October 14, 2024
For a single-seated valve, when the medium is in a flow-to-open configuration, the valve is stable; however, in a flow-to-close configuration, the valve's stability is compromised. Double-seated valves have two plugs: the lower plug operates in a flow-to-close configuration, while the upper plug operates in a flow-to-open configuration. At small openings, the flow-to-close plug tends to induce oscillations in the valve, which is why the upper seat is not suitable for small opening operations.
The advantage of a double-seated valve is its balanced force structure, which allows for large differential pressure. However, the downside is that the two sealing surfaces cannot make contact simultaneously, resulting in significant leakage. If used forcibly as a shutoff valve, it would be ineffective. Even with modifications (e.g., double-sealed sleeve valves), it remains unsuitable.
The plug in a linear valve moves vertically for throttling, while the medium flows in and out horizontally. The flow path inside the valve body is complex (shaped like an inverted "S"), which creates dead zones where medium sediment can accumulate, eventually leading to clogging. In contrast, rotary valves throttle in a horizontal direction, allowing the medium to flow in and out horizontally, which can easily carry away impurities. The simpler flow path and fewer areas for sediment accumulation make rotary valves better at preventing clogging.
The design is based on a simple mechanical principle: sliding friction is greater than rolling friction. In linear control valves, the stem moves up and down. If the packing is tightened slightly, it grips the stem tightly, causing significant backlash. To reduce backlash, the stem is designed to be very thin, and low-friction materials such as PTFE are used for the packing. However, this also means the stem is more prone to bending and the packing has a shorter lifespan. To solve this issue, rotary valve stems are used, which are 2-3 times thicker than linear valve stems. Long-lasting materials like asbestos are used for the packing, providing better rigidity and longer packing life with lower friction and minimal backlash.
The higher differential pressure for shutoff in rotary valves is due to the small resultant force generated by the medium on the valve plug or disk, which minimizes the torque on the rotating shaft. Thus, rotary valves can withstand higher differential pressures.
Desalinated water contains low concentrations of acids or alkalis that can corrode rubber, causing swelling, aging, and reduced strength. Both lined butterfly valves and diaphragm valves exhibit poor performance due to the rubber's susceptibility to corrosion. Although lined diaphragm valves have been improved with corrosion-resistant materials, such as fluorine linings, the diaphragms still tend to crack under repetitive bending, shortening the valve's lifespan. Currently, water treatment-specific ball valves are used, which can last 5-8 years.
Sleeve valves, introduced in the 1960s and widely used in the 1970s, were seen as potential replacements for single and double-seated valves. However, single, double-seated, and sleeve valves continue to coexist. Although sleeve valves offer improvements in throttling, stability, and maintenance, they share similar weight, anti-clogging, and leakage characteristics with single and double-seated valves, thus failing to fully replace them.
Shutoff valves require minimal leakage, and although soft sealing offers good shutoff performance, it is not durable and less reliable. Hard sealing, such as in lightweight all-purpose control valves with wear-resistant alloy protected sealing surfaces, provides higher reliability and lower leakage, making it more suitable for shutoff applications.
Compared to calculations, valve selection is more important and complex because calculations involve only simple formulas, while selection depends on the accuracy of the process parameters. Selection requires consideration of various factors, and mistakes can lead to improper selection, resulting in wasted resources and suboptimal performance, including issues with reliability, lifespan, and operational quality.
For pneumatic valves, piston actuators can fully utilize the air supply pressure, resulting in a more compact size and higher thrust compared to diaphragm actuators. The O-rings in piston actuators are also more reliable than diaphragms, leading to their growing popularity.
Keypoint： Control valves, Pneumatic Valves, Single and Double-Seated Valves
