An Overview of API 620: Design and Construction of Large, Welded, Low-Pressure Storage Tanks

Introduction

The American Petroleum Institute (API) developed API Standard 620 to govern the design, fabrication, inspection, and testing of large, welded, low-pressure storage tanks. These tanks are essential for industries that store gases, petroleum products, chemicals, and other materials at low pressures and often at ambient or slightly elevated temperatures.

Scope of API 620

API 620 covers the design and construction of large aboveground storage tanks made of carbon steel, austenitic stainless steel, and certain nickel alloys. Key highlights include:

• Pressure range: Up to 15 psig (pounds per square inch gauge).
• Diameter: No specific limit — often exceeding 300 feet (91 meters).
• Temperature range: Down to -325°F (-198°C) for cryogenic service.
• Tank types: Vertical, cylindrical, flat-bottom tanks; including double-wall designs.

Applicability

API 620 applies to tanks intended for:

• Low-pressure storage of liquids or gases.
• Refrigerated or cryogenic service.
• Large-capacity storage requiring specialized design.

It does not apply to:

• Pressures exceeding 15 psig.
• Small pressure vessels (governed by ASME Section VIII).

Usage

Tanks built under API 620 are used when:

• Low internal pressures are involved.
• Large storage volumes are needed.
• Special operating conditions (low temperatures, seismic loads) must be addressed.

The standard provides rules for:

• Design stress calculations.
• Materials selection.
• Welding and fabrication procedures.
• Inspection and testing methods.
• Environmental and load considerations.

Applications

Key Sections of API 620

API 620 is organized into several important sections, each covering critical aspects of tank construction:

Section 1: General

• Defines the scope, terminology, and application limits.

• Lists referenced standards and codes.

Section 2: Materials

• Specifies requirements for carbon steels, austenitic stainless steels, and nickel alloys.

• Material selection based on temperature range and service conditions.

• Charpy impact testing for low-temperature applications.

Section 3: Design Requirements

• Methods for calculating design stresses.

• Rules for internal pressure, external pressure (vacuum conditions), and hydrostatic loading.

• Roof designs: Cone roofs, dome roofs, and suspended decks.

• Shell thickness calculations, including corrosion allowances.

Section 4: Fabrication and Welding

• Welding qualifications according to ASME Section IX.

• Welding procedures for different materials and thicknesses.

• Requirements for radiographic testing (RT) and ultrasonic testing (UT) of welds.

Section 5: Examination and Testing

• Procedures for non-destructive examination (NDE).
• Hydrostatic testing requirements (test pressure usually higher than design pressure).
• Vacuum box testing for weld leak detection.
• Visual inspection guidelines.

Section 6: Marking and Reports

• Requirements for nameplates and tank marking.
• Mandatory documentation and inspection reports.

Appendices

• Appendix R: Guidelines for cryogenic tanks (low temperature, double-wall designs).

• Appendix Q: Requirements for liquefied gas storage tanks.
• Appendix S: Allowable stresses for certain materials under cryogenic conditions.
• Appendix E: Seismic design provisions.

Each appendix is critical when specialized applications like LNG storage or earthquake-resistant tanks are involved.

Key Differences from API 650