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Summary of common problems of regulating valve and selection principle of regulating valve

The regulating valve, also known as the control valve, changes the fluid flow by means of power operation by receiving the control signal output by the regulating control unit. The regulating valve generally consists of an actuator and a valve. According to the power used by the actuator, the control valve can be divided into three types: pneumatic control valve, electric control valve and hydraulic control valve. In addition, according to their functions and characteristics, linear characteristics, equal percentage characteristics and parabolic characteristics are three kind.

What are the common problems of regulating valve?
1. Is it easy to oscillate when the double-seat control valve is in the working state of small opening?
The reason why the double-seat regulating valve cannot be used with a small opening is determined by its valve seat structure. The double-seat control valve has two spools, the upper spool is open, the lower spool is closed, and the spool of the control valve is open, the stability is better, so when the spool of the double-seat control valve is close to the flow When working in the small opening range of the closed state, the spool will oscillate. Therefore, in the selection of the control valve, the use of the double-seat control valve in the small opening range should be avoided.

2. Can the double-seat regulating valve be used as a shut-off valve?
The valve core of the double-seat control valve has the advantage of a force-balanced structure, which allows a wide range of pressure difference before and after the valve. Since the two sealing surfaces cannot be in good contact at the same time, the leakage is large and cannot be cut off in the pipeline. , Even if the sleeve improvement is now added to the double seat valve, it cannot change the limitations caused by its structural design.

3. Comparison of anti-blocking performance between straight stroke regulating valve and angular stroke valve?
The spool of the straight-stroke control valve moves vertically, the fluid moves horizontally, and the flow channel structure in the valve cavity is not a straight-through type, generally in an S shape, which deforms into many blind spots. Precipitation, and then easy to cause blockage like. The spool of the angular stroke valve is in horizontal rotation, and the medium is also in the horizontal flow direction, so it is not easy to have a dead angle, and the sediment is easily taken away by the fluid. Therefore, the anti-blocking performance of the straight stroke regulating valve is much better than that of the quarter-turn valve.

4. What is the reason for the thin stem of the straight-stroke regulating valve?
Due to the principle of large sliding friction and small rolling friction. The valve stem of the straight stroke valve slides up and down, which generates a large friction force. Therefore, the valve stem is designed to be small, and the tetrafluoro packing with small friction coefficient is used to reduce the friction force. However, the valve stem is thin, easy to bend, and the packing life is short.

5. Is the cut-off pressure difference of the angular stroke valve large?
Since the resultant force generated by the medium acting on the spool has a very small torque on the valve shaft, it can withstand a large pressure difference, so the cut-off pressure difference of the angular stroke valve is large.

6. Why should the shut-off valve use hard seals as much as possible?
Only in terms of cutting effect, soft-sealed valves have better performance than hard-sealed valves. However, the wear resistance and reliability of soft-sealed valves are much worse than that of hard-sealed valves. Therefore, in terms of double standards of leakage and reliability, hard-sealed shut-off valves are better than soft-sealed shut-off valves. Many shut-off valves now use surfacing wear-resistant alloys, which greatly improves the sealing performance and is sufficient to meet the requirements of shut-off valves.

Selection principle of regulating valve
1. According to the process conditions, select the appropriate structural form and material.

2. According to the characteristics of the process object, select the flow characteristics of the regulating valve.

3. According to the process operation parameters, select the appropriate diameter of the regulating valve.

4. According to the requirements of the process, select the required auxiliary devices.

5. Reasonable selection of implementing agencies. The response speed of the actuator should be able to meet the process

For the requirements of controlling the travel time, the selected control valve actuator should be able to meet the requirements of the valve travel and process for the leakage level. In some occasions, if the pressure regulating valve (including the vent valve) is selected, the actual possible pressure difference should be considered for appropriate amplification, that is, the actuator is required to provide a larger force. Otherwise, when there is an abnormal situation in the process, the actual pressure difference between the front and rear of the regulating valve is large, and there is a danger that the valve cannot be closed or opened.

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