Category
Storage Tanks
Subcategory
Low Pressure
Applicable standards
5
Issuing bodies
2
Description
Welded low-pressure storage tank (≤15 psig) — ambient service or refrigerated outer shell.
Applicable Standards (5)
API — 3 standards
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.
API 625
Tank Systems for Refrigerated Liquefied Gas Storage
Single-, double- and full-containment refrigerated tank systems for LNG, LPG, ethylene, ammonia, butadiene, etc. Integrates inner tank, outer tank, foundation, insulation, vapor barrier, and ancillaries.
Key requirements (10)
- Tank systems for refrigerated liquefied gases — covers full / double / single containment.
- Inner tank materials: 9 % Ni steel (ASTM A553) for cryo (LNG, ethylene); SS for very low temp; CS for warmer (NH3, LPG).
- Outer tank: prestressed concrete (full containment) or steel (single/double).
- Insulation in annulus: perlite (loose-fill), cellular glass, PIR foam, depending on system.
- Vapor barrier mandatory for refrigerated systems to prevent atmospheric moisture ingress.
- Heated foundation required for cryogenic service to prevent ground frost heave.
- In-tank submerged pumps mandatory — no bottom or side connections below liquid level.
- Boil-Off Gas (BOG) management: vapor return, BOG compressor, recondenser typical.
- Roof: suspended deck (full containment) with insulation above.
- References API 620 Annex Q/R for inner tank design rules.
Note: Primary US/Global standard for cryogenic and refrigerated bulk storage tanks.
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.
ASTM — 2 standards
ASTM A516 / A516M
Pressure Vessel Plates, Carbon Steel, for Moderate- and Lower-Temperature Service
Most-used carbon steel plate for pressure vessel shells (Gr 60, 65, 70).
ASTM A537 / A537M
Pressure Vessel Plates, Heat-Treated, Carbon-Manganese-Silicon Steel
Heat-treated higher-strength carbon-mangan-silicon plate for PV.