"The roof and skirt are designed to travel (float) as the volume inside the tank fluctuates," according to Gary Zinn, sales director, Industrial Markets for Tnemec.
"No design has yet solved the persistent problem of rain water accumulation and ponding on top of the roof," Zinn
"This water combines with industrial and cooling tower fallout and escaped hydrocarbon vapor solutions which attack the substrate.
In addition to ultraviolet (UV) light degradation, roof and sidewall coatings are often subject to elevated temperatures for crude and distillate storage of 120 to 140 degrees F. It is easy to see that all exterior surfaces of floating-roof tanks should be regularly maintained."
Exterior steel on floating roofs is also subjected to salt-fog conditions found in coastal environments.
"It's estimated that in Louisiana's coastal environment, approximately 1.0 mil of protective coating material can be lost each year to erosion," Zinn
"So a dry film thickness of 16 to 20 mils is recommended to achieve a projected 20-year life cycle with proper maintenance."
"In meeting specification requirements of major oil companies and fuel jobbers, coating systems for floating-roof storage tanks must meet a range of performance criteria," Zinn
"Using this combination, total film thickness on the roof is approximately 20 mils," Zinn
The interior walls and floor areas are exposed to varying degrees of corrosion, from storage of sour crude (sulfur and salt laden), and higher vapor pressure volatile liquids.
"In the past, some engineers did not coat the interior surfaces with the belief that residues and a thin film of corrosion inhibiting petroleum will protect the surface from corrosion," Zinn
"This was proven not to be the case, even in those areas continually in contact with oil, because of the extremely corrosive nature of dissolved oxygen, hydrogen sulfide and naphthenic acids in some crude."
A design feature which greatly increases corrosion of interior surfaces is the freeboard area of the walls exposed by movement of the roof.
This zone may be alternately immersed in the product, physically abraded by the moving skirt, and exposed to the atmosphere as levels in the tank fluctuate.
"It is prudent to upgrade interior areas when maintenance pulldowns permit because inspection intervals can be between 10 to 25 years," Zinn