Tank bottoms

To a large exteat, the vapor pressure of the substance stored determines the shape and, consequently, the type of tank used. The roof shape of a tank may be used to classify the type of tank. This classification is self-explanatory to tank fabricators and erectors. Also important is the tank bottom. 

Tank Bottoms. The shape of cylindrical tank closures, both top and bottom, is a strong function of the internal pressure. Because of the varying conditions to which a tank bottom may be subjected, several types of tank bottoms (Fig. 7 Table 4) have evolved. These may be broadly classified as flat bottom, conical, or domed or spherical. Flat-bottom tanks only appear flat. These usually have designed slope and shape and are subclassifted according to the following flat, cone up, cone down, or single slope. 

Fig. 7. Tank bottom designs (a) flat bottom (b) cone-up bottom (c) cone-down bottom (d) cone-down bottom with sump and (e) single slope. In (c), buried lines are not used frequently because of the difficulty of inspection and possibiHty of accelerated corrosion.

Tank bottom slope is important because sediment, water, and heavy phases settle at the bottom. Corrosion is usually the most severe at the bottom, and the design of the bottom can have a significant effect on the life of the tank. In addition, if the Hquid stock is changed, it is usually desirable to remove as much as the previous stock as possible. Therefore, designs that allow for the removal of water or stock and the ease of tank cleaning have evolved. In addition, specialized tank bottoms have resulted from the need to monitor and detect leaks. Tank bottoms in contact with the soil or foundations are one of the primary sources of leaks from aboveground tanks. 

Cone Up. Cone-up tank bottoms are built to have a high point in the center of the tank (Fig. 7b). This is accompHshed by crowning the foundation and constmcting the tank on the crown. The slope is limited to about 1—2 in. (2.5—5 cm) for every 10 ft (3 m) of mn. Therefore, the bottom may appear flat, but heavy stock or water tends to drain to the edge where it can be removed almost completely from the tank. 

In the large-diameter vertical cylindrical tanks, because hoop stress is proportional to diameter, the thickness is set by the hydrostatic hoop stresses. Although the hydrostatic forces increase proportionally with the depth of Hquid in the tank, the thickness must be based on the hydrostatic pressure at the point of greatest depth in the tank. At the bottom, however, the expansion of the shell owing to internal hydrostatic pressure is limited so that the actual point of maximum stress is slightly above the bottom. Assuming this point to be about 1 ft (0.305 m) above the tank bottom provides tank shells of adequate strength. The basic equation modified for this anomaly is... 

Federal Regulations. Federal regulations tend not to be aimed at spill and tank-bottom leak prevention but rather on spill response. These therefore address issues such as containment of spills, financial habihty and responsibihty, discharge of contaminated stormwater, reporting, and response requirements. 

Spill Anatomy and Remediation. Contrary to past arguments that leaks or spills from aboveground tanks would stay near the surface, they go straight down into the aquifer and spread out. Various obstacles, such as clay lenses, rock, or impermeable layers of sod, simply divert the downward path. Slow leaks from tank bottoms tend to form a narrow plume, whereas larger spills cover much wider areas. When the contaminant reaches groundwater, it tends to be dispersed in the direction of the groundwater current and movement. 

Hydrocarbon sensors (qv) placed directiy below the tank bottoms can be effective. However, old contamination or contamination from other tanks or piping can yield misleading results. In addition, the low permeabUity of some areas in the soil can prevent the migration of vapors to the sensing ports under the tank bottom. 

By listening to the sound emitted from leaking tanks, it is possible to estimate not only the existence of, but also the location of, leaks in tank bottoms. Much work needs to be done in this area before it can be considered rehable. 

Pressure Tanks Vertical cylindrical tanks constructed with domed or coned roofs, which operate at pressures above several hundred pascals (a few pounds per square foot) but which are still relatively close to atmospheric pressure, can be built according to API Standard 650. The pressure force acting against the roof is transmitted to the shell, which may have sufficient weight to resist it. If not, the uplift will act on the tank bottom. The strength of the bottom, however, is limited, and if it is not sufficient, an anchor ring or a heavy... 

Figure 6.6 Clustered sulfate-reducer pits on a carbon steel tank bottom.

API RP 652, Lining of Aboveground Petroleum Storage Tank Bottoms, 1st ed., April 1991 (ANSI/API Std 652-1991). 

Complete off-bottom suspension means moving all particles off the tank bottom. 

Complete motion on the tank bottom refers to all particles suspended off the bottom or rolling on the bottom. 

Storage tanks are used throughout the refining process to store crude oil and intermediate process feeds for cooling and further processing. Finished petroleum products are also kept in storage tanks before transport off site. Storage tank bottoms are mixtures of iron rust from corrosion, sand, water, and... 

The rotating type may be driven from either the top or the bottom whilst the vibrating type is always driven from the top. The removal of the tank head cover from top driven filters is generally more complex than those driven from the bottom. On the other hand bottom driven filters are more susceptible to slurry leaks. The position of the cake outlet depends on the construction of the tank bottom. There are two types available ... 

For tanks and vessels, welded units are preferred over riveted or bolted designs. Fastener joints provide sites for crevice corrosion. Undrainable horizontal flat tops of tanks should be avoided unless proper drainage schemes are included in the design. Tank bottoms should be sloped toward... 

Oil-water mixmres and emulsions, e.g., from separators, tank bottoms, ballast water, etc. Heating of such materials is often necessary to separate oil and water. 

Figure 5-5F. Lifter turbine is efficient for pumping large volumes against static heads of less than 36 inches. As shown, It Is used below a draft tube. Inverted, It is used above an orifice plane In tank bottom. Courtesy of Lightnin (formerly Mixing Equipment Co.), a unit of General Signal.

Baffles that extend from the liquid level dowai, but not to, the tank bottom allow heavy swrirling action in the bottom of the tank, but no vortex at the top. Wdien baffles extend from the bottom up, but not to the liquid level, some v ortex and sw irling action will take place at the top. 

A tank bottom of dished or spherical shapes is usually better than a flat bottom as it requires less horsepower for the system. 

The turbine should be positioned to give 35% of the liquid below and 65% above the turbine. For depths exceeding these values, multiple impellers should be used. For total liquid depths less than the effective height of a turbine impeller, the turbine should be located 0.35 Z from the tank bottom, unless there is an ov erriding restraint. 

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