What Is a Storage Tank?
A storage tank is a large container used to store liquids (crude oil, refined products, water, chemicals) at or near atmospheric pressure. Storage tanks are standard components of tank farms in refineries, terminals, upstream production facilities, and petrochemical plants.
The two main design standards are API 650 (for atmospheric storage tanks, up to 2.5 psig internal pressure) and API 620 (for low-pressure storage tanks, up to 15 psig). Tanks operating above 15 psig fall under ASME Section VIII and are classified as pressure vessels.
API 650 vs. API 620
| Feature | API 650 | API 620 |
|---|---|---|
| Pressure range | Atmospheric to 2.5 psig | 2.5 to 15 psig |
| Temperature | Ambient to 260 degrees C (500 degrees F) | Ambient to 120 degrees C (250 degrees F) |
| Shape | Vertical cylindrical, flat bottom | Vertical cylindrical, may have domed roof |
| Roof types | Cone, dome, floating (internal/external) | Self-supporting dome or cone |
| Wall design | Variable thickness (thicker at bottom) | Designed for internal pressure loads |
| Foundation | Concrete ringwall or earth pad | Concrete ringwall or piled foundation |
| Common use | Crude oil, refined products, water | LPG, refrigerated liquids, low-pressure gas |
Tank Roof Types
| Roof Type | Description | Application |
|---|---|---|
| Cone roof (fixed) | Self-supporting conical roof welded to shell | Low vapor pressure products, water |
| Dome roof (fixed) | Geodesic aluminum or steel dome | Larger tanks, reduced structural weight |
| External floating roof | Open-top tank with pontoon or pan roof floating on liquid | Crude oil, gasoline—reduces vapor emissions |
| Internal floating roof | Fixed roof with floating deck inside | Products requiring weather protection + emission control |
| Membrane roof | Flexible membrane on fixed structure | Special applications, LNG containment |
Key Design Parameters
| Parameter | Typical Value |
|---|---|
| Diameter range | 3 m to 120+ m |
| Height range | 6 m to 20+ m |
| Shell material | A36, A283 Gr.C, A516 Gr.70 |
| Bottom plate | 6 mm minimum thickness (API 650) |
| Annular plates | Thicker ring at shell-to-bottom junction |
| Corrosion allowance | 1.5 to 3.0 mm (shell), 1.5 mm (bottom) |
| Hydrostatic test | Fill with water to maximum design liquid level |
| In-service inspection | API 653 (external and internal examination intervals) |
Storage tanks interface with the piping system through nozzle connections for inlet, outlet, drain, vent, and instrumentation. Tank nozzles are typically flanged connections sized and rated per the pipe class specification for the connected piping. Level instruments, temperature gauges, and flow measurement devices are mounted directly on tank nozzles.
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