Working principle and fault analysis of pneumatic valve
Working principle of pneumatic valve
The so-called pneumatic valve, specifically refers to the cylinder as an actuator, to compressed air as a source of power to achieve the valve drive, in order to achieve the switch for regulation. 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 safe and reliable, rapid response and simple control. In the process of specific operation, the adjusting chamber will be input 0.02-0.10mpa signal pressure, the film will appear thrust, in the process of thrust plate moving down the spring will be compressed to achieve the valve stem, push rod and spool moving down, to achieve the purpose of valve regulation.
2 common pneumatic valve failure and treatment methods
Through the study and analysis of the pneumatic valve fault state, know that the possible fault has valve stuck, valve does not move, valve leakage and shock, so it is necessary to work out countermeasures according to the actual fault, in order to achieve a stable 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, then it means that there may be a larger viscosity of the material in the valve body, and the situation of pneumatic valve stuck mainly in the early operation of the system. In this case, the valve or secondary line should be quickly switched on and off to flush out the media blocking the valve. At the same time, the stem can also be clamped by pipe tongs and applied signal pressure to counter rotation of the stem. If still cannot solve above problem, so the way that can increase through drive power, will undertake repeating fluctuation moves. It should be noted that due to the strong professionalism of the above operations, it is necessary to solve and deal with this fault with the help of professionals.
2.2 no action of valve
For this fault, the first step is to determine whether the air source pressure is normal, on this basis to find the air source fault. When the air pressure in the valve is normal, it is necessary to judge whether the amplifier of the electrical/gas converter has output. When there is no output, there may be two situations: first, the moisture in the air is compressed and placed at the ball valve of the amplifier; Second, the amplifier constant throttle hole plug. In this regard, it is necessary to perform clean air source, remove debris and dredge constant throttle hole and other ways. If pass above processing after the valve still does not have an action, need dismount valve and executive comprehensive inspection.
2.3 loud valve noise
When the fluid flows through the valve, once the pressure difference between the front and the back is relatively obvious, it will cause cavitation, which is mainly based on the valve core and seat, and then make the fluid noise. If the flow capacity value is too large, then it is necessary to reset the regulating valve and select the regulating valve that is suitable and consistent with the flow capacity value, so as to effectively reduce noise. The following methods for noise elimination are specifically introduced and analyzed :(1) resonant noise elimination method. Only when the valve resonates can the energy be superimposed, producing a noise of 100 decibels. There is a big difference in the performance, some vibration is strong, some vibration is weak, strong vibration noise is not big, weak vibration, noise is big; Some are loud and vibrating. This noise tends to produce a single tone sound, and when the resonance is removed, the noise is reduced. (2) elimination of cavitation noise method. As an important part of hydrodynamic noise, cavitation erosion has more significant influence on the intensity of noise. During cavitation, bubble rupture will generate high velocity impact force virtually, and then strong turbulence will be generated locally, and cavitation noise will be generated accordingly. The noise produces a rattling sound very similar to that of sand and gravel. Eliminating cavitation erosion can effectively reduce the noise; (3) adopt sound-absorbing material method. Attractive materials as a very common sound processing method, the treatment effect is very good. Noise can be effectively reduced by encircling the noise source with attractive materials. Because the noise will spread, it will attract the material around which part, will eliminate the noise of this part. This method is more used in the case of less noise, the main reason is that this method consumes a lot of money; (4) series silencer method. The series silencer method is more suitable for aerodynamic noise silencing, this method can be very good noise elimination. For the higher mass flow valve before and after the comparison, you can clearly consider the method 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 decibels.
2.4 valve leakage
After studying and analyzing the leakage of pneumatic valves, the following situations are known :(1) packing leakage. Due to the plastic deformation of the packing, it will be in full contact with the 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 highly permeable fluid medium, leakage will occur. In this regard, it is recommended to use flexible graphite as the filler, the fundamental reason is that this material has the characteristics of low friction, good air tightness, and maintenance is more convenient, to achieve the extension of valve life; (2) valve leakage. Because the length of the stem does not match the inside of the valve body, it is easy to have a gap between the seat and the spool, resulting in the leakage of the valve. At the same time, when the gas valve stem is short, this situation will occur. In order to solve this problem, the valve stem should be adjusted to the appropriate length. (3) seat, spool deformation leakage. The core factor in this situation is corrosion due to problems in the forging phase of production, and leakage due to the constant flushing of the valve by the fluid medium. In view of this situation processing stage, when the seat, spool deformation is light, can be achieved by sandpaper grinding to improve the sealing finish, to achieve the goal of improving the sealing performance of the valve. And seat, spool damage is more serious case, it is necessary to replace the accessories.
There are many reasons that lead to valve oscillation. For example, when the valve is in the working state, the change of flow velocity, pressure and flow resistance is very drastic under the condition of improper selection. At the same time, when the system frequency and valve frequency is the same, there will be oscillation. In general, due to the causes of valve oscillation are very many, it is necessary to develop targeted countermeasures according to the actual problems. For the valve vibration is relatively small, can be used to enhance the stiffness of the valve, such as the use of pistons to execute the 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 valve base, pipeline vibration is more intense, the most effective way is to achieve support to eliminate the increase in vibration.
2.6 valve positioner
The fault types of valve positioner mainly include the following four types: 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 gas source blocked, resulting in the valve positioner can not work normally; Second, because the valve positioner is the use of mechanical force balance principle, so it is easy to be affected by vibration, temperature, resulting in the failure of the valve positioner; Third, the spring elastic coefficient under the influence of external factors, there will be non-linear valve situation; Fourth, there is a big difference between the intelligent positioner and the ordinary positioner, so it is very easy to lose control of the electrical converter when the valve positioner is used in the case of emergency stop, the result is very dangerous. In this regard, in order to be able to solve the valve positioner fault timely as much as possible, we should develop targeted measures according to the specific situation, in order to play out the availability and reliability of the intelligent positioner.
In general, the role of pneumatic valves in the field of automatic process control 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 handling has a strong professional. In this regard, in addition to the need to master the corresponding theoretical expertise, but also need to constantly summarize the experience of fault treatment and induction, in order to achieve the improvement of pneumatic valve maintenance level. Under the background of reasonable troubleshooting, not only the system failure rate can be reduced, but also the valve utilization rate can be significantly improved, which is conducive to the smooth and safe operation of the system, in order to create higher benefits.