If you’re specifying a welded steel storage tank, the first engineering decision is which code governs the design: API 650 or AWWA D100. Both produce safe, durable tanks. They solve different problems for different industries — and choosing the wrong one can mean over-engineering, under-engineering, or a rejected permit.
This guide breaks down where each standard applies, what it requires, and how to pick the right one for your project.
What Is API 650?
API 650 — Welded Tanks for Oil Storage is published by the American Petroleum Institute. Despite the name, it’s used far beyond oil storage. API 650 is the dominant design standard for large, field-erected, vertical, cylindrical steel tanks storing liquids at atmospheric pressure — including petroleum products, chemicals, water (non-potable), biofuels, and industrial process liquids.
Key characteristics:
- Atmospheric pressure (internal pressure ≤ 2.5 psig; see API 620 for higher pressures)
- Welded steel construction
- Design temperatures from −40°F to 500°F (standard; higher with Appendix M)
- Comprehensive seismic design per Appendix E
- Wind load design per Appendix F or ASCE 7
- Optional annexes for floating roofs, aluminum domes, stainless steel, duplex steel, and elevated temperatures
API 650 is an industry standard, not a government regulation — but most jurisdictions adopt it by reference for petroleum and chemical storage. Many owners also specify it for non-regulated industrial water tanks when potable water certification isn’t required.
What Is AWWA D100?
AWWA D100 — Welded Carbon Steel Tanks for Water Storage is published by the American Water Works Association. It covers the design, fabrication, and erection of welded steel tanks used in water supply systems — standpipes, ground-level reservoirs, and elevated tanks.
Key characteristics:
- Atmospheric storage of water (potable, treated, raw, fire protection)
- Welded steel construction
- Seismic design per ASCE 7 (referenced, not self-contained like API 650 Appendix E)
- Hydrodynamic analysis for sloshing loads
- Specific requirements for roof types, overflow, venting, and access
- Interior coating must meet NSF/ANSI 61 for potable service
- Tank painting and cathodic protection requirements
AWWA D100 is adopted by reference in most state plumbing and building codes for public water system storage. If the tank feeds a municipal or regulated community water system, AWWA D100 is almost always the required standard.
API 650 vs AWWA D100: Side-by-Side
| Requirement | API 650 | AWWA D100 |
|---|---|---|
| Primary application | Petroleum, chemicals, industrial liquids | Potable water, fire protection water |
| Governing body | American Petroleum Institute | American Water Works Association |
| Max design pressure | 2.5 psig (atmospheric) | Atmospheric |
| Seismic design | Self-contained (Appendix E) | References ASCE 7 |
| Wind design | Appendix F or ASCE 7 | ASCE 7 |
| Material specs | SA-36, SA-283, SA-516, SA-240 (stainless), others | SA-36, SA-283, SA-516 primarily |
| Coating requirements | Owner-specified | NSF/ANSI 61 required for potable service |
| Inspection standard | API 653 (in-service) | AWWA D100 + utility SOPs |
| Floating roof provisions | Yes (Appendix C, H) | No |
| Elevated tank provisions | No (see API 620 / AWWA D100) | Yes |
| Common capacity range | 10,000 gal – 10M+ gal | 50,000 gal – 5M gal |
When to Use API 650
Choose API 650 when the stored product is:
- Crude oil, refined petroleum, biofuels, or chemicals
- Non-potable industrial water (process, cooling, wash)
- Any liquid where a floating roof or vapor containment is required
- High-temperature service (above ambient)
Also choose API 650 when the jurisdictional authority or the owner’s engineering specification calls for it — which is common in refinery, terminal, pipeline, and petrochemical work.
When to Use AWWA D100
Choose AWWA D100 when the tank is part of a:
- Municipal or community potable water system
- Fire protection water supply (often paired with NFPA 22 requirements)
- State-regulated public water utility
If the local jurisdiction or the state drinking water agency requires AWWA D100 certification, there’s no engineering judgment call — the standard is mandatory.
Where It Gets Complicated
Non-potable water in an industrial facility
API 650 and AWWA D100 can both technically govern a non-potable water tank at an industrial plant. Many owners default to API 650 for consistency with the rest of the facility. But if the tank will ever be reclassified for potable service, building to AWWA D100 from the start avoids an expensive retrofit of coatings and accessories.
Fire protection water
NFPA 22 governs fire protection water tanks specifically. NFPA 22 allows both AWWA D100 and API 650 as the underlying design standard, but adds its own requirements for capacity, heating, seismic restraint, and connections. MMI builds NFPA 22-compliant tanks to either base standard.
Dual-service or hybrid tanks
Some tanks store potable water and provide fire protection simultaneously. These are engineered to AWWA D100 for the potable requirements and NFPA 22 for the fire protection requirements — the more stringent provision governs each design element.
Seismic design
API 650 Appendix E has a self-contained seismic procedure specific to flat-bottom tanks. AWWA D100 references ASCE 7 and includes its own hydrodynamic sloshing analysis. In high-seismic zones (California, Nevada, Pacific Northwest), the seismic design can drive shell thickness, anchorage, and foundation cost. Getting this right at the engineering stage saves money — over-designing for the wrong code is just as wasteful as under-designing.
How MMI Tank Handles Code Selection
Because we build to both API 650 and AWWA D100 — and hold ASME S, U, and National Board R stamps — MMI’s engineering team selects the governing code based on:
- Stored product — petroleum vs. potable water vs. industrial liquid
- Jurisdictional requirements — state and local code adoptions
- Owner specifications — facility engineering standards
- Seismic and wind loading — site-specific conditions per ASCE 7
- Future service — will the tank’s use change over its lifespan?
We present the recommendation with a clear rationale so owners and engineers can sign off with confidence — not guesswork.
FAQ
Can a single tank be designed to both API 650 and AWWA D100?
Not formally — each standard has its own design methodology. However, an engineer can design a tank to the more conservative requirements of both, which is sometimes done for dual-service tanks. The tank is still officially built to one governing standard.
Which standard is more conservative?
It depends on the design element. AWWA D100’s seismic and sloshing provisions are often more demanding for water storage. API 650’s material and welding provisions may be more stringent for certain chemical services. Neither is universally “tougher.”
Does MMI build to API 620 as well?
API 620 covers low-pressure storage (up to 15 psig) — refrigerated tanks, cryogenic vessels, and gas holders. Contact our engineering team to discuss API 620 applications.
What inspection standard applies after the tank is in service?
API 650 tanks are inspected per API 653 (Tank Inspection, Repair, Alteration, and Reconstruction). AWWA D100 tanks follow the utility’s inspection program, often referencing AWWA standards and the authority having jurisdiction.
Next Steps
Whether your project calls for API 650, AWWA D100, or you’re not sure yet — contact MMI Tank or call (602) 272-6000. Our engineers will review your application, confirm the governing code, and provide a budgetary estimate so you can move forward with the right tank built to the right standard.
