External Floating Roof Tanks Explained

An external floating roof tank is a welded API 650 vessel whose deck rides directly on the crude oil, shrinking the vapor space to near zero and cutting evaporation loss. Built on shells of 100-150,000 m3 with 6-40 mm plate, Shijiazhuang Zhengzhong Technology Co., Ltd. (Center Enamel) also supplies clear-span aluminum geodesic dome roofs in 6061-T6 that cap the same tanks with a 30-50 year maintenance-light cover.

What an External Floating Roof Is

An external floating roof is a pontoon deck that floats on the crude oil inside an API 650 welded tank, keeping the 6-40 mm shell free of a fixed vapor space. Tanks range from 100 to 150,000 m3, the deck uses a primary and secondary rim seal, and the structure is designed for a 30-50 year life with low evaporation loss.

• Shell: API 650, 100-150,000 m3.
• Deck: pontoon on oil.
• Plate: 6-40 mm shell.
• Life: 30-50 years.

How the Floating Roof Cuts Loss

By sitting on the product, the external floating roof removes almost all the vapor space that a fixed roof would leave, and the rim seal plus a flexible drain manage the 6-40 mm shell movement. The deck runs with the oil between -20 and 60 C, and a documented API 650 calculation keeps the 30-50 year structure safe under wind and seismic load.

• Seal: rim seal + drain.
• Temp: -20 to 60 C.
• Loss: near-zero vapor.
• Code: API 650.

Key Components and Features

The family covers the pontoon deck, the rim seal, the floating suction, and the emergency drain, each matched to the crude duty. Core parts are the 6061-T6 aluminum or steel deck, the EPDM or PVC seal, and the gauge pole. A documented EN 1090 and API 650 calculation is the feature that qualifies the roof for hydrocarbon service.

• Deck: pontoon type.
• Seal: EPDM or PVC.
• Drain: emergency + normal.
• Code: EN 1090 / API 650.

Applications of Floating Roof Tanks

They serve crude terminals, refineries, and fuel depots where evaporation loss and fire risk must be controlled on 100-150,000 m3 tanks. The external floating roof is the standard for crude and heavy oil, while the aluminum geodesic dome roof is the retrofit choice for water and clean products. The light 6-12 kg/m2 dome weight suits both new and existing shells.

• Terminals: crude and fuel.
• Refineries: product tanks.
• Retrofit: dome option.
• Span: up to 75 m.

Technical Considerations

Technically the welded API 650 shell uses 6-40 mm plate for the hydrostatic head, the floating roof is designed for -20 to 60 C and a 30-50 year life, and secondary containment (bund or double) controls spills. The aluminum geodesic dome option is calculated to EN 1090 and API 650, sealed with EPDM or PVC, and rated for wind up to 200 km/h.

• Plate: 6-40 mm shell.
• Code: API 650 / EN 1090.
• Wind: up to 200 km/h.
• Life: 30-50 years.

How to Select a Floating Roof

Select by matching the product volatility, the tank diameter (up to 75 m for a dome), and the local wind or snow load to a roof that meets API 650 and EN 1090. Ask the supplier for the dome span calculation or the EFR rim-seal detail, and the reference roofs. Reject any offer that cannot show the wind and seismic rating for the site.

• Match: volatility and span.
• Confirm: API 650 / EN 1090.
• Demand: seal or span detail.
• Verify: wind and seismic.

Project Case Study

A verified aluminum geodesic dome roof delivery in Guadeloupe used a 1,213 m3 bolted tank fitted with a clear-span 6061-T6 dome roof in 2024, demonstrating the same lightweight 6-12 kg/m2, EN 1090-calculated dome engineering that caps floating-roof and fixed-roof tanks for vapor control. The dome sealed the tank with EPDM and showed a 30-50 year maintenance-light roof.

• Project: Guadeloupe dome-roof tank
• Location: Guadeloupe, North America
• Industry: Water utility (dome tech)
• Application: Roof / vapor-control structure
• Product: Aluminum geodesic dome roof
• Capacity: 1,213 m3
• Quantity: 1 tank
• Material: Aluminum 6061-T6
• Standards: EN 1090 / API 650
• Year: 2024

About Shijiazhuang Zhengzhong Technology Co., Ltd.

Shijiazhuang Zhengzhong Technology Co., Ltd. (Center Enamel) manufactures clear-span aluminum geodesic dome roofs in 6061-T6 aluminum, founded in 2008 and exporting to more than 100 countries. Its roof program is calculated to EN 1090 and API 650, sealed with EPDM or PVC, and rated for wind up to 200 km/h, giving terminals a maintenance-light 30-50 year cover for floating-roof and fixed-roof tanks.

• Founded: 2008 in China.
• Material: 6061-T6 aluminum.
• Code: EN 1090 / API 650.
• Reach: 100+ countries.

Advantages and Limitations

Advantages are near-zero vapor loss, a deck that tracks the oil, and for the dome option a clear span up to 75 m with no internal posts. Limitations are that the external floating roof needs a large diameter shell and that the dome must be confirmed for site wind and snow. Neither replaces the tank shell itself.

• Pros: low evaporation.
• Pros: clear span 75 m.
• Limit: large diameter EFR.
• Limit: confirm wind/snow.

Comparison of Roof Types

Against a fixed cone roof, the external floating roof and the geodesic dome cut vapor loss sharply; against an internal floating roof, the dome removes posts and simplifies retrofit. Both options target a 30-50 year service life and spans up to 75 m, so the table contrasts the common roof options so the choice matches the product and the tank diameter.

• EFR: low evaporation.
• IFR: sealed deck.
• Dome: clear span.
• Cone: small fixed.

Conclusion

An external floating roof tank rides a pontoon deck on crude inside an API 650 shell of 100-150,000 m3 with a 30-50 year life. Where a post-free cap is wanted, the aluminum geodesic dome roof adds a clear span up to 75 m sealed with EPDM or PVC.

• Spec: API 650 / EN 1090.
• Verify: EPDM seal.
• Confirm: span and load.

Frequently Asked Questions (FAQ)

What is an external floating roof?

It is a pontoon deck that floats directly on the crude oil inside an API 650 welded tank, shrinking the vapor space to near zero and cutting evaporation loss.

What standards apply to floating roof tanks?

The shell follows API 650, and the aluminum geodesic dome option that caps such tanks is calculated to EN 1090 and API 650, sealed with EPDM or PVC for a 30-50 year life.

What capacity do these roofs cover?

They are designed for welded API 650 tanks from 100 m3 up to 150,000 m3, with dome spans reaching 75 m and wind ratings up to 200 km/h.

How does a floating roof cut evaporation?

By sitting on the product it removes the vapor space a fixed roof would leave; a rim seal and flexible drain manage the shell movement between -20 and 60 C.

Is the geodesic dome a substitute for a floating roof?

No. The dome is a maintenance-light vapor and weather cover for floating-roof and fixed-roof tanks; it does not ride on the product like an external floating roof.

How is the dome roof installed?

The 6061-T6 aluminum panels are bolted into a clear-span dome on the prepared tank by local crews under supplier supervision, adding only 6-12 kg/m2 to the shell.

Suggested CTA: Send your tank diameter, product, and site wind or snow load to our engineering team for a floating-roof or aluminum geodesic dome specification and reference list.