|The highest-quality coatings
are electrostatically applied
and thermally cured under
factory-controlled conditions to
ensure consistent application.
Ensuring that dry bulk storage tanks and silos endure for many years and require little maintenance is dependent on their resistance to three key elements: corrosion, abrasion, and chemicals. Corrosion is the principal enemy of the tank. It shortens the life of the tank and increases the possibility of contamination of the stored dry bulk product. Abrasion wears the tank wall, hampers flow, and may cause corrosion. Stored dry bulk chemicals are aggressive toward tanks, and like abrasion, may eventually cause corrosion.
Corrosion is a problem whether the dry bulk material requires frequent loading and unloading, or is limited to standing storage over long periods of time. Only one defense exists to protect your tank against corrosion: the coating. If you’re considering the purchase of a new tank or silo, the coating will likely be one of your most important considerations.
Coatings can be found on both the interior and exterior of a storage tank. Many high-tech coatings exist today that offer tremendous resistance against corrosion and abrasion. Specialized coatings also provide chemical resistance for strong liquids, such as acids and bases, as well as extreme temperatures and other factors. The slickness of the coatings promotes flow to ensure all product passes out of the tank or silo, leaving minimal residue.
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—for exceptionally large volumes—concrete. What differentiates manufacturers is the coating. Some examples of coatings include epoxy (such as Trico-Bond EP), glass fused-to-steel (such as Vitrium), high-heat coatings, cold weather coatings, and coatings designed to withstand high levels of acidity.
Today’s most technologically advanced coatings are made of epoxy, which is most commonly applied electrostatically or as a powder or liquid. In general, electrostatic application coats the edges better than liquid epoxy, which tends to draw away from edges. 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.
The latest technology for dry bulk storage is Trico-Bond EP coating. A highly engineered thermosetting-modified epoxy powder, it has undergone more than five years of testing before entering the market. It has met FDA requirements for food contact and is utilized as an interior liner and exterior primer.
Dry Storage Considerations
Two considerations are important when reviewing tanks for dry storage: flow promotion and abrasion resistance. A tank buyer should first determine a product’s abrasiveness and bulk density and consider its flow requirement. Is the product free-flowing or non-free-flowing? A buyer not familiar with his or her product’s flow characteristics should have it professionally flow tested before buying a tank.
Flow testing will determine the proper angle for the hopper and hopper outlet, and will provide requirements for flow or blending equipment needed to ensure proper flow. A flow expert can determine the proper flow angle based on the slickness of the tank coating chosen. For example, in testing, Trico-Bond EP outperformed a 2B stainless-steel finish in promoting flow for several stored products. Slickness is a key element in maximizing a flow system; a relatively smooth wall and slick coating will achieve good flow promotion, continuing to pull out product and minimize residue.
Abrasion resistance is commonly tested by the “falling sand” test. This measures the stream of falling sand required to wear through a coating. At a normal 3-mil dry-film thickness, many older coatings require 20 to 200 L of sand before wearing through. In contrast, today’s high-quality coatings may require up to 600+ L of sand. Manufacturers should provide the testing data for the coating they use.
The Safety Factor
|Factory-applied coatings are optimal because coatings applied in the field may have missed spots, and their cure is affected by dust, wind, and humidity.|
Coatings also play a role in safety. Some coatings are designed for storing a wide variety of products. When it comes to safety, the most important consideration is making sure tank coating is suitable for the application. Coatings are application specific. If improperly applied, some coatings may react poorly with certain products.
It is unsafe to use an old storage tank for a product for which the tank was not designed and tested. Companies considering recycling a used tank for storing a new product (such as using a liquid storage tank for dry material) must be sure to first have the tank evaluated for safety, flow, corrosion resistance, and chemical resistance. Incorrect use of a tank could result in clogs, massive product drop, harm to product, or serious injury to personnel.
If the tank is used for a food application, the coating should be FDA compliant. Likewise, liquid tanks must be NSF certified for drinking water storage.
The Key Difference Is Application
|Electrostatic application of powder epoxy coats the edges better than liquid epoxy, which tends to draw away from edges.|
Tank manufacturers often choose a coating system based on the cost, their available space for installing the coating system, and the coating’s variety of uses. The key difference in coatings lies in the application process.
The highest-quality coatings are applied at the factory under environmentally controlled circumstances to ensure the most consistent application. Taken one step further, the best coatings are also thermally cured at the factory. Some manufacturers apply the coatings at the factory, but then allow them to air-dry and cure with ambient heat, which exposes the cure to environmental factors such as dust and humidity. Other manufacturers outsource the coating process. 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. This is often a feature of factory-welded and -bolted tanks. Field-welded and concrete tanks most often receive their coatings on-site in the field once the tank has been erected. These tanks may need to undergo a chemical process to protect the coating while the tank is being erected. (For example, heat from welding may damage the coating.) Tank buyers reviewing this type of tank must make sure there are adequate quality control measures listed in the specification and consider third-party inspections. They should be aware that once a tank has been erected in the field, there are often areas of the tank that are extremely difficult to sandblast or fully prepare for field coating. Even “missing the smallest spots” leaves exposed areas open to corrosion, residue, or both. Also, the thickness of coatings applied in the field cannot be controlled, which may result in uneven application. Finally, buyers should keep in mind that the curing process in the field will be affected by weather and the environment—dust, humidity, and wind.
Are coatings for tanks always necessary? “Black tanks,” or tanks without coatings, store products that are themselves protectants. Tanks that store soybean oil or abrasives do not need a coating.
Would a company ever consider moving its tank or silo in the future? 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 are more mobile, because each panel is individually coated. If the need would arise to expand the tank, a tank with a factory coating allows for the addition of rings (think upward) without affecting the coating on the original tank walls. A tank with field-applied coating will require recoating after the expansion is complete.
Cost Review: Compare Total Life Cycles
|Corrosion is the principal enemy of the tank. Abrasion wears the tank wall, hampers flow, and may cause
corrosion. The best defense is a factory-applied coating.
If a tank manufacturer promises a low cost for a tank and coating, the buyer should make sure that the competitive products it is comparing are equal over the total life cycle of the tank. A higher-quality coating may have a higher up-front cost but may require less lifetime maintenance and recoating. Companies should ask the tank manufacturer about the cost and likelihood of recoating, which is usually an expensive process. They should take into consideration the specifications of each tank being offered, especially the coating application sections. A good quality coating often means lower maintenance, offering fewer headaches during the tank’s life cycle.
Locating an Experienced Vendor
It is important to gain experience before choosing a tank vendor and coating. Experienced vendors often offer the highest-quality coatings and understand the nuances in products that are employed for dry bulk storage. End-users should consider the experience of the tank manufacturer—how long the company has been fabricating tanks, whether its coatings are factory applied and thermally cured, its quality control measures, the volume of tanks it has sold and the countries in which it has sold its products, and the markets to which the manufacturer sells. Tank manufacturers with third-party accreditation, such as the ISO 9000 quality certification, should also be considered.
For more information about choosing tanks and coatings, visit www.tanks.com.
David Wheat is director of dry bulk sales for Columbian TecTank, Kansas City, KS. He can be reached at [email protected] or 620-421-0200. Columbian specializes in the design, fabrication, factory coating/thermal curing, and erection of tanks and silos for the dry bulk industry. The company manufactures bolted tanks and shop-welded tanks of carbon steel, aluminum, and stainless steel featuring factory-applied and thermally cured epoxy coatings. For more information, visit www.columbiantectank.com.