03

2020

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06

Solutions to poor stability of electric control valves

Author:


Electric regulating valves, also known as control valves, are the final control components in the field of industrial automation process control that use power operation to change process parameters such as medium flow, pressure, temperature, and liquid level by receiving control signals output from the regulating control unit. Generally composed of an actuator and a valve. Because of its heavy weight, it is difficult to pinpoint the problem and is often thankless. It also involves system operation, system completion, adjustment quality, environmental pollution, etc.

Electric regulating valves, also known as control valves, are the final control components in the field of industrial automation process control that use power operation to change process parameters such as medium flow, pressure, temperature, and liquid level by receiving control signals output from the regulating control unit. Generally composed of an actuator and a valve. Because of its heavy weight, it is difficult to pinpoint the problem and is often thankless. It also involves system operation, system completion, adjustment quality, environmental pollution, etc.

 

01
Method of changing the direction of action of 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. Work on the valve
 
Under the above-mentioned unbalanced force conditions, the method of changing its direction of action is usually used to change the closed flow type to the open flow type. Generally speaking, the stability problem of the valve can be easily solved.

 

 

02
   How to avoid working in the unstable zone of the valve itself
 

Some valves are limited by their own structures 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, it is at 80 to 90% opening. 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.

 

 

03
   Replace the valve with good stability
 

 

A 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.

 

04
   Increase 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.

 

 

05
   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 needs to be fine-tuned, but the flow adjustment of the control valve changes greatly, or the system itself is already responding quickly. The system and the regulating valve have 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. The methods are:

①Change linear characteristics to logarithmic characteristics;

② Those with positioners can be changed to converters and relays.

 

06
   Symmetrically tightened bolts using thin gasket sealing method
 

In the regulating valve structure with "O" ring seal, when thick gaskets with large deformation (such as winding sheets) are used, if the compression is asymmetrical and the force is asymmetrical, the seal will easily be damaged, tilted and deformed. Seriously affect 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.

 

 

07
   Increase cover width to reduce 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. When the valve is working, the valve core is subject to lateral force and flows out from the inflow direction. If the valve core is square, the larger the matching gap of the valve core, the more serious this unilateral phenomenon will be. In addition, deformation, non-concentricity, or small chamfering of the valve core sealing surface (usually 30° chamfering for guidance) will result in valve core sealing when it is close to closing. The chamfered end face is placed on the sealing surface of the valve seat, causing the valve core to jump when closing, or even not closing at all, greatly increasing valve leakage. The 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, which has sufficient guidance to ensure that the valve core is guided into the valve seat and maintains good sealing surface contact.

 

 

08
   Change the flow direction and solve the problem of promoting relations
 

In order to improve the cutting effect, the position type valve is usually used as a closed flow 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.

 

09
   Overcome fluid damage method   
 

The most typical valve 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. the solution:

①Improve the material hardness of the guide part;

② Increase the middle size of the upper and lower balls of the valve core to make it thick;

③Select other valves instead. If a sleeve valve is used, the fluid flows in from around the sleeve, and the lateral thrust on the valve plug is greatly reduced.

 

10
   Method to overcome the rotational force generated by fluid to rotate the valve core
 

For the valve core with a "V" shaped port, due to the asymmetry of the inflow of the medium, the tangential force of the valve core acting on the "V" shaped port is inconsistent, resulting in a rotational force that causes it to rotate. 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. The solutions are:

① Turn 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;

⑤ If the rotation is caused by resonance, eliminating the resonance can solve the problem.