Application of Metal Seal Butterfly Valve in Cryogenic Equipment
Metal-sealed butterfly valves have certain necessary requirements in low-temperature equipment. The following will analyze the design requirements of the valve body stem bushing and the requirements of low-temperature butterfly valve sealing performance.
1. Design requirements for valve body and stem bushing
1) The structure and shape of the low temperature valve shell. The correct selection of materials is of great significance to the normal and reliable operation of the valve. Compared with globe valve and gate valve, the structure of butterfly valve not only avoids the deformation caused by irregular shape, uneven shell wall thickness, cold shrinkage at low temperature, and temperature difference stress, but also due to the small size of the butterfly valve, the valve body The shape is basically equal to the left and right, so the heat capacity is small; the consumption of pre-cooling is also small; the shape is regular and convenient for cooling measures of the valve. For example, the newly developed DD363H butterfly valve is designed and manufactured according to the particularity of the low temperature valve in order to ensure the reliable use of the valve at low temperature. For example, the shell material selects 1Cr18Ni9Ti austenitic stainless steel with cubic lattice.
2) Selection of valve stem bushing: According to user reports, some low-temperature valves are in operation, and the rotating parts of the valve are viscous, and the occlusion phenomenon occurs from time to time. , as well as the processing accuracy and other reasons. In the development of cryogenic valves, a series of measures have been taken to prevent the above phenomena. For example: We choose SF-1 type composite bearing with small 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 low temperature valves.
Metal-sealed butterfly valves have features that some ordinary valves do not have. In particular, 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 sealing, so it does not need to rely on the force of the medium, so it can be used for bidirectional 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
2. Requirements for low temperature butterfly valve sealing performance
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 at low temperature. When the temperature of the medium drops to the point where the material undergoes a phase change, the volume changes, and the sealing surface with high grinding precision is warped and deformed, resulting in poor low-temperature sealing. We have carried out low temperature test on DN250 valve, the medium is liquid nitrogen (-196 ℃), the disc material is 1Cr18Ni9Ti (without low temperature treatment), and it is found that the warpage deformation of the sealing surface is about 0.12mm, which is the main cause of internal leakage.
The newly developed butterfly valve is changed from a flat seal to a conical seal. The valve seat is an oblique conical elliptical sealing surface, which forms a sealing pair with a circular elastic sealing ring embedded in the butterfly plate. The sealing ring can float radially in the disc groove. When the valve is closed, the elastic sealing ring first comes into contact with the short axis of the elliptical sealing surface, and gradually pushes the sealing ring inward with the rotation of the valve stem, forcing the elastic ring to contact the long axis of the inclined conical surface again, eventually resulting in an elastic sealing The ring is in full contact with the elliptical sealing surface. Its sealing is achieved by the deformation of the elastic ring. Therefore, when the valve body or disc is deformed at low temperature, it will be absorbed and compensated by the elastic sealing ring, and there will be no leakage and jamming. When the valve is opened, the elastic deformation disappears immediately, and there is basically no relative friction during the opening and closing process, so the service life is long.
2) External leakage of the valve: One is that when the valve and the pipeline are flanged, the leakage is caused by the asynchronous shrinkage of the connecting gasket, the connecting bolt, and the material of the connecting piece at low temperature. Therefore, we changed the connection method between the valve body and the pipeline from the flange connection to the welded structure to avoid low temperature leakage. The second is the leakage at the valve stem and packing. Generally, F4 is used for packing of most valves, because of its good self-slip performance, small friction coefficient (f = 0.05 to 0.1 for steel), and unique chemical stability, so it is widely used. However, F4 also has its shortcomings. First, the tendency of cold flow is large; second, the coefficient of linear expansion is large, which leads to leakage due to cold shrinkage at low temperature, resulting in a large amount of icing at the valve stem, so that the valve fails to open. The low-temperature butterfly valve developed for this purpose adopts the self-shrinking sealing structure, that is, the feature of the large expansion coefficient of F4 is used, and the purpose of sealing at normal temperature and low temperature is achieved through the reserved gap.