Keywords:pneumatic valve;fault;working principle CLC number:TH134 Document code:A Article ID:1671-2064(2018)03-0074-02
1 The working principle of the pneumatic valve The so-called pneumatic valve, specifically refers to the cylinder as the actuator, using compressed air as the power source to achieve the valve to achieve the adjustment of the switch. When the regulation pipeline receives the control signal from the automation control system, it will adjust the temperature, pressure, flow and other related parameters. In general, pneumatic valves possess features such as safety, reliability, quick response, and simple control. In the specific operation process, after the signal pressure of 0.02-0.10 MPa is input to the adjusting air chamber, the film will have a thrust force, and the spring will be compressed during the downward movement of the thrust disk to realize the valve stem, the push rod and the valve core. Move down to achieve valve adjustment.
2 Pneumatic valve common faults and processing methods Through the research and analysis of the fault status of the pneumatic valve, we learned that the possible faults include valve jamming, valve inaction, valve leakage and shock, etc. Therefore, it is necessary to base on actual faults. Formulate countermeasures to achieve a smooth and safe system operation and create higher benefits.
2.1 Pneumatic valve jamming If the stem action becomes dull, it means that there may be more viscous material in the valve body, and the jamming of the pneumatic valve mainly occurs at the initial stage of system operation. In this case, the valve or auxiliary line should be opened and closed quickly to flush the medium that blocked the valve. At the same time, the valve stem can also be clamped by a pipe wrench, and signal pressure can be applied to rotate the valve stem in reverse. If it is still not possible to solve the above problems, then it can be moved up and down repeatedly by increasing the driving power. It should be noted that since the above operations are highly specialized, they need to be completed with the help of professionals when solving and handling this failure.
2.2 The valve does not move For this fault, the first step is to determine whether the source pressure is normal or not. Based on this, the gas source fault is searched. When the air pressure in the valve is normal, it is necessary to judge whether the amplifier of the electric/air converter has an output. In the absence of output, there may be two situations: First, the water in the air is compressed at the ball valve of the amplifier; second, the constant throttle of the amplifier is blocked. In this regard, it is necessary to perform various methods such as cleaning the gas source, removing impurities, and clearing the constant orifice. If the valve still has no action after the above treatment, it is necessary to disassemble the valve and perform a full inspection.
2.3 The valve noise is large When the fluid flow passes through the valve, once the pressure difference between the front and rear is obvious, the cavitation phenomenon will be caused. The cavitation is mainly generated based on the valve core and the valve seat, thereby making the fluid appear noise. If the value of the flow capacity is too large, it is necessary to reset the regulating valve and select the regulating valve that is suitable and consistent with the flow capacity value. This can effectively reduce the noise. The specific method for eliminating noise is as follows. Introduction and analysis: (1) Elimination of resonance noise method. Only when the valve is in resonance can the energy be overlapped and a noise of 100% bee can be generated. Among them, there are great differences in performance. Some of them have strong vibrations, some have weak vibrations, strong vibrations have little noise, vibrations have weak ones, and loud noises; others have very large vibrations and noises. This noise tends to produce a kind of monotonous sound. After eliminating the resonance, the noise will be reduced. (2) Elimination of cavitation noise. As an important part of fluid dynamic noise, cavitation has a more prominent influence on the strength of noise. When cavitation occurs, the bursting of bubbles will virtually produce high-speed impact forces, which in turn will generate strong turbulence, resulting in cavitation noise. This noise produces a rattling sound that is very similar to that of a gravel sound. Eliminating cavitation can effectively reduce noise; (3) using sound-absorbing material method. As a very common sound processing method, the attraction material plays a very good treatment effect. By using the attraction material to surround the noise source, noise can be effectively reduced. As noise spreads, which part of the area the material is attracted to, the noise in this area will be eliminated. This method uses even less noise. The main reason is that this method consumes a lot of money; (4) Tandem muffler method. The tandem silencer method is more applicable to the noise reduction of aerodynamic noise. This method can very well eliminate noise. For the front and back comparison of valves with higher mass flow, it can be clearly considered that the method is very comprehensive, not only effective, but also very cheap. With this method, the decibels of noise can be effectively reduced, but from an economic point of view, the noise is generally limited to 25 dB.
2.4 Valve Leaks After researching and analyzing the situation of the leakage of pneumatic valves, the following conditions are known: (1) Packing leakage. Due to the plastic deformation of the filler, it is completely in contact with the valve stem after pressure is applied. It should be noted that because the contact between the two is not very uniform, some parts are in tight contact and some parts are loose in contact. Under this background, leakage may occur under the influence of high pressure, high temperature, and highly permeable fluid media. In this regard, it is recommended to use flexible graphite as a filler. The fundamental reason is that this material has the characteristics of small friction and good air tightness, and it is also more convenient to maintain and achieve the prolongation of valve life; (2) valve leakage. Because the length of the valve stem does not match the valve body, a gap between the valve seat and the valve core is likely to occur, resulting in a valve internal leakage. At the same time, this happens when the valve stem of the air valve is short. In order to solve this problem well, it is necessary to adjust the valve stem to a suitable length; (3) The valve seat and the valve core are subject to deformation and leakage. The core factor in this situation is the presence of problems in the forging phase of production and the occurrence of corrosion. At the same time, the fluid medium can also leak when it continuously flushes the valve. In view of this situation processing stage, when the valve seat and the valve core are deformed lightly, the sanding and polishing can be used to achieve the improvement of the seal finish to achieve the valve sealing performance improvement goal. In the case of serious damage to the valve seat and the valve core, replacement of the accessories is required.
2.5 Oscillation caused by the valve oscillation is a lot of reasons, such as: improper selection, the valve is in the working state, the flow rate, pressure and flow resistance changes are very severe, then when the valve can withstand the limit of the stiffness, There will be oscillations. At the same time, when the system frequency is the same as the valve frequency, oscillation may occur. In general, there are many factors that cause the valve to oscillate. Therefore, it is necessary to formulate targeted countermeasures based on actual problems. For the case of relatively slight vibration of the valve, the method of increasing the rigidity of the valve may be used, for example, the piston is used to execute the structure and the valve with greater rigidity is selected; when the system frequency and the valve frequency are the same, the valve can be replaced; when the valve is When the vibration of the base and pipeline is severe, the most effective way is to achieve support to eliminate the increase of vibration.
2.6 The position of the valve positioner for the positioner of the valve is mainly composed of the following four types: First, when the nozzle baffle technology is used, the nozzle hole is very small, which is the core factor leading to the failure, and it is easy for the air source to be blocked. Staying in the situation, resulting in the valve positioner can not work properly; Second, because the valve positioner uses a mechanical force balance principle of work, it is vulnerable to vibration, temperature, resulting in failure of the valve positioner; The elastic coefficient of the spring under the influence of external factors, there will be a situation of non-linear valve; Fourth, there is a big difference between the intelligent positioner and the ordinary positioner, so the use of the valve positioner in the emergency stop situation It is very easy to get out of control of the electrical converter and the consequences are very dangerous. To this end, in order to be able to solve the problem of valve positioner failure as soon as possible, specific measures should be formulated to take advantage of the availability and reliability of the intelligent positioner.
3 Conclusion In general, the role of pneumatic valves is to automate the process parameters of the medium flow, pressure, temperature, liquid level and other process parameters. Due to the current type of failure of the pneumatic valve is more, and in the troubleshooting process has a strong professional. In this regard, in addition to the need to grasp the appropriate theoretical expertise, but also need to continue to summarize the experience of failure handling and induction, in order to achieve the maintenance of the level of pneumatic valves. In the context of reasonable failure resolution, not only can the reduction of the system failure rate be achieved, but also significant improvements in the valve utilization rate, which contributes to the smooth and safe operation of the system to create higher benefits.
References  Ren Wengang, Zhu Qiang. Performance Measurement and Evaluation System of Pneumatic Regulating Valve Based on LabVIEW[J]. Instrumentation Technology, 2016, (09): 15-19.
 Song Haicheng. Common Faults and Treatments of Pneumatic Control Valves [J]. Automation Technology and Applications, 2015, (07): 89-92.
 Zhu Jin. Failure analysis and maintenance method of pneumatic regulating valve[J]. Journal of Nanjing Institute of Industry Technology, 2016, (02): 10-12.
 Guo Song, Zhang Zhiyong, Gao Lihua. Analysis of Routine Maintenance and Common Faults of Pneumatic Control Valves[J]. SME Management and Technology, (second edition), 2012, (02): 284-285.
 Zheng Ying. Analysis of common faults and causes of pneumatic control valves [J]. Science and Technology Information, 2013, (16): 370.
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