Aluminum Floating Roofs: Design, Seals and Emission Control

Aluminum floating roofs are used in industrial storage tanks to reduce the vapor space above stored liquids and support the control of evaporation losses and volatile organic compound emissions.

Unlike a conventional fixed roof, an internal floating roof rests on or floats close to the liquid surface inside the tank. As the product level rises and falls, the roof moves with it.

The roof itself is only one part of the system. Effective performance also depends on the rim seal, deck fittings, tank geometry, stored product, inspection regime and, in many projects, an aluminum geodesic dome installed above the floating roof.

How aluminum floating roofs control vapor space

A fixed-roof tank can contain a substantial vapor space above the liquid. Product movements, temperature changes and pressure variations can contribute to vapor displacement.

An internal floating roof follows the liquid surface, reducing the volume of the vapor space directly above the product.

The roof does not create a perfectly sealed system by itself. Vapors can still pass through:

• The annular space between the roof and tank shell.
• Gaps in the rim seal.
• Columns and guide poles.
• Gauge wells.
• Roof legs.
• Hatches.
• Deck seams.
• Damaged membranes.
• Worn or incorrectly installed fittings.

Internal and external floating roofs

Pontoon aluminum floating roofs

A pontoon floating roof uses buoyant compartments connected by a structural deck or grid. The pontoons provide flotation while the deck covers the liquid surface.

Potential reasons for selecting a pontoon design include:

• Low structural weight.
• Modular installation.
• Access through existing tank openings.
• Compatibility with many tank geometries.
• Replacement of individual components.
• Integration with different rim-seal systems.
• Use in new tanks or retrofit projects.

Full-contact aluminum floating roofs

A full-contact roof places the underside of the deck directly against the stored liquid over most or all of its surface.

Possible advantages include:

• Reduced vapor space beneath the deck.
• Direct support from the stored liquid.
• Modular panel construction.
• Reduced number of open deck spaces.
• Suitability for selected high-volatility products.
• Integration with rim seals and tank fittings.

Pontoon versus full-contact roof

Why the floating roof seal is critical

A floating roof must move freely as the liquid level changes. This requires a gap between the outer edge of the roof and the tank wall.

The floating roof seal closes this annular space while allowing vertical movement.

Its performance depends on maintaining contact despite:

• Shell irregularities.
• Weld seams.
• Changes in rim space.
• Roof movement.
• Product turbulence.
• Temperature changes.
• Mechanical wear.

Primary floating roof seals

Seal selection should consider product compatibility, rim-space variation, shell condition, installation method and inspection access.

Secondary and external floating roof seals

A secondary seal is installed above the primary seal. It adds another barrier across the rim space and can help protect the primary system from weather, debris and direct exposure.

Secondary seals may be relevant when:

• Required by the applicable regulation.
• The tank stores volatile products.
• The primary system needs additional protection.
• Emission-control performance must be improved.
• The tank is being upgraded during maintenance.
• Rim gaps need better coverage.

How a compression plate seal works

A compression plate seal uses flexible or sprung metal plates that press a sealing membrane or lip against the tank wall.

The plates accommodate movement and variations in the shell while maintaining distributed contact around the tank circumference.

• Tank diameter.
• Available rim space.
• Shell irregularity.
• Roof movement.
• Stored product.
• Chemical resistance.
• Wind and weather exposure.
• Installation constraints.
• Inspection requirements.
• Replacement strategy.

Internal floating roof seals

Internal roofs also require a seal around their perimeter.

Possible configurations include:

• Mini-shoe primary seals.
• Flexible primary seals.
• Foam-filled seals.
• Wiper secondary seals.
• Column seals.
• Flexible gaiters.

The engineer should also review:

• Column seals.
• Ladder wells.
• Gauge poles.
• Roof legs.
• Deck penetrations.
• Anti-rotation devices.
• Pressure-vacuum behavior.
• Fixed-roof ventilation.

The role of an aluminum geodesic dome

An aluminum geodesic dome is a self-supporting fixed cover formed by a triangulated structural system.

When installed above an internal floating roof, the dome can:

• Protect the roof from rain and snow.
• Limit water ingress.
• Reduce exposure to wind and debris.
• Remove the need for internal roof columns in many designs.
• Provide a lightweight cover for existing tanks.
• Support modular installation.
• Reduce weather-related maintenance.

Benefits of an integrated roof, dome and seal system

Information required before specifying a system

1. Tank dimensions. Diameter, height and operating levels.
2. Existing roof. Fixed, external floating roof or existing dome.
3. Tank construction. Shell material and applicable construction standard.
4. Stored product. Composition, aromatic content and vapor pressure.
5. Operating conditions. Temperature and minimum and maximum liquid levels.
6. Tank geometry. Out-of-roundness, internal columns and supports.
7. Fittings. Nozzles, gauge poles, ladders, legs and penetrations.
8. Existing seal. Type, condition and available rim space.
9. Environmental loads. Wind, snow and seismic conditions.
10. Access. Manholes, lifting restrictions and installation area.
11. Shutdown plan. Whether the tank can be taken out of service.
12. Regulatory framework. Environmental, safety and inspection requirements.

Retrofit considerations

Installing a new aluminum floating roof or seal in an existing tank requires more than checking the nominal diameter.

The assessment should examine:

• Shell deformation.
• Internal obstructions.
• Existing corrosion.
• Roof clearances.
• Tank-bottom condition.
• Access through the manhole.
• Removal of old components.
• Hot-work limitations.
• Ventilation and confined-space procedures.
• Installation while in service or out of service.
• Compatibility with the fixed roof or dome.
• Future inspection access.

Inspection and maintenance

Floating roof performance can change over time due to mechanical wear, chemical exposure and tank-shell movement.

Inspection should consider:

• Visible seal damage.
• Rim gaps.
• Membrane condition.
• Plate deformation.
• Shoe contact.
• Loose fasteners.
• Pontoon integrity.
• Panel condition.
• Deck fittings.
• Column seals.
• Drainage.
• Roof level.
• Product above the deck.
• Corrosion or deposits.

The facility should follow the applicable legal requirements, tank standard, manufacturer instructions and site inspection plan.

TAC Enviro storage-tank systems

TAC Enviro publishes a portfolio that includes:

Technical note: final roof, dome and seal selection must be based on project-specific engineering, product compatibility, applicable standards and local regulatory requirements.

Frequently asked questions