Many RFQs say "tank" when they need a "vessel," or "vessel" when a cheap atmospheric tank would do. The line is not diameter-it is design pressure and intended failure mode.
Atmospheric storage tank (API 650 / GB 50128 / EN 14015)
Operating pressure: approximately atmospheric, usually < 500 mmWG
Driven by liquid head, wind, seismic, snow
Bottom supported, shell courses tapered by hydrostatic load
No ASME nameplate, no relief valve calculation in basic scope
Examples: 10,000 m³ diesel tank, 2,000 m³ fire water tank, 5,000 m³ edible oil tank
Pressure vessel (ASME VIII Div.1 / GB 150)
Design pressure: 0.1 MPa, 0.6 MPa, 1.6 MPa, 4.0 MPa and above
Driven by hoop stress P×D/2t
Requires documented MAWP, corrosion allowance, PWHT if thickness/charpy triggers it
Requires pressure relief device sizing per code
Examples: instrument air receiver, separator, surge drum, reactor
The dangerous middle zone
Some buyers specify a "0.5 barg tank" as atmospheric because it is big. But once internal pressure can exceed 0.034 MPa (≈ 5 psig) under blocking/thermal expansion, you need relief analysis. A large tank with a blocked vapor line can bulge the roof-fixed cone roofs are not pressure vessels.
Relief and venting
Atmospheric tank: conservation vent / P-V vent, flame arrestor, emergency roof manhole
Pressure vessel: spring relief valve sized to heat input, blocked outflow, fire case
Mixing them up is a compliance defect, not a wording issue.
Document stack difference
Storage tank MDR: mill cert, WPS/PQR, RT/UT, coating DFT, hydrotest, settlement report.
Pressure vessel MDR: above + design calculation, MAWP statement, impact test, PWHT chart, relief valve sizing, nameplate data, statutory inspection.
How to write the PO
"1 × 10,000 m³ API 650 diesel tank" → storage tank scope
"1 × 2 m³ @ 0.8 MPa instrument air receiver, ASME VIII Div.1" → pressure vessel scope
"40 × ellipsoidal dished heads, 2:1, δ=14 mm, SA-516 Gr.70" → head supply scope
FAQ snippet
"Can a storage tank be pressurized for nitrogen blanketing?" Yes, low-pressure blanket 50–100 mbar is normal on edible oil and jet fuel tanks, but the roof and seal are designed for it. If blanket pressure can exceed the roof design, you no longer have an atmospheric tank-you have a low-pressure vessel.
ZN Metal Structure runs both families in one shop: atmospheric tanks to API 650 / GB 50128, vessels to ASME VIII / GB 150, heads pressed in-house-so the buyer does not accidentally procure a "tank" that should have been a "vessel."
