Valve frequently asked questions 9 questions 9 answers
1. Why is the double seat valve easy to oscillate when working with a small opening? the
For a single core, when the medium is a flow-open type, the valve stability is good; when the medium is a flow-close type, the valve stability is poor. The double-seat valve has two spools, the lower spool is closed and the upper spool is open. In this way, when working at a small opening, the flow-closed spool will easily cause the valve to vibrate. This is the 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
The advantage of the double-seat valve core is the force balance structure, which allows a large pressure difference, but its outstanding disadvantage is that the two sealing surfaces cannot be in good contact at the same time, resulting in large leakage. If it is artificially and forcibly used in cut-off occasions, the effect is obviously not good, even if many improvements (such as double-sealed sleeve valves) have been made for it, it is not advisable.
3. Which straight-stroke regulating valve has poor anti-blocking performance, and quarter-stroke valve has good anti-blocking performance? the
The spool of the straight-stroke valve is throttled vertically, while the medium flows in and out horizontally. The flow path in the valve cavity must turn and turn backwards, which makes 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, and if things go on like this, it will cause blockage. The throttling direction of the quarter-turn valve is the horizontal direction. The medium flows in horizontally and flows out horizontally. It is easy to 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 valve stem of the straight stroke regulating valve thin? the
It involves a simple mechanical principle: the greater the sliding friction, the less rolling friction. The stem of straight stroke valve moves up and down. If the stuffing slightly pressed tighter, it would wrap up the stem tightly, resulting in a large hysteresis. To this end, the valve stem is designed to be thin and small and the stuffing use PTFE that has a small friction coefficient in order to reduce the hysteresis. But the problem that derives from this is that a thin valve stem is easy to bend and the stuffing has a short service life. To solve this problem, the best way is to use rotary valve stem, a regulating valve similar to the rotary stroke ones. Its valve stem is 2 to 3 times thicker than that of the straight stroke valve, and the graphite packing with long service life is used to improve the stiffness of the valve stem. Well, the packing has a long service life, but its friction torque is small and the hysteresis is small.
5. Why is the cut-off pressure difference of quarter-turn valves large? the
Quarter-turn valves have a large cut-off pressure difference because the resultant force generated by the medium on the valve core or valve plate produces a very small moment on the rotating shaft, so it can withstand a large pressure difference.
6. Why do rubber-lined butterfly valves and fluorine-lined diaphragm valves have a short service life for desalinated water? the
The desalinated water medium contains low concentrations of acid or alkali, which are highly corrosive to rubber. Corrosion of rubber is manifested as expansion, aging, and low strength. Butterfly valves and diaphragm valves lined with rubber are poor in use. The essence is that the rubber is not resistant to corrosion. The rear rubber-lined diaphragm valve was improved to a fluorine-lined diaphragm valve with good corrosion resistance, but 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 5-8 years.
7. Why should the cut-off valve be hard-sealed as much as possible? the
The cut-off valve requires the lower the leakage, the better. The leakage of the soft-sealed valve is the lowest. Of course, the cut-off effect is good, but it is not wear-resistant and has poor reliability. Judging from the double standards of small leakage and reliable sealing, soft seal cut-off is not as good as hard seal cut-off. For example, the full-featured ultra-light regulating valve is sealed and protected by wear-resistant alloys, with high reliability and a leakage rate of 10-7, which can already meet the requirements of shut-off valves.
8. Why did sleeve valves replace single and double seat valves fail? the
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 valves. Seat valve became the second generation product. Today, 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 improves the throttling form, and its stability and maintenance are better than that of the single-seat valve, but its weight, anti-blocking and leakage indicators are consistent with the single- and double-seat valves. How can it replace the single- and double-seat valves? Woolen cloth? Therefore, it can only be used together.
9. Why is model selection more important than calculation? the
Compared with calculation and selection, selection is much more important and more complicated. Because the calculation is just a simple formula calculation, it does not lie in the accuracy of the formula itself, but in whether the given process parameters are accurate. Model selection involves a lot of content, and a little carelessness will lead to improper selection, which will not only cause waste of manpower, material resources, and financial resources, but also cause unsatisfactory use results, which will bring some problems in use, such as reliability, life, operation quality etc.
Disclaimer: This article is reproduced from the Internet. The content of the article is for learning and communication purposes only. Air Compressor Network remains neutral to the views in the article. The copyright of the article belongs to the original author and the platform. If there is any infringement, please contact to delete