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Insights into the basics of process design of atmospheric storage tanks and an example of how to prepare a process datasheet are presented here
Storage tanks are widely used in the petroleum refining and petrochemical sectors to store a variety of liquids, from crude petroleum to finished product (Figure 1). This article presents the basic process of designing atmospheric storage tanks (ASTs), as well as a discussion about preparing a process datasheet. An example is used to illustrate the points made.
When to opt for ASTs
In simple terms, storage tanks that are freely vented to the atmosphere are known as (aboveground) atmospheric storage tanks (ASTs). They have a vertical cylindrical configuration and can be easily identified by the open vent nozzle or "gooseneck" vent pipe on the tank roof. ASTs may be shop-welded or fieldwelded and are customarily fabricated from structural quality carbon steel, such as A-36 or A-283 Gr.C. The vertical cylindrical shape and relatively flat bottom helps to keep costs low.
ASTs store low-vapor-pressure fluids that do not pose any environmental, hazard or product-contamination issues, so they can be freely vented to the atmosphere. However, when storing certain fluids, such as when vapors of the stored liquid are flammable or when oxidation of liquid may form hazardous compounds, it is undesirable to have the tank vapor space freely vented. In such cases, inert gas blanketing of the vapor space may be used. Tanks with inert-gas blanketing are also often included in this category. A blanketing system is normally designed so that it operates at slightly higher than atmospheric pressure, therefore preventing outside air from entering the vessel.
Typically, ASTs are considered to have an operating pressure ranging from 0 to 0.5 psig. Tanks designed to operate at pressures between 0.5 and 15 psig are termed as low-pressure storage tanks. Designs above 15 psig are treated as pressure vessels.
Tank roof types
There are two basic types of vertical-tank roof designs - fixed or floating roof.
Fixed roof. In this design, the tank roof is welded with the shell and the roof remains static.
Floating roof (internal or external). In this design, the tank roof floats on the liquid surface and rises and falls with changes in liquid level. The internal floating-roof...