API650 vs API653: Which Standard Applies to Your Tank?

If you operate or specify bulk storage tanks, two American Petroleum Institute standards govern almost every decision you'll make about them. API650 governs new tank construction. API653 governs in-service inspection, repair, and reconstruction. The transition between them happens at the moment a tank is placed into service.

What is API650?

API650 (full title "Welded Tanks for Oil Storage") is the American Petroleum Institute standard for the design, fabrication, erection, and testing of welded carbon-steel above-ground atmospheric storage tanks. It is the construction code for new tanks.

The standard covers:

• Design. Shell thickness calculations, bottom design, roof design (fixed cone, fixed dome, geodesic dome, internal floating roof, external floating roof), wind and seismic loading, foundation requirements.
• Materials. Acceptable plate materials, connection materials, welding consumables, with explicit requirements for low-temperature service.
• Fabrication. Plate cutting, forming, edge preparation, dimensional tolerances.
• Erection. Bottom plate welding sequences, shell course erection methods, roundness and plumbness tolerances.
• Testing. NDT requirements (visual, magnetic particle, dye penetrant, radiography), hydrotest pressures, settlement monitoring.

If you are commissioning a new tank, the design and construction documentation must demonstrate API650 compliance, or compliance with a recognised national equivalent that achieves the same intent. In South Africa, SANS 10131 references API650 for the technical body of work.

What is API653?

API653 (full title "Tank Inspection, Repair, Alteration, and Reconstruction") is the standard governing what happens to a tank after it has been placed into service. It applies to all welded steel storage tanks originally constructed to API650 (and to API 12C, the predecessor standard).

The standard covers:

• Inspection intervals. External inspection at least every 5 years; internal out-of-service inspection at intervals derived from measured corrosion rates (typically 10 to 20 years for a tank with a competent coating system); routine in-service inspection (monthly) by the owner.
• Inspection methods. Visual, ultrasonic thickness mapping (UT), magnetic flux leakage (MFL) for floor scanning, settlement surveys.
• Calculations. Minimum required shell thickness (the t_min calculation), corrosion rate determination, settlement evaluation methods.
• Repair methods. Floor plate replacement, shell strake repair, weld build-up, insert plate replacement, fittings repair.
• Alteration and reconstruction. Adding capacity, changing service, dismantling and re-erection.
• Inspector qualifications. The inspector must be an API653 Authorised Inspector certified through the API ICP examination.

If you are deciding whether a tank is still fit for service, scoping a refurbishment, or planning a major repair, API653 is the governing standard.

How they interact across a tank's lifecycle

The two standards run in sequence. A tank is built once under API650, then stays in service under API653 for the rest of its life. API653 frequently references back to API650 though: when a major repair amounts to a partial reconstruction, you execute the work to the original API650 detail.

In practice:

• Year 0: Tank designed and built to API650. Hydrotest, commissioning, in-service.
• Year 5: First external inspection per API653.
• Year 10-20: First internal out-of-service inspection per API653, with thickness mapping and settlement survey.
• Year 15-25: First major refurbishment, scoped under API653, with the actual repair detail (e.g. floor plate replacement) executed to API650 construction methods.
• Year 30-50: Decision point: continue under API653 with another major refurbishment, or replace the tank.

Quick comparison table

Aspect	API650	API653
Scope	New tank construction	In-service tank inspection, repair, alteration, reconstruction
Applies to	Tank being designed or built	Tank already in service
Key calculation	Required shell thickness for design pressure	Minimum acceptable shell thickness given measured corrosion
Required role	Design engineer	API653 Authorised Inspector
Typical interval	One-time at construction	Inspections every 5-20 years; repairs as required
Documentation	Construction record, hydrotest pack	Inspection report, repair plan, MoC, NDT, hydrotest pack
South African overlay	SANS 10131 (above-ground petroleum tanks)	SANS 10089 (storage and distribution)

Which standard applies to your situation?

If you can answer "yes" to any of these, you're in API650 territory:

• You are tendering for a new tank build.
• You are reviewing the design of a tank that has not yet been hydro-tested.
• You are specifying a major reconstruction where most of the original tank is being replaced.

Otherwise, if the tank exists and is or has been in service, you're in API653 territory:

• The 5-yearly external inspection is due.
• The next internal out-of-service inspection is due.
• Thickness measurements have flagged shell or floor corrosion below threshold.
• You are scoping a refurbishment.
• You are making a repair-vs-replace decision.
• You want to change the tank's service (e.g. from diesel to jet fuel) or add capacity.

FAQ

---

Need help applying these standards to a specific tank? Webmech delivers work under both standards. Get in touch with the team or read the related Knowledge Hub articles below.