Why can't a double-seat control valve be used as a shut-off valve?
The advantage of a double-seat valve is its force-balanced structure, allowing for a large pressure differential. However, its significant disadvantage is that the two sealing surfaces cannot simultaneously make good contact, resulting in significant leakage. Forcing it into shut-off applications is obviously ineffective, even with improvements (such as double-seat sleeve valves).
Why are double-seat control valves prone to oscillation at small openings?
For single-seat valves, stability is good when the medium is flow-open; stability is poor when the medium is flow-closed. A double-seat valve has two valve cores, the lower core in a flow-closed position and the upper core in a flow-open position. Therefore, at small openings, the flow-closed core easily causes valve vibration, which is why double-seat valves cannot be used for small openings.
Why do linear control valves have poor anti-clogging performance, while rotary control valves have good anti-clogging performance?
In a linear stroke valve, the valve core throttles vertically, while the medium flows in and out horizontally. This inevitably leads to bends and turns in the flow path within the valve cavity, making the flow path quite complex (shaped like an inverted S). This creates many dead zones, providing space for medium sedimentation, which can eventually cause blockages. Angle stroke valves, on the other hand, throttle horizontally. The medium flows in and out horizontally, easily carrying away impurities. Simultaneously, the flow path is simpler, and there is less space for medium sedimentation, resulting in better anti-clogging performance.
Why do angle stroke valves have a larger shut-off pressure differential?
Angle stroke valves have a larger shut-off pressure differential because the resultant force generated by the medium on the valve core or valve plate produces a very small torque on the rotating shaft. Therefore, they can withstand a larger pressure differential.
Why is the valve stem of a linear stroke control valve thinner?
This involves a simple mechanical principle: sliding friction is high, rolling friction is low. In a linear stroke valve, the valve stem moves up and down. Even a slight tightening of the packing will cause the valve stem to be tightly wrapped, resulting in a larger hysteresis. Therefore, the valve stem is designed to be very thin, and PTFE packing with a low coefficient of friction is commonly used to reduce hysteresis. However, this results in a thin stem that is prone to bending, and the packing life is also short. The best solution to this problem is to use a rotary valve stem, i.e., an angular stroke control valve. Its stem is 2 to 3 times thicker than that of a linear stroke valve, and it uses long-life graphite packing, resulting in good stem rigidity and long packing life.
What is a hard seal?
Shut-off valves require the lowest possible leakage. Soft-seal valves have the lowest leakage, and their shut-off effect is excellent, but they are not wear-resistant and have poor reliability. Considering both low leakage and reliable sealing, soft-seal shut-off is not as good as hard-seal shut-off. For example, a full-function ultra-lightweight control valve, with a seal protected by a wear-resistant alloy, has high reliability and a leakage rate of 10-7, which already meets the requirements of a shut-off valve.
