With the rapid development of industrial technology, the more stringent requirements of the valve industry, especially the use of butterfly valves in the cryogenic medium, to meet the general valve performance is cold outside, more importantly, in the low-temperature state the reliability of the valve seal, flexibility of movement and cryogenic valves some other special requirements.
Butterfly valve has a number of advantages of compact structure, small size, light weight (with the same pressure, compared to the same diameter of the valve to reduce the 40% to 50%) the fluid resistance is small, open and close rapidly, so widely used. But the 80% cut-off valve or gate valve, butterfly valve of a small number of our valves used in some low-temperature device, such as the chemical industry of the days of burning liquefaction plants, air separation equipment, as well as pressure swing adsorption equipment. The main reason is metal sealing butterfly valve sealing performance under low temperature conditions bad, as well as some other phenomenon caused by leakage and leakage in the medium due to the irrational structure, safety and normal operation of the serious impact of these low-temperature equipment can not meet the low-temperature equipment requirements.
Increasing based on the continuous development of China's low-temperature device, cryogenic valves, metal sealing butterfly valve structural improvements, development of high sealing performance of the the three eccentric pure metal butterfly, this butterfly regardless of medium, high or low temperatures are to meet their needs. Now combined with its structural characteristics, only briefly introduce low temperature performance.
A low temperature dish valve seal performance requirements:
The cryogenic valves leaking main reason there are two cases, one is internal leakage; second is leaking.
1) The valve endoleaks main reason is due to the deformation of seal pair in the low-temperature state. When the the medium temperature dropped to the material generated phase change caused by the change in volume, so the sealing surface of the original high precision grinding warpage caused by poor low temperature sealing. The low temperature test, we had DN250 valve disc material medium of liquid nitrogen (-196 ℃) 1Cr18Ni9Ti (not found sealing surface warpage after low-temperature treatment) amounted to 0.12mm, which is the main reason causing internal leakage.
New butterfly valve developed by a flat seal to the cone seal. The seat is an oblique cone oval sealing surface, and embedded in the butterfly board round elastic sealing rings seal pair. Seal ring floating in the radial direction of the disc slot. When the valve is closed, the elastic sealing ring first and the minor axis of the elliptical sealing surface contact, with the rotation of the stem will gradually sealing ring pushed inwardly, forcing the resilient ring again and the major axis of the oblique conical surface contact, eventually leading to the elastic sealing ring all contact with oval sealing surface. Its sealing rely on the elastic ring is deformed to achieve. Therefore, when the valve body, or the butterfly plate deformation at low temperatures, will be elastomeric sealing ring to absorb compensation, will not produce the phenomenon of leaks and stuck. This elastic deformation immediately disappear when the valve is open, substantially no relative friction in the opening and closing process, so long service life.
2) leakage of the valve: First valves and pipe flange connection, the connection of the litter, connecting bolts, and the connector between the low temperature material contraction not synchronized to produce relaxation and lead to leakage. We therefore of the body and the conduit connected by flange connections changed welded structures, to avoid the leakage of cryogenic. The second is a leak at the stem and packing. Usually most valve packing using F4, because its performance since the sliding friction coefficient is small (on-steel coefficient of friction f = 0.05 to 0.1), but also has a unique chemical stability and therefore widely used. But F4 inadequacies First, the cold flow tendencies; linear expansion coefficient at low temperatures Lengsu cause leakage, resulting in a large number of stem Department icing to make the valve open failure. The purpose of self-shrinking sealing structure that F4 expansion coefficient characteristics of cryogenic butterfly valve developed for this purpose, to stay through the gap to reach room temperature, low temperature can be sealed.
Valve body, stem bushing design requirements:
1) low temperature valve housing structure and shape. Material selection right or not reliable operation of the normal valve has an extremely important significance. Structural characteristics and cut-off valve of the butterfly valve, gate valve, compared not only to avoid the irregular shape, uneven shell thickness, the deformation caused by the cold shrink, thermal stress generated at low temperatures, but also due to the small size of butterfly valve, the valve body The basic shape so known for, and thus the thermal capacity; consumption to cold; regular shape and easy measures cold valve. Such as the newly developed DD363H type butterfly valve in order to ensure the reliable use of the valve at low temperatures, in full accordance with the particularity of the cryogenic valve design and manufacturing, such as: housing material selection 1Cr18Ni9Ti austenitic stainless steel has a cubic lattice.
2) stem bushing options: users reflect, some cryogenic valves during operation, the rotating parts of the valve occurred viscous bite phenomenon, mainly due to: the choice of matching material is unreasonable to stay cold the gap is too small reasons, and machining accuracy. Developed cryogenic valves, adopted a series of measures to prevent the above phenomenon. For example: We selected stem upper and lower bushing having a small coefficient of friction and self-lubricating properties of the SF-1 type composite bearings, this can apply to the special needs of the cryogenic valves.
Metal seal butterfly valve has the characteristics of ordinary valves are not available. Especially small flow resistance, reliable seal, open and close rapidly, long life. The company developed the three eccentric metal seal butterfly valve sealing force from the deformation of the elastic ring to seal, and therefore does not need the help of the media force, it can do two-way sealing. According to some of the features of the butterfly valve will be more important. In the future there will be more butterfly valve is applied to the low-temperature equipment.
