If you’re specifying a new storage tank and keep running into these two standards, here’s the short version: API 650 vs API 620 mostly comes down to pressure and temperature. API 650 covers standard, atmospheric storage tanks. API 620 covers larger tanks built for higher pressure or extreme cold.
Both standards are published by the American Petroleum Institute (API), and both apply to welded steel storage tanks. But they were written for very different jobs. Picking the wrong one can mean a tank that fails inspection, doesn’t meet your product’s requirements, or costs far more than it needs to.
This guide breaks down what each standard actually covers, how to tell which one your project needs, and what to expect around cost, process, and long-term upkeep.
API 650 vs API 620
- API 650 covers tanks operating at or near atmospheric pressure — up to about 2.5 psig (pounds per square inch gauge) — across a temperature range of -40°F to 500°F. It’s the standard used for most fuel, oil, water, and chemical storage tanks.
- API 620 covers larger tanks designed for higher internal pressure — up to about 15 psig — and much colder service, down to -325°F. It’s the standard behind most cryogenic and LNG storage tanks.
- If your product sits at normal atmospheric pressure and a moderate temperature, you almost certainly need API 650.
- If your product requires refrigeration, pressurization, or extreme cold — think LNG, LPG, or ammonia — you’re in API 620 territory.
What Is API 650?
API 650, titled Welded Tanks for Oil Storage, sets the minimum requirements for the material, design, fabrication, erection, and inspection of vertical, cylindrical, aboveground welded storage tanks. According to the American Petroleum Institute, the standard exists to support the safety, sustainability, and environmental performance of oil storage tanks.
These tanks are built to hold contents at pressures approximating atmospheric — generally not exceeding the weight of the roof plates — though limited exceptions apply when extra design requirements are met.
API 650 tanks are typically built from carbon steel, stainless steel, or aluminum, and they can operate across a wide temperature band, from well below freezing up to 500°F. There’s no fixed size limit, so these tanks range from small shop-built units to very large field-erected tanks.
Most of the tanks used at fuel terminals, bulk plants, and industrial sites — including the API tanks Tex Tanks fabricates — are built to this standard. If you want a broader rundown of how API-rated tanks compare to other tank categories, our guide to API tank types and applications covers that in more depth.
When API 650 Is the Right Fit
- Bulk fuel and crude oil storage
- Potable or process water storage
- Biofuel and ethanol storage
- General chemical storage at atmospheric pressure
- Refinery, terminal, and pipeline facility tanks
What Is API 620?
API 620, Design and Construction of Large, Welded, Low-Pressure Storage Tanks, was originally published in 1956 and has been revised through multiple editions since, according to Inspectioneering. It picks up where API 650 leaves off, covering tanks designed for higher internal pressures and lower operating temperatures.
API 620 tanks are rated for pressures up to roughly 15 psig and can be engineered for temperatures as low as -325°F, which is what makes them suitable for cryogenic liquids like LNG, LPG, and liquid nitrogen.
Because of the demands of low-temperature and higher-pressure service, API 620 tanks are typically built from carbon steel, low-temperature nickel alloys, or austenitic stainless steel, and many use double-wall or insulated construction for cryogenic products. These tanks are almost always field-erected due to their size.
This isn’t a niche or optional standard — federal pipeline safety regulators formally incorporate API 620 by reference into U.S. regulations, which underscores how central it is to safe large-scale storage.
When API 620 Is the Right Fit
- LNG and LPG storage
- Ammonia and other refrigerated gases
- Cryogenic liquid storage
- Large-volume tanks requiring pressurized or insulated, double-wall construction
Key Differences Between API 650 and API 620
| Factor | API 650 | API 620 |
|---|---|---|
| Full title | Welded Tanks for Oil Storage | Design and Construction of Large, Welded, Low-Pressure Storage Tanks |
| Max internal pressure | ~2.5 psig | Up to ~15 psig |
| Temperature range | -40°F to 500°F | -325°F to 250°F |
| Common materials | Carbon steel, stainless steel, aluminum | Carbon steel, low-temperature nickel alloys, stainless steel |
| Typical products stored | Crude oil, refined fuel, water, chemicals, biofuel | LNG, LPG, ammonia, cryogenic liquids |
| Construction method | Shop-built or field-erected | Almost always field-erected |
| Insulation | Not typically required | Common for cryogenic service |
Which Standard Do You Need?
A tank engineer will run the actual numbers before finalizing a design, but a few questions will point you in the right direction early on:
- What are you storing? Fuel, oil, water, or general chemicals usually point to API 650. Cryogenic gases point to API 620.
- What pressure will the vapor space reach? Anything above roughly 2.5 psig moves you toward API 620.
- How cold does the product get in storage? Standard ambient service stays within API 650. Sub-zero or cryogenic service needs API 620.
- How large is the tank, and will it be shop-built or field-erected? Larger, field-erected tanks are more likely to fall under API 620.
If you’re unsure where your project lands, it’s worth reviewing your requirements with a fabricator that works in both tank categories before committing to a design.
Cost Considerations
Costs vary more by project specifics than by which standard applies, but a few factors consistently move the price:
- Tank size and storage capacity
- Material selection (carbon steel vs. low-temperature alloys vs. stainless)
- Required wall thickness for the design pressure
- Shop fabrication vs. field erection
- Insulation and double-wall requirements for cryogenic service
- Welding qualification, non-destructive testing, and inspection requirements
- Site preparation, foundation work, and installation logistics
As a general rule, API 620 tanks cost more per unit of capacity than API 650 tanks, since they call for more engineering, specialized materials, and additional testing. The only reliable way to get an accurate number is a project-specific quote based on your actual product, capacity, and site conditions.
Design, Fabrication, and Installation Process
Both standards follow a similar general sequence, even though the engineering details differ:
- Needs assessment — define the product, required pressure, temperature range, and capacity.
- Engineering design — the tank is designed to the appropriate standard based on those requirements.
- Material procurement — steel or alloy selection is finalized based on the design conditions.
- Fabrication — smaller API 650 tanks are often shop-built; larger tanks and most API 620 tanks are field-erected.
- Welding qualification and testing — welders and procedures are qualified, and welds undergo non-destructive examination.
- Hydrostatic or pneumatic testing — the completed tank is tested to confirm it holds pressure safely.
- Certification and documentation — material certs, inspection reports, and test results are compiled to document compliance.
- Installation and commissioning — the tank is set, connected, and brought into service.
Timelines depend heavily on tank size, material availability, and whether the tank is shop-built or field-erected, so it’s worth discussing scheduling directly with your fabricator early in the process.
Inspection and Ongoing Maintenance
Getting the design standard right is only the first step — tanks built to either API 650 or API 620 need ongoing inspection once they’re in service. That’s typically governed by a separate standard, API 653, which covers inspection, repair, alteration, and reconstruction of tanks already in operation.
A basic maintenance program usually includes:
- Routine external visual inspections
- Scheduled internal inspections
- Corrosion monitoring on the shell, bottom, and roof
- Cathodic protection testing where applicable
- Coating and lining upkeep
Risks of Choosing the Wrong Standard
- Under-designing a tank for its actual operating pressure or temperature can lead to structural failure, safety hazards, and failed inspections.
- Over-designing a tank — building to API 620 when API 650 would have been sufficient — adds unnecessary cost, material, and lead time.
- Permitting issues can arise if a tank isn’t built to the standard your local fire marshal, environmental agency, or insurer expects for that product and service.
Frequently Asked Questions
Can a single tank be built to both API 650 and API 620? No. A tank is engineered and certified to one standard based on its specific pressure and temperature service. A facility’s overall tank farm, however, can include a mix of both — API 650 tanks for atmospheric products and API 620 tanks for pressurized or cryogenic ones.
Is API 620 always more expensive than API 650? Generally, yes, because of the added engineering, specialized materials, and testing involved. Actual cost still depends on size, materials, and site conditions, so it’s best to compare project-specific quotes rather than assume.
Which standard applies to ethanol or biofuel tanks? Most biofuel and ethanol storage operates at atmospheric pressure and ambient temperature, so API 650 usually applies unless the specific product requires refrigeration or pressurization.
Do I need other permits or codes beyond API 650 or API 620? Yes. These API standards cover tank design and construction, but most projects also need to meet local fire codes and environmental regulations, and in-service tanks are typically maintained under API 653.
Talk to a Tank Specialist Before You Finalize a Design
The right standard depends on what you’re storing, at what pressure, and at what temperature — so it’s worth confirming your specs with an engineer or fabricator before locking in a design.
Tex Tanks works with clients across a range of industries to design and source tanks built to the correct standard from day one. Browse the full lineup of storage tank systems, or request a quote for your next project.