Two Main Reasons of Cryogenic Valves' Leakage
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Two Main Reasons of Cryogenic Valves’ Leakage
Nowadays, as the technique of cryogenic valves develops rapidly, this kind of valve still has some problems remained to be solved. Including internal leakage and external leakage, the leakage of the cryogenic valve is one of those problems.
Internal leakage of cryogenic valves
The main reason for the internal leakage of cryogenic valves is the sealed auxiliary deforming at the low temperature.
The phase transition of valve materials, which is formed by the decrease of the medium temperature, changes the volume of the cryogenic valve and causes the warping deformation of the sealing surface with precision grinding. Thus, the cryogenic valve has poor sealing performance at the low temperature.
In the low-temperature test, the cryogenic valve DN250, whose media are liquid nitrogen (-196℃) and disc material is 1Cr18Ni9Ti (without low-temperature treatment), is found that the warping deformation of the valve's sealing surface can reach about 0.12mm. The deformation is defined as the main reason for the internal leakage.
External leakage of the cryogenic valve
First, the cryogenic valve leaks when it is connected to the pipeline with flanged connection and the gasket, connecting bolt and fittings are not contracted at the same time at the low temperature. Therefore, the flanged connection should be changed to the welded connection for preventing the leakage at the low temperature.
Second, there is the leakage between the valve stem and the stuffing box. In general, most cryogenic valves utilize F4 packing due to its unique chemical stability and small friction coefficient (the friction coefficient of the steel is between 0.05 and 0.1).
However, several disadvantages of F4 as valve packing still remain such as the tendency of cold flow properties and the large coefficient of linear expansion. At the low temperature, since the cooling contraction of F4 can cause the leakage of the cryogenic valve and then freeze the valve stem, the cryogenic valve will not be opened as normal. Therefore, the cryogenic valve keeps a certain space to seal at either the normal or low temperature by adopting self-sealing structures, namely, utilizing F4's property of large coefficient of linear expansion.
Internal leakage of cryogenic valves
The main reason for the internal leakage of cryogenic valves is the sealed auxiliary deforming at the low temperature.
The phase transition of valve materials, which is formed by the decrease of the medium temperature, changes the volume of the cryogenic valve and causes the warping deformation of the sealing surface with precision grinding. Thus, the cryogenic valve has poor sealing performance at the low temperature.
In the low-temperature test, the cryogenic valve DN250, whose media are liquid nitrogen (-196℃) and disc material is 1Cr18Ni9Ti (without low-temperature treatment), is found that the warping deformation of the valve's sealing surface can reach about 0.12mm. The deformation is defined as the main reason for the internal leakage.
External leakage of the cryogenic valve
First, the cryogenic valve leaks when it is connected to the pipeline with flanged connection and the gasket, connecting bolt and fittings are not contracted at the same time at the low temperature. Therefore, the flanged connection should be changed to the welded connection for preventing the leakage at the low temperature.
Second, there is the leakage between the valve stem and the stuffing box. In general, most cryogenic valves utilize F4 packing due to its unique chemical stability and small friction coefficient (the friction coefficient of the steel is between 0.05 and 0.1).
However, several disadvantages of F4 as valve packing still remain such as the tendency of cold flow properties and the large coefficient of linear expansion. At the low temperature, since the cooling contraction of F4 can cause the leakage of the cryogenic valve and then freeze the valve stem, the cryogenic valve will not be opened as normal. Therefore, the cryogenic valve keeps a certain space to seal at either the normal or low temperature by adopting self-sealing structures, namely, utilizing F4's property of large coefficient of linear expansion.
