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

Thread Content

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, leading 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 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 piping. Before putting it into operation, open the control valve fully; allow the medium to flow for a while before proceeding with 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 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; therefore, there must not be any debris in the fluid. In the event of such blockage, install a filter on the pipeline ahead of the valve to ensure smooth flow of the medium. For control valves used with positioners, when the positioner does not function properly, a clogged throttle in its air supply line 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 that have been washed away in the pipes, cause blockages or jams due to their inability to pass through the throttling opening, it is possible to use throttling elements with a larger throttling gap – valve cores or sleeves with window-like or open-shaped throttling areas. Since the throttling area is concentrated rather than distributed circumferentially, such problems 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 switch to a sleeve valve, the problem was resolved immediately. 　　 (5) Medium scouring method: Utilizing 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 resistance to blockage. Common methods include: ① Modifying it for use in a flow-closed configuration ； ②Use a streamlined valve body ； ③Place the throttle at the area most subject to erosion; 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 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. Solutions for vibration: (1) Increasing stiffness method. For oscillations and mild vibrations, the stiffness can be increased to eliminate or reduce them; methods such as using springs with higher stiffness or switching to piston actuators are feasible. 　　　　 (2) Increasing damping method: Increasing damping means increasing the friction against vibrations; for example, the plug of a sleeve valve can be sealed using an “O”-ring, or graphite packing with high friction can be employed. 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 vibrations occur, the vibrations can be reduced by increasing the guiding dimensions and decreasing the fit clearance. 　　　　 (4) Method of changing the shape of the throttling element to eliminate resonance: Since the so-called source of vibration in control valves occurs 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 residential area, a self-acting pressure control valve was installed; resonance caused humming noise that disturbed the workers’ rest. By milling 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. For example, change the plug-type valve core to a “V”-shaped groove valve core, and change the double-seat plug-type valve to a sleeve-type one ； 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. 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 inherent frequencies that vary; replacing the type of control valve is the most effective way to eliminate resonance fundamentally. A valve that exhibits **extreme vibration**—intense vibrations (which can even damage the valve), excessive rotation (to the point where the valve stem gets shaken or twisted off), and loud noise (exceeding 100 decibels)—will see immediate improvement if replaced with a valve having a significantly different structure. In such cases, 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 problems occurred: the DN300 pipeline vibrated, the valve plug rotated, the noise level exceeded 100 decibels, and the resonance opening range was 20–70%. Given the large resonance opening, 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 core 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) Avoiding vibration caused by external wave impacts External wave impacts can cause vibration in the valve, and this is something that must be avoided during the normal operation of control valves; if such vibration occurs, appropriate measures should be taken.