A large-diameter high-velocity vent pipe fitted to a tanker's mast to discharge cargo vapours safely above ignition sources.
In practice
IMO recommendations and ISGOTT specify minimum mast riser heights, outlet velocities, and location relative to accommodation and ignition sources. The valve at the base of the mast riser is normally kept closed when tanks are under inert gas control and opened only in specific venting phases of tank cleaning. Engineers and tanker officers must ensure the riser valve is operable and that any flame screen fitted at the outlet is clean and undamaged, as a blocked screen can cause dangerous back-pressure to build up in the tank during venting operations.
Regulatory detail & full definition
A mast riser, also called a high-velocity vent or masthead vent, is a large-diameter pipe fitted to a tanker's mast or king post to discharge cargo vapours at high velocity — typically exceeding 30 metres per second — and at a height well above the main deck. The high exit velocity disperses vapour-air mixtures rapidly into the free atmosphere above the ignition zone of the vessel, reducing the risk of vapour cloud ignition compared with low-velocity tank vents. Mast risers are used primarily when tanks are not under inert gas protection, during port operations where PV valve discharge at deck level might present a hazard.
IMO recommendations and ISGOTT specify minimum mast riser heights, outlet velocities, and location relative to accommodation and ignition sources. The valve at the base of the mast riser is normally kept closed when tanks are under inert gas control and opened only in specific venting phases of tank cleaning. Engineers and tanker officers must ensure the riser valve is operable and that any flame screen fitted at the outlet is clean and undamaged, as a blocked screen can cause dangerous back-pressure to build up in the tank during venting operations.