Technical Articles Impact on Working Volume, Emissions and Operating Costs Floating Roof Design 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. External Floating Roof (Image Courtesy of Mesa Rubber) Image 1 - Typical Floating Roof Designs Internal Floating Roof (Image Courtesy of Sandborn Roofs) Issue 1 - May 2009 www.sandbornroofs.com
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Technical Articles
Impact on Working Volume, Emissions and Operating CostsFloating Roof Design
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 a n y d i r e c t e x p o s u r e t o 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.
External Floating Roof(Image Courtesy of Mesa Rubber)
Image 1 - Typical Floating Roof Designs
Internal Floating Roof(Image Courtesy of Sandborn Roofs)
Issue 1 - May 2009
www.sandbornroofs.com
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. F loat ing 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 chemica ls (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 af fect ing 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 cons t ru c t i on ( i f i t s 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.
Image 2 - Tank Working Volume by Floating Roof Design
Steel PontoonExternal Floating Roof
Full ContactInternal Floating Roof
Low Fill Height
Max Fill Height
36” ~ 54” Loss
Emission Pathsat Leg Locations
Elevated Liquid Surface Temperaturein Direct Sunlight
Elevated Seal Losses caused by windand vacuum effect.
Fixed Roofs Shelters productfrom Direct Sunlight
and wind effects.
Sealed CablesNo Emission Paths
Technical Articles
150' dia. x 48' Shell Barrels % Volume151,092 100%113,319 75%124,336 82%11,017 7%
Total Tank CapacitySteel Pontoon EFRFull Contact IFR
Difference
Chart A
Based on 3'-6” Low Operating, 4'-0” Head Loss54” EFR vs 12” IFR
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TypeYears 1 5 10 15 20 25
Steel Pontoon EFR 7,480 37,400 74,800 112,200 149,600 187,000Full Contact IFR 5,490 27,450 54,900 82,350 109,800 137,250
Difference 1,990 9,950 19,900 29,850 39,800 49,750
Emission Losses (lbs per Year)
Chart B
Based on Crude Oil (RVP 5), Results generated from EPA Tanks 4.0.9d Software215’ dia. x 60’ Tank Size
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.
Technical Articles
Chart C
20 40
$144,700 $289,400
$54,700 $109,400
$90,000 $180,000Difference
YearsMaintenance Costs
Steel Pontoon EFR Perimeter Seals & Hose Drain
Full Contact IFR Perimeter Seals & Columns Seals
Materials
*Materials Only, Averaged based on 20 year lifespanof seal & drain materials. 215' dia. x 60' Tank Size.
2008 Pricing. US Dollars
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 d e s i g n a n d i t s o n g o i n g maintenance costs, emissions and operational limits all play an influential part of the process of deciding which system provides you the best solution.
About the Author
Sandborn Roofs is the manufacturer of the patented Sandborn full contact internal floating roof system.
Sandborn Roofs is also a major Canadian supplier of mechanical seal systems used on internal and external carbon steel floating roofs, foam log seal kits, wedge wiper seal materials and external floating roof hose drain systems.
Sandborn Roofs is a full service provider for floating roof installations and inspections with experienced staff to provide reliable and talented source of labor, supervisors and floating roof inspectors.
For more information on Sandborn Roofs and our products, visit our website at: www.sandborn.ca
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Issue 1 - May 2009
www.sandbornroofs.com
Sandborn Roofs Inc.801-25th Avenue
Nisku, Alberta, Canada
T9E 7Z4
Telephone 780 955 8761
Fax 780 955 8781
• • Toll Free 1 866 955 8781•
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