First, the design of the low temperature valves
The design of the cryogenic valve has the following requirements, depending on the working conditions:
- (1) The valve should have a 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 can reduce the thermal efficiency. In addition, excessive inflow will also cause the internal fluid to evaporate rapidly, causing abnormal pressure rise and posing a hazard.
- (3) The low temperature medium should not have harmful effects 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) The valve assembly working at low temperature cannot be lubricated, so structural measures are required to prevent the friction piece from being scratched.
Second, the material selection of the low temperature valve structure
◆ Cryogenic valve body material
(1) The factors should be considered in the selection of the valve body material.
Apart from austenitic steel, copper, aluminum, etc., which has a face-centered cubic lattice, the general steel material has low-temperature brittleness at low temperatures, thereby reducing the strength and service life of the valve.
When selecting the body material, first select materials suitable for working at low temperatures. Aluminum does not exhibit low temperature brittleness at low temperatures, but the hardness of aluminum and aluminum alloys is not high, and the wear resistance and scratch resistance of aluminum sealing surfaces are poor. Therefore, the use of low temperature valves has certain limitations, only used in low pressure and in small diameter valves.
Materials that work at low temperatures are guaranteed to have low temperature properties, primarily to ensure their cold impact strength.
The material for valve internals 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 operating temperatures are limited to -30 ° C and -50 ° C, respectively. Nickel steel containing 3.5% Ni can be used at -100 ° C, and nickel steel containing 9% Ni can be used at -192 ° C. Austenitic stainless steel, nickel, monel, Hastelloy, titanium, aluminum alloy and bronze can be used at lower temperatures (-273 ° C).
- The minimum operating temperature of the valve;
- The mechanical properties required for metal materials to maintain working conditions at low temperatures, especially impact toughness, relative elongation and structural stability;
- Good abrasion resistance under low temperature and no oil lubrication;
- Good corrosion resistance;
- The welding performance of the material must also be considered when using welded joints.
(2) Selection of material for valve body, valve cover, valve seat and valve flap (gate)
The selection principle of these main parts materials is: ferritic steel is used when the temperature is higher than -100 °C; austenitic steel is used when the temperature is lower than -100 °C; copper and aluminum materials are available for low pressure and small diameter valves.
Select the appropriate material based on the minimum operating temperature.
(3) Material of valve stem and fastener.
- A. When the temperature is higher than -100 °C, the valve stem and bolt materials are made of alloy steel such as Ni, Cr-Mo, etc., and go through heat treatment to improve the tensile strength and prevent thread biting. 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 valve stem and the filler to scratch each other, causing the filler to leak. Therefore, the surface of the stem must be plated with hard chrome (0.04-0.06mm thick) or nitrided and nickel-plated to increase surface hardness. In order to prevent the nut and the bolt from being killed, the nut is generally made of Mo steel or Ni steel, and the surface of the thread is coated with molybdenum disulfide.
- B. Selection of low temperature valve gasket and packing material: As the temperature decreases, the shrinkage of the fluoroplastic is large, which will result in a decrease in sealing performance and easy to 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, a long neck valve cover structure is used to separate the packing.