As we all know, suitable for medium temperature -40 ℃ ~ -196 ℃ valve called cryogenic valve, and such valves are generally used long bonnet cover. Low-temperature valve with long-necked valve cover provisions Low-temperature valve, including low temperature emergency shut-off valve, low temperature stop valve, low temperature check valve, LNG cryogenic valve, NG special cryogenic valve, mainly for 300,000 tons of ethylene, liquefied natural gas and other chemical plant on. The output of liquid low temperature medium such as ethylene, liquid oxygen, liquid hydrogen, liquefied natural gas, liquefied petroleum products, etc., not only flammable and explosive, but also when heating to gasification, gasification, volume expansion hundreds of times. The low-temperature valve regulates the use of a long-necked bonnet because: (1) The long-necked bonnet functions as a stuffing box to protect the cryogenic valve because the tightness of the stuffing box is one of the keys to the cryogenic valve. If the stuffing box at the leak, will reduce the cooling effect, resulting in liquefied gas gasification. Under low temperature conditions, as the temperature decreases, the elasticity of the filler gradually disappears and the leakage prevention performance decreases. As the medium leaks, the filler and the valve stem freeze, affecting the normal operation of the valve stem and also moving the valve stem up and down. Packing scratched, causing serious leaks. Therefore, to ensure that the filler 臬 parts of the temperature above 8 ℃. (2) The structure of long-necked bonnet is easy to wrap around cold-proof materials to prevent cold energy loss of cryogenic valve. (3) low temperature valve long neck structure allows easy removal of the valve main parts through the valve cover for quick replacement. As process piping and valves in equipment cold sections are often installed in 'cold boxes', the bonnet can extend through the 'cold box' wall. When replacing the valve main parts, it is only necessary to replace the valve cover without disassembling the valve body. Body and pipe welding into one, as much as possible to reduce the leakage of cold boxes to ensure the sealing of the valve.