Valves suitable for medium temperature -40℃～-196℃ are called low temperature valves, and such valves generally use long-necked bonnets.
The long-neck bonnet is used to stipulate that the cryogenic valve includes cryogenic emergency shut-off valve, cryogenic globe valve, cryogenic check valve, LNG special cryogenic valve, NG special cryogenic valve, etc., which are mainly used in chemical plants such as 300,000 tons of ethylene and liquefied natural gas. The output liquid low-temperature media such as ethylene, liquid oxygen, liquid hydrogen, liquefied natural gas, liquefied petroleum products, etc., are not only flammable and explosive, but also gasify when heated, and the volume expands hundreds of times when gasified.
Long neck bonnets are required because:
(1) The long neck bonnet has the function of protecting the low temperature valve stuffing box, because the tightness of the stuffing box is one of the keys to the low temperature valve. If there is a leak at this stuffing box, it will reduce the cooling effect and cause the liquefied gas to vaporize. At low temperature, as the temperature decreases, the elasticity of the packing gradually disappears, and the leak-proof performance decreases accordingly. Due to the leakage of the medium, the packing and the valve stem freeze, which affects the normal operation of the valve stem and also causes the valve stem to move up and down. Packing scratched, causing serious leakage. Therefore, it is necessary to ensure that the temperature of the filling part is above 8 °C.
(2) The long-neck valve cover structure is convenient for wrapping cold insulation materials to prevent the loss of cold energy of low temperature valves.
(3) The long neck structure of the cryogenic valve is convenient for quick replacement of the valve main part by removing the valve cover. Since the process pipes and valves in the cold section of the equipment are often installed in the \’cold box\’, the long-necked valve cover can protrude through the \’cold box\’ wall. When replacing the main valve parts, it is only necessary to remove and replace the valve cover without disassembling the valve body. The valve body and the pipeline are welded into one body, which reduces the leakage of the cold box as much as possible and ensures the tightness of the valve.
Post time: Oct-11-2022