First, the design requirements of low temperature valves
The design of the cryogenic valve has the following requirements, depending on the conditions of use:
1) The valve should have long-term working ability in low temperature medium and ambient temperature.
2) Valves should not be a significant source of heat for cryogenic systems. This is because the inflow of heat, in addition to reducing the thermal efficiency, such as excessive inflow, also causes the internal fluid to evaporate rapidly, causing an abnormal increase in pressure, posing a hazard.
3) The low temperature medium should not have a detrimental effect on the operation of the wheel and the sealing performance of the packing.
4) Valve assemblies that are in direct contact with the cryogenic medium shall have an explosion-proof and fire-resistant structure.
5) Valve assemblies that operate at low temperatures cannot be lubricated, so structural measures are required to prevent friction from scratching.
Second, the material selection of the cryogenic valve
1. Cryogenic valve body material
(1) The main material should be considered from the metal phase. In addition to the austenitic steel with a face-centered cubic lattice, copper, aluminum, etc., the general steel will exhibit low-temperature brittleness at low temperatures. Reduce the strength and service life of the valve. When selecting the host material, first select materials suitable for working at low temperatures. Aluminum does not exhibit low temperature brittleness at low temperatures.
However, due to the low hardness of aluminum and aluminum alloys, the aluminum sealing surface has poor wear resistance and scratch resistance, so there is a certain limit in the use of low temperature valves, and it is only used in low pressure and small diameter valves.
Materials that work at low temperatures are guaranteed to have low temperature properties, primarily to ensure their cold impact strength. The trim must be properly selected to have sufficient cold impact strength to prevent breakage. Alloy steels of C and Cr quickly lose their impact strength at temperatures below -20 ° C, so the use temperatures are limited to -30 ° C and -50 ° C, respectively. Ni
Nickel steel with a volume of 3.5% can be used at -100 ° C, and nickel steel with a Ni content of 9% can be used at -192 ° C. Austenitic stainless steel, nickel, monel, Hastelloy, titanium,
Aluminum alloys and bronzes can be used at lower temperatures (-273 ° C).
In addition, the selection of materials for cryogenic valves should also consider the following factors:
1) the minimum operating temperature of the valve;
2) the mechanical properties required for the metal material to maintain working conditions at low temperatures, in particular impact toughness, relative elongation and structural stability;
3) It has good abrasion resistance under low temperature and no oil lubrication;
4) has good corrosion resistance;
5) The welding properties of the material must also be considered when using welded joints.
(2) Selection of materials for valve body, valve cover, valve seat and valve flap (gate) The selection principle of these main parts and materials is: ferritic steel is used when the temperature is higher than -100 °C; the temperature is lower than -100. Austenitic steel is used at °C; copper and aluminum materials are available for low pressure and small diameter valves. The design is based on the selection of the appropriate material based on the minimum use temperature.
(3) When the material temperature of the valve stem and fastener is higher than -100 °C, the valve stem and bolt material are made of alloy steel such as Ni, Cr-Mo, etc., after proper heat treatment to improve the tensile strength and prevent thread biting. Injury and so on.
When the temperature is lower than -100 ° C, it is made of austenitic stainless acid-resistant steel. However, the hardness of the 18-8 acid-resistant steel is low, which will cause the stem and the filler to scratch each other, causing the filler to leak.
Therefore, the surface of the stem must be hard chrome plated (0.04-0.06mm thick) or nitrided and nickel-plated to improve surface hardness. In order to prevent the nut and bolt from seizing, the nut is generally made of Mo steel or Ni steel, and the surface of the thread is coated with molybdenum disulfide.
2, the selection of low temperature valve gaskets, filler materials As the temperature is reduced, the shrinkage of fluoroplastics is large, which will reduce the sealing performance and cause leakage. Asbestos packing cannot avoid osmotic leakage. Rubber has a swelling property for liquefied natural gas and cannot be used at low temperatures.
In the design of the low temperature valve, on the one hand, the structural design ensures that the packing is operated close to the ambient temperature. For example, the long neck valve cover structure is used to make the stuffing box as far as possible from the low temperature medium, and on the other hand, when selecting the packing Low temperature properties of the filler. Asbestos filler impregnated with polytetrafluoroethylene is generally used in the cryogenic valve.
Flexible graphite is an excellent sealing material that has recently been developed. This material is impermeable to gases and liquids and has a 10%-15% elasticity in the thickness direction.
The seal can be achieved with a lower fastening pressure. It is also self-lubricating and acts as a valve packing to prevent wear on the packing and stem.