Application of all-metal hard-sealed butterfly valves in low-temperature valves

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　　Application of all-metal hard-sealed butterfly valves in low-temperature valves? With the rapid development of the economy and industrial technology, as well as the constant emergence of new high-tech advancements and increasing demand for valves, the valve industry is faced with significant challenges as well as opportunities. Especially when butterfly valves are used with low-temperature media, in addition to meeting the performance requirements of ordinary valves at normal temperatures, it is more important to ensure the reliability of the valve’s sealing at low temperatures, its flexibility in operation, as well as meeting other specific requirements for valves used in low-temperature environments. 　　With the continuous development of low-temperature equipment in our country, the requirements for low-temperature valves are increasing. As a result, structural improvements were made to metal-sealed butterfly valves, leading to the development of triple-eccentric all-metal butterfly valves with high sealing performance. These valves can meet the needs regardless of whether the medium is at high or low temperatures. Now, taking into account its structural characteristics, only a brief introduction to its low-temperature performance will be provided. I. Requirements for the sealing performance of low-temperature disc valves: There are mainly two reasons for leaks in low-temperature valves; one is internal leakage ； The second is external leakage. 1. Internal leakage in the valve: The main cause is deformation of the sealing pair at low temperatures. 　　When the temperature of the medium drops to a level that causes a phase change in the material, this results in volume changes, which in turn cause the sealing surfaces that originally had high precision to warp and deform, leading to poor sealing at low temperatures. We conducted low-temperature tests on DN250 valves using liquid nitrogen at -196°C; the butterfly plate material was 1Cr18Ni9Ti (without low-temperature treatment). It was found that the warping deformation of the sealing surface reached approximately 0.12 mm, which is the main cause of internal leakage. 　　The newly developed butterfly valve has been changed from a planar seal to a conical seal. The valve seat is an oblique conical elliptical sealing surface, which forms a sealing pair with the circular elastic sealing ring mounted on the butterfly plate. The sealing ring can float radially within the butterfly plate groove. When the valve is closed, the elastic sealing ring first makes contact with the short axis of the elliptical sealing surface. As the valve stem rotates, it gradually pushes the sealing ring inward, forcing it to make contact with the long axis of the inclined conical surface, until ultimately the elastic sealing ring is in full contact with the elliptical sealing surface. Its sealing is achieved through the deformation of an elastic ring. 　　Therefore, when the valve body or butterfly disc deforms at low temperatures, this deformation is absorbed and compensated by the elastic sealing ring, preventing leakage and jamming. When the valve is opened, this elastic deformation disappears immediately, and there is virtually no relative friction during the opening and closing process, thus ensuring a long service life. 2. External leakage of the valve. 　　One reason is that when valves and pipelines are connected using flanges, leakage occurs due to relaxation resulting from the asynchronous contraction of the materials in the gasketing, connection bolts, and fittings at low temperatures. Therefore, we changed the connection method between the valve body and the pipeline from flange connection to a welded structure, thereby avoiding leaks at low temperatures. The second is leakage at the valve stem and packing. F4 is generally used as the packing for most valves, as it possesses good self-lubricating properties and a low friction coefficient (the friction coefficient against steel is 0.05–0.1). Its unique chemical stability also contributes to its widespread use. However, F4 also has its shortcomings: first, it has a strong tendency to experience cold flow; second, its linear expansion coefficient is high, which leads to contraction at low temperatures and results in leakage. This causes significant freezing at the valve stem, thereby preventing the valve from opening properly. The low-temperature butterfly valve developed for this purpose features a self-compressing sealing structure; by taking advantage of the high expansion coefficient of F4, it achieves sealing capabilities at both normal and low temperatures through a pre-designed gap. II. Design requirements for the valve body and stem bushings 1. Structural shape of the low-temperature valve housing. 　　The correct selection of materials is of utmost importance for the proper and reliable operation of valves. Compared to globe valves and gate valves, the structural features of butterfly valves not only prevent deformation caused by irregular shapes, uneven wall thicknesses, cold contraction at low temperatures, and thermal stress, but also enable a lower heat capacity due to their small size and basically symmetrical shape, which results in less energy required for pre-cooling. Their regular shape further facilitates the implementation of insulation measures for the valves. The newly developed DD363H type disc valve by Shanghai Weiwangdi Valves is designed and manufactured with regard to the specific requirements of low-temperature valves, in order to ensure its reliable operation at low temperatures; for example, the material used for the valve body is 1Cr18Ni9Ti austenitic stainless steel with a cubic crystal structure. 2. Selection of valve stem bushing. 　　According to user reports, in some low-temperature valves, sticking occurs at the rotating parts during operation, and seizure happens from time to time. The main reasons for this are improper selection of matching materials, too small a cold gap, as well as issues related to machining precision. A series of measures were taken when developing low-temperature valves to prevent the occurrence of the above phenomena. For example, we have chosen SF-1 type composite bearings with a low friction coefficient and self-lubricating properties for the upper and lower bushings of the valve stem, which allows them to meet the specific requirements of valves used in low-temperature environments. Metal-sealed butterfly valves possess characteristics that some ordinary valves do not have. In particular, it features low flow resistance, reliable sealing, fast opening and closing, and a long service life.