March 6, 2025
Pneumatic valve is a kind of valve with the help of compressed air as the driving force and then realize the opening and closing, can realize the control of different types of fluids such as gas, liquid, steam, etc., has the advantages of simple structure, fast response speed, high control accuracy, safety and reliability, etc., the following will be for you to talk about the pneumatic valve fault location: FC, FO, FL, FLC, FLO, AFL/EFC and AFL/EFO
First, the type and working principle of pneumatic valve
Pneumatic valve is mainly composed of three major components: pneumatic actuator, valve body, accessories. The actuator contains spring, diaphragm / piston, pneumatic rod, handwheel, etc., mainly clean compressed air as a driving force, accept 20 ~ 100kpa gas signal or 4 ~ 20mA electrical signal, and then drive the valve body for action, the spool and the valve seat to adjust the flow area to achieve the purpose of regulating the flow.
The valve body is mainly composed of valve flap, valve cage, valve stem, valve seat, valve cage pressure ring and other components; accessories mainly contain pressure reducing valve, solenoid valve, positioner, current/voltage converter, filters, flow amplifiers and so on.
The working principle of the pneumatic valve is through compressed air to promote the actuator pneumatic piston or gears and other mechanical parts of the movement to achieve the opening and closing of the valve, can be specifically subdivided into: air supply - control signal input - positioner movement - the valve opens and closes several steps. According to the requirements of the required rotating torque of the valve, the design adjusts the number of combinations of cylinders, and by changing the position of the inlet and outlet air, the direction of rotation of the spindle can be changed.
Pneumatic valve mainly has the following advantages: First, the pneumatic valve action is rapid, can be completed in a relatively short period of time to regulate the command; Second, the pneumatic valve can cooperate with large cylinders to achieve a larger torque of the thrust; Third, the pneumatic valve can be in all kinds of harsh conditions for a long time in a safe and stable operating state.
Pneumatic valve according to the role of the form, generally can be divided into gas open, gas shut type. The choice of gas open and gas shut-off is based on the safety of process production to consider the point of view.
When the gas source cut off, the valve is in the closed position or open position safety.
Air to Open type (Air to Open) is when the air pressure on the diaphragm head increases, the valve acts in the direction of increasing the opening, and when it reaches the upper limit of the input air pressure, the valve is in the fully open state. Conversely, when the air pressure decreases, the valve moves in the direction of closing, and when there is no air input, the valve is fully closed. Therefore, sometimes the air-open type valve is also known as the failure to close type (Fail to Close FC).
Air off type (Air to Close) action direction is exactly the opposite of air open type. When the air pressure increases, the valve to close the direction of action; air pressure decreases or no air pressure, the valve to open the direction or full open until. Therefore, sometimes also known as failure to open type (Fail to Open FO).
Double-action type. Valve opening and closing action all driven by the air source / compressed air, the basic principle of action and single-acting the same, just double-acting pneumatic valve actuator does not have a spring, in principle and structure is relatively more complex.
Then in the process of use, there are usually a few fault location for (FO, FC, FL), in the valve fault off/fault open in the fault refers to: the valve action when the gas source failure.
For the pneumatic valve failure position, mainly divided into several situations:
1, pneumatic valve device interlocking action, the valve position should be the following cases
FC - gas source loss, the valve is in the closed position
FO - the loss of gas supply, the valve is in the open position
FL - loss of air supply, the valve is in the position of the moment and has been maintained
FLC - loss of gas supply, the valve holds position but tends to close, the valve is in the closed position (the gas in the cylinder is consumed)
FLO - loss of gas supply, the valve holds position but tends to open, the valve is in the open position (gas consumption in the cylinder is complete)
2, regulating valves or switching valves involved in the device interlocking action, the valve position should be the following cases
FC - loss of gas supply or solenoid valve loss of power, the valve is in the closed position
FO--loss of gas supply or solenoid valve power failure, the valve is in the open position.
AFL / EFC - 1. gas source loss of solenoid valve is not de-energized, the valve to keep the position; 2. regardless of whether the loss of gas source solenoid valve de-energized, the valve is in the closed position
AFL/EFO - 1. gas source loss of solenoid valve is not de-energized, the valve is held in position; 2. regardless of whether the gas source is lost solenoid valve de-energized, the valve is in the open position
Pneumatic valve through the output signal to realize the valve cut off, connect, regulate and other functions, its opening and closing speed is relatively fast, commonly used in fast two cut off the use, but also can be used to regulate the flow use, with different accessories, you can not reach a variety of different control methods.
Second, the common faults of pneumatic valve
1 Pneumatic valve leakage increase failure and causes
Pneumatic valve leakage depends mainly on the valve switch. Summarized, the increase in leakage of pneumatic valves is mainly due to the following two factors: one is the spool of the pneumatic valve due to the use of a long time and wear and tear, resulting in the valve is not closed tightly, thus causing an increase in the amount of leakage; the second is if the valve is mixed with foreign objects or the internal liner is sintered, or in the control of the pressure of various media between the role of the medium when the media pressure difference is large, it will lead to Rigidity decreases, causing the valve can not be completely closed, and ultimately cause the pneumatic valve leakage increases.
2 pneumatic valve action unstable faults and causes
Whether it is signal pressure instability or gas source pressure instability may cause the pneumatic valve action instability. The signal pressure instability will cause the regulator output instability, and when the gas source pressure instability, due to the small capacity of the compressor will lead to the failure of the pressure reducing valve. It is also possible that the pneumatic valve action is unstable caused by the gap between each other when the position of the amplifier spray group baffle plate in the positioner is not parallel. In addition, the output pipe or the output line is not tight will also cause the pneumatic valve action is not stable; amplifier ball valve due to friction with a foreign body to form a gap, will also have an impact on the stability of the pneumatic valve.
3 pneumatic valve vibration faults and causes
Pneumatic valve in the working process is easily affected by the surrounding environmental factors. Liner and spool in the work for a long time, under the action of friction, the two will form a gap, pneumatic valve around the existence of another vibration of mechanical equipment, pneumatic valve installation position imbalance will lead to pneumatic valve vibration. In addition, when the size of the pneumatic valve is not properly selected or the closing direction of the single-seat valve and the direction of the media flow is not consistent with the same will also cause the pneumatic valve vibration.
4 pneumatic valve slow action faults and reasons
In the pneumatic valve movement process, the importance of the valve stem is indisputable. When the stem bends, it will increase the friction generated by its round-trip action, causing pneumatic valve sluggish action. When the graphite and asbestos packing lubricant, PTFE packing is not normal will also cause pneumatic valve sluggish action, when there is dust inside the valve body, pneumatic valve installed inside the positioner, etc. will increase the pneumatic valve stem running resistance, thus causing pneumatic valve sluggish action.
5 pneumatic valve does not move and the reasons
Air source and signal as a prerequisite to ensure the smooth operation of the pneumatic valve and the basis. If the positioner is no air source, the pressure reducing valve may fail, and in serious cases, may also lead to filters and pipeline clogging; if the positioner has an air source, but no output, it will block the positioner's throttle holes; if the air source and the signal is not available, when the temperature is low, the air source is not open, it will be due to the decompression valve and the air duct clogging and malfunctioning, which ultimately leads to compressor failure. If there is an air source but no signal, it will damage the regulating mesh diaphragm and cause the bellows of the positioner to leak.
Third, the common faults of pneumatic valve processing methods
1 Pneumatic valve leakage increased processing methods
For the pneumatic valve leakage increase this fault, if the valve spool is worn, need to immediately replace the new spool, reduce the leakage caused by internal leakage increased; if it is because the valve is mixed with foreign matter, need to apply cleaning items in time to clean and wash, will be excluded from the foreign matter; if it is the medium pressure difference is large, need to improve the pneumatic valve actuator, increase the air source, reduce leakage. In addition to the installation of pneumatic valves, should also ensure that the selection of the valve stem length is moderate, in order to prevent the valve is not completely closed caused by the leakage.
2 pneumatic valve action unstable fault handling methods
For the signal pressure instability caused by the action of the pneumatic valve instability, need to ensure that the power network system is running stable; for the gas source pressure instability caused by the action of the pneumatic valve instability, the positioner needs to be re-adjusted, and if necessary, can also be replaced with a new positioner, so as to ensure the stability of the gas source pressure. Can also reinstall the valve stem or increase the lubricant to reduce the friction of the valve stem contact parts, reduce the instability of the pneumatic valve action, but also to the positioner out of the pipe position of the precision to be adjusted to eliminate the failure of the pneumatic valve instability.
3 pneumatic valve vibration fault processing methods
For the liner and spool friction caused by the pneumatic valve vibration, need to immediately replace the liner; for the pneumatic valve around the existence of other vibration mechanical equipment caused by the pneumatic valve vibration, need to carefully find out the source of vibration and eliminate the vibration; for the pneumatic valve seat is not stable vibration, need to replace the new seat; for the single-seat valve shut-off direction and the direction of the flow of the media caused by the vibration of the direction of the flow of the media does not coincide with the need to analyze and judge the pneumatic valve, and the pneumatic valve is not stable, need to be replaced. Analysis and judgment, the correct installation direction of the pneumatic valve to be adjusted.
4 pneumatic valve slow action fault handling methods
Pneumatic valve slow action is mainly related to diaphragm damage, so it should be replaced in a timely manner with a new diaphragm; and carefully check whether the graphite and asbestos packing lubricant and PTFE packing is normal, if necessary, will be replaced, to ensure that they are in a normal state; valve body in a timely manner inclusions in the foreign body cleaned up to ensure that the cleanliness of the valve body; the valve stem to be dealt with to reduce the friction between the valve stem and the surrounding components to solve the problem of slow action of pneumatic valves. Pneumatic valve action slow failure.
5 pneumatic valve does not act fault processing methods
For the existence of a gas source but the pneumatic valve does not act on the fault, you need to check the command line one by one, timely troubleshooting. For the pneumatic valve positioner no input and display, you need to replace the new positioner in time; for the spool and stem serious deformation, you need to replace the new spool and stem in time to ensure that the handwheel is placed in the appropriate position.
