TAKING THE necessary steps in storage tank maintenance prevents product spills and results in bonuses for the environment and the financial bottom line. Tank leaks can be ameliorated by companies being alert to corrosion and applying appropriate protection to the tank skin.
The proper skin protection not only holds off corrosion, but makes for aesthetically pleasing storage tanks, which gives a company an extra value when soliciting customers or when prospective buyers are looking at the property. The plus for good public relations is invaluable.
With more than 500,000 above-ground storage tanks dotting the landscape in the United States and 14 million gallons of product reported lost from leakage in 1998-1999, keeping tanks in good condition becomes imperative, said Keith Rachel of Sigma Coatings USA.
Even new tanks are subject to corrosion, said Philip Myers of Chevron Products Company. "About one to five percent of tank bottoms fail when new," he said. "They are very small leaks, but they have to be filled."
Joining Discussion Rachel and Myers discussed tank maintenance at the Independent Liquid Terminals Association annual meeting June 14-17 in Houston, Texas. Joining them in the discussion were Ken Armstrong of Munters Moisture Control Services; David Hunter of Corrpro Companies Inc; and Stephen Klugherz of USF Surface Preparation Co. The information was tailored for industry managers to keep them abreast of storage tank superintendence and state-of-the-art equipment.
"Aggressive pitting in isolated areas is generally the problem," said Rachel. There are typically two types of pitting, open and cavernous. While the former is more obvious, the latter can be insidious because it is hidden underneath a vulnerable piece of tank skin. The lower two to three feet of the shell at the bottom of the tank are the most susceptible to corrosion.
The tanks' enemy comes with the territory - moisture, salinity, and oxygen, all pervasive contributors to pitting, Myers noted.
And, Armstrong added, high humidity can cause problems, particularly when coatings are being applied. The tank may not feel wet to the touch, but even infinitesimal amounts of moisture can cause corrosion and interfere with the setup of the coating.
A coating that is properly applied should last about 10 years. Today's cost for painting a 600,000-gallon tank is estimated at about $200,000. Expending that kind of capital for preservation of the tank skin calls for good management, said Hunter.
Among good management policies is the initial one involved in the coating - surface preparation, coating mixture, coating appropriateness for the surface, and a painter with expertise. Follow up with inspectors who won't miss a defect.
"Test the paint for its reaction to the product that will be in the tank," he added. Warranties should be examined, environmental concerns should be addressed, and containment for spills noted.
"It's a dirty, messy job," he said, adding, "It all happens at the spray gun - whether the paint is mixed right and applied correctly."
Coating Epoxies Rachel listed the advantages of solvent-free epoxies. "They are safe; there are no dangers of fire or explosion when they are used," he said. In addition, airless sprayers can be used and the light glossy colors make future inspections easier. Solvent-free epoxies adhere well and are resistant to chemicals.
Other construction considerations include significant upgrading of tank bottoms. Managers were advised to avoid putting new steel floors directly on top of the original tank bottom.
The two layers should be separated with a polyethylene liner and a layer of concrete. "Some use sand instead of concrete," said Myers.
A double-bottom tank can enhance savings because the tank doesn't have to be replaced and requires less inspection frequency. In Alaska, tanks with a double bottom are exempt from lining requirements, another advantage, Myers said.
Klugherz discussed the use of a centrifugal wheel blast machine used in surface preparation and operated by a remote control device. Abrasives are cycled back into the machine, filtered, and thrown again. No debris escapes into the environment, and worker exposure is avoided. He said the procedure reduces cleanup time and disposal costs. Two people are required for a job that would normally call for eight to ten.
The closed loop system can handle about one pound of waste per 40 square feet. Some machines can blast 4,000 to 5,000 feet per day, depending on the size of the machine. He estimated the cost at 14-20 cents per square foot. "The machines can't reach roof, ladders, or be used on exteriors, but that technology is on the way," said Klugherz. o