The definition of cryogenic valves in the petrochemical industry is defined in accordance with the design temperature of the transport medium, generally referred to as cryogenic valves applied to the medium temperature -40 ℃ or less, and the valves applied to the medium temperature -101 ℃ or less are called ultra-low temperature valves.
Ultra-low temperature ball valves are mainly used in liquefied natural gas, liquefied petroleum gas and air separation industry devices, the output of liquid cryogenic media are: liquid oxygen, liquid hydrogen, liquefied natural gas, liquefied petroleum products. These media are not only flammable and explosive, but also in the temperature or flash evaporation will occur when the gasification, gasification of the volume of the rapid expansion, if the delivery of these fluids in the valve has a closed valve cavity and the structure of the design is not reasonable, will result in overpressure of the valve cavity, resulting in leakage of the media, or even cracking of the valve to cause accidents.
Cryogenic ball valve series is mainly applied to ethylene, liquefied natural gas and other chemical plants, the output of liquid cryogenic media such as ethylene, liquid oxygen, liquid hydrogen, etc.. This kind of medium is not only flammable and explosive, but also to gasify when warming up, gasification, volume expansion of hundreds of times. And these media permeability, easy to leak, processing and manufacturing is more difficult.
Cryogenic ball valve features:
1. the material of the pressure-bearing parts can withstand the expansion and contraction caused by changes in the temperature of the medium, the structure of the sealing part of the temperature change will not produce permanent deformation. Used in -100 ℃ below the working conditions, valve parts in the finishing process should be deep cold treatment, that is, the parts are placed in liquid nitrogen tank for cooling, when the parts temperature reaches -196 ℃, start holding l-2h. Then take out of the box outside the natural treatment to room temperature, repeat the cycle 2 times.
2. The valve cover adopts the long neck structure, the purpose of which is to protect the function of the packing box, so that the packing box is in a position far away from the low temperature to ensure the sealing effect of the packing. At the same time, it can also be wrapped around the cold-retaining materials to prevent the loss of cold energy. The neck length of the long neck (H. see below) can be selected according to the use of temperature and the thickness of the cold-retaining material. When the packing sealing effect is reduced, grease can be added from the middle of the packing box (see combined packing structure) to form an oil sealing layer to reduce the differential pressure of the packing box and improve the sealing performance.
3. the use of temperature below -100 ℃, the valve stem material after chrome plating or nitriding treatment to improve the surface hardness of the valve stem, improve the reliability of the packing seal.
4. low-temperature ball valves have to prevent abnormal pressure structure, due to low-temperature valve medium gasification, its volume of radical expansion, the pressure will be abnormally high, when the valve cavity pressure rises, the valve cavity and the inlet side of the connection, or in the valve at the inlet end of the placement of the relief valve, to ensure that the valve is safe to use.
5. cryogenic ball valves with gaskets have at room temperature, cryogenic state and temperature changes can be reliable sealing livestock and resilience.