Atmospheric pressure and low pressure storage tanks are common equipment in process industry. Atmospheric pressure, low pressure tank in the use of the process often due to changes in the liquid level in the tank, or changes in the external temperature and other reasons, resulting in the expansion or contraction of the gas in the tank, the gas phase pressure in the tank also fluctuates, the fluctuation of gas pressure is easy to make the tank overpressure or vacuum, serious will cause the tank overpressure drum or low pressure dented tank. In order to prevent the instability of the storage tank such as overpressure or negative pressure, the process design usually adopts the way of installing a breathing valve on the top of the tank to maintain the pressure balance of the storage tank, ensure that the storage tank is not damaged during overpressure or vacuum, protect the safety of the storage tank, and reduce the volatilization and loss of the materials in the storage tank, which plays a certain role in promoting safety and environmental protection. The internal structure of the breathing valve is essentially composed of a pressure valve disc (that is, the exhalation valve) and a vacuum valve disc (that is, the suction valve), and the pressure valve disc and the vacuum valve disc can be arranged side by side or overlapping. Its working principle: when the tank pressure and atmospheric pressure are equal, the valve disc of the pressure valve and the vacuum valve are closely matched with the seat, and the sealing structure on the side of the seat has the "adsorption" effect, so that the seat is tight and not leaking. When the pressure or vacuum degree increases, the valve disc begins to open, because there is still a "adsorption" effect on the side of the seat, so it can still maintain a good seal.
When the pressure in the tank rises to a constant pressure value, the pressure valve is opened, and the gas in the tank is discharged into the outside atmosphere through the exhalation valve (that is, the pressure valve). At this time, the vacuum valve is closed due to the positive pressure in the tank. On the contrary, when the pressure in the tank drops to a certain vacuum degree, the vacuum valve opens due to the positive pressure of atmospheric pressure, and the outside gas enters the tank through the suction valve (that is, the vacuum valve), and the pressure valve is closed. At any time, the pressure valve and the vacuum valve should not be open at the same time. When the pressure or vacuum in the tank is reduced to the normal operating pressure state, the pressure valve and the vacuum valve are closed to stop the expiratory or inspiratory process.
The breathing valve plays a sealing role in the normal state, and only begins to work under the following conditions: (1) When the storage tank outputs the material, the breathing valve begins to suck human air or nitrogen into the tank. (2) When the material is filled into the tank, the breathing valve begins to exhale the gas in the tank to the outside of the tank. (3) Due to climate change and other reasons to increase or decrease the steam pressure of the material in the tank, the breathing valve exhales steam or inhales human air or nitrogen (usually called thermal effect). (4) In the event of a fire, the liquid evaporation in the tank increases dramatically due to the heat of the exhaled gas, and the breathing valve begins to exhale out of the tank to avoid damage to the tank due to overpressure. (5) In other working conditions, such as pressurized transportation of volatile liquids, chemical reactions of internal and external heat transfer devices, and operational errors, the breathing valve is exhaled or sucked accordingly to avoid damage to the storage tank due to overpressure or ultra-vacuum.