Working principle and failure analysis of pneumatic valve


1 Working principle of pneumatic valve

The so-called pneumatic valve, specifically refers to the cylinder as the actuator, compressed air as the power source to achieve the drive of the valve, in order to achieve the adjustment of the switch. When the regulating pipe receives the control signal sent by the automatic control system, it will adjust the temperature, pressure, flow and other related parameters. In general, pneumatic valves have the characteristics of safety and reliability, rapid response and simple control. In the specific process of operation, the adjustment chamber input 0.02-0.10MPa signal pressure, the film will appear thrust, in the thrust disc downward movement process will be compressed on the spring, to achieve the stem, push rod and spool downward movement, to achieve the purpose of valve regulation.

2 Common pneumatic valve failures and treatment methods

Through the study and analysis of the pneumatic valve fault state, it is understood that the possible faults are valve jam, valve action, valve leakage and shock, so it is necessary to formulate countermeasures according to the actual fault, in order to achieve the smooth and safe operation of the system, to create higher benefits.

2.1 The pneumatic valve is stuck

If the action of the valve stem appears dull, it means that there may be a large viscous substance in the valve body, and the situation of pneumatic valve jam mainly appears in the early stage of system operation. In this case, the valve or secondary line should be quickly switched on and off to flush out the blocking valve medium. At the same time, the stem can be clamped with pipe tongs, and signal pressure can be applied to reverse force rotation of the valve stem. If you still can't solve the above problems, then you can increase the driving power to repeatedly move up and down. It should be noted that the above operations are highly professional, so the help of professionals is needed to solve and handle the fault.

2.2 Valve does not operate

For this fault, the first step is to determine whether the air source pressure is normal, and on this basis to find the air source fault. When the gas source pressure in the valve is normal, it is necessary to determine whether the amplifier of the electric/gas 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 amplifier constant throttle hole is blocked. To this end, it is necessary to clean the air source, remove debris and dredge the constant throttle hole and other ways. If the valve still does not operate after the above treatment, the valve needs to be disengaged and a comprehensive inspection is performed.

2.3 Loud valve noise

When the fluid flows through the valve, once the pressure difference between the front and back is obvious, it will cause cavitation, which is mainly based on the valve core and seat, and then make the fluid appear noise. If the flow capacity value is too large, then it is necessary to reset the regulating valve, choose and flow capacity value to adapt to and match the regulating valve, so as to effectively reduce the noise, the following noise elimination method for specific introduction and analysis: (1) eliminate resonance noise method. Only when the valve resonates can the energy be overlapped to produce a noise of 100 decibels. There are great differences in performance, some vibration is strong, some vibration is weak, strong vibration noise is not big, weak vibration, noise is larger; Some of them are very loud in terms of vibration and noise. This noise tends to produce a single tone sound, and after the elimination of resonance, the noise will be reduced somewhat; (2) cavitation noise elimination method. Cavitation, as an important component of hydrodynamic noise, has a more prominent effect on the intensity of noise. During cavitation, the bubble rupture will produce a high-speed impact force imperceptibly, and then local strong turbulence will be generated, resulting in cavitation noise. This noise produces a rattling sound, which is very similar to the sound of sand and gravel. Eliminating cavitation can effectively reduce noise; (3) Adopting sound-absorbing material method. Suction material as a very common sound processing method, the effect is very good. The noise can be reduced effectively by means of attracting material to surround the noise source. Because the noise will spread, and then attract material surrounded in any part, will eliminate the noise in this part. This method is more used in the case of low noise, the main reason is that this method consumes a lot of money; (4) series silencer method. Series silencer method is more applicable to the aerodynamic noise silencer, this method can be very good to eliminate noise. For higher quality flow valve before and after comparison, can be clear that the method of consideration is very comprehensive, not only the effect is good, but also very cheap. With this method, the noise level can be effectively reduced, but from an economic point of view, the noise limit is usually reduced to 25 dB.

2.4 Valve Leakage

After studying and analyzing the leakage of pneumatic valve, it is understood that there are the following conditions: (1) packing leakage. Due to the plastic deformation of the packing, it will be in full contact with the stem after being subjected to 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 permeable fluid medium, leakage will occur. In this regard, it is suggested to use flexible graphite as filler, the fundamental reason is that this material has the characteristics of low friction, good air tightness, and maintenance is more convenient, to realize the extension of valve life; (2) valve leakage. Because the length of the stem does not match the valve body, it is easy to have a gap between the valve seat and the spool, resulting in valve leakage. At the same time, when the gas - off valve stem is short, this situation will also occur. In order to be able to solve this situation, it is necessary to adjust the valve stem to the appropriate length; (3) Valve seat and spool deformation leakage. The core factor in this case is corrosion caused by problems in the forging stage during production, and leakage occurs when the fluid medium continuously washes the valve. In view of this situation, when the valve seat and spool deformation is light, the sealing finish can be improved by sandpaper grinding, so as to achieve the goal of improving the valve sealing performance. In the case of serious damage to the valve seat and spool, it is necessary to replace the parts.

2.5 Oscillation

There are many reasons for valve oscillation. For example, in the case of improper selection, the flow rate, pressure and flow resistance of the valve are very drastic when it is in working condition. Then, when the valve can bear the limit of stiffness, there will be oscillation. 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 specific countermeasures according to the actual problems. For the valve vibration is relatively slight, can be used to improve the valve stiffness, such as the use of piston execution structure, the choice of greater stiffness of the valve; When the system frequency is the same as the valve frequency, the valve can be replaced; When the vibration of the valve base or pipe is severe, the most effective way is to realize the support to eliminate the increase of vibration.

2.6 Valve positioner

In view of the fault types of valve positioner, it mainly includes the following four kinds: First, when using nozzle baffle technology, the nozzle hole is very small is the core factor leading to the fault, it is easy to appear the air source is blocked, resulting in the valve positioner can not work normally; Second, because the valve positioner uses the mechanical force balance principle to work, so it is easy to be affected by vibration and temperature, resulting in the failure of the valve positioner; Third, the elastic coefficient of the spring under the influence of external factors, there will be non-linear valve; Fourth, there is a big difference between intelligent positioner and ordinary positioner, so it is easy to use the valve positioner in the case of emergency stop electrical converter out of control, the consequences are very dangerous. In this regard, in order to solve the valve positioner fault as far as possible, targeted measures should be developed according to the specific situation, in order to play the availability and reliability of intelligent positioner.

3 Conclusion

In general, the role of pneumatic valves is the automatic process control in the field of medium flow, pressure, temperature, liquid level and other process parameters. Because 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 professional knowledge, at the same time also need to constantly summarize and conclude the experience of fault handling, in order to achieve the improvement of pneumatic valve maintenance level. In the context of reasonable troubleshooting, not only can reduce the system failure rate, but also significantly improve the utilization rate of the valve, contributing to the smooth and safe operation of the system, in order to create higher benefits.