What is a pneumatic control valve

What is a 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. The regulating valve in chemical production is essential in the regulating system, and it is an important part of the industrial automation system. Pneumatic control valve is to use compressed gas as power source, cylinder as actuator, and with the help of valve positioner, converter, solenoid valve, holding valve, gas tank, gas filter and other accessories to drive the valve, realize the switching value or proportional regulation, and receive the control signal of industrial automation control system to complete the regulation of pipeline medium: flow, pressure, temperature, pressure, etc Liquid level and other process parameters. Pneumatic control valve is characterized by simple control, fast response, and intrinsically safe, no need to take additional explosion-proof measures.

20210524221753 37536 - What is a pneumatic control valve

Classification of pneumatic control valve

Pneumatic control valve can be described as a wide range of types, a very wide range of uses, not only commonly used in the chemical and food industry, but also often seen in printing and dyeing, water treatment and other industries. However, with the continuous improvement of process flow and performance of various types of complete sets of equipment, the types of control valves are increasing, and there are a variety of classification methods. If you don’t know all the classification of pneumatic control valve, but you must know the structure classification.
(1) Pneumatic control valve is classified according to the form of pneumatic actuator

  • ① : membrane actuator. It is also divided into direct installation (positive action and reaction) and side installation (positive action and reaction).
  • ② : piston actuator. It is also divided into proportional formula (positive action and reaction) and binary formula.
  • ③ : long stroke actuator.
  • ④ : rolling film actuator.

(2) Pneumatic control valves are classified according to regulation forms

  • ① : adjustment type
  • ②: cut-off type
  • ③: adjustment cut-off type

(3) Regulating valves are classified according to their moving forms

  • ① : straight stroke
  • ②: angular stroke

(4) Pneumatic control valves are classified according to spool shape

  • ① : flat valve core
  • ②: plunger valve core
  • ③: window valve core
  • ④ : sleeve shaped valve core
  • ⑤: multistage shaped valve core
  • ⑥: deflection shaped valve core
  • ⑦ : butterfly valve core
  • ⑧: spherical valve core

(5) Classification by flow characteristics

  • ① : straight line
  • ②: equal percentage
  • ③: parabola

(6) Classification according to the type of upper valve cover

  • ① : common type
  • ② : diffuse (endothermic)
  • ③ : long neck type
  • ④ : bellows seal type

Working principle of pneumatic control valve

Pneumatic control valve is to use compressed air as the power source, cylinder as the actuator, and with the help of electrical valve positioner, converter, solenoid valve, holding valve and other accessories to drive the valve, realize the switch or proportional regulation, receive the control signal of industrial automation control system to complete the regulation of pipeline medium: flow, pressure, temperature and other process parameters. Pneumatic control valve is characterized by simple control, fast response, and intrinsically safe, no need to take additional explosion-proof measures.

Pneumatic control valve is usually composed of pneumatic actuator and control valve. Pneumatic actuator can be divided into single acting type and double acting type. There is a return spring in single acting actuator, but there is no return spring in double acting actuator. The single acting actuator can automatically return to the initial opening or closing state of the valve in case of loss of origin or sudden failure.

Pneumatic control valve is divided into two types according to the action form, namely the so-called normally open type and normally closed type. The pneumatic opening or closing 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

The air opening type (normally closed type) is that when the air pressure on the membrane head increases, the valve moves in the direction of increasing opening. When it reaches the upper limit of input air pressure, the valve is in full open state. On the other hand, when the air pressure decreases, the valve moves in the closing direction. When there is no air input, the valve is fully closed. Generally, we call the air opening control valve a fault closing valve.
The action direction of Qiguan type (normally open type) is just opposite to Qikai type. When the air pressure increases, the valve moves to the closing direction; When the air pressure decreases or does not exist, the valve shall be opened or fully opened. In general, we call the gas off control valve the fault open valve.
The selection of gas on / off is based on the safety of process production. When the air source is cut off, is the regulating valve in the closed position safe or the open position safe.
For example, in the combustion control of a heating furnace, the control 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 choose the gas open valve, because once the gas supply is stopped, it is more appropriate for the valve to be closed than for the valve to be fully open. If the gas source is interrupted and the fuel valve is fully opened, the excessive heating will be dangerous. Another example is a heat exchange equipment cooled by cooling water. The hot material is cooled by heat exchange with the 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 off (i.e. fo) regulating valve.

Valve positioner

The valve positioner is the main accessory of the control valve, which is greatly matched with the pneumatic control valve. It receives the output signal of the regulator, and then uses its output signal to control the pneumatic control valve. After the action of the control valve, 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 delay of the regulating signal, speed up 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 actuator is divided into pneumatic actuator and electric actuator, including straight stroke and angular 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 should be a certain height from the ground, and there should be a certain space above and below the valve to facilitate the disassembly and repair of the valve. For the control valve with pneumatic valve positioner and hand wheel, it must be easy to operate, observe and adjust.
  • (2) The control valve should be installed on the horizontal pipeline and vertical to the pipeline. Generally, it should be supported under the valve to ensure stability and reliability. For special occasions, when the control valve needs to be horizontally installed on the vertical pipeline, the control valve should also be supported (except for small diameter control valve). During installation, avoid additional stress on the control valve).
  • (3) The working environment temperature of the control valve should be (- 30 ~ + 60) and the relative humidity should not be greater than 95% and 95%.
  • (4) There should be a straight pipe section at the front and back of the control valve, and the length should not be 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 control valve is different from that of the process pipe, the reducing pipe shall be used for connection. When the small diameter control valve is installed, the screw connection can be used. The flow direction arrow on the valve body shall be consistent with the flow direction.
  • (6) The bypass pipe should be set. The purpose is to facilitate switching or manual operation, and the regulating valve can be overhauled without stopping.
  • (7) Before the installation of the control valve, the foreign matters in the piping system, such as dirt and welding slag, shall be thoroughly removed.

Common faults and treatment of pneumatic control valve

The control valve doesn’t work

First, confirm whether the air source pressure is normal and find out the air source fault. If the air pressure is normal, judge whether the amplifier of positioner or electro pneumatic converter has output; If there is no output, the constant orifice of the amplifier is blocked, or the moisture in the compressed air accumulates at the ball valve of the amplifier. Dredge the constant orifice with small fine steel wire to remove dirt or clean the air source.
If all of the above are normal and there is signal but no action, then the actuator is faulty or the valve stem is bent, or the valve core is stuck. In this case, the valve must be removed for further inspection.

Control valve blocked

If the reciprocating stroke of the valve stem is slow, there may be viscous substances in the valve body, coking and blocking, or the packing is pressed too tightly, or the PTFE packing is aging, and the valve stem is bent and scratched. The blocking fault of the control valve mostly occurs in the newly put into operation system and in the early stage of overhaul. Because of the blockage caused by welding slag and rust in the pipe at the throttle and guide position, the medium circulation is not smooth, or the packing is too tight in the maintenance of the control valve, resulting in the increase of friction, leading to the phenomenon of small signal not acting and large signal acting too much.
In this case, the auxiliary line or control valve can be opened or closed quickly, so that the stolen goods can be washed away by the medium from the auxiliary line or control valve. In addition, the valve stem can be clamped with pipe clamp, and the valve stem can be rotated with positive and negative force under the condition of additional signal pressure to make the valve core flash over the card. If the problem can not be solved, the problem can be solved by increasing the air source pressure, increasing the driving power and moving up and down repeatedly for several times. If it still can’t work, the control valve needs to be disassembled. Of course, this work needs strong professional skills and must be completed with the help of professional and technical personnel, otherwise the consequences will be more serious.

Valve leakage

The leakage of control valve generally includes the internal leakage of control valve, the leakage of packing and the leakage caused by the deformation of valve core and valve seat. The following are analyzed respectively.
1. Valve internal leakage
The length of the valve stem is not suitable, the valve stem of the air opening valve is too long, and the upward (or downward) distance of the valve stem is not enough, resulting in a gap between the valve core and the valve seat, which can not be fully contacted, resulting in loose and internal leakage. Similarly, the valve stem is too short, can also lead to a gap between the spool and seat, can not fully contact, resulting in tight and leakage. Solution: should shorten (or extend) the stem of the control valve, so that the length of the control valve is appropriate, so that it does not leak.
2. Packing leakage
After the packing is put into the stuffing box, the axial pressure is exerted on it through the gland. Because of the plastic deformation of the packing, it produces radial force and makes 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. Control valve in the process of use, there is a relative movement between the stem and packing, this movement 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 control valve is also the place where more leakage occurs. The main cause of packing leakage is interface leakage, and there will be leakage for textile packing (the pressure medium leaks along the tiny gap between the packing fibers). The interface leakage between the valve stem and the packing is caused by the gradual attenuation of the packing contact pressure and the aging of the packing itself. At this time, the pressure medium will leak outward along the contact gap between the packing and the valve stem.
In order to make the packing easy to install, chamfering at the top of the stuffing box, and placing a metal protection ring with small clearance at the bottom of the stuffing box, it should be noted that the contact surface between the protection ring and the packing should not be inclined, 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 stuffing should be finished to improve the surface finish and reduce the stuffing wear. The packing is made of flexible graphite because of its good air tightness, small friction, small change in long-term use, small abrasion and burning loss, easy maintenance, no change in friction after re tightening the gland bolt, good pressure resistance and heat resistance, no erosion of internal medium, and no pitting or corrosion of metal in contact with the valve stem and stuffing box. In this way, the sealing of the valve stem packing box is effectively protected, the reliability of the packing seal is ensured, and the service life is greatly improved.
3. Deformation and leakage of valve core and valve seat
The main reason for the 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. And the corrosive medium through, the erosion of the fluid medium will also cause the leakage of the control valve. Corrosion mainly exists in the form of erosion or cavitation. When the corrosive medium passes through the control 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. With the passage of time, the valve core and valve seat do not match, there is a gap, and leakage occurs due to lax closing.
The material selection of valve core and valve seat should be well controlled. Select corrosion-resistant materials, and resolutely eliminate 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. In case of serious damage, replace the valve with a new one.

Oscillation

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 oscillation 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. Improper selection of the control valve results in drastic changes of flow resistance, flow velocity and pressure when it works in small opening. When it exceeds the valve stiffness, the stability becomes poor, and oscillation occurs when it is serious.
There are many reasons for the oscillation, which should be analyzed in detail. If the vibration is slight, it can be eliminated by increasing the stiffness, such as selecting the control valve with large stiffness spring and using the piston actuator structure; If the pipeline and base vibrate violently, the vibration interference can be eliminated by adding support; When the frequency of the valve is the same as that of the system, replace the control valve with different structure; The oscillation caused by working in small opening is caused by improper selection. Specifically, it is because the valve’s flow capacity C value is too large, so it must be re selected. The valve with small flow capacity C value should be selected, or split range control or sub master valve should be adopted to overcome the oscillation caused by working in small opening.

The control valve is noisy

When the fluid flows through the control valve, if the pressure difference between the front and back is too large, cavitation will occur for the valve core, valve seat and other parts, which will make the fluid produce noise. When the flow capacity value is larger, the control valve with appropriate flow capacity value must be re selected to overcome the noise caused by the small opening of the control valve. Here are several methods to eliminate the noise.
1. Method of eliminating resonance noise
Only when the control valve resonates, there will be a strong noise of more than 100 dB due to the superposition of energy. Some have strong vibration and little noise, while others have weak vibration and very big noise; Some have high vibration and noise. This noise produces a single tone sound with a frequency of 3000-7000 Hz. Obviously, if the resonance is eliminated, the noise will disappear naturally.
2. Method of eliminating cavitation noise
Cavitation is the main source of hydrodynamic noise. When cavitating, the bubble ruptures and produces high-speed impact, which produces strong turbulence locally and cavitation noise. This kind of noise has a wide frequency range and produces rattle sound, which is similar to the sound produced by sand in fluid. Eliminating and reducing cavitation is an effective way to eliminate and reduce noise.
3. Using thick wall pipeline method
Using thick wall tube is one of the ways to deal with sound path. The noise can be increased by 5 dB by using thin wall tube and reduced by 0-20 dB by using thick wall tube. The thicker the wall of the same pipe diameter is, the larger the pipe diameter of the same wall thickness is, the better the noise reduction effect is. For example, when the wall thickness of DN200 pipe is 6.25, 6.75, 8, 10, 12.5, 15, 18, 20 and 21.5mm, the noise can be reduced to – 3.5, – 2 (i.e. increase), 0, 3, 6, 8, 11, 13 and 14.5 DB respectively. Of course, the thicker the wall, the higher the cost.
4. Using sound-absorbing material method
This is also a common and most effective way to deal with the sound path. Sound absorbing material can be used to cover the noise source and the pipeline behind the valve. It must be pointed out that since the noise will spread over a long distance through fluid flow, the effectiveness of noise elimination will end where the sound-absorbing material is wrapped and where the thick wall tube is used. This method is suitable for the situation that the noise is not very high and the pipeline is not very long, because it is a more expensive method.
5. 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 the most effective and economical for the places with high mass flow rate or high pressure drop ratio before and after the valve. The noise can be greatly reduced by using absorption type series muffler. However, from the economic point of view, the attenuation is generally limited to about 25 dB.
6. Sound insulation box method
Use sound isolation box, house and building to isolate the noise source inside, so that the noise of the external environment can be reduced to the acceptable range.
7. Series throttle method
When the pressure ratio of the control valve is high (△ P / P 1 ≥ 0.8), the series throttling method is used to disperse the total pressure drop on the control valve and the fixed throttling element behind the valve. Such as the use of diffusers, multi hole limiter, which is the most effective way to reduce noise. In order to get the best efficiency of the diffuser, it is necessary to design the diffuser (the shape and size of the entity) according to the installation of each piece, so that the noise level produced by the valve is the same as that produced by the diffuser.
8. Choose low noise valve
According to the gradual deceleration of the fluid through the valve core and valve seat zigzag flow path (multi channel, multi channel), the low noise valve can avoid supersonic flow at any point in the flow path. There are various forms and structures of low noise valve (specially designed for the system) for use. When the noise is not great, low noise sleeve valve can reduce the noise by 10 ~ 20 dB, which is the most economical low noise valve.

Failure of valve positioner

The common positioner works on the principle of mechanical force balance, i.e. nozzle baffle technology
(1) Because of the mechanical force balance principle, there are many movable parts, which are easily affected by temperature and vibration;
(2) Using 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 principle of force balance, the elastic coefficient of the spring will change in the bad site, resulting in the nonlinearity 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 ordinary positioner. The comparison between the given value and the actual value is purely electric signal, not force balance. Therefore, it can overcome the disadvantage of force balance of conventional positioner. However, when it is used for emergency shutdown, such as emergency shut-off valve and emergency vent valve, these valves are required to be stationary at a certain position. Only when an emergency occurs, they need to act reliably. If they stay at a certain position for a long time, it is easy to make the electrical converter out of control, resulting in the dangerous situation of small signal not acting. In addition. Because the position sensing potentiometer used for valve works in the field, 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.

Application and characteristics of pneumatic control valve

A. The purpose and characteristics of pneumatic control valve is a kind of right angle rotary structure, which can be used with valve positioner to realize proportional adjustment; V-type valve core is most suitable for all kinds of adjustment occasions, with large rated flow coefficient, large adjustable ratio, good sealing effect, sensitive adjustment performance, small volume, and can be installed vertically and horizontally. Suitable for controlling gas, steam, liquid and other media.
B. Features: it is a right angle rotary structure, which is composed of V-shaped valve body, pneumatic actuator, positioner and other accessories; There is an approximate constant flow rate characteristic; Adopting double bearing structure, it has small starting torque, good sensitivity and induction speed; Super shearing ability.
C. The pneumatic piston actuator uses compressed air as the power source to drive the crank arm to rotate 90 degrees through the movement of the piston, so as to make the valve open and close automatically. It consists of adjusting bolt, actuator box, crank arm, cylinder block, cylinder shaft, piston, connecting rod and cardan shaft.
D. Working principle of pneumatic control valve: pneumatic control valve is composed of actuator and regulating mechanism. The actuator is the thrust component of the control valve, which generates the corresponding thrust according to the control signal pressure to promote the action of the control mechanism. The valve body is the regulating part of the pneumatic regulating valve, which directly contacts with the regulating medium to regulate the flow of the fluid.

Installation principle of pneumatic control valve

  • (1) The installation position of the pneumatic control valve should be a certain height from the ground, and there should be a certain space above and below the valve to facilitate the disassembly and repair of the valve. For the control valve with pneumatic valve positioner and hand wheel, it must be easy to operate, observe and adjust.
  • (2) The control valve should be installed on the horizontal pipeline and vertical to the pipeline. Generally, it should be supported under the valve to ensure stability and reliability. For special occasions, when the control valve needs to be horizontally installed on the vertical pipeline, the control valve should also be supported (except for small diameter control valve). During installation, avoid additional stress on the control valve).
  • (3) The working environment temperature of the control valve should be between – 30 and + 60, and the relative humidity should not be more than 95%.
  • (4) There should be a straight pipe section at the front and back of the control valve, and the length should not be 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 control valve is different from that of the process pipe, the reducing pipe shall be used for connection. When the small diameter control valve is installed, the screw connection can be used. The flow direction arrow on the valve body shall be consistent with the flow direction.
  • (6) The bypass pipe should be set. The purpose is to facilitate switching or manual operation, and the regulating valve can be overhauled without stopping.
  • (7) Before the installation of the control valve, the foreign matters in the pipeline, such as dirt and welding slag, shall be thoroughly removed.

Matters needing attention in the use of pneumatic control valve

1. The valve should be stored in a dry room, and both ends of the passage must be blocked. No stacking
2. The long-term storage control valve should be checked regularly to remove the dirt, and the moving parts and processing surface should be coated with anti rust oil to prevent rust.
3. The valve should be installed on the horizontal pipeline, and should be installed vertically. Stem up.
4. Install according to the medium flow direction indicated by the arrow in the figure.

Basic connection dimensions of pneumatic control valve

Nominal diameter DN(mm)
25
32
40
50
65
80
100
125
150
200
250
300
400
Rated flow coefficient Kv
Single seat
8
12
20
32
50
80
120
200
280
450
700
1100
Two seater
10
16
25
40
63
100
160
250
400
630
1000
1600
2700

How to choose the right pneumatic control valve

In the process of industrial production, there are many kinds of control valve body, including straight single seat, straight double seat, angle, diaphragm, small flow, tee, eccentric rotation, butterfly, sleeve, spherical and so on. Before selecting the valve, carefully analyze the medium, process conditions and parameters of the control process, collect enough data, understand the requirements of the system for the control valve, and determine the type of valve to be used according to the collected data. Especially in the control link of pneumatic system, a good control valve is needed. In the process of selection, what functional issues should we pay attention to? Here is a brief introduction.
1. Regulation function

  • ① The control valve should be stable;
  • ② Small opening has good regulation performance;
  • ③ Select the required flow characteristics;
  • ④ Meet the adjustable ratio;
  • ⑤ Small resistance and large flow ratio (the ratio of rated flow parameters and nominal diameter of control valve);
  • ⑥ Adjust the speed.

2. Leakage and cut off pressure difference
These are two inseparable and interrelated factors. The leakage should meet the process requirements, and the reliability of the sealing surface should be protected; The cut-off pressure difference (the pressure difference when the valve is closed) must be put forward (unfortunately, this parameter is not included in the calculation specification of regulating valve of many design institutes), so that the selected regulating valve has enough output force to overcome it, otherwise it will lead to the selection of larger or smaller actuator.
3. Anti blocking
Even if it is a clean medium, there is also a problem of blockage, which is that the unclean things in the pipeline are brought into the control valve by the medium, causing blockage. This is a common fault, so the anti blocking performance of the control valve should be considered. Generally, the anti blocking performance of the angle stroke control valve is much better than that of the straight stroke control valve, so the angle stroke control valve will be used more and more in the future.
4. Corrosion resistance
It includes erosion resistance, cavitation resistance and corrosion resistance. It mainly involves the selection of materials and the service life of the control valve, at the same time, it also involves the economic problems. The essence of this problem should be that the selected valve has good corrosion resistance and reasonable price. If you can choose all PTFE valve, you should not choose all corrosion resistant alloy control valve; If the angle high-pressure control valve with good anti cavitation effect and simple structure can be selected (to meet the service life of about two years), other high-pressure control valves with complex structure and expensive price should not be selected.
5. Withstand voltage and temperature
This involves the selection of the nominal pressure and working temperature of the control valve. In terms of pressure resistance, if the pressure is high, it is not difficult, mainly because the pressure difference will produce cavitation; In terms of temperature resistance, it is usually very easy to solve the problem below 450 ℃, and it is not difficult to solve the problem between 450 ℃ and 600 ℃, but when it is above 600 ℃, the contradiction will be prominent; When the temperature is 80 ℃, soft sealing material is usually not recommended for shut-off control valve, so hard sealing shut-off should be considered.
6. Weight and appearance
This problem is very intuitive, must be good appearance, light weight control valve is welcomed by the manufacturers. Here to change a prejudice that the control valve is a “big big thick”, heavy or poor appearance is not great. Now we attach great importance to it, so that the control valve should have the characteristics of miniaturization, lightweight, instrumentation.

Source: China Control Valve Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.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.)

If you want to have more information about the article or you want to share your opinion with us, contact us at sales@steeljrv.com

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