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TANK FARM DESIGN

Storage tanks are a critical part of bulk blending plant operation. They may be used to store base blending components, solvents, additives, acids, caustic, chemicals, or finished products. In addition, they are often used as blending vessels. Storage tanks are expensive to build and they require periodic maintenance to keep them in proper operating condition. For all these reasons, it is important that they be properly sized and utilized to maximize the return on investment.

 Tank Farm

Beamer & Associates, Inc. uses a number of computerized tools to help determine how many tanks are needed for a given process, what their capacities should be, and how they should be used. The method used for the analysis varies with the intended function of each storage tank.


TANK CAPACITIES

Tanks to be used for the storage of raw materials are sized based on the projected daily consumption of the materials to be stored, the receipt parcel size, and the reliability of the supply. The calculation sizes each tank to receive the anticipated parcel size, plus sufficient capacity to operate for given number of days without going empty.

TANK DESIGN

Tanks that will be used to store finished products fall into two categories; dedicated and swing. Dedicated tanks are used for finished products that are packaged or shipped in high frequency, such that near-continuous supply is needed. Since each dedicated tank is used for only one product, a small amount of batch-to-batch residue is normally not a problem. For this reason, dedicated tanks are typically designed with flat bottoms. Swing tanks are used for multiple products that are packaged or shipped in lower frequency. In these cases, batch-to-batch cross-contamination can be an issue, so swing tanks are typically emptied completely between batches. To accommodate this requirement, they are normally designed with cone or dish bottoms. In both cases (Dedicated or Swing), the size of the tank is based on the size of the finished product batches that it will receive and store. Typically a range of sizes is used in a tank farm to accommodate a variety of different batch sizes.

TANK DIMENSIONS

Once the tank capacities are determined, the next step is to determine tank diameters. For practical, as well as aesthetic reasons, it is desirable to build tanks of similar heights. Tanks of equal height permit easier and less costly construction of access platforms and catwalks. They are also more efficient to operate, since the operator can move from the top of one tank to another more easily. For this reason, tank heights tend to be standardized in a tank farm and tank diameters vary with capacity.

TANK SEPARATION

Tank-to-tank separation is important for operations, piping design, and maintenance access. In addition, there are minimum separations that may be required to meet local fire codes and other regulations. Minimum tank separations can vary, depending on the material to be stored in the tank and capacity of the tank.

SPILL CONTAINMENT

Spill containment is required for most tank farms, and normally consists of a concrete or earthen dike wall build around the perimeter of the tank farm. The required capacity of the dike wall can vary according to local codes, but should be at least 110% of the capacity of the largest tank, plus some allowance for rain fall. In some parts of the United States, the rain fall allowance may be 4 inches or more. That would mean the dike wall would be four inches higher than that needed to contain 110% of the largest tank. The floor of the tank farm should be constructed of a impermeable substance to prevent any spilled material from leaching into the soil. For small to medium sized tank farms, this requirement is typically met through the use of concrete or asphalt paving installed from dike wall to dike wall, and between all the tanks. To minimize tank-bottom corrosion from rain water that may collect inside the containment area, tank foundations are typically designed to elevate the tanks above the floor of the tank farm.
 Tank Foundations
Tank Foundations with Spill Containment Dike Wall


PIPING DESIGN

Piping design in the tank farm depends on the intended location of the transfer pumps. If the pumps are located together on a "pump pad" or outside the diked area, then pump suction lines tend to be quite long. In these cases, NPSH (Net Positive Suction Head) becomes critical and piping between the tanks and the pumps must be designed accordingly. Low NPSH normally forces suction piping to be larger in diameter and lower to the ground. Piping layout become critical to minimize the need for pipes to "cross over' one another, and thus avoid the creation of high and low points in the piping. If the pumps are located near the tanks, then NPSH is maximized and the piping inside the tank farm is from the discharge side of the pump. In this case the piping configurations can be routed overhead and precise piping layouts are less important.
   
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