The vapor return valve is also called the vapor equalizing connection or vapor equalization valve. Vapor return valves are required on all ASME propane tanks and will generally be used during periods of hot weather. In the event that liquid must be evacuated from the container, the vapor return valve will be used.
The vapor return valve is used primarily to relieve the pressure in the tank being filled to prevent overpressure and potential activation of the safety relief valve during periods of hot weather. The vapor hose from the delivery truck is connected to the tank's vapor return valve during the filling process and the excess pressure is vented back to the truck. This allows the vapor pressure to equalize between the tank and truck preventing an over-pressure situation at the tank. Basically, the vapor return valve is a manually operated safety relief valve that prevents prevents propane from being released into the atmosphere.
If the pressure in a propane tank gets too high, the safety relief valve will open allowing propane to release into the atmosphere. When propane companies fill tanks on hot days they will often connect the vapor hose to the vapor return valve. This allows vapor to be safely recovered and contained in the delivery truck. Many people think that the propane they are buying is being pumped back into the truck and they are losing gas that they have just paid for. This is not the case as only the excess vapor pressure is being relieved from the tank being filled. Think of it this way:
You go to a gas station in the morning and fill your car with gas, place the gas cap back on the tank opening and drive to work. If you were to go out to your car at lunch and open the gas cap you would hear a hissing sound as you turned the cap. This hissing sound would be gasoline vapors escaping from your gas tank and you didn't lose any gas. It's the same principle with regard to propane but because propane is such a high pressure gas that will vaporize in the atmosphere, the excess pressure is removed during the filling process.
The vapor removed from the tank through the vapor return valve is much less than if is removed through the safety relief valve.