Valves suitable for operating in medium temperatures ranging from -40℃ to -196℃ all adopt long-neck valve covers. The use of long-neck valve covers stipulates that low-temperature valves include low-temperature emergency shut-off valves, low-temperature stop valves, low-temperature check valves, LNG-specific low-temperature valves, NG-specific low-temperature valves, etc., and are mainly used in chemical plants such as those for 300,000 tons of ethylene and liquefied natural gas. The output of liquid low-temperature media such as ethylene, liquid oxygen, liquid hydrogen, liquefied natural gas, and liquefied petroleum products are not only flammable and explosive, but also need to vaporize when heated, and during vaporization, the volume expands hundreds of times. 

The reason for using the long-neck valve cover is: 

(1) The long-neck valve cover has the function of protecting the low-temperature valve packing box, as the sealing performance of the packing box is one of the key factors of the low-temperature valve. If there is leakage at the packing chamber, it will reduce the insulation effect and cause the liquefied gas to vaporize. Under low-temperature conditions, as the temperature decreases, the elasticity of the packing gradually disappears, and the leak prevention performance also declines. Due to the leakage of the medium, the packing and the valve stem will freeze together, affecting the normal operation of the valve stem. At the same time, due to the up and down movement of the valve stem, the packing will be scratched, causing serious leakage. Therefore, the temperature of the packing area must be above 8℃. 

(2) The long-neck valve cover structure facilitates the winding of insulation materials, preventing the loss of cold energy from the low-temperature valve. 

(3) The long-neck structure of the low-temperature valve facilitates the quick replacement of valve components by removing them through the valve cover. Since the process pipelines and valves in the cold section of the equipment are often installed in 'cold boxes', the long-neck valve cover can extend through the 'cold box' wall. When replacing the main components of the valve, it is only necessary to remove and replace through the valve cover, without the need to disassemble the valve body. The valve body is welded integrally with the pipeline, which minimizes the leakage of the cold box and ensures the sealing performance of the valve. 

Selection of materials for low-temperature valves: 

The valve body and valve cover are made of: LCB (-46℃), LC3 (-101℃), and CF8 (304) (-196℃). 

2. Gate plate: Stainless steel clad with cobalt-based hard alloy. 

3. Valve seat: Stainless steel clad with cobalt-based hard alloy. 

4. Valve stem: 0Cr18Ni9. 

Low-temperature valve testing equipment: 

Low-temperature valve test device 

2. Liquid nitrogen storage device 

3. Low-temperature treatment tank 

4. Low-temperature test bench to ensure the performance of the valve under low-temperature operating conditions 

Low-temperature valve test: 

The main components of the low-temperature valve are subjected to low-temperature treatment, and a low-temperature impact test is conducted on each batch for sampling purposes to ensure that the valve does not crack under low-temperature conditions and can withstand the impact of low-temperature media. 

Perform the following tests on each valve: 

Temperature-controlled shell strength test; 

2. Sealing test under normal temperature and low pressure; 

3. Normal temperature and low-pressure sealing test; 

4. Low-temperature sealing and air-tightness test (when there is an upper seal); 

5. Conducting low-temperature gas sealing tests, etc., to ensure that the entire low-temperature valve complies with the standards stipulated. 

6. The main components are subjected to low-temperature treatment and a sampling test for low-temperature impact is conducted for each batch to ensure that the valve does not crack under low-temperature conditions and can withstand the impact of low-temperature media. 

7. The low-temperature (cryogenic) valves have undergone low-temperature treatment and impact tests in accordance with the relevant material specifications. 

8. The static electricity function is more powerful. The conductive resistance between the valve body and the valve stem or between the internal components and the valve body is less than 1 ohm.