API 650 Storage Tanks: Types, Design & Selection Guide

Quick Answer

API 650 (Welded Tanks for Oil Storage) is the international standard for the design, fabrication, erection, inspection, and testing of welded atmospheric storage tanks — those with design pressure up to about 2.5 psig. It covers four main configurations: cone roof (fixed roof, for low-vapour-pressure products like diesel, kerosene, and fuel oil), internal floating roof (a fixed cone roof with an internal floating cover for evaporation control — gasoline, naphtha, jet fuel), external floating roof (no fixed roof; the roof rises with product level, for high-volatility products, cutting vapour losses by around 95%), and double-deck floating roof (heavier construction, used for fire-water and demanding service). Shell courses are tapered — thickest at the bottom, thinnest at the top — to match hydrostatic loading, with the variable-design-point method used on larger tanks to save steel. Permitted materials include carbon steel (SA-283 Gr C, SA-285 Gr C, SA-516 Gr 70), stainless (SA-240 304/316L), and aluminium, with corrosion allowance typically 1.5 mm for sweet service and 3 mm for sour. Two sister standards complete the picture: API 620 covers low-pressure tanks from about 2.5 to 15 psig including refrigerated and cryogenic service, and API 653 governs in-service inspection, repair, and alteration. Note that API 650 applies only to welded tanks — bolted tanks fall under AWWA D103 or similar.

A tank farm is the least glamorous part of a refinery and one of the most consequential. Tanks hold the plant's entire working inventory, they sit outside exposed to wind, sun, and seismic loading, they corrode from the underside where nobody can see, and when one fails the consequence is measured in thousands of cubic metres of product on the ground and, occasionally, in a fire that takes days to put out.

API 650 exists because of that consequence. It is the standard that turns "a big steel cylinder" into an engineered structure — specifying how shell thickness varies with height, how the bottom is laid out and protected, how roofs are designed for wind and snow, how openings are reinforced, how welds are qualified and examined, and how the finished tank is tested before it holds product. In the United States, OSHA 1910.106 and NFPA 30 both write that tanks shall be designed in accordance with API 650, which makes it de facto law; internationally it is the default specification in EPC scope.

For project engineers, tank farm operators, EPC contractors, and procurement teams — this guide covers API 650 tanks: the tank and roof types and which product each suits, how the shell and bottom are designed, materials and corrosion protection, foundations, venting, testing, and how API 650 relates to API 620 and API 653.

For related plant context, see Chemical Factory Setup and Plant Construction & Erection Equipment.

Tank and Roof Types

The roof choice is driven by the vapour pressure of the stored product and the emissions regime.

Cone Roof (Fixed Roof)

A cylindrical shell with a permanently welded conical roof. The simplest and most widely used configuration.

• Minimum slope around 9.5° for self-draining
• Suits low vapour pressure products: diesel, kerosene, fuel oil, water, many chemicals
• Cost-effective and simple to construct
• Requires normal and emergency venting to manage vapour accumulation
• Vapour space above the liquid means breathing losses on filling and thermal cycling

Dome (Umbrella) Roof

A curved self-supporting roof.

• Lower steel weight than an equivalent supported cone
• Better performance in seismic zones
• Used where roof loading or seismic demand favours the geometry

Internal Floating Roof (IFR)

A fixed cone roof with a floating cover inside, resting on the liquid surface.

• Controls evaporation losses while the fixed roof handles weather, snow, and rain
• Standard for gasoline, naphtha, jet fuel and other volatile products
• Aluminium internal floating roofs are commonly 6 mm minimum thickness, in pontoon or double-deck construction
• The rolling ladder and gauge well must be bonded for lightning protection

External Floating Roof (EFR)

No fixed roof — the floating roof itself is the top of the tank and rises and falls with product level.

• Reduces vapour losses by around 95%, meeting emissions regulations such as EPA NSPS
• Used for high-volatility products and large crude storage
• Requires rim seals, drainage, and roof drains that must be maintained
• Exposed to weather; snow and rainwater loading must be considered

Double-Deck Floating Roof

A heavier two-deck construction with greater buoyancy and rigidity.

• Better stability and insulation of the product surface
• Used for fire-water service and demanding applications

Roof Type Selection

Shell, Bottom and Roof Design

Shell Courses

The shell is built from horizontal courses (rings) of plate, welded together. Because hydrostatic pressure is greatest at the base and falls to zero at the top, courses are tapered — thickest at the bottom, thinnest at the top.

Two calculation methods are used:

• One-foot method — simpler, suitable for smaller tanks
• Variable-design-point method — optimises thickness course by course as pressure decreases toward the top; used on larger tanks to save significant steel tonnage

Bottom and Annular Plates

The bottom is normally a flat steel plate on a compacted foundation or concrete slab, laid with attention to slope and drainage to prevent water pooling and corrosion.

Annular plates — a ring of thicker plate under the first shell course — are required when the bottom shell course exceeds 25 mm or the stored fluid is water. Minimum annular plate thickness is typically 8 mm.

The underside of the bottom plate is the tank's most vulnerable area because it cannot be inspected in service, which is why cathodic protection on the underside is mandatory and leak detection liners are commonly installed for environmental compliance.

Roof Plates and Loading

• Minimum roof plate thickness 5 mm including corrosion allowance
• Live load on roof plates limited to 1.2 kPa (25 psf)
• A 3:1 safety factor against buckling under dead plus live load is required
• Roofs may be self-supporting (cone, dome, umbrella) or structurally supported on rafters and columns

Openings and Reinforcement

All openings larger than 50 mm NPS must be reinforced with built-up pad plates or thick-shell insert plates. The reinforcement area must equal the area of metal removed by the cut-out. Nozzles, manways, vents, and fittings are arranged for safe operation, inspection, and maintenance.

Loads Considered

• Hydrostatic pressure from the stored liquid — the primary load driving shell thickness
• Dead loads — shell, bottom, roof, insulation, and all permanent appurtenances
• Live loads — roof loading, snow
• Wind and seismic — API 650 gives detailed attention to environmental effects
• Internal and external pressure within the atmospheric limit

Materials and Corrosion Protection

Permitted materials:

• Carbon steel: SA-283 Grade C, SA-285 Grade C, SA-516 Grade 70
• Stainless steel: SA-240 304 / 316L
• Aluminium (including internal floating roofs)

See Carbon Steel Plate and Stainless Steel Plate for material detail.

Corrosion allowance by service:

• Sweet service: typically 1.5 mm
• Sour service: typically 3 mm
• High-corrosion service: greater, per assessment

Protection measures:

• Cathodic protection on the underside of the bottom plate — mandatory
• Internal coating selected for the product service
• External coating per the plant's corrosion protection specification
• Leak detection liner under the bottom for environmental compliance

Toughness: design metal temperature (DMT) governs impact testing requirements. Per API 650, impact test temperature is set at or below the lowest expected one-day mean temperature plus 15°F.

Foundations

Options include:

• Crushed stone ringwall — common and economical
• Concrete ringwall
• Concrete slab
• Pile-supported — for poor soils

Settlement is a primary concern. Edge settlement is surveyed during the first hydrotest, with re-levelling required if differential settlement exceeds about 25 mm (1 inch). During hydrotest, differential settlement between adjacent measurement points is commonly limited to around 13 mm (½ inch).

Venting

Fixed roof tanks breathe as they fill, empty, and undergo thermal cycling. Inadequate venting is a classic cause of tank collapse under vacuum or over-pressure.

• Normal vents sized per API 2000 for in-breathing and out-breathing due to pump rates and thermal change
• Emergency venting — manhole cover or gauge hatch arrangements that must handle a flash fire scenario, typically around 2.5 in H₂O set pressure
• Foam dam (typically 0.187 in plate) is mandatory on tanks of 100 ft diameter and above per NFPA 11

For the overpressure protection principles behind venting, see Pressure Relief & Safety Valves.

Fabrication, Inspection and Testing

Welding is qualified per ASME Section IX (WPS and PQR with essential variables recorded); NDE follows ASME Section V.

Testing includes:

• Hydrostatic test — fill to 1.25 × design height, hold a minimum of 24 hours
• Vacuum box test on bottom welds
• Magnetic particle examination where specified
• Settlement survey during hydrotest
• Radiography of shell seams per the standard's requirements

Only authorised facilities may apply the API Monogram, and the quality assurance programme must meet API requirements.

API 650 vs API 620 vs API 653

Above 15 psig, tanks fall outside API 620 and are designed as pressure vessels to ASME Section VIII.

Note: API 650 applies to welded steel tanks only. Bolted steel tanks for water and dry storage fall under AWWA D103 or similar standards — a bolted tank cannot correctly be called an API 650 tank.

API 653 In-Service Management

• Inspection intervals are determined by API 653
• Risk-based inspection using API RP 581 calculates probability of failure from corrosion rate and consequence from spill volume, allowing a shift from time-based to condition-based inspection
• Repair limits are defined — for example weld-build overlay limited to 50% of shell thickness with maximum patch dimensions, and replacement insert plates required to be a minimum distance from existing weld seams
• Early detection of bottom-side pitting is the priority, since it causes environmental release

Combining protective coatings with cathodic protection can extend effective corrosion allowance life beyond 30 years.

Common Specification Mistakes

After 15+ years supplying materials and equipment to tank and terminal projects:

Mistake 1: Wrong Roof Type for the Product

Fixed cone roof specified for gasoline storage. Evaporation losses are high, emissions limits are exceeded, and product is lost to atmosphere.

Prevention: Match roof type to vapour pressure — internal floating roof for volatile refined products, external floating roof for crude and high-volatility service.

Mistake 2: Inadequate Venting

Vents sized without reference to actual pump-in and pump-out rates. The tank buckles under vacuum during rapid emptying or over-pressures during filling.

Prevention: Size normal vents per API 2000 using actual pumping rates plus thermal effects, and provide emergency venting for the fire case.

Mistake 3: Skipping Underside Protection

Bottom plate installed without cathodic protection or leak detection. Underside corrosion progresses invisibly until product reaches the ground.

Prevention: Cathodic protection on the bottom underside is mandatory; add a leak detection liner where environmental exposure warrants it.

Mistake 4: Ignoring Settlement During Hydrotest

Settlement survey not performed or differential settlement accepted beyond limits. The tank distorts, floating roof binds, and shell stresses rise.

Prevention: Survey edge settlement during the first hydrotest and re-level if differential settlement exceeds the specified limit.

Mistake 5: Under-Reinforced Openings

Nozzle cut into the shell without adequate reinforcement. Stress concentration cracks the shell around the opening.

Prevention: Reinforce all openings above 50 mm NPS with pad plates or insert plates providing reinforcement area equal to the metal removed.

Mistake 6: Wrong Standard for the Pressure

API 650 tank specified for a service that will run above 2.5 psig, or a refrigerated product.

Prevention: Use API 620 for 2.5–15 psig and refrigerated/cryogenic service, and ASME Section VIII above 15 psig.

Mistake 7: Insufficient Corrosion Allowance for Sour Service

Sweet-service corrosion allowance (1.5 mm) applied to a sour product. The tank reaches minimum thickness far earlier than planned.

Prevention: Apply 3 mm for sour service or more per corrosion assessment, and plan API 653 inspection intervals accordingly.

Supply from Kasko Makine

Kasko Makine supplies storage tank materials, components, and fabrication support for refinery, terminal, petrochemical, water, and industrial projects:

Tank materials:

• Carbon steel plate: SA-283 Gr C, SA-285 Gr C, SA-516 Gr 70, A36
• Stainless plate: SA-240 304 / 304L / 316 / 316L
• Structural sections for roof framing, wind girders, and stairs
• Annular plate and bottom plate

Tank components:

• Nozzles, manways, and reinforcing pads
• Flanges and fittings (see Pipe Flanges)
• Vents, gauge hatches, and emergency vent covers
• Roof drains, rim seals, and floating roof components
• Spiral stairs, platforms, handrails, and rolling ladders
• Foam dams and firefighting connections
• Cathodic protection components

Fabrication:

• Shell plate rolling and forming
• Shop-fabricated tank components
• Field erection support
• Welding per ASME Section IX, NDE per Section V

Engineering support:

• Roof type selection for the stored product
• Shell course thickness (one-foot and variable-design-point methods)
• Material and corrosion allowance selection
• Venting review per API 2000
• Foundation type guidance
• API 650 vs API 620 vs ASME VIII scope confirmation
• API 653 inspection and repair planning

Certification: EN 10204 Type 3.1/3.2 material test certificates, WPS/PQR documentation, NDE reports, hydrostatic test records, settlement survey records, heat traceability, third-party inspection available

Logistics: Tank materials and components shipped from Istanbul to projects across Africa, the Middle East, Central Asia, and beyond. Plate and standard components 6-12 weeks; fabricated components and complete tank packages by project schedule.

Need storage tank materials or fabrication? Send us the tank capacity and dimensions, stored product and its vapour pressure, design and operating temperature, site wind and seismic data, applicable standard (API 650 / 620), and required roof type to info@kaskomakine.com or WhatsApp +90 (537) 521 1399. We'll confirm the configuration, specify materials and corrosion allowance, and provide a quotation within 72 hours.

Continue Reading: Equipment & Materials Guides

• Carbon Steel Plate — SA-516 and tank plate grades
• Stainless Steel Plate — Stainless tank materials
• Pipe Flanges — Tank nozzle connections
• Pressure Relief & Safety Valves — Overpressure protection and venting
• Plant Construction & Erection Equipment — Tank erection
• Chemical Factory Setup — Plant layout and storage

Frequently Asked Questions

Q: What is API 650?
A: API 650 (Welded Tanks for Oil Storage) is the international standard for the design, fabrication, erection, inspection, and testing of welded atmospheric storage tanks used for oil and refined petroleum products. It covers tanks with design pressure up to approximately 2.5 psig, addressing shell and bottom thickness, roof design, foundation requirements, permitted materials, welding qualification per ASME Section IX, NDE per ASME Section V, and testing including hydrostatic test and vacuum box examination. The standard covers cone roof tanks, internal floating roof tanks, external floating roof tanks, and double-deck floating roof tanks at refineries, terminals, and petrochemical plants. In the US, OSHA 1910.106 and NFPA 30 reference API 650, making it effectively mandatory. API 650 applies only to welded tanks — bolted tanks fall under AWWA D103.

Q: What is the difference between API 650 and API 620?
A: The distinction is design pressure and service. API 650 covers welded atmospheric storage tanks with design pressure up to about 2.5 psig — typically flat-bottom cylindrical tanks with cone, dome, internal floating, external floating, or open-top roofs, used for crude, refined products, water, and chemicals at ambient conditions. API 620 covers welded low-pressure storage tanks from about 2.5 to 15 psig, including refrigerated and cryogenic service such as LPG and liquefied gases. API 620 tanks typically use dome or pressure-resistant roofs forming part of the pressure boundary, thicker shell plates, and integrated insulation systems for low-temperature service. Above 15 psig, equipment falls outside API 620 and is designed as a pressure vessel to ASME Section VIII.

Q: What are the main types of API 650 storage tanks?
A: API 650 covers four main configurations. Cone roof tanks have a permanently welded fixed roof (minimum slope around 9.5° for self-draining), suiting low-vapour-pressure products such as diesel, kerosene, fuel oil, and water. Internal floating roof tanks combine a fixed cone roof with an internal floating cover to control evaporation, standard for gasoline, naphtha, and jet fuel. External floating roof tanks have no fixed roof — the floating roof rises with product level, cutting vapour losses by around 95% and meeting emissions regulations, used for crude and high-volatility products. Double-deck floating roof tanks use heavier two-deck construction for greater buoyancy and stability, used for fire-water service and demanding applications. Dome or umbrella roofs offer lower steel weight and better seismic performance.

Q: How is API 650 tank shell thickness determined?
A: Shell thickness is driven primarily by hydrostatic pressure from the stored liquid, which is greatest at the base and falls to zero at the top. The shell is therefore built from horizontal courses that are tapered — thickest at the bottom, thinnest at the top. Two calculation methods are used: the one-foot method, which is simpler and suits smaller tanks, and the variable-design-point method, which optimises each course as pressure decreases toward the top and is used on larger tanks to save significant steel tonnage. Thickness calculations also account for dead loads (shell, roof, insulation, appurtenances), live loads, wind and seismic effects, the specified corrosion allowance, and the material's allowable stress at design metal temperature.

Q: What corrosion allowance is used for API 650 tanks?
A: Corrosion allowance depends on the service. Typical values are 1.5 mm for sweet service and 3 mm for sour service, with greater allowances specified for high-corrosion applications based on a corrosion assessment. Corrosion allowance is added to the pressure-design thickness of shell courses, bottom plates, and roof plates — the minimum roof plate thickness of 5 mm, for example, is stated as including corrosion allowance. Beyond the allowance itself, protection measures are essential: cathodic protection on the underside of the bottom plate is mandatory since that surface cannot be inspected in service, internal coatings are selected for the product, and leak detection liners are commonly installed. Combining coatings with cathodic protection can extend effective corrosion allowance life past 30 years.

Q: What is API 653?
A: API 653 is the sister standard to API 650, governing in-service inspection, repair, alteration, and reconstruction of aboveground storage tanks throughout their operational life. While API 650 covers new construction, API 653 determines inspection intervals, defines acceptable repair methods and limits — such as weld-build overlay limited to 50% of shell thickness with maximum patch dimensions, and replacement insert plates required a minimum distance from existing weld seams — and sets criteria for continued service. It also applies to tanks built to unknown standards provided the geometry meets basic requirements. Risk-based inspection using API RP 581 allows operators to shift from time-based to condition-based inspection by calculating probability of failure from corrosion rate and consequence from potential spill volume.