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Why do two-seat valves tend to oscillate when operating at a small opening degree? For single-core valves, when the medium flows in an open direction, the valve maintains good stability； When the medium is of the flow-closed type, the valve has poor stability. A two-seat valve has two spools: the lower spool is in a flow-blocking position while the upper spool is in a flow-allowing position. As a result, when operating at low opening degrees, the flow-blocking spool can easily cause vibration in the valve, which is why two-seat valves cannot be used at low opening degrees. Why cannot double-seal valves be used as shut-off valves? The advantage of a double-seat valve’s spool is its force-balanced design, which allows for high pressure differences; however, its major drawback is that the two sealing surfaces cannot make good contact at the same time, resulting in significant leakage. If it is artificially and forcibly used for cutting off flow, the results are obviously poor, and it is not advisable even with many improvements made to it (such as double-sealed sleeve valves). Straight-stroke control valves have poor anti-clogging performance, while angle-stroke valves have good anti-clogging performance. In straight-stroke valves, the valve element performs vertical throttling, whereas the medium flows in and out horizontally; as a result, the flow channels within the valve chamber must make turns, making the flow path of the valve quite complex (with a shape similar to an inverted “S”). As a result, there are many dead zones that provide space for the medium to settle, which over time leads to blockages. The throttling direction of a rotary valve is horizontal; the fluid flows in horizontally and exits horizontally, which makes it easy to carry away any impurities. Additionally, the flow path is simple, and there is little space for the fluid to settle, so rotary valves have good anti-clogging properties. Why is the stem of a straight-stroke control valve relatively thin? It relates to a simple mechanical principle: sliding friction is high while rolling friction is low. In a straight-stroke valve, as the valve stem moves up and down, the packing becomes slightly compressed, which causes it to wrap tightly around the valve stem and results in a larger backset. To this end, the valve stem is designed to be very thin, and PTFE packing with a low friction coefficient is often used to reduce backlash; however, the problem arising from this is that a thin valve stem is prone to bending, and the packing also has a short lifespan. The best way to solve this problem is to use a travel valve stem, that is, a control valve of the angle-type. Its stem is 2 to 3 times thicker than that of a linear-type valve stem, and graphite packing with a long service life is used; the stem has good stiffness, the packing lasts longer, which results in lower friction torque and less hysteresis. Why do angle-action valves have a high shut-off pressure difference? The reason why angle-action valves have a high shut-off pressure difference is that the resultant force exerted by the medium on the valve core or valve plate generates a very small torque on the rotating shaft; as a result, such valves can withstand high pressure differences. Why do rubber-lined butterfly valves and fluorine-lined diaphragm valves have a short service life when used with desalinated water media? The desalinated water medium contains low concentrations of acids or bases, which are highly corrosive to rubber. The corrosion of rubber is manifested as swelling, aging, and low strength; the poor performance of butterfly valves and diaphragm valves lined with rubber is essentially due to the fact that rubber is not resistant to corrosion. The back-gasket diaphragm valve was improved to a fluorinated diaphragm valve with better corrosion resistance, but the diaphragm of this fluorinated diaphragm valve cannot withstand being folded up and down and thus gets damaged, leading to mechanical failure and a reduced service life of the valve. The best solution now is to use a special ball valve for water treatment, which can last for 5 to 8 years. Why hard-sealed valves should be preferred for cut-off valves Cut-off valves require as low a leakage rate as possible. Soft-sealed valves have the lowest leakage rates, so their cut-off performance is good; however, they are not wear-resistant and have poor reliability. From the dual criteria of low leakage volume and reliable sealing, soft-seal cutoff is not as good as hard-seal cutoff. Such as fully functional ultra-lightweight control valves, which are sealed and protected by wear-resistant alloys, offering high reliability with a leakage rate of 10–7; they are already sufficient to meet the requirements of shut-off valves. Why haven’t sleeve valves succeeded in replacing single- and double-seat valves? Introduced in the 1960s, sleeve valves were widely used at home and abroad in the 1970s, and they accounted for a large proportion in the petrochemical plants built in the 1980s. At that time, many people believed that sleeve valves could replace single- and double-seat valves and become the next generation of valve products. These days, that is not the case; single-seat valves, double-seat valves, and sleeve valves are all used equally. This is because the sleeve valve only improves the throttling mechanism, stability, and maintainability compared to single-seat valves; however, its weight, clogging resistance, and leakage performance are on par with those of single-seat and double-seat valves. How can it then replace them? So, it can only be used together. Why is selection more important than calculation? Compared to calculation, selection is far more important and complex. Because the calculation is merely a simple formula-based process; what matters is not the accuracy of the formula itself, but whether the given process parameters are accurate. The process of selection involves many factors; even the slightest carelessness can lead to an inappropriate choice. This not only results in waste of human, material, and financial resources but also leads to suboptimal performance, causing various issues related to use such as reliability, lifespan, and operational quality.