Application of metal seal butterfly valve in cryogenic equipment

With the rapid advancement of industrial technology, the valve industry has faced increasingly stringent demands, especially for valves used in low-temperature environments. In particular, butterfly valves designed for cryogenic applications must not only meet general performance standards but also ensure reliable sealing at extremely low temperatures, maintain flexibility in operation, and satisfy other unique requirements specific to cryogenic systems. Despite their advantages—such as compact design, lightweight, and minimal fluid resistance—butterfly valves are still underutilized in many cryogenic applications. Currently, over 80% of valves used in critical cryogenic equipment like natural gas liquefaction units, air separation systems, and pressure swing adsorption devices in the chemical industry are gate or globe valves, with only a small percentage being butterfly valves. This is primarily due to the poor sealing performance of traditional metal-sealed butterfly valves at low temperatures, which can lead to internal leakage, structural deformation, and operational failures. These issues significantly impact the safety and efficiency of low-temperature systems, making it difficult to meet the growing demand for reliable cryogenic valves. To address these challenges, significant structural improvements have been made to metal-sealed butterfly valves. One notable innovation is the development of a high eccentricity butterfly valve, which has been patented in China. This design allows the valve to perform effectively across a wide range of temperatures, from high to low. Here’s a brief overview of its low-temperature performance: First, the sealing performance of low-temperature valves is crucial. There are two main causes of leakage: internal leakage and external leakage. 1) Internal leakage typically results from thermal contraction and deformation of the sealing surfaces. When the temperature drops, materials may undergo phase changes, leading to volume expansion or contraction. This can cause warping of the sealing surfaces, resulting in poor sealing. For example, during testing of a DN250 butterfly valve using liquid nitrogen (-196°C), the sealing surface of a 1Cr18Ni9Ti disc (not treated for low temperatures) deformed by approximately 0.12 mm, causing internal leakage. To solve this, a new design has been introduced, changing the seal from a flat to a conical shape. The valve seat features an elliptical conical sealing surface, while the disc is equipped with a flexible sealing ring that can move radially within the groove. As the valve closes, the elastic ring first contacts the minor axis of the elliptical surface and then gradually moves to the longer axis, ensuring full contact and effective sealing. This design compensates for any deformation caused by low temperatures, preventing leakage and jamming. Additionally, there is minimal friction during operation, contributing to a long service life. 2) External leakage often occurs at flange connections or around the stem packing. At low temperatures, materials such as gaskets and bolts may contract unevenly, leading to loosening and potential leaks. To mitigate this, the connection between the valve body and pipeline has been changed from flanged to welded, eliminating the risk of low-temperature leakage. Regarding stem packing, traditional PTFE (Teflon) is commonly used due to its low friction and chemical stability. However, PTFE tends to shrink at low temperatures, leading to leakage and ice buildup on the stem, which can prevent the valve from opening. To address this, a self-shrinking sealing structure has been developed, utilizing the high expansion coefficient of PTFE to maintain a tight seal even at low temperatures. In addition, the design of the valve body and stem bushing plays a critical role in low-temperature performance. The choice of materials is essential for ensuring reliable operation. For instance, 1Cr18Ni9Ti austenitic stainless steel, with its face-centered cubic crystal structure, is often selected for its excellent low-temperature properties. The compact size of butterfly valves also helps reduce heat loss and makes cooling more efficient. Moreover, some users have reported issues such as sticking or jamming in low-temperature valves, often due to improper material selection, insufficient cold clearance, or poor machining accuracy. To prevent these problems, advanced bearing materials like SF-1 composite bearings have been used in the upper and lower bushings of the stem, offering low friction and self-lubrication suitable for extreme conditions. In summary, modern metal-sealed butterfly valves now offer superior performance compared to conventional valves. They provide low flow resistance, reliable sealing, quick operation, and extended service life. The three eccentric design ensures that sealing is achieved through the deformation of an elastic ring rather than relying on media pressure, allowing for bidirectional sealing. With these advancements, butterfly valves are becoming increasingly popular in cryogenic applications, and their use in low-temperature equipment is expected to grow in the future.

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