For normal venting api 650 g 8 3 and h 6 2 specifies vents in the fixed roof tank shell and internal floating roof.
Floating roof tank breather vent.
The vents in the floating roof protect the roof and rim seal and accommodate liquid movement and thermal breathing.
Floating roof tanks are commonly protected against lightning ignition by bonding the floating roof to the seal shoes at no less than 3m 10ft intervals use of insulating sections in the hanging linkages covering sharp points on hangers with insulating materials and installation of electrical bond straps across each pinned hanger joint.
Tanks with external floating roofs are used primarily to contain liquids with high vapor pressures when the vapor emissions from fixed roof tanks would exceed the standards set by the local jurisdiction.
Depending upon how a tank receives product there are instances where entrained vapour may be released into the tank from the filling pipeline.
The rim vents are fitted with a hinged pallet lift set to lift when the set pressure in the tank exceeds the set pressure allaowing the air gas to flow.
For a tank with a fixed roof and internal floating roof but without a gas blanket.
The floating roof rim vent has a cast body which can be flanged or threaded for connection to the storage tank the weatherhood protects the exhaust port of the rim vent from atmospheric conditions.