Storage Tanks in Oil & Gas – Types, Design, Inspection & Maintenance

Storage tanks are critical components of oil & gas, petrochemical, and industrial facilities. They are used to safely store a wide variety of liquids and gases, including crude oil, refined petroleum products, chemicals, and water. This page offers a complete guide to the design, classification, operation, and maintenance of industrial storage tanks, with a focus on practical engineering applications and compliance with international standards.

Purpose and Importance of Storage Tanks

In any process facility, storage tanks serve as buffers between production, processing, and distribution. They ensure continuous plant operation by compensating for flow rate variations and facilitating inventory management. From upstream crude oil storage to downstream blending and distribution terminals, tanks play a key role in both safety and logistics.

Their design must take into account fluid properties, environmental conditions, operating pressures, and industry codes. Poorly designed or maintained tanks can lead to leaks, contamination, fire hazards, or even catastrophic failures—making engineering diligence critical.

Classification of Storage Tanks

Storage tanks are typically classified based on the type of fluid they store, pressure requirements, and roof configuration. The two most common categories are atmospheric and pressurized tanks.

  • Atmospheric Storage Tanks operate at or near ambient pressure and are used for fluids like crude oil, diesel, or water. They are usually designed according to API 650 or similar standards.

  • Pressurized Tanks are designed to handle higher pressures and are suitable for storing LPG, ammonia, or other volatile substances. These tanks follow design standards such as API 620 or ASME Section VIII.

Tanks can also be aboveground or underground, depending on site conditions, safety requirements, and environmental regulations.

Types of Storage Tanks

Fixed Roof Tanks

These are the most common tanks used for storing stable liquids such as water, fuel oil, or diesel. The tank has a permanently attached roof and is often fitted with a breather valve to allow vapors to escape as liquid levels change.

Floating Roof Tanks

Floating roof tanks are designed for volatile liquids like gasoline or crude oil. The roof floats directly on the liquid surface, minimizing vapor space and reducing evaporation losses. They can be internal or external floating roof tanks, depending on whether the roof is inside a fixed shell.

Dome Roof and Cone Roof Tanks

These are variations of fixed roof tanks where the roof is shaped for structural efficiency and vapor containment. Dome roofs are often used in refrigerated tanks or tanks holding light hydrocarbons.

Spherical and Bullet Tanks

Used for high-pressure storage of gases such as LPG or hydrogen, spherical tanks (gas spheres) and horizontal bullet tanks are designed to evenly distribute stress and withstand internal pressure.

Storage Tank Design Considerations

Proper tank design ensures safety, stability, and compliance with codes. Important design parameters include:

  • Tank Diameter and Height: Determined by storage capacity and site constraints.
  • Corrosion Allowance: Additional thickness is added to account for long-term material loss.
  • Nozzles and Manways: Located for operational access, sampling, draining, and venting.
  • Foundation Design: Tanks are typically installed on concrete or compacted soil pads with settlement and drainage in mind.
  • Seismic and Wind Loads: Especially critical in earthquake-prone or windy regions.

Design calculations must consider pressure, temperature, corrosion, fluid characteristics, and environmental conditions.

Storage Tank Materials

Tank material selection is based on the chemical compatibility of the stored product. Most tanks are made from carbon steel, which is suitable for a wide range of hydrocarbons and chemicals. Stainless steel is used when corrosion resistance is paramount—such as for storing acidic or caustic substances.

For specialized applications, FRP (fiberglass reinforced plastic) and lined tanks are used. These materials are lightweight, corrosion-resistant, and ideal for certain chemical or wastewater applications.

Tank Inspection and Testing

Regular inspection and testing are essential to ensure safe operation and regulatory compliance. Storage tanks are inspected both externally and internally at fixed intervals.

Common Inspection Methods:

  • Visual Inspection for corrosion, cracks, and structural damage
  • Ultrasonic Thickness Testing (UTT) to measure wall thickness and detect thinning
  • Magnetic Particle or Dye Penetrant Testing for crack detection
  • Hydrostatic Testing to verify tank integrity before commissioning or after major repairs
  • API 653 Inspections for aboveground atmospheric tanks, including fitness-for-service evaluations

Digital inspection tools, drones, and robotic crawlers are increasingly used for more efficient, accurate, and safer inspections.

Maintenance and Repair of Storage Tanks

Tank maintenance includes both routine and corrective actions aimed at extending service life and preventing failures. Common maintenance activities include:

  • Repainting and coating applications to prevent corrosion
  • Replacing gaskets, nozzles, or floating roof seals
  • Welding patch plates on damaged shell or bottom areas
  • Cleaning tanks before inspection or change of service
  • Cathodic protection system checks and maintenance

Maintenance shutdowns should be planned well in advance with detailed scope, job cards, permits, and safety measures in place.

Safety and Environmental Protection

Storage tanks must be equipped with features to minimize safety risks and environmental impact. This includes:

  • Overfill protection systems like alarms, sensors, and emergency shutoff valves
  • Vapor recovery systems to reduce VOC emissions
  • Bund walls and containment dykes to control spills
  • Flame arrestors and pressure relief valves for fire protection
  • Access ladders, platforms, and safety rails for operator safety

Compliance with standards such as API, OSHA, NFPA, and local environmental regulations is mandatory.

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FAQs – Storage Tanks

What standard governs storage tank design?
API 650 is the most commonly used standard for atmospheric aboveground storage tanks.

How often should tanks be inspected?
External inspections are done annually, while internal inspections typically occur every 10 years or sooner depending on service conditions and RBI results.

Can floating roof tanks prevent vapor emissions?
Yes, floating roof tanks significantly reduce vapor losses, especially for volatile organic compounds (VOCs).