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Valve selection, maintenance and care! Chemical workers must know! 10 common problems encountered when using valves?

1. Why is the double-seat valve easy to oscillate when working with a small opening?

For a single core, when the medium is flow-open, the valve has good stability; when the medium is flow-closed, the valve has poor stability. The double-seat valve has two valve cores. The lower valve core is closed and the upper valve core is open. In this way, when working at a small opening, the closed-flow valve core will easily cause vibration of the valve. This is a double-seat valve. The reason why it cannot be used for small opening work.

2. Why can’t the double seal valve be used as a shut-off valve?

The advantage of the double-seat valve core is the force-balanced structure, which allows a large pressure difference. However, its outstanding disadvantage is that the two sealing surfaces cannot make good contact at the same time, causing large leakage. If it is used artificially and compulsorily in cutting-off situations, the effect will obviously not be good. Even if many improvements are made for it (such as double sealing sleeve valve), it is not advisable.

3. Which straight-stroke control valve has poor anti-clogging performance, while the angle-stroke valve has good anti-clogging performance?

The valve core of the straight stroke valve is a vertical throttling, while the medium flows in and out horizontally. The flow path in the valve cavity must turn and reverse, making the flow path of the valve quite complicated (the shape is like an inverted "S" shape). In this way, there are many dead zones, which provide space for the precipitation of the medium, which in the long run will cause blockage. The throttling direction of the quarter-turn valve is the horizontal direction. The medium flows in horizontally and flows out horizontally, which can easily take away the dirty medium. At the same time, the flow path is simple and there is little space for the medium to settle, so the quarter-turn valve has good anti-blocking performance.

4. Why is the stem of a straight stroke regulating valve thinner?

It involves a simple mechanical principle: large sliding friction and small rolling friction. The valve stem of a straight stroke valve moves up and down. If the packing is pressed a little tighter, it will wrap the valve stem very tightly, resulting in a large hysteresis. To this end, the valve stem is designed to be very small, and the packing often uses PTFE packing with a small friction coefficient in order to reduce the hysteresis. However, the problem derived from this is that a thin valve stem is easy to bend, and the packing life is also short. The best way to solve this problem is to use a rotary valve stem, that is, a quarter-stroke regulating valve. Its valve stem is 2 to 3 times thicker than a straight-stroke valve stem, and it uses graphite packing with long service life. The rod has good stiffness, long packing life, small friction moment and small hysteresis.
 5. Why is the cut-off pressure difference of quarter-turn valves larger?

The cut-off pressure difference of the quarter-turn valve is large because the resultant force generated by the medium on the valve core or valve plate exerts a very small torque on the rotating shaft. Therefore, it can withstand a large pressure difference.

6. Why do rubber-lined butterfly valves and fluorine-lined diaphragm valves have short service lives when used in salt water media?

**The salt water medium contains low concentrations of acid or alkali, which are highly corrosive to rubber. Corrosion of rubber manifests as expansion, aging, and low strength. Butterfly valves and diaphragm valves lined with rubber have poor performance. The essence is that rubber is not resistant to corrosion. The rear rubber-lined diaphragm valve was improved to a fluorine-lined diaphragm valve with good corrosion resistance. However, the diaphragm of the fluorine-lined diaphragm valve could not withstand folding up and down and was broken, causing mechanical damage and shortening the life of the valve.

The best way now is to use a special ball valve for water treatment, which can be used for 5 to 8 years.

7. Why should shut-off valves use hard seals as much as possible?

The leakage of the shut-off valve is required to be as low as possible. The leakage of the soft seal valve is absolute, and the shut-off effect is certainly good, but it is not wear-resistant and has poor reliability. From the dual standards of small leakage and reliable sealing, soft seal cutting is not as good as hard seal cutting. For example, the full-function ultra-light control valve is sealed and protected by wear-resistant alloy. It has high reliability and a leakage rate of 10-7, which can already meet the requirements of a shut-off valve.

8. Why did sleeve valves fail to replace single and double-seat valves?

Sleeve valves, which came out in the 1960s, were widely used at home and abroad in the 1970s. Sleeve valves accounted for a large proportion of petrochemical plants introduced in the 1980s. At that time, many people believed that sleeve valves could replace single and double-seat valves. , becoming a second-generation product. Until now, this is not the case. Single-seat valves, double-seat valves, and sleeve valves are all used equally. This is because the sleeve valve only has improved throttling form, stability and maintenance than single-seat valves, 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? Woolen cloth? Therefore, they can only be used together.

9. Why is selection more important than calculation?

Compared with calculation and selection, selection is much more important and complicated. Because the calculation is just a simple formula calculation, it does not depend on the accuracy of the formula, but on whether the given process parameters are accurate. Selection involves a lot of content, and if you are not careful, it will lead to improper selection, which will not only cause a waste of manpower, material resources, and financial resources, but also the use effect is not ideal, causing several use problems, such as reliability, lifespan, and operation. Quality etc.

10. Why are piston actuators used more and more in pneumatic valves?

For pneumatic valves, the piston actuator can make full use of the air source pressure, making the actuator smaller and larger than the diaphragm type. The O-ring in the piston is also more reliable than the diaphragm, so its use will become more and more popular. many.

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For a single core, when the medium is flow-open, the valve has good stability; when the medium is flow-closed, the valve has poor stability. The double-seat valve has two valve cores. The lower valve core is closed and the upper valve core is open. In this way, when working at a small opening, the closed-flow valve core will easily cause vibration of the valve. This is a double-seat valve. The reason why it cannot be used for small opening work.

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