Methods to prevent clogging of control valves and solutions to vibration problems

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Methods to prevent clogging of control valves: (1) Cleaning method. Weld slag, rust, debris, etc. in the pipelines can cause blockages or jams at the throttle openings, guide sections, and balance holes on the lower valve cover; this can lead to 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 the unit for cleaning to remove any debris; if the sealing surface is damaged, it must 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 piping. Before putting it into operation, open the control valve fully; allow the medium to flow for a while before starting normal operation. 　　 (2) 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 introduced 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 having to adjust the regulating valve, thereby enabling it to function normally. 　　 (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 any debris in the fluid. In the event of such blockage, install a filter on the pipeline before the valve to ensure smooth flow of the medium. For 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 the locator is in use, the air supply must be properly managed; the common approach is to install an air filter and pressure reducing valve on the air supply pipeline before the locator. 　　 (4) Method of increasing the throttling gap
When solid particles in the medium, or weld slag and rust particles washed off inside the pipeline, fail to pass through the throttling orifice, resulting in blockages or jamming, it is advisable to use a throttling element with a larger throttling gap—such as a valve core or sleeve with openings or slits. Since the throttling area is concentrated rather than distributed circumferentially, such 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, 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. 　　 (5) Medium flushing method: Utilizing 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: ① Modifying it for use in a flow-closed configuration ； ②Use a streamlined valve body ； ③Place the throttle orifice at the most severely eroded area; when using this method, it is important to improve the erosion resistance of the material used for the throttle component. 　　 (6) Change from straight-through to corner flow method. The straight-through configuration results in an inverted S-shaped flow pattern; the flow path is complex, with numerous dead zones in the upper and lower chambers, which provide areas for the medium to settle. Angular connection: the medium flows as if through a 90° elbow, resulting in good scouring performance, a small dead zone, and ease of designing it in a streamlined shape. Therefore, when a slight blockage occurs with a straight-through control valve, it can be replaced with an angle valve. Solutions to vibrations (1) Increasing stiffness method: For oscillations and minor vibrations, increasing the stiffness can eliminate or reduce them. For instance, using springs with higher stiffness or switching to a piston actuator are both viable solutions. 　　　　 (2) Method of increasing damping: Increasing damping means enhancing the friction against vibrations. For instance, an “O”-ring seal can be used for the valve plug in a sleeve valve, and graphite packing with high frictional properties can also be utilized. This approach can indeed help to eliminate or reduce minor vibrations. 　 　　　 (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, in the event of minor mechanical vibrations, the vibrations can be attenuated by increasing the guide dimensions and reducing the clearance. 　　　　 (4) Changing the shape of the throttle element to eliminate resonance: Since the so-called vibration source in control valves occurs at the throttle orifice where there is high-speed flow and rapid pressure changes, altering the shape of the throttle element can change the frequency of this vibration source. This solution is relatively easy to implement when the resonance isn’t very intense. The specific method is to turn the valve core surface by 0.5–1.0 mm within the vibration opening range. In a factory 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) Method of eliminating resonance by replacing the throttling element: 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 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. For another example, a laboratory at an aviation academy used a DN200 sleeve valve; the plug of this valve rotated violently and could not be used. 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 natural 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 of a different design; the severe 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%. Considering the high resonance opening degree, a double-seat valve was used instead, and as a result, the resonance disappeared and the valve operated normally. 　　　　 (7) 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. ①The 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 have 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 decompression. 　　　　 (8) Method of avoiding excitation by external vibration sources  External vibration sources can cause valve vibrations; this is clearly something that should be avoided during the normal operation of control valves. If such vibrations occur, appropriate measures must be taken.