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Cryogenic valve design requirements and materials selection
First, the design requirements of cryogenic valves: According to the conditions of use, the design of cryogenic valves have the following requirements: 1) The valve in the low temperature medium and ambient temperature should have the ability to work long hours. 2) Valves should not be a significant source of heat for cryogenic systems. This is because the inflow of heat in addition to reducing thermal efficiency, such as excessive influx, but also the rapid evaporation of the internal fluid, resulting in abnormal pressure, causing danger. 3) Low temperature media should not have a detrimental effect on handwheel handling and packing seal performance. 4) The valve assembly directly in contact with low temperature medium shall have an explosion-proof and fireproof structure. 5) The valve assembly working at low temperature can not be lubricated, so structural measures need to be taken to prevent abrasion of the friction parts. Second, the choice of materials, cryogenic valve 1, the main body of the material cryogenic valve (1) the main material selection should be considered from the metallographic considerations, in addition to metal materials with face-centered cubic lattice austenitic steel, copper, aluminum, The general low temperature steel brittleness, lowers the strength and life of the valve. When choosing the main material, we must first select the material suitable for working at low temperature. Aluminum does not appear low temperature brittleness at low temperature, but due to the hardness of aluminum and aluminum alloy is not high, wear-resistant aluminum surface, scratch resistance is poor, so the use of low-temperature valves have some restrictions, only low pressure and Small diameter valve used. Materials working at low temperature to ensure its low temperature performance, mainly to ensure its impact strength. Trim must be properly selected so that it has sufficient cold impact strength to prevent breakage. Alloy steels C and Cr lose their impact strength sooner than -20 ° C, so the service temperature is limited to -30 ° C and -50 ° C, respectively. Nickel steels containing 3.5% Ni can be used up to -100 ° C and nickel steels containing 9% Ni can be used up to -192 ° C. Austenitic stainless steel, nickel, Monel, Hastelloy, titanium, aluminum alloys and bronze 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 service temperature of the valve; 2) the mechanical properties required to maintain the working conditions of the metal material at low temperatures, especially the impact toughness, relative elongation and texture Stability; 3) Good resistance to abrasion at low temperature and without oil lubrication; 4) Good corrosion resistance; 5) Welding properties of materials need to be considered. (2) The choice of material of valve body, valve cover, valve seat and disc (gate) material, the principle of the selection of materials of these main body parts is as follows: Ferritic steel is selected when the temperature is higher than -100 ℃; 100 ℃ when the choice of austenitic steel; low-pressure and small-caliber valves can choose copper and aluminum and other materials. Design time according to the lowest temperature to select the appropriate material. (3) Valve rod and fastener material selection temperature is higher than -100 ℃, the stem and bolt material using Ni ,, Cr-Mo and other alloy steel, the appropriate heat treatment to improve the tensile strength and prevent thread bite Injury and so on. Temperature below -100 ℃, the use of austenitic stainless steel acid-resistant steel. However, 18-8 acid-resistant steel hardness is low, will cause the stem and filler mutual bruise, resulting in filler leakage. Therefore, the stem surface must be hard chromium plating (coating thickness 0.04-0.06mm), or nitriding and nickel-phosphorus treatment to increase the surface hardness. In order to prevent nuts and bolts from seizing, the nut is generally made of Mo steel or Ni steel, while the surface of the thread coated with molybdenum disulfide. 2, low-temperature valve gasket, filler material selection: With the temperature decreases, a large amount of shrinkage of fluoroplastic, sealing performance will decline, easily lead to leakage. Asbestos filler can not avoid permeability leakage. Rubber liquefied natural gas has swelling, can not be used at low temperatures. Cryogenic valve design, on the one hand by the structural design to ensure that the filler is close to ambient temperature work, for example, the use of long-necked bonnet structure, the packing letter from.