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Industry Innovator: Ultraflote

Internal Floating Covers for Ethanol Storage Tanks:

Clean Air, Safety, and Profitability

In the 1920s the petroleum industry began installing floating covers on crude oil and petroleum products to prevent fires and loss of product due to evaporation. For many decades the cost of a floating cover was justified on the basis of safety and profitability.

Times changed; floating covers are now required by local and federal environmental clean air standards, but still serve their original purpose of making the tank safer and conserving product.

Ultraflote was established in 1972, and since then Ultraflote® Internal Floating Covers have been installed inside petroleum and chemical tanks throughout the world. Most ethanol plants in North America have Ultraflote® Internal Floating Covers installed in the denaturant tank, 190 proof tank, 200 proof tank, and final product tanks. The company’s corporate office is in Houston and manufacturing is done in Hastings, Nebraska.

The floating cover does not prevent loss of product by holding the contents under pressure, such as a stove-top pressure cooker. There is essentially no pressure differential between the under side and top side of the floating cover. The floating cover works by preventing the circulation of air at the surface of the product.

The physical mechanism for the escape of product is by diffusion. Diffusion rates are kept low by keeping the openings small or by making a long tortuous path. Visualize a covered glass of water and the only opening is a long straw; if the straw does not extend into the liquid, the pressure under the cover of the glass of water is the same as the pressure in the room. Evaporation rates from that covered glass of water will be very low because the diffusion rate up the long straw is very low. The smaller the straw diameter and the longer the straw, the lower the diffusion rate.

The overall effectiveness of the Ultraflote® Internal Floating Cover has been measured under controlled laboratory conditions in a test program that used a 20’ diameter specially constructed test tank. An Ultraflote® Internal Floating Cover prevents better than 99 percent of the loss of product that would have occurred had there been no internal floating roof. Emission losses in an actual operating tank are so low that the loss cannot be accurately measured and are instead predicted on a theoretical basis.

Most floating covers have adjustable legs. With adjustable legs, the floating cover can be supported in the maintenance position with the deck skin 6’-6” from the tank floor. In the operating position, the legs are typically set at 3’-6” so that the floating cover travels as low as possible without blocking a tank shell manway. In the low position, the owner utilizes the maximum amount of tank volume without landing the floating cover.

Cable suspended aluminum floating covers, in lieu of legs, have become increasingly popular, especially with larger diameter tanks. Stainless steel cables take the place of the legs and each cable is attached to a neck that is welded to the exterior of the tank roof. A neck with a cap provides a weather tight cover and allows the cable lengths to be adjusted from the tank roof exterior while the floating cover is floating. This system has the advantage of allowing a lower operating level for the floating cover and therefore greater usable tank capacity. Secondary advantages are that cables eliminate the emission loss from the legs and the tank is easier to clean because the floating cover legs do not interfere with the cleaning operation.

The Ultraflote® Internal Floating Cover pontoons are designed for 100 percent excess buoyancy. They are specially designed aluminum tubes with end caps and structural ears on the end caps that allow the pontoons to be bolted together. The attachment of pontoons is such that the pontoons are allowed to flex at each connection. A flexible floating cover design enhances the performance and extends the life. If the product in the tank has standing waves, the floating cover will move with those waves.

The aluminum pontoons support an aluminum grid made up of long clamp beams and deck skin. The aluminum deck skin for an Ultraflote® Internal Floating Cover comes in rolls of sheet 0.023” thick by 84” wide. The clamp beams are positioned at the edge of two sheets so that the edges of both sheets are overlapped, clamped, and bolted together. The clamp beam extrusion is one of the keys to the strength of the floating cover. The clamp beam and bolting must be sufficiently secure that the sheets cannot separate. When in service the floating cover deck skin is in membrane tension, similar to a drumhead. The clamp beams allow the sheets of aluminum skin to act structurally as though the floating cover was assembled with only one wide sheet. A load on the deck is therefore transferred out to the edge of the floating cover where the load is restrained by a rim plate.

When a new Ultraflote® Internal Floating Cover is installed, or a repair or modification is required, the installation crew work through the tank shell manway so that cutting into the tank is not required.

Although clean air requirements now mandate a floating cover, the original role of the floating cover has not diminished. The floating cover continues to provide a safe operating tank and enhanced profitability by preventing loss of product. The floating cover has become even more essential than it was 37 years ago when the first Ultraflote® Internal Floating Covers were placed in service.


Contact info:

Ultraflote Corporation

Houston, TX

Key Contact:

Steve Hall, Vice President, Sales

Ph: 713-461-2100

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The American Coalition for Ethanol publishes Ethanol Today magazine each month to cover the biofuels industryís hot topics, including cellulosic ethanol, E85, corn ethanol, food versus fuel, ethanolís carbon footprint, E10, E15, and mid-range ethanol blends.
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