Vapor Return Valve: Generally known as the vapor equalizing link or vapor equalization valve, this attribute is needed on all ASME propane tanks. They are often deployed in the event that the liquid during the container becomes extremely overheated and have to be evacuated within the container. Do your investigation: https://120gallonverticalpropanet00998.alltdesign.com/facts-about-buy-propane-tanks-revealed-46239493