In the field of oil storage, floating roof storage tanks have received significant attention due to their outstanding performance and wide range of applications. The design of this type of storage tank is ingenious, effectively reducing evaporation losses of oil while ensuring the quality of the stored oil, making it an indispensable and important equipment in modern petroleum storage. This article provides a detailed introduction to the structure, features, classification, and significance of floating roof storage tanks in oil storage.
Floating roof storage tanks, abbreviated as floating tanks, are a special type of oil storage tank. The main difference between floating tanks and ordinary fixed-roof tanks is that floating tanks are designed with a “floating roof device” that can “float” on the oil surface and rise or fall with the oil level inside the tank. This design allows floating tanks to significantly reduce the gas space above the liquid during oil storage, thereby greatly lowering oil evaporation losses. Floating roof storage tanks are widely used for storing light oils such as gasoline, aviation kerosene, diesel, as well as crude oil.
Floating roof storage tanks are mainly divided into external floating roof storage tanks and internal floating roof storage tanks.
External floating roof storage tanks consist of a floating roof that floats on the surface of the medium and a vertical cylindrical tank wall. The roof rises and falls with the increase or decrease of the medium in the tank. An annular sealing device between the outer edge of the roof and the tank wall ensures that the medium inside the tank is always directly covered by the roof, thereby reducing medium evaporation. The floating roofs of external tanks are divided into single-deck floating roofs and double-deck floating roofs:
Single-deck floating roof: Composed of several independent compartments forming a ring-shaped pontoon, with a single top plate on the inner side of the ring. The bottom of the single top plate is reinforced with multiple circular steel rings. The advantages of this type of floating roof are low cost and convenient maintenance.
Double-deck floating roof: Composed of an upper plate, lower plate, and compartment edge plates, divided into several independent ring-shaped compartments by radial and circumferential partitions. The advantages of double-deck floating roofs are large buoyancy and good drainage performance.
External floating tanks generally have large volumes. The bottom plate adopts arched edge plates, and the tank wall uses a straight design. Butt welds must be ground smooth to ensure a flat internal surface. To increase wall rigidity, the top of the tank wall requires wind rings and reinforcement rings based on the wind load of the area. The tank's breathing device is installed on the floating roof. Compared with fixed-roof tanks, external floating tanks greatly reduce oil losses and are safer.
Internal floating roof storage tanks are fixed-roof tanks equipped with an internal floating roof, combining the advantages of both fixed-roof and floating roof storage tanks. There is a fixed top cover above the internal floating roof, which protects the floating roof from wind and rain, ensuring the quality of the oil stored under the floating roof. The internal floating roof of internal tanks differs from the floating roof of external tanks' upper openings; therefore, it does not bear rain or snow loads. The load on the internal floating roof is small, and the structure is simple and lightweight, eliminating the need for accessories such as a central drainage pipe or rotating ladder. Internal floating roof storage tanks are easy to construct and maintain, and sealing materials can avoid aging caused by sunlight.
The main feature of internal floating roof storage tanks is the addition of a floating roof inside a fixed-roof tank that covers the liquid surface and rises or falls with the liquid level, while ventilation holes are added at the upper part of the tank wall. Like fixed-roof tanks, the capacity of internal floating roof storage tanks is limited by structural and economic constraints. Internal floating roof storage tanks are mainly used for storing light chemical liquids such as gasoline and light oil. By effectively isolating the medium from air, the internal floating roof reduces the risk level of fire and explosion to a certain extent.
The basic components of floating roof storage tanks are similar to ordinary tanks, mainly including:
Foundation: The foundation of floating roof storage tanks transfers the load of the tank to the intermediate soil layers. From bottom to top, it generally includes natural soil, lime soil, sand cushion, and asphalt-sand cushion layers.
Bottom Plate: The bottom plate is laid at the bottom of the tank by welded steel plates, transmitting the weight of the oil and the tank structure. The thickness is generally between 0.5–1.2 cm.
Tank Wall: The tank wall is made of several circumferential plates connected to form a layer. Several layers are welded to form the main structure of the tank. Layers are connected using interlocking, sleeve, butt, or hybrid methods.
Floating Roof: The floating roof is the key core component of floating roof storage tanks and is divided into external floating roofs and internal floating roofs. As the roof “floats” on the liquid surface, its sealing performance is crucial. The roof uses elastic packing, with pressure strips pressing against the tank wall and bolts fixing the roof. The roof rises and falls with the liquid surface. When lowered to a certain position (approximately 1.8 m), support posts hold it in place.
To ensure normal operation, floating roof storage tanks are equipped with specific auxiliary devices:
Pressure Balancing Device: The pressure balancing device ensures that oil can flow smoothly into and out of the tank after the roof is supported.
Internal Floating Roof Top Cover Equipment: The top cover equipment maintains pressure balance between the internal floating roof and the upper top cover, ensuring normal movement of the internal floating roof.
Static Electricity Elimination Device: Static electricity can accumulate between the floating roof and tank wall, creating fire hazards. Therefore, a grounding wire is installed on the roof to eliminate static generated by roof movement.
Although floating roof storage tanks cost more to build than ordinary fixed-roof tanks, they compensate by reducing evaporation losses. This is especially significant for oil depots and transfer stations with frequent oil transactions and noticeable evaporation losses. Additionally, for light oils such as gasoline and aviation kerosene, which have strict quality requirements for water and sulfur content, internal floating roof storage tanks can significantly reduce losses while ensuring oil quality.
The sealing device is a key component for reducing evaporation. There is a 200–300 mm gap between the floating roof edge and the tank wall, with a sealing device fixed on the roof. The seal types include mechanical, tubular, and elastic packing. Tubular and elastic seals are widely used because they are soft, facilitate roof movement, and provide good sealing. To further reduce evaporation during storage, a secondary seal can be added, known as a double seal.
Floating roof storage tanks are increasingly applied due to their advantages in reducing evaporation losses, ensuring oil quality, and enhancing safety. External floating tanks are commonly used for large storage tanks (at least 10,000 m³) of crude oil, fuel oil, and heavy oil. Internal floating tanks are mainly used for storing light chemical liquids such as gasoline and light oil. With the development of the petroleum industry, floating roof storage tanks, as an efficient oil storage solution, will become increasingly important.
Floating roof storage tanks play a vital role in petroleum storage due to their unique design and excellent performance. Both external and internal floating roof storage tanks effectively reduce the gas space above the liquid, lowering evaporation losses while ensuring oil quality. When selecting storage tanks, factors such as storage requirements, oil properties, and economic considerations should be comprehensively evaluated. For large-scale storage of crude oil, fuel oil, and heavy oil, external floating roof storage tanks are ideal; for light chemical liquids, internal floating roof storage tanks are more suitable. Regardless of the type, tanks should be well-designed, constructed with high quality, and equipped with complete auxiliary devices to achieve safe, efficient, and economical oil storage.
