Above-ground storage tank fabrication and field erection · reference card
Field-erected and shop-fabricated tanks welded under API 650. The four monitoring pain points that cost tank fabricators most, and which Therness signal addresses each.
A storage tank is one of the largest single welded structures most fabricators build, and its welding has two characters inside one construction code. Floor and bottom seams are welded onto a foundation from one side, with no way to inspect or repair the underside once the plate is down. Shell courses stack a tank’s whole height, and conditions at the top are not the conditions at the bottom.
Field erection is the harder half of the job. A shop-built tank sees a controlled environment throughout; a field-erected one sees weeks of wind, temperature swing and rain on the same procedure qualified indoors. Interpass and preheat evidence taken at the joint, not assumed from the qualification record, is what tells the two apart.
Leak testing and weld-quality evidence answer different questions. A floor seam that holds vacuum has proven it is tight at the test pressure, not that every pass stayed inside its qualified variables — see the API 650 inspection guide for how the NDE matrix and acceptance criteria fit together across bottom, shell, roof and nozzle welds.
| Code | Why it applies |
|---|---|
| API 650 | The primary construction code for new-build, welded steel storage tanks, covering material, design, welding qualification and NDE by joint category. |
| API 653 | Governs in-service inspection, repair, alteration and reconstruction of a tank already in service. |
| AWWA D100 | The equivalent welded-tank construction standard for water storage rather than petroleum service. |
| ISO 3834-2 | Comprehensive welding quality-system requirements increasingly required alongside the tank-specific construction code. |
| Pain point | Consequence | Signal that sees it |
|---|---|---|
| Floor and bottom seams are welded onto a foundation from one side, with no access to inspect or repair the underside | A root-side imperfection found by leak testing means cutting into a completed floor to reach it. | Thermal or pool evidence captured on the single-pass root while it is welded, before service covers it. |
| Shell courses are welded vertically, and interpass conditions change from the bottom course to the top of a tall tank | Heat input drifts unnoticed up the shell, and the top courses see a different thermal history than the bottom. | Interpass and heat input measured per pass across the full height, not spot-checked once. |
| Field erection runs for weeks outdoors, in weather a shop procedure qualification never saw | A procedure qualified indoors can sit outside its qualified interpass window on site without anyone measuring it. | Non-contact interpass measurement taken at the joint on site, not assumed from the qualification record. |
| Vacuum-box and hydrostatic testing are the leak-tightness evidence a tank owner sees on floor welds | A tank can pass leak-tightness testing while carrying a weld-quality gap the test was never designed to close. | Per-pass process evidence answering a different question than a leak test, captured while the seam is made. |
It confirms leak-tightness at the specified test pressure, not general weld quality. A joint can hold vacuum and still carry an imperfection a different examination method would find.
The construction code is the same either way. What differs is environmental exposure during welding, which is why interpass and preheat evidence matters more in a field-erected tank's record.
Yes, depending on plate thickness, joint type and service. Monitoring does not change how much examination is required, only how early a suspect weld is flagged before examination confirms it.
Therness measures what happens while the arc is running — parameters, thermal signature, per-weld records — so the problems on this page surface on the shift that made them.
Progetto cofinanziato nell'ambito del PR Piemonte FSE+ 2021-2027,
Priorità I, Obiettivo Specifico a), Azione 4 – "Sostegno alla nascita delle start up"
