Principle, installation and maintenance knowledge of pneumatic control valve
What is pneumatic control valve?
Pneumatic control valve is one of the industrial process control instruments widely used in petroleum, chemical, electric power, metallurgy and other industrial enterprises. In chemical production, regulating valve is essential in regulating system. It is an important part of industrial automation system, such as automation of production process. Now let’s have a comprehensive understanding of pneumatic control valve.
Pneumatic control valve is to use compressed air as power source, cylinder as actuator, and with the aid of electric valve positioner, converter, solenoid valve, position maintaining valve and other accessories to drive the valve, realize switch or proportional regulation, receive the control signal of industrial automation control system to complete the regulation of various process parameters of pipeline medium: flow, pressure, temperature, etc. Pneumatic control valve is characterized by simple control, fast response, and intrinsic safety, without additional explosion-proof measures.
Working principle of pneumatic control valve (Figure)
Pneumatic control valve is usually composed of pneumatic actuator and regulating valve connection, installation and commissioning. Pneumatic actuator can be divided into single acting actuator and double acting actuator. There is a return spring in the single acting actuator, but there is no return spring in the double acting actuator. The single acting actuator can automatically return to the opening or closing state initially set by the valve in case of loss of origin or sudden failure.
According to the action form, pneumatic control valve can be divided into air open type and air close type, that is, the so-called normally open type and normally closed type. The air open or air close of pneumatic control valve is usually realized by the positive and negative action of actuator and different assembly methods of valve structure.
Action mode of pneumatic control valve
Air open type (normally closed type) is when the air pressure on the membrane head increases, the valve acts in the direction of increasing the opening, when the upper limit of the input air pressure is reached, the valve is in the fully open state. Conversely, when the air pressure decreases, the valve acts in the closing direction, and when there is no air input, the valve is fully closed. Usually, we call the air open type control valve a fault closed type valve.
The action direction of air close type (normally open type) is just opposite to that of air open type. When the air pressure increases, the valve acts in the closing direction; when the air pressure decreases or does not, the valve moves in the opening direction or fully open. Usually, we call the air closed regulating valve a fault open valve.
When the air supply is cut off, whether the regulating valve is in the closed position or the open position is safe.
For example, for the combustion control of a heating furnace, the regulating valve is installed on the fuel gas pipeline to control the fuel supply according to the temperature of the furnace or the temperature of the heated material at the outlet of the heating furnace. At this time, it is safer to select the air opening valve, because once the air supply stops, it is more appropriate for the valve to be closed than the valve to be fully open. If the air source is interrupted and the fuel valve is fully opened, it will cause danger of over heating. Another example is a heat exchange equipment cooled by cooling water. The hot material is cooled by heat exchange with cooling water in the heat exchanger. The regulating valve is installed on the cooling water pipe. The temperature of the material after heat exchange is used to control the cooling water volume. When the air source is interrupted, the regulating valve should be in the open position, which is safer. It is better to select the air closed (FO) regulating valve.
The valve positioner is the main accessory of the regulating valve, which is widely used with the pneumatic regulating valve. It receives the output signal of the regulator, and then controls the pneumatic regulating valve with its output signal. When the regulating valve acts, the displacement of the valve stem is fed back to the valve positioner through the mechanical device, and the valve position status is transmitted to the upper system through the electrical signal. According to its structure and working principle, valve positioner can be divided into pneumatic valve positioner, electro pneumatic valve positioner and intelligent valve positioner.
The valve positioner can increase the output power of the regulating valve, reduce the transmission lag of the regulating signal, accelerate the movement speed of the valve stem, improve the linearity of the valve, overcome the friction force of the valve stem and eliminate the influence of the unbalanced force, so as to ensure the correct positioning of the regulating valve.
The pneumatic actuator and the electric actuator are divided into direct stroke and angle stroke. It is used to open and close all kinds of cutting doors and air boards automatically and manually.
Installation principle of pneumatic control valve
- (1) The installation position of the pneumatic control valve shall be a certain height from the ground, and there shall be a certain space between the upper and lower parts of the valve, so as to facilitate the disassembly and repair of the valve. For the regulating valve equipped with pneumatic valve positioner and hand wheel, it must be easy to operate, observe and adjust.
- (2) The regulating valve shall be installed on the horizontal pipeline and vertical to the pipeline. Generally, it shall be supported under the valve to ensure stability and reliability. For special occasions, when the regulating valve needs to be installed horizontally on the vertical pipeline, the regulating valve shall also be supported (except for small caliber regulating valve). During installation, additional stress shall be avoided to the regulating valve).
- (3) The working environment temperature of the regulating valve shall not be more than 95% and 95% relative humidity at (- 30 ~ + 60).
- (4) There should be straight pipe section at the front and back of the regulating valve, with the length not less than 10 times of the pipe diameter (10d), so as to avoid the short straight pipe section of the valve affecting the flow characteristics.
- (5) If the diameter of the regulating valve is different from that of the process pipe, the reducing pipe shall be used for connection. When installing the small caliber regulating valve, it can be connected with thread. The flow direction arrow on the valve body shall be consistent with the flow direction.
- (6) To set the bypass pipe. The purpose is to facilitate switching or manual operation, and the control valve can be maintained without stopping.
- (7) Before the installation of the regulating valve, the foreign matters in the pipeline, such as dirt, welding slag, etc., shall be completely removed.
Common faults and handling
Control valve does not operate
First, confirm whether the air source pressure is normal and find out the air source fault. If the air source pressure is normal, judge whether the amplifier of the positioner or the electric / pneumatic converter has output; if there is no output, the constant orifice of the amplifier is blocked, or the water in the compressed air accumulates at the ball valve of the amplifier. Use small and fine steel wire to dredge the constant orifice, remove dirt or clean the air source.
If all the above are normal and there is signal but no action, the actuator fails or the valve stem is bent, or the valve core is stuck. In this case, the valve must be removed for further inspection.
Blocking of regulating valve
If the reciprocating stroke of the valve rod is slow, there may be substances with large viscosity in the valve body, coking and blocking, or the packing is too tight, or the PTFE packing is aging, the valve rod is bent and scratched, etc. The blocking fault of regulating valve mostly occurs in the newly put into operation system and in the early stage of overhaul and operation. Due to the blockage of welding slag and rust in the pipeline at the throttle and guide parts, the medium flow is not smooth, or the packing is too tight during the maintenance of regulating valve, resulting in the increase of friction, leading to the phenomenon that small signal does not act and large signal acts too much.
In such case, the auxiliary line or regulating valve can be opened and closed quickly to let the stolen goods be washed away by the medium from the auxiliary line or regulating valve. In addition, the valve stem can be clamped with pipe tongs. Under the condition of external signal pressure, rotate the valve stem with positive and negative force to make the valve core flash through the clamp. If the problem cannot be solved, the problem can be solved by increasing the air source pressure and increasing the driving power to move up and down repeatedly for several times. If it still can’t act, the control valve needs to be disassembled. Of course, this work needs strong professional skills and must be completed with the help of professional technicians, otherwise the consequences will be more serious.
Leakage of regulating valve
The leakage of control valve generally includes internal leakage of control valve, leakage of packing, and leakage caused by deformation of valve core and valve seat, which are analyzed respectively below.
The length of the valve rod is not suitable, the valve rod of the air opening valve is too long, and the upward (or downward) distance of the valve rod is not enough, resulting in a gap between the valve core and the valve seat, and insufficient contact, resulting in an internal leakage. Similarly, the valve stem of the air shutoff valve is too short, which may also cause the gap between the valve core and the valve seat, and the valve cannot be fully contacted, resulting in lax closing and internal leakage. Solution: shorten (or extend) the valve stem of the control valve to make the length of the control valve suitable, so that it no longer has internal leakage.
After the packing is installed into the stuffing box, axial pressure is exerted on it through the gland. Due to the plastic deformation of the packing, the radial force is generated and the packing is in close contact with the valve stem, but the contact is not very uniform. Some parts are loose, some parts are tight, and even some parts are not in contact at all. During the use of the regulating valve, there is a relative movement between the stem and the packing, which is called axial movement. In the process of use, with the influence of high temperature, high pressure and strong permeability of the fluid medium, the stuffing box of the regulating valve is also the place with more leakage. The main cause of packing leakage is interface leakage. For textile packing, there will be leakage (the pressure medium leaks out along the tiny gap between the packing fibers). The interface leakage between the valve stem and the packing is caused by the gradual decline of the contact pressure of the packing and the aging of the packing itself. At this time, the pressure medium will leak out along the contact gap between the packing and the valve stem.
In order to make the packing easy to install, chamfer the top of the stuffing box, and place a metal protection ring with a small erosion resistance gap at the bottom of the stuffing box. Note that the contact surface between the protection ring and the packing cannot be an inclined surface, so as to prevent the packing from being pushed out by the medium pressure. The surface of the contact part between the stuffing box and the packing should be finished to improve the surface finish and reduce the packing wear. Flexible graphite is selected as the packing because of its good air tightness, small friction, small change in long-term use, small wear and burn, easy maintenance, and the friction does not change after the gland bolt is re tightened, with good pressure resistance and heat resistance, free from the erosion of internal media, and the metal contacting with the valve rod and stuffing box does not have pitting or corrosion. In this way, the seal of the packing box of the valve rod is effectively protected, the reliability of the packing seal is guaranteed, and the service life is greatly improved.
Deformation and leakage of valve core and valve seat
The main reason for leakage of valve core and valve seat is that the casting or forging defects in the production process of control valve can lead to the strengthening of corrosion. The passage of corrosive medium and the flushing of fluid medium will also cause the leakage of regulating valve. Corrosion mainly exists in the form of erosion or cavitation. When the corrosive medium passes through the regulating valve, it will produce erosion and impact on the valve core and valve seat materials, making the valve core and valve seat oval or other shapes. As time goes on, the valve core and valve seat do not match, there is a gap, and the valve is not closed tightly, resulting in leakage.
The material selection of valve core and valve seat shall be well controlled. Select corrosion-resistant materials, and resolutely remove the products with pitting, trachoma and other defects. If the deformation of valve core and valve seat is not too serious, fine sandpaper can be used for grinding to eliminate traces and improve the sealing finish, so as to improve the sealing performance. If the damage is serious, replace the valve with a new one.
The spring stiffness of the control valve is insufficient, and the output signal of the control valve is unstable and changes rapidly, which is easy to cause the vibration of the control valve. In addition, the frequency of the selected valve is the same as that of the system or the pipeline and base vibrate violently, which makes the control valve vibrate accordingly. If the valve is not selected properly, there will be severe changes in flow resistance, flow rate and pressure when the valve is operated at a small opening. If the valve’s rigidity is exceeded, the stability will become poor, and it will oscillate when it is serious.
Because there are many reasons for the oscillation, it is necessary to analyze the specific problems. If the vibration is slight, it can be eliminated by increasing the rigidity, for example, selecting the regulating valve with large rigidity spring and using the piston executive structure; if the pipeline and base vibrate violently, the vibration interference can be eliminated by increasing the support; if the frequency of the valve is the same as that of the system, the regulating valve with different structure can be replaced; if the vibration is caused by the small opening, it is caused by improper selection, specifically, the valve If the C value of the flow capacity is too large, it is necessary to select a new type. If the C value of the flow capacity is small, or adopt the split range control or adopt the sub main valve to overcome the oscillation caused by the small opening of the control valve.
The regulating valve is noisy
When the fluid flows through the regulating valve, if the pressure difference between the front and the back is too large, cavitation will occur on the valve core, valve seat and other parts, making the fluid produce noise. When the flow capacity value is large, it is necessary to re select the regulating valve with appropriate flow capacity value, so as to overcome the noise caused by the operation of the regulating valve at a small opening. Here are some methods to eliminate the noise.
Eliminating resonance noise
Only when the control valve resonates can the energy be superposed to produce a strong noise of more than 100 decibels. Some of them are characterized by strong vibration and low noise, some of them are weak, but the noise is very large, and some of them are large. This noise produces a single tone sound with a frequency of 3000-7000 Hz. Obviously, eliminate resonance, and the noise will disappear naturally.
Method of eliminating cavitation noise
Cavitation is the main hydrodynamic noise source. During cavitation, high speed impact is produced by bubble rupture, which makes local strong turbulence and cavitation noise. This kind of noise has a wide frequency range, producing lattice sound, similar to the sound produced by sand in the fluid. Eliminating and reducing cavitation is an effective way to eliminate and reduce noise.
Use thick wall pipeline method
Using thick wall pipe is one of the ways to deal with the sound path. Using thin wall can increase the noise by 5 dB, and using thick wall tube can reduce the noise by 0 ~ 20 dB. The thicker the wall with the same pipe diameter, the larger the pipe diameter with the same wall thickness, the better the noise reduction effect. For example, when the wall thickness of DN200 pipe is 6.25, 6.75, 8, 10, 12.5, 15, 18, 20 and 21.5mm respectively, the noise can be reduced by – 3.5, – 2 (i.e., increased), 0, 3, 6, 8, 11, 13 and 14.5db respectively. Of course, the thicker the wall, the higher the cost.
Using sound-absorbing material method
It is also the most common and effective way to deal with the sound path. Sound absorbing materials can be used to cover the noise source and pipeline behind the valve. It must be pointed out that where sound-absorbing materials are packed and thick wall pipes are used, the effectiveness of noise elimination will be terminated, because noise will spread over a long distance through fluid flow. This method is suitable for the situation of low noise and short pipeline, because it is a more expensive method.
Series muffler method
It can effectively eliminate the noise inside the fluid and suppress the noise level transmitted to the solid boundary layer. This method is most effective and economical for places with high mass flow or high pressure drop ratio before and after the valve. The noise can be greatly reduced by using absorption type series muffler. However, in terms of economy, it is generally limited to attenuation to about 25 dB.
Soundproof box method
The use of loudspeakers, houses and buildings to isolate the noise source inside, so that the noise of the external environment can be reduced to an acceptable range.
When the pressure ratio of the regulating valve is high (△ P / P1 ≥ 0.8), the series throttling method is adopted, that is, the total pressure drop is distributed on the regulating valve and the fixed throttling element behind the valve. For example, the most effective way to reduce noise is to use a diffuser and a multi hole restriction plate. In order to get the best diffuser efficiency, the diffuser (solid shape and size) must be designed according to the installation of each piece, so that the noise level generated by the valve is the same as that generated by the diffuser.
Choose low noise valve
In order to avoid supersonic flow at any point in the flow path, the low-noise valve will slow down gradually according to the zigzag flow path (multi channel, multi channel) of the valve core and valve seat. There are various forms and structures of low-noise valve (designed for special system) for use. When the noise is not very large, low noise sleeve valve can reduce the noise by 10-20 dB, which is the most economical low noise valve.
Valve positioner failure
The common positioner adopts the mechanical force balance principle, i.e. nozzle baffle technology, which mainly has the following fault types:
- (1) Because the mechanical force balance principle is adopted, there are many movable parts, which are easily affected by temperature and vibration, resulting in the fluctuation of the control valve;
- (2) With the nozzle baffle technology, because the nozzle hole is very small, it is easy to be blocked by dust or unclean air source, so that the positioner can not work normally;
- (3) Using the balance principle of force, the spring’s elastic coefficient will change in the bad site, resulting in the nonlinear control of the control valve and the decline of the control quality.
- (4) Intelligent positioner is composed of microprocessor (CPU), a / D, D / a converter and other components. Its working principle is different from that of ordinary positioner. The comparison between the given value and the actual value is purely electric signal, not force balance. So it can overcome the disadvantage of force balance of conventional positioner. However, when it is used in emergency stop situations, such as emergency cut-off valve, emergency vent valve, etc., these valves are required to be stationary at a certain position. Only when an emergency occurs, they need to act reliably and stay at a certain position for a long time, which is easy to cause the electrical converter out of control and the small signal does not act. In addition. Because the position sensor potentiometer used for valve works on site, the resistance value is easy to change, resulting in the dangerous situation that small signal does not act and large signal is fully open. Therefore, in order to ensure the reliability and availability of intelligent positioners, they must be tested frequently.
Source: China Valve Manufacturer – Yaang Pipe Industry Co., Limited (www.metallicsteel.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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