Solutions to common faults of control valves

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Solutions to valve vibration: 1. The method of increasing stiffness – For oscillations and mild vibrations, increasing the stiffness can be used to eliminate or reduce them; options 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 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: Axial plug valves generally have small guiding dimensions, while the clearance between all components of such valves is usually large, ranging from 0.4 to 1 mm; this helps to induce mechanical vibrations. Therefore, when mild mechanical vibrations occur, the vibrations can be reduced by increasing the guiding dimensions and decreasing the fit clearance. 　　4. Changing the shape of the throttling element to eliminate resonance: Since the so-called source of vibration in control valves lies at the throttle area where flow velocity is high and pressure changes rapidly, altering the shape of the throttling element can change 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. The principle of eliminating resonance by replacing the throttle element is the same as that in 4.5, point 4), except that it involves replacing the throttle element. 　　The methods include: ① Changing the flow characteristic, from logarithmic to linear, or from linear to logarithmic; ② Changing the type of valve core. For example, changing the plug type to a \"V\"-groove valve core, changing the double-seat valve plug type to a cartridge type; replacing the windowed cartridge with a cartridge having small holes, etc. In a nitrogen fertilizer plant, a DN25 two-seat valve had its stem breaking at the connection point with the valve disc frequently. 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 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 plant, 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 occurrence rate was between 20% and 70%. Given this high level of resonance, a double-seat valve was used instead, and as a result, the resonance disappeared and the valve operated normally. 　　7. To reduce cavitation vibration caused by the rupture of cavitation bubbles, it is natural to seek ways to minimize 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 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 staging or *pressure reduction. 　　9. Avoid vibration caused by external excitation waves from the vibration source. This is something that must be avoided during the normal operation of control valves; if such vibration occurs, appropriate measures should be taken. Solutions when a control valve has poor stability: 1. Change 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 tends to cause the valve to close, the valve’s stability 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 changing it 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. 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. 　　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; ② The stability is poor in the vicinity where the slope of the unbalanced force changes intermittently. For butterfly valves, the alternating point is around 70 degrees; for two-seat valves, it is at an opening degree of 80–90%. 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 it with a valve that has good stability; such valves exhibit minimal changes in unbalanced forces and provide 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 cartridge valves will definitely improve stability. 4. The ability of the actuator based on the method of increasing spring stiffness to resist the impact of load changes on stroke depends on the spring stiffness; the greater the stiffness, the lesser the impact on 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 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, this is problematic. For example, if fine tuning of flow rate is needed but the valve’s ability to adjust the flow rate changes significantly, or if the system itself is already a fast-response system yet the control valve is equipped with a positioner to further accelerate its operation, these situations are all undesirable. This will result in overshoot and vibrations, etc. To address this, the response speed should be reduced. 　　The solutions are: ① Change the linear characteristic to a logarithmic one; ② Those with positioners can be replaced by converters or actuators. 6. Change the flow direction, address the issue of premature shutdown, and 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 surging. The solution to the aforementioned phenomenon is to change the flow direction to one away from it, 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 *A typical valve is a double-seat valve; 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. 　　Solutions: ① Increase the hardness of the material in the guiding area. ② Enlarge the size between the upper and lower balls of the valve core to make it thicker. ③ Use another type of 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. The method of overcoming the rotational force generated by the fluid to rotate the valve core applies to valves 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 unequal, resulting in a rotational force that causes it to spin. 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: ① Rotating the valve core in the opposite direction by an angle to balance the tangential forces acting on it; ② Securing the connection between the valve stem and the push rod further, and adding a anti-rotation clamp if necessary; ③ Replacing the valve core with a plug-type valve core; ④ Using or switching to a sleeve-type structure; ⑤ If the rotation is caused by resonance, eliminating the resonance will resolve the issue. 9. Adjust the friction of the butterfly valve disc to overcome opening oscillation. Butterfly valves that use soft seals such as \"O\"-rings, sealing rings, and linings achieve a very satisfactory shut-off effect when closed, as the deformation of these soft seals allows the valve disc to close properly and cover it completely. 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. This intense and sudden opening can reach 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 squeezed 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 performance is achieved while also allowing the valve to start operating properly. The specific methods include: ① Adjusting the amount of interference; ② Reducing the difficulties associated with opening the valve due to excessive closure of the valve plate, by using limits or by adjusting the pre-tensioning force and output force of the actuator