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, and the compressed air as the power source to realize the valve drive, in order to realize the adjustment of the switch. When the regulating pipe receives the control signal from the automatic control system, the temperature, pressure, flow and other related parameters will be adjusted. Generally speaking, the characteristics of pneumatic valve are safety, reliability, quick reaction and simple control. During the specific operation, after the regulating chamber is input to the signal pressure of 0.02-0.10mpa, the film will have thrust. During the downward movement of the thrust disc, the spring will be compressed to realize the downward movement of the valve stem, push rod and valve core to achieve the purpose of valve adjustment.

2 common faults and treatment methods of pneumatic valve

Through the research and analysis of the failure state of pneumatic valve, it is found that the possible faults include valve jam, valve inaction, valve leakage and vibration, etc. Therefore, it is necessary to formulate countermeasures according to actual fault to achieve stable and safe operation of the system and create higher benefits.

2.1 pneumatic valve stuck

If the valve stem is slow, it means that there may be more viscous material in the valve body, and the situation of pneumatic valve blocking mainly occurs in the early stage of system operation. In case of this situation, the valve or auxiliary line should be quickly opened and closed to flush the medium blocking the valve. At the same time, the valve rod can be clamped by pipe clamp and signal pressure can be applied to rotate the valve rod with counter force. If the above problems are still unable to be solved, the up and down movements can be repeated by increasing the driving power. It should be noted that because the above operation has strong professionalism, it is necessary to complete the problem with the help of professional personnel when solving and handling this fault.

2.2 valve does not act

For this fault, the first step is to determine whether the air source pressure is normal, and then find the gas source fault. When the air supply pressure in the valve is normal, it is necessary to judge whether the amplifier of the electric / gas converter has output. There are two possible cases when there is no output: one is to put the water in the air and put it at the ball valve of the amplifier after the water is compressed; the other is that the constant orifice of the amplifier is blocked. Therefore, it is necessary to clean the air source, remove debris and dredge the constant throttle hole. If the valve still does not act after the above treatment, the valve needs to be opened and fully inspected.

2.3 valve noise

When the fluid flow passes through the valve, once the pressure difference is obvious before and after, cavitation will be caused. Cavitation is mainly based on the valve core and valve seat, which makes the fluid appear noise. If the circulation capacity value is too large, it is necessary to reset the regulating valve and select the regulating valve which is suitable for and consistent with the circulation capacity value, so that the effective noise can be reduced. The following detailed introduction and analysis are made on the noise elimination method: (1) elimination of resonance noise method. Only in the resonance state of the valve can the energy be overlapped, and then a hundred decibels of noise can be generated. There are great differences in the performance, some of which are strong, some are weak, the strong ones are not big, the vibration is weak, and the noise is relatively large; some are very loud in vibration and noise. This noise often produces a single tone sound, and after eliminating resonance, the noise will also weaken; (2) the method of eliminating cavitation noise. Cavitation, as an important part of hydrodynamic noise, has a more prominent influence on the intensity and intensity of noise. When cavitation occurs, bubble rupture will produce high-speed impact force invisibly, and then strong turbulence will be generated locally, and cavitation noise will be generated. This noise produces lattice sound, very similar to the sound of sand and stone. The noise can be reduced by eliminating cavitation; (3) sound absorption material method is adopted. As a very common sound processing method, attractive materials have a very good effect. By means of the attractive material, the noise source can be effectively reduced. Because noise can spread, and then attract the material to surround which part, it will eliminate the noise. This method is more used in the case of low noise, the main reason is that the method consumes a lot of funds; (4) series muffler method. The series muffler method is more suitable for the noise elimination of aerodynamic noise, which can eliminate noise very well. The comparison between the valve with high mass flow rate and the comparison between the two methods can be made clear that the consideration of this method is very comprehensive, not only the effect is good, but also the cost is very low. The method can reduce the noise DB effectively, but from the economic point of view, it is usually limited to 25 dB.

2.4 valve leakage

After studying and analyzing the leakage of pneumatic valve, we can find out the following: (1) packing leakage. Due to the plastic deformation of the packing, it will contact the valve stem completely after being under pressure. It should be noted that because the contact between the two is not very uniform, some parts contact is tight, and some parts contact is relatively loose. In this context, the leakage will occur under the influence of high pressure, high temperature and strong permeability of fluid medium. Therefore, it is suggested that flexible graphite be used as packing, the root reason is that the material has the characteristics of small friction and good air tightness, and the maintenance is also convenient, which can extend the valve life; (2) leakage inside the valve. Because the length of valve stem does not match with the valve body, it is easy to appear the gap between the valve seat and valve core, which leads to the leakage in the valve. At the same time, this can occur when the valve stem of the air shut-off valve is short. In order to solve this problem, the valve stem should be adjusted to the proper length; (3) the valve seat and valve core are deformed and leaking. The core factor of this situation is the corrosion of the forging stage, and the leakage of the fluid medium will occur when the valve is constantly scoured. In this case treatment stage, when the deformation of valve seat and valve core is relatively light, the sealing smoothness can be improved by sandpaper grinding, so as to achieve the goal of improving the sealing performance of the valve. In case of serious damage to valve seat and valve core, it is necessary to replace the accessories.

2.5 oscillation

There are many reasons for valve oscillation. For example, under the condition of improper selection, the changes of flow rate, pressure and flow resistance are very violent when the valve is in working state. If the valve can bear the limit of rigidity, it will oscillate. At the same time, when the system frequency and valve frequency are the same, there will be oscillation. Generally speaking, there are many factors that cause valve oscillation, so we need to formulate targeted countermeasures according to the actual problems. In view of the slight vibration of the valve, the way to improve the valve stiffness can be adopted, such as the piston structure and the valve with greater rigidity; when the system frequency is the same as the valve frequency, the valve can be replaced; when the vibration of valve base and pipe channel is more severe, the most effective way is to realize the increase of support and vibration elimination Plus.

2.6 valve positioner

According to the failure types of valve positioner, it mainly includes the following four types: first, when using nozzle baffle technology, the small nozzle hole is the core factor leading to failure, and it is easy to appear the situation that the air source is blocked, which leads to the valve positioner not working normally; secondly, it is easy to work because the valve positioner is operated by mechanical force balance principle The valve positioner is affected by vibration and temperature, which leads to the failure of valve positioner; thirdly, the elasticity coefficient of spring will appear the non-linear condition of valve under the influence of external factors; fourthly, there is a big difference between intelligent positioner and ordinary positioner, so it is easy to lose control of electrical converter when using valve positioner in case of emergency stop In addition, the consequences are very dangerous. In order to solve the valve positioner fault as soon as possible, the measures should be made according to the specific situation to give full play to the availability and reliability of the 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. Because the current types of pneumatic valve faults are more, and in the process of fault handling has a strong professionalism. In order to improve the maintenance level of pneumatic valve, we need to master the corresponding theoretical knowledge, and at the same time, we need to summarize and summarize the experience of fault treatment. In the context of reasonable fault resolution, not only can the system failure rate be reduced, but also has a significant improvement in valve utilization, which is helpful to the smooth and safe operation of the system, in order to create higher benefits.