by Peter Vodak
All liquids - whether wastewater, chemical, potable water or others - are aggressive toward the tanks in which they are stored. Such aggression results in corrosion, the principal enemy of the tank. Corrosion shortens the life of the tank, increases maintenance, and may contaminate the tank’s contents.
Municipal liquid tanks are most often purchased for use as standpipes, reservoirs and secondary containment. If your municipality is considering the purchase of a new liquid storage tank, the type of coating found on the inside will be one of the most important factors you consider. Today’s tank coatings are technically advanced, and they are a municipality’s best line of defense against corrosion for the protection of liquid material or water supplies.
Interior coatings provide protection for submerged surfaces, protecting the inside of your tank from materials that pass through, and protecting the materials themselves from outside contaminants. Quality coatings provide chemical resistance from any harsh substances that might be found in the stored material. Specialized coatings also may provide chemical resistance for strong liquids, such as acids and bases.
While the old adage that “a tank is a tank” is no longer true, tanks do often share the same materials of construction. The majority of tanks and silos are constructed of carbon steel, stainless steel, aluminum and concrete. What differentiates manufacturers is the coating. Today’s most technologically advanced coatings are of epoxy, which is most commonly applied electrostatically or as powder or liquid. Once applied and cured, epoxy coatings will bend without tearing or breaking. On the exterior, epoxy coatings may be combined with urethane topcoats to provide protection against the environment.
Protective Coatings
There are several factors to review when it comes to liquid tank coatings. Most importantly, the coating must offer proven corrosion resistance over long periods of time. This may seem elementary, but not all coatings are alike. Manufacturers should provide testing data for the coating they use.
All tanks marketed for municipal potable water storage must be ANSI/NSF Standard 61 certified for contact with potable water. The Standard 61 certification involves an extraction test which is run on samples of the coating that have been applied and cured in accordance with the manufacturer’s instructions. Harmful materials (solvents, heavy metals, etc.) must not leach out of the coating above the maximum allowable levels when the coating is exposed to water. The certification is based on a minimum tank volume, so one needs to make sure the coating is certified for the desired size of tank.
It is of utmost importance to ensure that the coating on the tank is specifically designed for the product to be stored. Potable water and wastewater tanks must have coatings that can stand up to disinfection and oxidizing agents such as chlorine and ozone. The storage of wastewater and wastewater sludges will require a coating that is designed to withstand the aggressive effects of ever-present materials such as fatty acids and hydrogen sulfide.
Specialized coatings designed to withstand high heat, extreme cold and high levels of acidity are available. Storing liquids in tanks with coatings not designed for the liquid will result in corrosion, potential safety hazards, greater maintenance requirements and a shorter tank lifespan.
Safety Factor
Coatings also play a role in safety. Coatings are application specific, and some coatings may react poorly with certain products. It is unsafe to use an old storage tank for a product for which it was not designed. If you are considering recycling a used tank for storing a new product (such as storing wastewater in an old potable water tank, or chemical storage in a wastewater tank), be sure to first have the tank evaluated for structural integrity, adequate design for the specific gravity of the intended product, and chemical and corrosion resistance of the coating. Incorrect use of a tank could result in severe corrosion, premature failure of the coating, harm to the product, or serious injury to personnel.
Application Process
The key difference in coatings today lies in the application process. The highest quality coatings are applied in the factory under environmentally controlled circumstances to ensure the most consistent application. Taken one step further, the best coatings are also thermally cured in the factory. Some manufacturers apply the coatings in the factory and then allow them to air-dry and cure with ambient heat, which exposes the quality of the cure to environmental factors such as dust and humidity. Other manufacturers outsource the coatings process altogether. The optimal coating solution is one that is both applied and thermally cured in controlled factory conditions, before the tank is shipped and erected in the field.
Tanks that typically receive their coatings onsite once the tank has been erected include field welded and concrete tanks. These tanks may need to undergo a chemical process to protect the coating while the tank is being erected (i.e., heat from welding may damage the coating). Be sure there are adequate quality control measures listed in the specification, and consider third party inspections.
Growing municipalities and metropolitan areas may encounter the need to move or relocate a storage tank. Moving a tank that has received its coating application in the field may result in damage and the need to recoat, as the painted coating may tear or break. Tanks featuring factory applied coatings may be moveable, because each panel is individually coated. If population growth would result in the need to expand a tank, a tank with a factory coating often allows municipalities to add rings (think upward) without affecting the coating on the original tank.
If choosing a bolted tank, be sure the bolts and hardware are corrosion resistant. Some tank manufacturers also offer corrosion-resistant bird and insect screens for protecting municipal water supplies.
Cost Review
When reviewing bids from tank manufacturers, make sure you are comparing apples to apples. If a tank manufacturer promises a low cost for a tank and corrosion-resistant coating, make sure you review the total lifecycle of the tank, not merely initial cost. A higher quality coating may have a higher up front cost, but offers lower life maintenance and recoating requirements - a long term savings for municipalities.
Manufacturer
It also may seem like common knowledge, but choosing a tank manufacturer with long-term experience in municipal tanks is a key consideration. How long has the company been manufacturing tanks? Do they use the most advanced coatings available? What are their quality control measures? How much of the manufacturing process do they outsource? What is their volume sold, in which markets and in what countries? Also, consider using a tank manufacturer with third party accreditation, such as ISO 9000 quality certification.
About the Author
Peter Vodak is Corporate Coating Engineer with CST Industries Inc. He is a NACE Certified Coating Inspector. Columbian TecTank, a division of CST, specializes in the design, fabrication and erection of tanks for liquid and dry bulk storage. The company manufactures bolted and shop-welded tanks of carbon steel, aluminum and stainless steel featuring factory applied and thermally cured epoxy coatings. Vodak may be contacted at [email protected].
