Vapor Return Valve: Generally known as the vapor equalizing connection or vapor equalization valve, this feature is required on all ASME propane tanks. They tend to be deployed in the event that the liquid inside the container gets to be quite overheated and have to be evacuated from the container. https://1000-gallon-above-ground88765.blogproducer.com/32418725/5-simple-statements-about-propane-tanks-for-sale-explained