API 620 Standard: The Ultimate Guide

Design and Construction of Large, Welded, Low-Pressure Storage Tanks - Introduction

Updated: 26-Aug-2025

Introduction

The API Standard 620, established by the American Petroleum Institute (API), is the internationally recognized code for the design, fabrication, and testing of large, welded, low-pressure storage tanks. This comprehensive guide provides a detailed overview of the API 620 standard, including its scope, key design requirements, material selection, and how it differs from other codes like API 650. Whether you’re an engineer specifying a new tank, an inspector verifying compliance, or a student learning about pressure vessel design, this resource covers the essential information you need to understand this critical standard.

API 620 Scope and Applicability

API Standard 620 provides the rules for constructing large, welded tanks that operate at internal pressures above atmospheric pressure but not exceeding 15 psig (pounds per square inch gauge). Its scope is defined by specific technical and operational limits:

  • Pressure Range: Primarily for tanks with an internal pressure between 0.5 psig and 15 psig. It can also be used for tanks designed for full vacuum (external pressure).

  • Temperature Range: It covers a wide range, from cryogenic temperatures as low as -325°F (-198°C) for storing liquefied gases like LNG, up to elevated temperatures allowed by the stress limits of the materials used.

  • Tank Types: The standard applies to vertical, cylindrical, flat-bottomed tanks. It includes detailed provisions for both single-wall and double-wall designs, the latter being critical for insulated and cryogenic service.

  • Applications: It is mandated for the storage of products where vapor pressure must be contained, such as liquefied natural gas (LNG), liquefied petroleum gas (LPG), refrigerated products, and many volatile chemicals.

Key Design Requirements and Formulas in API 620

The core of API 620 is its detailed engineering rules for ensuring tank integrity under internal pressure and hydrostatic loads. Unlike its counterpart API 650, which uses more prescribed formulas, API 620 requires a design-by-analysis approach, providing engineers with specific formulas to calculate stresses and determine wall thickness.

1. Shell Thickness Calculation:

The required minimum thickness for cylindrical shell courses is determined by the combined load of internal pressure (P) and the hydrostatic head of the liquid (H). The fundamental formula is:

                                t = (P * D) / (2 * S * E – 0.2 * P) + (H * D) / (2 * S * E) + CA

Where:

  • t = Required shell thickness (in inches or mm)
  • P = Design internal pressure (in psi or kPa)
  • D = Nominal diameter of the tank (in inches or mm)
  • S = Allowable stress value for the material (from API 620 tables)
  • E = Joint efficiency factor (typically 0.7 to 1.0, based on weld type and inspection)
  • H = Height from the bottom of the course under consideration to the top of the shell (in feet or meters) x liquid density
  • CA = Corrosion allowance (specified by the user)

2. Allowable Stress Values (S):

A fundamental part of API 620 design is the use of conservative allowable stress values for materials, which are tabulated in the standard for various temperature ranges. These values include a safety factor to ensure reliability. The designer must select the correct `S` value based on the material grade and design temperature.

3. Roof Design:

The standard provides rules for multiple roof types, with dome roof thickness being a common search topic. The approximate formula for the required thickness of a spherical segment dome roof is:

                                t_roof = (P * R) / (2 * S * E)

Where:

  • R = Radius of curvature of the dome roof
  • P, S, E are as defined above.

Important Note: These are simplified conceptual formulas for educational overview. The actual API 620 standard contains more complex, precise formulas and detailed constraints that must be followed for code-compliant design. This overview does not replace the need for the full standard.

 

API 620 vs. API 650: Key Differences Explained

A fundamental question for engineers is whether a storage tank falls under API 620 or API 650. The choice is not optional; it is dictated by the tank’s intended service conditions. The key differences are rooted in their core design purposes:

FeatureAPI 620API 650
Primary PurposeDesign for significant internal pressure (and vacuum).Design for atmospheric storage with minimal internal pressure.
Pressure RangeUp to 15 psig and full vacuum.Limited to a maximum of 2.5 psig (with special considerations).
Temperature RangeDesigned for a broad range, including cryogenic service (down to -325°F / -198°C).Primarily for ambient temperature storage.
Design PhilosophyDesign-by-analysis. Engineers perform specific stress calculations using formulas provided in the standard.Design-by-rule. Uses pre-defined, empirical formulas and tables for most applications.
Common ApplicationsStorage of liquids with high vapor pressure: LNG, LPG, refrigerated gases, volatile chemicals.Storage of crude oil, gasoline, water, and other liquids at near-atmospheric pressure.
Roof TypesOften uses dome roofs (for pressure) and double-wall designs (for cryogenics).Typically uses cone roofs or external floating roofs.
Cost & ComplexityGenerally more expensive and complex to design and fabricate due to thicker shells, specialized materials, and more rigorous inspection.Generally more economical and simpler to construct for atmospheric service.

The Simple Rule of Thumb:

  • Does your tank need to hold significant pressure or contain a refrigerated/cryogenic product? → API 620
  • Is your tank storing a product at atmospheric pressure and temperature? → API 650

This comparison provides a foundational understanding. For a detailed breakdown of selection criteria, see our dedicated guide on choosing between API 620 and API 650.

API 620 Materials and Toughness Requirements

API 620 provides rigorous rules for material selection based on service conditions, particularly pressure and temperature.

  • Approved Materials: The standard covers a wide range of materials, including:
    • Carbon Steels: Most common for non-cryogenic, ambient temperature services.
    • Alloy Steels: Used for higher strength or improved properties.
    • Austenitic Stainless Steels (e.g., 304, 316, 321): For corrosion resistance and cryogenic service.
    • Nickel Alloys (e.g., 9% Nickel): Essential for extreme cryogenic applications like LNG storage.
  • Impact Testing (Charpy V-Notch): This is a non-negotiable requirement for most applications and a key interview question.
    • Why? To prevent brittle fracture. Materials can lose toughness and become prone to sudden catastrophic failure at low temperatures.
    • When is it required? API 620 mandates Charpy impact testing for:
      1. All materials for cryogenic service.
      2. Carbon steel plates thicker than a specified limit (e.g., typically 1 inch or 25.4mm, but varies with grade and stress).
      3. Any material used at a temperature where it could be susceptible to brittle fracture.
    • The Rule: The minimum design metal temperature (MDMT) determines the required impact test temperature. The material must have adequate toughness at or below the MDMT.

Fabrication, Welding, and Inspection (NDT) in API 620

API 620 has stringent rules to ensure the quality of fabrication and welds, which are the primary source of potential failure.

  • Welding Procedures (WPS/PQR): All welding must be performed using qualified procedures. The Welding Procedure Specification (WPS) and supporting Procedure Qualification Record (PQR) must be qualified according to ASME Section IX standards.

  • Welders and Operators: All personnel must be qualified according to ASME Section IX to perform the specific welding processes used.

  • Non-Destructive Examination (NDE): API 620 requires extensive NDT to verify weld integrity. Key methods include:

    • Radiographic Testing (RT): Required for all butt-welded joints in the shell and critical roof welds. A specific percentage of welds (often 100% for critical joints) must be radiographed to check for internal defects like slag, porosity, or cracks.

    • Ultrasonic Testing (UT): May be used in addition to or in place of RT for certain applications.

    • Liquid Penetrant Testing (PT) or Magnetic Particle Testing (MT): Used for inspecting fillet welds, nozzle welds, and other surface-connected defects.

  • Leak Testing: All welds must be tested for leaks. For pressurized shells, this is done via a hydrostatic test (filling with water to a pressure above the design pressure). For non-pressurized welds (like floor plates), vacuum box testing or air soap testing is used.

API 620 Appendices Explained (Q, R, L, S, V)

While the main body of API 620 covers general requirements, mandatory appendices provide rules for specific services. Which appendix applies is a fundamental design choice.

  • Appendix A: Thermally Stress-Relieved Austenitic Stainless Steel. Rules for when post-weld heat treatment (PWHT) of stainless steels is required.
  • Appendix C: Internal Pressure Coefficients for Shell Design. Provides methods for analyzing shell stresses.
  • Appendix Q: Requirements for Liquefied Hydrogen Storage. Highly specialized rules for one of the most challenging cryogenic services.
  • Appendix R: Low-Pressure Storage Tanks for Refrigerated Service. This is a critical appendix. It provides additional, mandatory requirements for tanks storing refrigerated liquefied gases (like LNG, ethylene, etc.), covering materials, design, and inspection for cold temperatures.
  • Appendix S: Allowable Stresses for Austenitic Stainless Steels and Nickel Alloys. Provides the specific “S” values used in design formulas for these materials.
  • Appendix V: Design of Storage Tanks for External Pressure. Rules for tanks designed to withstand full vacuum.

Common Applications of API 620 Tanks

API 620 tanks are engineered for specific challenging services where containment of pressure and/or low temperature is paramount. They are a critical piece of infrastructure in the following industries:

IndustryApplicationWhy API 620 is Required
Oil & GasStorage of Liquefied Natural Gas (LNG), Liquefied Petroleum Gas (LPG), butane, and propane.Contains the vapor pressure of the liquefied gas at cryogenic (LNG) or ambient (LPG) temperatures.
Chemical ProcessingBulk storage of volatile chemicals and refrigerated gases (e.g., ammonia, ethylene, chlorine).Withstands internal pressure from vaporization and specifies materials compatible with the chemical.
Power GenerationLNG storage tanks at power plants that use natural gas.Provides the large-volume, low-pressure cryogenic storage needed to fuel the plant.
Cryogenic ApplicationsStorage of liquid nitrogen, oxygen, argon, and other industrial gases.Designed for the extreme cold and pressure of these liquefied gases.
Other IndustriesStorage of fermentation gases in biofuels, vapor containment in wastewater treatment.Used anywhere a large volume of gas m

Inspection, Certification, and How to Get the Standard

The journey of an API 620 tank from design to operation concludes with rigorous verification and certification.

  • Authorized Inspector: Fabrication must be monitored by an API-certified Authorized Pressure Vessel Inspector. This independent inspector verifies that all construction and examination activities comply with the certified design and the requirements of API 620.

  • Certification and Nameplate: Upon successful completion of all inspections and hydrostatic tests, the tank is stamped with an API Monogram and a nameplate is affixed. This nameplate includes critical data like design pressure, temperature, capacity, and the manufacturer’s details. This is the formal certification of compliance.

  • How to Purchase the Official API 620 Standard:
    API standards are copyrighted documents. The only legal way to obtain the full, current, and definitive version of API Standard 620 is to purchase it directly from the official source, the American Petroleum Institute (API). This ensures you have the latest code revisions, addenda, and errata.
    👉 Purchase API Standard 620 from the official API.org website

Conclusion

This overview covers the critical aspects of the API 620 standard—from its scope and design rules to material selection, fabrication, and application. It is a comprehensive code designed to ensure the safety, reliability, and integrity of large, low-pressure storage tanks operating under demanding conditions. For engineers, a firm grasp of API 620’s principles is essential for specifying, designing, and approving these critical industrial assets.

Whether for oil storage, LNG terminals, or chemical processing plants, API 620 provides the framework necessary to build reliable, compliant, and long-lasting storage solutions.