The purpose of vapour return systems is to ensure that cargo vapours are not released to the atmosphere. Vapour return lines on chemical tankers are either connected to the vessel’s PV line or, if the vessel is fitted with an inert gas system, to an extension of that system.
The vapour return lines are led to the manifold where a connection is provided to connect to the shore vapour collecting line.
As the tank is loaded the shore recovers the displaced gas from the tank being loaded where it is stored, treated or disposed of under controlled conditions. The vapour return system is intended to maintain a slight over pressure in the cargo tank(s) which is below the pressure setting of the tank’s PV valve.
In certain circumstances, the vessel is required to accept cargo vapours from shore while the ship is discharging, although the operating principle remains the same.
In order to ensure that the balance of gas between ship and shore is maintained within safe limits, over and under pressure sensors are provided on the ship’s vapour return line to provide a warning should the operational limits be exceeded. The tank’s PV valve remains the essential safety device should the pressure or vacuum in the tank exceed the designed set levels.