Why use a long-necked bonnet for low temperature valves?

Valves that are suitable for medium temperatures of -40℃ to -196℃ are referred to as cryogenic valves. Such valves typically employ long-necked bonnets. In the field of engineering applications, cryogenic valves are extensively utilized in piping systems for the conveyance of cryogenic liquids or gases. In comparison to valves operating at room temperature, the working environment of cryogenic valves is more challenging, necessitating the use of safety and performance measures while meeting the higher sealing and pressure resistance requirements. The use of a long-necked bonnet is a crucial aspect in ensuring the performance of cryogenic valves.

The use of long-necked bonnet provisions of cryogenic valves, including cryogenic emergency shut-off valves, cryogenic globe valves, cryogenic check valves, LNG special cryogenic valves, NG special cryogenic valves, and others, is primarily employed in 300,000-tonne ethylene, liquefied natural gas, and other chemical installations. The output of liquid cryogenic media, including ethylene, liquid oxygen, liquid hydrogen, liquefied natural gas, and liquefied petroleum products, is not only flammable and explosive but also exhibits a significant temperature rise, resulting in a volume expansion of hundreds of times.

Advantages of adopting a long neck valve cover:

(1) The long-necked bonnet serves to protect the packing box of the cryogenic valve, as the sealing of the packing box is a crucial aspect of the cryogenic valve. Leakage at the stuffing box will result in a reduction of the cooling effect and may lead to liquefied gas gasification. In the low-temperature state, with the temperature decreasing, the packing elasticity gradually disappears, and the leakage performance decreases due to media leakage caused by packing and stem icing. This affects the normal operation of the valve stem. Furthermore, the up and down movement of the valve stem and the pulp filler scratches result in serious leakage. Therefore, it is essential to ensure that the filler environment is maintained at a temperature above 8 ℃.

(2) The long-necked bonnet structure facilitates the winding of cold materials to prevent cold energy loss in low-temperature valves.

(3) The long-necked structure of the low-temperature valve facilitates the replacement of main valve parts by removing the bonnet. As the process piping and valves in the cold section of the equipment are often installed in the cold box, the long-neck valve cover can be extended through the wall of the cold box. When replacing the main part of the valve, it is only necessary to remove it through the bonnet without dismantling the valve body. This is achieved by welding the valve body to the pipework as a single unit, which minimizes leakage from the cold box and ensures that the valve is leak-tight.