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Floating roof tanks
Floating roof tanks are a type of atmospheric storage tank.
When product vapor pressure is greater than 0.5 psia (more in some states) but less than 11.1 psia, the U.S. Environmental Protection Agency permits the use of a floating-roof as the primary means of vapor control from the storage tank. Floating-roof tanks are not intended for all products. In general, they are not suitable for applications in which the products have not been stabilized (vapors removed). The goal with all floating-roof tanks is to provide safe, efficient storage of volatile products with minimum vapor loss to the environment.
Design requirements for external floating roofs are provided in Appendix C of the API Standard 650. The external floating roof floats on the surface of the liquid product and rises or falls as product is added or withdrawn from the tank.
The internal floating roof tank (IFRT) was developed in the mid-1950s to provide protection of the floating roof from the elements, including lightning strikes to the floating roof. The tank vapor space located above the floating roof and below the fixed-roof includes circulation vents to allow natural ventilation of the vapor space reducing the accumulation of product vapors and possible formation of a combustible mixture. Fig. 1 shows a typical internal floating-roof tank.
Closed floating roof tank (CFRT)
The closed floating roof tank (CFRT) is similar to an IFRT. It uses an internal floating roof but eliminates natural ventilation of the tank vapor space. Instead, the CFRT is equipped with a pressure-vacuum (PV) vent and may even include a gas blanketing system such as that used with fixed roof tanks. Emissions from a CFRT are virtually the same as those from an IFRT, however, can be easily collected for further treatment if necessary. One such closed roof tank for benzene storage with associated vapor recovery equipment is shown in Fig. 2.
Floating roof tank networking capacity
Determining what tank size is required for the desired net storage capacity must consider several factors. Internal or external floating-roof tank shell height must account for the space required by the floating roof as shown in Fig. 3.
The tank working capacity is obtained by operating a floating-roof tank between the maximum high gauge and recommended low landing position for the specific floating-roof tank design. A floating roof should be landed only if the tank is to be removed from service for routine inspection or maintenance activities. Landing the floating roof during normal tank operations should be avoided. Product losses increase whenever the roof is not in complete contact with the liquid surface.
In general, floating-roof tanks have been used only at terminal or refinery locations where larger storage capacities are needed. Increased emphasis on the control of evaporative emissions from storage tanks might change the roll of floating-roof tanks in the future with the increased use in smaller tanks. Internal floating roofs have been used in tanks as small as 15 ft in diameter to minimize product losses.
Product vapor control with floating roof tanks
In general, the floating roof covers the entire liquid surface except for a small perimeter rim space. Under normal floating conditions, the roof floats essentially flat and is centered within the tank shell. There should be no vapor space underneath a welded-steel floating roof. Under normal conditions, the amount of product vapor that might become trapped beneath the floating roof should be insignificant. However, if large quantities of flash vapor or other noncondensable vapors become trapped, the floatation stability of the roof can be affected. These conditions should be avoided if possible.
It is important to understand how a floating roof works and why details are so important in the design of a floating-roof storage tank. The study of evaporative emissions from storage tanks and possible methods to control or eliminate these emissions has been the focus of an extensive series of analytical studies, field, and laboratory testing programs sponsored by the American Petroleum Institute.
Calculating evaporative losses
API Publications 2517 (EFRT), 2518 (FRT), and 2519 (IFRT) summarized methods for calculating evaporative losses from the storage and handling of petroleum liquids. These were first published in 1962 and then updated in 1991. Most recently, Publications 2517 and 2519 were consolidated in April 1997 in "Evaporative Loss From Floating-Roof Tanks," Chap. 19.2 of the API Manual of Petroleum Measurement Standards.
The new publication updates the evaporative loss estimation procedures for EFRTs, IFRTs, and CFRTs. The results continue to be used as the basis for the U.S. Environmental Protection Agency (U.S. EPA) publication on air pollution emission factors.
It has been demonstrated that evaporative emissions from a fixed-roof tank can be reduced by over 98% through the use of a properly designed and maintained external floating roof tank, assuming the same product and ambient conditions.
Evaporative emissions, although greatly reduced, cannot be entirely eliminated. Normal practice is to use floating-roof tanks only to store products that are considered "stabilized" such that large quantities of vapor will not be introduced underneath the floating roof. In cases when the product entering the tank is at a condition that produces flashing conditions, vapors produced will be captured underneath the floating roof. Evaporation and associated product losses still occur from the rim space, standard roof deck fittings, product that remains on the tank shell, and tank operations that require the tank to be emptied and the floating roof landed on its supports.
Evaporative Loss Measurement. 1997. In Manual of Petroleum Measurement Standards, Ch. 19, Sec. 2-3. 1997. Washington, DC: API.
API RP12R1, Setting, Maintenance, Inspection, Operation, and Repair of Tanks in Production Service, fifth edition. 1997. Washington, DC: API.
API RP575, Inspection of Atmospheric and Low-Pressure Storage Tanks, first edition. 1995. Washington, DC: API.
API RP651, Cathodic Protection of Aboveground Petroleum Storage Tanks, second edition. 1997. Washington, DC: API.
API RP652, Lining of Aboveground Petroleum Storage Tank Bottoms, first edition. 1991. Washington, DC: API.
API RP2003, Protection Against Ignitions Arising Out of Static, Lightning, and Stray Currents, fifth edition. 1991. Washington, DC: API.
API Spec. 12B, Bolted Tanks for Storage of Production Liquids, fourteenth edition. 1995. Washington, DC: API.
API Spec. 12D, Field-Welded Tanks for Storage of Production Liquids, tenth edition. 1994. Washington, DC: API.
API Spec. 12F, Shop-Welded Tanks for Storage of Production Liquids, eleventh edition. 1994. Washington, DC: API.
API Standard 650, Welded Steel Tanks for Oil Storage, tenth edition. 1998. Washington, DC: API.
API Standard 653, Tank Inspection, Repair, Alteration, and Reconstruction, second edition. 1995. Washington, DC: API.
API Standard 2000, Venting Atmospheric and Low-Pressure Storage Tanks (Nonrefrigerated and Refrigerated), fifth edition. 1998. Washington, DC: API.
API Standard 620, Design and Construction of Large, Low-Pressure Storage Tanks, tenth edition. 2002. Washington, DC: API.
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