With the rapid development of industrial technology, more stringent requirements have been put forward for the valve industry, especially the use of butterfly valves in cryogenic media. In addition to meeting the performance of general valves at room temperature, it is more important in low temperature conditions. The reliability of the lower valve seal, the flexibility of action and some other special requirements for cryogenic valves.

Because the butterfly valve has a series of advantages such as compact structure, small size and light weight (compared with the same pressure and the same diameter gate valve can be reduced by 40% to 50%), the fluid resistance is small, and the opening and closing is rapid. Therefore, it is widely used. However, in my country, more than 80% of the valves used in some low-temperature equipment, such as natural gas liquefaction equipment, air separation equipment, and pressure swing adsorption equipment, are stop valves or gate valves, and the number of butterfly valves used is very small. The main reason is that the metal-sealed butterfly valve has poor sealing performance under low temperature conditions, and other internal leakage and external leakage of the medium due to unreasonable structure, which seriously affects the safety and normal operation of these low-temperature equipment, and cannot meet the low temperature requirements. Equipment requirements.

According to the continuous development of my country's cryogenic devices, the requirements for cryogenic valves are increasing. Therefore, the structure of the metal sealing butterfly valve is improved, and a triple eccentric pure metal butterfly valve with high sealing performance has been developed. This kind of butterfly valve is either high or low temperature. Can meet its needs. Now combined with its structural characteristics, only a brief introduction to low-temperature performance is given.

1. Requirements for the sealing performance of low-temperature butterfly valves:

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

1) The main reason for the internal leakage of the valve is the deformation of the sealing pair in the low temperature state. When the temperature of the medium drops to a phase change of the material, the volume changes, which causes the sealing surface with high grinding accuracy to warp and deform, resulting in poor low-temperature sealing. We have conducted low-temperature tests on DN250 valves. The medium is liquid nitrogen (-196℃). The butterfly plate material is 1Cr18Ni9Ti (without low-temperature treatment). We found that the sealing surface warpage and deformation reached about 0.12mm, which is the main cause of internal leakage.

The newly developed butterfly valve is changed from flat sealing to conical sealing. The valve seat is an oblique cone ellipse sealing surface, which forms a sealing pair with a round elastic sealing ring embedded on the butterfly plate. 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. As the valve stem rotates, the sealing ring is gradually pushed inward, forcing the elastic ring to contact the long axis of the oblique cone surface, and finally the elastic sealing ring All contact with the oval sealing surface. Its sealing is achieved by the deformation of the elastic ring. Therefore, when the valve body or butterfly plate is deformed at low temperature, it will be absorbed and compensated by the elastic sealing ring, and leakage and jamming will not occur. When the valve is opened, this elastic deformation disappears immediately, and there is basically no relative friction during the opening and closing process, so the service life is long.

2) Valve leakage: First, when the valve and the pipeline are connected by flanges, leakage occurs due to the unsynchronized contraction of gaskets, connecting bolts, and connecting parts at low temperatures. Therefore, we changed the connection between the valve body and the pipeline from a flange connection to a welded structure to avoid low-temperature leakage. The second is leakage at the valve stem and packing. Generally, most valve packing adopts F4 because of its good self-sliding performance, low friction coefficient (friction coefficient for steel f=0.05～0.1), and unique chemical stability, so it is widely used. However, F4 also has shortcomings. First, the tendency of cold flow is large; second, the linear expansion coefficient is large, and the cold shrinkage at low temperature causes leakage, causing a large amount of ice at the valve stem, so that the valve opening fails. The low-temperature butterfly valve developed for this purpose adopts a self-shrinking sealing structure, that is, taking advantage of the large expansion coefficient of F4, and achieves the purpose of sealing at room temperature and low temperature through the reserved gap.

2. Design requirements for valve body and stem bushing:

 1) The structural shape of the cryogenic valve shell. The correct choice of materials is of great significance to the normal and reliable operation of the valve. Compared with globe valves and gate valves, the structural characteristics of butterfly valves not only avoid deformation caused by irregular shape, uneven shell wall thickness, cold shrinkage at low temperatures, and temperature stress, but also due to the small size of the butterfly valve, the valve body The shape is basically the same on the left and right, so the heat capacity is small; the pre-cooling consumption is also small; the shape is regular and it is convenient for the cold preservation of the valve. In order to ensure the reliable use of the valve at low temperatures, the newly developed DD363H butterfly valve is designed and manufactured in accordance with the particularity of the cryogenic valve. For example, the shell material is 1Cr18Ni9Ti austenitic stainless steel with cubic lattice.

2) Selection of stem bushing: According to users' feedback, some low-temperature valves are in operation, and the rotating parts of the valve are sticky and seizure occurs from time to time. The main reason is: the selection of matching materials is unreasonable, and the cooling gap is too small. , And processing accuracy and other reasons. In the development of cryogenic valves, a series of measures have been taken to prevent the occurrence of the above phenomena. For example, we have selected SF-1 composite bearings with low friction coefficient and self-lubricating performance for the upper and lower bushings of the valve stem, which can be suitable for some special needs of cryogenic valves.

The metal-sealed butterfly valve has the characteristics that some ordinary valves do not have. Especially the flow resistance is small, the sealing is reliable, the opening and closing is rapid, and the service life is long. The sealing force of the triple eccentric metal sealing butterfly valve developed by our company comes from the deformation of the elastic ring to achieve the seal, so it does not need to rely on the force of the medium, so it can be used for two-way sealing. According to some characteristics of the butterfly valve, more people will pay attention to it. In the future, more butterfly valves will be applied to cryogenic equipment.