What Are the Different Types of Oil Tanks?

What Are the Different Types of Oil Tanks?

Oil tanks are categorized by their fabrication method (welded vs. bolted), roof structure (fixed vs. floating), and physical placement (aboveground vs. underground). The optimal choice for an industrial facility depends on the specific gravity of the liquid, vapor pressure, volume requirements, and strict adherence to international engineering standards like API 650.

1. Fabrication Methods: Welded vs. Bolted Steel

In industrial Engineering, Procurement, and Construction (EPC) projects, how a tank is built dictates its installation speed, structural integrity, and total cost of ownership.

• Welded Steel Tanks: The definitive industry standard for massive bulk liquid terminals, crude oil, and refined petrochemicals. Built to API 650 or EN 14015 standards, welded tanks provide a monolithic structure capable of handling extreme capacities, high temperatures, and high pressure. They require significant on-site "hot work" (welding) and rigorous non-destructive testing (NDE), but they offer unmatched durability for heavy-duty applications.

• Bolted Steel Tanks: Engineered to standards like AWWA D103, these modular tanks utilize factory-applied coatings—such as Glass-Fused-to-Steel (GFS) or Fusion-Bonded Epoxy (FBE)—which provide exceptional corrosion resistance. Because they are shipped as panels and assembled on-site using mechanical fasteners and gaskets, they require virtually no hot work. While most commonly used for industrial wastewater, anaerobic digesters, and process water, they are increasingly deployed for specific industrial liquids in remote locations where rapid installation and modular scalability are critical.

2. Roof Designs and Vapor Control

For Aboveground Storage Tanks (ASTs) holding volatile organic compounds (VOCs) like crude oil, gasoline, and aviation fuel, the roof design is strictly regulated to prevent evaporative emissions and product loss.

• Fixed Roof Tanks: These feature a permanently attached cylindrical steel shell and a cone or dome roof. Because a vapor space exists between the liquid surface and the roof, they are reserved for low-volatility liquids such as diesel fuel, heating oil, and asphalt.

• External Floating Roof Tanks (EFRT): These massive tanks have no fixed roof. Instead, a pontoon or double-deck roof floats directly on the surface of the stored liquid. As the tank fills or drains, the roof moves vertically, effectively eliminating the vapor space. This design is critical for storing large volumes of crude oil and preventing evaporative losses.

• Internal Floating Roof Tanks (IFRT): Combining the protective benefits of a fixed roof with the vapor control of a floating deck, IFRTs feature a permanent outer roof (protecting the product from weather and debris) and a lightweight floating deck inside. They are the standard choice for highly volatile refined products like gasoline and ethanol.

3. Location: Aboveground vs. Underground

• Aboveground Storage Tanks (AST): Situated above grade, ASTs are much easier to inspect, maintain, and repair. They range from small utility tanks to 500,000-barrel giants. Environmental regulations mandate secondary containment systems (like dykes or berms) to capture potential spills.

• Underground Storage Tanks (UST): Buried beneath the surface, USTs are primarily used at commercial fueling stations or facilities with limited footprint. Because of the risk of undetectable leaks contaminating groundwater, modern USTs require rigorous dual-wall construction and continuous automated leak detection systems.

Frequently Asked Questions (FAQ)

What is the difference between an API 650 tank and an AWWA tank?

API 650 is the standard for welded steel tanks used primarily for oil and petroleum storage, focusing on monolithic integrity and atmospheric pressure containment. AWWA standards (like D103) govern bolted steel tanks, which are factory-coated and modularly assembled, typically used for water, wastewater, and bioenergy storage.

Why use a floating roof instead of a fixed roof for crude oil?

Crude oil contains volatile organic compounds that evaporate easily. A fixed roof leaves a large vapor space where these gases can expand and escape. A floating roof rests directly on the liquid surface, eliminating the vapor space, significantly reducing emissions, and preventing product loss.

Can bolted steel tanks be used for oil storage?

While bolted tanks are the premier choice for water, wastewater, and anaerobic digesters due to their rapid, hot-work-free installation, they are generally not the primary choice for bulk crude oil or highly volatile petroleum storage. Welded API 650 tanks remain the standard for heavy petroleum, though bolted tanks may be used for specific heavy oils, lubricants, or temporary industrial liquid storage depending on gasket compatibility.