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May 1, 2001 12:00 PM

Gallagher goes on to list basic design principles for external and internal floating roofs: They must be designed to remain buoyant on liquid with a specific gravity of 0.7 under specified design operating conditions. They should maintain full liquid contact to minimize evaporation and reduce product side corrosion.

Floating roofs and their accessories must accept the full range of roof movements during unattended operation from low operating level to the maximum design liquid level. They should keep product emissions to a minimum. They should be designed to provide an extended service life with minimum maintenance.

Additional requirements for external floating roofs take weather conditions into consideration. API 650 Appendix C requires that an external floating roof must have sufficient buoyancy to remain floating on liquid with a specific gravity of 0.7 when the single-deck roof drain is inoperative and the roof is exposed to 10 inches (250 mm) rainfall in 24 hours. Double-deck roofs may be designed for a lesser amount provided the roof is equipped with emergency roof drains to prevent over-accumulation of water.

Design and construction details used in double-deck floating roofs are important. As with the variations in design for single-deck roofs, different manufacturer details can make a difference in the mechanical stability and overall performance of a double-deck floating roof.

More design options are available for internal floating-roof tanks. However, design options for a fixed-roof tank are strictly limited. Internal floating roofs are available in conventional welded steel construction, but also may be constructed from lightweight aluminum or composite fiberglass. Specific design criteria for internal floating roofs are presented in API 650, Appendix H — Internal Floating-Roofs.

“External floating roofs can be — and have been — used as internal floating roofs,” Gallagher says. “In general, this occurs when a fixed roof is added to an existing external floating-roof tank. For new construction, the internal floating roof is designed as such and will have slight differences in construction details in response to reduced loading conditions.”

Selection of a floating roof design from the various types available should be done carefully. Petroleum storage tanks serve many purposes in a refinery, marine terminal, or pipeline terminal environment. Tank operations, the type and quality of product being stored, and site-specific conditions must be considered.

“Selection of the floating roof design and associated equipment can ensure an excellent return on the investment, or it can provide a costly maintenance headache for many years,” Gallagher says.

Mechanical stability of the floating roof is probably the most important parameter of any floating roof. Any load acting on the floating roof that is not balanced will force the roof to float over to one side of the tank or, in the most severe cases, the roof will tilt due to unbalanced loads.

Product factors that should be considered when choosing a roof include product composition and chemical stability, specific gravity, viscosity, and wax content.

“Increased storage and handling risks are associated with new crude oils that are modified at the well or mine site to ensure that the end product can be safely transported,” Gallagher says. “All product characteristics can be affected by site ambient conditions. Variations in ambient temperature should be considered when reviewing the potential design impact on product true vapor pressure and product viscosity.”

Product composition is important because of the potential effects certain chemicals will have on the service life of some of the equipment associated with the floating roof. The greatest potential for problems is with non-metallic materials used with the floating-roof perimeter seals. Some chemicals can destroy commonly used seal materials after only a few weeks of exposure.

Other considerations when choosing a floating roof are product specific gravity and true vapor pressure. In most cases, floating roofs are designed for a specific gravity of 0.7. It is unusual to find a product with a lighter specific gravity but it is possible, Gallagher says.

It is important that the roof designer know the minimum specific gravity since this will determine how far a given roof will float into the product. The distance from the product surface to the top of the floating roof deck is one measure of overall roof stability. This information also is used to check the relative position of the rim seal with respect to the liquid surface.

Product true vapor pressure (TVP) is the single most critical design parameter when selecting the type of floating roof. Most current environmental regulations limit the product TVP to less than 11.1 psia. As the TVP increases above 11 to 12 psia, daily heating of the product under a center deck will produce enough vapors to balloon the deck. It is common for these vapors to condense during the cooler evening hours, allowing the roof to resume a normal flat shape.

“It must be emphasized that if the tank is in an area of significant rainfall, ballooning of a single-deck roof may not permit normal water drainage to the primary roof drain,” Gallagher says. “An unbalanced load can quickly be developed that can sink the floating roof.”

A secondary consideration with the high vapor pressure products is that with increasing TVP, the overall effectiveness of any floating roof design is reduced. More evaporation will occur, and this vapor will escape to the atmosphere above the floating roof. Air pollution and the risk for a fire are increased under these conditions.

“If the TVP is going to be high, approaching 11.1 psia or greater, consideration should be given to using a double-deck floating roof,” he says. “A double deck will maintain its ability to drain water while containing some amount of product vapor. The double-deck roof can help reduce vapors from product heating due to the insulating effect the design provides to the product surface. Properly designed, the double-deck floating roof can be the most stable design available.”

Conditions upstream from the tank farm should be considered when selecting the type of floating roof. Process flow rates must be identified, as well as any process that might result in large quantities of vapor being mixed with the product fill stream.

“Each of these conditions can be addressed in the roof selection process and during the detailed design of the storage tank,” Gallagher says. “If the application is an internal floating roof and there is a possibility that significant quantities of vapor may be included in the fill stream, a pan or lightweight floating roof should not be considered. The minimum suggested roof design would be a pontoon single-deck roof, or in extreme cases, a double-deck roof equipped with surface discretionary vents.”

Floating-roof storage tanks and associated equipment must be kept up to date and a working knowledge of the equipment maintained by refiner and terminal operator. New roof and tank designs are being developed for more efficient product storage.

“Conventional floating-roof tanks operate with a low landing position of about three feet (less than one meter) and require five feet (1.5 meters) or more vertical clearance for the floating roof,” Gallagher says. “New designs and equipment options are available that would permit a floating roof to land almost on the tank bottom, increasing the storage and operating efficiency of the system. However, some of these options need further development and testing.”