Linings Provide Added Measure of Safety for Many Storage Tanks
Apr 1, 2000 12:00 PM
LININGS for aboveground storage tanks (ASTs) were around even before World War II. However, demand for linings has grown significantly in recent years along with the overall population of storage tanks.
The Environmental Protection Agency estimates that there are at least 1.5 million aboveground storage tanks in the United States. Petroleum storage tanks alone number more than 700,000 in North America as a whole, according to the American Petroleum Institute.
Three primary designs of ASTs are used today: fixed-roof tanks (also called cone-roof tanks), external floating roof tanks (EFRT), and internal floating roof tanks (IFRT). Open-top tanks or reservoirs were used up until the 1950s but have been phased out of service.
The fixed-roof tank (FRT) was developed to overcome the disadvantages of open-top tanks. An FRT provides superior containment of evaporative vapors but does not suppress evaporation from the complete exposure of the stored hydrocarbon, according to a presentation given by J F Delahunt at Corrosion 99, which was sponsored by the National Association of Corrosion Engineers. Delahunt works for Carmagen Engineering Inc, Madison, New Jersey.
The first EFRTs were put into service in the 1920s. They were designed to significantly reduce the evaporation losses that were common with FRTs. The roof floats with the liquid level, which rises and falls, eliminating the vapor space. Disadvantages of the EFRT include the possibility of aggressive internal shell corrosion due to exposure to the elements and abrasion of the tank roof as it moves up and down.
Developed in the late 1950s and early 1960s, the IFRT has an external covering that protects the internal floating roof from the elements and lightning strikes. This arrangement minimizes evaporation losses and limits the exposure of the shell to the weather.
Corrosion Threat Corrosion problems seem to turn up most frequently in EFRTs. Delahunt pointed out that the rate of corrosion in a carbon steel EFRT used to store light petroleum products varies by geographic location and the atmospheric conditions at each location.
For example, it was estimated that internal corrosion in a carbon steel tank ranged from 10 to 20 mils per year (mpy) on the Caribbean island of Aruba. Corrosion was judged to be six mpy in the northeastern United States and 15 mpy along the Gulf Coast.
Linings provide a cost-effective means of protecting against the corrosion and abrasion that occur in EFRTs. The use of coatings and linings in aboveground storage tanks is not a new concept.
In fact, protective coatings were first applied within storage tanks around 1915, but many were unsuccessful. Vinyl and gunite gave some of the best results and were used into the 1960s. For gunite, lining life was 15 to 20 years.
By the late 1940s, thin-film protective coatings were being tested for use in EFRTs. Materials included polyvinyl chloride and inorganic zinc-rich coatings (IZRC). Most were applied by brush and roller.
From the mid- to late-1950s, additional tank lining materials were developed. These included amine-epoxies, polyamide epoxies, polyurethanes, and epoxy phenolics.
Waterbased IZRC showed some of the best results in tests, Delahunt said. IZRC tank linings are still in use today and have an estimated life in excess of 10 years.
FRP Materials Fiberglass-reinforced plastic (FRP) materials also achieved success as lining materials. FRP has become a popular alternative for tank bottom repair, because full replacement can be a costly and time-consuming process.
Thick-film FRP linings can be considered secondary bottoms tightly bonded to the storage tank, according to Bob Hummel, The Sherwin-Williams Co, Cleveland, Ohio. He discussed the advantages and disadvantages of FRP linings during Corrosion 99.
When properly selected and applied, FRP linings have been proven to prevent leakage due to internal corrosion of steel tank bottoms. They also help reduce the potential for leakage if perforation of the steel bottom occurs due to soil-side corrosion.
On the plus side, tests show that FRP linings have the ability to bridge holes up to eight inches in diameter under 82 psi for a double laminate and 37 psi for a single laminate. FRP linings of vinyl ester resin can resist numerous corrosive materials at elevated temperatures.
FRP has a 35-plus-year history of success as a lining material.
Because they can be installed quickly, FRP linings minimize installation downtime. A storage tank can be put back into service much faster than with other lining materials or replacement of the bottom with new steel.
FRP is not without its disadvantages, though. When pigmented gel coats are used, it is not possible to see a hole through the steel bottom or determine the existence or extent of bottom-side corrosion. Cyclic loading may affect the long-term ability of FRP linings to bridge a large, clear opening.
As with any lining system, applicators may not understand the proper installation procedure or the importance of having clean, contaminant-free substrates. Most laminate failures are due to application problems.
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