Internal floating roofs (IFR) first found their place in the oil industry to conserve product and prevent fires. Later, they were required for controlling hydrocarbon emissions into the atmosphere, as regulated by the U.S. EPA. An IFR tank has a floating deck below a fixed roof. The floating deck rises and falls along with the liquid level, leaving no room for a vapor zone. Depending on the product and the plant environment, floating roofs are readily available in steel or aluminum. Fiber reinforced plastic can also be an option in highly corrosive environments.
For most applications, an IFR is the most cost-effective solution to regulating vapor emissions. However, they have their own limitations and disadvantages. One disadvantage is they reduce the tank’s working capacity by limiting the maximum product depth (freeboard) and may also require a minimum product depth (heel). IFRs are not suitable with unstable products and the EPA regulations specify a floating roof cannot be used to control vapor emissions if the product vapor pressure is in excess of 11 pounds per square inch absolute (759 millibars). Additionally, the bubbling of unstable liquids can produce enough vapor pressure beneath the floating roof to damage the roof itself or the perimeter seal, causing failure of the floating roof.
Among various types of IFRs available, aluminum internal floating roofs are widely used due to their cost effectiveness. There are two basic types: full contact honeycomb-sandwich and skin and pontoon type. As the name suggests, a honeycomb-sandwich roof is in full contact with the product with a honeycomb hex-core sandwiched between aluminum sheeting. Similarly, a pontoon roof has an aluminum skin and roof supported on pontoons and a structural grid. This type of roof leaves some saturated vapor space beneath the skin.
API 650 Annex H provides minimum requirements for IFRs. Exceeding the minimum requirements may be required to meet operating conditions and can increase longevity. The standards of design need to be set based on system requirements and a cost benefit analysis for the durability of the roof. The perimeter seal is a very susceptible part, which demands precise design and detailing. The seal must be suitable for the expected service, resistant to the wear and tear due to cycles of movement along the tank shell, and possess low sliding friction for proper roof function. Aluminum has low tolerance to turbulence due to its low density and is another design aspect that needs consideration in overall design of the tank system. In conjunction with API 650, the designer must also refer to the “Aluminum Association Aluminum Design Manual” for allowable design strengths for the various grades of aluminum, minimum safety factors for various applications and design formulas.
Corrosion of aluminum can be a cause of concern depending on the product and tank environment. Presence of salt, chlorides or sulfur can cause aluminum oxidation, and a pH under four or over eight can also cause similar corrosion. Galvanic corrosion also can be triggered between the tank and the floating roof in steel tanks. Corrosion of the aluminum skin can cause compounding problems and it is difficult to detect in hydrocarbon tanks. Any damages due to corrosion in the skin will cause vapor to leak, causing emissions or accumulation of product on top of the skin causing it to sink in some instances. Even a small pinhole due to corrosion can let the product enter the pontoons causing maintenance concerns. While skin and pontoon type roofs are widely used for their cost effectiveness and constructability, they are less forgiving to any lapses in construction details.
Other alternatives like tank blanketing can be considered for emission control; nitrogen is one of the most common gases used for blanketing. If the blanket pressure needed to reduce the maximum vapor concentration allowed by regulations does not exceed the design pressure of the tank, blanketing can be a viable option. Understandably, it may not be the best option for atmospheric tanks.
For more information on this topic, contact Aditya Atluri at [email protected] or call (502) 964-3361.
For more information about the National Institute for Storage Tank Management, visit www.nistm.org or call (800) 827-3515.
** Published in BIC Magazine, February 2014