Breather Valve

The breather valve are designed to protect ASTs from the excessive pressure and/or vacuum. When a  tank is being filled, the vapor that filled the space above the liquid surface is compressed and it this pressure were allowed toexceed the storage tank design pressure, then it would explode or rupture. In addition, if the temperature of the storage tank or vessel increases, then expansion and vaporization may cause the pressure to rise. 

Conversely, emptying liquid from the tank, or a decrease of temperature, cause a vacuum to be created.