The mast riser is normally used during loading for tank vapour pressure control. Its exit location, being at least 6 meters above the deck, allows for the free flow of the vapours displaced from the cargo tanks by the incoming liquid crude oil at the rate of loading of the cargo.
The rate of displacement of VOC (Volatile Organic Compounds) vapours from the cargo tank system will be the same as the loading rate but the concentration of VOC vapours in the displaced stream will be greater dependent upon the extent and rate of evolution of VOC vapours from the incoming cargo that would add to the volume of gas/vapour mixture already existent in the cargo tank prior to loading.
The concept of mast riser is slowly and steadily decreasing, i have often come across vessels not fitted with a mast riser at all. The venting is expected to be carried out by P/V valves only whilst loading or by the vapour return line.
In case you dont have a mast riser you ll be having a high velocity vent on top of each tank . Whereas mast riser is common for all tanks .