It is desirable to minimize this potential difficulty with small diame- ter floating roofs by avoiding roof ladders, swing lines, and closed-type roof drains whenever possible. This loss of buoyancy can result in erratic roof movement during the filling or emptying of small-diameter (10 to 15 feet) tanks. A disadvantage of internal floating roofs is that the seals cannot be maintained while the tank is in service.Īs the diameter of a floating roof decreases, the buoy- ant force that floats the roof decreases in relation to the frictional resistance to vertical movement at the periphery of the roof. Tanks with internal floating roofs are used when the stock contained in the tank is sensitive to water contamination (such as jet fuel), or if other factors such as very heavy snow loads would complicate the design of an external floating roof.Īir scoops are required to ventilate the space between an internal floating roof and the fixed roof above it, to prevent the accumulation of vapors in an explosive mixture with air.

They permit use of an existing cone roof tank when the service requires a floating roof.They are generally preferred to tanks with internal floating roofs, because they are more economical to construct and much easier to inspect and maintain.Īlthough external floating roofs are preferred, internal floating roofs are also useful: 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. The floating roofs can be either external (i.e., with open top tanks) or internal (i.e., inside fixed roof tanks). Floating roof tanks cost more to construct than fixed roof tanks for the same storage capacity.

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