API 650 vs API 653: Building a Tank vs Keeping It in Service

Short answer: API 650 governs how a welded steel atmospheric storage tank is designed, fabricated, erected, inspected and tested when it is new. API 653 takes over once the tank is in service and covers inspection, repair, alteration, relocation and reconstruction. The dividing line is commissioning. API 653 still refers back to API 650 for repair materials, welding and new components, so the two standards work together for the tank's whole life.

That split matters for owners, EPC and fabrication QA teams and terminal operators, because the two standards are written for different people, answer different questions and are referenced by different regulations. This guide explains what each standard covers, how they connect at repair and reconstruction, which NDE each calls for, and how US and Canadian rules use them. It refers to API 650 (current fourteenth edition, August 2025) and API 653 (current fifth edition, 2014, with addenda through Addendum 4 of July 2025) in general terms. Confirm details in your licensed copies before applying them to a tank.

What API 650 covers

Direct answer: API 650 sets minimum requirements for the materials, design, fabrication, erection and testing of new vertical, cylindrical, aboveground, closed- and open-top welded carbon or stainless steel storage tanks, at internal pressures approximating atmospheric, with uniformly supported bottoms, in non-refrigerated service. Its base scope runs to a maximum design temperature of 200°F (93°C); annexes extend it.

API describes API 650 as a purchase specification: it is written so that a purchaser and a manufacturer can agree on a tank of adequate safety and reasonable economy for storing petroleum and other liquids. That shapes how it reads. Much of the standard is a menu of options the purchaser selects on the data sheet, such as design method, corrosion allowance, roof type, annexes for special service, and the extent of inspection beyond the minimums. The manufacturer then designs and builds to those selections.

Main areas of API 650 include:

• Materials: permitted plate, pipe, forging and bolting specifications and toughness requirements tied to design metal temperature and thickness.
• Design: shell thickness by course for product and hydrostatic test conditions, bottom and annular plate requirements, roofs, nozzles and openings, wind and seismic loads, and annexes for things such as internal floating roofs, small tanks and stainless or elevated-temperature service.
• Fabrication and erection: tolerances for plumbness, roundness and peaking/banding, welding procedure and welder qualification by reference to ASME Section IX, and joint details.
• Inspection and testing of the new tank: radiographic examination of shell butt welds at the frequency the standard sets, examination of other welds by methods such as magnetic particle, liquid penetrant, vacuum box or tracer gas, and a hydrostatic test of the completed tank.
• Marking and certification: the nameplate and the manufacturer's certification that the tank meets the standard.

The people who use API 650 most are tank designers, fabricators and erectors, the owner's project engineers and construction QA/QC inspectors. Its working life ends, in practical terms, when the tank is accepted and commissioned. After that, API 650 remains relevant mainly as the reference API 653 points back to.

What API 653 covers

Direct answer: API 653 covers tanks that are already in service. It applies to carbon and low-alloy steel tanks built to API 650 or its predecessor API 12C, and sets minimum requirements for keeping welded or riveted, non-refrigerated, atmospheric aboveground tanks fit for continued service through inspection, repair, alteration, relocation and reconstruction.

API 653 is a maintenance and inspection standard. It answers questions API 650 never asks: how often a tank must be inspected, what inspections are routine and which need the tank out of service, how to calculate the minimum acceptable thickness of a corroded shell or bottom, how to judge settlement, and when a repair or alteration needs a hydrostatic test. It places ultimate responsibility on the owner/operator and is written for organizations that employ or have access to an authorized inspection agency, with an authorized inspector (usually an API 653-certified inspector) performing and approving the inspections the standard assigns to that role.

Main areas of API 653 include:

• Suitability for service: evaluation of the roof, shell, bottom and foundation, including minimum shell thickness for continued service, bottom plate minimum remaining thickness, and brittle fracture considerations.
• Inspection: routine in-service checks by owner personnel, periodic external inspections by an authorized inspector, ultrasonic thickness monitoring of the shell, and internal (out-of-service) inspections whose interval depends on bottom corrosion rate and bottom design, with a risk-based alternative.
• Materials, design, welding and examination for repairs and alterations: largely by reference to API 650.
• Repair, alteration and reconstruction: shell and bottom repairs, nozzle work, hot taps, roof repairs, and dismantling and re-erecting a tank at a new site.
• Hydrostatic testing and leak testing: when a full hydrostatic test is required after repair or alteration, and when an engineering evaluation may exempt it.
• Settlement evaluation: how to survey and assess uniform, planar tilt, out-of-plane, edge and bottom settlement.
• Records and reports: the construction, inspection history and repair/alteration records the owner keeps.

Side-by-side comparison

Where the two standards connect: repairs, alterations and reconstruction

Direct answer: API 653 does not repeat construction rules; it points to API 650 for them. New plates, nozzles and welds added during repair, alteration or reconstruction are generally expected to meet API 650 requirements for materials, design, welding and examination, adjusted by API 653's own rules for in-service work.

In the API 653 editions we reviewed, new repair materials are expected to meet the current edition of API 650, and exemptions from a hydrostatic test after repair require approval by an engineer experienced in tank design, again with reference to API 650 principles. Check the current API 653 edition and addenda for the exact wording. In practice this produces some common patterns:

• Shell plate replacement: replacement plate is ordered to an API 650-listed material, welded with procedures and welders qualified to ASME Section IX, and examined to the API 653 repair rules, which reference API 650 acceptance standards for radiography and other NDE.
• New or modified nozzles: reinforcement and weld details follow API 650 design, with API 653 rules on spacing from existing welds and on hot tapping.
• Bottom replacement: a new bottom is built to API 650 bottom and annular plate requirements, with weld testing such as vacuum box, and the API 653 hydrostatic test rules decide whether a full test is needed.
• Reconstruction: a tank dismantled and re-erected at a new site is evaluated under API 653 and rebuilt essentially as a new tank under API 650-based rules, with the reconstructing organization responsible for the work.
• Re-rating: changing the product specific gravity, fill height or design temperature requires re-evaluation of the shell under API 653's continued-service thickness rules, which use different allowable stresses from API 650's new-construction design rules.

That last point is a frequent source of confusion. API 650 thickness rules decide what a new tank must be built to; API 653 thickness rules decide what a corroded tank may be allowed to operate at. The numbers are not interchangeable, and using API 650 design thickness as an in-service retirement limit is overly conservative, while using API 653 limits to design a new tank is not permitted.

A worked example, described qualitatively

A terminal buys a new crude tank. The purchaser's data sheet selects API 650, a corrosion allowance on the shell and bottom, an external floating roof, and supplementary inspection beyond the minimum. During erection the fabricator's examiners radiograph shell butt welds at the required frequency, vacuum-box test bottom lap welds, and examine nozzle and shell-to-bottom welds by the methods the standard and purchaser require. The tank passes its hydrostatic test, is nameplated and enters service. From this moment API 650 has done its job.

The owner's API 653 program now takes over. Owner personnel perform routine in-service checks from the ground. An authorized inspector performs periodic external inspections and the shell is monitored by external UT thickness readings at intervals the standard sets. At the first internal inspection, examiners scan the floor with MFL, prove up indications with UT, measure the shell, inspect the annular ring and check settlement. The inspector uses the measured bottom corrosion rate to determine the next internal interval under the standard's rules.

Years later, the floor shows soil-side pitting near the shell. The owner decides on an annular ring repair and some floor plate replacement. The repair design follows API 653's repair rules, which call up API 650 materials and welding; the new welds are examined accordingly; and API 653's hydrostatic test rules decide whether a full test is needed or whether the repair qualifies for exemption with engineering approval. Both standards are active on the same job, with different roles.

NDE methods and what each standard asks them to find

Direct answer: API 650 NDE confirms that new welds were made correctly. API 653 NDE measures how the tank has changed: metal loss, cracking, settlement and coating breakdown. Some methods appear in both, but the purpose and acceptance logic differ.

• Radiography (RT): the main volumetric method for new shell butt welds under API 650; used under API 653 for repair and alteration welds where the standard requires it.
• Ultrasonic testing (UT): thickness monitoring of shell and roof in service, prove-up of floor indications found by scanning, and in some cases a substitute for radiography on repair welds where the standard allows.
• Magnetic flux leakage (MFL) floor scanning: rapid screening of tank floors for product-side and soil-side metal loss during API 653 internal inspections, with UT to size indications.
• Vacuum box and tracer gas: leak testing of bottom welds in new construction and after bottom repairs.
• Magnetic particle (MT) and liquid penetrant (PT): surface crack detection on shell-to-bottom welds, nozzle welds and repair welds, including back-gouged and root passes where required.
• Visual testing (VT): the base of both standards, from fit-up and weld profile in construction to the external and internal inspections of API 653.
• Settlement survey: elevation measurements around the shell and across the bottom, evaluated under API 653's settlement methods.

Method limits matter. MFL is a screening tool affected by coatings, plate condition and access near the shell; UT readings need clean coupling surfaces and correct calibration; radiography requires licensed crews and exclusion zones. API 653 relies on the inspector to judge whether the data collected is adequate for the bottom thickness and corrosion rate decisions.

US and Canadian regulatory overlay

PHMSA breakout tanks. 49 CFR 195.132(b) requires aboveground breakout tanks first placed in service after October 2, 2000 to be designed and constructed to an applicable API standard, with API 650 covering vertical, cylindrical, welded steel tanks at internal pressures approximating atmospheric (API 620, API 12F and API 2510 cover other tank types). 49 CFR 195.432 then requires in-service atmospheric and low-pressure steel breakout tanks to be inspected according to API 653, excluding the standard's risk-based alternative for internal inspection intervals, alongside an inspection at least once each calendar year at intervals not exceeding 15 months.

EPA SPCC. 40 CFR 112.8(c)(6) requires facilities to test or inspect each aboveground container for integrity on a regular schedule and whenever material repairs are made, with frequency, type of testing and personnel qualifications determined in accordance with industry standards. For field-erected tanks that usually means API 653; for shop-built tanks it is often STI SP001.

State rules. Some states have their own aboveground storage tank programs with inspection or registration requirements. Confirm with the state environmental or fire marshal authority.

Canada. Storage tanks are regulated provincially and through fire codes and environmental rules, and the National Fire Code of Canada and provincial adoptions reference recognized tank standards. Canadian owners commonly use API 650 for new construction and API 653 for in-service inspection, but confirm which documents your province or regulator references.

Common mistakes

• Using API 650 design thickness as the in-service retirement limit, or the reverse.
• Assuming API 653 applies to shop-built or refrigerated tanks; check scope and consider STI SP001 or other standards.
• Ordering repair materials without checking that they are listed in the current API 650.
• Skipping the hydrostatic test decision record after a repair; the exemption route under API 653 needs documented engineering approval.
• Applying API 653's risk-based internal interval option to PHMSA breakout tanks, which 49 CFR 195.432 excludes.
• Working from outdated editions or ignoring addenda; API 653 Addendum 4 (2025) revised tank-bottom thickness measurement text.

How Atlantis supports this

Atlantis NDT performs the NDE both standards call for: weld examination support during API 650 construction (VT, MT, PT, UT, vacuum box, and RT through radiography crews licensed where the work is) and API 653 in-service work, including UT thickness surveys, tank-floor MFL scanning with UT prove-up, and repair weld examination. Technicians are ASNT-certified under ASNT Level III oversight. Results go to the owner's API 653 authorized inspector, who stays inspector of record and makes the code decisions; we are not the authorized inspector or inspection agency. With 1,500+ inspection activities delivered, we respond to quote requests within 24 hours. See aboveground storage tank inspection, MFL tank floor testing and NDE for API 653 programs, or request a tank NDE quote.

Related: API 650 glossary entry, API 653 tank inspection guide, API 653 inspection intervals explained, API 653 repairs and hydrotest, STI SP001 vs API 653.

FAQ

What is the difference between API 650 and API 653?

API 650 covers design, fabrication, erection and testing of new welded atmospheric storage tanks. API 653 covers inspection, repair, alteration, relocation and reconstruction of those tanks after they enter service.

What are API 650 tanks?

Vertical, cylindrical, aboveground welded carbon or stainless steel tanks at internal pressures approximating atmospheric, with uniformly supported bottoms, in non-refrigerated service, built to API 650. Its base scope runs to 200°F (93°C), with annexes for other conditions.

What is the current edition of API 650?

The fourteenth edition, published in August 2025. Check API for addenda and errata.

Does API 650 cover tank inspection?

Only the inspection and testing of new construction: weld examination, dimensional checks and the hydrostatic test. In-service inspection is covered by API 653.

Can an API 650 tank be inspected with API 653?

Yes. API 653 was written for tanks built to API 650 and its predecessor API 12C, and it is the usual in-service standard for them.

Do repairs under API 653 have to meet API 650?

Largely. API 653 refers to API 650 for materials, design and welding of new components, with its own rules for in-service repairs, examination and hydrostatic testing.

Does a repaired tank need a hydrostatic test?

API 653 requires a full hydrostatic test after major repairs or alterations unless the work meets the standard's conditions for exemption, which require documented engineering evaluation and approval.

Who can perform API 653 inspections?

The inspections API 653 assigns to the authorized inspector are performed by a qualified authorized inspector, normally API 653-certified. NDE examiners collect data under the inspector's direction.

Which standard applies to shop-built tanks?

Many shop-fabricated tanks are built to standards such as UL 142 or API 12F and inspected under STI SP001. Check the tank's nameplate and your SPCC plan.

Planning construction QC or an out-of-service inspection? Book NDE for an API 653 inspection or ask about API 650 construction weld examination.