An internal floating roof, or IFR, is a deck that floats directly on the liquid surface inside a tank that also has a fixed cone or dome roof over the top. It combines the vapor-loss control of a floating roof with the weather protection of a fixed roof. This guide explains how internal floating roofs are built, how the rim seal cuts standing and working losses, and why operators choose an IFR over an external floating roof.
Internal Floating Roof in one line: An internal floating roof is a lightweight deck - either a pontoon type or a full-contact type - that floats on the stored liquid inside a tank that has its own fixed roof above. It rises and falls with the level so almost no vapor space sits over the product, and a flexible rim seal bridges the gap between the deck edge and the shell, which together sharply reduce evaporative emissions of volatile products.
There are two broad families of internal floating roof. A pontoon-style deck uses annular or distributed buoyancy compartments and a single-plate deck spanning between them, so the deck skin sits slightly above the liquid across most of its area. A full-contact deck, often a honeycomb aluminum panel or a foam-filled sandwich, floats with its entire underside in contact with the liquid, leaving essentially no vapor pocket beneath the deck itself.
Full-contact decks are popular in retrofits because they are light, come in bolted aluminum panel kits, and can be built inside an existing fixed-roof tank through a manway. Pontoon decks tend to be more robust and are common in heavier-duty or steel-deck installations. Both ride on adjustable legs that let the deck land on supports for maintenance and inspection when the tank is emptied.
The deck must accommodate everything that penetrates it - the fixed-roof support columns, the gauge well, guide poles, and level instrument stilling wells. Each penetration gets its own seal or sleeve, because any uncovered gap is a path for vapor to escape into the space above the deck.
The single most important emission-control component is the rim seal that closes the annular gap between the moving deck and the fixed shell. As the deck rises and falls, this gap changes slightly and the tank shell is never perfectly round, so the seal must be flexible enough to stay in contact through the full travel. Designs range from a foam-filled wiper seal to a mechanical shoe seal, sometimes in a primary and secondary arrangement for tighter control.
Storage losses come in two forms. Standing losses occur while the tank sits, as vapor works past the seal and out of the fixed-roof vents. Working losses occur during filling and emptying, when liquid movement and the changing wetted shell drive additional evaporation. By keeping the deck riding on the liquid and sealing the rim, an IFR removes the large vapor space a plain fixed-roof tank would have, which is where the bulk of those losses would otherwise come from.
The fixed roof above the deck is vented with circulation vents around its perimeter and often a center vent, so the space above the deck stays near atmospheric and does not accumulate a flammable vapor concentration. That vented space is what distinguishes an internal floating roof tank from a simple fixed-roof tank.
An external floating roof tank has no fixed roof at all - the floating deck is exposed to the weather. That exposure means rain and snow load the deck, dust and water can foul the seals, and the deck must have drains and be designed to shed weather. An internal floating roof puts a fixed roof over everything, so the deck and seal are protected from rain, snow, and wind-driven emissions, and the design is often preferred in regions with heavy weather or where a cleaner product must be kept free of rainwater.
Retrofitting an IFR into an existing fixed-roof tank is also a common way to reduce emissions and product loss on tanks that were originally plain cone roofs, without having to convert to a fully external floating design. The trade-off is the added deck, seals, and inspection burden inside the tank.
For remote operations, a cloud SCADA such as Merobix trends the tank level that tells operators where the deck is riding, so they can avoid landing the deck on its legs unintentionally or floating it too high toward the fixed roof. Continuous level and, where instrumented, seal or deck condition data let a team manage IFR tanks across a field remotely, catching a sticking deck or an abnormal level trend before it becomes a seal or overfill problem.
Both have a deck that floats on the liquid, but an internal floating roof sits inside a tank that also has a fixed roof over the top, while an external floating roof is exposed to the open sky with no fixed cover. The fixed roof on an IFR protects the deck and seal from rain, snow, and wind, which is a major reason IFRs are chosen.
The rim seal closes the annular gap between the moving deck and the fixed tank shell as the deck rises and falls. It must flex to follow the out-of-round shell and changing gap while staying in contact, which is what keeps vapor from escaping past the deck edge and drives down both standing and working losses.
A full-contact deck floats with its entire underside on the liquid, leaving essentially no vapor pocket beneath the deck, and it is typically a light aluminum panel kit that assembles inside an existing tank, which makes it well suited to retrofits. A pontoon deck is generally heavier and more robust and is common in steel-deck and heavier-duty service.
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