10 problems often encountered in the use of valves

1. Why is the double seat valve easy to oscillate when it works with small opening? For single core, when the medium is flow open type, the stability of the valve is good; when the medium is flow closed type, the stability of the valve is poor. The double seat valve has two valve cores. The lower valve core is in the flow closed position and the upper valve core is in the flow open position. In this way, when the valve core is in the flow closed position, it is easy to cause the vibration of the valve, which is the reason why the double seat valve cannot be used in the small open position.

2. Why can't double seal valve be used as cut-off valve? The advantages of double seat valve element are force balance structure and large allowable pressure difference, while its outstanding disadvantage is that the two sealing surfaces cannot contact well at the same time, resulting in large leakage. If it is used artificially and compulsorily in the cut-off situation, obviously the effect is not good, even if many improvements (such as double seal sleeve valve) have been made for it, it is not desirable.

3. What direct stroke control valve has poor anti blocking performance and angle stroke valve has good anti blocking performance? The valve core of the straight stroke valve is vertically throttled, while the medium flows in and out horizontally. The flow passage in the valve chamber must turn and reverse, which makes the flow path of the valve quite complex (the shape is like inverted "s"). In this way,

4. Why is the stem of straight stroke regulating valve thin? It involves a simple mechanical principle: large sliding friction and small rolling friction. When the stem of the straight stroke valve moves up and down, and the packing is slightly compressed, it will pack the stem tightly, resulting in a large return difference. For this reason, the valve rod is designed to be very small, and the packing is often filled with Teflon with small friction coefficient, so as to reduce the backlash. However, the problem is that the valve rod is thin, easy to bend, and the packing life is short. To solve this problem, the best way is to use the travel valve stem, i.e. the angle stroke type regulating valve. Its stem is 2-3 times thicker than the straight stroke valve stem, and the graphite packing with long service life is selected. The rigidity of the stem is good, the packing life is long, the friction torque is small, and the backlash is small.

5. Why is the cut-off pressure difference of angle stroke valve large? The cut-off pressure difference of angle stroke type valve is large, because the resultant force of the medium on the valve core or valve plate has very small moment on the rotating shaft, so it can bear a large pressure difference.

6. Why does desalted water medium use rubber lined butterfly valve and fluorine lined diaphragm valve with short service life? The medium of desalted water contains low concentration of acid or alkali, which are corrosive to rubber. The corrosion of rubber is characterized by expansion, aging and low strength. The use effect of rubber lined butterfly valve and diaphragm valve is poor. The essence is that rubber is not resistant to corrosion. The rear rubber lined diaphragm valve is improved to be a fluorine lined diaphragm valve with good corrosion resistance. However, the diaphragm of the fluorine lined diaphragm valve can not withstand the folding up and down and is broken, which causes mechanical damage and shortens the service life of the valve. Now the best way is to use water treatment ball valve, which can be used for 5 to 8 years.

7. Why should the shut-off valve be hard sealed? The lower the leakage is, the better the cut-off valve requires. The leakage of the soft seal valve is the lowest. The cut-off effect is certainly good, but it is not wear-resistant and has poor reliability. According to the double standards of small leakage and reliable sealing, soft sealing is not as good as hard sealing. For example, the fully functional ultra light regulating valve, sealed and stacked with wear-resistant alloy protection, high reliability, leakage rate up to 10-7, has been able to meet the requirements of the shut-off valve.

8. Why the sleeve valve instead of single or double seat valve did not work out as expected? 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 and become the second generation of products. Up to now, it is not so. Single seat valve, double seat valve and sleeve valve are all used equally. 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 the single seat valve and the double seat valve. How can it replace the single seat valve and the double seat valve? Therefore, it 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 only a simple formula calculation, its own is not the accuracy of the formula, but the accuracy of the given process parameters. There are many contents involved in model selection. A little carelessness will lead to improper model selection, not only resulting in human, material and financial resources

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