API 650 Tank Design and Construction

API 650 — Welded Tanks for Oil Storage — is the most widely used design standard for large, field-erected, vertical, cylindrical steel tanks storing liquids at atmospheric pressure. Despite its title, API 650 governs far more than oil tanks. It’s the standard of record for crude storage, refined product terminals, chemical tanks, industrial water reservoirs, biofuel storage, and virtually any large atmospheric welded steel tank in North America.

MMI Tank designs, fabricates, and erects API 650 tanks from our 8-acre Phoenix, Arizona facility. Our in-house engineering team produces complete API 650 design packages — shell course calculations, wind and seismic analysis, nozzle reinforcement, foundation loads, and stamped drawings — before the first plate is cut.

What API 650 Covers

  • Tank type: Vertical, cylindrical, flat-bottom, welded steel
  • Pressure: Atmospheric to 2.5 psig internal (see API 620 for higher pressures)
  • Temperature: −40°F to 500°F (standard; Appendix M for elevated temperatures)
  • Materials: Carbon steel (SA-36, SA-283, SA-516), stainless steel (SA-240), duplex, and others
  • Seismic: Self-contained seismic design per Appendix E (sloshing, overturning, anchorage)
  • Wind: Appendix F or ASCE 7
  • Roofs: Cone, dome, internal floating, external floating, aluminum geodesic, open top

Key API 650 Appendices

Appendix Subject When Required
Appendix A Optional design basis (allowable stress) Alternative to one-foot method
Appendix C External floating roofs Volatile liquid storage with emission controls
Appendix E Seismic design All tanks in seismic zones (most western U.S. sites)
Appendix F Wind load design Alternative to ASCE 7 wind
Appendix H Internal floating roofs Gasoline, jet fuel, and other volatile products
Appendix M Elevated temperature service Tanks operating above 200°F
Appendix R Duplex stainless steel Aggressive corrosion environments
Appendix S Austenitic stainless steel Chemical and food-grade service

MMI’s API 650 Engineering Process

1. Design Input

We start with your storage requirements: product, capacity, operating temperature, fill/draw rates, site seismic and wind data, and any owner specifications or regulatory requirements. If you have a spec, we design to it. If you need us to develop the spec, we do that too.

2. Shell Design

Shell course thicknesses are calculated per the one-foot method or variable-design-point method (Appendix A), depending on tank diameter and owner preference. We optimize plate thicknesses to balance material cost against fabrication and erection efficiency.

3. Seismic Analysis (Appendix E)

For most western U.S. sites, seismic design is the governing load case. API 650 Appendix E requires analysis of:

  • Impulsive and convective (sloshing) hydrodynamic forces
  • Overturning moment and anchor bolt design
  • Freeboard for sloshing wave height
  • Foundation and ringwall reactions

MMI’s engineers perform the full Appendix E analysis and provide anchor bolt schedules and foundation reaction loads to your civil engineer or our in-house foundations team.

4. Roof Design

Roof type selection depends on stored product, vapor pressure, emission requirements, and owner preference. We design and build:

  • Self-supporting cone roofs (small-to-mid diameter)
  • Rafter-supported cone roofs (large diameter)
  • Aluminum geodesic dome roofs (column-free interior, lightweight)
  • Internal floating roofs (vapor emission control)
  • External floating roofs (large crude and refined product tanks)

5. Fabrication and Erection

Plates are cut, beveled, and rolled in our Phoenix shop. Field erection uses conventional bottom-up or jacking methods. All welds performed by AWS-certified welders; NDE per API 650 requirements (RT, UT, MT, PT as applicable). Full weld maps and inspection documentation provided at turnover.

Common API 650 Applications

  • Crude oil storage — production, gathering, and terminal tanks
  • Refined products — gasoline, diesel, jet fuel, heating oil
  • Produced water and frac water — upstream and midstream water management
  • Chemical storage — caustic, acid, solvent, and process chemicals
  • Industrial water — cooling, process, wash, and non-potable water
  • Biofuels — biodiesel, ethanol, feedstock
  • Mining solutions — leach solutions, process water, reagents

For potable water storage, see AWWA D100 / D103 water treatment tanks.

API 650 vs. Other Tank Standards

Standard Use API 650 When… Use the Other When…
AWWA D100 Non-potable water, chemicals, petroleum Potable water or fire protection water
API 620 Atmospheric pressure (≤ 2.5 psig) Low-pressure service up to 15 psig
ASME Sec. VIII Atmospheric flat-bottom tank Pressure vessel (any pressure above atmospheric)
API 12B Large field-erected welded construction Small bolted production tanks

Read our full comparison: API 650 vs AWWA D100: When to Use Which Standard →

In-Service Inspection: API 653

Once an API 650 tank is in service, API 653 governs inspection, repair, alteration, and reconstruction. MMI’s National Board R stamp authorizes us to perform API 653 repairs and alterations on existing tanks — floor replacement, shell repairs, nozzle modifications, relining, and seismic retrofits.

API 650 Tank FAQ

What is the maximum size for an API 650 tank?
There is no code-imposed maximum. API 650 tanks exceeding 300 feet in diameter and holding over 10 million gallons are in service worldwide. Practical limits are set by site, foundation, and economics.

Does API 650 cover pressure vessels?
No. API 650 covers atmospheric tanks (up to 2.5 psig internal). For higher pressures, see API 620 (up to 15 psig) or ASME BPVC Section VIII (any pressure). MMI holds ASME S and U stamps for pressure vessel fabrication.

What NDE is required for API 650 welds?
API 650 specifies minimum NDE requirements by joint category: vertical shell butt welds require RT or UT; horizontal shell-to-shell joints, annular plate joints, and roof-to-shell joints have specific inspection requirements. MMI performs all NDE in-house with certified Level II technicians.

Is Appendix E seismic analysis required in Arizona?
Yes. Most of Arizona falls within seismic design categories that trigger Appendix E requirements. All western U.S. states in MMI’s service territory require seismic analysis for API 650 tanks.

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