Normal venting (liquid movement and thermal) and emergency venting for fire exposure, for atmospheric and low-pressure storage tanks
◉ API STANDARD 2000, 5TH EDITION, APRIL 1998 · NOT A REGULATORY SUBMISSION TOOL
⚠ This edition is several revisions behind current. Source: API Standard 2000, 5th Edition (April 1998), read directly from its own text and formulas — not from any secondary summary. The 6th edition (2009) and 7th edition (2014, reaffirmed 2020) both followed, and multiple sources describe an 8th edition published August 2026 with a new Annex H on flame arresters (replacing API RP 2210) — that last point comes from a single vendor site rather than API directly, so treat it as likely rather than confirmed, but either way this 1998 copy is not the current edition. The 7th edition in particular revised the thermal breathing factors based on updated heat-transfer modeling, which this tool's Table 2A values predate. Confirm against the edition your project specifies before relying on this for a real vent selection.
This tool covers normal venting (§4.3.2) and fire-exposure emergency venting (§4.3.3.2) for tanks without a weak (frangible) roof-to-shell attachment — a properly designed frangible-roof tank per API 650 needs no additional emergency venting capacity (§4.3.3.1). It does not cover refrigerated tanks, viscous/high-pour-point stocks, or the other special circumstances in §4.2.5.
Identification
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1. Normal Venting – Liquid Movement (§4.3.2.1.1, §4.3.2.2.1, §4.3.2.3.1)
2. Normal Venting – Thermal (§4.3.2.1.2, §4.3.2.2.2/§4.3.2.3.2, Table 2A)
Linearly interpolated between Table 2A's listed capacities. Column 3 (outbreathing, flash point ≥ 100°F) is roughly 60% of Column 2, per the table's own note, but the two are tabulated independently here, not derived by that ratio.
3. Total Normal Venting Capacity (§4.3.2)
3b. Tank Blanketing Valve Capacity (Vendor Guidance, not API 2000 Itself)
API 2000 itself states that "the design of a gas repressuring system to eliminate the requirement for vacuum relief valves is beyond the scope of this standard" (§4.2.5.3) — a blanketing valve supplements, and is not a substitute for, the vacuum relief device. The displacement rate below (8.0 SCFH per US gpm of emptying rate) is arithmetically identical to API 2000's own inbreathing rate of 5.6 SCFH per barrel/hour once converted to gpm (5.6 ÷ 42 × 60 = 8.0 exactly) — it is not a separate or looser criterion, just the same requirement in units more natural for a pump rate. The thermal component reuses Table 2A Column 2, already computed in card 2 above.
Actual valve selection (orifice size, Cv, droop at a given set pressure and supply pressure) is manufacturer- and model-specific — this tool does not include any single manufacturer's capacity curves, and you should size against the capacity table or curve for the specific blanketing regulator under consideration. Two general principles worth carrying into that selection: size the valve against the minimum available supply pressure, and separately check its capacity at maximum supply pressure to understand the flow if the regulator fails wide open, since failure of the blanketing valve is itself a cause of overpressure that §4.2.5.1 says must be considered.
4. Emergency Venting – Fire Exposure, General Formula (§4.3.3.2.1, Eq. 1A/1B)
T (\u00b0R) = T(\u00b0F) + 459.67. Water-application and depressuring/emptying credits (F = 1.0) require the facilities described in Table 4A's own footnotes d and e to actually be present and reliable — API 2000 does not recommend a blanket reduction just because water spray equipment exists.
Table 3A itself is tabulated at F = 1.0; this tool scales its interpolated value by your entered F, consistent with how Equation 2A (SCFH = 1107 × F × A0.82, for A ≥ 2800 ft² at 1–15 psig) applies F directly. Use this only when hexane's properties (144 BTU/lb, M = 86.17, 60°F vapor) are a reasonable stand-in for the actual stored liquid — otherwise use the general formula in card 4.
6. Wetted Surface Area Rules (Table 3A, Footnote a)
Vertical tanksTotal shell area up to 30 ft (9.14 m) above grade; ground-supported bottom excluded; elevated bottom included per engineering judgment
Horizontal tanks75% of total surface area, or the area up to 30 ft (9.14 m) above grade, whichever is greater
Spheres and spheroids55% of total surface area, or the area up to 30 ft (9.14 m) above grade, whichever is greater