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2022

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07

Solutions when the stability of the regulating valve is poor | Common troubleshooting of the regulating valve

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1 ) Method of changing the action direction of the unbalanced force In the stability analysis, it is known that when the unbalanced force acts in the same direction as the valve closing direction, that is, when the valve has a closing tendency, the valve stability is poor. When the valve operates under the above-mentioned unbalanced force conditions, the method of changing its direction of action is usually changed from the flow-closed type to the flow-open type.

1 ) Method of changing the action direction of the unbalanced force In the stability analysis, it is known that when the unbalanced force acts in the same direction as the valve closing direction, that is, when the valve has a closing tendency, the valve stability is poor. When the valve operates under the above-mentioned unbalanced force conditions, the method of changing its direction of action is usually changed from the flow-closed type to the flow-open type. Generally speaking, the stability problem of the valve can be easily solved.

2 ) Avoid working in the unstable zone of the valve itself . Some valves are limited by their own structure and have poor stability when working at certain openings. ① For double-seat valves, the opening is within 10%, because the flow is open at the upper ball and the flow is closed at the lower ball, causing instability problems; ② The stability is poor near the alternating slope of the unbalanced force change. For example, for butterfly valves, the crossover point is around 70 degrees; for double-seat valves, the opening is between 80 and 90%. When encountering this type of valve, working in the unstable zone will inevitably lead to poor stability, so just avoid working in the unstable zone.

3 ) Replace the valve with good stability. The valve with good stability has smaller changes in unbalanced force and good guidance. Among commonly used ball valves, sleeve valves have this major feature. When the stability of single or double-seat valves is poor, replacing them with sleeve valves will definitely improve the stability.

4 ) Increase the spring stiffness method. The ability of the actuator to resist the impact of load changes on the stroke depends on the spring stiffness. The greater the stiffness, the smaller the impact on the stroke and the better the stability of the valve. Increasing the spring stiffness is a common and simple method to improve the stability of the valve. For example, changing the spring with a spring range of 20 to 100KPa to a large stiffness spring of 60 to 180KPa. This method is mainly used for valves with a positioner. Otherwise, use The valve must be equipped with a positioner.

5 ) Reduce response speed method: When the system requires that the response or adjustment speed of the control valve should not be too fast, the response and adjustment speed of the valve are fast. For example, the flow rate needs to be fine-tuned, but the flow adjustment of the control valve changes greatly, or The system itself is already a fast response system, but the regulating valve has a positioner to speed up the valve action, which is disadvantageous. This will cause overshoot, vibration, etc. For this, the response speed should be reduced. Methods include: ① Change linear characteristics to logarithmic characteristics; ② Those with positioners can be changed to converters and relays.

6 ) Tighten the bolts symmetrically and use thin gasket sealing method. In the structure of the regulating valve with "O" ring seal, when using thick gaskets with large deformation (such as winding sheets), if the compression is asymmetrical, the force will be asymmetrical. , it is easy for the seal to be damaged, tilted and deformed, seriously affecting the sealing performance. Therefore, when repairing and assembling this type of valve, the compression bolts must be tightened symmetrically (note that they cannot be tightened at once). It would be better if the thick gasket can be changed to a thin gasket, which can easily reduce the inclination and ensure sealing.

7 ) Ways to increase the width of the sealing surface to prevent the flat valve core from jumping when it is closed and reduce its leakage. The flat valve core (such as the valve plug of the two-position valve and sleeve valve) has no guide and guide curved surface in the valve seat. Because when the valve is working, the valve core is subject to lateral force from the inflow direction to the outflow direction. The larger the valve core matching gap is, the more serious this unilateral phenomenon will be. In addition, it will be deformed, not concentric, or the valve core sealing surface will be inverted. The angle is small (usually 30° chamfering for guidance), so when it is close to closing, the chamfered end surface of the valve core sealing surface is placed on the valve seat sealing surface, causing the valve core to jump when closing, or even not closing at all, causing the valve to The amount of leakage increases significantly. The simplest and most effective solution is to increase the size of the valve core sealing surface, so that the minimum diameter of the valve core end face is 1 to 5 mm smaller than the valve seat diameter, and has sufficient guidance to ensure that the valve core is guided into the valve seat and maintains Good sealing surface contact.

8 ) Change the flow direction to solve the problem of closing and eliminating surge. In order to improve the cutting effect, the two-position valve is usually used as a flow-closing type. For liquid media, due to the flow-closing unbalanced force, the valve core is pressed closed, which has a closing effect, also known as a suction effect, speeding up the action of the valve core, resulting in a slight water hammer, causing system surge. The solution to the above phenomenon is to simply change the flow direction to open and the surge can be eliminated. This method can also be considered to solve problems like this that affect the normal operation of the valve due to accelerated closing. 

9 ) The most typical valve to overcome the fluid destruction method is a double-seat valve. The fluid enters from the middle, the valve core is perpendicular to the inlet, and the fluid bypasses the valve core and is divided into upper and lower streams and flows out. The fluid impacts the valve core, causing it to move toward the outlet side, causing friction and damaging the guide surfaces of the valve core and bushing, resulting in abnormal operation. High flow rates may also cause the valve core to bend, erode, or even break in severe cases. Solutions: ① Increase the hardness of the material in the guide part; ② Increase the middle size of the upper and lower balls of the valve core to make it rough; ③ Use other valves instead. If a sleeve valve is used, the fluid flows from around the sleeve, and the lateral thrust on the valve plug is greatly reduced.

10 ) The method of overcoming the rotational force generated by the fluid to rotate the valve core. For the valve core with a "V" shaped port, due to the asymmetry of the medium inflow, the tangential force of the valve core acting on the "V" shaped port is inconsistent, resulting in a The rotational force of rotation. Especially for valves with DN≥100. As a result, the connection between the valve and the actuator push rod may become disconnected, and the springless actuator may cause the diaphragm to twist. Solutions include: ① Rotate the counter-rotation direction of the valve core by an angle to balance the tangential force acting on the valve core; ② Further lock the connection between the valve stem and the push rod, and if necessary, add an anti-rotation splint; ③Replace the "V"-shaped opening valve core with a plunger-shaped valve core; ④Adopt or change to a sleeve-type structure; ⑤If the rotation is caused by resonance, eliminating the resonance can solve the problem.

11 ) Adjust the friction of the butterfly valve plate to overcome the opening bounce method. For butterfly valves that use soft seals such as "O" rings, sealing rings, and linings, when the valve is closed, due to the deformation of the soft seal, the valve plate is closed in place and covers the valve. plate, can achieve very ideal cutting effect. But when the valve wants to open, the force of the actuator to open the valve plate continues to increase. When the friction force of the soft seal on the valve plate becomes equal, the valve plate starts. Once activated, this friction decreases dramatically. In order to achieve the balance of force, the valve plate opens violently. When the unbalanced torque of this force and the medium acting on the corresponding opening is balanced with the opening torque of the actuator, the valve stops at this opening. The opening of this violent and sudden jump can be as high as 30 to 50%, which will cause a series of problems. At the same time, due to the large changes in the soft seal when closing, it is easy to deform the valve body or be squeezed or strained by the valve plate, which affects the service life. The solution is to adjust the friction force of the soft seal on the valve plate startup, which can not only ensure that the required cut-off requirements are met, but also enable the valve to start more normally. Specific methods include: ① Adjust the interference amount; ② Reduce the difficulty caused by excessive closing of the valve plate by limiting the position or adjusting the preload force and output force of the actuator.