Working principle of self-operated regulating valve, 10 common problems of self-operated regulating valve

- Oct 17, 2019-

The self-operated regulating valve is also called a self-operated control valve. It is a new type of regulating valve. As the name suggests, it does not require an external power supply and a secondary instrument. It relies on the pressure and temperature of the medium flowing through the valve as the energy to drive the valve to work automatically. The feedback signal (pressure, differential pressure, temperature) at the output of the valve is transmitted to the actuator through the signal tube, and the valve flap is driven to change the opening of the valve to achieve the purpose of regulating pressure, flow and temperature.


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According to the structural function, the self-operated regulating valve can be generally divided into: self-operated temperature regulating valve, self-operated pressure regulating valve, self-operated flow regulating valve, etc., which can be applied to most fluid media for automatic adjustment. It can effectively convert the energy energy of the fluid medium into a driving force, thereby controlling the valve to open and close.


Working principle of self-operated regulating valve:


When the medium fluid flows from the front of the valve through the spool seat, it is converted to post-valve pressure. Then, through the pipeline input upper chamber acts on the top tray, and the force generated will be opposite to the reaction force of the spring. This determines the relative position of the valve seat and thus the post-valve pressure. When the pressure behind the valve increases, the force acting on the top plate also increases, so that the valve core is closed to the position of the valve seat, so that the interval between the valve core and the valve seat is reduced, and the flow resistance becomes larger. reduce. Until the force on the top plate is balanced with the spring reaction force, the post-valve pressure drops to a preset value. When the pressure behind the valve is lowered, the direction of action is opposite to that previously mentioned. This is the working process of the self-regulating valve.


For the traditional control valve, the self-operated regulating valve does not require external energy. The output signal of the regulated medium can effectively adjust the properties of the fluid medium, which not only saves a lot of extra accessories, but also reduces the cost. The use of energy caters to the national call for energy conservation and emission reduction.


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Top 10 common problems with self-regulating valves:


First, why the self-operated regulating valve is easy to oscillate when working at a small opening. For a single core, when the medium is flow-open type, the valve stability is good; when the medium is flow-closed, the stability of the valve is poor. The double seat valve has two spools, the lower spool is closed, and the upper spool is open. Thus, when working at a small opening, the flow-closed spool is liable to cause vibration of the valve. This is a double seat valve. Can not be used for small opening work.


Second, why the double-sealed valve can not be used as a shut-off valve The advantage of using a double-seat valve spool is that the force balance double-seal valve structure allows a large pressure difference, and its outstanding disadvantage is that the two sealing faces cannot be in good contact at the same time, resulting in large leakage. . If it is artificially and forcibly used to cut off the occasion, it is obviously not effective, even if it has made many improvements (such as double-sealed sleeve valves), it is not advisable.


Third, what is the straight-stroke regulating valve anti-blocking performance, the anti-blocking performance of the angular stroke valve is good. The straight-stroke valve spool is vertical throttling, and the medium is horizontally flowing in and out, the flow passage in the valve cavity must turn and turn, making the valve The flow path becomes quite complicated (the shape is like the inverted 'S' type). Thus, there are many dead zones that provide space for the precipitation of the medium, which in the long run causes blockages. The direction of the angular stroke valve throttling is the horizontal direction. The medium flows horizontally and horizontally, which makes it easy to take away the dirty medium. At the same time, the flow path is simple, and the space for the medium to precipitate is also small, so the anti-blocking performance of the angular stroke valve is good.


Fourth, why the straight stroke control valve stem is thin It involves a simple mechanical principle: large sliding friction, small rolling friction. The valve stem of the straight stroke valve moves up and down, and the packing is slightly pressed tightly, which will tightly seal the valve stem and produce a large backlash. For this reason, the valve stem is designed to be very small, and the filler is also commonly used with a small friction coefficient of tetrafluorofiller to reduce the backlash, but the problem is that the valve stem is thin, the bend is easy, and the filler life is short. The best way to solve this problem is to use the travel valve stem, that is, the angular stroke type regulating valve. Its valve stem is 2 to 3 times thicker than the straight stroke valve stem, and the graphite packing with long life is selected. Well, the filler has a long service life, and the friction torque is small and the backlash is small.


5. Why is the cut-off pressure difference of the angular stroke type valve larger? The cut-off pressure difference of the angular stroke type valve is large because the resultant force of the medium on the valve core or the valve plate generates a very small torque to the rotating shaft, and therefore, it can withstand a large pressure difference.


6. Why does the demineralized water medium use a rubber-lined butterfly valve and a lining fluorine diaphragm valve with a short service life? Demineralized media contain low concentrations of acids or bases which are highly corrosive to rubber. The corrosion of rubber is characterized by expansion, aging, and low strength. The use of rubber-lined butterfly valves and diaphragm valves is inferior in that the rubber is not resistant to corrosion. The back-lined rubber diaphragm valve is improved to a fluorine-lined diaphragm valve with good corrosion resistance, but the diaphragm of the fluorine-lined diaphragm valve can be broken by being folded up and down, causing mechanical damage and shortening the life of the valve. The best way to do this now is to use a special ball valve for water treatment, which can be used for 5 to 8 years.


7. Why should the shut-off valve be as hard as possible? The cut-off valve requires the lower the leak, the better, the leakage of the soft-sealed valve is the lowest, the cutting effect is of course good, but it is not wear-resistant and the reliability is poor. From the double standard of small leakage, sealed and reliable, the soft seal cut is not as good as the hard seal cut. Such as full-featured ultra-light regulating valve, sealed and protected by wear-resistant alloy, high reliability, leakage rate of 10 to 7, has been able to meet the requirements of the shut-off valve.


8. Why did the sleeve valve replace the single and double seat valve but did not have the sleeve valve that was introduced in the 1960s? It was widely used at home and abroad in the 1970s. In the petrochemical plant introduced in the 1980s, the ratio of the sleeve valve was large. At the time, many people believe that the sleeve valve can replace the single and double seat valve and become the second generation product. This is not the case today, single-seat valves, double-seat valves, and sleeve valves are equally used. This is because the sleeve valve only improves the throttling form, stability and maintenance better than the single seat valve, but its weight, anti-blocking and leakage indicators are consistent with single and double seat valves. How can it replace single and double seat valves? What? Therefore, they can only be used together.


9. Why is selection more important than calculation? Compared with selection, selection is much more important and complicated. Because the calculation is just a simple formula calculation, it is not in itself the accuracy of the formula, but whether the given process parameters are accurate. There are many contents involved in the selection. If you are not careful, it will lead to improper selection, which not only causes waste of manpower, material resources and financial resources, but also has unsatisfactory effect on use, which brings about some use problems such as reliability, longevity and operation. Quality and so on.


10. Why is the piston actuator used more and more in the self-operated control valve? For the pneumatic valve, the piston actuator can make full use of the air source pressure, so that the size of the actuator is smaller than the diaphragm type, and the thrust is larger. The O-ring in the piston is also more reliable than the film, so it will be used more and more.


Self-operated regulating valve anti-blocking anti-card six methods:


1. Washing method: The welding slag, rust, slag, etc. in the pipeline cause blockage or jam in the throttle hole, the guiding part, and the balance hole of the lower bonnet, causing strain and scratches on the curved surface of the valve core and the guide surface. . In this case, it must be unloaded for cleaning, remove the slag, if the sealing surface is damaged, it should be ground; at the same time, the bottom plug is opened to wash off the slag falling into the lower bonnet from the balance hole, and the pipeline Rinse. Before commissioning, let the regulating valve open completely, and the medium will flow into the normal operation after a period of time.


2, medium flushing method: the use of the medium's own flushing energy, scouring and taking away things that are easy to precipitate, easy to block, thereby improving the valve's anti-blocking function. Common methods are:


a changed to flow closed type;


b using a streamlined valve body;


c Place the orifice at the most scouring point. Use this method to pay attention to improve the erosion resistance of the throttle.


3, straight through the angle method: straight through the inverted S flow, the flow path is complex, the upper and lower chambers have more dead zones, providing a place for the precipitation of the medium. The angular connection, the medium is like flowing through the 90 °C elbow, the scouring performance is good, the dead zone is small, and it is easy to design into a streamline shape.


4. Increase the throttling gap method: the solid particles in the medium or the slag and rust that are washed away in the pipeline can not be blocked or stuck due to the throttle, and the section with large throttle gap can be used instead. Flow element—The throttle area is a window opening, an open type valve core, and a sleeve. Since the throttle area is concentrated rather than circumferentially distributed, the fault can be easily eliminated. If it is a single or double seat valve, the plunger valve core can be changed to a "V" shaped valve core, or changed to a sleeve valve.


5. External scouring method: When some mediums that are easy to precipitate and contain solid particles are adjusted by ordinary valves, they are often blocked at the throttle and guide, and the gas and steam can be flushed at the bottom of the lower bonnet. When the regulating valve is blocked or jammed, open the external gas or steam valve to complete the flushing operation without moving the regulating valve to make the valve operate normally.


6. Installation of pipeline filter method: For small diameter regulating valve, especially ultra-small flow regulating valve, the throttle gap is very small, even if there is a little bit of slag in the medium will be blocked, it is best to install a pipe on the front of the valve. Filter to ensure the smooth passage of the media. With the regulating valve used by the positioner, the positioner does not work properly, and the blockage of the air passage throttle is the most common fault.