Internal floating roofs (IFR) first found their place in the oil industry to conserve product and prevent fires. Later, they were required to control hydrocarbon emissions to the atmosphere, as regulated by the U.S. EPA . An IFR tank has a floating membrane under a fixed roof. The floating sheet rises and falls along with the liquid level, leaving no room for a vapor zone. Depending on the product and plant environment, internal floating roofs are available in steel, aluminum, and stainless steel. Fiber Reinforced Plastic , Glass Reinforced Plastic or Glass Fiber Reinforced Epoxy Vinyl Ester IFRs can also be an option in highly corrosive environments.
All internal floating roofs (IFR), when properly designed and installed to suit tank conditions and stored product specifications, mitigate the emission of hydrocarbon vapors with similar levels of effectiveness. All can be equipped with suitable sealing seals , be fitted with adjustable or fixed support legs, almost all can be suspended from the tank roof, all can be fitted with a range of accessories and all are of modular design, prefabricated for final assembly on site. Selection of the type of floating internal membrane is generally an operator preference and, provided the IFR is purchased from a reputable supplier with a good track record, all types of floating internal roof should be fit for purpose.
For most applications, a internal floating roof is the most cost-effective solution for regulating vapor emissions. However, they have their own limitations and disadvantages. One of the disadvantages is that that they reduce tank storage capacity by limiting the maximum product height (dead zone) and may also require a minimum product height. IFRs are not suitable with unstable products and EPA regulations specify that a floating roof cannot be used to control vapor emissions if the vapor pressure of the product is greater than 11 PSI absolute (759 millibars). In addition, bubbling of unstable liquids can produce sufficient vapor pressure under the floating roof to damage the roof itself or the perimeter seal, causing failure of the internal floating roof.
Among the various types of internal floating roofs available, aluminum internal floating ceilings are the most widely used due to their cost-effectiveness. There are two basic types: full contact and pontoon. As the name implies, the full contact IFR is in in total contact with the product. Similarly, a pontoon type IFR has an aluminum deck and a roof supported by pontoons and a structural grid. This type of IFR leaves a saturated vapor space under the deck.
Annex H of API 650 establishes the minimum requirements for internal floating roofs. Exceeding the minimum requirements may be necessary to meet operating conditions and may increase longevity. Design standards should be established based on system requirements and a cost-benefit analysis for internal floating roof durability. The perimeter seal is a very susceptible part, requiring precise design and detailing. The seal must be suitable for the intended service, resistant to wear due to movement cycles along the tank shell, and possess low sliding friction for proper roof operation. Aluminum has low tolerance to turbulence due to its low density and is another design aspect that must be considered in the overall design of the tank system. Along with API 650, the designer should also consult the Aluminum Association Aluminum Design Manual for allowable design strengths for various grades of aluminum, minimum safety factors for various applications, and design formulas.
The correct choice must take into account some important variables such as: tank type, tank age, operating cycle (rotation), expected service life, type of material, volume, product stored, design, location, weather conditions, product environment and temperature, liquid movement system (swirl), maintenance plan, flow rates, design pressures, fire fighting system (if any), cost effectiveness, among others.
As mentioned above, internal floating roofs are basically divided into two groups:
In the case of a full contact IFR, the roof is always in contact with the surface of the stored liquid. In the other, the deck is supported on floats or pontoons, and these are floating on the surface of the liquid. In this group, the deck has no contact with the liquid.
Item "H.2" of API 650, 13th edition (appendix "H"), defines the most common types of IFR, which we will discuss below (perimeter seals have not been represented):
Type H.2.2.a (pan)
Type "H.2.2.a": Internal floating pan-type roof. This roof has its edge above the liquid, is hermetically open, having its lower part in direct contact with the fluid, resembling this container (pan), hence its name. They are usually made of steel..
For this type of internal floating roof, the customer should be informed that this design does not have multiple flotation compartments necessary to meet the requirements of H.22.214.171.124, which defines that all IFRs with multiple flotation compartments must be able to float without any damage, when any of the other two have been flooded.
Type H.2.2.b (open compartments)
Type "H.2.2.b": "Open" internal floating roof. It is similar to type "a", full contact, but with bulkheads or compartments in the deck, usually made of simple steel or aluminum sheets. This type of IFR is usually made of steel or aluminum.
Type H.2.2.c (peripheral pontoons)
Type "H.2.2.c": "Peripheral pontoon" type floating inner roof, is also full contact, characterized by closed peripheral flotation compartments, forming a hollow ring, with the center of the deck open without compartments, also typically made of steel, with a single deck.
Type H.2.2.d (double deck)
Type "H.2.2.d": Internal floating roof with two decks (double deck) is similar to type "c" (full contact), with several compartments, but with two decks. The entire upper part (upper deck) is closed, and the lower part (lower deck) is in full contact with the fluid. They have high mechanical and structural strength. This type of internal membrane is usually made of steel.
Type H.2.2.e (pontoons and deck)
Type "H.2.2.e": Pontoon type floating roof, the only type that is not full contact. It consists of typically tubular floats, spaced apart, attached to membrane support profiles, or metal film (deck). Only the floats (pontoons) of the floating inner membrane remain in contact with the liquid. Below the metal jacket there is a vapor space up to the liquid surface. They are usually made of aluminum, stainless steel or a combination of both metals (hybrid versions).
Type H.2.2.f (panel type)
Type "H.2.2.f": Internal Floating Roof "panel" type, is formed by hollow metallic or non-metallic floating panels in the form of a tray, forming a sandwich, with internal separators, or core, like a honeycomb, which can be metallic or not. It is also full contact with the fluid and is normally manufactured in aluminum, although other materials, such as polymeric composites, are acceptable, provided they meet the technical requirements and are approved by the customer.
Type H.2.2.g (hybrid type)
Type "H.2.2.g": Internal Floating Roof, hybrid type, is a combination of types "b" and "c", whose peripheral and central points can be open or closed, provided they are duly approved by the customer. They are also full contact with the fluid.
Type "H.2.2.h": other floating inner membrane materials or designs, other than those described in this document, may be accepted if specified by the customer and provided they meet the applicable technical requirements. Fiber Reinforced Plastic , Glass Reinforced Plastic or Glass Fiber Reinforced Epoxy Vinyl Ester IFRs can also be an option in highly corrosive environments.
Corrosion of aluminum can be a cause for concern depending on the product and tank environment. The presence of salt, chlorides or sulfur can cause aluminum oxidation, and a pH below four or above eight can also cause similar corrosion. Galvanic corrosion can also be triggered between the tank and the floating roof in steel tanks. Corrosion of the aluminum internal floating roof can cause aggravated problems and is difficult to detect in hydrocarbon tanks. Any damage due to corrosion on the jacket will cause vapor leaks, causing emissions or product buildup on top of the jacket causing it to sink in some cases. Even a small hole due to corrosion can allow product to enter the pontoons, causing maintenance problems.
The pontoon type IFR is usually inserted into the tank through a 24" manway. It is also the easiest to install.
Other full contact internal floating roofs, whether honeycomb, open compartment or polyurethane foam panel type, usually require specific manholes larger than 24". They also require more assembly time.