Storage Tank Classification

🛢️ STORAGE TANK CLASSIFICATION

Storage tanks are categorized based on their location, design pressure, roof structure, bottom configuration, and safety requirements. These classifications help select the right tank for a particular service, fluid type, and environmental condition.

🧱 BASED ON SAFETY

Tanks are classified by their containment method and protection against leakage or fire hazards. The safety level depends on the stored material’s hazard potential.

• Single Wall: Simple construction with a single containment shell; used for non:toxic, non:flammable liquids. Example: Grain storage, water tank

• Double Wall: Two concentric shells; the outer wall acts as secondary containment to prevent environmental leakage. Example: Ammoina, LNG tanks

• Diked: Surrounded by a containment dike or bund wall to collect spillage; common in petroleum storage terminals. Example: Disel tanks, Asphalt tank

• Vaulted: Installed within a concrete vault for additional protection against fire and impact; often used underground or in tight spaces. Example: where strict leakage requirement.

⚙️ BASED ON PRESSURE

Storage tanks are also grouped by the internal design pressure they can safely handle. This affects wall thickness, material selection, and applicable design codes.

• Atmospheric (≤ 2.5 psi):Operate at or near atmospheric pressure; typically API 650 or AWWA D100 designs. Example: Water storage, crude oil.

• Low Pressure (≤ 15 psi):Designed for slightly pressurized storage of volatile products like gasoline or ethanol; often API 620. Example: Egg shape digester, noded spheriod.

• Pressure (> 15 psi):Built to ASME standards to hold high:pressure gases or liquids; often referred to as pressure vessels. Example: Spherical storage for liquified gases such as LIN/LOX/LC02

🏗️ BASED ON ROOF DESIGN

The roof design is chosen to control vapor losses, resist wind loads, and accommodate floating or fixed configurations depending on the liquid’s volatility.

1. Floating Roof Tanks

Used for volatile liquids to minimize vapor emission by floating directly on the liquid surface.The roof rides up and down inside the tank shell as the oil level changes. The floating roof design as the beginning in 1919, the first tank with floating roof was constructed by CB&I in 1923.

• External Floating Roof: Exposed to the atmosphere; ideal for large crude oil tanks.

• Internal Floating Roof: Protected beneath a fixed roof; reduces evaporation and contamination.

• Double Deck: Two parallel decks provide buoyancy and reduce deformation.

• Pontoon Roof: Equipped with hollow pontoons to support the roof on the liquid surface.

• Skin & Pontoon: Combination of flat skin plate with circumferential pontoons for stability.

• Pan Roof: Shallow dished roof design with limited buoyancy, used in small floating tanks.

• Bulkhead Pan Roof: Reinforced pan with bulkheads; improves rigidity.

• Honeycomb Roof: Lightweight composite structure; high stiffness to weight ratio.

• Plastic Sandwich Roof: Corrosion resistant and thermally insulated composite panels.

2. Fixed Roof Tanks

Used for less volatile liquids or where vapor recovery systems are applied.

• Flat Roof: Simple and economical; used for small diameter tanks typically <10 ft.

• Self-Supported Cone Roof: Conical roof supported only by the shell; common in moderate-sized tanks with diameter ranging between 10 to 30 ft. Slope of the roof can be increased to avoid rafter or columns. API 650 limits the maximum corroded thickness to ½”.

• Centre Supported Cone Roof: Includes a central column for large diameter tanks.

• Supported Cone Roof: Uses rafters or beams under the roof for structural support.

• Externally Supported Cone Roof: External trusses support the roof; provides internal clearance.

• Dome or Umbrella Roof: Curved shape provides strength and excellent drainage; ideal for large tanks.

⚖️ BASED ON BOTTOM PLATE DESIGN

Bottom configurations influence tank drainage, settlement control, and foundation interaction. The choice depends on stored liquid type and site conditions.

• Flat Bottom:Economical design for smaller tanks on stable foundations.

• Conical Bottom (Cone Up):Upward slope allows sediment collection and simplifies cleaning.

• Conical Bottom (Cone Down):Downward slope for gravity drainage of liquids.

• Single Slope Bottom:Inclined in one direction for complete emptying through a drain. Generally used in smaller tanks.

• Domed or Spheroid Bottom:Curved bottom improves pressure distribution and resistance to uplift. It is costly and difficult to construct, so its use is restricted to smaller sizes.