Category
Safety & Relief
Subcategory
Tank Vent
Applicable standards
4
Issuing bodies
2
Description
Pressure-vacuum vent valve for atmospheric / low-pressure storage tanks.
Applicable Standards (4)
API — 3 standards
API 2000 / ISO 28300
Venting Atmospheric and Low-Pressure Storage Tanks
Normal and emergency venting requirements for atmospheric and low-pressure tanks.
Key requirements (7)
- Venting requirements for atmospheric & low-pressure tanks (also as ISO 28300).
- Normal venting: thermal in-breathing (cooling) and out-breathing (heating + product movement).
- Emergency venting: external fire exposure — table-based wetted-area heat input.
- Heat input formula: Q = 21,000·F·A^0.82 (BTU/hr) where A = wetted area (ft²), F = environment factor.
- Pilot-operated PV vents allowed in lieu of weighted pallet vents.
- Maximum allowable pressure typically 2.5 psig (matches API 650 limit).
- Combined pressure & vacuum venting capacity sized for worst-case scenario.
API 650
Welded Tanks for Oil Storage
Atmospheric, vertical, cylindrical above-ground welded steel tanks for liquid storage.
Key requirements (11)
- Atmospheric tanks — design pressure –0.25 in WC to 2.5 psig (≈17 mbar) max.
- Design temperature: ambient up to 93 °C (200 °F); higher → API 620 Annex M.
- Bottom plate min thickness: 6 mm (1/4") nominal.
- Shell design: 1-foot method (Section 5.6.3) or variable design point (5.6.4).
- Joint efficiency: 0.85 (lap-welded) or 1.0 (butt-welded with full RT).
- Hydrotest: filled with water to design liquid level; 24-hour holding test.
- Wind girders / top angle stiffeners: Section 5.9.6.
- Annex E: seismic; Annex C: external floating roof; Annex H: internal floating roof.
- Annex F: internal pressure up to 18 in WC; Annex M: elevated temperature; Annex S: stainless.
- Default allowable stress: 2/3 yield or 2/5 UTS (whichever lower) at design temp.
- Foundation: Annex B guidance (ringwall, slab, piled).
Note: Default global standard for atmospheric oil/petroleum storage tanks.
API 620
Design and Construction of Large, Welded, Low-Pressure Storage Tanks
Atmospheric and low-pressure (≤15 psig) welded storage tanks. Annex Q covers refrigerated LNG/LPG; Annex R covers low-pressure refrigerated storage.
Key requirements (7)
- Low-pressure tanks: pressures > 2.5 psig up to 15 psig (≈ 1 barg).
- Design temperatures: ambient down to –50 °F (–46 °C) by default; Annex Q to –270 °F (–168 °C); Annex R for refrigerated.
- Annex Q: refrigerated LNG, ethylene with 9 % Ni or stainless inner shell.
- Annex R: low-pressure refrigerated storage of LPG, NH3, etc.
- Stress allowable: ≤ 1/3 UTS or 5/12 yield (lower governs).
- Joint efficiency E: 1.0 with full RT for primary stress areas.
- Hydrotest: pneumatic + hydrostatic combination commonly used.
ISO — 1 standard
ISO 28300
Petroleum, petrochemical and natural gas industries — Venting of atmospheric and low-pressure storage tanks
International equivalent of API 2000.