API standard · reference card
API 650: scope, tank type and welding qualification route
Does API 650 govern the tank in front of you, or API 620 or API 653 instead? Scope decision flow for the welded-tank standard and the codes it invokes.
- Designation
- API 650
- Issued by
- API
- Title
- Welded Tanks for Oil Storage
- Edition
- Confirm the edition and addenda your purchase specification cites.
API 650 governs new-build, welded steel storage tanks, and most scope confusion around it comes from three neighbouring codes rather than the standard itself. A tank already in service moves to API 653 the moment repair is the question, not new construction. A tank outside the pressure and temperature range this standard assumes moves to API 620. And equipment under real internal pressure, rather than atmospheric storage, is a pressure vessel under ASME BPVC Section VIII — a different construction logic entirely.
Inside API 650’s own scope, a tank is not one welding problem. Bottom, shell, roof and nozzle welds are four separate joint categories, each with its own inspection logic — a vacuum-box test on a bottom seam checks leak-tightness, not what a radiographic plan on a shell butt weld checks. Assigning the category before planning examination is the step that gets skipped.
Therness equipment does not replace NDE. It evidences that the process stayed inside its qualified range for the joint category being welded — heat input and interpass measured continuously on long shell-course seams, tied to the welding process record checked against the NDE plan.
Scope: does it apply to your job?
Applies to
- New-build, welded steel storage tanks for petroleum, petrochemical and similar products, plus water and other non-volatile services, field-erected or shop-assembled
- Welder and welding procedure qualification for tank fabrication, by reference to ASME Section IX or an equivalent scheme the purchaser accepts
- NDE for bottom, shell, roof and nozzle or attachment welds, each treated as its own joint category
Does not apply to
- In-service inspection, repair, alteration or reconstruction of a tank already built — that is API 653
- Low-pressure or refrigerated storage above this standard's range — that is API 620
- Pressure vessels proper, governed by ASME BPVC Section VIII
Applicability decision flow
| Question | If yes | If no |
|---|---|---|
| Is this a new-build, welded steel tank for atmospheric or low-pressure storage — field-erected or shop-assembled? | API 650 governs either way; shop assembly follows the code's own annex, not a separate standard. | A tank already in service follows API 653; one outside this standard's pressure and temperature range follows API 620. |
| Is the equipment an atmospheric storage tank rather than a pressure vessel? | Stay in API 650. | A vessel under real internal pressure is ASME BPVC Section VIII's scope, not this one. |
| Which joint category is being welded — bottom, shell, roof, or nozzle? | Plan NDE per that category; the four are not one blanket plan. | Assign the category before planning examination. |
| Are the welders and WPS qualified per ASME Section IX for this material, thickness and position? | Proceed under the qualified procedure. | Qualify first; API 650 does not qualify anything on its own. |
What proves conformity, and what only supports it
Conformity is decided by inspection and testing against the criteria the standard and the contract set. The records below are evidence that the process was held where it was qualified; they support an inspection result, they never replace one.
| Requirement | Record produced | Source |
|---|---|---|
| Demonstrating a shell-course seam stayed inside its qualified heat input and interpass band across a long multi-pass weld | Arc energy and interpass temperature measured per pass, continuously, not spot-checked | WeldTrace |
| Confirming preheat was reached and held before the arc struck on a shell course thick enough to require it | Full-field thermal image of the joint immediately before and through welding | HeatCam IR-C |
Questions engineers ask about API 650
Does API 650 cover a tank built for refrigerated storage?
No. Refrigerated and low-temperature service beyond this standard's range moves to API 620, built on a different pressure and temperature basis.
Which code governs a weld repair on a tank already in service?
API 653, not API 650. Once a tank is built and commissioned, repair and alteration fall under the in-service standard's own requalification logic.
Does API 650 apply one NDE extent to the whole tank?
No. Bottom, shell, roof and nozzle welds are separate joint categories, each with its own examination requirement.
Evidence the process, not just the paperwork
Therness records what the machine actually delivered on every weld — parameters, thermal signature and a per-weld trail — so a conformity argument rests on measurements instead of recollection.
