Types of Oil Storage Tanks: Designs, Applications, and Selection Criteria

An oil storage tank is a heavy-duty industrial containment vessel designed to securely hold crude oil, refined petroleum products, diesel, lubricants, and intermediate hydrocarbons during extraction, processing, refining, and distribution. Because petroleum products possess varying flash points, vapor pressures, and chemical properties, different tank designs are required to minimize evaporation losses, control emissions, and ensure absolute operational safety.

Oil storage tanks are primarily classified based on their structural configuration and how they manage vapor pressures:

A fixed roof tank features a permanently attached roof—typically cone-shaped, dome-shaped, or umbrella-shaped—welded directly to the cylindrical steel shell.

• Best Used For: Storing heavy fuel oils, lubricants, bitumen, and non-volatile petroleum products with high flash points where vapor accumulation is minimal.

• Operational Mechanism: They operate at or near atmospheric pressure and utilize pressure-vacuum relief vents to accommodate slight volumetric changes caused by temperature fluctuations or liquid movement.

External floating roof tanks feature an open-topped cylindrical shell with a deck that floats directly on the surface of the stored liquid, rising and falling with the fluid level.

• Best Used For: High-volume storage of volatile crude oil, gasoline, and naphtha where minimizing vapor space is critical.

• Operational Mechanism: By eliminating the vapor pocket between the liquid surface and the roof, these tanks significantly reduce hydrocarbon emissions (volatile organic compounds) and lower explosion risks. A specialized rim seal system closes the gap between the floating deck and the tank wall.

Combining features of both designs, an internal floating roof tank features a permanent fixed outer roof with an independent floating deck installed inside the vessel.

• Best Used For: Storing medium-to-low flash-point fuels, aviation turbine fuel (ATF), and specialized gasolines in areas subject to heavy rainfall, snow, or strict environmental emission laws.

• Operational Mechanism: The outer fixed roof protects the internal floating deck from weather elements (like rain and wind), while the internal floating deck traps vapors and reduces evaporative losses.

Spherical tanks are heavy-duty, pressurized containment structures designed to store volatile liquids under high vapor pressure.

• Best Used For: Storing liquefied petroleum gas (LPG), propane, butane, and natural gas liquids.

• Operational Mechanism: The spherical geometry distributes internal stress uniformly across the vessel walls, providing the high structural integrity required to contain pressurized gases safely.

Beyond structural shape, oil storage tanks are categorized by where they sit relative to ground level:

• Aboveground Storage Tanks (ASTs): The most common configuration in refineries and tank farms. They offer lower upfront installation costs, easier visual inspections, and straightforward maintenance access.

• Underground Storage Tanks (USTs): Primarily utilized at retail fueling stations or smaller industrial facilities where surface space is constrained. They require strict environmental monitoring, corrosion protection, and regulatory registration to prevent subsurface leaks.

Q: What is the primary difference between a fixed roof tank and a floating roof tank?

A: A fixed roof tank has a permanently sealed roof attached to the shell, making it suitable for stable, non-volatile oils. A floating roof tank has a deck that rests directly on the liquid surface, moving up and down to eliminate vapor pockets, which prevents excessive evaporation and emissions when storing volatile products like crude oil or gasoline.

Q: Why are spherical tanks used for storing petroleum products like propane?

A: Propane and liquefied petroleum gases (LPG) remain gaseous at standard atmospheric temperatures and pressures. Spherical tanks provide uniform stress distribution across the vessel walls, making them the most structurally sound shape for containing high-pressure volatile liquids safely.

Q: How do internal floating roof tanks differ from external floating roof tanks?

A: External floating roof tanks are open to the sky, meaning the floating deck is exposed directly to weather elements. Internal floating roof tanks feature a traditional fixed roof covering an internal floating deck, protecting the deck from rainwater, snow, and wind while still capturing hydrocarbon vapors.

Q: What industry standards govern the design and construction of oil storage tanks?

A: The American Petroleum Institute (API) establishes the primary global benchmarks. Key standards include API 650 for welded steel tanks for oil storage, API 620 for low-pressure storage vessels, and API 653 for tank inspection, repair, and reconstruction.