If you walk into any refinery, LNG terminal or chemical plant, you’ll see two kinds of large steel silhouettes on the skyline: flat-bottomed, open-top or cone-roof tanks (API 650) and the smaller, chubbier “dome” tanks that look like squat thermos flasks (API 620). Both are welded, both store liquids, both are built to American Petroleum Institute rules—but they live in totally different pressure neighbourhoods.
API 650 = atmospheric to 2.5 psi (17 kPa), bulk crude, diesel, fire-water, ethanol.
API 620 = low-pressure, ‑50 °C to +200 °C, refrigerated LPG, ethane, butane, ammonia, liquid CO₂, LNG at –162 °C, internal pressure up to 15 psi (103 kPa).
Pick the wrong code and you either over-pay for steel you don’t need (620) or create a bomb (650 at 10 psi). The rest of this article shows you exactly where the codes diverge so you can defend your choice in front of the owner, the insurer and the regulator.
Pressure & Temperature Limits
API 650, Annex F, stops dead at 2.5 psi internal and 1.0 psi external. That’s it. Go one decimal higher and you are outside the code. The roof-to-shell joint is designed to “frangibly” fail first, turning the tank into a pressure-relief device. It’s a feature, not a bug.
API 620, on the other hand, is written around pressure vessels. It starts where 650 gives up: 2.5 psi all the way to 15 psi. Anything above 15 psi falls under ASME VIII. Temperature-wise, 650 is happy between ‑40 °C and 260 °C; 620 is validated down to ‑50 °C (Annex Q) and up to 200 °C, with special rules for cryogenic nickel steels and aluminium alloys down to ‑198 °C.
Bottom line: if your product boils above ambient, you need 620. If it’s stored “dead” liquid at close to atmospheric boiling point, 650 is king.
Geometry & Bottom Design
API 650 tanks are flat-bottomed cylinders. The code assumes the bottom plate sits on a sand pad or concrete ringwall and takes no pressure-induced uplift. That’s why you see 80 m-diameter crude tanks—flat bottoms make cheap footprints.
API 620 tanks are never flat-bottomed. They use conical, hemispherical or torispherical heads top and bottom, just like a giant ASME vessel lying on its side. The bottom head resists internal pressure trying to balloon the tank upward. You’ll see 620 tanks standing on skirts or legs, not on sand. Diameter is usually < 15 m; height < 25 m. The squat shape keeps hoop stress within the 15 psi cap without needing absurd plate thickness.
Materials & Impact Rules
API 650 lets you use vanilla carbon steel—SA-36, SA-283, SA-516-70—down to ‑29 °C without impact testing. Below that, you buy a normalized, Charpy-tested plate or switch to SA-537 CL1. For most refinery tanks in the U.S. Gulf Coast, that means ordinary A516-70 is fine.
API 620 is pickier. Any steel below ‑20 °C must be impact-tested in the weld and HAZ. Annex Q gives you 9 % nickel, 5083 aluminium, or 304/316 stainless options for cryogenic service. The code also caps tensile strength: 80 ksi (550 MPa) max to avoid brittle fracture at low temperature. Translation: 620 material costs 30–70 % more per tonne before you even strike an arc.
Shell Thickness & Stress Basis
API 650 uses the “1-foot method” or the variable-design-point method, whichever gives the thinner shell. Allowable stress is 23,000 psi (158 MPa) for A516-70 in the first edition, rising to 25,000 psi if you use Annex V (higher-strength steel). Corrosion allowance is owner-specified; 1.5 mm per side is typical.
API 620 sets allowable stress at the lesser of 25 % of ultimate or 62.5 % of yield at temperature. For A516-70 at ambient that works out to 20,000 psi—lower than 650. Add the pressure term (PD/2t) and you quickly need thicker walls. A 10 psi, 12 m-diameter 620 tank often ends up with 22 mm first course versus 12 mm for the same diameter 650 tank at 2.5 psi.
Roof Types – Fixed vs Dome
API 650 gives you three choices: self-supporting cone, dome, or umbrella; or a structurally supported cone. All are thin plates (5 mm min) designed to fail first in a fire or over-pressure event.
API 620 roofs are pressure-retaining heads—2:1 ellipsoidal, torispherical, or hemispherical—calculated per ASME VIII-1. They don’t “fail first”; they hold pressure. You can still add a frangible roof-to-shell joint, but only if you want emergency venting at 1.5 times design pressure.
Seismic & Wind Approach
Both codes reference ASCE 7 for wind, but seismic is where they split. API 650 Annex E gives a simplified response-spectrum method tuned for flat-bottom tanks: check overturning moment, check shell compression, check anchor bolt uplift, done.
API 620 Annex L makes you run a full finite-element spectral analysis because the dome geometry and higher internal pressure change the whole dynamic response. You also have to prove the skirt-to-head joint can take the moment—something 650 never worries about. Expect an extra week of engineering and a thicker anchorage detail.
Inspection & Testing Gap
API 650: 10 % RT on horizontal seams, 100 % on verticals, vacuum box the floor, hydro-test for 24 h, measure settlement.
API 620: 100 % RT/UT on all Category A and B welds (shell-to-head, nozzle-to-shell), plus 100 % MT/PT on all attachment welds. Pneumatic test is allowed if water weight would overload the foundation, but only at 1.1 times design pressure with API 579 fracture-mechanics review. Translation: NDE budget doubles, test duration stretches to 2–3 weeks.
Cost & Schedule Snapshot
Rule-of-thumb numbers, U.S. Gulf Coast, Q2-2025:
- 50,000 bbl API 650 crude tank, fixed roof, $35 MM, 20 weeks EPC.
- 15,000 m³ API 620 double-wall LNG tank, $110 MM, 42 weeks EPC.
Material, NDE, and specialist cryogenic insulation drive the delta. If you don’t need the pressure or the cold, 650 is a bargain.
When to Pick 650 or 620
Pick API 650 when:
- Storage pressure ≤ 2.5 psi
- Temperature ‑40 °C to 260 °C
- Flat bottom, large diameter, bulk volume
- Product is atmospheric-boiling liquid (crude, diesel, water, ethanol)
Pick API 620 when:
- Storage pressure 2.5–15 psi
- Temperature below ‑40 °C or above 100 °C with pressure
- Product is refrigerated (LPG, LNG, ammonia, CO₂)
- You can live with a smaller, thicker, domed tank and higher cost
Hybrid edge cases:
- Butane at 4 psi and ‑7 °C → 620 mandatory
- Heavy crude with 1 psi blanket gas → 650 is fine
- Ethanol at 1.8 psi and 45 °C → 650 still cheaper, but check vapor-handling system capacity.
FAQs
Can I convert an API 650 tank to 10 psi?
No. The roof-to-shell joint, shell thickness, and anchorage are not rated for it. You would need a full re-rating to API 620 or ASME VIII—usually cheaper to build new.
Which code requires fracture mechanics?
API 620 for tanks below ‑20 °C or using high-strength steel above 60 ksi yield. API 650 only requires Charpy testing below ‑29 °C.
Is insulation mandatory in API 620?
Only if you store cryogenic liquid. The code gives you Annex Q details, but the insulation itself is specified by the owner.
Can I use stainless steel in API 650?
Yes, Annex S allows 304/316/2205 duplex. You still follow 650 thickness rules, not ASME VIII.
Which tank needs more maintenance?
API 620—double-wall cryogenic tanks need annual annular-space nitrogen checks, inner-shell fatigue monitoring, and periodic insulation vacuum tests. API 650 needs floor scans every 10 years; otherwise it’s hose it out and go.
Conclusion
Choosing between API 650 and API 620 is not a spreadsheet exercise—it’s a boundary-condition decision. Pressure, temperature, product boiling point, and project budget draw the line for you. Remember: 650 is the workhorse for atmospheric bulk storage; 620 is the thermos bottle for cold or pressurized products. Get the choice right and you save millions, pass PHA audits, and sleep soundly during hurricane season. Get it wrong and you either over-build a museum piece or under-build a future headline. When in doubt, run the vapor-pressure curve at the highest expected solar temperature—if it crosses 2.5 psi, you’ve already answered the question.
