API 650 vs API 620: Which Standard for Your Storage Tank?

Mr. Asim Chandra

Chief Strategist · 42+ years in Industrial EPC

Every above-ground steel storage tank in a refinery, terminal, fertiliser complex or chemical plant is designed to one of two American Petroleum Institute standards: API 650 or API 620. The choice between them is not a preference — it is dictated by two numbers: the vapour pressure the tank must contain and the metal temperature at which it must operate. Specify the wrong standard and you either over-spend on a tank that never needed low-pressure design, or you build a tank that cannot legally or safely hold its product.

This guide gives you the exact boundaries between the two standards, explains the low-temperature and cryogenic appendices of API 620, shows where BS 7777 / EN 14620 enters the picture, and lays out a simple selection logic you can apply to any product.

What Is API 650?

API 650 is the standard for atmospheric storage tanks — the familiar vertical, flat-bottom, welded steel tanks that hold crude oil, diesel, petrol and water at every refinery and terminal. It applies when internal pressure is essentially atmospheric, up to a maximum of 2.5 psig with Annex F.

"API 650, 'Welded Tanks for Oil Storage', governs vertical, cylindrical, flat-bottom, above-ground welded steel tanks operating at internal pressures approximating atmospheric — extendable to 2.5 psig (18 kPa) under Annex F. It is the standard behind the vast majority of the world's crude oil, refined product, chemical and water storage tanks."

The base standard covers design metal temperatures up to 93°C (200°F); Annex M extends it to 260°C (500°F) for hot products such as bitumen and residues, and Annex S covers austenitic stainless steel construction for corrosive services. Roof options — fixed cone roof, external floating roof (EFR), internal floating roof (IFR) and double-deck floating roof (DDFR) — are selected by product volatility and emission requirements, not by the standard boundary itself. In India, API 650 is typically specified together with IS 803, with OISD-118 governing layout and OISD-244 in-service requirements.

What Is API 620?

API 620 is the standard for large, welded, low-pressure storage tanks — tanks whose gas or vapour space operates above 2.5 psig up to 15 psig, or whose contents are refrigerated far below ambient temperature. It is the governing standard for ammonia, LPG, LNG and liquid-gas storage tanks.

"API 620, 'Design and Construction of Large, Welded, Low-Pressure Storage Tanks', covers above-ground steel tanks with gas or vapour pressures from just above atmospheric up to 15 psig (103 kPa) and metal temperatures from +121°C down to -196°C. Its Annex R and Annex Q provisions make it the governing standard for refrigerated ammonia, propane, LNG, ethylene, oxygen and nitrogen tanks."

Unlike API 650, API 620 is not limited to flat-bottom cylinders — it permits curved-bottom and dome-topped geometries, and its rules are built around a full membrane-stress analysis of the tank under combined gas pressure and liquid head. The standard's real significance in modern projects, however, is its two low-temperature annexes (historically called Appendix R and Appendix Q), which carry tank design down into refrigerated and cryogenic territory where API 650 cannot go.

API 650 vs API 620: What Are the Key Differences?

The short answer: API 650 is for atmospheric tanks up to 2.5 psig at near-ambient temperatures; API 620 is for low-pressure tanks up to 15 psig and for refrigerated or cryogenic products down to -196°C. The table below compares the two standards point by point.

What Do API 620 Annex R and Annex Q Cover?

Annex R covers refrigerated tanks with design metal temperatures from +4°C down to -51°C (-60°F) — ammonia, propane and butane service. Annex Q covers cryogenic tanks from -51°C down to -196°C (-325°F) — LNG, ethylene, liquid oxygen and liquid nitrogen — with materials that stay tough at those temperatures.

The two annexes (called Appendix R and Appendix Q in older editions) split the low-temperature world by metallurgy:

• Annex R (-51°C to +4°C): impact-tested carbon and low-alloy steels remain viable. This is the home of refrigerated ammonia at -33°C, propane at -42°C and butane at around -6°C — the fertiliser and LPG terminal workhorses.
• Annex Q (-196°C to -51°C): ordinary steels become brittle, so the annex requires 9% nickel steel, austenitic stainless steel or aluminium for primary components. This covers LNG at -165°C, ethylene at -104°C, liquid oxygen at -183°C and liquid nitrogen at -196°C.

Refrigerated tanks to Annex R/Q are usually built double-wall: an inner tank in low-temperature material holding the product, an outer tank in carbon steel containing the insulation (commonly perlite in the annular space), with the whole system sitting on a heated or elevated foundation to prevent ground frost heave. For complete refrigerated systems — tank plus outer containment, insulation and appurtenances — API 625 is the umbrella standard that ties the API 620 metallic tank into the containment design.

When Does BS 7777 (Now BS EN 14620) Apply?

BS 7777 is the British standard for flat-bottom, vertical, cylindrical storage tanks in low-temperature service, and it defined the industry's containment categories: single, double and full containment. It was superseded by BS EN 14620 in 2006, but Indian and Middle Eastern specifications still cite it widely.

Where API 620 focuses on the metallic tank, BS 7777 / EN 14620 is organised around the containment philosophy of the whole installation:

• Single containment: one liquid-tight inner tank; any outer shell holds only insulation. Spills are managed by a bund.
• Double containment: a secondary wall can hold the liquid if the inner tank leaks, but not the vapour.
• Full containment: the outer tank — steel or pre-stressed concrete — holds both liquid and vapour in a leak event. This is today's default for large ammonia and LNG terminals close to populated or congested plant areas.

In practice, large low-temperature tanks in India are specified as hybrids: the metallic inner tank designed to API 620 Annex R or Q, with the containment system, outer tank and interfaces governed by BS 7777 / EN 14620 or API 625. A contractor bidding cryogenic tank construction must be fluent in both families of standards, because the QA/QC, welding and testing regimes are stitched together from both.

How Do You Choose Between API 650 and API 620?

Work through three questions in order: What is the product's vapour pressure at storage temperature? If it needs more than 2.5 psig, API 650 is out. What is the storage temperature? Below roughly -5°C refrigerated service, API 620 Annex R or Q governs. Above 15 psig? Neither standard applies — you need an ASME Section VIII pressure vessel.

The selection logic that covers nearly every product:

1. Atmospheric products at ambient temperature — crude, diesel, petrol, ATF, naphtha, water, chemicals with low vapour pressure: API 650, with roof type (CR/EFR/IFR/DDFR) chosen for vapour control. See our storage tank construction and petroleum tank pages for product-wise details.
2. Slightly pressurised vapour spaces (2.5–15 psig) — some petrochemical intermediates and gas-blanketed chemical tanks: API 620 base rules.
3. Refrigerated products (-51°C to +4°C) — ammonia, propane, butane stored cold at near-atmospheric pressure: API 620 Annex R, usually double-wall, often with BS 7777 / EN 14620 containment.
4. Cryogenic products (below -51°C) — LNG, ethylene, oxygen, nitrogen: API 620 Annex Q with 9% Ni steel or stainless inner tank, full-containment design per EN 14620 / API 625 for large terminals.
5. Pressurised liquefied gases at ambient temperature — LPG, propane, butane above 15 psig vapour pressure: not a tank at all, but an ASME Section VIII sphere or mounded bullet.

One economic note: refrigerated API 620 storage beats pressurised ASME storage on cost once volumes get large — a single refrigerated double-wall tank can hold what would otherwise need a whole farm of spheres. That crossover, typically in the thousands of tonnes, is why fertiliser plants store ammonia at -33°C rather than under pressure.

What Is HAIL's Experience with API 650 and API 620 Tanks?

HAIL has delivered 2,000+ storage tanks across India and overseas — API 650 atmospheric tanks up to 110 m diameter in cone roof, EFR, IFR and DDFR configurations, and double-wall cryogenic and refrigerated tanks to API 620 Annex Q/R and BS 7777 for ammonia, LPG, oxygen, nitrogen and ethylene service.

On the API 650 side, HAIL builds, tests and commissions tanks for crude oil, diesel, petrol, ATF, naphtha, kerosene, furnace oil, bitumen, methanol, ethanol, chemicals and water — to API 650, IS 803 and OISD-244 — for clients including IOCL, BPCL, HPCL, Reliance, Nayara Energy and Nirma, including complete tank farm and terminal packages with piping, pump houses and fire-water systems. On the API 620 / BS 7777 side, HAIL constructs double-wall low-temperature tanks for ammonia (-33°C), LPG (-42°C), ethylene (-104°C), liquid oxygen (-183°C) and liquid nitrogen (-196°C), and is certified for API 650 and API 620 work alongside its ASME U-Stamp and PESO SMPV(U) licence.

HAIL's low-temperature credentials extend to the end of the tank life cycle as well: the company executed India's first ammonia tank decommissioning by reverse jacking — a 5,000 MT double-wall ammonia tank at Birla Copper, Dahej — with zero safety incidents. Understanding how these tanks are built to Annex R rules is precisely what makes it possible to take them apart safely.

Frequently Asked Questions