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   As we all know, butterfly valve has a series of advantages such as compact structure, small size, light weight, low fluid resistance, rapid opening and closing. However, in the chemical industry with low-temperature devices used in more than 80% of the valves are globe valves or gate valves, the number of butterfly valves is very small. Analyze the reason is mainly metal sealed butterfly valve sealing performance is poor in low temperature conditions, as well as other reasons due to structural irrationality caused by media leakage and leakage, seriously affecting the safety and normal operation of these low-temperature equipment, cannot meet the requirements of low-temperature equipment.

   Due to the continuous development of cryogenic devices, the increasing demand for cryogenic valves, in order to meet the needs of market economic development, through the structural improvement of the metal-sealed butterfly valve, developed a three eccentric, pure metal, high sealing performance of the butterfly valve, regardless of whether the medium is a high-temperature or low-temperature can meet its needs.

   一、Requirements for sealing performance of low-temperature butterfly valves:

   There are two main reasons for leakage in cryogenic valves, one is internal leakage; the other is external leakage.

1) Valve internal leakage is the main reason for the sealing vice in the low temperature state deformation caused. When the medium temperature drops to make the material phase change caused by volume changes, so that the original grinding precision of the sealing surface has a high warping deformation caused by low-temperature sealing bad. We have DN250 valve for low-temperature test, the medium for liquid nitrogen (-196 ℃) butterfly plate material for the 1Cr18Ni9Ti (no low-temperature treatment) found that the sealing surface warping deformation amounted to 0.12mm or so, which is caused by the main cause of internal leakage.

   The newly developed butterfly valve is changed from plane sealing to cone sealing. The valve seat is an oblique cone oval sealing surface, and embedded in the butterfly plate on the circular elastic sealing ring to form a sealing vice. The sealing ring can float radially in the butterfly plate groove. When the valve is closed, the elastic sealing ring first contacts the short axis of the elliptical sealing surface, and with the rotation of the valve stem gradually pushes the sealing ring inward, forcing the elastic ring to contact the long axis of the inclined conical surface, which ultimately results in the contact of the elastic sealing ring with the elliptical sealing surface. Its sealing is achieved by the deformation of the elastic ring. Therefore, when the valve body or the butterfly plate is deformed under low temperature, it will be absorbed by the elastic sealing ring to compensate, and will not produce leakage and jamming phenomenon. When the valve is opened, this elastic deformation disappears immediately, and there is no relative friction in the process of opening and closing, so it has a long service life.

2) Valve leakage: One of them is that when the valve is connected with the pipeline by flange, the connection gasket, connection bolts and connectors will relax due to the asynchronous contraction of the materials under low temperature. Therefore, we change the connection between valve body and pipeline from flange connection to welded structure to avoid low temperature leakage. The second is the valve stem and packing leakage. general most of the valve packing F4, because of its good self-slip performance, friction coefficient is small, and has a unique chemical stability, so it is widely used. But F4 also has shortcomings, one is the cold flow tendency; Second, the linear expansion coefficient is large, at low temperatures resulting in cold shrinkage leads to leakage, resulting in a large number of iced at the valve stem, to make the valve open failure. The low-temperature butterfly valve developed for this purpose adopts the self-shrinking sealing structure, i.e., it makes use of the characteristics of F4 with a large expansion coefficient, and achieves the purpose of sealing at room temperature and low temperature through the reserved gap.

二、Design requirements for valve body and stem bushings:

1) Cryogenic valve shell structure shape. Material selection is correct or not on the normal and reliable work of the valve has an extremely important significance. Butterfly valve structure features and globe valve, gate valve, compared to not only avoid the irregular shape, shell wall thickness is not uniform, in the low temperature generated by the cold shrinkage, temperature stress caused by the deformation, but also due to the butterfly valve volume is small, the shape of the valve body left and right is basically symmetrical, and thus the heat capacity of a small; pre-cooling consumption is also small; the shape of the rules and to facilitate the valve's cold protection measures.

2) Selection of stem bushing: According to user feedback, some low-temperature valves in operation, the valve's rotating parts of the viscosity, bite phenomenon occurs from time to time, the main reason is that: pair of material selection is unreasonable, leaving too small a cold gap, as well as processing accuracy and other reasons. In the development of low-temperature valves, a series of measures to prevent the above phenomenon. For example: we choose the valve stem on the lower liner with a coefficient of friction less than the self-lubricating properties of SF-1 type composite bearings, which can be applied to some of the special needs of low-temperature valves.

    Metal sealed butterfly valves have features that some ordinary valves do not have. In particular, it has small flow resistance, reliable sealing, rapid opening and closing, and long service life. The sealing force of the three eccentric metal-sealed butterfly valve developed and produced by our company comes from the deformation of the elastic ring to achieve sealing, so it does not need to rely on the medium force, so it can be used for two-way sealing. With the maturity and promotion of metal-sealed butterfly valve technology, there will be more and more suitable butterfly valves applied to cryogenic equipment in the future.