Storage tank testing

Storage tank testing is the set of construction acceptance, leak-tightness, settlement, and in-service integrity tests that verify a storage tank's welds, floor, shell, and foundation meet its design code on first build, and that the corrosion, settlement, and weld degradation the tank develops in service stay within the in-service inspection code between outages. The governing design and construction codes are API 650 (Welded tanks for oil storage, API publications) and in China SH/T 3046 (Petrochemical vertical cylindrical welded storage tanks — design) with construction to GB 50128 (Vertical cylindrical welded storage tanks — construction); the in-service inspection codes are API 653 and EEMUA 159, with the non-destructive examination to NB/T 47013. Storage tank testing is not a re-run of pressure-vessel testing — a storage tank operates at atmospheric or near-atmospheric pressure, so it does not owe the pressure-vessel hydrotest at design pressure; instead it owes the water-fill test with settlement observation, the floor-plate thickness survey, and the weld NDT to the tank-grade acceptance — all of which a pressure vessel does not owe in that form. It therefore shares its weld-NDT and fracture methods with our Steel structure testing and Fracture analysis of metals programmes — the site-welded tank shell is a large steel structure, and a weld crack that grows to rupture is the same failure mode we chase metallurgically — while the shell-plate material itself follows our Seamless steel pipe testing for plate-grade material verification, and the tank's attached piping and isolation valves are qualified alongside our shut-off valve testing.

What Makes a Storage Tank a Distinct Test Subject?

A vertical-cylindrical atmospheric storage tank is a large, thin-walled, site-welded shell holding a liquid head, not a pressure (storage tank background). Its failure modes are different from a pressure vessel's, and so are its tests:

• The load is the liquid head and the wind, not an internal pressure. A pressure vessel's design is governed by hoop stress from internal pressure; a storage tank's shell is governed by the hydrostatic hoop stress from the stored liquid (highest at the bottom course) plus wind and seismic on a large diameter-to-thickness shell. So the tank's acceptance test is a water fill, not a pressure hydrotest.
• The floor is the failure hotspot, and it is only accessible when the tank is emptied. Tank-floor corrosion (bottom-side, from soil-side moisture; and top-side, from settled water under the product) is the leading cause of tank leakage, and the floor can only be inspected when the tank is out of service. So the floor-plate thickness survey (magnetic-flux leakage scan + ultrasonic on the annular ring) is the headline in-service test, run once per outage.
• The foundation settles, and uneven settlement stresses the shell. A large tank on soil settles under load, and differential (radial) settlement bends the shell and can rupture a floor weld. So settlement observation during the water-fill test is mandatory, and the radial settlement difference must stay within the code limit.

The fact the SERP obscures: a storage-tank report that quotes only "hydrotest passed" is unverifiable. The water-fill test with the settlement observation, the floor-plate thickness map, and the weld NDT grade (NB/T 47013 acceptance) are the properties that decide whether the tank will hold — and they are tests a pressure-vessel acceptance (which a tank is not) does not produce.

What Are the Construction Acceptance Tests?

The tests run on first build, to GB 50128 / API 650:

• Water-fill (hydrostatic) test — the tank is filled with water in defined steps to the test level (commonly the design liquid level), held, and inspected for leaks at every shell weld, floor weld, and roof weld. The fill is stepped because the load on the foundation rises with level, and the settlement is read at each step. Unlike a pressure-vessel hydrotest, there is no pressure gauge to read — the test is leak-tightness and settlement, not pressure-holding.
• Settlement observation — survey benchmarks on the shell are read at each fill step and after the hold period; the radial settlement difference (the difference in settlement between points around the circumference) must stay within the GB 50128 / API 650 limit. A tank that settles uniformly is fine; a tank that settles differentially tilts, bends the shell, and can tear a floor weld — so the differential limit is the acceptance, not the absolute settlement.
• Weld non-destructive examination (NB/T 47013) — the shell, floor, and roof welds are examined by radiography, ultrasonic, magnetic-particle, or penetrant to the tank-grade acceptance: surface NDT (MT/PT) to Class I, radiography (RT) to Class III, ultrasonic (UT) to Class III. The locations and percentages are defined by the design code (API 650 / SH/T 3046) and depend on the weld location and plate thickness — thicker shell plates owe a higher NDT percentage, and the floor-to-shell (annular) weld is always 100 % surface-examined.
• Geometric shape and dimension check — verticality, roundness, and local deviation of the shell, verified against the code tolerances. A shell out of roundness is weaker against wind and harder to fit with a floating roof.
• Roof test — for fixed and floating roofs, a strength and (for fixed roofs) internal-pressure test to the design value, plus a floating-root buoyancy and pontoon-integrity test.

What Are the In-Service Inspection Tests?

Once a tank is in service, the inspections move to the API 653 / EEMUA 159 programme — run at defined intervals (commonly 10–20 years for external, shorter for internal/out-of-service), and focused on the degradation a tank develops in service:

• Floor-plate thickness survey (the headline in-service test) — with the tank emptied and cleaned, the floor is scanned by magnetic-flux leakage (MFL) for大面积 corrosion mapping, with ultrasonic thickness measurement on the annular ring and on suspect points flagged by the MFL scan. The remaining floor thickness is mapped against the minimum-fit-for-service and the next-inspection critical thickness — a floor below the minimum must be repaired or the plate replaced before the tank returns to service.
• Shell thickness survey — ultrasonic thickness at defined grids on each shell course, tracking general thinning and locally corroded areas (especially at the liquid-air interface and at the bottom course where water settles).
• Weld NDT on suspect welds — MT/PT on surface cracks (especially stress-corrosion cracking in ammonia and other specific services), and RT/UT where the design-code NDT was deferred or where a weld is suspect.
• Settlement re-survey — re-reading the benchmarks to confirm the tank has not tilted further in service; differential settlement that has grown beyond the code limit requires realignment or a foundation intervention.
• Roof and floating-roof inspection — roof-plate thickness, pontoon integrity (for floating roofs), drain-system integrity, and seal-condition for floating roofs.
• Cathodic-protection verification (where fitted) — the floor's underside is protected by CP, and the CP system must be tested to confirm it is protecting the floor.

A common field failure — a tank that develops a floor leak between outages — is traceable to the floor-plate survey: the tank was inspected only externally, the floor was not scanned because it required an outage, and bottom-side corrosion thinned the floor below critical between inspections. That is why the floor survey is the headline in-service test and why it can only be run when the tank is emptied.

How Do the Standards Differ — and Which Applies?

A specification that names only "API" or "GB" without the standard number is unverifiable — API 650 (new build) and API 653 (in-service) are different programmes, and GB 50094 (spherical) does not apply to a vertical-cylindrical tank. The standard must be named, and the new-build vs in-service distinction must be made.

Frequently Asked Questions

What standard governs storage tank testing?
API 650 (new-build welded tanks for oil storage) and in China SH/T 3046 (design) + GB 50128 (construction); in-service inspection to API 653 / EEMUA 159; NDT to NB/T 47013. GB 50094 is for spherical tanks only and does not apply to vertical-cylindrical tanks.

Why does a storage tank not owe a pressure hydrotest?
Because a storage tank operates at atmospheric or near-atmospheric pressure — the load is the liquid head, not an internal pressure. So the acceptance test is a water-fill for leak-tightness and settlement, not a pressure-hold at a design pressure. A pressure-vessel hydrotest does not apply because the tank is not a pressure vessel.

What is the floor-plate survey and why is it the headline in-service test?
With the tank emptied, the floor is scanned by magnetic-flux leakage for corrosion mapping and ultrasonic on the annular ring and suspect points. Floor corrosion (bottom-side from soil moisture, top-side from settled water) is the leading cause of tank leakage, and the floor is only accessible when the tank is out of service — so the floor survey is the test that decides whether the tank can return to service, and it is the one most often deferred and most often the root cause of a between-outage leak.

What is settlement observation and why does it matter?
During the water-fill test (and re-surveyed in service), benchmarks on the shell are read at each fill step, and the radial settlement difference must stay within the code limit. A tank that settles differentially tilts, bends the shell, and can tear a floor weld — so the differential limit is the acceptance, not the absolute settlement.

To what grade are tank welds accepted?
Per NB/T 47013: surface NDT (MT/PT) to Class I, radiography (RT) to Class III, ultrasonic (UT) to Class III. The NDT locations and percentages depend on the weld location and plate thickness — thicker shell plates owe a higher percentage, and the annular floor-to-shell weld is always 100 % surface-examined.

Are spherical tanks tested to the same standard?
No. Spherical tanks (used for pressurized gases like LPG, propane, ammonia) are pressure vessels and follow GB 50094 (spherical storage tank construction) with a pressure hydrotest, not the atmospheric-tank water-fill and floor-survey programme. A spherical tank is a pressure vessel; a vertical-cylindrical atmospheric tank is not.

Our Testing Capabilities

Beijing ZKGX Research (ISO/IEC 17025 testing laboratory) provides storage tank testing across construction acceptance and in-service integrity:

• Water-fill (hydrostatic) test with stepped filling and settlement observation to GB 50128 / API 650 — radial settlement difference against the code limit.
• Weld non-destructive examination to NB/T 47013 — MT/PT (Class I), RT (Class III), UT (Class III), at the design-code locations and percentages for API 650 / SH/T 3046.
• Floor-plate thickness survey — magnetic-flux leakage scan for corrosion mapping, ultrasonic thickness on the annular ring and flagged points, against the minimum-fit-for-service.
• Shell thickness survey — ultrasonic thickness at defined grids per course, especially at the liquid-air interface and bottom course.
• Settlement re-survey in service (API 653 / EEMUA 159), and roof / floating-roof inspection.
• Spherical storage tanks — separate programme to GB 50094 with pressure hydrotest and weld NDT.

If you have a storage tank to accept on construction (water-fill + settlement + weld NDT), an in-service tank to inspect (floor + shell survey), or a spherical tank to test to GB 50094, contact our testing team to scope the applicable standard, the NDT acceptance, and the in-service interval.