Features of Forged Steel Cryogenic Globe Valves

Features of Forged Steel Cryogenic Globe Valves

Dec 03, 2020


Forged steel cryogenic globe valves are suitable for the management system of cryogenic liquid storage and transportation equipment, which have the characteristics of flexible opening and closing and reliable sealing. They can also be used for other management systems for low temperature and cryogenic media. Normal temperature globe valves refer to valves whose medium operating temperature are between -40 and +500 degrees. The material of the valve body is also different. Cryogenic globe valves mainly refers to globe valves whose medium temperature is between -196 to-150 degrees in the low-temperature liquid medium system. Cryogenic valves especially ultra-low temperature valves have extremely low operating temperatures. When designing this kind of valve, in addition to following the general valve design principles, there are also some special requirements.
 
Design requirements for cryogenic valves
 According to service conditions, the design of cryogenic valves has the following requirements:
  • The cryogenic valve should have the ability to work for a long time in low temperature medium and ambient temperature.
  • The cryogenic valve should not be a significant heat source for the cryogenic system. This is because the inflow of heat reduces the thermal efficiency. If the inflow is too much, the internal fluid will evaporate rapidly, causing abnormal pressure rise and causing danger.
  • The low-temperature medium should not have a harmful effect on the operation of the hand wheel and the sealing performance of the packing.
  • The cryogenic valve assembly being directly in contact with the cryogenic medium should have an explosion-proof and fire-proof structure.
  • The valve assembly working at low temperature cannot be lubricated, so structural measures need to be taken to prevent the friction parts from scratching.
Material selection of cryogenic valves
Main materials of cryogenic valves

(1) Factors to be considered in the selection of main materials
From the perspective of metallography, in addition to austenitic steel, copper and aluminum with face-centered cubic lattice, general steel materials will exhibit low-temperature brittleness at low temperatures, reducing the strength and service life of the valve. When choosing the main material, first choose the material that is suitable for working at low temperature. Aluminum will not exhibit low-temperature brittleness at low temperatures, but the wear resistance and scratch resistance of the aluminum sealing surface are poor due to the poor hardness of aluminum and its alloys. Therefore, there are certain restrictions on the use of low-temperature valves. Cryogenic valves made of aluminum and aluminum alloys with low pressure and small diameters can only be chosen.
 
Materials that work at low temperatures must ensure their low temperature performance, mainly to ensure their cold impact strength. The material of the valve internal parts must be selected correctly to make them have sufficient cold impact strength to prevent fracture. C and Cr alloy steel lose their impact strength quickly when the temperature is lower than -20℃, so the operating temperature is limited to -30℃ and -50℃ respectively. Nickel steel containing 3.5% Ni can be used in a temperature of -100°C, and nickel steel containing 9% Ni can be used in a temperature of -192°C. Austenitic stainless steel, nickel, monel, hastelloy, titanium, aluminum alloy and bronze can be used in a lower temperature of -273°C. In addition, the following factors should also be considered for the material selection of forged steel cryogenic globe valves:
  • The minimum operating temperature of the valve
  • The mechanical properties required by the metal material to maintain working conditions at low temperatures, especially the impact toughness, relative elongation and structural stability
  • Being of good abrasion resistance at low temperatures and without oil lubrication
  • Having good corrosion resistance
  • Considering the welding performance of the material when adopting welding
 
(2) The selection of materials for the valve body, valve bonnet, valve seat and valve disc or wedge The selection principles of the materials of these main parts are roughly as the following:
  • Ferritic steel should be used when the temperature is higher than -100℃.
  • Austenitic steel should be used when the temperature is lower than -100℃. 
  • Copper and aluminum can be used for low pressure and small diameter valves.
  • When designing, select the appropriate material according to the lowest operating temperature.
 
(3) Material selection of valve stems and fasteners
When the temperature is higher than -100℃, the valve stem and bolt material shall be Ni, Cr-Mo and other alloy steels, and the valve stem and bolt should be heat treated properly to improve the tensile strength and thread bites. When the temperature is lower than -100℃, valve stems and fasteners should be made of austenitic stainless acid-resistant steel. However, the 18-8 acid-resistant steel has low hardness, which will cause the valve stem and packing to scratch each other, causing leakage at the packing. Therefore, the surface of the valve stem must be plated with hard chromium or nitrided and nickel-phosphorus plated to improve the surface hardness. The thickness of the coating is from 0.04 to 0.06mm. 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. Material Selection of gaskets and packing for cryogenic valves
As the temperature decreases, the fluorine plastic shrinks greatly, which will reduce the sealing performance and easily cause leakage. Asbestos filler cannot avoid permeability leakage. Rubber has swelling properties to liquefied natural gas and cannot be used at low temperatures. In the design of cryogenic valves, the structural design ensures that the packing is close to the ambient temperature. For example, the valve bonnet with a long neck structure is used to separate the packing box.
 

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