Storage Tanks
What Is a Storage Tank
A storage tank is a large-capacity vessel that holds liquids, compressed gases, or vapor-phase products at or near atmospheric pressure (for fixed/floating roof types) or at elevated pressures (for spheres and bullets). In the oil and gas industry, storage tanks are required infrastructure at every stage of the value chain, from crude oil production sites and pipeline terminals to refineries, petrochemical plants, and product distribution depots.
Storage tanks serve several critical functions in the petroleum supply chain.
| Function | Role |
|---|---|
| Inventory management | Buffers between continuous production processes and batch-mode transportation (tanker loading, pipeline batch sequencing) |
| Product segregation | Keeps different crude grades, refined products, and chemical feedstocks separated to maintain quality specifications |
| Operational flexibility | Provides surge capacity for flow-rate fluctuations, planned shutdowns, and emergency scenarios |
| Safety containment | Holds hazardous liquids and pressurized gases within engineered systems that include pressure relief, fire protection, overfill prevention, and secondary containment |
The design, materials, and appurtenances of a storage tank depend on the product stored (its vapor pressure, specific gravity, corrosivity, and flash point), the operating conditions (pressure, temperature, seismic zone, wind exposure), and the applicable codes and standards (API 650, API 620, ASME Section VIII, EN 14015).
Key Takeaway: Storage tanks are purpose-built vessels that range from small atmospheric shop-built units to massive field-erected floating roof tanks exceeding 120 meters in diameter. Selecting the correct tank type requires balancing vapor-loss control, safety, product compatibility, and economics.
Fixed Roof Tank: Cross-Section
The diagram below shows the principal components of a cone-roof fixed storage tank, the most common type in the oil and gas industry for products with low vapor pressure (diesel, fuel oil, water, crude oil).
Types of Storage Tanks
Storage tanks are classified primarily by their roof configuration and design pressure. The choice of tank type is driven by the vapor pressure of the stored product, environmental regulations on emissions, safety requirements, and cost.
Fixed Roof Tanks
Fixed roof tanks are the most common and economical type of above-ground storage tank. The roof is permanently attached to the shell, creating an enclosed vapor space above the liquid surface. These tanks operate at or near atmospheric pressure (up to approximately 2.5 kPa gauge / 0.36 psig per API 650).
Cone Roof Tanks
The cone roof is the most prevalent fixed-roof design. The roof consists of structural rafters and plates arranged in a conical shape with a typical slope of 3/4 inch per foot (approximately 1:16). The roof is supported either by internal columns and rafters (column-supported) for larger diameters or is self-supporting (frameless) for smaller diameters (typically below 18 m / 60 ft).
Cone roofs are simple to construct and carry the lowest capital cost per unit volume. They suit products with low vapor pressure (flash point above 37.8 degC / 100 degF), including diesel, fuel oil, lube oil, water, and low-RVP crude oil. However, they are vulnerable to breathing losses from diurnal temperature cycling and working losses from fill/draw operations. Maximum diameter typically reaches ~90 m (300 ft), though most installations fall between 10 and 60 m.
Dome Roof Tanks
A dome (or spherical segment) roof has a curved, self-supporting structure that eliminates the need for internal columns and rafters. Dome roofs can withstand higher internal pressures than cone roofs (up to ~6.9 kPa / 1.0 psig) and provide better structural resistance to external loads such as vacuum, wind uplift, and snow.
Dome roofs offer greater internal volume efficiency because there are no column obstructions. They also resist vacuum conditions more effectively, making them a good choice for products requiring inert gas blanketing (nitrogen pad). Fabrication cost runs higher than cone roofs for equivalent diameters.
Umbrella Roof Tanks
The umbrella roof is a modified self-supporting design with radial sections curved in a segmental pattern. It is a cost-effective alternative to the dome roof for smaller-diameter tanks and is commonly used for water storage.
External Floating Roof Tanks (EFRT)
External floating roof tanks have a roof that floats directly on the liquid surface and rises or falls with the liquid level. Because the roof eliminates the vapor space above the liquid, EFRTs reduce evaporative emissions by 95% or more compared to fixed roof tanks, making them the standard choice for high-vapor-pressure products.
The floating roof is equipped with a rim seal that bridges the annular gap between the roof edge and the tank shell, preventing vapor escape while allowing the roof to travel freely. Weather shields mounted above the rim seal protect against rain intrusion.
Pontoon-Type Floating Roof
The single-deck (pontoon) roof consists of a thin steel deck plate supported by annular pontoons around its perimeter. The pontoons provide buoyancy, while the center deck (single-skin) rests directly on the liquid surface.
Pontoon roofs cost less than double-deck designs but bring trade-offs: the center deck can flex under loads, and rainwater accumulation on the center deck is a persistent concern that demands an effective roof drain system.
Double-Deck Floating Roof
The double-deck roof has an upper deck and a lower deck separated by bulkheads, creating a series of sealed compartments. This design provides superior buoyancy, structural rigidity, and thermal insulation.
The insulating air gap between decks reduces solar heat gain and further lowers emissions. Double-deck roofs cost more than pontoon types but offer a greater safety margin and are better suited for very large tanks (60-120+ m diameter).
EFRT Characteristics
EFRTs handle crude oil, gasoline, naphtha, jet fuel, and condensate in capacities from 10,000 to over 1,500,000 bbl (1,600 to 240,000+ m3). Primary seal types include mechanical shoe, resilient foam-filled, and liquid-mounted designs. A secondary seal mounted above the primary further cuts emissions by 50-90%. Rim-mounted foam dams and fire-fighting foam systems are standard safety features. The exposed roof surface must resist wind, rain, and snow loading.
Internal Floating Roof Tanks (IFRT)
An internal floating roof tank combines a fixed roof (cone or dome) with a lightweight internal floating roof (or “floater”) that rides on the liquid surface inside the tank. The fixed roof protects the floating roof from weather, wind, and debris, while the floating roof minimizes the vapor space and controls emissions.
The internal floater is typically constructed from aluminum pontoons, honeycombed aluminum panels, or steel panels. Rim seals between the floater edge and the shell work similarly to EFRT seals. The fixed outer roof can be a standard cone/dome or a geodesic aluminum dome (retrofit). IFRTs suit products with moderate-to-high vapor pressures that also need weather protection — gasoline, jet fuel, chemical solvents, and ethanol are common applications. Emission performance approaches that of an EFRT with a secondary seal. Automatic roof vents in the fixed roof allow vapor to escape and prevent overpressure, while free vents on the floater remain sealed.
Spherical Tanks (Horton Spheres)
Spherical storage tanks (often called Horton spheres after their inventor Horace E. Horton) store products under elevated pressures, typically in the range of 100-1,700 kPa (15-250 psig). The sphere is the most efficient pressure-containing shape because it distributes internal stresses equally in all directions, allowing for thinner wall thicknesses relative to equivalent cylindrical vessels.
Spheres follow API 620 (Appendix Q for refrigerated storage) or ASME Section VIII, Division 1 or 2. Typical diameters run 10 to 25 m (33 to 82 ft) with capacities from 5,000 to 75,000 bbl (800 to 12,000 m3). Shell thickness ranges from 20 to 90 mm depending on diameter, pressure, and material. Six to twenty tubular or wide-flange steel columns with bracing support the vessel. Equipment includes pressure relief valves, liquid level gauges, temperature indicators, and access platforms. Common products are LPG (propane, butane), NGL, anhydrous ammonia, ethylene, and hydrogen. Fire protection relies on water deluge spray systems with fixed nozzles covering the entire sphere surface. Seismic requirements follow API 620 Annex L or site-specific analysis.
Horton spheres are commonly found in refinery tank farms, gas processing plants, and LPG distribution terminals where pressurized storage of volatile liquids or liquefied gases is required.
Bullet Tanks (Horizontal Pressure Vessels)
Bullet tanks are horizontal cylindrical pressure vessels mounted on saddle supports. They store smaller volumes of LPG, propane, butane, and anhydrous ammonia at pressures between 700 and 1,700 kPa (100-250 psig).
Bullet tanks follow ASME Section VIII, Division 1 (or Division 2 for optimized designs). Typical capacities fall between 500 and 60,000 gallons (2 to 230 m3). The vessels are shop-fabricated and transported to site, so dimensions are limited by highway transport constraints. Heads are hemispherical or 2:1 ellipsoidal. Per-unit cost for smaller volumes runs lower than spheres, and bullets are often grouped in rows (bullet farms) for LPG storage at gas plants and distribution terminals. Each tank carries relief valves, excess-flow valves, liquid-level gauges, and emergency shut-off valves.
Underground Storage Tanks (USTs)
Underground storage is employed where surface footprint constraints exist, where environmental or security considerations require below-grade containment, or for strategic reserves.
| Underground Storage Type | Description |
|---|---|
| Lined steel tanks | Double-walled steel tanks buried below grade for fuel storage at retail stations (subject to EPA UST regulations in the U.S.) |
| Salt caverns | Solution-mined cavities in underground salt formations for strategic petroleum reserves and NGL/LPG storage (e.g., U.S. Strategic Petroleum Reserve at Bryan Mound, Big Hill) |
| Rock caverns | Excavated cavities in hard rock, lined or unlined, for crude oil and product storage (common in Scandinavia, Southeast Asia) |
| Concrete pits | Reinforced concrete structures, often for water, wastewater, or secondary containment basins |
Underground storage can provide excellent thermal insulation, natural pressure containment (for caverns), protection from weather and external threats, and minimal surface footprint.
Floating Roof Tank: Cross-Section
The diagram below illustrates the key components of an external floating roof tank (EFRT), the preferred design for crude oil and high-vapor-pressure product storage.
Spherical Tank (Horton Sphere): Diagram
The diagram below shows a spherical pressure storage vessel (Horton sphere) used for LPG, NGL, and other pressurized products.
Tank Type Comparison
The table below compares the principal characteristics of the main storage tank types used in the oil and gas industry.
| Characteristic | Fixed Roof (Cone/Dome) | External Floating Roof (EFRT) | Internal Floating Roof (IFRT) | Spherical (Horton Sphere) | Bullet Tank |
|---|---|---|---|---|---|
| Design Pressure | Atmospheric (~2.5 kPa) | Atmospheric | Atmospheric | 100-1,700 kPa (15-250 psig) | 700-1,700 kPa (100-250 psig) |
| Typical Products | Diesel, fuel oil, water, low-RVP crude | Crude oil, gasoline, naphtha, condensate | Gasoline, jet fuel, solvents, ethanol | LPG, NGL, butane, propane, ammonia | LPG, propane, butane, ammonia |
| Capacity Range | 100-500,000 bbl | 10,000-1,500,000+ bbl | 5,000-500,000 bbl | 5,000-75,000 bbl | 500-15,000 gal (typical) |
| Vapor Loss | High (breathing + working losses) | Very low (95%+ reduction) | Low (90%+ reduction) | None (sealed system) | None (sealed system) |
| Relative Cost | Lowest | Moderate-high | Moderate | High | Moderate |
| Key Standard | API 650 | API 650 | API 650 | API 620 / ASME VIII | ASME VIII Div. 1 |
| Key Advantage | Simplicity, low cost | Emission control, large capacity | Weather protection + emission control | Efficient pressure containment | Shop-fabricated, transportable |
| Key Limitation | High evaporative losses | Exposed to weather, rain on roof | Complex internal access for maintenance | High cost, field-erected | Size limited by transport |
Standards and Specifications
The design, fabrication, inspection, and operation of storage tanks are governed by several industry standards. The table below summarizes the most important codes.
| Standard | Title / Scope |
|---|---|
| API 650 | Welded Tanks for Oil Storage. Covers the design, fabrication, erection, and testing of above-ground, vertical, cylindrical, closed- and open-top welded tanks for atmospheric-pressure storage of petroleum, chemicals, and water. This is the primary standard for fixed and floating roof tanks. |
| API 620 | Design and Construction of Large, Welded, Low-Pressure Storage Tanks. Covers tanks operating at pressures up to 103 kPa (15 psig). Includes appendices for refrigerated storage (Appendix Q) and pressurized spheres. |
| API 653 | Tank Inspection, Repair, Alteration, and Reconstruction. Provides requirements for maintaining the integrity of existing above-ground storage tanks after they have been placed in service. Covers inspection intervals, fitness-for-service evaluation, repair methods, and reconstruction. |
| API 2000 | Venting Atmospheric and Low-Pressure Storage Tanks. Establishes requirements for normal and emergency venting of above-ground storage tanks to prevent overpressure or vacuum damage. |
| API 2610 | Design, Construction, Operation, Maintenance, and Inspection of Terminal and Tank Facilities. Provides guidance for safe and efficient operation of petroleum storage terminals. |
| NFPA 30 | Flammable and Combustible Liquids Code. Covers the storage, handling, and use of flammable and combustible liquids, including tank spacing, diking, fire protection, and ventilation requirements. |
| ASME BPVC VIII | Boiler and Pressure Vessel Code, Section VIII. Governs the design and fabrication of pressure vessels, applicable to bullet tanks and spheres when operating above 15 psig. Divisions 1 (rules-based) and 2 (alternative rules with stress analysis). |
| EN 14015 | Specification for the Design and Manufacture of Site Built, Vertical, Cylindrical, Flat-Bottomed, Above Ground, Welded, Steel Tanks for the Storage of Liquids at Ambient Temperature and Above. The European counterpart to API 650. |
| UL 142 | Steel Aboveground Tanks for Flammable and Combustible Liquids. Applies to shop-built atmospheric tanks, commonly used for fuel storage at commercial and industrial facilities. |
Tank Components
Shell Plates and Courses
The tank shell is the vertical cylindrical wall constructed from multiple horizontal bands of steel plate called courses (or strakes). Each course is a ring of plates joined by vertical butt welds, and successive courses are joined by horizontal butt welds.
In an API 650 tank:
- Shell thickness is calculated based on the hydrostatic head at each course elevation, using either the 1-foot method (simplified) or the variable-design-point method (optimized, thinner plates at upper courses)
- The bottom course is the thickest (highest hydrostatic load) and the top course is the thinnest
- Minimum shell thickness: 5 mm (3/16 in.) for tanks up to 15 m (50 ft) diameter; 6 mm (1/4 in.) for 15-36 m; 8 mm (5/16 in.) for 36-60 m
- Shell plates are typically 1.8-3.0 m (6-10 ft) high and up to 12 m (40 ft) long (limited by rolling and transport)
- All shell-to-shell butt welds are 100% radiographed per API 650, Section 8
Bottom Plates
The tank bottom is a steel plate assembly resting on the foundation. It consists of bottom sketch plates, annular plates, and a sloped profile:
- Bottom sketch plates are rectangular plates (typically 6 mm / 1/4 in. minimum thickness) laid across the tank floor in a pattern and lap-welded together
- Annular plates form a ring of thicker plates at the tank perimeter (directly beneath the shell-to-bottom weld) to resist the concentrated shell load; annular plate thickness ranges from 6 mm to 12 mm depending on shell thickness and hydrostatic load
- The bottom is typically sloped (cone-up toward center or cone-down toward a sump) at 1:96 to 1:120 to facilitate drainage
- Bottom plates are fillet-welded (not butt-welded) and are subject to vacuum-box testing or oil-leak testing during construction
Roof
Tank roofs fall into two broad categories.
Self-supporting roofs use no internal structural support (columns or rafters). They are limited to smaller diameters — typically less than 18 m / 60 ft for cone roofs, though dome and umbrella roofs span wider. They must withstand a minimum uniform live load of 1.0 kPa (20 psf) for snow and maintenance access.
Column-supported roofs rely on internal columns (pipe or structural sections) carrying rafters and roof plates. They are required for larger-diameter tanks. The column layout is typically radial (spokes) with circumferential purlins (girders). Roof plate minimum thickness is 5 mm (3/16 in.) or 4.7 mm (No. 7 gauge). At the perimeter, the roof-to-shell junction acts as a frangible joint (weak seam) that fails before the shell-to-bottom joint in case of overpressure, preventing catastrophic tank failure.
Foundation
The tank foundation transfers the tank weight and contents to the soil. Common foundation types are summarized below.
| Foundation Type | Description |
|---|---|
| Ringwall | A concrete ring beam beneath the tank shell perimeter, with compacted fill (sand or gravel) inside the ring. Most common type for large tanks. Width and depth depend on tank diameter, soil bearing capacity, and seismic requirements. |
| Slab (mat) | A full reinforced concrete slab beneath the entire tank bottom. Used when soil bearing capacity is low or when a leak-detection system is required beneath the tank. |
| Pile-supported | Driven or drilled piles supporting a ring beam or slab. Used on soft or compressible soils (e.g., coastal or marshy areas). |
| Compacted earth pad | A graded and compacted earth or gravel pad. Suitable for smaller tanks on competent soils. |
All foundations must accommodate the oilsand pad (a thin asphalt or oiled-sand layer between the concrete and the tank bottom) to prevent corrosion of the bottom plates.
Appurtenances
Tank appurtenances are the auxiliary components mounted on or within the tank.
| Component | Function |
|---|---|
| Nozzles | Flanged openings for inlet, outlet, drain, fill, level gauge, temperature, and sampling connections. Sizes range from 2 in. to 36 in. or larger. Reinforcing pads are required (per API 650, Section 5.7) when the opening exceeds a threshold size. |
| Manholes | Shell manholes (typically 24 in. or 600 mm) and roof manholes for personnel access during inspection and maintenance. Shell manholes are reinforced and flanged with bolted covers. |
| Vents | Pressure/vacuum (P/V) vents limit the pressure and vacuum in the vapor space. Emergency vents (open or frangible) provide large-area relief during fire exposure. Free vents (open breathers) serve tanks with internal floating roofs. |
| Gauging devices | Manual gauge hatches (roof-mounted for dip-tape measurement), automatic tank gauges (radar, servo, or magnetostrictive), level indicators, and high-level alarms. |
| Mixers and agitators | Side-entry mixers for blending or preventing sludge settling (common in crude oil tanks). Jet mixers (eductors) for water-draw systems. |
| Heating coils | Internal steam or hot-water coils for maintaining product temperature (heavy crude, fuel oil, asphalt). |
| Foam systems | Fixed foam chambers and rim-pour foam systems for fire suppression on floating roof tanks. |
Materials Selection
Shell and Structural Steel
The most common materials for atmospheric storage tank shells (API 650) are:
| Material Grade | Specification | Min. Yield (MPa) | Typical Use |
|---|---|---|---|
| A36 | Structural steel | 250 | Small tanks, structural members |
| A283 Grade C | Low/intermediate tensile steel | 205 | Tank shells (general service) |
| A516 Grade 60 | Pressure vessel plate, C-steel | 220 | Tank shells, higher-pressure service |
| A516 Grade 70 | Pressure vessel plate, C-steel | 260 | Thicker shells, lower temperatures |
| A537 Class 1 | Heat-treated C-Mn-Si steel | 345 | High-stress applications, seismic zones |
| A572 Grade 50 | HSLA structural steel | 345 | Wind girders, stiffening rings |
For pressurized spheres and bullets, materials are selected per ASME Section VIII and include SA-516, SA-537, SA-612, and SA-738 grades, with impact testing required for design metal temperatures below -29 degC (-20 degF).
Bottom Plate Materials
Bottom plates are typically A36 or A283 Grade C. In corrosive service (sour crude, produced water), a corrosion allowance of 1.5-3.0 mm is added to the calculated thickness, or a corrosion-resistant liner (fiberglass, epoxy coating) is applied.
Roof Materials
| Roof Component | Material |
|---|---|
| Fixed roof plates | A36 or A283 Grade C, minimum 5 mm (3/16 in.) thickness |
| Floating roof pontoons and deck plates (steel) | A36 or A283, typically 5-6 mm |
| Aluminum floating roofs | 5000-series aluminum alloys (5052, 5083, 5086) for internal floating roofs; lightweight, corrosion-resistant, and spark-resistant. Aluminum geodesic domes for retrofit IFRT applications. |
Coatings and Linings
Corrosion protection is required for tank longevity.
| Protection Method | Details |
|---|---|
| External coatings | Primer (inorganic zinc or epoxy zinc-rich) plus intermediate (epoxy) plus topcoat (polyurethane or aliphatic polyurethane). Total DFT: 250-400 microns. Color is typically white or light gray for solar reflectance (reduces product heating and emissions). |
| Internal linings | Epoxy (amine-cured or novolac) or glass-flake-reinforced vinylester for chemical resistance. Required when storing corrosive products, sour crude, or ethanol. DFT: 300-750 microns. Surface preparation: SSPC-SP10 (near-white blast) minimum. |
| Cathodic protection | Impressed-current or sacrificial-anode systems protect the tank bottom exterior (soil side) and internal bottom surfaces. Required by many operators and regulatory agencies. |
| Thermal spray aluminum (TSA) | Applied to carbon steel surfaces in splash zones and under insulation as a long-term corrosion barrier. |
Design Considerations
Product Properties
The physical and chemical properties of the stored product dictate the tank type, materials, and accessories.
| Property | Impact on Tank Design |
|---|---|
| Vapor pressure (RVP) | Products with RVP above 11 kPa (1.5 psia) are candidates for floating roof tanks to minimize emissions. Very high vapor-pressure products (LPG, NGL) require pressurized spheres or bullets. |
| Specific gravity | Determines hydrostatic load on shell and bottom plates, affecting thickness calculations. Heavy crude (SG > 0.95) imposes higher loads than lighter products. |
| Flash point | Products with flash points below 37.8 degC (100 degF) are classified as flammable and require flame arrestors, inert gas blanketing, and electrically classified areas. |
| Corrosivity | Sour crude (high H2S), naphthenic acid-containing crudes, and acidic chemicals require corrosion-resistant materials or linings. |
| Pour point / viscosity | High-viscosity or high-pour-point products (heavy crude, fuel oil, asphalt) may require heated storage with internal coils and insulation. |
Seismic Design
Seismic design of storage tanks is addressed in API 650, Annex E (Seismic Design of Liquid Storage Tanks) and is mandatory in seismically active regions. The main concerns fall into five areas.
| Concern | Description |
|---|---|
| Sloshing | Earthquake-induced lateral forces cause the liquid to slosh, generating convective wave loads on the shell and roof. Adequate freeboard must separate the maximum liquid level from the roof (fixed-roof tanks) or the top of the shell (floating-roof tanks) to prevent roof damage or liquid spillage. |
| Overturning moment | The combined hydrodynamic force (impulsive + convective) creates an overturning moment at the base that must be resisted by anchor bolts or self-weight. |
| Shell compression | Seismic overturning can induce compressive stresses in the lower shell courses. The shell must be checked against elastic buckling (elephant’s foot buckling). |
| Foundation sliding | The base shear force must be transferred to the foundation without sliding. Anchor bolts or friction resistance (for unanchored tanks) must be verified. |
| Piping flexibility | All piping connections must accommodate relative displacement between the tank and the piping during an earthquake (flexible connections, expansion loops). |
Wind Loads
Wind loading on large-diameter tanks can induce shell buckling (external pressure instability) and overturning. API 650 requires:
- Wind girders (intermediate stiffening rings) at specified intervals along the shell height to prevent buckling of the empty or partially filled tank under design wind speed (typically 190 km/h / 120 mph, 3-second gust, unless site-specific data requires higher)
- Top wind girder (compression ring) at the top of the shell
- Verification of overturning stability under wind load (especially for empty tanks)
- Wind loads on floating roof tanks include consideration of the exposed shell above the floating roof at the lowest operating level
Settlement and Foundation Design
Differential settlement of the foundation can cause four main problems.
| Problem | Effect |
|---|---|
| Shell distortion | Out-of-roundness and localized stress concentrations |
| Bottom plate deformation | Bulging, wrinkling, and overstress of bottom-to-shell welds |
| Roof misalignment | Binding of floating roofs in the shell |
| Nozzle overstress | Displacement-induced loads on rigid nozzle connections |
API 653 provides settlement limits: maximum localized settlement of the shell should not exceed L^2 / (2 * E * t * D) to avoid yielding, where L is the arc length between high and low points, E is the modulus, t is the shell thickness, and D is the tank diameter.
Fire Protection
Fire protection systems for storage tank farms follow NFPA 30 and API 2610 requirements.
| System | Application |
|---|---|
| Fixed foam | Foam chambers (Type II) mounted on the tank shell rim deliver AFFF or fluoroprotein foam to the liquid surface. Foam dam heights, application rates, and discharge durations follow NFPA 11. |
| Rim seal fire protection | Semi-fixed or fixed foam pour systems deliver foam to the seal area of floating roof tanks, where most tank fires originate. |
| Cooling water | Ring-mounted spray nozzles on adjacent tanks (exposure tanks) deliver cooling water to prevent fire spread. Application rate: typically 10 L/min/m2 (0.25 gpm/ft2) of exposed shell surface. |
| Water deluge | Fixed spray nozzle systems covering the entire surface of spherical and bullet tanks, activated by fire detection systems or manual valves. Per NFPA 15. |
| Fire walls and dikes | Physical barriers between tanks and tank groups to contain spills and limit fire spread. |
Secondary Containment
Secondary containment systems are required by environmental regulations (EPA 40 CFR 112, NFPA 30) to prevent spills from reaching the environment.
| Containment Type | Description |
|---|---|
| Earthen dikes (berms) | The most common form for large tank farms. The diked area must hold at least 100% of the volume of the largest tank within the enclosure plus the volume displaced by other tanks, foundations, and piping. |
| Concrete containment walls | Used where space is limited or earthen dikes are not feasible. Provides a compact, impermeable barrier. |
| Remote impounding | Spills are channeled through a drainage system to a remote impounding basin, separating the spill from the heat source (burning tank). Required by API 2610 for certain tank sizes and products. |
| Double-bottom tanks | A secondary steel bottom with leak detection sensors between the primary and secondary bottoms. Increasingly required for environmental protection over sensitive groundwater areas. |
| Liner systems | HDPE or clay liners beneath earthen dikes and tank pads to prevent groundwater contamination from slow leaks. |
Applications
Tank Farms at Refineries
Refineries operate crude tankage to receive and buffer incoming crude oil, intermediate tankage for process units (distillation column feeds and products), and product tankage for finished fuels and chemicals awaiting shipment. A typical large refinery (250,000 bpd capacity) may have 50-150 storage tanks of various types with combined capacity of 5-15 million barrels.
| Tank Service | Typical Type and Size |
|---|---|
| Crude oil | Large EFRTs (80,000-500,000 bbl each), typically 40-100 m diameter |
| Products (gasoline, diesel, jet fuel) | IFRTs or EFRTs (20,000-200,000 bbl) |
| Slop, drain, and blend | Fixed roof (5,000-50,000 bbl) |
| LPG | Horton spheres (10,000-50,000 bbl), typically 4-8 units per refinery |
Crude Oil Storage at Terminals
Pipeline terminals and marine terminals use large tank farms to buffer between pipeline transportation and tanker loading/offloading. These facilities typically group multiple EFRTs of 500,000 to 1,500,000 bbl capacity, connected by manifold systems to pipeline headers and marine loading arms. Blending and quality control systems manage product specifications, and vapor recovery units (VRUs) capture displaced vapors during filling operations.
Product Storage at Distribution Points
Downstream distribution depots (fuel terminals, airport fuel farms, industrial fuel storage) typically use smaller tanks:
- IFRTs for gasoline and jet fuel (10,000-100,000 bbl)
- Fixed roof tanks for diesel, heating oil, and lubricants (1,000-50,000 bbl)
- Shop-built tanks (UL 142) for smaller commercial fuel storage (500-30,000 gallons)
- Above-ground storage tanks (ASTs) with secondary containment per SPCC regulations
LPG Storage (Pressurized Spheres and Bullets)
Liquefied petroleum gas (propane, butane, and their mixtures) is stored under pressure to maintain the liquid phase. Horton spheres handle volumes of 5,000-75,000 bbl at gas processing plants, refineries, and large distribution terminals, typically in groups of 2-8 connected to a common manifold. Bullet tanks serve smaller distribution points, retail propane facilities, and industrial sites, commonly installed in rows (bullet farms) of 4-20 units. For very large LPG volumes (100,000+ bbl), fully refrigerated atmospheric-pressure tanks (double-wall, insulated, API 620 Appendix Q) become more economical than pressurized spheres.
Strategic Petroleum Reserves
Nations maintain strategic petroleum reserves (SPR) for energy security. Salt caverns — solution-mined cavities in salt domes, each holding 5-40 million barrels of crude oil — are the most cost-effective approach where geology permits. The U.S. SPR stores approximately 700 million barrels in four salt-dome sites along the Gulf Coast (Bryan Mound, Big Hill, West Hackberry, Bayou Choctaw). Countries without suitable salt geology rely on above-ground tank farms (Japan, South Korea, and parts of Europe) or rock caverns (Scandinavia and Singapore).
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