There are two main reasons for the leakage of cryogenic valves, one is internal leakage; The second is leakage.
1) Internal leakage of the valve
The main reason is the deformation of the sealing pair at low temperature.
When the temperature of the medium drops to the point where the phase change occurs in 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 carried out a low temperature test on the DN250 valve, the medium is liquid nitrogen (-196 °C), the butterfly plate data is 1Cr18Ni9Ti (not undergoing low temperature treatment), and it is found that the warping deformation of the sealing surface is about 0.12mm, which is the main reason for the formation of internal leakage.
The newly developed butterfly valve was changed from a flat seal to a cone seal. The valve seat is an oblique conical oval sealing surface, which is formed by a perfectly circular elastic sealing ring embedded in the butterfly plate. The sealing ring can float radially in the butterfly groove. When the valve is closed, the elastic sealing ring first touches the short axis of the oval sealing surface, and gradually pushes the sealing ring inward with the rolling of the valve stem, forcing the elastic ring to touch the long axis of the oblique conical surface, and finally causes the elastic sealing ring and the oval sealing surface to all touch. 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 there will be no leakage and jamming. When the valve is opened, this elastic deformation disappears immediately, and there is basically no relative friction in the process of opening and closing, so the service life is long.
2) Leakage of the valve.
One is that when the flange connection method is selected for the valve and pipeline, the leakage occurs due to the unsynchronized shrinkage of the connecting bedding, connecting bolts, and connecting parts at low temperatures. Therefore, we changed the connection method between the valve body and the pipeline from flange connection to welded structure to avoid low-temperature leakage.
The second is the leakage of the valve stem and the packing. Generally, F4 is used as the packing material of most valves, because of its good self-sliding performance, small friction coefficient (friction coefficient f=0.05-0.1 for steel), and unique chemical stability, so it is widely used.
However, F4 also has shortcomings, one is that the tendency of cold flow is large; Second, the linear expansion coefficient is large, and the cold shrinkage occurs at low temperature, resulting in leakage, forming a large amount of ice at the valve stem, so that the valve is open and out of order. For this reason, the low-temperature butterfly valve developed adopts a self-shrinking sealing structure, that is, using the characteristics of F4's large expansion coefficient, and the intention of sealing at room temperature and low temperature can be achieved through the gap left.
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