The working medium of cryogenic valves is not only low in temperature, but also most of them are toxic, flammable and explosive, and have strong permeability, which determines many special requirements for valve materials. The mechanical properties of steel at low temperature are different from those at normal temperature. In addition to strength, the important index of low temperature steel is its low temperature impact toughness. Cryogenic valves are installed at normal temperature and work at low temperature, with a wide range of temperature changes. If the design and installation are improper, it is easy to generate thermal stress or deformation. At the same time, the operating part of the valve is at normal temperature, and the circulating part is at low temperature. In order to reduce the cold loss, the valve stem is often made very long, and it is easy to deform and get stuck. The low temperature valve is often flexible at normal temperature, but it is very tight at low temperature and even cannot be opened.
There are three main reasons why the valve is stuck at low temperature:
1. During installation, the valve and the pipeline are not properly configured to cause prestress; or the cold compensation capability of the pipeline is poor, and the valve position changes at low temperature; or the valve lacks a bracket, resulting in deformation at low temperature; or the valve is improperly fixed, and the cooling box is deformed at low temperature It affects the concentricity of the valve stem and the valve body.
2. In design, due to the different materials of the valve stem and the valve sleeve, the linear expansion coefficient is different. Generally, the valve stem is made of stainless steel, with a linear expansion coefficient of 1.73×10-6°C-1; the valve sleeve is made of brass, with a linear expansion coefficient of 19.9×10-6°C-1, that is, the shrinkage of brass is larger than that of stainless steel, and it may be possible at low temperatures. Bite the thread. Especially when the dark rod structure and fine thread are used, the temperature change range of the screw thread is large, the thread clearance is small, and the phenomenon of biting is more likely to occur.
3. During operation, due to incomplete heating at the valve, or water entering the valve packing, freezing at low temperature, or closing the valve too tightly at room temperature, the thread is bitten, etc.
