Floating covers

The foam concentrate used with subsurface systems should be a fluoroprotein type for best results, although some AFFF foams are listed for substitutable application (because of their "fuel shedding" properties). The minimum foam solution rate should be 0.3 gpm/ft (12 Ipm/m ). The supply of foam liquid should be adequate to operate the system for 20 minutes. The foam injection point must be above the level of any residual water in the bottom of the tank. Subsurface foam application is not recommended for open or covered floating roof tanks or cone roof tanks with internal floating covers. 

Internal (or covered) floating-roof tanks Vent fire Overfill ground fire Obstructed rim seal fire Obstructed full liquid surface fire Many fires in this type of tank occur as a result of overfilling. Tank will be extremely difficult to extinguish if entire liquid surface becomes involved. Fires in tanks with pan type covers can be expected to develop into obstructed full liquid surface fires. 

Covered floating roof tank—an atmospheric storage tank with a fixed roof and steel internal floating roof of the single-deck pontoon or double-deck design. 

Larson, R.J., Vapor Accumulations within Covered Floating Roof Tanks, Annual Meeting of API Codes and Standards Committee, Los Angeles, CA, May 4, 1966. 

API 2015B Cleaning Open Top and Covered Floating Roof Tanks. 

Two types of designs are commonly applied. For cone top tanks or internal floating roof tanks with other than pontoon decks, multiple foam makers are mounted on the upper edge of the tank shell. These systems are designed to deliver and protect the entire surface area of the liquid of the tank. For open and covered floating roof tanks with pontoon decks, the foam system is designed to protea the seal area. Foam makers are mounted on the outside of the tank shell near the rim and foam is run down inside to the seal area that is provided with vertical barrier adjacent to the seal area i.e., a foam dam to hold the foam in the seal area. This method tends to cause the movement of cooler product to the surface to aid in extinguishment of... 

Shells, Channels, Covers, Floating Heads, Tubesheets, and Flanges Pipe... 

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