Ⅰ. Why can't double-sealed valves be used as shut-off valves? 

The advantage of the double-seat valve valve core is its balanced force structure, which allows for a large pressure difference. However, its prominent drawback is that the two sealing surfaces cannot simultaneously make good contact, resulting in significant leakage. If it is used in situations where cutting is artificially or forcibly required, the effect is obviously poor. Even if many improvements have been made to it (such as the double-sealing sleeve valve), it is still not advisable. 

II. Why does the double-seat valve tend to oscillate when operating at a small opening? 

For a single core, when the medium is of the open-flow type, the valve stability is good; when the medium is of the closed-flow type, the valve stability is poor. A double-seat valve has two valve cores. The lower core is in the closed-flow state, and the upper core is in the open-flow state. Thus, when working at a small opening degree, the closed-flow type valve core is more likely to cause the valve to vibrate. This is the reason why the double-seat valve cannot be used for small opening degree operations. 

III. The straight-shaft regulating valve has poor anti-blocking performance, while the angular-shaft valve has better anti-blocking performance. 

The valve core of a linear motion valve is vertically throttled, while the medium flows horizontally in and out. The flow channels inside the valve cavity must make turns and bends, making the valve's flow channels quite complex (shaped like an inverted "S"). As a result, there are many dead zones, providing space for the sedimentation of the medium. Over time, this causes blockages. In a rotary motion valve, the throttling direction is horizontal, with the medium flowing horizontally in and out. This easily removes unclean medium and the flow path is simple, with very little space for the medium to settle, so the rotary motion valve has good anti-blocking performance. 

IV. Why did the use of sleeve valves fail to replace single and double seat valves as expected? 

The sleeve valve, which was introduced in the 1960s, was widely used both domestically and internationally in the 1970s. In the petrochemical plants that were introduced in the 1980s, the proportion of sleeve valves was relatively large. At that time, many people believed that the sleeve valve could replace the single-seat and double-seat valves and become the second-generation product. Up to now, all three types of valves - single-seat valve, double-seat valve, and sleeve valve - are used equally. This is because the sleeve valve merely improved the throttling form, stability, and maintenance performance compared to the single-seat valve, but its weight, anti-blocking, and leakage indicators are consistent with those of the single and double-seat valves. How could it replace the single-seat and double-seat valves? Therefore, they can only be used together. 

V. Why is selection more important than calculation? 

Compared with calculation, selection is more important and much more complex. Because calculation is merely a simple formula calculation, its significance does not lie in the accuracy of the formula, but in whether the given process parameters are accurate. The content involved in selection is much more extensive. If not handled carefully, it may lead to improper selection, not only resulting in waste of manpower, material resources and financial resources, but also leading to unsatisfactory usage effects and some usage problems, such as reliability, lifespan, and operation quality. 

VI. Why do the service life of rubber-lined butterfly valves and fluorine-lined diaphragm valves used in desalinated water media be relatively short? 

The demineralized water medium contains low concentrations of acids or bases, which have a significant corrosive effect on rubber. The corrosion of rubber manifests as expansion, aging, and low strength. The use of rubber-lined butterfly valves and diaphragm valves results in poor performance due to the fact that the rubber is not corrosion-resistant. The improvement of the later rubber-lined diaphragm valve to a fluorine-lined diaphragm valve with better corrosion resistance did not solve the problem; instead, the membrane of the fluorine-lined diaphragm valve could not withstand repeated folding and was damaged, causing mechanical damage and shortening the valve's lifespan. Currently, the best solution is to use water treatment-specific ball valves, which can be used for 5 to 8 years. 

VII. Why should the shut-off valve be equipped with hard seals as much as possible? 

The cutting valve requires the lowest possible leakage. The leakage of the soft-sealing valve is the lowest. The cutting effect is of course good, but it is not wear-resistant and has poor reliability. From the dual criteria of low leakage and reliable sealing, the soft-sealing cutting is inferior to the hard-sealing cutting. For example, the fully functional ultra-lightweight regulating valve has a reliable seal and is protected by wear-resistant alloy, with a leakage rate of 10 to the power of -7, which can already meet the requirements of the cutting valve. 

VIII. Why is the stem of a linear motion control valve relatively thin? 

It involves a simple mechanical principle: sliding friction is greater while rolling friction is smaller. In a linear valve, the valve stem moves up and down. If the packing is slightly tightened, it will tightly enclose the valve stem, resulting in a large backlash. To address this, the valve stem is designed to be very small, and the packing commonly uses fluorine tetrafluoroethylene (PTFE) with a low friction coefficient to reduce the backlash. However, the problem arising from this is that the valve stem is thin and thus prone to bending, and the packing has a short lifespan. The best solution to this problem is to use rotary valve stems, that is, angle-travel type regulating valves. Their valve stems are 2 to 3 times thicker than those of linear valves, and they use long-lasting graphite packing. The valve stem has good rigidity and a long lifespan, and the small friction torque results in a smaller backlash. 

IX. Why is the cutting pressure difference of the angular motion type valve relatively large? 

The cutting pressure difference of the angular motion type valve is relatively large because the resultant force generated by the medium on the valve core or valve plate has a very small torque on the rotating axis. Therefore, it can withstand a larger pressure difference.