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Figure 7.3. Water Draw-Off System Detail. 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. 75 UFC 3-460-03 21 JANUARY 2003 Figure 7.4. Floating Pan. Figure 7.5. Floating Pan Detail. 76 UFC 3-460-03 21 JANUARY 2003 Figure 7.6. Floating Pan Seal. Figure 7.7. New Standard Tank Seal. 77 UFC 3-460-03 21 JANUARY 2003 7.2.1.3. Automatic Float Gauge. Figures 7.8 and 7.9 show the type of gauge used on an open-top floating-roof tank. The gauge is actuated by a float in a well in the deck. The float is connected to the gauge tape by a stainless steel cable. By connecting the float to the cable with a turnbuckle, it is possible to make quick adjustments for over-reading or under-reading (lengthen the cable for under-reading; shorten the cable for over-reading). The tape is counterweighted and both tape and counterweight are enclosed in a weatherproof housing. The tape is read through a window. The gauge head (Figure 7.9) uses a spring-actuated storage sheave to take up the tape instead of counterweights; the tape is passed over a sprocket sheave that registers the liquid level in the tank on counter wheels for a more accurate reading. On tanks with floating pans, the tape is attached directly to the honeycomb pan (Figure 7.10). NOTE: This design will not read fuel level below the setting of the pan legs, and is not a substitute for automatic tank gauging. Figure 7.8. Automatic Float Gauge. 78 ... - tailieumienphi.vn
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