Vapor return line

A vapor return line is the pipe that returns cargo vapor between ship and shore during loading and discharge on tankers and gas carriers, so the gas displaced from the receiving tank is collected rather than vented. As liquid enters a tank it pushes out an equal volume of vapor; the return line sends that vapor back to the source tank or to a shore vapor-recovery unit, controlling emissions and tank pressure. It is required for vapor balancing on cargoes covered by MARPOL Annex VI Reg 15 volatile organic compound controls and by terminal vapor-emission-control regulations.