Taking the most commonly used steel vertical cylindrical oil tank as an example, an oil storage tank primarily consists of a tank foundation, floor, wall panels, roof, and accessories.
The tank foundation bears the weight of the tank shell and the stored oil, transferring this weight to the soil. The general order of construction, from bottom to top, is a plain soil layer, a lime soil layer, a sand cushion layer, and an asphalt anti-corrosion layer. The foundation must have a load-bearing capacity of no less than 10 tons per square meter. If there is groundwater below, a minimum separation of 30 centimeters is required. A rational arrangement of the foundation soil layers ensures a certain degree of resilience, allowing it to withstand any settlement that may occur after the tank is filled. When constructing in soft soil areas, a reinforced concrete ring beam can be constructed around the foundation to prevent the sand cushion layer around the tank bottom from being squeezed out if the foundation sinks.
Bottom Plate
The bottom plate itself is not subject to loads. The pressure from the tank and the materials within it is directly applied to the tank foundation through the bottom plate. However, because the lower layer of the bottom plate contacts the foundation, it is susceptible to erosion from deposited materials and moisture in the foundation soil. Therefore, even if it is not subject to loads, 4-6 mm thick steel plates are used. Tanks with a volume of 50,000 cubic meters or more use 8 mm thick steel plates. The stress conditions around the tank are more complex, so additional thickness is used, typically 2-4 mm.
Tank Wall
The tank wall is the primary load-bearing component of the tank. As the liquid level increases, the pressure at the bottom increases (p = ρgh). Therefore, the thickness of the tank wall steel plate is thicker at the bottom and thinner at the top. The thickness of the steel plate indicated in the general specifications is the thickness at the thinnest point at the top. Depending on the tank size, the thickness ranges from 4 mm to 32 mm. The steel plates of the tank wall are welded together to form a single unit. The upper and lower layers are arranged in various ways: alternating, sleeve, butt-jointed, and mixed. Alternating rivet connections are inconvenient to construct and are rarely used. The sleeve type uses an upper ring plate that extends into the lower plate, with lapped circumferential welds. The ring diameter decreases toward the top. The sleeve type is the most commonly used due to its ease of construction and excellent weld quality. The butt-jointed type requires more precise construction, requiring the upper and lower layers to be of equal size. It is primarily used in floating roof oil storage tanks. The hybrid type, as the name suggests, combines several connection methods and is no longer used.
Tank Roof
A tank with a domed roof is called a "dome tank." A dome is a fixed roof that is required to withstand high internal pressures (ranging from 2 kPa to 10 kPa). The steel plate thickness is generally 4-6 mm. The advantages of a dome roof are low cost and simple construction, but it also results in higher evaporation losses. Another type of roof is a "floating roof," where the roof rises and falls with the liquid level. This type of roof can significantly reduce oil evaporation losses during storage, but it requires high technical skills and is more expensive to construct. Accessories
To ensure safe use of storage tanks, the following accessories are installed on the tank: Manhole: A manhole is an opening above the tank floor, used for personnel to enter and exit during installation, maintenance, and cleaning; Light hole: Installed on the tank roof for lighting and ventilation during cleaning; Oil metering hole: A device on the tank roof used for metering; Safety valve, a safety device for oil in and out; Breathing valve, a safety device used to protect the oil tank from the breathing effect caused by the temperature difference between day and night; Ladders and railings, devices for personnel to enter and exit the oil tank, etc.
