Common troubleshooting methods for pneumatic control valves
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2014-07-08
Common troubleshooting methods for pneumatic control valves
For reference only, please refer to the actual usage situation and issues.
1) External scouring method When using ordinary valves to regulate media that are prone to sedimentation and contain solid particles, they often block the throttle and guide ports, and can be externally connected with flushing gas and steam at the bottom plug of the lower valve cover. When the valve becomes blocked or stuck, opening the external gas or steam valve can complete the flushing work without moving the regulating valve, ensuring the normal operation of the valve.
2) Installation of pipeline filter method For small caliber control valves, especially ultra small flow control valves, their throttling clearance is particularly small, and there should be no small residue in the medium. In case of blockage, install a filter on the pipeline in front of the valve to ensure smooth passage of the medium. The regulating valve used with a locator is not working properly, and the blockage of its gas path throttling port is a common fault. Therefore, when working with a locator, it is necessary to handle the air source properly. The usual method is to install an air filter pressure reducing valve on the air source pipeline in front of the locator.
3) Increase throttle clearance method If solid particles in the medium or welding slag and rust that have been washed away in the pipeline cannot pass through the throttling port, causing blockages, blockages, and other faults, they can be replaced with throttling components with large throttling gaps - valve cores and sleeves with window or opening areas. Due to their concentrated throttling area rather than circular distribution, the fault can be easily eliminated. If it is a single or double seat valve, the plunger shaped valve core can be changed to a "V" shaped valve core, or it can be changed to a sleeve valve, etc. For example, a double seat valve in a certain chemical plant often gets stuck. It is recommended to use a sleeve valve instead, and the problem is immediately resolved.
4) Medium erosion method Utilize the scouring energy of the medium itself to flush and remove easily precipitated and clogged substances, thereby improving the anti clogging function of the valve. Common methods include: ① switching to a closed flow type; ② Adopting a streamlined valve body; ③ Place the throttle port at a location with severe erosion. When using this method, attention should be paid to improving the erosion resistance of the throttle material.
5) Change direct to angular method The direct flow is an inverted S flow, with complex flow paths and multiple dead zones in the upper and lower chambers, providing a place for the precipitation of the medium. Angular connection, the medium flows like a 90 elbow, with good scouring performance, small dead zone, and easy to design as a streamline. Therefore, when using a straight-through regulating valve that causes slight blockage, it can be changed to an angle valve for use at ℃.
Solutions for poor sealing performance (5 methods) 1) Grinding method Fine grinding can eliminate traces, reduce or eliminate sealing gaps, improve the smoothness of sealing surfaces, and improve sealing performance.
2) Using Unbalanced Force to Increase Sealing Pressure Method The sealing pressure generated by the actuator on the valve core is constant. When the unbalanced force causes a top opening trend on the valve core, the sealing force of the valve core decreases with the two forces. On the contrary, there is a pressure closing trend on the valve core, and the sealing force of the valve core is the sum of the two forces. This greatly increases the sealing pressure, and the sealing effect can be improved by 5-10 times or more than the former. Generally, single seal valves with dg ≥ 20 are the former case, usually in the flow open type, If the sealing effect is considered unsatisfactory, changing to a flow closed type will double the sealing performance. Especially for two-position shut-off regulating valves, they should generally be used according to the flow closed type.
3) Method of improving the sealing force of the actuator Improving the sealing force of the actuator on the valve core is also a common method to ensure valve closure, increase sealing pressure, and improve sealing performance. Common methods include: ① Moving the working range of the spring; ② Switch to a small stiffness spring; ③ Add accessories, such as with locators; ④ Increase the pressure of the air source; ⑤ Switch to an actuator with greater thrust.
4) Adopting single sealing and soft sealing methods For regulating valves with dual seals, a single seal can be used, which usually improves the sealing effect by more than 10 times. If the imbalance force is large, corresponding measures should be taken. For hard sealed valves, a soft seal can be used, which can also improve the sealing effect by more than 10 times.
Switch to a valve with good sealing performance In case of necessity, it is possible to consider switching to valves with better sealing performance. For example, ordinary butterfly valves can be switched to elliptical butterfly valves, and then shut-off butterfly valves, eccentric rotary valves, ball valves, and specially designed shut-off valves can also be used.
Solution to leakage of regulating valve (6 methods) 1) Add sealing grease method For valves that do not use sealing grease, adding sealing grease can be considered to improve the sealing performance of the valve stem.
Adding filler method To improve the sealing performance of the packing on the valve stem, the method of adding packing can be adopted. Usually, double or multi-layer mixed fillers are used, and simply increasing the quantity, such as increasing 3 to 5 pieces, does not have a significant effect.
3) Replacing graphite packing method The widely used tetrafluoro filler, due to its working temperature ranging from -20 to+200, has a significant decrease in sealing performance, rapid aging, and short lifespan when the temperature changes significantly between the upper and lower limits. Flexible graphite fillers can overcome these shortcomings and have a long service life. Therefore, some factories have completely changed the PTFE packing to graphite packing, and even newly purchased control valves have replaced the PTFE packing with graphite packing for use. However, the use of graphite fillers has a large return difference, and some may even experience crawling at the beginning, which must be taken into account.
4) Change the flow direction and place P2 at the valve stem end When Δ P is large and P1 is also large, sealing P1 is obviously more difficult than sealing P2. Therefore, the method of changing the flow direction can be adopted, changing P1 from P2 at the valve stem end to P2 at the valve stem end. This is more effective for valves with high pressure and large pressure difference. For example, for corrugated pipe valves, sealing should usually be considered
5) The lens gasket sealing method is used for the sealing of the upper and lower covers, as well as the sealing of the valve seat and the upper and lower valve bodies. If it is a flat seal, the sealing performance is poor under high temperature and pressure, causing leakage. Lens gasket sealing can be used instead, achieving satisfactory results.
Replace the sealing gasket Until now, most sealing gaskets still use asbestos sheets, which have poor sealing performance and short lifespan at high temperatures, leading to leakage. In this situation, spiral wound gaskets, "O" rings, etc. can be used instead, and many factories have now adopted them.
Solution for regulating valve vibration:
Increasing stiffness method
For oscillations and slight vibrations, increasing stiffness can be used to eliminate or weaken them, such as selecting springs with high stiffness or switching to piston actuators.
2) The method of increasing damping is to increase the friction against vibration. For example, the valve plug of a sleeve valve can be sealed with an "O" ring, and graphite packing with high friction can be used. This can still have a certain effect on eliminating or weakening slight vibrations.
3) Increase guide size and reduce fit gap method
Axial plug valves generally have smaller guide dimensions, and all valves have larger fit clearances, ranging from 0.4 to 1mm, which is helpful for generating mechanical vibration. Therefore, when slight mechanical vibration occurs, the vibration can be weakened by increasing the guide size and reducing the fit clearance.
Change the shape of the throttling element and eliminate resonance method
Because the so-called vibration source of the regulating valve occurs at the throttle port with high-speed flow and rapid pressure changes, changing the shape of the throttle element can change the frequency of the vibration source, which is easier to solve when the resonance is not strong. The specific method is to turn the valve core surface within the range of vibration opening by 0.5~1.0mm. For example, a self operated pressure regulating valve was installed near the family area of a certain factory, and the resonance generated a howling sound that affected workers' rest. After we dropped the valve core curved surface by 0.5mm, the resonance howling sound disappeared.
5) Replacing throttle components to eliminate resonance method! The principle is the same as 4) in 4.5, except for replacing the throttling component. The methods include: ① Change the flow characteristics, logarithmic to linear, linear to logarithmic; ② Replace the valve core form. If the shaft plug shape is changed to a "V" groove valve core, and the double seat valve shaft plug type is changed to a sleeve type; Change the sleeve with a window to a sleeve with a small hole, etc. For example, in a nitrogen fertilizer plant, a DN25 double seat valve often vibrates and breaks at the connection between the valve stem and the valve core. After confirming resonance, we changed the linear characteristic valve core to a logarithmic valve core, and the problem was solved. For example, in a laboratory of a certain aviation college, a DN200 sleeve valve was used, and the valve plug caused strong rotation and could not be put into use. After changing the sleeve with a window to a sleeve with a small hole, the rotation immediately disappeared.
6) Replace the type of regulating valve to eliminate resonance Different structural forms of regulating valves naturally have different natural frequencies, and changing the type of regulating valve is an effective method to fundamentally eliminate resonance. A valve that resonates extremely strongly during use - it vibrates strongly (can damage the valve in severe cases), rotates strongly (even the valve stem is broken or twisted), and produces strong noise (up to over 100 decibels). As long as it is replaced with a valve with a significant structural difference, it immediately takes effect, and the strong resonance miraculously disappears. For example, a DN200 sleeve valve is selected for the new expansion project of a Vinylon plant. All the above three phenomena exist. The DN300 pipeline jumps with it, the valve plug rotates, the noise is more than 100 decibels, and the resonance opening is 20~70%. Considering the large resonance opening, the resonance disappears after using a double seat valve, and the operation is normal.
7) Reducing cavitation vibration method For the cavitation vibration caused by the rupture of cavitation bubbles, it is natural to find ways to reduce cavitation The impact energy generated by bursting bubbles does not act on the solid surface, especially on the valve core, but is absorbed by the liquid. Sleeve valves have this characteristic, so the plug type valve core can be changed to a sleeve type Take all measures to reduce cavitation, such as increasing throttling resistance, increasing the pressure at the constriction port, grading or series pressure reduction, etc.
8) Avoiding vibration source wave strike method The vibration of the valve caused by external vibration sources is clearly something that should be avoided during normal operation of the regulating valve. If this vibration occurs, corresponding measures should be taken.
Solutions to High Noise in Regulating Valves (8 Methods) Resonance noise elimination method Only when the regulating valve resonates, there is energy superposition that produces strong noise of over 100 decibels. Some exhibit strong vibration with little noise, while others exhibit weak vibration with very high noise; Some vibrations and noises are relatively large. This noise produces a single tone sound with a frequency of generally 3000 to 7000 Hz. Obviously, by eliminating resonance, the noise naturally disappears. The methods and examples are shown in 4), 5), and 6) in 4.5 above.
2) Method of eliminating cavitation noise
Cavitation is the main source of Fluid power noise. During cavitation, the bubble rupture will produce a high-speed impact, which will produce strong turbulence locally and cavitation noise. 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 effective method for acoustic path processing. 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 This method is applicable to the silencing of aerodynamic noise. It can effectively eliminate the noise inside the fluid and suppress the noise level transmitted to the solid Boundary layer. This method is 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.
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 0 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 the best 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 an economical low noise valve.
Methods to improve lifespan (8 methods) 1) Extended service life method with large opening Let the regulating valve work as close to the opening as possible from the beginning, such as 90%. In this way, damage such as cavitation and erosion occurs on the valve core head. As the valve core is damaged and the flow rate increases, the corresponding valve is closed a little more. This continues to damage and gradually closes, making the entire valve core fully utilized until the root and sealing surface of the valve core are damaged and cannot be used. At the same time, the large opening work has a large throttle clearance and weakened erosion, which increases the service life of the valve by more than 1-5 times compared to working at the middle and small opening from the beginning. If a certain chemical plant adopts this method, the service life of the valve has been increased by two times.
Reducing S and increasing working opening to improve service life method
Reducing S means increasing the loss of the system except for the regulating valve, reducing the pressure drop distributed to the valve. To ensure that the flow passes through the regulating valve, the opening of the regulating valve must be increased. At the same time, the pressure drop on the valve decreases, weakening cavitation and erosion. The specific methods include: setting an orifice plate behind the valve to throttle and consume pressure drop; Close the manual valves connected in series on the pipeline until the regulating valve achieves an ideal working opening. This method is very simple, convenient, and effective when selecting a large valve and working with a small opening at the beginning.
3) Reducing the aperture, increasing the working opening, and improving the service life method By reducing the diameter of the valve to increase the working opening, the specific methods are as follows: ① Change to a valve with a smaller diameter, such as changing from DN32 to DN25; ② The valve body remains unchanged, replace the valve core and valve seat with a small valve seat diameter. During a major overhaul of a chemical plant, the throttling component dgl0 was replaced with dg8, which doubled the service life.
4) Method of Transferring Failure Location to Improve Life Transfer the severely damaged area to a secondary location to protect the sealing and throttling surfaces of the valve core and seat. Increasing Throttling Channel and Improving Life Method
The simple way to increase the throttling channel is to thicken the valve seat, causing the valve seat hole to grow, forming a longer throttling channel. On the one hand, it can cause the sudden expansion and delay of the flow closed throttle, which plays a role in transferring the damaged position and keeping it away from the sealing surface; On the other hand, it also increases throttling resistance, reduces the degree of pressure recovery, and weakens cavitation. Some design the valve seat hole as a stepped or wavy shape to increase resistance and weaken cavitation. This method is often used on high-pressure valves in imported devices and when improving old valves, and is also very effective.
6) Changing the flow direction to increase lifespan method The flow opening type flows towards the open direction, and cavitation and erosion mainly act on the sealing surface, causing damage to the valve core root and valve seat sealing surface quickly; The flow closed type flows towards the closed direction, and cavitation and erosion act below the sealing surface of the valve seat after throttling, protecting the sealing surface and the root of the valve core, and extending its service life. When the problem of extending the service life of a valve used in a flow open type is prominent, simply changing the flow direction can extend the service life by 1-2 times.
7) Using special materials to increase lifespan method To resist cavitation (such as honeycomb shaped small points) and erosion (streamlined small grooves), special materials that resist cavitation and erosion can be used to manufacture throttling components. This special material includes 6YC-1, A4 steel, Stellite, hard alloy, etc. To resist corrosion, materials that are more resistant to corrosion and have certain mechanical and physical properties can be used instead. This material is divided into two categories: non-metallic materials (such as rubber, PTFE, ceramics, etc.) and metallic materials (such as Monel, Hastelloy, etc.).
8) The method of changing valve structure to improve service life is to adopt the method of changing valve structure or selecting valves with longer service life to achieve the goal of improving service life, such as selecting multi-stage valves, anti cavitation valves, corrosion resistant valves, etc.
