Type the four words into a search bar and the first page is an odd cocktail of Python SDKs and 600-page PDFs. The algorithm is not at fault; the English language is. “API” once meant only the American Petroleum Institute, but the rise of REST endpoints hijacked the acronym. For bulk-liquid storage, however, the stakes are literal tons of steel and pressure. A single mis-citation can void insurance or invite a OSHA citation. This article removes the noise and tells the story behind each standard so you can quote the right paragraph in front of an inspector without sounding like a chatbot.
Which Exact API Standards Cover Storage Tanks
Skip the scroll. Here are the five citations inspectors actually write on NCR forms:
| API Standard | Typical Service | Max Pressure | Key Clause |
| API 650 | Atmospheric oil, water, chemicals | 2.5 psig (17 kPa) | 1.1.1 Scope |
| API 620 | Refrigerated, low-pressure, cryogenic | 15 psig (103 kPa) | 1.2 |
| API 653 | In-service inspection, repair, alteration | n/a | 1.1.1 |
| API 2000 | Venting for atmospheric & low-pressure | n/a | 1.1.2 |
| API 2350 | Overfill protection | n/a | 1.2 |
If you’re quoting a new fixed-roof crude tank in North Dakota, 99 % of the time the purchaser’s data sheet will simply state: “Design & build per API 650, latest edition.” Everything else is supplemental for API storage Tanks,
API 650 Deep Dive: Design, Materials, Fabrication & Testing
Scope & Pressure Limits
API 650 covers vertical, cylindrical, above-ground, closed- and open-top tanks. Internal pressure must stay below 2.5 psig—that’s roughly the draught of a soft-drink can. Anything higher triggers API 620 rules.
Material Grades & Shell Thickness
Common plate specs:
- ASTM A36M (basic carbon steel)
- ASTM A516 Gr 70 (low temp)
- ASTM A537 Class 1 (higher strength, reduces wall thickness 12–18 %)
Minimum shell thickness is the greater of:
- tmin = 6 mm (¼ in) for tanks ≤ 15 m (50 ft) diameter
- t calculated = (2.6 D (H – 1) G) / (S E) (Imperial, API 650 5.6.3.2)
Welding & NDE
- Butt joints: full-penetration, full-fusion, double-welded.
- Spot RT: 10 % of vertical seams per 30 m (100 ft) of weld; full RT for bottom annular plates ≥ 13 mm (½ in).
- Magnetic-particle every intersection of horizontal & vertical seams.
Leak-Detection Appendices
- Appendix I: bottom corrosion allowance + underside coating.
- Appendix T: leak-detection port in annular plate, popular in ethanol terminals.
API 620 Deep Dive: Low-Pressure & Cryogenic Service
When to Choose API 620 Over 650
Rule of thumb: if the design pressure exceeds 2.5 psig or the liquid is colder than –40 °C, switch to 620. Think LNG, liquid ammonia, butane spheres.
Pressure & Temperature Envelope
- Internal pressure: up to 15 psig.
- Metal temperature: –270 °C to 260 °C (–450 °F to 500 °F).
- Required Charpy impact testing for plate ≤ 13 mm at –50 °F.
Alloy Selection
- 9 % nickel steel for LNG inner shell.
- 5083-O aluminum for inner dome when weight-critical on barges.
API 653 Deep Dive: In-Service Integrity Management
Mandatory Inspection Intervals
- External visual: every 5 years (or 15 years if RBI justifies).
- Internal or ultrasonic thickness: ≥ 10 years unless risk-based interval approved.
- Hydrostatic test repeat: when calculated tmin < 0.5 in and corrosion rate > 5 mpy.
Minimum Thickness Calculations
Use API 653 4.3.3:
tmin = 2.6 D (H – 1) G / (S E) + CA
Compare against the critical zone: lowest measured ultrasonics minus future corrosion allowance (FCA).
Repair Acceptance
- Weld-built overlay: ≤ 50 % of shell thickness, max patch 12 in × 12 in.
- Replacement insert plates: ≥ 2 in (50 mm) from existing weld seams.
Companion API Standards (Quick Reference)
| Code | Topic | Why It Matters |
| API 651 | Cathodic protection | Tank bottom external corrosion control |
| API 652 | Tank bottom linings | Internal corrosion, sour crude |
| API 2550 | Calibration | Custody-transfer volume tables |
| API 2517/2519 | Evaporative loss | Emissions reporting to EPA |
Stuck choosing API Code? Our engineers will guide in 24 hours – Free consultation
Code Compliance Checklist for Tank Owners & EPCs
Pre-Construction Decisions
- Roof type: cone, dome, internal floating, external floating?
- Bottom type: annular plate yes/no?
- Design metal temperature (DMT) for toughness requirements.
Welding Procedure Qualifications
- WPS qualified to ASME IX; PQR essential variables recorded.
- Impact test temperature ≤ lowest expected one-day mean + 15 °F per API 650 4.2.9.
Hydrostatic & Pneumatic Testing
- Fill to 1.25 × design height; hold 24 h minimum.
- Settlement survey: ≤ ½ in (13 mm) differential between adjacent measurement points.
Documentation Package
- Data report (API 650 Appendix L)
- Radiographic film map
- Mill test certificates
- As-built drawings stamped by PE
FAQs
Is API 650 a law or a guideline?
Guideline unless adopted by reference. OSHA 1910.106 and NFPA 30 write “tanks shall be designed in accordance with API 650,” making it de-facto law in the U.S.
What is the latest edition of API 650?
13th Edition, March 2020 + Errata 1 (2021). Addenda are free downloads for licensed users.
Can I use API 653 on non-API 650 tanks?
Yes. API 653 explicitly states it applies to any steel tank built to an unknown standard provided the geometry meets basic cylindrical requirements.
Difference between API 620 and ASME VIII?
ASME VIII is for pressure vessels; API 620 is for low-pressure flat-bottom storage tanks. Bottom-to-shell joint detail and foundation loading methods differ.
Conclusion
Select the correct code early, document every decision, and future inspectors will bless your name. Choose poorly and the tank will remind you every time the wind blows or the temperature drops. Steel has a long memory, but the American Petroleum Institute gave us a vocabulary for speaking with it politely. Use that vocabulary and your storage tank will outlive the next technology cycle, quietly doing its job while the world argues again about what “API” really means.
