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Facts At Your Fingertips: Roof Classifications of Atmospheric Storage Tanks

By Scott Jenkins, Chemical Engineering magazine |

Storage tanks that are freely vented to the atmosphere are known as aboveground atmospheric storage tanks (ASTs). ASTs have a vertical cylindrical configuration and can be identified by the open vent nozzle on the tank roof. This one-page reference outlines the classifications for ASTs based on roof type, and provides information on tank construction. Tank roof types Two basic types of vertical-tank roof designs exist — fixed or floating roof. Fixed roof. In this design, the tank roof is welded, and the shell and the roof remain static. Fixed-roof tanks consist of a cylindrical shell with a permanently welded roof that can be flat, conical or dome-shaped. As a rule of thumb, fixed-roof tanks are used to store liquids with true vapor pressures (TVP) of less than 10 kPa (absolute) [2]. TVP is the absolute pressure when the vapor is in equilibrium with liquid at a constant temperature. 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. These tanks reduce evaporation losses and control breathing losses while filling. Floating roofs are preferred for tanks storing petroleum products with a TVP of 10.3 to 76.5 kPa absolute. For liquids…
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