Morrison Bros. 244OM-0100AV 4-Inch, 8-Ounce per Square Inch, Emergency Vent - Male NPT with Viton A "O" Ring

 Emergency vents are installed on above-ground storage tanks as a code requirement. Emergency vents provide high-capacity pressure relief if the tank is exposed to fire. When the internal tank pressure reaches the opening pressure of the vent, the weighted cover moves up on the center pin allowing air to escape. When the internal tank pressure falls, the cover automatically lowers back onto the body and is automatically reset.