Tank vapours can be released and sent clear of the decks during loading through large, high velocity vent. The type shown above has a moving orifice, held down by a counter weight to seal around the bottom of a fixed cone. Pressure build up in the tank, as filling proceeds, causes the moving orifice to lift. The small gap between orifice lip and fixed cone gives high velocity to the emitted vapour. It is directed upwards with an estimated velocity of 30 meters per second. Air drawn in by the ejector effect dilutes the plume. The conical flame screen fixed to the moving orifice to give protection against flame travel will, like the moving parts, require periodic cleaning to remove gummy deposit. The cover is closed (as shown) when the vessel is on passage. A simpler design of a high velocity vent, having two weighted flaps which are pushed open by the pressure build up to achieve a similar nozzle effect.
References “General Engineering Knowledge” by H.D. McGeorge