IESCON provides engineering and industrial solutions for storage tanks, piping systems, and oil and gas facilities. When planning a welded storage tank, understanding API 650 vs API 620 is essential because the two standards address different pressure, temperature, design, and service conditions. Choosing the appropriate standard should be based on the tank’s operating requirements, stored product, design conditions, and project specifications.
What Are API 650 and API 620?
API 650 and API 620 are American Petroleum Institute standards used for the design and construction of aboveground welded storage tanks, but they are intended for different service conditions.
API 650, titled Welded Tanks for Oil Storage, establishes minimum requirements for the material, design, fabrication, erection, and inspection of vertical, cylindrical, aboveground welded storage tanks operating at pressures approximately equal to atmospheric pressure. API’s scope also covers non-refrigerated service with a maximum design temperature of 93°C (200°F), subject to the standard’s requirements.
API 620, titled Design and Construction of Large, Welded, Low-Pressure Storage Tanks, covers large welded, low-pressure carbon-steel aboveground storage tanks. API identifies applications involving liquids with gases or vapors above the liquid surface and tanks designed for gas or vapor storage. The standard includes provisions for specific low-temperature and refrigerated applications through its annexes.
In simple terms:
API 650 → Atmospheric or near-atmospheric storage
API 620 → Low-pressure storage requiring a higher-pressure design approach
The final selection should always be confirmed against the current edition, project specifications, design conditions, and applicable regulations.
API 650 vs API 620: Key Differences
The most important difference is the operating pressure and the resulting design requirements.
| Factor | API 650 | API 620 |
| Primary application | Welded oil and liquid storage tanks | Large welded low-pressure tanks |
| Typical pressure regime | Approximately atmospheric | Low pressure above atmospheric |
| Tank configuration | Vertical, cylindrical, aboveground | Large welded aboveground tanks |
| Typical service | Petroleum and liquid products | Liquids with vapor space and certain gas/liquid services |
| Temperature scope | Non-refrigerated service up to 200°F under the standard | Includes low-temperature and refrigerated applications through applicable provisions |
| Design approach | Atmospheric storage | Low-pressure storage |
| Selection priority | Normal storage conditions | Pressure, temperature, and product requirements |
API describes API 650 as a standard for welded oil-storage tanks and API 620 as a standard for large welded low-pressure storage tanks.
Therefore, the question is not simply which standard is “better.” The correct question is:
Which standard matches the tank’s design conditions?
Design and Construction Requirements
Both standards address engineering and construction requirements, but their design bases differ.
API 650 establishes requirements covering:
- Materials.
- Design.
- Fabrication.
- Erection.
- Inspection.
- Tank components.
- Welding-related requirements.
API’s published scope states that API 650 is intended for vertical, cylindrical, aboveground welded storage tanks with uniformly supported bottoms.
API 620 addresses large welded low-pressure tanks and includes requirements appropriate to the pressure and temperature conditions of that service.
For both standards, tank engineering must consider factors such as:
- Tank diameter and height.
- Product density.
- Operating temperature.
- Design pressure.
- Wind conditions.
- Seismic conditions where applicable.
- Corrosion allowance.
- Foundation conditions.
- Roof configuration.
- Shell thickness.
- Nozzle and connection requirements.
A project engineer should therefore establish the design basis before selecting the applicable tank standard.
Operating Pressure and Temperature Differences
Pressure and temperature are two of the most important factors when comparing API 650 and API 620.
API 650
API 650 is intended primarily for tanks operating at pressures approximating atmospheric pressure. The API scope states that internal pressure does not exceed the weight of the roof plates under the basic scope, although higher pressure can be permitted when additional requirements are satisfied.
The standard applies to non-refrigerated service with a maximum design temperature of 93°C (200°F) under its stated scope.
API 620
API 620 is designed for low-pressure storage. API’s published material identifies tanks with gas or vapor-space pressures up to 15 psi gauge under the referenced standard scope and metal temperatures up to 250°F, with additional annexes addressing specific low-temperature and refrigerated services.
This makes pressure and temperature a critical starting point when determining which standard may be appropriate.
Materials and Welding Requirements
Material selection is an important part of storage tank engineering.
The selected material must be suitable for:
- Design temperature.
- Product characteristics.
- Mechanical loads.
- Corrosion conditions.
- Welding requirements.
- Applicable impact-testing requirements.
Both API 650 and API 620 contain requirements related to materials, fabrication, welding, and inspection, but the detailed requirements depend on the selected standard, material grade, thickness, temperature, and service conditions.
Welding procedures and welder qualifications should be established according to the applicable project requirements and referenced codes.
Quality control can include:
- Material identification.
- Material certification review.
- Welding procedure qualification.
- Welder qualification.
- Weld inspection.
- Non-destructive examination where required.
- Dimensional inspection.
- Final documentation.
API also maintains related standards for tank inspection, repair, alteration, and reconstruction, including API 653.
Storage Tank Applications: API 650 vs API 620
API 650 is widely associated with aboveground welded storage tanks for petroleum and other liquid products.
Typical applications can include:
- Crude oil storage.
- Petroleum product storage.
- Water and utility storage where applicable.
- Chemical liquid storage.
- Fuel storage.
- Tank farms.
- Industrial storage facilities.
API 620 is used where the tank requires a low-pressure design approach.
Its published scope covers large welded low-pressure tanks containing petroleum intermediates, finished products, and other liquids commonly handled by different branches of the industry.
The key principle is that the stored product alone does not determine the standard. The pressure, temperature, tank configuration, and project design conditions must also be evaluated.
API 650 vs API 620 for Different Types of Stored Products
A useful way to approach the decision is to start with the product and then evaluate the operating conditions.
| Stored Product / Service | Initial Consideration |
| Crude oil | API 650 may be appropriate for atmospheric storage |
| Petroleum products | API 650 may be suitable depending on conditions |
| Low-pressure hydrocarbon service | API 620 may be considered |
| Refrigerated products | API 620 provisions or another applicable tank-system standard may need evaluation |
| Liquefied gases | Requires specialized design evaluation and applicable provisions |
| Water or industrial liquids | Standard selection depends on project requirements |
API’s standards portfolio separately identifies API 625 for refrigerated liquefied gas storage systems, demonstrating why some refrigerated services require consideration beyond a simple API 650/API 620 comparison.
Inspection and Testing Requirements
Inspection is essential for both API 650 and API 620 tanks.
A typical tank quality program may include:
Before Fabrication
- Review material certificates.
- Verify material grades.
- Review approved drawings.
- Confirm welding procedures.
- Confirm inspection plans.
During Fabrication and Erection
- Dimensional inspection.
- Weld inspection.
- Visual inspection.
- Non-destructive examination where required.
- Shell alignment checks.
- Nozzle inspection.
- Roof inspection.
After Construction
Depending on the tank and project requirements, testing may include:
- Hydrostatic testing.
- Leak testing.
- Vacuum testing.
- Pressure-related testing where applicable.
- Settlement monitoring.
- Final dimensional inspection.
The specific inspection and testing program must follow the applicable edition of the standard and project specifications.
Cost and Project Considerations
Cost should not be the first criterion when choosing between API 650 and API 620.
A lower-cost design that does not satisfy the required pressure or temperature conditions is not an appropriate solution.
Project cost can be influenced by:
- Tank diameter.
- Tank height.
- Steel quantity.
- Material grade.
- Shell thickness.
- Roof design.
- Pressure requirements.
- Temperature requirements.
- Welding requirements.
- Inspection requirements.
- Foundation design.
- Internal equipment.
- Nozzles and piping connections.
- Construction location.
- Schedule.
Practical Example
Suppose two projects require large petroleum storage tanks.
Project A: Atmospheric storage of a petroleum liquid at moderate temperature.
Project B: Storage involving a controlled low-pressure vapor space and more demanding temperature conditions.
Even if both tanks have similar capacities, their design requirements can be significantly different. The second project may require evaluation under API 620 rather than simply applying API 650.
API 650 vs API 620: Advantages and Limitations
API 650 Advantages
- Well established for atmospheric liquid storage.
- Broad application to petroleum and other liquid products.
- Provides requirements for design, fabrication, erection, and inspection.
- Suitable for many conventional storage-tank projects.
API 650 Limitations
It is not a universal standard for every tank. Projects involving significant internal pressure, refrigerated service, or specialized conditions may require another standard or additional design requirements.
API 620 Advantages
- Designed specifically for large welded low-pressure tanks.
- Addresses higher internal pressure than the basic API 650 scope.
- Includes provisions for certain low-temperature and refrigerated services.
- Suitable for specific gas, vapor, and liquid storage applications.
API 620 Limitations
It involves a different and potentially more demanding design basis. It should not be selected simply because a project requires a large tank.
How to Choose Between API 650 and API 620
Use a structured engineering decision process.
Step 1: Define the Product
Identify exactly what will be stored.
Step 2: Establish Operating Conditions
Determine:
- Operating pressure.
- Design pressure.
- Operating temperature.
- Design temperature.
Step 3: Determine Tank Configuration
Define:
- Diameter.
- Height.
- Roof type.
- Bottom configuration.
- Foundation conditions.
Step 4: Review Special Conditions
Consider:
- Refrigeration.
- Vacuum conditions.
- Corrosive products.
- Seismic requirements.
- Wind loads.
- Low ambient temperatures.
Step 5: Review Applicable Standards
Compare the project requirements with the current applicable API standard and any referenced codes.
Step 6: Confirm With the Engineering Team
The final selection should be documented in the tank design basis and project specifications.
API 650 vs API 620: Which Storage Tank Standard Fits Your Project?
A simplified decision guide is:
Is the tank primarily an atmospheric-pressure storage tank for liquid products?
→ API 650 may be the appropriate starting point.
Does the tank require low-pressure service above the normal atmospheric storage range?
→ API 620 should be evaluated.
Does the project involve refrigerated or liquefied-gas service?
→ Evaluate the applicable API 620 provisions and other relevant tank-system standards, such as API 625 where applicable.
Does the tank involve unusual pressure, temperature, geometry, or product conditions?
→ A qualified engineering team should establish the applicable design code and requirements before fabrication begins.
The important point is that API 650 vs API 620 is not simply a choice between two equivalent alternatives. Each standard has a defined scope, and the tank must be designed according to the conditions it is expected to withstand.
How IESCON Can Support Storage Tank Projects
IESCON provides engineering and industrial capabilities for projects involving storage tanks, piping systems, and industrial facilities.
A tank project requires coordination between:
- Engineering.
- Material selection.
- Fabrication.
- Welding.
- Inspection.
- Erection.
- Piping connections.
- Testing.
- Documentation.
For an API-based tank project, the engineering team should establish the applicable standard before fabrication starts. This helps align the design, material specifications, welding requirements, inspection plan, and construction activities.
IESCON’s industrial capabilities can support projects where tank construction and piping need to be coordinated as part of a broader industrial facility.
Frequently Asked Questions About API 650 and API 620
1. What is the main difference between API 650 and API 620?
The main difference is their design scope and pressure regime. API 650 primarily addresses welded aboveground storage tanks operating at approximately atmospheric pressure, while API 620 addresses large welded low-pressure storage tanks.
2. Is API 650 suitable for crude oil storage?
API 650 can be applicable to welded aboveground tanks used for petroleum and other liquid products when the tank’s pressure, temperature, configuration, and other conditions fall within the standard’s scope.
3. When should API 620 be considered?
API 620 should be evaluated when a large storage tank requires a low-pressure design approach or when the service involves specific low-temperature or refrigerated conditions addressed by the standard.
4. Is API 620 always more expensive than API 650?
Not necessarily. Cost depends on tank size, materials, pressure, temperature, design requirements, fabrication, inspection, foundation, and project conditions. The applicable standard should be selected based on technical requirements rather than price alone.
5. Can one storage facility use both API 650 and API 620 tanks?
Yes, a facility can contain tanks designed to different standards when their individual service conditions justify it. Each tank should have its own documented design basis and applicable standard.
Conclusion
Understanding API 650 vs API 620 is essential when designing a welded storage tank because the standards address different operating conditions and design requirements. The correct selection depends on pressure, temperature, product, tank configuration, and project specifications. IESCON can support storage tank and industrial projects through engineering, tank construction, piping, and related execution capabilities.
Need the Right Storage Tank Solution?
Planning a new storage tank or industrial facility? Contact IESCON to discuss your tank requirements, operating conditions, piping scope, and project specifications and determine the appropriate engineering and construction approach for your application.
