A steel storage tank's coating is only as good as its ...

Because unwanted corrosion can contaminate liquids, it&#;s important to ask suppliers questions that allow the confident choice of a coating with proven field performance. Moreover, how the coating is applied further separates the quality of one tank from another. Coating application will take place either at the factory or in the field.

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Surface preparation

Uncoated steel forms a thin corrosion layer as soon as the raw sheets are laid out. Tank manufacturers should remove this layer prior to coating application. Some manufacturers prepare the surface with chemicals to prepare the surface, which is cost effective, but of limited efficacy.

A more vigorous surface preparation, such as blasting with grit, not only removes surface corrosion but also roughens the steel surface so a coating can better grip or adhere to it. A proper sandblast procedure yielding the maximum cleaning and surface profile should meet standards such as "NACE No. 2/SSPC-SP 10 Near White Cleanliness" to ensure the coating adheres properly.

Once the surface is prepared, because of its vulnerability to environmental contamination, the clock is ticking. Flash rust can occur in as little as 30 minute&#;s time. This aspect of the coating process is often overlooked, yet it&#;s critical to avoid compromising the coating&#;s quality. The length of time between surface preparation and paint application affects the expected field longevity. Some users specify a first coating be applied less than 15 minutes after the surface preparation is complete.

Airborne particles, as environmental contamination, can negatively impact coating surface adhesion. Tank manufacturers employ significant quality control measures, including cleaning and filtering equipment, to prevent air particulates from interfering with coating adhesion.

Factory-coated tanks will undergo surface preparation as part of their application process. Tanks that receive field coatings should be inspected to ensure good preparation on all surfaces, including the underside of the floor, which is not easy to reach, as well as on the underside of roofs, which requires blasting shot upward. Good surface preparation includes uniform grit blast coverage even in hard-to-reach spots and achieving a uniform mill profile for maximum coating adhesion.

Testing applied coatings

Throughout a factory-applied coating process, incremental tests ensure that it advances without compromise. Wash water temperature and pH, proper grit blast profile, paint booth temperature, humidity and air quality are all potential test parameters.

Applied to individual tank panels under environmentally controlled conditions, coatings are closely monitored throughout the process. Two common factory-applied coatings are high quality glass-fused-to-steel (porcelain enamel) and powdered epoxy.

High-tech coating lines control the process by measuring coating quality and thickness, capturing overspray, monitoring air particulates and testing for uniformity. Glass-fused-to-steel coatings are applied in layers of two to three coats, including a top coat. Powdered epoxy coatings are applied electrostatically to ensure uniform thickness and good adhesion to corners and edges.

Some epoxy-coated tanks have a coat on the tank exterior in lieu of a top coat of acrylic polyurethane. However, acrylic polyurethane offers better UV resistance, which results in less chalking and fading.

Factory application

Fusing glass to steel requires molecular interaction, resulting in a coating that blends glass and steel so that one cannot be separated from the other. The fusing takes place at the steel surface at F, forming a tight chemical comingling of materials. Glass frit cannot be fused to a substrate in the field, therefore glass-fused-to-steel coatings are only factory applied.

Some glass-fused-to-steel tank manufacturers fire the panels after two coat applications, then follow with a third coat and second firing. The most advanced tank manufacturers have technology that fuses three coats to the steel during one firing. This reduces the cost and time required for the coating application, as well as the risk of contamination between coats.

How the curing proceeds varies from tank to tank. Factory-applied epoxy coatings sometimes are cured by ambient air; however, a thermal cure is preferred. Thermal curing takes panels through ovens at temperatures up to of 400 F.

Epoxy coatings applied and cured under factory-controlled conditions are proven to be durable and long lasting. But beware of tank vendors who claim their epoxy is "fused" to steel, as this is technically impossible.

Field work

In the field, tanks often are sand blasted and coated with epoxy, using a hand-held sprayer. Some localities require sand blasting and that paint overspray be captured by tenting, to prohibit silica or VOC release into the atmosphere.

Field installation often includes applying two coats, typically a primer followed by a top coat of such as acrylic polyurethane. It is possible, although difficult, to monitor and control air particulates during field-coating application. Contamination is a concern, as curing is subject to local ambient conditions such as air temperature and humidity.

Field applied coatings most commonly rely on ambient air to cure, which can take up to 72 hours. The ambient air curing process is subject to weather and environmental conditions, including temperature, humidity, wind and dust.

All tank coatings should be tested for quality. Missed spots often are not detectable by the human eye. Rust and corrosion will start at a pin point-sized missed spot &#; or "holiday" &#; and spread. While corrosion creep will not occur with glass-fused-to-steel coatings, all coatings &#; including glass &#; must be holiday tested for missed spots prior to the tank&#;s first use.

Test time

The basic test for holidays uses a 12-volt tester. A wet sponge with an electrical charge is moved across the surface. The metal behind the coating also receives a charge. Wherever the wet sponge touches bare metal, a circuit is completed and the holiday is made known by either an audible or visual indicator.

A more stringent test, however, is the high-voltage spark detector. This test uses the material&#;s dielectric strength to determine the minimal coating thickness. Both a wand and the tank wall receive a charge. If the electrical current reaches bare metal or detects thinly applied coating, a spark appears, showing the exact location. High-voltage defect testing validates coating uniformity across surfaces, including tough-to-reach areas. Thinly applied coating may allow premature corrosion. High-voltage defect testing is not common among tank manufacturers, and its use is another way to measure tank-coating quality.

For field applications, both tests may be employed. However, a high voltage spark detector test should be requested. When coating with a hand-held sprayer, it&#;s difficult to ensure uniform surface application. Outside the factory setting, the best scenario includes hiring an independent certified coatings inspector to monitor the coating-application process, ensuring proper tenting, surface preparation, checking mill thickness and other parameters.

Many coatings manufacturers recommend using an NSF-certified inspector. It may be tempting to inspect coatings yourself, but a trained expert certifies that coating is properly applied, ensuring the longest possible life for your tank.

Final words

Your tank is only as strong as its coating. The better the application process, the longer the tank will last. While it&#;s tempting to choose a tank based solely on up-front cost, do your research based on total life cycle cost.

A field-applied coating may require repainting sooner than a factory-applied coating. Ask vendors to explain their coating process. Ask for data and case histories about how coating performs over time. Ask about maintenance and recoating requirements. Finally, investigate quality control and testing measures used.

While most tank manufacturers claim to have the "best" coating, this can only be proven through years of performance. Therefore, audit or visit the factory where your coating will be applied, or visit a site where a tank is receiving a field-applied coating, to validate the steps taken during the coating application process and ensure your tank manufacturer delivers a solution for long-lasting, durable storage.

Mark Eklund P.E. serves as materials specialist for CST Storage. With more than 35 years of experience serving the wastewater treatment market, Mark provides design leadership and solutions management for CST Storage. Mark earned a BS degree in civil engineering with an environmental emphasis from the University of Wisconsin. Contact: [ protected] or 913-621-.

CST Storage specializes in the manufacture and erection of complete storage systems &#; enamel and epoxy coated bolted tank designs, domes and covers, integrated reclaiming systems and more for liquid and dry bulk storage worldwide. CST Storage tanks and covers are manufactured at ISO Certified facilities and are found in 125 countries worldwide.

The other challenge was the very poor soil on which the water tank was installed. Cleveland says construction involved driving concrete precast piles into the ground to support the tank and associated pump station. This impacted both the cost of the project and its schedule. &#;It&#;s also a very noisy operation,&#; he says, so communication with the neighbors and sensitivity to working hours were important considerations.

What Are Your Options?
Containment Solutions Inc. manufactures fiberglass tanks in sizes up to 60,000 gallons that can be tethered together for larger storage and buried underground. According to David Heiman, director of marketing, business for its tanks has been exploding in the last 10 years.

Heiman attributes this growth to the increased popularity of green projects, including water capture and water conservation, which are important for point collection with Leadership in Energy and Environmental Design (LEED) buildings applying for certification. Reducing the consumption of potable water for non-potable use by capturing rainwater and/or graywater for irrigation will earn points toward gold, silver, or platinum certification.

Historically, municipal water agencies were not interested in fiberglass water tanks, but Heiman says this is changing in a time with little or no funding. He says these agencies are looking for alternate water conservation methods and are encouraging customers to conserve water in tanks. New developments are installing fiberglass tanks in rural areas where there are no municipal services.

Customers prefer multiple large tanks, says Heiman. The ability to build a hybrid system for customers, such as schools and hospitals who are routinely buying fiberglass tanks creates opportunities for water conservation, particularly in summer peak demand periods. Rainwater can be captured in one tank, graywater in a second, and water condensate in a third. Heiman says the University of Binghamton in New York did just that. It recently bought four 50,000-gallon tanks with separate outlets to store rainwater in some and graywater in others.

American Structures designs, fabricates, installs, and services bolted and welded stainless steel storage tanks for the agricultural, industrial, municipal, and wastewater industries. Its water tanks are used to store potable, recycled, and stormwater, as well as water for fire protection. They are made to order, and come in all sizes.

Earl Wildenberg, president of American Structures, says, &#;People are getting really excited about stainless steel. It doesn&#;t have to be painted; it is resistant to corrosion, rust, and relatively maintenance-free; it is environmentally friendly; it blends into the landscape.&#;

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Municipal water agencies don&#;t have to worry about refurbishing a stainless steel tank in five years, he adds. Water agencies are now looking at the cost of refurbishing existing tanks and comparing those numbers with the costs of a stainless steel tank.

Wildenberg says the company has installed a lot of smaller tanks, which can be designed for expansion later. The tank is unbolted at the bottom, the tank is jacked up and new panels are installed. They recently installed two 100,000-gallon tanks in North Dakota for a mining company. There are no differences in installing small or large tanks.

&#;It&#;s like building a garage,&#; he says. &#;Once you build an envelope, you can put anything in it.&#;

United Tank Systems is a distributor for Fusion Tanks, manufacturer of glass-fused steel tanks, headquartered in England. It also manufactures both epoxy-lined steel tanks and stainless steel tanks in all sizes. Joel Portmann, company president, says glass-fused bolted steel water tanks are like putting water in a glass.

Glass-fused steel tanks have advantages over other types of tanks. They can be built up to accommodate growth, they can be moved to another location for other uses, and they have near-zero life-cycle costs, compared to concrete and welded steel tanks, which must be recoated about every 10 years and be maintained to avoid biofilm growth, says Portmann.

What About the Price?
Glass-fused steel tanks are price competitive with concrete tanks, stainless steel and welded steel tanks as size increases beyond 200,000 gallons, but pricing is dependent on region and customer requirements, Portmann says. For example, in Wisconsin, a tank has to be built to handle snow on its roof, but in California, seismic regulations require a heavier tank&#;these variations all impact price. The cost of stainless steel tanks is tied to the price of the metal itself and becomes less competitive pricewise as the size increases.

&#;There have been times when stainless steel is a lot cheaper and times when it is more expensive,&#; says Portmann. Operations and maintenance is similar to that recommended for glass-fused steel tanks.

Wildenberg agrees that stainless steel is cost-competitive initially and has low operations and maintenance costs since it is resistant to corrosion and rust, making it cheaper to maintain.

Another element affecting price is the ability to assemble the tank at the factory if it is small enough and can be trucked to the site, thereby decreasing costs. The cutoff is 20,000 gallons, Portmann says. However, transportation of the tank materials outside of the region where they are manufactured also increases the total price.

Portmann says epoxy-lined steel tanks are the least expensive type, and are usually purchased by private companies for chemical and other liquid storage uses. New improved epoxies are now coming on the market. The original epoxies are holding up well on tanks, he says, although some require special coatings. It is still too soon to say how well the new epoxies will endure, he says.

Pricing can be a challenge, says Heiman, and can vary depending on the fittings and accessories customers request for the tanks. Fiberglass is easy to customize, but still economies of scale rule, he says. If changes are called for in the field during installation, they can be made there, in contrast to concrete or steel tanks.

Prices for fiberglass tanks are comparable to those for concrete tanks, and with water rates rising, paybacks are improving, Heiman says. Life-cycle costs for fiberglass tanks are much lower, since they don&#;t rust, because biofilm such as bacteria and algae can find no home in the underground environment due to their smooth inner surfaces. Heiman says biofilm needs a textured wall to grow on, as well as sunlight. Once installed correctly, fiberglass tanks will operate for decades without maintenance, he says.

Heiman contractors like fiberglass tanks because of the ease with which they are installed. Anchors can be transported on the same truck as the tanks, and smaller cranes are needed.

Good O&M Will Protect Assets
Operations and maintenance (O&M) standards for water tanks developed by the American Water Works Association (AWWA) have been established long enough that they have become the industry standard and adopted by virtually all the states and some countries abroad.

AWWA M42, Manual for Steel Water Storage Tanks is referenced several times in this story. Standards are found in D100 for welded steel, D102 for coating steel water storage tanks, C652 for disinfection of water storage facilities, and G200 for distribution systems operation and management. Standards for concrete tanks can be found in D110 and D115, and standards for fiberglass tanks are found in D120-09.

The following discussion was drawn from a white paper written by Randall L. Moore, vice president of special products and technical services at Utility Service Company, and an April Web seminar organized by AWWA featuring Moore as moderator with speakers Kirt Ervin, vice president for Water Quality at Utility Service Company, and Mark Johnson, director of metropolitan water operations at the Massachusetts Water Resources Authority.

The white paper, &#;A Sustainable Approach to Potable Water Quality Management,&#; can be found on the PAX Water Technologies website, under resources/white papers. The Web seminar, &#;Tank Inspections and Sustainable O&M Best Practices to Manage Water Quality&#; is available through AWWA&#;s education services. Additional comments were provided by Moore and Johnson in interviews.

Moore cautions that it is best to confirm you are viewing the most recent updates when you read the standards. The most recent update of AWWA&#;s reference book, Steel Water Storage Tanks: Design, Construction, Maintenance, and Repair, for example, was released in , he says.

Moore, Ervin, and Johnson stressed chemical cleaning, active mixing, and tank asset management as tools that will provide plant managers and operators the ability to comply with the myriad new regulations governing contaminants and disinfection by-products.

Inspections Are Important
In the Web seminar, Ervin recommends that all water agencies have a written inspection plan outlining frequency, procedures, and maintenance of records. A good inspection program can extend the life of an existing tank indefinitely, he says.

Types of inspections include weekly visual, plus once-per-year inspections via remotely operated vehicles if sedimentation is a problem, in which tanks can remain full and sediments can be removed. Dive inspections can be done every three years or longer. When tanks are drained for chemical washouts, every aspect of the tank, and distribution system can be inspected.

Johnson says that remotely operated vehicles (ROVs) are doing a great job now. A crawler tank can soak up sediment to produce a clean tank as a result. Once materials dredged from the tank are allowed to settle and dry out the remaining liquid can be discharged, he says.

Areas that should be inspected on a regular basis, says Ervin, include the roof and its coverings, low spots on roof plates, access hatches that may get unlatched and vents, which should be kept in good condition. Overflow pipes should have a good catch basin.

Openings through roofs or access hatches have allowed birds, squirrels, beer cans, and, yes, even eels, to get into the water. Roofs with low spots are prone to rust where water and snow can gather. Vents should be inspected at least once a year, and kept in good condition. A good vent has a fine mesh screen preventing wind-borne particles from entering the tank.

Johnson recommends taking outbound water samples from tanks weekly, and having them laboratory tested for bacteria. The monitored data and chlorine residuals, for example, can be plotted each week. He recommends using 1.0 milligram per liter (mg/l) chlorine residual as a measurement for an action threshold.

Furthermore, this schedule is a weekly opportunity to confirm access is intact, gates are locked, the fences are secure, there are no obvious tank problems, screens, vents, and hatches all secure, and there is no leakage due to rust or penetrations. Finally, Johnson cautions that tank ladders should not become cable trays for cell vendors. Renting space to cell vendors is a great source of needed income, he says, but don&#;t forget that these are water tanks, not antenna bases.

Johnson recommends AWWA&#;s G200-09, &#;Distribution systems Operations and Maintenance,&#; section 4.3.1, which provides a great inspection checklist.

Maintain Water Quality
When potable water sits in a storage tank, problems occur and the water stagnates. Johnson says to try and keep the age as low as possible. He quoted a white paper published by EPA and a Water Research Foundation study, which reviewed turnover rates and found they varied from one to three days and as much as five days. The Water Research Foundation study found that a turnover rate of 2.5 days of 40% of water in the tank became the optimum level.

Tanks that are well mixed are sustainable, because they consume less disinfectant, minimize the formation of disinfectant by-products, and avoid the need to empty the tank should nitrification occur, which depletes disinfectant residual. With active mixing, the common operational practices of &#;deep cycling&#; storage tanks, onsite chlorine boosting, and tank/main flushing used to remedy problems due to poor mixing can be reduced or eliminated within the storage tank.

Also, an active mixing system can minimize icing in tanks located in northern climates and reduce ice damage to the tank interior coatings, thereby helping to minimize biofilm growth and corrosion by-products, ultimately improving water quality.

Three water quality management tools can be used to maintain water quality. Washouts at least every three years are recommended in AWWA&#;s M42 Manual for Steel Water Storage Tanks to remove sediments and clean the tank. But neither pressure washing nor the disinfection process will kill and remove biofilm on interior surfaces.

As biofilm builds up, an increasing amount of disinfectant is required. However, a low-pressure NSF standard 60-certified chemical washout can remove all biological growth from the empty tank, as well as the mineral staining typically associated with the biofilm.

Removing biofilm and stain, while the water tank is empty makes inspection and failure repairs in the coating much easier and more thorough. Maintaining the integrity of the coating will minimize biological regrowth by eliminating anchor sites where colonization occurs. Eliminating surface roughness will reduce biofilm growth.

An active mixing system certified by NSF Standard 61 which operates 24 hours a day seven days a week is highly recommended to eliminate thermal stratification and achieve homogenous water chemistry within the storage tank. Moore sites a study, which shows that passive mixing of the water when the tank fills will restratify both thermally and chemically within four hours after pumping water into the tank.

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