81

June 2011 • TA N K S TO R A G E Floating rooF design

External floating roof or full contact internal floating roof? These tests

compare emission levels, maintenance costs and the impact the

decision has on working volumes

Decision timeFloating roofs have

been in existence for over half a century,

providing operators and regulatory bodies a reliable, time proven safety and emission control system.

The 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 do not 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 a

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 16psf. 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”-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”-24” above the pontoon level. For external floating roof pontoon designs, that total volume loss profile equates to about 42”-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 2.5psf. 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 8”-10” in diameter and immerse in the stored product approximately 4”-5”

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

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 % Volume

151,092 100%

113,319 75%

124,336 82%

11,017 7%

Total Tank Capacity

Steel Pontoon EFR

Full Contact IFR

Difference

Chart A

Based on 3'-6” Low Operating, 4'-0” Head Loss54” EFR vs 12” IFR

Issue 1 - May 2009

www.sandbornroofs.com

Typical floating roof designs

Tank working volume floating roof design

Floating rooF design TA N K S TO R A G E • June 2011

82

inches. Honeycomb Panels or composite foam filled panels float in full contact with the liquid surface and only immerse into the product by about 0.5”-1”. Internal floating roof systems only require a single liquid mounted seal, which typically extends above the liquid level by 8”-12”. For an internal floating roof, depending on the perimeter seal system used, the total volume loss profile will vary from about 9”-18” above product level.

The difference between a carbon steel external floating roof and an internal floating roof volume loss height is anywhere from roughly 36”-54”.

Depending on tank diameter, that represents a significant amount of working volume loss for identical sized aboveground storage tanks.

With all floating roof systems, the floating roof has to remain floating and not land to work effectively. Floating roof tanks have restrictions on how high the tank can be filled 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), vapour pressure, atmospheric 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 vapours 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 travelling across the top of the tank. internal floating roofs (IFR) 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 aupported 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 is 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 do

not 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 215ft by 60ft 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 the best solution. n

For more information:www.sandborn.ca

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 Pontoon

External Floating Roof

Full Contact

Internal 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 % Volume

151,092 100%

113,319 75%

124,336 82%

11,017 7%

Total Tank Capacity

Steel Pontoon EFR

Full Contact IFR

Difference

Chart A

Based on 3'-6” Low Operating, 4'-0” Head Loss54” EFR vs 12” IFR

Issue 1 - May 2009

www.sandbornroofs.com

Type

Years 1 5 10 15 20 25

Steel Pontoon EFR 7,480 37,400 74,800 112,200 149,600 187,000

Full 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

Years

Maintenance 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

• • • •

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•

Type

Years 1 5 10 15 20 25

Steel Pontoon EFR 7,480 37,400 74,800 112,200 149,600 187,000

Full 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

Years

Maintenance 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

• • • •

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•

*Materials Only, Averaged based on 20 year lifespan of seal & drain materials. 215’ dia. x 60’ Tank Size. 2008 Pricing. US Dollars

Based on 3’-6” Low Operating, 4’-0” Head Loss 54” EFR vs 12” IFR

Based on Crude Oil (RVP 5), Results generated from EPA Tanks 4.0.9d Software 215’ dia. x 60’ Tank Size

Chart A: the difference in net operating volume between apontoon EFR system and full contact IFRsystem

Chart C: 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

Chart B: a sample VOC emission calculation for a steel leg supported pontoon EFR versus a cable supported full contact aluminum IFR system