UFC 3-460-03
21 JANUARY 2003
7.2. Types of Tanks.
7.2.1. Aboveground.
7.2.1.1. Floating Roof. These types of tanks are in general use for storage of light-weight volatile
liquids and jet fuels. The tank is designed to decrease vapor space over the stored liquid. The
problem of rainfall or melting snow accumulating on the top roof deck of the open-top floating-
roof tank is improved by sloping the roof to a center sump. The sump is connected to a hose or
multi-jointed pipe extending through the fuel to an outside water draw-off valve. Because this
valve must be closed and locked when unattended, water contamination remains a problem where
rainfall is heavy over short periods. Most floating-roof tanks have aluminum fixed roofs installed
over the open top where excessive water contamination of fuel is a possibility. For maintenance
requirements and responsibilities see paragraph 10.6.2. For all new construction, construct a cone
roof tank with internal aluminum honeycomb floating pan (see MIL-HDBK-1022A).
7.2.1.2. Roof Seal. The efficiency of an open-top floating roof in preventing evaporation losses,
entrance of precipitation, and reducing the possibility of rim fires, depends largely on the
effectiveness of the seal closing the space between the rim of the roof and the tank shell. If the
seal does not prevent the escape of vapors around the sealing ring, evaporation will occur. The
sealing ring must fit the tank shell snugly. The type of seal generally used with the open-top
floating-roof tank has a continuous steel ring with vertical flexures about 0.55 meter (22 inches)
apart (Figure 7.4). A continuous, gastight, weatherproof, synthetic-rubber-coated fabric closes the
space between the sealing ring and the rim of the roof. The sealing ring is supported and held
firmly but gently against the tank shell by pantograph hangers. Because these hangers apply a
uniform, outward radial pressure at each flexure in the sealing ring, they tend to keep the roof
property centered in the tank. In freezing weather, the seal must be kept free of ice. Moderate use
of calcium chloride crystals is permitted at the discretion of the BCE. The sealing ring must move
freely on the tank shell during filling as well as during removal of fuel from the tank. Open-top
floating-roof seals are also of the type shown in Figure 7.5. Seals for the cone roof tanks that have
been converted with the floating pan "floater" usually have the type of rim seal shown in
Figure 7.6. Cone roof tanks built to the new standard are equipped with honeycomb aluminum
pans and dual pan-to-tank seals as shown in Figure 7.7.
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