Bleeder vents (external) and tank circulation vents (internal) — API 650 §C.3.9 and §H.5.2
◉ API 650 APPENDIX C & H STYLE CHECK · NOT A REGULATORY SUBMISSION TOOL
⚠ Indicative tool only. All figures were checked directly against the standard's page images (this was a scanned/OCR'd PDF), not just extracted text. Confirm this is your current edition of API 650 before relying on this for a real design.
The key thing this tool exists to clarify: API 650's requirements for the vapor space under/around a floating roof are fundamentally different depending on whether the roof is external (open to atmosphere, §C.3.9) or internal (enclosed beneath a fixed roof, §H.5.2). Neither one sizes vents based on primary seal type (vapor-mounted / liquid-mounted / mechanical shoe) — that distinction drives evaporative loss (already covered by the Emission Calculator's AP-42/MPMS 19.2 tables), not vent sizing itself. External roof bleeder vents have no formula at all — it's an explicit Manufacturer judgment call. Internal roof circulation vents do have a real formula, and it's driven by tank diameter alone.
Identification
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Floating Roof Type
Bleeder Vents (§C.3.9)
There is no sizing formula for this. §C.3.9 states: "The Manufacturer shall determine and recommend the number and sizes of bleeder vents to be provided based on maximum filling and emptying rates specified." This is confirmed by API's own official interpretation (Technical Inquiry 650-I-12/2):
Q: Must the thermal in-breathing/out-breathing requirements as per API Std 2000 also need to be considered during design of bleeder vents? A: No, §C.3.9 does not require venting per API Std 2000.
In other words: this is deliberately left to the roof Manufacturer's engineering judgment, based on the Purchaser-specified fill/empty rates — it is not derived from the same thermal-breathing methodology as fixed-roof tank venting (the Tank Venting Calculator elsewhere in this app). If a client is asking "why doesn't the API 2000 approach apply here," this is the citable answer.
Reference: bleeder vents (leg-activated) on a floating roof open automatically when the roof lowers to within 150 mm (6 in.) of its lowest operating position, and close when it rises more than 150 mm (6 in.) above that position (§H.5.2.1.4 — written for internal floating roofs but describing the same general activation logic used for external roof leg/vent hardware).
Tank Circulation Vents (§H.5.2.2)
Required total net open peripheral vent area = 0.06 m² per metre (0.2 ft² per foot) of tank diameter — driven by diameter alone, not seal type. Plus a separate fixed-minimum center vent.
§H.5.2.2.1 specifies a 13 mm (½ in.) coarse-mesh screen but doesn't give its open-area fraction directly — get this from the actual screen product's data sheet, or leave at 1 to see the gross (unscreened) requirement.
Peripheral Vents (§H.5.2.2.1)
Required total net open area—
Minimum number of vents (spacing & count rule)—
Required net open area per 10 m (32 ft) interval—
If evenly distributed: net open area per vent—
Governing net area per vent (recommended)—
Governing gross (pre-screen) area per vent—
Center Vent (§H.5.2.2.2)
Minimum net open area (fixed, independent of D)—
Minimum gross (pre-screen) area—
§H.5.2.2.3: if circulation vents are not installed, gas blanketing (or another acceptable method to prevent a combustible gas mixture from developing within the tank) is required, and the tank must additionally be protected by pressure-vacuum vents per §5.8.5, based on information provided by the Purchaser. That PV vent sizing is a separate calculation from this one — §5.8.5 isn't in scope of this tool yet.