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Detailed explanation of installation, debugging, and troubleshooting of pneumatic control valves

2021-10-23 01:47:27
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Pneumatic control valves are one of the industrial process control instruments widely used in industrial enterprises such as petroleum, chemical, power, and metallurgy. In chemical production, the control valve is indispensable in the control system. It is an important part of the industrial automation system, such as the hand and foot of the production process automation. Next, we will take you to a comprehensive understanding of pneumatic control valves.

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Working principle:
Pneumatic control valve is a type of valve that uses compressed air as the power source, cylinder as the actuator, and uses electrical valve positioners, converters, solenoid valves, holding valves and other accessories to drive the valve, achieving on-off or proportional adjustment. It receives control signals from industrial automation control systems to adjust various process parameters such as flow, pressure, temperature, etc. of the pipeline medium.The characteristics of pneumatic control valves are simple control, fast response, and intrinsic safety, without the need for additional explosion-proof measures.

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Working principle of pneumatic control valve (diagram)

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Pneumatic regulating valves are usually composed of a pneumatic actuator and a regulating valve connected, installed, and debugged. Pneumatic actuators can be divided into single acting and double acting types. The single acting actuator has a return spring inside, while the double acting actuator does not have a return spring inside. The single acting actuator can automatically return to the initial open or closed state of the valve in case of loss of origin or sudden failure.

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Pneumatic control valves are divided into two types based on their action forms: air open and air close, which are called normally open and normally closed. The air open or air close of pneumatic control valves is usually achieved through the positive and negative effects of the actuator and different assembly methods of the valve structure.

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Action mode of pneumatic control valve
Air open type (normally closed type) refers to the action of the valve in the direction of increasing opening when the air pressure on the membrane head increases. When the input air pressure limit is reached, the valve is in a fully open state. Conversely, when the air pressure decreases, the valve moves towards the closed direction, and when there is no input air, the valve is fully closed. Usually, we refer to pneumatic control valves as fault closed valves.
The direction of action of the air close type (normally open type) is exactly opposite to that of the air open type. When the air pressure increases, the valve moves towards the closed direction; When the air pressure decreases or does not exist, the valve should be turned towards the open direction or fully open. We usually refer to an air closed regulating valve as a fault open valve.

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The selection of gas on/off is based on the safety perspective of process production. When the air supply 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, a regulating valve is installed on the fuel gas pipeline to control the supply of fuel based on the temperature of the furnace or the temperature of the heated material at the outlet of the heating furnace. At this point, it is safer to choose an air operated valve because once the air supply is stopped, it is more suitable for the valve to be closed than fully open. If the gas source is interrupted and the fuel valve is fully open, it can cause danger of excessive heating. Another example is a heat exchange equipment cooled by cooling water, where the hot material exchanges heat with the cooling water in the heat exchanger and is cooled. The regulating valve is installed on the cooling water pipe, and the temperature of the heat exchanged material is used to control the cooling water volume. When the gas source is interrupted, the regulating valve should be in the open position for safer operation, and an air closed (i.e. FO) regulating valve should be selected.

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Valve Positioner
The valve positioner is the main accessory of the regulating valve, which is greatly matched 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 a mechanical device, and the valve position status is transmitted to the upper system through an electrical signal. Valve positioners can be divided into pneumatic valve positioners, electro-pneumatic valve positioners, and intelligent valve positioners based on their structural form and working principle.
The valve positioner can increase the output power of the regulating valve, reduce the transmission delay 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 unbalanced force, thereby ensuring the correct positioning of the regulating valve.
There are two types of actuators: pneumatic actuator and electric actuator, with straight stroke and angular stroke. Used to automatically and manually open and close various types of cutting doors, wind panels, etc.
Installation principles of pneumatic control valves
(1) The installation position of the pneumatic control valve should be at a certain height from the ground, and there should be a certain space left above and below the valve for valve disassembly and repair. For regulating valves equipped with pneumatic valve positioners and handwheels, it is necessary to ensure easy operation, observation, and adjustment.
(2) The regulating valve should be installed on a horizontal pipeline, perpendicular to the pipeline from top to bottom, and generally supported under the valve to ensure stability and reliability. For special occasions where it is necessary to install the regulating valve horizontally on a vertical pipeline, the regulating valve should also be supported (except for small diameter regulating valves). When installing, it is necessary to avoid adding additional stress to the regulating valve.
(3) The working environment temperature of the regulating valve should be within (-30~+60), with a relative humidity of no more than 95% and 95%, and a relative humidity of no more than 95%.
(4) There should be a straight pipe section at the front and rear positions of the regulating valve, with a length not less than 10 times the pipe diameter (10D), to avoid the valve's straight pipe section being too short and affecting the flow characteristics.
(5) If the diameter of the regulating valve is different from the process pipeline, a reducing pipe should be used for connection. When installing small diameter regulating valves, threaded connections can be used. The flow direction arrow on the valve body should be consistent with the flow direction.
(6) To set up a bypass pipeline. The purpose is to facilitate switching or manual operation, and the regulating valve can be repaired without stopping.
(7) Before installing the regulating valve, it is necessary to thoroughly remove foreign objects such as dirt, welding slag, etc. from the pipeline.
Common Fault and Handling 1: The Regulating Valve Does Not Operate
Firstly, confirm whether the air supply pressure is normal and locate the air supply fault. If the air source pressure is normal, determine whether the amplifier of the locator or electric/pneumatic converter has output; If there is no output, the constant throttle hole of the amplifier is blocked, or water in the compressed air accumulates at the amplifier ball valve. Use small fine steel wire to clear the constant throttle hole, remove dirt or clean the air source.
If all of the above are normal and there is a signal but no action, then the actuator is faulty, the valve stem is bent, or the valve core is stuck. In this case, the valve must be opened for further inspection.
2. The regulating valve is blocked
If the reciprocating stroke of the valve stem is sluggish, there may be substances with high viscosity in the valve body, coking blockage or excessive pressure on the packing, aging of PTFE packing, bending and scratching of the valve stem, etc. The jamming faults of regulating valves mostly occur in newly put into operation systems and in the early stages of major maintenance and operation. Due to the blockage of welding slag, rust, etc. in the throttle and guide parts of the pipeline, the medium flow is not smooth, or the filler is too tight during the maintenance of regulating valves, resulting in increased friction, leading to the phenomenon of small signal not acting and large signal acting excessively.
In such situations, the secondary line or regulating valve can be quickly opened or closed to allow the stolen goods to be washed away by the medium from the secondary line or regulating valve. In addition, a pipe wrench can be used to clamp the valve stem, and under the condition of external signal pressure, the valve stem can be forcefully rotated in both directions to allow the valve core to flash past the clamp. If the problem cannot be solved, the problem can be solved by increasing the air supply pressure and driving power by repeatedly moving up and down several times.If it still cannot move, it is necessary to disassemble the control valve. Of course, this work requires strong professional skills and must be completed with the assistance of professional technical personnel, otherwise the consequences will be even more serious.
3 valve leakage
The leakage of regulating valves generally includes internal leakage of regulating valves, leakage of packing, and leakage caused by deformation of valve core and valve seat. The following will be analyzed separately.
(1) Valve leakage
The length of the valve stem is not suitable, and the valve stem of the air opening valve is too long. 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 cannot be fully contacted, leading to laxity and internal leakage.Similarly, if the valve stem of an air shut-off valve is too short, it can also lead to gaps between the valve core and valve seat, which cannot be fully contacted, resulting in incomplete closure and internal leakage. Solution: The valve stem of the regulating valve should be shortened (or extended) to make the length of the regulating valve suitable and prevent internal leakage.
(2) Packing leakage
After the packing is loaded into the stuffing box, axial pressure is applied to it through the gland. Due to the plastic deformation of the packing, it generates radial force and is in close contact with the valve stem. However, this contact is not very uniform, with some parts in loose contact, some parts in tight contact, and even some parts not in contact at all. During the use of the regulating valve, there is relative movement between the valve stem and the packing, which is called axial movement. During use, with the influence of high temperature, high pressure, and highly permeable fluid media, the control valve packing box is also a location where leakage occurs more frequently. The main reason for filler leakage is interface leakage, and for textile fillers, there may also be leakage (pressure medium leaks outward along the small gaps between the filler fibers). The interface leakage between the valve stem and the packing is caused by the gradual attenuation of the contact pressure of the packing, the aging of the packing itself, and other reasons. At this time, the pressure medium will leak outward along the contact gap between the packing and the valve stem.
In order to facilitate the installation of the packing, chamfer the top of the packing box and place a metal protective ring with a small erosion resistance gap at the bottom of the packing box. Note that the contact surface between the protective ring and the packing should not be inclined to prevent the packing from being pushed out by medium pressure. The surface of the contact part between the stuffing box and the packing should be finely machined to improve surface smoothness and reduce packing wear. Flexible graphite is selected as the filler because of its good airtightness, low friction, small changes in long-term use, low wear and burn, and easy maintenance. The friction force does not change after the gland bolts are re tightened, and the pressure and heat resistance are good. It is not corroded by internal media, and the metal in contact with the valve stem and packing box does not undergo pitting or corrosion. This effectively protects the sealing of the valve stem packing box, ensures the reliability of the packing seal, and greatly improves its service life.
(3) Deformation and leakage of valve core and valve seat
The main cause of valve core and valve seat leakage is the strengthening of corrosion caused by casting or forging defects during the production process of regulating valves. The passage of corrosive media and the flushing of fluid media can also cause leakage of the regulating valve. Corrosion mainly exists in the form of erosion or cavitation. When corrosive media passes through the regulating valve, it will cause erosion and impact on the valve core and valve seat materials, causing the valve core and valve seat to form an oval or other shape. Over time, it will result in mismatched valve core and valve seat, gaps, and leakage if not tightly closed.
Pay attention to the material selection of the valve core and valve seat.Choose corrosion-resistant materials and resolutely eliminate products with defects such as pitting and trachoma. If the deformation of the valve core and valve seat is not too severe, fine sandpaper can be used to grind, eliminate traces, improve the sealing smoothness, and improve the sealing performance. If the damage is severe, the valve should be replaced with a new one.
4 oscillations
The spring stiffness of the regulating valve is insufficient, and the unstable output signal of the regulating valve can easily cause oscillation. The frequency of the selected valve is the same as the system frequency, or the pipeline or base vibrates violently, causing the regulating valve to vibrate accordingly. Improper selection results in severe changes in flow resistance, flow rate, and pressure when the regulating valve operates at a small opening. When the stiffness of the valve is exceeded, the stability deteriorates, and in severe cases, oscillation occurs.
Due to the multiple reasons for oscillation, it is necessary to analyze the specific problems in detail. For those with slight vibration, stiffness can be increased to eliminate it, such as selecting a regulating valve with a high stiffness spring or using a piston actuator structure; Severe vibration of pipelines and bases can be eliminated by adding support to eliminate vibration interference; When the frequency of the valve is the same as that of the system, replace the regulating valve with a different structure; The oscillation caused by working at a small opening is caused by improper selection. Specifically, it is due to the large flow capacity C value of the valve, which must be re selected. Choose a valve with a smaller flow capacity C value, use split range control, or use a sub mother valve to overcome the oscillation caused by the regulating valve working at a small opening.
5. The regulating valve has high noise
When the fluid flows through the regulating valve, if the pressure difference between the front and rear is too large, cavitation phenomenon will occur on the valve core, valve seat, and other components, causing noise in the fluid.If the flow capacity value is selected too high, it is necessary to reselect a regulating valve with a suitable flow capacity value to overcome the noise caused by the regulating valve working at a small opening. Below are several methods to eliminate noise.
(1) Resonance noise elimination method
Only when the regulating valve resonates, there is a strong noise of over 100 decibels generated by energy superposition. Some exhibit strong vibration with minimal noise, while others exhibit weak vibration with very high noise;Some have significant vibrations and noise. This noise produces a single tone sound, with a frequency typically ranging from 3000 to 7000 Hz.Obviously, by eliminating resonance, the noise naturally disappears.
(2) Method of eliminating cavitation noise
Cavitation is the main source of hydrodynamic noise. During cavitation, the bubble ruptures and produces high-speed impacts, causing strong turbulence and cavitation noise locally. This type of noise has a wide frequency range and produces a grating sound, similar to the sound emitted by fluids containing sand and gravel. Eliminating and reducing cavitation is an effective way to eliminate and reduce noise.
(3) Using thick wall pipeline method
One of the methods for acoustic path treatment is to use thick walled pipes.The use of thin-walled pipes can increase noise by 5 decibels, while the use of thick walled pipes can reduce noise by 0-20 decibels. The thicker the wall of the same pipe diameter, the larger the pipe diameter of the same wall thickness, and the better the noise reduction effect. When the wall thicknesses of DN200 pipes are 6.25, 6.75, 8, 10, 12.5, 15, 18, 20, and 21.5mm, the noise can be reduced by -3.5, -2 (i.e. increased), 0, 3, 6, 8, 11, 13, and 14.5 decibels, 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 sound path processing method.Sound absorbing materials can be used to wrap the noise source and the pipeline behind the valve. It must be pointed out that since noise can propagate over long distances through fluid flow, the effectiveness of noise reduction ends wherever the sound-absorbing material is packaged and thick walled pipes are used. This method is suitable for situations where the noise is not very high and the pipeline is not very long, as it is a more expensive method.
(5) Series silencer method
Suitable for noise reduction as an aerodynamic noise source, it can effectively eliminate noise inside the fluid and suppress noise levels transmitted to the solid boundary layer. This method is most effective and economical for areas with high mass flow rate or high pressure drop ratio before and after the valve. The use of absorption type series silencers can significantly reduce noise. However, from an economic perspective, it is generally limited to attenuation to about 25 decibels.
(6) Soundproof box method
Use speakers, houses, and buildings to isolate noise sources inside, reducing the noise in the external environment to an acceptable range for people.
(7) Series throttling method
In situations where the pressure ratio of the regulating valve is high (△ P/P1 ≥ 0.8), the series throttling method is used to disperse the total pressure drop between the regulating valve and the fixed throttling element behind the valve. Using diffusers and multi hole flow limiting plates is the most effective way to reduce noise. In order to achieve better diffuser efficiency, it is necessary to design the diffuser (physical shape and size) based on the installation of each component, so that the noise level generated by the valve is the same as that generated by the diffuser.
(8) Choose a low noise valve
The low noise valve gradually decelerates according to the zigzag flow path (multi orifice, multi groove) of the fluid passing through the valve core and valve seat to avoid supersonic flow at any point in the flow path. There are various forms and structures of low noise valves (designed for specialized systems) available for use. When the noise is not very loud, choosing a low noise sleeve valve can reduce the noise by 10-20 decibels, which is the most economical low noise valve.
Valve positioner malfunction
The ordinary locator operates on the principle of mechanical force balance, namely nozzle baffle technology, and mainly has the following types of faults:
(1) Due to the use of mechanical force balance principle, there are many movable components that are easily affected by temperature and vibration, resulting in fluctuations in the regulating valve;
(2) Adopting nozzle baffle technology, due to the small nozzle hole, it is easy to be blocked by dust or unclean air source, which prevents the locator from working properly;
(3) Using the principle of force balance, the elastic coefficient of the spring can change in harsh environments, causing nonlinearity in the control valve and resulting in a decrease in control quality.
(4) The intelligent locator is composed of components such as a microprocessor (CPU), A/D, D/A converter, etc. Its working principle is completely different from ordinary locators. The comparison between the given value and the actual value is purely an electric signal, no longer a force balance. Therefore, it can overcome the disadvantage of force balance in conventional locators. But when used in emergency parking situations, such as emergency shut-off valves, emergency vent valves, etc., these valves are required to remain stationary in a certain position, and only when an emergency situation occurs, reliable action is required. Staying in a certain position for a long time can easily cause the electrical converter to lose control and cause a dangerous situation where small signals do not operate.In addition. The position sensing potentiometer used for valves is prone to changes in resistance values due to its on-site operation, resulting in dangerous situations where small signals do not operate and large signals are fully open. Therefore, in order to ensure the reliability and availability of intelligent locators, they must be frequently tested.

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