Looking for the book \"Practical Techniques for Control Valves\"

It can be purchased online from Dangdang: victory:

Here is a list of 60 methods for dealing with faults in control valves: 60 ways to handle common problems with control valves. In industrial automation instruments, control valves are considered relatively bulky, and due to their simple structure, they often don’t receive much attention. However, it’s located on process pipelines where operating conditions are complex; once a problem occurs, everyone becomes busy trying to handle it. Due to their bulkiness, it’s difficult to pinpoint the problems; efforts often prove futile. These issues also involve system commissioning, system integrity, regulation quality, and environmental pollution. The treatment methods for the 60 common faults described below come, in the vast majority of cases, from the author’s own work experience. They can serve as a reference for analyzing and dealing with faults in control valves, and are thus useful for field maintenance workers and technical personnel. 4.1 Methods to increase lifespan (8 methods) 1) Method of extending lifespan by operating at high opening degree: Have the control valve operate at the maximum opening degree 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 closure 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, operating at a large opening results in a larger throttling gap, which reduces erosion; this increases the valve’s lifespan by 1 to 5 times or more compared to operating it at intermediate or small openings from the start. If a chemical plant adopts this method, the service life of the valves is doubled. 2) Method of reducing S to increase the operating opening and extend lifespan: By reducing S, the losses in the system other than the control valve are increased, which lowers the 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 leads to a decrease in cavitation and erosion. Specific methods include: installing a orifice plate behind the valve to create pressure drop through throttling ； Close the manually operated valve connected in series on the pipeline until the control valve reaches a more desirable operating opening. When the valve is initially operated at a small opening despite being oversized, this method is very simple, convenient, and effective. 3) Method of reducing the diameter to increase the operating opening and extend service life: This involves reducing the valve’s diameter to increase its operating opening. Specific measures include: ① Replacing the valve with one having a smaller diameter; for example, replacing a DN32 valve with a DN25 one ； ②The valve body remains unchanged; only the valve core and seat with a smaller diameter are replaced. For instance, during a major overhaul at a chemical plant, replacing the throttle element dgl0 with dg8 doubled its service life. 4) Method of improving lifespan by shifting the location of damage: Moving the areas severely damaged to less critical positions in order to protect the sealing surfaces and throttling surfaces of the valve core and seat. 5) Method of increasing lifespan by enlarging the throttling channel: The simplest way to do this is to thicken the valve seat, thereby increasing the diameter of the valve seat hole and creating a longer throttling channel. On the one hand, it can delay the sudden expansion that occurs after throttling in a flow-blocking manner, 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 designs feature stepped or wavy shapes in the valve seat hole, aiming 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. 6) Method of improving lifespan by changing the flow direction: 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-closed type, flow moves in the closed direction; 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. 7) Method of improving lifespan by using special materials: To resist cavitation (damage in the form of honeycomb-like spots) and erosion (streamline-shaped 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.). 8) Method of improving 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. 4.2 Methods to prevent clogging (sticking) of control valves that stick or get blocked frequently (6 methods) 1) 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, leading to scratches and abrasions on the valve core’s surface 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 remove the component for cleaning in order to get rid of 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 into service, fully open the control valve; allow the medium to flow for a period of time before resuming normal operation. 2) External flushing method: When using ordinary valves to regulate certain media that are prone to sedimentation and contain solid particles, blockages often occur at the throttle orifices and guides. In such cases, flushing gas and steam can be introduced externally through 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. 3) Pipe filter installation method: For small-diameter control valves, especially those with extremely low flow rates, the throttling gap is very small, and there must not be even the slightest amount of debris in the fluid. In case of such blockages, it is advisable to install a filter on the pipeline before the valve to ensure the smooth passage of the medium. For control valves used with positioners, when the positioner does not function properly, the most common fault is a blockage in the air circuit throttle. Therefore, when a positioner is in use, it is necessary to properly manage the air supply; a common approach is to install an air filter and pressure reducing valve on the air supply pipeline before the positioner. 4) Increasing the throttling gap: If solid particles in the medium, or welding slag and rust that have been washed into the pipes, cause blockages or jams due to their inability to pass through the throttling orifice, it is possible to use a throttling element with a larger throttling gap – such as a valve core or sleeve with window-like or open-shaped throttling areas. Since the throttling area in this case is concentrated rather than distributed circumferentially, faults can be easily resolved. For single or double-seat valves, the plunger-type valve core can be replaced with a valve core having a “V”-shaped port, or it can be changed to a cartridge valve or similar. For example, at a chemical plant, a double-seat valve kept getting stuck. After it was recommended to switch to a sleeve valve, the problem was immediately resolved. 5) Medium scouring method: Utilizes the scouring energy of the medium itself to scour away and remove substances that tend to precipitate or cause blockages, thereby enhancing the valve’s anti-blocking capability. Common methods include: ① modifying it for use in a flow-closed configuration ； ②Adopts a streamlined valve body ； ③Place 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. 6) Changing from straight-through to corner flow method: The straight-through configuration involves an inverted S-shaped flow pattern, resulting in a complex flow path and numerous dead zones in the upper and lower chambers, which provide areas for the deposition of the medium. Angular connection: the medium flows as if through a 90° elbow, offering good scouring performance, a small dead zone, and allowing for easy design in a streamlined shape. Therefore, when a slight blockage occurs with a straight-through control valve, it can be replaced with an angle valve. 4.4 Solutions to external leakage in control valves (6 methods) 1) Adding sealing grease: For valves that do not currently use sealing grease, adding it can be considered to improve the sealing performance of the valve stem. 2) 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 mixture of fillers is used; however, simply increasing their quantity—for instance, from 3 pieces to 5 pieces—does not yield significant results. 3) Graphite packing replacement method: 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 is large when using graphite fillers, and some systems even exhibit crawling behavior at first; this must be taken into consideration. 4) Change the flow direction and place P2 at the valve stem end. When △P is relatively large and P1 is also large, it is evidently more difficult to seal P1 than to seal 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 usually be taken into consideration. 5) 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. In the case of a flat seal, its sealing performance is poor under high temperature and pressure, leading to leaks; using a lens gasket for sealing can yield satisfactory results. 6) Replace the 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. 4.5 Solutions to control valve vibration (8 methods) 1) 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. 2) Increasing damping: Increasing damping means increasing the friction against vibrations; for example, the plug of a sleeve valve can be sealed with an “O” ring, or graphite packing with high friction can be used. This can help to eliminate or reduce minor vibrations to some extent. 3) Increasing the guiding dimensions and reducing the clearance method: 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 facilitates the occurrence of mechanical vibrations. Therefore, when mild mechanical vibration occurs, the vibration can be reduced by increasing the guiding dimensions and reducing the fit clearance. 4) The method of changing the shape of the throttling element to eliminate resonance: Since the so-called source of vibration in control valves lies at the throttling area where flow is rapid and pressure changes sharply, changing the shape of the throttling 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 noises that disturbed the workers’ rest. By grinding 0.5 mm off the surface of the valve core, the resonant humming noise disappeared. 5) Resonance elimination by replacing the throttle element: The principle is the same as that in 4.5, point 4), except that the throttle element is replaced. The methods include: ① Changing the flow characteristic, from logarithmic to linear, or from linear to logarithmic ； ②Change the type of valve core. If the plug type is changed to a “V”-groove valve spool, and the double-seat valve plug type is changed to a cartridge type ； Replace the sleeve with an opening window with a sleeve having small 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. 6) Replace the type of control valve to eliminate resonance. Control valves with different structural designs have inherent frequencies that vary; replacing the type of control valve is the most effective way to eliminate resonance fundamentally. 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 severe resonance disappears miraculously. In a newly expanded project at a vinylon plant, a DN200 globe valve was initially selected. All three aforementioned phenomena occurred: the DN300 pipeline vibrated, the valve plug rotated, and the noise level exceeded 100 decibels. The resonance occurred at an opening range of 20–70%. Given that this resonance range was quite large, a double-seat valve was substituted instead. Consequently, the resonance ceased, and the valve has been operating normally since then. 7) Methods for reducing cavitation vibration: For cavitation vibration caused by the collapse of cavitation bubbles, efforts should naturally be made to minimize cavitation. ①The impact energy generated by the bursting of bubbles should not act on the solid surface, especially on the valve stem; instead, it should be absorbed by the liquid. Sleeve valves possess this feature, which allows the plug-type valve element to be replaced with a sleeve-type one. ②All 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. 8) Avoid vibration caused by external shock waves. External shock waves can cause vibration in the control valve, and this is something that must be avoided during the normal operation of the control valve; if such vibration occurs, appropriate measures should be taken. 4.6 Solutions to excessive noise in control valves (8 methods): 1) Eliminating resonance noise – Strong noise levels of over 100 decibels occur only when the control valve is in a state of resonance, as energy accumulates in that condition. Some exhibit strong vibration with low noise, while others have weak vibration but very high noise levels ； Some have significant vibration and noise. 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. Methods and examples are given in 4), 5), and 6) of section 4.5 above. 2) Method to eliminate cavitation noise: Cavitation is the main source of hydrodynamic noise. During cavitation, the bursting of bubbles generates high-speed shock waves, causing intense local turbulence and resulting in cavitation noise. This noise has a wide frequency range and produces a rattling sound, similar to the noise generated by sand and stones present in the fluid. Eliminating and reducing cavitation is an effective way to eliminate and reduce noise. 3) Using the thick-walled pipeline 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 to 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. 4) The use of sound-absorbing materials is also a common and highly effective method for handling sound paths. The noise source and the pipelines behind the valve can be covered with sound-absorbing materials. It must be noted that since noise can travel over long distances via 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 is not high and the pipeline length is not long, as it is a relatively costly approach. 5) Series muffler method. This method is suitable for attenuating aerodynamic noise. It can effectively eliminate noise within the fluid and suppress the noise level transmitted to the solid boundary layer. For applications with high mass flow rates or a high pressure drop across the valve, this method is the most effective and economical. Using absorptive series silencers can significantly reduce noise. However, from an economic perspective, it is generally limited to attenuating to about 25 decibels. 6) Soundproof box method: Soundproof boxes, houses, and buildings are used to enclose the noise source, thereby reducing the noise from the external environment to an acceptable level. 7) Series throttling method: In cases where the pressure ratio of the control valve is high (△P/P1≥0.8), the series throttling method is employed, which involves distributing the total pressure drop across both the control valve and a fixed throttling element located after the valve. 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. 8) 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 to 20 decibels; it is the most cost-effective low-noise valve available. 4.7 Solutions when the stability of the control valve is poor (5 methods) 1) 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 conditions of such unbalanced forces, the approach adopted is to change the direction of their action; usually, this involves changing the valve from a flow-blocking type to a flow-allowing type, and this generally allows the stability issue of the valve to be resolved easily. 2) Avoiding operation in the valve’s own unstable region: Some valves, due to limitations in their structure, exhibit poor stability when operating at certain opening degrees. ①For 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 ； ②Near the region where the slope of the unbalanced force variation becomes oscillatory, its stability is poor. For example, in the case of butterfly valves, the alternating point is around 70 degrees ； The double-seat valve is at 80–90% opening. When encountering such valves, operating in the unstable region will inevitably result in poor stability; avoiding operation in that unstable region is sufficient. 3) Replace with a valve that has good stability. A valve with good stability exhibits minimal changes in unbalanced force and has excellent 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 cartridge valves will definitely improve stability. 4) Method of increasing spring stiffness: The ability of the actuator to resist the effect of load variations on its stroke depends on the spring stiffness. The greater the stiffness, the less impact there is on the stroke, and the better the valve stability. 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 separate positioner must be installed on the valve. 5) 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 tuning of flow rate is needed but the flow regulation capability of the control valve is quite large, or when the system itself is already a fast-response system yet a positioner is used on the control valve to accelerate its operation, these situations are unfavorable. This will cause overshoot, vibrations, etc. To address this, the response speed should be reduced. The solutions are: ① Change the linear characteristic to a logarithmic characteristic ； ②The positioner-equipped unit can be changed to a converter or relay. 4) Symmetrically tighten the bolts using a thin washer sealing method. In the structure of control valves with O-ring seals, when thick gaskets that can undergo significant deformation (such as spiral-wound gaskets) are used, asymmetrical tightening leads to uneven stress distribution. This can easily cause damage, tilting, and deformation of the seal, thereby 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. 5) A method to increase the width of the sealing surface in order to prevent the flat valve core from jumping during closure and to reduce leakage. In flat-type valve cores (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 core, pushing it from the inlet side toward the outlet side. The greater the clearance between the valve core 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 core (usually a 30° chamfer is used for guidance) can cause the chamfered end of the valve core’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 core 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. 6) Changing the flow direction to address the issue of premature shutdown and to eliminate surging; two-position valves are typically used in a flow-blocking mode to improve the shut-off effect. In liquid media, the unbalanced forces resulting from flow closure press the valve core shut, exerting a force that promotes closing; this is also known as suction effect. It accelerates the movement of the valve core, causing slight water hammer and leading to system surge. The solution to the aforementioned phenomenon is to change the flow direction to outward, 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. 7) Method of overcoming fluid disruption: The most typical valve in this category 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 valve core and exits in two streams, one above and one below. The flow impact on the valve core pushes it toward the outlet side, causing friction that damages the guiding surfaces of the valve core and the bushing, leading to abnormal operation. High flow rates can also cause the valve core to bend or erode, and in severe cases, even break. Solution: ① Increase the hardness of the material in the guiding area ； ②Increase the size between the upper and lower balls of the valve core to make it thicker ； ③Use another valve as a substitute. When a sleeve valve is used, the fluid flows in from around the sleeve, thereby **reducing** the lateral force on the valve plug. 8) Methods for overcoming the rotational force generated by the fluid to rotate the valve core. In the case of a valve core with a “V”-shaped port, due to the asymmetric flow of the medium, the tangential forces acting on the valve core at the “V”-shaped port are not uniform, resulting in a rotational force that causes it to spin. This is especially true for valves with DN≥100. As a result, this may cause the valve to become disconnected from the actuator rod, and springless actuators may cause the diaphragm to twist. Solutions include: ① Rotating the valve core in the opposite direction by an angle to balance the tangential forces acting on it ； ②Further secure the connection between the valve stem and the push rod; if necessary, add a anti-rotation clamp ； ③Replace the valve core with a V-shaped opening with a plunger-type valve core ； ④Adopt or switch to a sleeve-type structure ； ⑤In the case of rotation caused by resonance, eliminating the resonance solves the problem. 9) Adjusting the friction of the butterfly valve disc to overcome the problem of erratic opening: For butterfly valves equipped with soft seals such as O-rings, sealing rings, and linings, when the valve is closed, the deformation of these soft seals ensures that the valve disc closes properly and is completely covered by them, thereby achieving a highly effective shutoff effect. However, when the valve is to be opened, the actuator must exert an increasing force on the valve disc; once this force equals the frictional force exerted by the soft seal on the valve disc, the valve disc 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 degree resulting from this violent and sudden jump can reach as much as 30–50%, which will cause 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 frictional force exerted by the soft seal on the valve disc during startup, which ensures that the required shut-off performance is achieved while also allowing the valve to start up properly. Specific methods include: ① Adjusting the interference amount ； ②By 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.

Do you have any materials on regulator training?

I bought one; the quality is average. It seems that control valves aren’t his strong point – he should focus on steam traps instead, haha :)

Sure, as long as it’s not already available on the forum and is practical.

I had previously only seen the catalog online and thought it sounded good; later I saw it in a bookstore as well, and indeed it wasn’t very in-depth. I originally wanted to understand the calculation for mixing valves

\"Practical Techniques for Control Valves\" should be uploaded at http://bbs.hcbbs.com/thread-148764-1-1.html

:Lol, thank you so much to the original poster for their help! ! ! ! Thank you~~~