1. Why can't double-sealed control valves be used as shut-off valves?
The advantage of the double-seated valve core is its force balance structure, which allows for a large differential pressure. However, its prominent disadvantage is that the two sealing surfaces cannot make good contact simultaneously, resulting in large leakage. If it is artificially and forcibly used for shut-off applications, the effect is obviously not good, even with many improvements (such as double-sealed sleeve valves), it is not advisable.
2. Why do double-seated control valves easily oscillate when working at small openings?
For a single core, the valve has good stability when the medium is flow-opening; the valve has poor stability when the medium is flow-closing. The double-seated valve has two valve cores, the lower valve core is flow-closing, and the upper valve core is flow-opening. Thus, when working at small openings, the flow-closing valve core easily causes valve vibration. This is why double-seated valves cannot be used for small opening operations.
3. Why do straight-stroke control valves have poor anti-clogging performance, while angular-stroke valves have good anti-clogging performance?
The valve core of a straight-stroke valve throttles vertically, while the medium flows in and out horizontally. The flow path inside the valve cavity inevitably turns and bends, making the valve's flow path quite complex (shaped like an inverted S). Thus, there are many dead zones, providing space for medium sedimentation, which leads to blockage over time. The throttling direction of the angular-stroke valve is horizontal. The medium flows in and out horizontally, easily carrying away unclean media. At the same time, the flow path is simple, and there is little space for medium sedimentation, so the angular-stroke valve has good anti-clogging performance.
4. Why do angular stroke valves have a larger shut-off differential pressure?
Angular stroke valves have a larger shut-off differential pressure because the resultant force generated by the medium on the valve core or plate produces a very small torque on the rotating shaft. Therefore, it can withstand a larger differential pressure.
5. Why are the valve stems of straight-stroke control valves thinner?
It involves a simple mechanical principle: sliding friction is large, and rolling friction is small. The valve stem of a straight-stroke valve moves up and down. If the packing is slightly tightened, it will wrap the valve stem very tightly, producing a large hysteresis. For this reason, the valve stem is designed to be very thin, and Teflon packing with a small friction coefficient is often used to reduce hysteresis. However, the problem arising from this is that a thin valve stem is prone to bending, and the packing life is also short. The best way to solve this problem is to use a rotary valve stem, i.e., an angular stroke control valve. Its valve stem is 2 to 3 times thicker than a straight-stroke valve stem, and it uses long-life graphite packing. The valve stem has good rigidity, and the packing has a long life. Its frictional torque is small, and the hysteresis is small.
6. Why do rubber-lined butterfly valves and fluorine-lined diaphragm valves have a short service life when used with demineralized water media?
Demineralized water media contains low concentrations of acids or alkalis, which have a greater 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 has poor results because the rubber is not corrosion-resistant. Later, the 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 cannot withstand being folded up and down and is broken, causing mechanical damage and shortening the valve's life. Now the best solution is to use a water treatment special ball valve, which can be used for 5 to 8 years.
7. Why should shut-off valves preferably use hard seals?
Shut-off valves require the lowest possible leakage. Soft-sealed valves have the lowest leakage, and the shut-off effect is of course good, but they are not wear-resistant and have poor reliability. From the dual standards of small leakage and reliable sealing, soft-sealed shut-off is not as good as hard-sealed shut-off. For example, the sealing of a full-function ultra-light control valve is protected by a wear-resistant alloy, has high reliability, and a leakage rate of 10-7, which can already meet the requirements of a shut-off valve.
