The cargo tanks of chemical tankers are required to be provided with venting systems which are independent of air pipes and vents serving other parts of the ship. The functions of venting are to relieve pressure and prevent the formation of a vacuum where pressure differential could impair the structure of the tank.
- Controlled tank venting system – A system fitted with pressure/vacuum relief valves. Controlled venting systems are required to be fitted to all tanks carrying cargoes which emit harmful or flammable vapours.
- Open tank venting system – A system which offers no restriction, except for friction losses and flame screens if fitted, to free flow of liquid vapour to or from the tank served during normal liquid transfer or ballasting operation. Open venting is specified for cargoes which pose little or no flammable or toxic hazard.