With the rapid development of China's economy and China's industrial technology, the ever-changing high-tech and people's demand for valves make the valve industry face enormous challenges and opportunities. In particular, the use of butterfly valves in low-temperature media, in addition to meeting the performance of general valves under normal temperature conditions, is more important, the reliability of valve sealing at low temperatures, the flexibility of action and some other for cryogenic valves. special requirements.
The butterfly valve has a series of advantages such as compact structure, small volume, light weight (40% to 50% reduction compared with the same pressure and the same diameter of the gate valve), such as small fluid resistance and rapid opening and closing, so it is widely used.
However, in China, more than 80% of the valves used in the chemical industry such as Tianhuo liquefaction equipment, air separation equipment and pressure swing adsorption equipment are shut-off valves or gate valves, and the number of butterfly valves is small. The main reason is that the sealing performance of the metal sealing butterfly valve is poor under low temperature conditions, and other phenomena such as leakage and leakage of the medium due to unreasonable structure, which seriously affect the safety and normal operation of these low temperature equipment, and cannot meet the low temperature. Equipment requirements.
According to the continuous development of cryogenic devices in China, the requirements for cryogenic valves are increasing. Therefore, the structural improvement of metal sealing butterfly valves has been developed. Three butterfly valves with high eccentricity and high sealing performance have been developed. These butterfly valves are high temperature or low temperature. Can meet their needs.
Now combined with its structural characteristics, only a brief introduction to low temperature performance.
First, the requirements for the sealing performance of low temperature disc valves:
There are two main reasons for the leakage of the cryogenic valve, one is internal leakage; the other is external leakage.
1) Internal leakage of the valve The main reason is that the sealing pair is deformed at a low temperature.
When the temperature of the medium drops to cause a phase change of the material, the volume change is caused, 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 °C). The material of the butterfly plate is 1Cr18Ni9Ti (not subjected to low temperature treatment). The warpage deformation of the sealing surface is about 0.12mm, which is the main cause of internal leakage.
The newly developed butterfly valve has been changed from a flat seal to a tapered seal. The valve seat is a slanted conical elliptical sealing surface, which forms a sealing pair with a perfect circular elastic sealing ring embedded in the butterfly plate. The seal ring can float radially within the disc groove. When the valve is closed, the elastic sealing ring first contacts the short axis of the elliptical sealing surface, and the sealing ring is pushed inwardly as the valve stem rotates, forcing the elastic ring to contact the long axis of the oblique conical surface, eventually resulting in an elastic sealing ring. All contact with the elliptical sealing surface. Its seal is achieved by the deformation of the elastic ring.
Therefore, when the valve body or the butterfly plate is deformed at a low temperature, it is absorbed and compensated by the elastic sealing ring, and no leakage or jamming occurs. When the valve is opened, this elastic deformation disappears immediately, and there is substantially no relative friction during the opening and closing process, so the service life is long.
2) The leakage of the valve.
One is that when the valve and the pipeline are flanged, the connection pad, the connecting bolt, and the connecting member are loosened and contracted at a low temperature to cause slack and lead to leakage. Therefore, we changed the connection 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 of the valve stem and the packing. Generally, most of the valves are made of F4. Because of its good self-slip performance, low friction coefficient (f = 0.05 to 0.1 for steel) and unique chemical stability, it is widely used.
However, F4 also has shortcomings. First, the cold flow tends to be large; second, the coefficient of linear expansion is large, and the cold shrinkage at low temperatures causes leakage, causing a large amount of ice at the valve stem, causing the valve to fail to open. The low-temperature butterfly valve developed for this purpose adopts a self-shrinking sealing structure, that is, the characteristic of using a large expansion coefficient of F4, and can achieve the purpose of sealing at normal temperature and low temperature through the reserved gap.
Second, the valve body, valve stem bushing design requirements 1) low temperature valve housing structure shape.
The correct choice of materials is of great importance to the normal and reliable operation of the valve. Compared with the stop valve and the gate valve, the structural characteristics of the butterfly valve not only avoid the irregular shape, the uneven wall thickness of the casing, the cold shrinkage caused by the low temperature, the deformation caused by the temperature difference stress, but also the valve body due to the small size of the butterfly valve. The shape is basically called, so the heat capacity is small; the pre-cooling consumption is also small; the shape rule is convenient for the valve to keep cold. The DD363H type disc valve newly developed by Haiweiwangdi Valve is designed and manufactured in accordance with the speciality of the cryogenic valve in order to ensure the reliable use of the valve at low temperature. For example, the shell material is selected to have a cubic lattice of 1Cr18Ni9Ti austenite. Body stainless steel, etc.
2) Selection of stem bushing.
According to the user's feedback, some low-temperature valves are in operation, the rotating parts of the valve are viscous, and the occlusion phenomenon occurs. The main reasons are: the selection of the matching materials is unreasonable, the residual cooling gap is too small, and the machining accuracy is caused.
In the development of cryogenic valves, a series of measures were taken to prevent the above phenomenon. For example, we have selected SF-1 type composite bearings with small friction coefficient and self-lubricating properties for the upper and lower bushings of the valve stem, which can be applied to some special needs of low temperature valves. Metal-sealed butterfly valves are characterized by some common valves. In particular, the flow resistance is small, the seal is reliable, the opening and closing is rapid, and the service life is long.