Floating roofs have been in existence for over half a century, providing operators and regulatory bodies a reliable, time proven safety and emission control system.   Floating roofs function by reducing the formation and release of environmentally harmful and potentially explosive volatile organic chemicals in above ground storage tank systems.

Floating roof design generally varies with the material used for construction.  External floating roofs which are exposed to the elements use carbon steel construction with pontoon or double deck designs.   These designs allow the floating roof to support potentially heavy loads of rain and snow and are also usually coated to prevent corrosion from weather exposure.  Carbon steel floating roofs use heavy leg supports when landed as well as large air-filled `compartments` known as pontoons for buoyancy.   External carbon steel floating roofs also incorporate some form of articulating pipe or flexible hose deck drainage system to remove excess precipitation from their surface.

For tanks that use fixed covered roofs, like steel cone roofs or aluminum geodesic domes, the internal floating roof systems are typically constructed using light-weight materials because they don`t have to support the environmental loads placed on the external floating roof systems.  While materials vary, internal floating roofs typically use aluminum as their structural material, though some use fiberglass, thin gauge stainless steel or other composite panels.   Depending on design, internal floating roof systems can usually be cable supported from the tank fixed roof structure, due to their low mass.  

One of the key differences between tanks with carbon steel external or aluminum internal floating roof is their working volume.  Each floating roof has a different cross sectional area displacing volume that would otherwise be filled with stored product, translating into lost working volume due to the profile depth of the floating roof system.

Carbon steel external floating roofs use large perimeter pontoons to not only support environmental live loads but also their own mass, which varies from about 12 to 16 psf.  Pontoon thickness varies based on design and tank diameter but generally ranges from 32” to 48” in depth.   The weight of the floating roof will immerse the pontoon anywhere from 4” to 12”.  Also, external floating roofs require primary and secondary perimeter seals to reduce emissions in the rim area (area between the pontoon and tank shell wall).   The primary seal is typically contained within the pontoon rim area while the secondary seal extends anywhere from 18” to 24” above the pontoon level.    For external floating roof pontoon designs, that total volume loss profile equates to about 42” to 72” or more above product level.

Internal floating roofs do not have any direct exposure to environmental loads like rain or snow, and are typically very light in mass, varying from about 1 to 2.5 psf.  Internal floating roofs use either air filled aluminum or stainless steel pontoons, air filled honeycomb panels or composite foam filled panels for buoyancy.   Air filled pontoons vary in size but are typically eight to ten inches in diameter and immerse in the stored product approximately 4” to 5” inches.   Honeycomb Panels or composite foam filled panels float in full contact with the liquid surface and only immerse into the product by about ½” to 1”.   Internal floating roof systems only require a single liquid mounted seal, which typically extends above the liquid level by 8” to 12”.  For internal floating roof a design and depending on the perimeter seal system used the total volume loss profile will vary from about 9” to 18” above product level.

The difference between a carbon steel external floating roof and a internal floating roof volume loss height is anywhere from roughly 36” to 54”.   Depending on tank diameter, that represents a significant amount of working volume loss for identical sized above ground storage tanks, as shown in Image 2.

With all floating roof systems, the floating roof has to remain floating and not land to work effectively.  Floating roofs tanks have restrictions on how high you can fill the tank with product.  External floating roof systems need to operate below the tank shell or foam ports.  Internal floating roof systems must not make contact with the underside of the tank fixed roof or support rafters.  As a result, the net capacity of tanks is restricted to a certain minimum (low) and maximum (high) operating levels.

Chart A shows the difference in net operating volume between a pontoon External Floating Roof (EFR) system and full contact Internal Floating Roof (IFR) system.  In this example, the IFR systems adds 7% more Net tank operating volume compared to the EFR systems on the same sized tank and operating low/high level conditions.

Floating roof design also has a significant impact on the total amount of volatile organic chemicals (VOCs) product emission losses.   VOC emissions vary based on several conditions including tank turnovers (shell losses), vapor pressure, atmos-pheric conditions (heat/wind) and floating roof design.   The floating roof components affecting emission loss include the perimeter seal design (single/double), support type (legs/cables/grid), penetration seals  for columns and ladders, as well as the floating roof deck construction (if its bolted/welded/sealed together).

Carbon steel EFR systems require steel legs for supports when landed due to their significant mass.   Most EFR systems use an adjustable leg with support collar penetration sleeves to allow the operators to increase or decrease the landed height from low operating to a higher maintenance position allowing clearance for workers underneath.  Adjustable leg supports provide an emission path for VOC vapors to escape into the atmosphere.

Aluminum Internal floating roofs can use thinner legs supports or suspended cable supports from the tank fixed roof.  Suspended cable supports are adjusted from the top of the tank fixed roof and have no emission paths from the sealed deck connections.

External Floating Roofs are also subject to higher overall emission losses due to the heating effects of solar radiation and vacuum effects of wind traveling across the top of the tank.   Internal Floating Roofs are sheltered from the direct wind and sunshine and as a result environmental factors have a much smaller impact on the VOC emission loss on IFR tanks.

Chart B is a sample VOC emission calculation for a Steel Leg Supported Pontoon EFR versus a Cable Supported Full Contact Aluminum IFR system on the same size tank.  In this sample, the EFR equipped tank system shows 27% higher emission rates than the IFR equipped tank system.

Another impact of floating roof design is the ongoing maintenance costs for the floating roof seals and drainage systems.  Perimeter seals on both designs, external or internal floating roofs may require replacement in approximately 10 to 20 year cycles.   EFR perimeter seal systems are larger than IFR seal systems and are significantly more expensive to replace on an on-going basis.  Some IFR storage tanks use several steel columns to support the fixed steel cone roof system, so each column will have a corresponding penetration seal that are typically replaced at the same time as the perimeter seal system.

External floating roof system tanks require a drain system, consisting of either an articulated pipe or flexible hose to drain the rainwater and melted snow off the surface of the floating roof.   These drainage systems need to be replaced after a few maintenance cycles to ensure they don`t leak and result in rainwater contamination of the stored product or possible contamination of groundwater (and the corresponding disposal costs).

Chart C shows the approximate maintenance costs of replacing the perimeter seal, penetration seal and drain systems on a large 215` x 60` diameter tank in 20 year service intervals.

In this example, the on-going seal maintenance costs of the IFR system is approximately 37% the equivalent costs of the EFR seal maintenance.   Depending on the design and construction quality of the aluminum IFR system, their expected life-span can be as long as their steel EFR counterparts.

While these comparisons concern floating roof design, tank operators will have to weigh the operational and lifetime benefits versus the difference in capital costs of tank construction between the two systems.

Both systems offer industry-proven solutions to safe storage of VOCs, however the floating roof design and its ongoing maintenance costs, emissions and operational limits all play an influential part of the process of deciding which system provides you the best solution.