API 650 vs API 620 Storage Tanks: How to Choose the Right Standard in 2026
API 650 vs API 620 Storage Tanks: Which Standard Do You Actually Need?
If you're specifying a new bulk liquid storage tank or replacing an aging vessel, the first question your engineering team needs to answer isn't about diameter or capacity. It's about pressure. Specifically: does your stored product's vapour pressure, at maximum expected operating temperature, stay at or below 2.5 psi (17 kPa)?
If yes, you're working with an API 650 application. That standard governs welded steel tanks designed for atmospheric storage, and it covers the vast majority of bulk liquid storage installations in Canada and globally. If your operating conditions push internal pressure above that threshold, up to a maximum of 15 psi (103 kPa), you're in API 620 territory. That standard covers low-pressure large storage tanks built for applications where vapour pressure or process conditions exceed what atmospheric design can safely contain.
The distinction matters beyond engineering paperwork. Choosing the wrong standard isn't just a specification error. It can void your facility's insurance coverage, trigger a provincial regulatory shutdown, and create genuine catastrophic risk in sectors like oil and gas or chemical processing. These aren't theoretical outcomes. They're the documented consequences of misapplied tank standards in operating facilities.
The single fastest screening method: identify your stored product's vapour pressure and its operating temperature at maximum expected site conditions, then compare those values against each standard's design envelope. That one calculation typically resolves the question before you've written a single line of your RFQ.
There's an added dimension for Canadian projects that US-sourced procurement guidance tends to skip. Both API standards must be read alongside provincial pressure vessel regulations and CSA requirements. Alberta, Ontario, British Columbia, and other provinces each have their own registration thresholds, inspection requirements, and safety authority oversight. Understanding that regulatory layer is not optional. You can review the regulatory framework governing Canadian tank installations to get oriented before engaging a fabricator.
The 6 Technical Criteria That Determine API 650 vs API 620
Selecting the right API standard for your storage tank project depends on six technical factors. Each one is worth evaluating independently, because any single criterion can override a decision that looks correct on the others.
1. Design Pressure
This is the primary differentiator, and it's binary in practice. API 650 tanks operate at or near atmospheric pressure, with internal design pressure not exceeding 2.5 psi (17 kPa). API 620 tanks are engineered for sustained internal pressures up to 15 psi (103 kPa). The engineering consequence of this gap is significant: API 620 requires heavier shell-course calculations, different weld joint efficiencies, and more complex pressure analysis than API 650. If your product's vapour pressure at maximum ambient temperature exceeds 2.5 psi, API 650 is not an option regardless of any other project constraint.
2. Design Temperature
API 650 is generally specified for stored liquids operating between -40°C and 93°C. That range covers the overwhelming majority of hydrocarbon, chemical, and process liquid storage in Canada. API 620 opens the door to cryogenic service, permitting design temperatures as low as -168°C. That range is specifically intended for LNG storage, refrigerated ammonia, and similar low-temperature applications. If your project involves any cryogenic product, API 620 with appropriate low-temperature steel specification is the only valid path.
3. Material Specifications
The two standards diverge sharply on acceptable steel grades. API 650 commonly uses ASTM A36 and A283 carbon steels for shell courses and annular plates, which are widely available and cost-effective for atmospheric service. API 620 mandates higher-strength, low-temperature steels such as ASTM A537. For cryogenic designs, the material specification escalates further to 9% nickel steel, which is substantially more expensive and requires specialised welding procedures. This material gap is a major driver of the cost premium between the two standards.
4. Tank Geometry and Capacity
API 620 tanks are typically large-diameter purpose-engineered vessels, often exceeding 60 metres in diameter, where internal pressure and geometric scale interact in ways that atmospheric tank design doesn't address. API 650 spans a far wider practical range, from small field tanks used at remote mine sites to major crude oil terminal tanks exceeding 100,000 barrels of capacity. For most Canadian industrial installations, API 650's geometry flexibility makes it the default unless pressure or temperature requirements force the issue.
5. Hydrostatic Testing Protocols
Inspection and testing requirements are more rigorous under API 620 because the tank is in pressure service. Inspectors verify shell integrity under design pressure conditions, not merely the water-fill acceptance test commonly applied under API 650. This affects both commissioning timelines and the qualifications required of the inspection authority. For Canadian projects, this testing regime also intersects with provincial registration requirements, as discussed below.
6. Industry and Regulatory Application
In practice, application mapping drives the initial standard selection before detailed engineering begins. API 650 dominates crude oil terminals, produced water storage, chemical feed tanks, food-grade liquid storage, and mine site fuel tanks. These are the sectors where our oil and gas tank solutions and chemical storage systems are most commonly deployed. API 620 concentrates in LNG terminals, refrigerated product storage, and high-vapour-pressure chemical facilities.
For Canadian projects specifically, provincial regulations introduce a sixth layer on top of the API standard itself. In Alberta, ABSA (Alberta Boilers Safety Association) requires registration of tanks that qualify as pressure vessels under provincial legislation. In Ontario, the equivalent authority is TSSA (Technical Standards and Safety Authority). British Columbia operates through BCSA. An API 620 tank almost always triggers provincial pressure vessel registration. Some larger API 650 tanks may also require registration depending on capacity thresholds. Understanding where each standard's jurisdiction begins and ends is essential before you finalise your procurement scope. Our IBC standards and compliance guide provides context on how Canadian regulatory layers interact with design standards, and our team can also support custom process equipment configurations that must satisfy multiple concurrent requirements.
One clarification worth making: CSA B625 governs portable tanks in Canada but doesn't replace API 650 or API 620 for fixed bulk storage. The two regulatory domains are separate, and conflating them creates compliance gaps that surface at the worst possible time, during provincial inspection.
5 Procurement Mistakes That Lead to the Wrong API Standard - and How to Avoid Them
Getting the standard wrong costs money, time, and sometimes your operating licence. Here are the five errors we see most often in procurement cycles, and how to avoid each one.
1. Relying on product name instead of vapour pressure calculation. Specifying API 650 because "it's just diesel" or "it's just water" without calculating actual vapour pressure at maximum ambient temperature is the single most consequential error in this process. A product that behaves atmospherically at 15°C may generate meaningful vapour pressure at 45°C on a hot summer day at a Canadian Prairie site. The calculation takes one engineering hour. Skipping it can result in a non-compliant installation that triggers regulatory shutdown or forces a full tank replacement. Our hazardous material container selection guide walks through a similar methodology for product-first evaluation.
2. Over-specifying API 620 to appear conservative. Requesting API 620 for a standard hydrocarbon tank because it seems like the safer option adds 30% to over 60% to fabrication cost and lead time, with no corresponding safety or regulatory benefit. That capital could fund secondary containment, level instrumentation, or foundation upgrades that actually reduce operational risk. Over-specification is a budget problem, not a safety solution.
3. Ignoring Canadian provincial pressure vessel registration. A facility that sources a US-fabricated API 620 tank without ABSA or TSSA registration faces immediate shutdown risk during the first provincial inspection. This isn't an administrative inconvenience. It's an operating liability. Registration requirements must be confirmed before the fabrication contract is placed, not after the tank arrives on site.
4. Assuming any fabricator can build both standards equally. Checking that a manufacturer holds CWB (Canadian Welding Bureau) certification covering the required weld procedures and material grades for your specific standard is a non-negotiable due diligence step. Many smaller fabricators hold API quality programs for only one standard. A fabricator certified only for API 650 cannot legally stamp an API 620 tank. This gap surfaces during third-party inspection and can halt your project at the worst possible moment.
5. Optimising for capital cost and ignoring lifecycle cost. A tank decision based purely on lowest fabrication price routinely produces the highest total cost of ownership over a 20- to 30-year asset life. Inspection intervals, shell-course replacement, recertification requirements, and unplanned downtime all factor into the real number. Our analysis of long-term container durability and total cost demonstrates how this pattern plays out across asset classes. The principle applies equally to API-standard fixed storage tanks. Pairing your tank selection with a maintenance and recertification services plan from day one changes the cost picture substantially.
API 650 vs API 620 Selection Checklist: 10 Questions to Ask Before You Specify
Before your engineering team writes a single line of tank specification, work through these ten questions. They're structured to surface the critical variables that determine which standard applies, and to flag the due-diligence gaps that cause procurement failures downstream.
1. What is the maximum internal operating pressure at peak ambient temperature? If it exceeds 2.5 psi (17 kPa), API 620 is required. At or below that threshold, API 650 applies. This is your primary filter.
2. What is the minimum design temperature? If cryogenic service below -40°C is required, API 620 with low-temperature steel specification is mandatory. API 650 doesn't cover that range.
3. What product is being stored? Volatile hydrocarbons, refrigerated ammonia, and LNG point toward API 620. Crude oil, diesel, produced water, ambient-condition chemicals, and food-grade liquids typically fall under API 650. Our steel vs. poly storage comparison guide covers material selection considerations that overlap with this product classification step.
4. What is the required tank capacity and diameter? API 620 is generally applied to very large tanks where internal pressure and geometric scale interact. API 650 covers the widest practical range of field and terminal tank sizes.
5. In which Canadian province is the tank being installed? Alberta (ABSA), Ontario (TSSA), British Columbia (BCSA), and other provinces each have registration and inspection requirements that overlay the API standard. These requirements affect manufacturer qualification and commissioning timelines.
6. Does your fabricator hold a current, audited API quality program for the applicable standard? Verify CWB certification scope and documented API quality program coverage before contract award. Don't accept a general fabrication capability statement as a substitute for a verified quality program specific to API 650 or API 620.
7. What is the planned service life and inspection interval? Longer asset lives favour higher-specification fabrication and documented quality control that supports future re-rating or inspection compliance.
8. Are there secondary containment, foundation, or seismic zone requirements? Site-specific conditions can interact with shell-course design, annular plate specifications, and foundation loading under the chosen standard. Surface these early to avoid change orders mid-fabrication.
9. Is the stored product classified as hazardous under TDG regulations? Transportation of Dangerous Goods classification can impose additional design or marking requirements beyond what the API standard itself mandates. Confirm this with your regulatory team before issuing specifications. Our IBC totes vs. drums comparison illustrates how TDG classification affects container selection across multiple container types.
10. Does your manufacturer have reference-backed experience building to the specific standard required? Generic fabrication capability isn't the same as documented API tank experience with verifiable inspection records and on-time delivery from a Canadian facility. The Hawman team has over 40 years of in-house Canadian manufacturing expertise and can provide engineering support from initial standard selection through fabrication and commissioning.
API 650 vs API 620: Frequently Asked Questions
Q: What is the main difference between API 650 and API 620 storage tanks?
API 650 covers welded steel tanks for atmospheric storage, with a maximum internal pressure of 2.5 psi (17 kPa). API 620 covers low-pressure large storage tanks rated up to 15 psi (103 kPa) internal design pressure. API 620 also permits cryogenic design temperatures as low as -168°C, while API 650 is generally limited to -40°C minimum. The stored product's vapour pressure and operating temperature are the primary factors that determine which standard is required.
Q: Can the same fabricator build tanks to both API 650 and API 620?
Yes, but only if the fabricator maintains separate, audited quality programs for each standard and holds certified welding procedures covering the required material grades for both. API 620 demands higher-strength, low-temperature steels and more rigorous inspection hold points than API 650. Before awarding a contract, always verify the fabricator's current API quality program scope. A fabricator certified only for API 650 cannot legally stamp an API 620 tank. Canadian manufacturers who build to both standards must maintain distinct documented quality systems for each, which is why verifying scope before contract award is a non-negotiable step.
Q: Do Canadian provinces require additional registrations beyond API 650 or API 620 compliance?
Yes. Provincial safety authorities, including ABSA in Alberta, TSSA in Ontario, and BCSA in British Columbia, require tanks that meet the definition of a pressure vessel under provincial boiler and pressure vessel acts to be registered, inspected, and stamped before commissioning. An API 620 tank almost always triggers this requirement. Some larger API 650 tanks may also require registration depending on capacity thresholds set by each province. Canadian procurement teams must confirm provincial requirements alongside API standard compliance. You can review the applicable Canadian regulatory requirements to understand how these layers interact before your project begins.
Q: Is API 650 or API 620 used for crude oil storage?
Crude oil storage tanks are almost universally built to API 650 because crude oil at ambient temperatures operates at or near atmospheric pressure, well within the API 650 design envelope. API 620 would only apply to crude oil or hydrocarbon storage if the product required pressurised containment above 2.5 psi, for example in high-vapour-pressure condensate service or refrigerated storage scenarios. Our custom API 650 storage tanks are engineered for exactly these standard atmospheric crude oil and hydrocarbon applications.
Q: How much more does an API 620 tank cost compared to an API 650 tank?
API 620 tanks typically cost 30% to over 100% more than equivalent API 650 tanks, depending on design pressure, minimum design temperature, required steel grade, and tank diameter. The cost premium comes from higher-grade steels such as ASTM A537 or 9% nickel for cryogenic designs, more complex engineering calculations, additional weld inspection requirements, and lengthier material procurement lead times. Specifying API 620 when API 650 is sufficient adds significant cost with no safety or regulatory benefit.
Q: What does 'shell course' mean in API tank design?
A shell course is one of the cylindrical plate rings that together form the vertical wall of a welded storage tank. Tanks are built in multiple courses stacked vertically, with each course requiring dimensional verification and inspection of its horizontal and vertical weld seams. As tanks increase in height, lower courses are designed with greater wall thickness to handle the higher hydrostatic pressure from the stored liquid above. API 650 and API 620 each specify minimum thickness and inspection requirements for every shell course.
Q: What are annular plates in a storage tank?
Annular plates are the ring-shaped steel floor plates that form the outermost portion of a tank's bottom, directly beneath and connected to the shell courses. They're thicker than the interior floor plates and are critical for the structural integrity of the shell-to-floor junction. Under API 650, annular plate dimensions, weld requirements, and inspection standards are explicitly specified. Proper annular plate design also supports leak detection and accommodates differential settlement under the tank foundation.
Ready to Specify the Right Tank for Your Project?
Whether your application clearly falls under API 650 or sits in the pressure and temperature range where the two standards need careful evaluation, getting the specification right from the start saves time, money, and regulatory headaches. Our engineering team at Hawman Container Services has over 40 years of in-house Canadian manufacturing expertise across both atmospheric and custom-engineered tank solutions.
If you're at the RFQ stage or still working through your design requirements, talk to our team. We'll help you confirm which standard applies, identify any provincial registration requirements for your site, and engineer a tank that meets your process needs and your compliance obligations, from a Canadian facility with no outsourcing.
Request a quote or engineering consultation and let's get your project specification right the first time.