leading to valve oscillation, for example, in the case of improper selection, when the valve is in the working state, the change of flow velocity, pressure and flow resistance is very violent, then when it exceeds the limit of the stiffness that the valve can withstand, the oscillation will occur. At the same time, when the system frequency is the same as the valve frequency, there will be oscillation. In general, there are many factors leading to valve oscillation, so it is necessary to formulate targeted countermeasures according to the actual problems. When the vibration of the valve is relatively slight, the way to improve the stiffness of the valve can be adopted, such as using the piston actuator structure and selecting the valve with greater stiffness; when the frequency of the system is the same as that of the valve, the valve can be replaced; when the vibration of the valve base and pipe is relatively severe, the most effective way is to realize the support to eliminate the vibration Plus.

2.6 valve positioner

According to the types of valve positioner failure, mainly includes the following four types: first, the use of nozzle baffle technology, the nozzle hole is very small is the core factor leading to failure, it is easy to appear that the gas source is blocked, resulting in the valve positioner can not work normally; second, because the valve positioner uses the mechanical force balance principle, it is easy to work Due to the influence of vibration and temperature, the valve positioner fails; thirdly, the spring's elastic coefficient is affected by external factors, and the valve will be nonlinear; fourthly, the intelligent positioner is quite different from the ordinary positioner, so it is easy to cause the electrical converter out of control when the valve positioner is used in emergency stop Besides, the consequences are very dangerous. In this regard, in order to be able to solve the valve positioner fault as soon as possible, we should formulate targeted measures according to the specific situation, so as to give play to the availability and reliability of intelligent positioner.

Conclusion

Generally speaking, the function of pneumatic valve is the process parameters of medium flow, pressure, temperature and liquid level in the field of automatic process control. Due to the current pneumatic valve fault type is more, and in the process of fault treatment has a strong professional. In this regard, in addition to the need to master the corresponding theoretical expertise, but also need to continue to summarize and summarize the experience of fault treatment, in order to achieve the improvement of the maintenance level of pneumatic valve. In the context of reasonable fault resolution, it can not only reduce the system failure rate, but also significantly improve the utilization rate of valves, which is conducive to the smooth and safe operation of the system, so as to create higher benefits.