Operational Requirements of Cryogenic Valves
Cryogenic valves, basically, are able to be operated at working temperatures below -29 degrees Celsius. Some special cryogenic valves can deliver liquid gases such as oxygen and natural gases as usual at the working temperature lower than -196 degrees Celsius.
In order to reduce the heat transmission from the external environment, the operating unit requires to be kept a certain distance from the valve body of the adiabatic apparatus. Thus, the extended bonnet neck of the cryogenic valve is adopted generally. The cryogenic valve should work under the complicated working condition, which means the valve needs to be kept workable all the time under either the external temperature or the ultra low temperature.
The valve disc, generally, is sealed by the contact pressure of the sealing ring. Moreover, if the acting force of the transmission actuator is fixed, the sealing performance of the sealing ring may be different under the working temperature. At the same time, the material performance of the sealing ring, especially the polymetric material, can be different. Thus, the cryogenic valve has the stringent requirement of its sealing ring, and the cemented carbide is recommended in general daily usage.
For the cryogenic valve, the opening and closing speed of its disc’s opening and closing is limited. The heat flow occurring occurs within the limited range causes the appearance appear of the liquid phase in some parts of the valve system, though the heat insulation measure is are already taken. Thus, when the valve is opened rapidly, the liquid might flow at a high speed, and the water hammer will occur if the flow of water is blocked by the barrier-the valve or by the resistance force of the valve.
Applied to the field of the refrigeration technology, the cryogenic valve is operated with the cryogen including nitrogen and freon. In the past, thecryogenic valve was made of the cast iron and it was suitable for the working temperature from -28 to 150 degrees Celsius while the cast iron is rarely used for the cryogenic valve now. However, there are more and more cryogenic valves made of ferrite nodular iron because the ferrite nodular iron has characteristics of great impact toughness, abrasion resistance and corrosion resistance.
The ultra low working temperature of the cryogenic valve should be primarily considered. The cryogenic valve itself can be placed in any position inside the cooler box. However, the stuffing box, the hand wheel and the indicator of the valve clack position are supposed to be placed outside the cooler box, for the convenience of the practical operation and the packing replacement.
In operation, it should be noticed that the valve component produced and inspected at room temperature will be operated with expansion and contraction at the low working temperature due to the variation of the working temperature. Therefore, the normal operation of the cryogenic valvecan be directly impacted.
Furthermore, on the basis of practical experience, we can know that, without the proper low-temperature treatment, even the cryogenic valvemade of bronze, cooper-nickel alloys and austenitic stainless steel will leak under the low-temperature condition because the valve component will deform due to the material phase change caused by the low temperature.