A Comparison of Floating Roof Designs
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Large storage tanks that hold volatile liquids are often required to use both an outer roof and an inner roof. The fixed outer roof protects the inner roof from weathering and corrosion, while the internal floating roof (IFR) keeps the liquid safe and vapor sealed inside. IFRs are designed to cover the entire surface of the stored liquid and move with the liquid level changes. Carbon steel floating roofs can be coated to reduce corrosion, and aluminum IFRs are installed without any coating.
Internal floating roofs prevent evaporation of the stored product, improving air quality and reducing the likelyhood of combustion.
There are several types of internal floating roofs (IFRs) used in industrial storage tanks:
Steel Pan Internal Floating Roof
This IFR consists of a steel deck or pan that floats on the surface of the stored liquid. A steel rim welded to the outer edge of the deck is supported with braces made of steel plate or angle. The bottom of the rim plate keeps the stored liquid off of the deck to provide buoyancy, and the top of the rim plate provides a surface to bolt the rim seal. Steel pan IFRs may be used in tanks that store crude oil, gasoline, and other petroleum products. They are cost-effective, relatively easy to install and maintain, and provide a high degree of vapor control to reduce emissions and improve safety. However, steel pan IFRs may be prone to sinking. If tank damage, extreme weather, or earthquake causes rainwater to leak onto the floating roof or causes liquid product to slosh onto the IFR deck, the floating roof may quickly sink because there is no excess buoyancy.
Steel Open-Top Pontoon Internal Floating Roof
An open-top pontoon IFR adds an inner rim plate to the pan-type IFR. The vertical steel plates welded between the inner rim and outer rim form pontoons that are connected continuously along the outside edge of the floating roof. The open-top pontoons allow the floating roof to take on a certain amount of liquid without sinking. The steel deck is also stiffened by the fully-welded pontoons, which improves performance during tank sloshing. The open-top pontoon reduces construction costs and allows the floating roof to be inspected quickly. However, the open-top design may be susceptible to rainwater accumulation. Overall, the steel open-top pontoon IFR is a reliable and effective solution for reducing vapor emissions.
Steel Pontoon Internal Floating Roof
Steel pontoon internal floating roofs (IFRs) are a type of IFR that consists of individual buoyant compartments or pontoons that are connected to form a continuous ring on top of the floating roof deck. The top of the pontoons are constructed from welded steel plates, are typically sloped like a shed roof, and have manway access points for inspection. The sealed pontoons provide buoancy to the IFR even if rainwater or stored liquid have accumulated on top of the IFR. It is relatively easy to install and maintain, and provides a high degree of vapor control to reduce emissions and improve safety. Steel pontoon IFRs are often preferred in high-wind or high-seismic areas where the stability of the floating roof is critical. However, like all types of IFRs, steel pontoon IFRs require regular inspection and maintenance to ensure their integrity and effectiveness.
Steel Reverse-Slope Pontoon Internal Floating Roof
Steel reverse-slope pontoon floating roofs are designed to force rainwater or stored liquid to the center of the floating roof to be drained away. It is typically used as an external floating roof, but can also be used as an IFR. Unlike the other floating roofs discussed above, the floating roof deck does not extend all the way across the tank. Instead, a seperate pontoon bottom is constructed. The angle of the pontoon top and bottom form a wedge that extends down into the stored liquid. The wide end of the wedge is at the tank shell, and the narrow end is welded to the floating roof deck. When the tank is filled with product, the pontoons force the floating roof to flex into a slight dish shape. A center drain pipe allows the IFR to shed rainwater and prevent accumulation. The increased size of the pontoons also strongly resist overturning in the event of an earthquake. The increased construction costs of the reverse-slope design make it less cost-effective when used as an IFR. However, the roof is no more more expensive to maintain than other types of steel IFRs.
Steel Double-Deck Pontoon Internal Floating Roof
In some cases, a customer may request a fully enclosed floating roof deck. A double-deck floating roof extends the design of a pontoon floating roof or reverse-slope floating roof by including additional rings of pontoons to cover the entire deck. Because there are a large number of pontoon compartments, the construction cost is quite high. Inspection of this type of roof is more difficult as well, because every pontoon must be tested for flammable vapor before before work can proceed.
Aluminum Full-Contact Internal Floating Roof
This type of IFR is designed to provide complete vapor control in liquid storage tanks. Aluminum full-contact IFRs are commonly used in tanks that store flammable and volatile liquids such as gasoline or jet fuel, where minimizing vapor emissions is critical. The roof is constructed from a series of buoyant panels that float on the surface of the liquid. The aluminum panels typically include a foam core. The modular panels allows for easy installation, and the lightweight design reduces the load on the tank shell. Many aluminum floating roofs are light enough to be suspended from the tank's fixed roof.
Aluminum Honeycomb Internal Floating Roof
Honeycomb floating roofs are also considered full-contact IFRs. The design is taken one step further by creating panels of factory seal-welded hexagons in a honeycomb pattern. The honeycomb IFR is designed with excess buoyancy in case several sealed compartments are damaged.
Aluminum Skin-and-Pontoon Internal Floating Roof
Aluminum skin-and-pontoon internal floating roofs have an aluminum roof deck mounted above buoyant pontoons. The skin is constructed of lightweight aluminum panels that are bolted together to form a continuous deck. The pontoons are constructed from long pipes with seal-welded caps. Aluminum skin-and-pontoon IFRs are commonly used in tanks that store flammable and volatile liquids where minimizing vapor emissions is critical. Unlike full-contact IFRs, there is no foam inside the pontoons. This allows the deck and pontoons to be degassed for repairs in order to extend the floating roof's service life.
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