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As crude oil prices decline, consumption of petroleum fuels is increasing. To supply refined products, refineries across the world are importing crude oil through very large crude carriers (VLCCs) and extra-large crude carriers (ELCCs) using single point mooring (SPM) systems, and storing crude oil in onshore tank farms before pumping it to refinery crude oil tanks. This work discusses the major design aspects and construction of large crude oil tanks, with an illustration of a typical external double-deck floating roof tank with an inside diameter of 79 m and a height of 18 m.
Type of tank. Generally, the crude oil vapor pressure in the tank is approximately 14.6 psi. To reduce vapor emissions and the loss of hydrocarbons from the tank, the type of tank (i.e., floating roof, fixed-cone roof, etc.) should be decided initially. For large crude oil storage tanks, double-deck floating roof (DDFR) tanks are selected for safety and to reduce evaporation losses. To maintain the pour point in the tank, side entry mixers and heaters are provided.
Storage tank sizing. Since the volume of crude oil handled is high, the sizes of storage tanks have also increased for smooth and continuous operations. A storage tank with an inside diameter of 79 m and a height of 18 m has become the standard size, with the ability to handle approximately 70 Mm3 of oil at a time. The volume of pumpable crude from the storage tank is calculated as the volume between low level (LL) and high level (HL).
An inlet pumping rate of 5 min from HL to high-high level (HHL) storage should be maintained so that the operator monitoring the level can cut off the inlet valve when it reaches HL. The closure time, or response time, of the inlet valve is based on a surge analysis of the inlet line. Generally, 2 sec per in. diameter of pipe is allowed for safe shutoff of the inlet valve. If the inlet valve is not shut off at HL and the height of fluid in the tank reaches HHL, then the immediate tank body valve located on the tank is shut off. This valve is called the remote operated shutoff valve (ROSOV) (fig. i). At least...