API 650/620 Tank Design – Step by Step Procedure

Designing storage tanks in the oil, gas, and chemical process industries involves meticulous engineering to ensure safety, regulatory compliance, and long-term reliability. This guide provides a complete, detailed walk-through of the tank design process using API 650 (for atmospheric tanks) and API 620 (for low-pressure tanks), with explanations of each step, formulas, and references to code clauses. Whether you’re a piping engineer, process engineer, or mechanical engineer with no prior tank design experience, this resource will help you understand the logic, math, and engineering decisions behind tank design.

Step 1: Define Tank Operating Requirements

Purpose: Establish the basis for design by understanding what the tank will store, under what conditions, and where it will be located.

Key Inputs Required:

• Stored Fluid Properties:

  • Type (e.g., crude oil, gasoline, hydrocarbon condensate, etc.)

  • Specific gravity (SG)

  • Temperature range (ambient to maximum design temperature)

  • Vapor pressure

  • Corrosiveness or chemical aggressiveness

• Tank Location Conditions:

  • Site ambient temperature (min and max)

  • Seismic zone classification

  • Wind speed (basic wind speed per ASCE 7 or local code)

  • Soil type (for foundation design)

• Operating Parameters:

  • Maximum liquid level (height)

  • Design internal pressure (API 620 for >2.5 kPa)

  • Design vacuum (external pressure)

  • Heating/cooling requirements (if any)

Step 2: Select the Applicable Design Code (API 650 vs. API 620)

API 650:

• For atmospheric storage tanks up to 2.5 kPa (0.25 bar) internal pressure.
• Suitable for petroleum products, water, and non-refrigerated chemicals.
• Commonly used in refineries, terminals, and bulk liquid storage.

API 620:

• For low-pressure storage tanks, from 2.5 kPa to 103 kPa (15 psig).
• Required for refrigerated liquids, LNG, and light hydrocarbons with low boiling points.
• Designed for lower temperature service (-325°F or -198°C in some cases).

Decision Criteria:

Note: If tank falls under both categories (e.g., intermediate pressure), API 620 should be chosen.

Step 3: Determine Basic Tank Dimensions

Purpose: Define the tank’s basic geometry to meet storage volume needs within space and height limitations.

Formula for Volume (Cylindrical Tank):

                                                           V = π × (D / 2)² × H

Where:

• V = Volume in m³
• D = Internal Diameter in meters
• H = Tank height (liquid level) in meters

Example:
To store 10,000 m³ of fluid, assume a tank diameter of 25 m.

                                        Step 1: Use the volume formula:
                                         V = π × (D / 2)² × H

                                        Step 2: Rearranged to find height:
                                        H = V / [π × (D / 2)²]

                                        Step 3: Substitute the values:
                                        H = 10,000 / [3.1416 × (12.5)²]
                                        H = 10,000 / [3.1416 × 156.25]
                                        H = 10,000 / 490.87
                                        H = 8.14 m

Note: Adjust tank height and diameter based on site constraints, mechanical design, and fluid behavior. An H/D ratio between 0.6 and 1.2 is generally preferred for stability and economy.

Step 4: Material Selection

Purpose: Choose appropriate materials for the tank shell, bottom, roof, and nozzles based on fluid properties, design temperature, and corrosion resistance.

Common Materials as per API 650:

How to Choose:

• Check API 650 Table 4-1 (for carbon steels), 4-2 (low-temp materials), and 4-3 (stainless steels).

• Select based on:

  • Design temperature (API 650 Table 4-2)

  • Corrosiveness of product

  • Weldability and availability in the region

For API 620:

• Material selection follows Annex Q for low temperature applications.

• Check allowable stress and toughness from API 620 Table Q-2 and Q-3.

Corrosion Allowance: Typically 1.5 mm for atmospheric tanks, but can be 3 mm or more for corrosive services or longer life expectations. Specified by the client or design basis.

Cladding or Lining:

• Stainless steel cladding may be used for internal corrosion protection.
• Linings like epoxy or rubber are recommended for aggressive chemicals.

Step 5: Design Pressure and Shell Thickness Calculation

Purpose: Ensure the shell is thick enough to withstand the hydrostatic head and internal/external pressures.

For API 650 Shell Thickness (Hydrostatic Method):

Shell Thickness Formula:
                                                   t = (H × SG × 9.81) / (2 × S × E − 1.2 × P) + CA

Where:

• t = required shell thickness (mm)
• H = liquid height (m)
• SG = specific gravity of fluid
• S = allowable stress of material (MPa)
• E = weld joint efficiency (typically 1.0)
• P = design internal pressure (MPa)
• CA = corrosion allowance (mm)

For API 620 (with Pressure):

Use design equations from API 620, Section 5.11 and 5.12.

Minimum Thickness: API 650 Clause 5.6.1 limits the minimum nominal thickness (e.g., 6 mm for carbon steel).

Shell Courses: Divide shell into rings/courses based on height. Use 1-ft method or variable design point method for each shell course.

Step 6: Roof Design

Purpose: Select and design a suitable roof type based on the stored product and required functionality.

Roof Types (API 650):

• Cone Roof: Standard for fixed roof tanks. Simple to fabricate.
• Dome Roof: Hemispherical for higher pressure or aesthetic needs.
• Umbrella Roof: Slope on all sides. Drains rainwater.
• Floating Roof: For volatile liquids to reduce vapor losses.

Design Considerations:

• Roof Slope: Typically 1:16 to 1:6 for cone roofs (API 650 Clause 5.10.2)
• Roof Plates: Min. thickness from API 650 Table 5-21
• Roof-to-shell connection: Must be designed to avoid uplift during wind/seismic

Floating Roofs:

• Single-deck or double-deck
• Include seal systems, guide poles, drains, pontoons

API 620 Roofs: Often involve internal or external domes due to pressure requirements.

Step 7: Wind and Seismic Design

Purpose: Ensure tank stability and structural integrity under environmental loads.

Wind Design:

• Wind pressure calculated as per ASCE 7 or local codes
• Uplift on the roof (especially for floating roofs)
• Overturning moment and anchorage required

Seismic Design:

• Follow API 650 Appendix E for seismic zones
• Compute base shear, overturning moment, and sloshing effects
• Use seismic coefficient (Ss, S1) from USGS or local code

Anchorage: Required if overturning moment exceeds tank dead weight.

Stiffeners: May be needed for shell buckling prevention (API 650 Clause 5.11)

Step 8: Nozzle Sizing and Placement

Purpose: Provide safe and functional inlets, outlets, drains, and vents.

Types of Nozzles:

• Fill/Discharge
• Overflow
• Manways (24″ typically)
• Drain nozzles (lowest elevation)
• Vents (API 2000 for vent sizing)
• Instrumentation (e.g., level gauges)

Design Considerations:

• Reinforcement pad calculation (API 650 Clause 5.7)
• Minimum distance from welds and edges
• Orientation based on accessibility and piping layout

Nozzle Load Checks:

• Perform WRC-107/297 or FEA for large pipe loads

Step 9: Foundation and Settlement Considerations

Purpose: Design a stable foundation to prevent uneven settlement or tank tilting.

Foundation Types:

• Ringwall (common for small/medium tanks)
• Slab with piles (for poor soils or large tanks)

Key Considerations:

• Soil bearing capacity
• Tank bottom corrosion protection (bitumen, HDPE liner, etc.)
• Sand pad leveling layer
• Anchor bolts in ringwall

API 650 Appendix B:

• Guidance for typical ringwall design
• Settlement criteria (total and differential)

Step 10: Appurtenances and Accessories

Examples:

• Spiral stairs or ladders (API 650 Clause 3.11)
• Handrails and platforms
• Level instruments (radar, float, sight glass)
• Earthing lugs
• Foam chambers for fire protection
• Breather valves (API 2000)

Painting & Coating:

• Epoxy or polyurethane coating inside/outside
• Cathodic protection for underground portions

Step 11: Testing and Inspection

Purpose: Verify tank integrity before commissioning

Types of Testing:

• Hydrostatic Test (API 650 Clause 7.3)
• Radiographic Examination (Clause 8.1)
• Vacuum box testing for bottom welds
• Magnetic particle or dye penetrant testing
• Coating thickness & holiday detection

Documentation:

• Welding Procedure Specifications (WPS)
• Inspection Test Plan (ITP)
• As-built drawings

Conclusion

Storage tank design as per API 650 and API 620 is a comprehensive engineering task that involves careful consideration of the product to be stored, site conditions, loading scenarios, and safety factors. By following a methodical approach—from identifying design inputs to calculating shell thickness and checking wind/seismic stability—you can ensure that the final tank is safe, code-compliant, and long-lasting. Engineers must always cross-check the applicable clauses from the standards and document each step of the design for future inspection and verification.

For real-world projects, it’s also important to coordinate across disciplines (civil, piping, E&I) to ensure seamless integration with the surrounding infrastructure. This guide can serve as a reference checklist for your next tank design project.