Which standard fits atmospheric fixed-roof tanks and which fits low pressure.
Pick the code by pressure
API 650 covers atmospheric and very low-pressure storage and is the usual reference for oil tanks. API 620 covers a higher pressure range for vessels that operate slightly above atmosphere.
Side by side
| Feature | API 650 | API 620 |
|---|---|---|
| Pressure range | Near atmospheric (up to ~2.5 psi with Appendix F) | Higher internal pressure, up to ~15 psi |
| Common roof shape | Fixed cone or dome, or floating | Mostly dome, for better pressure resistance |
| Typical service | Crude oil, petroleum products, water | Low-pressure liquefied gas, refrigerated storage |
If the design pressure exceeds API 650's Appendix F ceiling, moving to API 620 — or even ASME Sec. VIII — becomes unavoidable. That call should be made at the datasheet stage, not after fabrication has started.
