Aluminum-floating roofs can achieve longer lives with improved design

June 1, 1998
Comparion of Internal-Floating Roofs [123,180 bytes] Users have many reasons for selecting various aluminum and steel floating-roof designs for aboveground storage tanks (AST). The American Petroleum Institute (API) 650 standard defines a minimum quality which permits users to select the lowest-cost alternative. When safety and total costs are most important, however, an improved design with higher costs may result in a substantially extended life.
Philip E. Myers
Chevron Products Co.
San Ramon, Calif.

George L. Morovich
Tank & Environmental Technologies Inc.
The Woodlands, Tex.

Earl J. Crochet
Plantation Pipe Line Co.

Users have many reasons for selecting various aluminum and steel floating-roof designs for aboveground storage tanks (AST).

The American Petroleum Institute (API) 650 standard defines a minimum quality which permits users to select the lowest-cost alternative. When safety and total costs are most important, however, an improved design with higher costs may result in a substantially extended life.

This conclusion of a two-part series compares the use of aluminum vs. steel for internal-floating roofs. The first article of this series compared covered tanks vs. uncovered tanks as well as aluminum vs. steel for fixed roofs (OGJ, May 18, 1998, p. 66).

The most common considerations for aluminum and steel floating-roof purchases are low capital and maintenance costs, long service life, compatible service conditions, product quality, and manufacturer warranties. Based on the user's experience and needs, the user specifies a minimum level of quality.

Internal-floating roofs

Whether constructed of steel or aluminum, an internal-floating roof is used to control evaporative loss of the stored product. The floating roof contains a buoyant deck surrounded by a peripheral seal. The seal must be suitable for the anticipated service and resistant to wear against the tank shell for thousands of cycles.

Internal-floating roof tanks meet requirements of API 650-Appendix H or API 650-Appendix C (for external-floating roofs which have been covered with fixed roofs).

Internal (Appendix H) types are designed for lower live loads than those of Appendix C. At a minimum, design live loads are 500 lb over 1 sq ft (at any point on the surface), and buoyancy is provided for 200% of dead load. Supports are designed to support a 12.5 psf live load, but this requirement may be eliminated with the use of floating-roof deck drains.

Although API 650 requires the floating roof design to support a 500 lb load, the standard permits this requirement to be waived for small-diameter tanks. This waiver is a concern if walking on the deck is permitted. For some companies, confined space entry on the floating-roof deck is prohibited under all operating or static circumstances to protect personnel.

The fact that the standard permits the design specific gravity to be increased presents another concern. Increased specific gravity reduces buoyancy. It is likely that some internal-floating roofs designed for a high specific gravity service, which cost less as a result of lower buoyancy, have been switched to a lighter product service without considering the resulting reduction of buoyancy.

The impact of the reduced buoyancy is magnified for aluminum roofs because of their low mass. The aluminum skin-and-pontoon roofs also introduce a reduced comfort factor because they are suspended over the product using a very thin membrane.

The internal-floating roof has a structural support system, which is primarily used only when the tank is removed from service. When the supports are engaged, the floating-roof relief vents open, eliminating the function of the deck as an emissions-control device.

Decks with two support elevations are desirable. During normal operation, the low-deck position maximizes tank-working capacity. This low-operating level is usually dictated by the elevation of inlets or the shell obstructions. The higher second position allows for tank-bottom maintenance, which requires 6-7 ft of clearance so workers can work upright under the roof.

Steel internal floating roof characteristics

Except for the steel-pan roof, which has a history of serious problems, steel roofs are usually durable for violent tank operations (e.g., the presence of gas bubbles, heavy mixing, high rates of pumping, and flashing liquids) because of their mass.

Because internal-floating roofs are required to have 100% excess buoyancy, steel use results in at least seven times more excess buoyancy than that required for aluminum.

Without bolted deck seams, which aluminum roofs have, welded-steel construction results in reduced emission variables. There is little possibility of liquid entering through deck seams onto the top of the roof. Liquid on the deck is a fire hazard.

Steel internal-floating roofs are generally the highest cost of all floating roofs. Cost for steel can be up to five times the initial cost of an aluminum-floating roof. Although steel has had a good history of durability, when it does fail, the repairs can be costly.

Although steel plate offers excellent corrosion resistance for most refined-product service (like caustic) they can corrode in some services. Consideration of a corrosion allowance, protective coating, and special weld details may be necessary when the steel is exposed to oily wastewater or sulfur compounds. Corrosion of steel is most aggressive with surfaces exposed to vapors (e.g., the outer rim and support housings) or those exposed to acidic-water bottoms (e.g., support legs).

A hydrotest is sometimes required to retrofit steel roofs because of the large shell openings required to construct the roofs.

During a fire, a partially sunk steel internal-floating roof blocks access to fire fighting foam, which makes the fire difficult to extinguish.

Aluminum internal- floating roof

Although most aluminum internal-floating roofs (AIFRs) meet the minimum requirements of API 650-Appendix H, many have performed poorly. Problems have been encountered as a result of aluminum's lower resistance to turbulence (lower mass), design deficiencies, poor structural details, and corrosion. A properly designed AIFR should meet the anticipated life of a steel roof.

The AIFR is a very low-cost system compared to steel roofs. When the selection is based strictly on cost, without any requirements beyond the minimum standards, the AIFR will have a reduced life expectancy. Higher standards will result in higher cost but longer life.

The aluminum-floating roof does not offer the high mass of steel to resist effects of turbulence. Turbulence is often an operational problem which can be avoided by using filling procedures which slow initial fill rates with inlet diffusers, minimizing gas slugs.

The presence of liquid on the deck can result from design deficiencies. One deficiency is using a design specific gravity above 0.7. Although API 650 suggests a specific gravity for proper operation in a full range of hydrocarbon service, it permits use of a higher specific gravity, which will reduce cost along with buoyancy and service flexibility.

Buoyancy problems attributed to excessive rim-seal friction are also design deficiencies. During seal replacements, a thorough inspection of the roof is required with special attention to any addition of weight or friction to the seal.

Another design deficiency results in structural buckling. The user should be aware that API 650 does not currently require that designs be based on any published data for fabricated aluminum alloys. This may mean that no factors of safety are used for design.

A large percentage of existing aluminum-floating roofs have pontoons which contain hydrocarbons and are a fire hazard during internal tank hotwork. Contaminated pontoons can be the result of poor weld procedures, faulty joint details, or corrosion. Corrosion of aluminum in liquid hydrocarbon service is difficult to detect.

Although an aluminum-internal-floating roof may be more susceptible to damage than steel, even the poorest designs rarely completely sink and can often be repaired. Additionally, the installation time, once fabricated, is far less than that for steel roofs.

All variations of AIFRs can be provided with a cable-supported roof system. This system allows for roof-level adjustments and floating-roof leg adjustments without confined space entry, as well as avoidance of emission factors for supports and ladders. In addition, the system results in unobstructed access across the tank bottom for future inspection and maintenance. It also eliminates the potential for the roof to spiral down, eliminates the potential for the supports to interface with corrosive water bottoms, and protects the bottom coatings from contact by the supports.

Types of steel internal-floating roofs

There are two basic types of steel internal-floating roofs: covered-external floating roofs (API 650-Appendix C) and lower-live load internal-floating roofs (API 650-Appendix H). There are four types of steel-floating roofs (Fig. 1 [82,086 bytes]):
  • The steel-pan roofs (Appendix C or Appendix H.2.a)
  • The steel-bulkheaded (with open-top compartments) roofs (Appendix H.2.b)
  • The steel-pontoon roofs (Appendix C or Appendix H.2.c)
  • The steel double-deck roofs (Appendix C or Appendix H.2.d).

Covered external floating roof (Appendix C)

Appendix C roofs are pan, pontoon, or double-deck external roofs that have been retrofit with a fixed roof. These external floating roofs were designed for a 25 psf live load while resting on supports.

The covered external-floating roof offers several advantages. Except for the external-pan roof (which is an obsolete design), these are the least likely internal-floating roofs to fail. The external live-load design gives Appendix C roofs deep rim, seal, and deck fittings and this leads to the lowest emission rating for these components.

Concerning disadvantages, this roof will have the highest initial cost and the greatest reduced-tank working capacity as a result of displacement (roof distance below the liquid surface) and profile (roof distance above the liquid surface). The design uses the most expensive peripheral seals (approx. 36-42 in. deep vs. 18 in. or less for Appendix H roofs), which impacts maintenance as well as initial cost.

Steel internal floating roof (Appendix H)

Appendix H roofs include pan, open-top bulkheaded, closed pontoon, or double-deck designs. Except for surfaces exposed to vapors or liquids, a lighter-gauge steel is permitted (minimum 0.094 in.) for Appendix H roofs than for that of Appendix C.

The advantages and disadvantages of the four steel internal-floating roof variations are:

  • Steel pan. The pan roof has no inherent buoyancy. A single leak can sink it. Flotation depends entirely on rim displacement because there are no internal bulkheads or pontoons. Except for the steel-pan roof, all other internal-floating roofs are designed to remain floating if any two compartments are flooded (See API 650-Appendix H.5.1.2.). As a result of the lack of compartmented buoyancy, this roof is highly subject to sinking due to turbulence, binding on columns, corrosion, faulty welds, or stress cracks. It is flexible and easily buckles. Many users prohibit the use of this type of floating roof because of serious failure histories and consequences. This roof, however, has the least expensive initial cost of the steel internal-floating roof options.

  • Steel bulkheaded with open-top compartments. The flotation of bulkheaded roofs depends on rim displacement with open-top interior bulkheads. This roof has enhanced buoyancy because of its individually bulkheaded compartments. The inner rim can provide sufficient strength to resist buckling forces if designed properly. Visual inspections from tank top can identify when liquid is on the deck allowing time for repairs. A disadvantage is that excess fire-fighting water or foam could sink the roof.

  • Steel pontoon. Flotation depends on rim displacement with bulkheaded-closed peripheral pontoons. This design is less susceptible to becoming awash with product, but the covered and seal-welded peripheral pontoons result in additional cost.

  • Steel double deck. Flotation depends on rim displacement with an interior bulkheaded double deck. This design is the least susceptible to becoming awash with product, but the covered and seal-welded bulkheaded compartments result in the highest cost. This design is preferred for turbulent applications.

Types of aluminum internal-floating roofs (AIFRs)

There are two basic types of aluminum internal-floating roofs: the honeycomb-sandwich (contact) panel and the skin and pontoon. Fig. 2 (7,961 bytes) Pontoons under the aluminum skin are supported with a grid structure. This skin-and-pontoon roof is under a steel-cone roof, identifiable by the columns holding up the roof. Photo courtesy of Matrix Service Inc. and Fig. 3 (10,352 bytes), Fig. 3a (5,686 bytes) Aluminum-honeycomb panels are laid in a checkered pattern. Unlike cone roofs, dome roofs allow a clear span under the fixed roof. Photo courtesy of Allentech Inc. , and Fig. 3b(8,871 bytes) show photos of these two designs. In addition, there are two types of honeycomb-sandwich panels: the adhesive-bonded panel design and the welded-panel design.

Honeycomb- sandwich panel

The aluminum sandwich panel is a full surface-contact design. It was preceded by a design (dating back to the late 1950s) which used rigid closed-cell urethane-foam panels (joined by aluminum tape), with a very thin aluminum skin. The original design evolved into two variations:
  • An adhesive-bonded panel (developed in the 1970s), which uses adhesives to seal panel modules
  • A welded panel (developed in the 1990s), which replaced exposed adhesive sealants with welded joints.
The panel-module skin of both designs is a laminated-honeycomb panel core. The panel core is laminated with aluminum skin, and the module is framed by structural extrusions.

The completed panel modules use structural frames which are clamped together in the field by bolting and a batten (with an elastomeric gasket).

The honeycomb walls, which provide panel strength, are bonded together with an adhesive which is not designed for hydrocarbon exposure. Therefore, it is mandatory that the panel modules maintain integrity or the honeycomb walls will delaminate.

Entry of hydrocarbons into the cells can be caused by either penetration through a small corrosion hole in the skin or failure of the adhesive polymer (in the adhesive-bonded panel design) at the edges where the panels are framed.

The honeycomb-floating roof can be assembled and installed through a roof-access panel quickly and easily. There is no need for a door sheet in the tank shell to install the aluminum roof, and a hydrotest may be avoided.

The honeycomb structure can transfer heat quickly in a fire so that the panels do not melt. Tests conducted by two manufacturers show this phenomenon, and NFPA (National Fire Protection Association) 11 is being modified to reflect this. These roofs function as a blanket during a fire to prevent spread to most of the liquid surface. However, with little buoyancy the weight of fire fighting water and foam could cause submergence allowing liquid on top of the panels and sustain a fire.

Lower profile and less displacement result in a 12-40 in. increase of tank working capacity over steel floating roofs. For small-diameter tanks, as a result of full surface contact and self-buoyant panels, the honeycomb-sandwich roof may be preferred over the skin-and-pontoon aluminum roof. Also, for small-diameter installations, because fixed costs are more significant, making material cost less of a factor, honeycomb-sandwich roofs may be more competitive.

Adhesive-bonded vs. welded panels

The adhesive-bonded panels are laminated with 0.014-in. panel skins (the minimum per API 650-H.4.4.e) and glued to panel frames with an adhesive polymer sealant.

Welded panel modules are laminated with heavier 0.040-in. panel skins, which is over 2.5 times the minimum panel thickness required. Panels are attached to the panel frames by welding (without exposed adhesives). Panel-module welds are shop pressure/soap bubble tested and designed with a test port for field verification of panel integrity.

The exposed-polymer adhesives used for adhesive-bonded panels must be compatible with the material being stored. Although the effects of fuel additives and temperature variations are not always known, the standard adhesive is known to fail in products with an elevated sulfide content. While the adhesives seem compatible with jet-fuel service, massive failures have occurred in crude-oil service as a result of separation of the panel frames and skins.

The inherent buoyancy of the adhesive-bonded panel is at least 1 in. The welded-panel design has a buoyancy of 2.8 in.

Additional buoyancy has been added to adhesive-bonded panel designs by using extra pontoons, but this has not been necessary for the welded-panel design. Excessive friction at the edge of adhesive-bonded panel roofs has sunk the roof into the liquid or lifted the roof making a gas pocket under the roof. Again, this is not a reported problem with the welded-panel design, which is a heavier panel construction with 350% buoyancy.

The welded-panel design roofs are equipped with perforated cells and a test plug to shop-test weld seams at the time of manufacture and to field verify that the panels are gas free upon future inspection. There is no easy way to inspect the cells of the adhesive-bonded design for the presence of fuel.

Aluminum-pontoon roof

The aluminum skin-and-pontoon roof has its deck above the liquid resulting in a saturated vapor space beneath the deck. This roof, which dates back to the early 1970s, uses a pontoon and structural grid to support an aluminum skin.

There are many poor-quality, pontoon-type, internal-floating roofs in service today. Their existence may be encouraged because the aluminum-pontoon roof is the lowest-cost floating roof alternative. Many roofs meet only the minimum requirements-or less, if a lower specific gravity was identified at the time of purchase.

Specifications for pontoon roofs should be set above minimum requirements if improved quality and an extended service life is desired.

Pontoons can fail from very small corrosion pinholes or poor welds that are not readily detectable by visual inspection. These holes may be the result of improper alloy, pontoon design, or service conditions.

Poor construction details allow fatigue cracking of the support-deck structure, which permits liquid to enter the pontoons. When hotwork or maintenance is performed within the tank, there is a probability of trapped liquids in pontoons. All flotation compartments (pontoons) should be manufactured with vapor test ports, located above the liquid surface, to test for hydrocarbon vapors before hotwork.

Although the deck is usually considered too thin to safely walk on, roofs should be designed for the 500 lb load specified by API 650 and equipped with flotation based on a design specific gravity of 0.7. Limits for spacing of the structural-grid members can eliminate some of the safety concerns. Corrosion or an initiated tear could result in personnel falling through a panel.

In a fire, the deck would melt, yielding a full-surface fire. In a fire, it is desirable for any components located above the liquid surface to melt away and provide access to the fire surface. However, the pontoons create problems with foam distribution and flame reignition. Internal-floating roof fires, however, are extremely rare.

In addition to the general advantages for aluminum roofs, the skin-and-pontoon roof has the advantage that it can be installed in tanks through very small openings, for example, a 20 in.-shell manway.

Pontoon roofs have low initial costs and a wide range of manufacturing designs. The fabrication and installation time can also be one of the quickest available.

Upgrades for the aluminum-pontoon floating roof

Several specifications, beyond the API minimum standards, can help aluminum-pontoon floating roofs to achieve longer life. Users should pay close attention to specific gravity, structural, material, and details:
  • Design specific gravity should be based on 0.7 for the internal-floating roof (to allow for adequate buoyancy and operation in a full range of hydrocarbon service). Purchasers often state the product specific gravity, but the floating roof design specific gravity should also be identified. Unfortunately, AIFRs purchased as the low bid may have reduced flotation.
  • Pontoon attachment details can be enhanced by reinforced pontoon end caps, with a minimum thickness (0.100 in.) of a corrosion resistant alloy (AA5000 series), and appropriate nonfatiguing details.

    Welding of aluminum should be performed by ASME-certified welders. Weld details and all structural components should be designed in accordance with the "Aluminum Design Manual: Specifications for Aluminum Structures," as published by the Aluminum Association.

  • Maximum spacing limits between pontoons should be 3.05 m (vs. 6.4 m observed) and between deck girders should be 1.55 m (vs. 2 m observed) to improve distribution of support-grid components.
  • Lengths of the pontoons should be limited to 7.5 m (vs. 11 m observed).
  • Only aluminum and stainless steel hardware should be used (no plated fasteners).
  • Maximum spacing for decking should be set to 60 in., a commonly available width. Although the use of wider decking will reduce the number of deck seams, it will also increase spacing between the deck structural-grid members (reducing the ability to walk near the girders).
  • Deck surface sheeting can be upgraded by requiring a higher minimum thickness and a corrosion-resistant alloy (e.g., 0.025 in., AA3003 Alclad).
  • Alloy use requires special attention. The Aluminum Association designates AA3000 and AA5000 series as corrosion-resistant. Although the AA6000 series is an excellent structural grade, it experiences light pitting and should not be used as a flotation component. For added corrosion protection, an Alclad material is economically used for decking. Alclad material is a composite AA3000 series alloy with an AA7072-alloy (low silicone/magnesium, with 1% zinc) surface which is corrosion resistant and exerts a galvanic effect protecting the core material.
  • An extruded structural rim (to distribute loads and buoyancy at the perimeter) with a minimum thickness of 0.100 in., which is standard minimum thickness for several manufacturers, should be required.
  • A stainless steel mechanical-shoe seal (for the best seal life) with a fabric vapor barrier (such as a reinforced flouropolymer) that is durable and compatible with a wide range of products should be required.
  • Use of 8-in. diameter pontoons results in over 30% more length to provide the same amount of buoyancy as 10-in. diameter pontoons.
  • Nut-and-bolt construction should be required for structural connections. The use of extruded-aluminum threads for structural attachments is unacceptable for structural connections.
  • Users should evaluate floating roof options based on experience and needs for a specific application, considering API 650 only as a minimum requirement.
  • The use of reliable liquid level alarm systems should be favored over overflow ports as level indicators. The use of an overflow port reduces operating capacity. Running the floating roof into the fixed roof presents less risk and potential cost than spilling product onto the ground.
  • Finally, the cable-suspended design should be considered for new construction.

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