01 Why are double-seat valves prone to oscillation when operating at small openings?
For single-seat valves, valve stability is good when the medium is flow-opening; it is poor when the medium is flow-closing. Double-seat valves have two valve cores: the lower valve core is in the flow-closing position, and the upper valve core is in the flow-opening position. Therefore, when operating at small openings, the flow-closing valve core is prone to causing valve vibration, which is why double-seat valves cannot be used for small openings.
02 Why can't double-seat valves be used as shut-off valves?
The advantage of a double-seat valve core is its balanced force structure, which allows for large pressure differentials. However, its significant disadvantage is that both sealing surfaces cannot maintain good contact at the same time, resulting in large leakage. If a double-seat valve is artificially and forcibly used for shut-off applications, the effect is obviously poor, even with numerous improvements (such as the double-seat sleeve valve).
03 Why do linear control valves have poor anti-clogging performance, while angular-turn valves have better anti-clogging performance?
The valve stem of a linear-stroke valve throttles vertically, while the medium flows in and out horizontally. The flow path within the valve cavity inevitably curves and bends, making the valve's flow path quite complex (e.g., an inverted "S" shape).
04 Why are linear-stroke control valve stems thinner?
This involves a simple mechanical principle: high sliding friction and low rolling friction. As the stem of a linear-stroke valve moves up and down, even slightly tightening the packing compresses it tightly, resulting in a large backlash. To address this, the stem is designed to be extremely thin, and the packing is often made of PTFE, a material with a low friction coefficient, to reduce backlash. However, this results in a thin stem that bends easily, shortening the packing's life. The best solution to this problem is to use a rotary valve stem, or a quarter-turn control valve. This stem is two to three times thicker than a linear-stroke stem, and uses long-lasting graphite packing. This results in greater stem rigidity and longer packing life, resulting in lower friction torque and smaller backlash.
05 Why do quarter-turn valves have a larger cutoff pressure differential?
Quarter-turn valves have a large cutoff pressure differential because the combined force of the medium on the valve core or disc creates a very small torque on the rotating shaft. Therefore, they can withstand a large pressure differential.
06 Does the service life of rubber-lined butterfly valves and fluorine-lined diaphragm valves shorten when used with desalted water?
Desalted water contains low concentrations of acid or alkali, which are highly corrosive to rubber. Rubber corrosion manifests itself as swelling, aging, and reduced strength. Rubber-lined butterfly valves and diaphragm valves perform poorly, essentially due to the rubber's lack of corrosion resistance. Later, rubber-lined diaphragm valves were upgraded to fluorine-lined diaphragm valves, which offer better corrosion resistance. However, the diaphragm of these fluorine-lined diaphragm valves cannot withstand the folding and can break, causing mechanical damage and shortening the valve's life. The best solution now is a ball valve designed for water treatment, which can last 5 to 8 years.
07 Why should hard seals be used for cutoff valves whenever possible?
Shutoff valves require the lowest possible leakage. Soft-seal valves offer the lowest leakage and provide excellent cutoff performance, but they are less wear-resistant and less reliable. From the dual perspectives of low leakage and reliable sealing, soft-seal shutoff valves are inferior to hard-seal shutoff valves. For example, full-function, ultra-lightweight control valves, which are sealed and protected by wear-resistant alloys, offer high reliability and a leakage rate of 10⁻⁷, which already meets the requirements of shutoff valves.
08. Did sleeve valves replace single- and double-seat valves, but they never lived up to expectations?
Sleeve valves, introduced in the 1960s, were widely used both domestically and internationally in the 1970s. Sleeve valves accounted for a significant proportion of petrochemical plants introduced in the 1980s. At that time, many believed that sleeve valves could replace single- and double-seat valves and become a second-generation product. Today, this is no longer the case. Single-, double-, and sleeve valves are all equally used. This is because sleeve valves only offer improved throttling, stability, and maintenance compared to single-seat valves. However, their weight, anti-clogging, and leakage performance are comparable to those of single- and double-seat valves. How could they replace single- and double-seat valves? Therefore, they can only be used together.
09. Why is selection more important than calculation? Compared to calculation and selection, selection is far more important and complex. Because calculation is simply a simple formula, its importance lies not in the accuracy of the formula itself but in the accuracy of the given process parameters.
10 Will Piston Actuators Be Used More and More in Pneumatic Valves?
For pneumatic valves, piston actuators can fully utilize the air source pressure, making the actuator smaller than diaphragm actuators and providing greater thrust. The O-rings in the piston are also more reliable than diaphragm actuators, leading to their increasing use.