Floating roofs

Nov. 1, 2005
THE USE of SUSPENDED aluminum floating roofs can reduce vapor losses more effectively, compared to conventional leg supported floating roofs. Additional

THE USE of SUSPENDED aluminum floating roofs can reduce vapor losses more effectively, compared to conventional leg supported floating roofs. Additional benefits include eliminating the need for confined space entry to adjust leg heights in an operational storage tank, according to John Oleyar of HMT Inc.

Oleyar discussed aluminum and other internal floating roofs made from various materials at the Independent Liquid Terminals Association International Operating Conference held earlier this year in Houston, Texas.

He pointed out that vapor losses from fixed-roof aluminum tanks can come from the leg housings penetrating the floating roof. This is eliminated when the roof is suspended since the supports do not penetrate the floating roof. Similar vapor loss occurs in shell and roof vents in tanks with an internal floating roof.

Today, about 30% of storage tank roofs being installed are designed with suspended roofs. One of the most popular choices is the deck-connected system. It includes a hanger system used to suspend the roof.

In addition to preventing emissions release, safe entry concerns are eliminated since the roof landing height can be adjusted from operational height (designed to maximize operating capacity) usually three feet to maintenance height (designed to allow routine maintenance) from the outside of the tank.

“This is not possible with a traditional leg supported floating roof,” Oleyar said.

Cable attachments used in these systems include fixed-roof, floating roof rim, floating roof pontoon, and floating roof connections.

To ensure that the cables are properly coupled, they should be color-coded according to their location.

To counter the effects of corrosive content in some storage tanks, the components of the internal floating roof that are immersed in stored product can be made from stainless steel.

Turning to standards for internal floating roofs for oil storage, Oleyar said they must be designed and built to float and rest in a horizontal plane (API 650), and all seams exposed to product vapor or liquid must be vapor tight. The design calculations are based on a specific gravity of 0.7.

The standard also calls for a rim or skirt to be provided around the roof perimeter. All conductive parts are to be interconnected and bonded into the outer tank shell.

The roof must be designed to support two men or 500 pounds over one foot square.

Materials required include steel that conforms to Section 2 of American Petroleum Institute (API) 650, aluminum to conform to Section 2 of American Society of Mechanical Engineers (ASME) B 96.1, stainless steel to conform to American Society for Testing and Materials (ASTM) A 240M/A 240, and plastic must conform to a number of ASTM requirements with regards to its density, compressive yield strength, shear strength, water vapor transmission, and porosity.

Common internal floating roofs problems include pontoon end cap failure, primarily due to pontoons being connected to the landing legs.

Sheeting also can fail. Rim damage occurs due to wave action/turbulence in the tank. Rim seals fail, deck seam loss occurs because of too many seams and poor clamping, and there can be a weak or flimsy structure that is unserviceable.