With the rapid development of industrial technology, the valve industry made more stringent requirements, especially for use in cryogenic medium butterfly, in addition to meet the general performance of the valve with a normal condition and, more importantly, in the low temperature under the valve seal reliability, flexibility and movement of cryogenic valves some other special requirements.
Because butterfly valve with a compact, small size, light weight (with the same pressure, the same diameter of the valve can reduce by 40% compared to 50%), fluid resistance, open and close rapidly, such as a series of advantages, it is widely used. However, some of the low-temperature devices, such as days of burning liquefied equipment, air separation equipment and pressure swing adsorption equipment, valves used in the chemical industry for more than 80% or cut-off valve or gate, using the number of small butterfly. The main reason is the metal sealing butterfly valve at low temperature sealing performance in poor condition, and some other factors irrational structure within the media and other causes leakage and leakage phenomenon, seriously affected the normal operation of these security and cryogenic equipment can not meet the low temperature equipment requirements.
According to the continuous development of low-temperature device for cryogenic valves demands increasing, so the metal sealing butterfly valve structural improvements, the development of the three eccentric high sealing performance of pure metal butterfly, this butterfly whether the medium is high or low temperatures are able to meet their needs. Now combined with its structural characteristics, low temperature performance only for a brief introduction.
Cryogenic butterfly valve sealing performance requirements: leakage of cryogenic valves reason there are two cases, one internal leakage; the second is body leakage.
1. The main reason is generated within the valve seal leakage caused by deformation-produced at low temperatures. When the medium temperature dropped to cause the material to produce the volume change during the phase transition, so that the original high precision grinding sealing surface warpage caused by low temperature sealing bad. We had low test DN250 valves, the medium is liquid nitrogen (-196 ℃) disc material is 1Cr18Ni9Ti (not after low temperature treatment) found that the sealing surface warpage amounted to about 0.12mm, which is the main cause of endoleaks. The newly developed valve replaced by a flat sealing cone seal. Seat is an oblique cone oval sealing surface, with butterfly plate fitted elastic sealing rings are circular seal pair. Seal ring groove radial floating disc. When the valve is closed, the first elastic sealing ring and the sealing surface of the minor axis of the ellipse in contact, with the rotation of the stem seal ring gradually pushed inward, forcing the major axis of the contact and the elastic ring, and then the oblique conical surface, resulting in an elastic sealing ring all contact with the sealing surface of the ellipse. It relies on the elastic sealing ring is deformed and achieved. Thus, when the valve body or disc deformation at low temperatures, the elastic sealing ring will be compensated to absorb, no leakage and stuck phenomenon. When the valve opens the elastic deformation disappear immediately, in opening and closing process is relatively little friction, so long life.
2. Leakage of the valve: one is the valve and the pipe flange connection, because the connection litter, connecting bolts, and the connections between the low temperature shrink material is not synchronized to produce relaxation and lead to leaks. So we put the valve and pipeline connection to the flange welded structure, to avoid the low leakage. The second is at the stem and packing leak. Most valve packing more generally using F4, because of its good performance from the sliding friction coefficient (the coefficient of friction of steel f = 0.05 ~ 0.1), but also has a unique chemical stability, so widely used. But there are inadequacies F4, one cold tendency of large; the second is the linear expansion coefficient, produce shrink at low temperatures cause leakage, resulting in a lot of ice at the stem to open the valve failure. To do this using a self-developed cryogenic valve seal structure that reduced use F4 expansion coefficient characteristics, through the gap to the left to reach room temperature, low temperature can be the purpose of sealing.
This article comes from Baidu document