API 650 vs. API 620: How Do I Know Which Standard My Tank Needs?

API 650 Scope: Flat-Bottom Tanks

API 650 applies to: Welded steel storage tanks with a flat (or nearly flat) bottom, open top, and atmospheric or low-pressure operation (typically up to 15 psi gauge). Roof can be fixed cone, fixed dome, floating, or open. Internal design for liquid storage. Originated for petroleum but now used for many liquids (chemicals, water, etc.).

Key characteristics of API 650 tanks:

• Flat bottom (no doming or sloping)
• Cylindrical shell (vertical walls)
• Atmospheric or low-pressure roof
• Design for hydrostatic loading (pressure increases linearly with depth)
• Simpler fabrication and inspection than pressure vessels

Why flat-bottom? Flat bottoms are easier and cheaper to fabricate than domed bottoms. They work for atmospheric/low-pressure storage where the bottom doesn't need to resist hoop stress. For high pressure, a domed bottom is more efficient.

API 620 Scope: Pressure Vessels and Domed-Bottom Tanks

API 620 applies to: Welded steel storage tanks designed for higher internal pressure (above atmospheric, up to roughly 100+ psi depending on design) and/or tanks with domed or conical bottoms. Also covers storage vessels for cryogenic liquids and low-temperature service. More conservative allowable stresses than API 650.

Key characteristics of API 620 tanks:

• Domed or conical bottom (resists internal pressure hoop stress)
• Cylindrical shell and domed top (for pressure containment)
• Higher design pressure (above ~5 psi gauge)
• More complex fabrication and stress analysis required
• Thicker material or more conservative allowables

Why domed-bottom? A domed bottom creates a pressure vessel — both the shell and the bottom can carry hoop stress. This is efficient for high-pressure service. Flat-bottomed vessels cannot use their bottom to carry hoop stress (it would crack); so for high pressure, domed geometry is necessary.

Classification Decision Tree

Question 1: Is the internal design pressure > ~5 psi gauge?

• Yes: Probably API 620. Go to Question 2.
• No: Probably API 650. Go to Question 3.

Question 2 (for high-pressure): Is the bottom flat or domed?

• Flat: API 650 may still apply for low-pressure (under ~5 psi). But if pressure is higher, thicker shell and/or reinforced flat bottom = hybrid design. Consult both codes.
• Domed or conical: API 620 applies. Design uses hoop stress in bottom to help carry pressure.

Question 3 (for low-pressure): Is there a domed or conical bottom?

• No (flat): API 650 is the standard.
• Yes (domed): Hybrid case. Domed bottom for strength and appearance even though pressure is low. Code choice is less clear; consult both and potentially design to the stricter standard (API 620) to be safe.

Allowable Stress: API 620 Is More Conservative

API 650 allowable stress (Sd): Based on the lower of 1/3 ultimate strength or 2/5 yield strength. For A283 Grade C, roughly 12.9 ksi at room temperature.

API 620 allowable stress: Often more conservative, based on 1/4 ultimate strength or 1/3 yield strength (varies by design pressure and temperature). For A283 Grade C, roughly 9–11 ksi. This is roughly 20–30% lower than API 650.

Implication: A tank designed to API 620 for a given pressure will require thicker shell than an API 650 design for the same pressure. Cost difference can be 15–30% higher for API 620.

Hybrid Cases: When Both Standards Matter

Case 1: Low-pressure atmospheric tank with domed bottom (for style or seismic resistance)

Pressure is below 5 psi (API 650 territory), but bottom is domed. Scope is ambiguous. Solution: Design the shell to API 650 (flat-bottom design principles), but verify the domed bottom against API 620 pressure-vessel requirements. This hybrid approach ensures both safety and economy.

Case 2: Tall atmospheric tank in high-seismic region**

Standard API 650 flat-bottom design, but seismic forces are extreme. The code specifies thickening equivalent to what API 620 would require for higher pressure. Design to API 650 but with API 620-level conservatism.

Case 3: Cryogenic liquid tank (LNG, LN2, etc.)

Always API 620, due to cold-temperature requirements, high stresses in inner tank (due to thermal cycling), and dual-containment vessel design.

Cost Implications of Code Choice

API 650 design: Lower material cost, faster fabrication (well-understood process), lower inspection stringency. Typical premium for higher-quality steel or coatings is modest.

API 620 design: Higher material cost (thicker or higher-grade steel), more complex fabrication and inspection (all welds may require 100% radiography), longer schedule. Premium over API 650 for same capacity: typically 15–30%.

When to choose API 620 despite the cost:

• Pressure is genuinely above 5 psi and owner expects it
• High-temperature or cryogenic service (mandatory)
• Owner/site engineer has regulatory preference for API 620 (safer/more conservative)
• Domed-bottom is required for site reasons (seismic, appearance, etc.)

Common Mistakes

Mistake 1: Assuming a tall atmospheric tank can't be API 620.** Tall tanks with seismic loads can benefit from (or require) API 620's conservative allowables. Consult the engineer before assuming API 650 is always cheaper.

Mistake 2: Choosing code based on cost rather than scope.** The correct code is determined by tank geometry, pressure, temperature, and service — not by what saves money. Choosing the wrong code can result in non-compliance and rework.

Mistake 3: Not involving the inspector/authority in code selection.** Some jurisdictions require API 620 for certain applications. Confirm code requirements early with the inspector/AHJ (Authority Having Jurisdiction).

Mistake 4: Mixing API 650 and API 620 design rules without clear intent.** Don't cherry-pick rules from both codes. Either design to one standard, or clearly state that the design is "API 650 + API 620 hybrid" with explicit bridging logic.

Mistake 5: Forgetting inspection/testing scope differs between standards.** API 620 often requires more extensive radiography and nondestructive testing than API 650. Budget for this difference in schedule and cost.

Practical Tips

• Before starting design, confirm with the owner/authority which code applies.** Get it in writing. This prevents rework mid-project.
• For any tank with internal pressure above 5 psi gauge, engage an API 620 engineer early.** Even if you think API 650 might apply, a 620 review prevents mistakes.
• For domed-bottom tanks at atmospheric pressure, explicitly state the design approach:** "Designed to API 650 with domed-bottom configuration per [approved detail]. Domed-bottom verified to withstand internal pressure of [P psi]."
• Document the code choice and scope in the design basis.** State why API 650 or API 620 was chosen, and what specific sections/requirements apply.
• For cost-sensitive projects, run a feasibility study: "Can this tank be designed to API 650 instead of 620?"** The answer depends on pressure, temperature, and service. A feasibility assessment early can save significant cost if the owner can accept API 650 scope.
• If hybrid design is necessary (API 650 shell + API 620 bottom), involve both specialists and document the integration clearly.** Confusion during fabrication is expensive.

Related reading: API 650 vs AWWA D100, Design Pressure Selection, and Tank Classification.