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Key inspection areas for control valves in case of failure: 1. The inner wall of the valve body. For control valves used in applications with high pressure differences and corrosive media, the inner wall is frequently subjected to the impact and corrosion caused by these media; therefore, it is essential to carefully check their resistance to pressure and corrosion. 2. Valve seat: During operation of control valves, as the medium seeps in, the internal surface of the threads used to secure the valve seat is prone to corrosion, which can cause the valve seat to loosen; this should be taken into consideration during inspections. For valves operating under high pressure differences, it is also necessary to check whether the sealing surface of the valve seat has been damaged. 3. Valve core: The valve core is the moving part of a control valve during operation, and it is subject to the most severe erosion and corrosion caused by the medium. During maintenance, it is necessary to carefully check whether all parts of the valve core have been corroded or worn out; especially under high pressure differences, wear on the valve core is more severe, and this issue needs to be taken into account due to cavitation. The valve core should be replaced when it is severely damaged. It should also be noted whether a similar phenomenon occurs with the valve stem, or if there is looseness in its connection to the valve core. 4. Are the \"O\"-ring seals and other gaskets aged or damaged? 5. Attention should be paid to the PTFE filler, whether the sealing lubricant has aged, and whether the mating surfaces are damaged; it should be replaced when necessary. I. Methods to increase lifespan 01: Extending lifespan by operating at full opening – Have the control valve operate at its maximum opening from the start, such as 90%. In this way, damages such as cavitation and erosion occur on the head of the valve core. As the valve core is damaged, the flow rate increases, causing the valve to close a little more; this process of continuous damage and gradual closing ensures that the entire valve core is utilized to the fullest extent, until the root of the valve core and the sealing surface are damaged and the valve can no longer be used. At the same time, when operating at a large opening, the throttle gap is larger, reducing erosion; this increases the lifespan by more than 1 to 5 times compared to operating the valve at intermediate or small openings from the start. If a chemical plant adopts this method, the service life of the valves is doubled. 02 Method of reducing S, increasing the operating opening, and extending lifespan: By reducing S, the losses in the system other than the control valve increase, which leads to a lower pressure drop across the valve. To ensure that flow passes through the control valve, its opening must be increased. At the same time, the reduced pressure drop across the valve also decreases cavitation and erosion. Specific methods include: using a orifice plate behind the valve to create pressure drop through throttling ； Close the manual valve connected in series on the pipeline until the control valve reaches an optimal operating position. For when the valve is initially selected to operate at a small opening degree, this method is very simple, convenient, and effective. 03 Method of reducing diameter, increasing operating opening, and extending lifespan: The operating opening is increased by reducing the diameter of the valve. The specific methods include: replacing the valve with one of a smaller diameter, such as changing from DN32 to DN25 ； The valve body remains unchanged; only the valve core and seat with a smaller diameter are replaced. During a major overhaul of a chemical plant, replacing the throttle element dgl0 with dg8 doubled its service life. 04 Method of improving lifespan by shifting the location of damage: Moving the areas most severely damaged to less critical positions in order to protect the sealing surfaces and throttling surfaces of the valve core and seat. 05 Method for increasing the length of the throttling passage to prolong service life. The simplest way to lengthen the throttling passage is to thicken the valve seat, thereby increasing the length of the valve seat hole and creating a longer throttling path. On the one hand, it can delay the sudden expansion that occurs after throttling in flow-blocking mode, thereby shifting the location of failure and keeping it away from the sealing surface ； On the other hand, it increases the throttling resistance, reduces the degree of pressure recovery, and thus attenuates cavitation. Some design the valve seat hole in a stepped or wavy pattern in order to increase resistance and reduce cavitation. This method is often used on the high-pressure valves in new installations and when upgrading old valves, and it is also very effective. 06 Method of changing the flow direction to extend lifespan: The fluid flows in the open direction; cavitation and erosion mainly affect the sealing surfaces, causing rapid damage to the root of the valve core as well as the sealing surfaces of the valve core and the valve seat ； In the flow-blocking type, flow moves in the direction of closure; cavitation and erosion occur after throttling, below the valve seat sealing surface, which protects the sealing surface and the root of the valve element, thereby extending its service life. For valves designed to be used in a flow-diverting manner, when extending their lifespan is a significant concern, simply changing the direction of flow can double or triple their service life. 07 Using special materials to improve durability: To resist cavitation (damage in the form of honeycomb-like spots) and erosion (streamlined small grooves), special materials resistant to cavitation and erosion can be used to manufacture the throttle components. Such special materials include 6YC-1, A4 steel, Stellite, and cemented carbide. To resist corrosion, materials that are more resistant to corrosion and possess 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.). 08 Methods to Improve Lifespan by Changing Valve Structure: The goal of increasing lifespan is achieved by altering the valve structure or by using valves with a longer service life, such as multi-stage valves, anti-cavitation valves, and corrosion-resistant valves. 09 Method of reducing stroke to extend diaphragm lifespan: For two-position control valves, when the operating frequency is extremely high, the diaphragm tends to rupture quickly due to repeated up-and-down folding; the point of failure is usually along the circumference of the diaphragm holder. The simplest and most effective way to increase diaphragm life is to reduce the stroke. The reduced stroke value is 1/4dg. For valves such as the dgl25, the standard stroke is 60 mm, but it can be reduced to 30 mm, a reduction of 50%. In addition, the following factor can also be considered: minimizing the pressure in the membrane chamber while meeting the conditions for opening and closing ； Improve the smoothness at the joint where the pallet meets the diaphragm. II. Methods to prevent clogging of control valves 01 Cleaning method: Weld slag, rust, debris, etc. in the pipelines can cause blockages or sticking at the throttle openings, guide areas, and balance holes on the lower valve cover. This can result in scratches and abrasions on the surface of the valve core and guide surfaces, as well as indentations on the sealing surfaces. This often occurs in newly commissioned systems and in the early stages after major repairs. This is the most common fault. In such cases, it is necessary to disassemble it for cleaning in order to remove debris; if the sealing surface is damaged, it should also be ground ； At the same time, open the bottom plug to flush out the debris that has fallen from the balance hole into the lower valve cover, and to clean the pipelines. Before putting it into operation, open the control valve fully; allow the medium to flow for a while before proceeding with normal operation. 02 External flushing method: When using ordinary valves to regulate media that tend to precipitate and contain solid particles, blockages often occur at the throttling ports and guide areas; in such cases, flushing gas and steam can be supplied at the bottom plug of the lower valve cover. When the valve becomes clogged or stuck, opening the external gas or steam valve allows for flushing to be carried out without moving the control valve, thereby restoring normal operation of the valve. 03 Installation of pipe filters: For small-diameter control valves, especially those with extremely low flow rates, the throttling gap is very small; therefore, there must not be any debris in the fluid. In the event of such blockage, it is best to install a filter on the pipeline before the valve to ensure smooth flow of the medium. In control valves used with positioners, when the positioner fails to function properly, a blocked throttle orifice in its pneumatic circuit is the most common fault. Therefore, when operating with a positioner, the air supply must be properly handled. The common practice is to install an air filter-regulator on the air supply line before the positioner. 04 Increasing the throttle gap: When solid particles in the medium, or weld slag and rust that have been washed away in the pipes, cause blockages or jams due to their inability to pass through the throttle opening, it is possible to use a throttle element with a larger throttle gap – namely, a valve core or sleeve with window-like or open-shaped throttle areas. Since the throttle area is concentrated rather than distributed around the circumference, such problems can be easily resolved. For single- or double-seat valves, the plunger-type valve core can be replaced with a “V”-shaped valve core, or changed to a sleeve valve, etc. For example, a chemical plant had a two-seat valve that would get stuck frequently; after it was recommended to replace it with a sleeve valve, the problem was resolved immediately. 05 Medium flushing method: Utilizes the flushing energy of the medium itself to flush away and remove substances prone to sedimentation and clogging, thereby enhancing the valve’s anti-clogging performance. Common methods include: repurposing it for use as a flow-closure type ； Use a streamlined valve body ； Place the throttle orifice at the location where erosion is most severe. When using this method, care should be taken to enhance the erosion resistance of the throttle material. 06 Changing from straight-through to corner flow method: The straight-through configuration results in an inverted S-shaped flow pattern, with a complex flow path and numerous dead zones in the upper and lower chambers, which provide areas for the deposition of the medium. Angular connections allow the medium to flow as if through a 90° elbow; they offer good flushing performance, minimal dead zones, and can be easily designed to be streamlined. Therefore, when a slight blockage occurs with a straight-through control valve, it can be replaced with an angle valve. III. Solutions to leakage in control valves 01 Adding sealing grease: For valves that do not currently use sealing grease, it is possible to add such grease to improve the sealing performance of the valve stem. 02 Method of adding packing: To improve the sealing performance of the packing against the valve stem, the method of adding more packing can be employed. Typically, a double-layer or multi-layer mixed packing arrangement is used; however, simply increasing the quantity—for instance, from 3 pieces to 5 pieces—does not yield significant results. 03 Method of replacing with graphite packing: The widely used PTFE packing, with an operating temperature range of –20 to +200°C, experiences a significant decline in its sealing performance when the temperature varies greatly between these limits; it ages rapidly and has a short lifespan. Flexible graphite fillers can overcome these disadvantages and have a long service life. As a result, some factories have replaced all PTFE packing with graphite packing, and even newly purchased control valves have their PTFE packing replaced with graphite packing before use. However, the hysteresis with graphite fillers is large, and some systems exhibit crawling behavior at first; this must be taken into consideration. 04 Change the flow direction and place P2 at the end of the valve stem. When △P is large and P1 is also high, sealing P1 is clearly more difficult than sealing P2. Therefore, the method of changing the flow direction can be adopted, by moving P1 to the valve stem end and P2 to the other end; this is particularly effective for valves with high pressure and large pressure differences. For example, with bellows valves, sealing P2 should generally be taken into consideration. 05 Use the lens gasket sealing method for the sealing of the upper and lower covers, as well as for the sealing between the valve seat and the upper and lower valve bodies. For flat sealing, under high temperature and pressure, the sealing performance is poor, leading to leakage. In such cases, switching to a lens gasket seal can yield satisfactory results. 06 Replacing gaskets: To this day, most gaskets are still made of asbestos sheets; under high temperatures, they have poor sealing performance and a short lifespan, which leads to leaks. In such cases, wound gaskets or \"O\"-rings can be used as alternatives; many factories are already adopting them. 07 Symmetrical bolt tightening using thin washer sealing: In control valve structures with O-ring sealing, when thick gaskets with high deformability (such as wound gaskets) are used, uneven compression and uneven stress distribution can easily lead to damage, misalignment, and deformation of the seal, severely affecting its sealing performance. Therefore, when repairing or assembling such valves, the compression bolts must be tightened symmetrically (be careful not to tighten them all at once). It would be better if thick gaskets could be replaced with thin ones, as this makes it easier to reduce the inclination and ensure a good seal. 08 Methods for increasing the width of the sealing surface in order to prevent the valve disc from jumping during closure and reduce leakage. In flat-type valve discs (such as those used in two-position valves or sleeve valves), there are no guiding or supporting surfaces within the valve seat. As the valve operates, lateral forces act on the valve disc, pushing it from the inlet side toward the outlet side. The greater the clearance between the valve disc and the valve seat, the more severe this unilateral movement becomes. Additionally, deformation, misalignment, or a small chamfer on the sealing surface of the valve disc (usually a 30° chamfer is used for guidance) can cause the chamfered end of the valve disc’s sealing surface to come into contact with the valve seat’s sealing surface when the valve is near closure. This results in the valve disc jumping during closure, or even failing to close properly, thereby increasing leakage. The simplest and most effective solution is to increase the size of the sealing surface on the valve stem, so that the minimum diameter of the end face of the valve stem is 1–5 mm smaller than the diameter of the valve seat; this provides sufficient guidance to ensure that the valve stem fits into the valve seat and maintains good contact between the sealing surfaces. IV. Solutions to valve vibration (8 methods) 01 Increasing stiffness: For oscillations and mild vibrations, increasing the stiffness can be used to eliminate or reduce them; methods such as using springs with higher stiffness or switching to piston actuators are feasible. 02 Increasing damping: Increasing damping means increasing the friction against vibrations; for example, the plug of a sleeve valve can be sealed using \"O\"-rings, or graphite packing with high friction can be employed. This can help to eliminate or reduce minor vibrations to some extent. 03 Increasing the guiding dimensions and reducing the clearance: Plug-type valves generally have small guiding dimensions, and the clearance between the various components of such valves is usually large, ranging from 0.4 to 1 mm; this condition contributes to the occurrence of mechanical vibrations. Therefore, when mild mechanical vibrations occur, the vibrations can be reduced by increasing the guiding dimensions and decreasing the fit clearance. 04 Changing the shape of the throttle element to eliminate resonance: Since the so-called source of vibration in control valves lies at the throttle opening where flow is rapid and pressure changes sharply, changing the shape of the throttle element can alter the frequency of this vibration source, making it easier to resolve the issue when resonance is not severe. The specific method is to turn the valve core surface by 0.5–1.0 mm within the vibration opening range. In a factory’s residential area, a self-acting pressure control valve was installed; resonance caused humming noise that disturbed the workers’ rest. After grinding 0.5 mm off the surface of the valve core, the resonant humming noise disappeared. 05 Method of eliminating resonance by replacing the throttling element: Methods include changing the flow characteristic, from logarithmic to linear, and from linear to logarithmic ； Change the type of valve core. If the plug type is changed to a “V”-groove valve core, and the double-seat valve plug type is changed to a cartridge type ； Replace the sleeve that opens a window with a sleeve that has holes, etc. In a nitrogen fertilizer plant, a DN25 two-seat valve experienced frequent breakage at the connection between the valve stem and the valve disc. After identifying resonance as the cause, we replaced the linear-characteristic valve disc with a logarithmic one, and the problem was resolved. Another example: in a laboratory at an aviation academy, a DN200 sleeve valve was used, but the valve plug rotated violently and could not be put into use. After changing the sleeve with openings to one with small holes, the rotation stopped immediately. 06 Changing the type of control valve to eliminate resonance: Control valves with different structural designs have inherent frequencies that vary naturally; changing the type of control valve is the most effective way to eliminate resonance at its root. A valve that experiences severe vibration – intense shaking (which can damage the valve in severe cases), strong rotation (to the point where the valve stem may break or twist), and produces loud noise (reaching over 100 decibels) – will see immediate improvement once it is replaced with a valve that has a significantly different structure; the intense resonance disappears miraculously. In the new expansion project of a vinylon factory, a DN200 sleeve valve was selected, and all three of the aforementioned phenomena occurred: the DN300 pipeline vibrated, the valve plug rotated, and the noise level exceeded 100 decibels. The resonance occurred at an opening degree of 20–70%. Given the high degree of resonance, a double-seat valve was used instead, and as a result, the resonance disappeared and the valve operated normally. 07 Method of reducing cavitation vibration: For the cavitation vibration caused by the collapse of cavitation bubbles, it is natural to seek ways to reduce cavitation. The impact energy generated by the bursting of bubbles is not allowed to act on the solid surface, especially on the valve core, but is instead absorbed by the liquid. Sleeve valves possess this feature, which allows the plug-type valve element to be replaced with a sleeve-type one. All methods to reduce cavitation should be employed, such as increasing throttling resistance, raising the pressure at the constriction, and using staged or series pressure reduction. 08 Avoid vibration caused by external shock waves: External shock waves can cause vibration in valves, and this is something that must be avoided during the normal operation of control valves. If such vibration occurs, appropriate measures should be taken. V. Solutions to High Noise Levels in Control Valves 01 Method of Eliminating Resonant Noise: Strong noise levels of over 100 decibels occur only when the control valve resonates, as energy builds up in that condition. Some exhibit strong vibration with low noise, while others have weak vibration but very high noise levels ； Some have relatively high vibration and noise levels. This noise produces a single-toned sound, with a frequency generally ranging from 3000 to 7000 Hz. Obviously, by eliminating resonance, the noise disappears as well. 02 Methods to eliminate cavitation noise: Cavitation is a major source of hydrodynamic noise. During cavitation, the bursting of bubbles creates high-speed shock waves, which induce intense local turbulence and result in cavitation noise. This noise has a relatively wide frequency range and produces a rattling sound, similar to the sound produced by sand and gravel in a fluid. Eliminating and reducing cavitation is an effective way to eliminate and reduce noise. 03 Using the thick-walled pipe method: Employing thick-walled pipes is one of the methods for acoustic path treatment. Using thin walls can increase noise by 5 decibels, while using thick-walled pipes can reduce noise by 0–20 decibels. The thicker the wall for a given diameter, and the larger the diameter for a given wall thickness, the better the noise reduction effect. For a DN200 pipe, when the wall thicknesses are 6.25, 6.75, 8, 10, 12.5, 15, 18, 20, and 21.5 mm respectively, the noise reduction can be -3.5, -2 (i.e., an increase), 0, 3, 6, 8, 11, 13, and 14.5 decibels respectively. Of course, the thicker the wall, the higher the cost. 04 Using sound-absorbing materials: This is also a common and highly effective method for handling sound paths. The noise source and the piping after the valve can be wrapped with sound-absorbing materials. It must be noted that since noise can travel over long distances through fluid flow, the effectiveness of noise elimination ends at the point where the sound-absorbing material is applied or where thick-walled pipes are used. This method is suitable for situations where the noise level isn’t very high and the pipelines aren’t very long, as it is a relatively costly approach. 05 Series-connected silencer method: This method is suitable for reducing aerodynamic noise; it can effectively eliminate noise within the fluid and suppress the noise level transmitted to the solid boundary layer. For locations with high mass flow rates or a high ratio of pressure drop before and after the valve, this method is the most effective and economical. Using absorptive series silencers can significantly reduce noise. However, from an economic standpoint, it is generally limited to a attenuation of about 25 decibels. 06 Soundproof enclosure method: Use soundproof enclosures, houses, and buildings to isolate the noise sources within them, thereby reducing external noise to a level that is acceptable to people. 07 Series throttling method: In cases where the pressure ratio across the control valve is high (△P/P1 ≥ 0.8), the series throttling method is employed; this involves distributing the total pressure drop between the control valve and a fixed throttling element located downstream of it. Using diffusers or porous flow-restricting plates is the most effective method among those for reducing noise. To achieve optimal diffuser efficiency, it is necessary to design the diffuser (its shape and size) based on the installation conditions of each unit, so that the noise level generated by the valve is the same as that generated by the diffuser. 08 Use low-noise valves. These valves work by allowing the fluid to slow down gradually as it passes through the complex flow paths within the valve core and seat (multiple channels and grooves), thereby preventing supersonic speeds from occurring at any point in those flow paths. There are various forms and structures of low-noise valves (some designed for specific systems) available for use. When the noise level is not very high, using a low-noise sleeve valve can reduce noise by 10–20 decibels; it is the most cost-effective low-noise valve available. VI. Solutions when the stability of the control valve is poor 01 Method of changing the direction of the unbalanced force: In stability analysis, it is known that when the unbalanced force acts in the same direction as the valve’s closing direction, that is, when it exerts a tendency to close the valve, the stability of the valve is poor. When the valve operates under the aforementioned unbalanced force conditions, the method of changing the direction of this force is employed; usually, this involves switching from a flow-blocking type to a flow-allowing type, and this generally allows the stability issue of the valve to be resolved easily. 02 Method to avoid operating in the valve’s own unstable zone: Some valves, due to limitations in their structure, exhibit poor stability when operating at certain opening degrees. For two-seat valves, when the opening degree is within 10%, flow occurs at the upper ball while it is blocked at the lower ball, resulting in instability ； Nearwhere the slope of the unbalanced force variation becomes alternating, its stability is poor. For example, in the case of butterfly valves, the alternating point is around 70 degrees ； The two-seat valve is at 80–90% opening. For such valves, operating in the unstable region inevitably results in poor stability; therefore, one should avoid operating in that region. 03 Replace with a valve of good stability. A valve with good stability exhibits minimal changes in unbalanced forces and provides good guidance. Among the commonly used ball valves, sleeve valves possess this significant feature. When single- and double-seat valves have poor stability, replacing them with a set of cartridge valves will definitely improve stability. 04 Increasing spring stiffness method: The ability of the actuator to resist the effects of load changes on its stroke depends on the spring stiffness; the greater the stiffness, the less impact there is on the stroke, and the better the stability of the valve. Increasing the spring stiffness is a common and simple method to improve valve stability; for example, replacing springs with a stiffness range of 20–100 KPa with springs having a higher stiffness of 60–180 KPa. This approach is mainly used for valves equipped with positioners; otherwise, a positioner must be installed separately on the valve. 05 Method of reducing response speed: When the system requires that the response or adjustment speed of the control valve not be too fast, yet the valve’s response and adjustment speed are actually high. For example, when fine adjustments to flow rate are needed, but the flow control capability of the valve results in large changes in flow rate; or when the system itself is already a fast-response system but the control valve is equipped with a positioner to further accelerate its operation, these situations are unfavorable. This will result in overshoot and vibrations, etc. To address this, the response speed should be reduced. One method is to change the linear characteristic to a logarithmic characteristic ； Those with positioners can be replaced by converters or actuators. VII. Handling of other malfunctions in control valves 01 Changing the flow direction to resolve premature closing issues and eliminate surging: To enhance the shut-off effect, two-position valves are typically used as flow-closing type valves. In liquid media, the unbalanced forces resulting from flow closure press the valve core shut, exerting a force that accelerates its closure; this is also known as suction effect. It speeds up the movement of the valve core, causing slight water hammer and leading to system surging. The solution to the above phenomenon is to change the flow direction to outward flow, and the surge will be eliminated. For problems similar to this, where the act of promoting closure affects the proper functioning of the valve, this approach can also be considered as a solution. 02 Methods to prevent plastic deformation: Plastic deformation causes the surface of one metal to scratch the surface of another metal component, or even cause them to stick together, leading to problems such as valves getting stuck, impaired functionality, damage to the sealing surfaces, increased leakage, and inability to rotate the two parts connected by threads (such as the upper and lower valve bodies in high-pressure valves). Plastic deformation is related to temperature, mating materials, surface roughness, hardness, and load. High temperatures cause the metal to anneal or soften, further exacerbating the tendency for plastic deformation. Methods to address valve failures caused by plastic deformation include: using materials with high hardness for easily scuffed areas, ensuring a hardness difference of 5–10 Rc ； The two parts are made of different materials ； Increase the gap ； Add lubricant ； Repair the damaged surface to improve smoothness and hardness: When the thread won’t turn, it’s necessary to weld it in one go. 03 Adding seal grease method: When factors such as inaccurate calculations or increased production result in a low flow coefficient of the valve, such that the desired flow rate cannot be ensured even when the valve is fully open, it becomes necessary to open a bypass to allow some of the flow to pass through. Typically, the bypass flow is <15–20% of the maximum flow. Here is a method for creating a bypass: since the flow resistance of the flow-closing type is low, its flow coefficient is 10–15% higher than that of the flow-opening type. Therefore, by changing the direction of flow, it is possible to use the flow-closing mode instead of the normal flow-opening mode, allowing the valve to handle a flow rate that is 10–15% higher. This prevents the bypass from being activated, and since it operates at a full opening, stability issues need not be considered. 04 Overcoming fluid disruption: The most typical valve is the double-seat valve, where the fluid enters from the middle; the valve core is perpendicular to the inlet, and the fluid flows around the core and exits in two streams, one above and one below. The fluid impacts the valve stem, pushing it toward the outlet side. This causes friction, damaging the guiding surfaces of the valve stem and bushing, leading to malfunction. At high flow rates, the valve stem may also bend, erode, or even break in severe cases. Solution: Increase the hardness of the material in the guiding area ； Increase the size between the upper and lower balls of the valve core to make it thicker ； Use other valves as substitutes. When a sleeve valve is used, the fluid flows in from around the sleeve, thereby **reducing** the lateral force on the valve plug. 05 Methods for overcoming the rotational force generated by the fluid to rotate the valve stem. For valve stems with a “V”-shaped opening, due to the asymmetrical inflow of the medium, the tangential forces acting on the valve stem at the “V”-shaped opening are unequal, thereby generating a rotational force that causes it to rotate. This is especially true for valves with DN≥100. As a result, it may cause the valve to disconnect from the actuator rod, and springless actuators may cause the diaphragm to twist. Solutions include: rotating the valve core in the opposite direction by an angle to balance the tangential forces acting on it ； Further secure the connection between the valve stem and the push rod; if necessary, add a anti-rotation clamp ； Replace the valve core with a V-shaped opening with a plunger-type valve core ； Adopt or switch to a sleeve-type structure ； In the case of rotation caused by resonance, eliminating the resonance solves the problem. 06 Adjusting the friction of the butterfly valve disc to overcome opening jitter: Butterfly valves equipped with soft seals such as \"O\"-rings, sealing rings, and linings are used; when the valve is closed, the deformation of these soft seals ensures that the disc closes properly and covers it completely, thereby achieving an excellent sealing effect. However, when the valve needs to be opened, the force exerted by the actuator to open the valve plate continues to increase. When this force becomes equal to the frictional force between the soft seal and the valve plate, the valve plate begins to move. Once activated, this friction force decreases sharply. To achieve force balance, the valve disc opens violently; when this force, together with the unbalanced torque generated by the medium at that opening degree, balances the opening torque of the actuator, the valve stops at that opening degree. The opening angle resulting from this violent and sudden jump can reach as much as 30–50%, which will lead to a series of problems. At the same time, during shutdown, significant changes occur in the soft seals, which can lead to permanent deformation or damage as a result of being compressed or strained by the valve plate, thereby affecting their lifespan. The solution is to adjust the friction force exerted by the soft seal on the valve disc during startup, which ensures that the required cutting action is achieved while also allowing the valve to start functioning properly. The specific methods include: adjusting the interference amount ； By using limits or adjusting the pre-tightening force and output force of the actuator, the difficulties arising from excessive closure of the valve plate when attempting to open it can be reduced. VIII. Fault Analysis of Pneumatic Control Valves 01 The control valve does not operate. Cause 1: No air supply or insufficient air pressure. Solution: First, check whether the air supply (instrument air) is unobstructed and whether the air pressure meets the requirements for operating this valve. Reason 2: There is air supply, but no output signal. Pressure measure: For mechanical controllers or positioners, a different mechanical pressure controller or positioner should be used. For the control valves of mechanical controllers, check whether the valves in the signal acquisition pipelines of the medium in the installation pipelines of these control valves are fully open or suffering from severe leakage; any abnormalities should be addressed promptly. Reason 3: The pressure of the output signal gas is normal, but it still does not operate. Measure: Check whether the diaphragm of the pneumatic diaphragm actuator has severe air leakage; if there is leakage, the diaphragm and related seals should be replaced promptly. If it is checked that the diaphragm is intact and there is no air leakage, then it is likely that the valve core of the main valve is stuck to the bushing and seat; the main valve should be disassembled for inspection and any debris removed. The valve stem is severely bent; the main valve should be disassembled for inspection, and replacement or repair should be carried out based on the results of the inspection. There is a leak in the signal gas supply pipeline; check and repair the leak. If the signal amplifier is faulty or not properly adjusted, and the main air supply does not reach the diaphragm chamber through the amplifier, it should be adjusted or replaced promptly. 02 Regulation oscillations occur and control is unstable when the control valve is in operation. Cause 1: Large fluctuations in air supply pressure, or abnormal operation of the filter or pressure regulator. Measure: Check the operating condition of the compressed air system. Check and replace the filter or pressure relief valve. Reason 2: The air supply pressure is stable, while the signal pressure is unstable. Measure: If stability is still not achieved after replacing the smart locator, the PID parameters should be adjusted. For mechanical controllers or positioners, the mechanical controller or positioner should be replaced, and the controller should be adjusted. If instability persists even after replacing the mechanical controller or positioner, the signal air amplifier should be checked and replaced, and then adjusted. Reason 3: Both the air supply and signal pressure are stable, but the control valve still operates unstably. Measure: Check the airtightness of the pneumatic diaphragm actuator. Carefully check the drive rod seal for any slight air leakage; if there is leakage, the seal should be removed and replaced ； Inspect the diaphragm for any scratches or punctures that may cause minor air leakage; if there is leakage, the diaphragm should be replaced. Carefully check whether there are any gaps at the connection point between the locator and the main valve; the relevant connectors between them should be replaced or tightened. Carefully check the signal air pipeline with soapy water for any minor leaks, and address them promptly if detected. The signal gas source amplifier is not properly tuned; adjust the balance screw of the signal gas source amplifier. If the resistance of the actuation mechanism of the control valve (including the main valve and the pneumatic diaphragm actuator) is too high, the main valve should be disassembled to inspect and replace any faulty components, including the valve core, valve seat, valve stem, valve stem seal, and drive rod. Check whether the pressure balance spring in the pneumatic diaphragm actuator is damaged, and inspect whether the spring has suffered fatigue deformation; if any abnormalities are found, the pressure balance spring should be replaced. 03 Slow operation of the control valve. Cause 1: Deposits inside the valve body are causing blockage. Measure: Disassemble the main valve body and clean the residues inside it. Reason 2: The valve stem packing has hardened due to deterioration, or the lubricant in the graphite or asbestos packing has dried out. Measure: Disassemble the main valve body and replace the valve stem packing. Reason 3: The packing is tightened too much, increasing frictional resistance. Measure: Loosen the nut used to compress the valve stem packing, operate the main valve back and forth several times, then adjust the nut to the appropriate torque. If Measure 1 does not work, replace the valve stem packing and adjust the valve stem packing compression nut to the appropriate torque. Reason 4: An uneven valve stem results in high frictional resistance. Measure: Disassemble the main valve and replace the valve stem. Reason 5: There is slight air leakage in the diaphragm of the pneumatic diaphragm actuator or in the signal air pipeline. Measurements: Replace the diaphragm of the pneumatic diaphragm actuator, and address leaks in the signal air pipeline. 04 The control valve operates normally, but the process control parameters are abnormal. Cause 1: The valve spool of the control valve has fallen off. Measure: Disassemble and inspect the main valve body of the control valve, and restore the valve spool in case it has fallen out, based on the inspection results. Reason 2: Relative displacement occurred at the connection between the valve spool and the valve stem, but it did not come loose. Measure: Disassemble and inspect the main valve body of the control valve, and restore the position of the valve spool based on the inspection results. Reason 3: The valve stem of the control valve is broken. Measures: Disassemble and inspect the main valve body of the control valve; replace the valve stem. Reason 4: There is dirt blocking inside the valve spool of the control valve. Measures: Disassemble and inspect the main valve body of the control valve; clean any dirt from inside the valve stem. Reason 5: Other control parameters of the controlled medium system are inappropriate; for example, the medium flow rate and pressure are not within the design specifications. Measures: Adjust the system control parameters within the design requirements. IX. What parts should be carefully inspected during the maintenance of pneumatic control valves? 1. Inner walls of the valve body: In situations involving high pressure differences and corrosive media, the inner walls of the valve body as well as the diaphragms in diaphragm valves are frequently subjected to the impact and corrosion of these media. Therefore, it is essential to thoroughly inspect their resistance to pressure and corrosion. 2. Valve core: The valve core is one of the moving parts of a control valve, and it is subject to severe erosion by the medium. During maintenance, it is necessary to carefully check whether various parts of the valve core are corroded or worn. Especially under conditions of high pressure differentials, the wear of the valve stem is exacerbated by cavitation-induced erosion. Severely damaged valve stems should be replaced. Inspect the packing: check whether the asbestos rope packing is dry. If PTFE packing is used, pay attention to whether it has aged and whether its mating surfaces are damaged. 3. The valve seat: As the medium seeps in during operation, the internal surface of the threads used to secure the valve seat is prone to corrosion, which can cause the valve seat to become loose. 4. Diaphragms and O-rings: vulnerable parts; check for signs of aging. 5. Packing: When graphite or asbestos is used as packing for the control valve, lubricating oil should be applied to the packing approximately every three months to ensure smooth operation of the valve. If it is found that the packing gland cap is pressed too low, the packing should be replenished ； If the polytetrafluoroethylene filler becomes hardened, it should be replaced promptly. 6. Air supply: For control valves equipped with positioners, the air supply should be checked regularly to ensure its quality, free from water and other impurities.