STI SP001 vs API 653: Which Standard Fits Your Tank

Short answer: Use API 653 for welded steel tanks built in the field to API 650 or its predecessor API 12C: the large, vertical, single-wall tanks at refineries and terminals. Use STI SP001 for shop-fabricated tanks such as UL 142 tanks, double-wall, rectangular and generator-base tanks, small field-erected tanks within SP001's size limits, and portable containers. Your SPCC Plan names the standard, and its inspector requirements then apply.

This comparison is for facility owners, EHS managers and Professional Engineers deciding which standard governs which tank. It also helps inspection planners who need to know what each standard asks NDE crews to do. It draws on the publishers' own descriptions (STI/SPFA and API) and on EPA's SPCC guidance. Both standards are copyrighted, so this page describes them in our own words and does not reproduce clauses or tables. Buy and read the current editions before you build a programme. API 653 has had several addenda, including Addendum 4 in July 2025, which revised the tank-bottom thickness measurement text. STI SP001's seventh edition was released in early 2024.

The one-minute decision

Direct answer: Start with how the tank was built. If it was erected on site from plates to API 650 or API 12C, API 653 is the natural fit. If it arrived on a truck, built in a shop to a UL or STI standard, SP001 is usually the fit. Size, wall type, regulator and owner policy then refine the choice.

The size, scope and category points above come from STI/SPFA's published comparison and SP001 summary. The API 653 scope description is STI's summary combined with API's own product description. Confirm both against the current documents. Industry summaries also quote gallon-based capacity limits for SP001. We have not verified those against the current edition, so we do not repeat them here.

What API 653 is built for

Direct answer: API 653 sets minimum requirements for keeping welded or riveted, non-refrigerated, atmospheric-pressure aboveground steel tanks fit for service after they were built. It covers inspection, repair, alteration, relocation and reconstruction. It is mostly applied to field-erected tanks built to API 650 or API 12C.

API 653's world is the large tank whose failure modes are bottom corrosion, shell thinning, settlement and roof damage. Its inspection structure has three layers. Routine in-service checks are made by the owner's personnel. Formal external inspections are made by an API 653 certified inspector while the tank is in service. Formal internal inspections, done out of service, assess the bottom from inside. STI's comparison summarises the API 653 schedule as monthly and annual operator checks, external inspections every five years, and a first internal inspection at ten years that can be extended to twenty years or more. The actual intervals depend on corrosion rates, release prevention barriers and other factors in the code. Read our API 653 interval explainer and external interval guide before you schedule, and confirm them in the current edition.

API 653 is heavy on engineering evaluation. It covers minimum shell thickness, bottom-plate remaining thickness, settlement evaluation and the corrosion rates that set the next internal inspection. That is why API 653 inspections usually involve a wide NDE scope: shell UT, floor MFL scanning with UT prove-up, vacuum box testing of floor welds and settlement surveys. EPA's SPCC fact sheet lists the same methods when it describes an API 653 programme. For background, see our API 653 tank inspection guide, API 650 vs API 653 and settlement measurement.

API 653 inspector certification is run by API's Individual Certification Programs. API describes it as ANSI-accredited, with a three-year certification term. The exam is delivered at Prometric test centres. Atlantis does not offer API certification training.

What STI SP001 is built for

Direct answer: STI SP001, the Standard for the Inspection of Aboveground Storage Tanks, was written for the shop-built tank designs that API 653 does not address. It covers welded carbon and stainless steel shop-fabricated tanks, small field-erected tanks and portable containers such as drums and IBCs. It inspects the whole tank system, not just the shell.

STI first published SP001 in 2000 to cover tank designs common in fuel storage, generator sets and small terminals. STI's comparison stresses three structural differences from API 653:

• System view. SP001 inspects the tank plus its spill control, piping connections and appurtenances: vents, emergency vents, overfill equipment and gauges.
• Categories. SP001 sorts tanks into three categories based on whether they have spill control and a continuous release detection method (CRDM). STI describes Category 1 tanks, which have spill control and CRDM, as potentially never needing to come out of service. Category 2 tanks have spill control but no CRDM. Category 3 tanks have neither and face the most demanding inspection requirements. The category and the tank's size drive the formal inspection schedule.
• Repairs. STI says listed shop-fabricated tanks should be repaired by the original manufacturer or in line with STI SP031, subject to the authority having jurisdiction. Field-welding repairs to a UL-listed tank under a field-erected tank code is not the SP001 route.

SP001 inspections come in layers. Periodic inspections are done by the owner's trained personnel (STI describes monthly and annual checklists). Formal external and formal internal inspections are done by a certified inspector, and leak tests are done where the standard calls for them. Industry summaries of the seventh edition report several changes: periodic inspections and leak tests may be done by a qualified party the owner designates, the grid pattern for formal internal inspections is described in more detail, MIC testing of tank-bottom water is suggested where MIC is suspected, and each formal inspection needs a written report. These are secondary reports. Confirm them in the standard itself.

Grey zones: tanks that could go either way

Direct answer: Small field-erected tanks within SP001's limits, API 650 tanks that happen to be small, and owners with a single corporate standard are the usual grey zones. Either standard can be defensible. The choice must be documented, applied consistently and carried out by an inspector qualified for that standard.

• Small field-erected tank. SP001 explicitly allows these within its size limits. Owners whose field-erected fleet otherwise runs under API 653 often keep them under API 653 for consistency. The Plan should say which applies.
• Shop-built vertical tank at a refinery. Corporate policy may push it into the API 653 programme. Check that the API 653 evaluation methods suit its construction, and that repairs respect any listing requirements.
• Double-wall tanks. STI's comparison notes these are outside the usual API 653 tank type. SP001 addresses the interstice and leak detection directly.
• Elevated and skid-mounted tanks. These are usually shop-built, so SP001 applies. The SPCC rule names both as design features that affect testing choices.
• Riveted or very old tanks. API 653 covers riveted tanks. Unknown construction history argues for a baseline inspection before choosing intervals.

Once the Plan names a standard for integrity testing, EPA treats the relevant portions as mandatory. Switching standards partway through a tank's life should be a documented Plan amendment, with the reason given.

How each standard fits the SPCC rule

Direct answer: The SPCC rule at 40 CFR 112.8(c)(6) requires integrity testing on a regular schedule, set according to industry standards. EPA names API 653 and STI SP001 as the two commonly used standards. Either satisfies the rule when it suits the container and is actually carried out. Deviations need a PE-certified environmental equivalence.

EPA's fact sheet describes API 653 as typically used for field-erected tanks, and SP001 as focused on welded metal shop-fabricated and small field-erected tanks plus portable containers. It also notes that SP001 lets owners inspect portable containers visually when secondary containment is adequate. For tanks over 5,000 gallons, most standards require more than an owner's visual inspection. A Tier I qualified facility following the self-certification template cannot deviate from the standard it adopts. A Tier II facility can deviate only with PE certification of each alternative. For the rule itself, see SPCC tank integrity testing: what 40 CFR 112 requires.

Pipeline breakout tanks are a separate matter. 49 CFR 195.432 requires operators to inspect in-service atmospheric and low-pressure steel breakout tanks according to API 653 (incorporated by reference). It also excludes API 653's risk-based alternative internal inspection interval. SP001 is not the federal reference there. See our 49 CFR 195 page.

NDE each standard calls on

Direct answer: API 653 inspections lean on quantitative NDE: shell UT, floor MFL with UT prove-up, vacuum box testing and settlement surveys. SP001 inspections lean more on visual inspection, leak detection checks and UT where the category and size call for it. Both use MT and PT around repairs and suspect welds.

Whichever standard applies, comparison-ready records are what make the next inspection cheaper. They mean the same grid, the same CML or plate identifiers and the same reporting format each time.

Common mistakes

Direct answer: The common mistakes are applying API 653 evaluation rules to a listed shop tank, applying SP001 to a large field-erected tank, using an inspector certified for the other standard, and never writing the choice into the SPCC Plan.

• Wrong inspector. An API 653 certification alone does not make someone an SP001 inspector. STI's route for API 653 inspectors is its adjunct certification.
• Field-welding a listed tank. A repair that ignores SP031 or the manufacturer can affect the tank's listing. Check with the authority having jurisdiction.
• No category assignment. SP001 schedules depend on the category, so an unassigned tank has no defensible schedule.
• Treating STI's API 653 interval summary as the code. Use the current API 653 text and its addenda for actual interval rules.
• Silent Plans. If the SPCC Plan does not say which standard applies to which tank, an inspector will ask why.

Canada and state programmes

Direct answer: In Canada, aboveground petroleum tanks fall under federal storage-tank regulations on federal lands and under provincial and territorial rules elsewhere. Many of these reference CCME guidance and recognise industry standards. In the US, some state tank programmes name API 653 or SP001 directly.

Before you assume either standard is "the" requirement, check the state or provincial rules for each site, and the fire code adopted locally for shop-built tanks. Owners running US and Canadian sites often keep one standard per tank type across the fleet and add jurisdiction-specific items where needed.

How Atlantis supports this

Atlantis NDT performs the NDE either standard calls for. That includes shell UT thickness surveys, floor MFL scanning with UT prove-up, vacuum box testing, MT and PT at welds and repairs, corrosion mapping, and visual examination of foundations and appurtenances, by ASNT-certified technicians under ASNT Level III oversight. Our readings are recorded on fixed grids so they serve as comparison records. The formal inspection is signed by your API 653 certified inspector or your STI-certified SP001 inspector, and your SPCC Plan and PE set scope and frequency. Atlantis does neither. See aboveground storage tank inspection and MFL floor scanning, or request a quote within 24 hours.

Frequently asked questions

What is the difference between STI SP001 and API 653?

SP001 covers shop-fabricated tanks, small field-erected tanks and portable containers, and inspects the whole tank system. API 653 covers field-erected tanks built to API 650 or API 12C and focuses on the tank's structural integrity, with detailed engineering evaluation.

Does SP001 or API 653 apply to my tank?

Start with construction. Shop-built usually means SP001, and field-erected to API 650 usually means API 653. Size limits, double-wall design, regulator and owner policy refine the choice. Document it in the SPCC Plan.

Which standard applies to shop-built tanks?

STI SP001 is the standard written for shop-fabricated tanks. Repairs to listed shop tanks go through the manufacturer or STI SP031.

Can an API 653 inspector inspect an SP001 tank?

For formal SP001 inspections, STI recognises its own certified inspectors and API 653 inspectors who hold its SP001 adjunct certification. API 653 certification alone is not the SP001 credential.

What are the API 653 vs SP001 inspection intervals?

SP001 intervals depend on tank category and size. API 653 intervals depend on corrosion rates, bottom protection and code limits. STI summarises API 653 as five-year externals and first internals at ten years, extendable. Confirm both in the current editions.

Does API 653 apply to double-wall tanks?

Double-wall tanks are generally shop-built, and STI notes they are outside the usual API 653 tank type. SP001 addresses them directly, including the interstitial space.

Which standard does EPA require for SPCC?

Neither by name. The SPCC rule requires integrity testing per industry standards, and EPA names API 653 and SP001 as the commonly used ones. A PE-certified equivalent programme is also possible.

Which standard applies to pipeline breakout tanks?

API 653, which 49 CFR 195.432 incorporates by reference for in-service atmospheric and low-pressure steel breakout tanks.

What edition of SP001 is current?

Industry sources report the seventh edition, released in early 2024. Check STI/SPFA for the current edition before you cite it.

Who repairs a UL 142 tank?

STI says the original manufacturer, or a repair in line with STI SP031, subject to approval by the authority having jurisdiction.

Not sure which standard your fleet is under? Send your tank list for NDE scoping, or ask for an API 653 floor and shell NDE quote.