9 Questions and Answers on Common Valve Problems

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1. Why do two-seat valves tend to oscillate when operating at a small opening? 　　For single-core systems, when the dielectric is of the flow-open type, the valve exhibits 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. 2. Why can’t double-seal valves be used as shut-off valves? 　　The advantage of the double-seat valve stem is its force-balanced structure, which allows for a large pressure difference. However, its major drawback is that the two sealing surfaces cannot make good contact simultaneously, resulting in significant leakage. If it is artificially and forcibly used for cutting off flows, the results are obviously poor; even with many improvements made to it (such as double-sealed sleeve valves), it is not a viable option. 3. Which straight-stroke control valves have poor anti-clogging performance, and which angle-stroke valves have good anti-clogging performance? 　　In a straight-stroke valve, the valve core performs vertical throttling, while the medium flows in and out horizontally; as a result, the flow channels within the valve chamber have to 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 enters horizontally and exits horizontally, which makes it easy to carry away any impure substances. Additionally, the flow path is simple, and there is little space for the fluid to settle, so rotary valves have good anti-clogging properties. 4. Why is the stem of a straight-stroke control valve relatively thin? 　　It involves 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 commonly used to reduce backlash; however, the problem that arises 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 pilot-operated valve stem, that is, a control valve of the angle-type. Its stem is 2 to 3 times thicker than that of a straight-stroke valve, and graphite packing with a long service life is used; the stem has good stiffness, the packing lasts longer, resulting in lower friction torque and less hysteresis. 5. Why do angle-type valves have a relatively high shut-off pressure difference? 　　The cut-off pressure difference of angle-action valves is relatively high, as the resultant force exerted by the medium on the valve core or valve plate generates 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 used in desalinated water media have a short service life? 　　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 fluorine-lined diaphragm valve with better corrosion resistance, but the diaphragm of this fluorine-lined diaphragm valve cannot withstand folding up and down and gets damaged, resulting in mechanical failure and a reduced lifespan for the valve. The best solution now is to use a special ball valve for water treatment, which can last for 5 to 8 years. 7. Why should hard seals be preferred when selecting shut-off valves? 　　Cut-off valves require as low a leakage rate as possible; soft-sealed valves have the lowest leakage rate, which means they provide good cut-off performance, but they are not wear-resistant and have poor reliability. From the dual criteria of low leakage volume and reliable sealing, soft-seal cutoff is inferior to 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 capable of meeting the requirements of shut-off valves. 8. Why hasn’t the sleeve valve succeeded in replacing single- and double-seat valves as expected? 　　The sleeve valve was introduced in the 1960s, saw widespread use at home and abroad in the 1970s, and accounted for a large proportion in the petrochemical plants installed in the 1980s. At that time, many people believed that the sleeve valve could replace single- and double-seat valves and become the next generation of valves. 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? Therefore, they can only be used jointly. 9. Why is selection considered more important than calculation? 　　Compared to calculation, selection is much more important and much more complex. Since the calculation involves merely a simple formula, its effectiveness does not depend on the precision of the formula itself, but rather on 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 wasted resources in terms of labor, materials, and money, but also leads to suboptimal performance, causing various issues related to use, such as reliability, lifespan, and operational quality.