Air Valve is a special valve used to prevent the negative pressure in the pipe caused by the decompression wave in the transient process. It is usually installed at a high point where the dynamic water pressure is low when the pipeline is in normal operation, and the liquid column separation may occur during the transient flow process. Air is sucked in when the pressure in the pipe is below atmospheric pressure, and air is discharged when the pressure in the pipe rises above atmospheric pressure. During the exhaust process, the valve can be automatically closed when the liquid in the pipe fills the pipe, and the liquid is not allowed to leak into the atmosphere.