Features of forged steel cryogenic globe valve and selection of structure

Forged steel cryogenic globe valve is suitable for the management system of cryogenic liquid storage and transportation equipment. It has the characteristics of flexible switch and reliable sealing. It can also be used for management systems for other low temperature and cryogenic media. Normal temperature shut-off valve means that the working temperature of the medium is between -40 and +500 degrees, and the material of the valve body is also different. The low temperature stop valve mainly refers to the stop valve in the low temperature liquid medium system with the medium temperature of -196~-150 degrees.

Forged steel cryogenic globe 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 innovations.

1. Design of cryogenic valve

According to the conditions of use, the design of cryogenic valves has the following requirements:

1) The valve should have the ability to work for a long time in the low temperature medium and the ambient temperature.

2) The valve should not become a significant heat source for the cryogenic system. This is because the influx of heat not only reduces the thermal efficiency, if the inflow is too much, the internal fluid will evaporate rapidly, causing abnormal pressure rise and causing danger.

3) The low temperature medium should not have a harmful effect on the operation of the hand wheel and the sealing performance of the packing.

4) The valve assembly directly in contact with the cryogenic medium should have an explosion-proof and fire-proof structure.

5) Valve assemblies that work at low temperatures cannot be lubricated, so structural measures need to be taken to prevent frictional parts from scratching.

2. Material selection of cryogenic valve structure

1. The main material of the cryogenic valve

(1) Factors to be considered in the selection of the main material From the consideration of metallography, in addition to the austenitic steel, copper, aluminum, etc. with face-centered cubic lattice, the general steel will have low temperature at low temperature. Brittleness, thereby reducing the strength and service life of the valve.

When choosing the main material, first choose the material suitable for working at low temperature. Aluminum will not exhibit low-temperature brittleness at low temperatures, but because the hardness of aluminum and aluminum alloys is not high, the aluminum sealing surface has poor wear resistance and scratch resistance, so there are certain restrictions on its use in low-temperature valves. Optional for small diameter valves.

Materials that work at low temperatures must ensure their low temperature performance, mainly to ensure their cold impact strength. The internal parts of the valve must be selected correctly to make it have sufficient cold shock strength to prevent fracture. C and Cr alloy steel loses its impact strength quickly when it is lower than -20℃, so the service temperature is limited to -30℃ and -50℃ respectively. Nickel steel with 3.5% Ni can be used to -100°C, and nickel steel with 9% Ni can be used to -192°C. Austenitic stainless steel, nickel, Monel, Hastelloy, titanium, aluminum alloy and bronze can be used to lower temperatures (-273°C).

1) The minimum operating temperature of the valve;

2)Mechanical properties required by metal materials to maintain working conditions at low temperatures, especially impact toughness, relative elongation and structural stability;

3) In the case of low temperature and no oil lubrication, it has good friction resistance;

4) Has good corrosion resistance;

5) When using welding connection, the welding performance of the material should also be considered.

(2) Selection of materials for valve body, valve cover, valve seat and valve disc (gate)

The selection principle of the materials of these main parts is roughly: 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. And other materials.

When designing, select appropriate materials according to the lowest operating temperature.

(3) Material of valve stem and fasteners

1. When the temperature is higher than -100℃, the valve stem and bolt material shall be Ni, Cr-Mo and other alloy steels, after proper heat treatment, in order to improve the tensile strength and prevent the thread bite. When the temperature is lower than -100℃, it is made of austenitic stainless acid-resistant steel. However, the 18-8 acid-resistant steel has low hardness, which will cause the valve stem and the packing to scratch each other, resulting in leakage at the packing. Therefore, the surface of the valve stem must be plated with hard chromium (the thickness of the coating is 0.04-0.06mm), or be nitrided and nickel-phosphorus plated to improve the 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. Selection of cryogenic valve gaskets and packing materials: 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, on the one hand, the structural design ensures that the packing is close to the ambient temperature. For example, a long-necked valve cover structure is used to separate the packing box.