Keith McGuire, PE
If you’re driving a Prius, you’re clearly the kind of person who has thought about your automobile’s effect on the environment. While using a Prius to store your product is not recommended (although let’s face it, we have all seen vehicles that are also used as a waste storage container - especially if you have teenagers) it is commonly accepted that the Prius brand leads the industry in “green” automotive technology. So when you’re choosing your next vehicle for storage, why would you not care about environmentally friendly choices in storage tanks and silos?
According to the American Iron and Steel Institute, for every ton of steel produced, approximately 10 million Btu’s of energy are required and approximately 1.7 tons of CO2 are produced. Your car, in comparison, will release 24 lb of CO2 for each gallon of fuel used. So saving a ton of steel is equivalent to saving 142 gallons of gasoline as well as the resulting CO2 emissions.
When it comes to tanks, including fit-up and welding, field-welded steel tanks and silos are thick and heavy, requiring the use of substantially more steel in manufacture than comparable factory-welded and steel-bolted tanks and silos for a given volume of storage. Similar results are applicable for aluminum and other alloys.
The processes required for constructing field-welded steel tanks and silos utilize arc-welding equipment and air compressors operated by gasoline or diesel powered industrial generators, which have no emission-control features. Additionally, shielding gasses, noxious fumes, and consumables are released during the welding process, which contaminate the air and the ground. Factory-welded silos, on the other hand, are fabricated in enclosed buildings utilizing local electrical power. Airborne contaminants are captured and filtered. The silos arrive as completed units and there is no field welding or on-site fabrication.
When it comes to coating, field-welded steel tanks and silos require field surface preparation prior to painting. The choice of blasting media is an important consideration. Sand-blasting media may contain silica dust, which is a known carcinogen if inhaled, and requires special site preparation and safety equipment for the workers. Special tarps around the entire tank are often necessary in field coating to contain the contaminants from the blasting operation. Complete cleanup and 100% containment is impossible, and wind, rain, temperature, and humidity all play a role on how well the environment may (or may not) be protected.
Conversely, factory-welded and bolted tank and silo steel surfaces are prepared by blasting with recycled steel grit and/or shot at the factory in a controlled environment with proper safety equipment and controlled air filtration for worker safety.
Liquid paints and epoxies contain solids that are suspended in solvents. Solvents contain volatile organic compounds (VOCs). The release of VOCs into the atmosphere constitutes a primary form of air pollution and can contribute to the formation of smog. Liquid paint and epoxies are the typical choice for coating welded tanks and silos in the field.
Using normal application rates, approximately 3.8 lbs of VOCs are released into the air for every 100 sq ft of painted surface using liquid paints and epoxies. Typically, the interiors and exteriors of field-welded steel tanks and silos are coated on-site using this process. Re-coating of field-welded tanks is typically performed every 8 to 12 years and the environmental impact is substantial over the life of the structure.
On the other hand, bolted steel tanks are fabricated and coated at the factory under a controlled environment. Utilizing coating processes containing 100% solids, such as epoxy powder and glass-fused-to-steel, no solvents are used, and thus no VOCs released into the environment.
Factory-welded tanks and silos typically are coated with liquid epoxies that are high-solids, low-VOC that can be oven-cured in a well-controlled and enclosed environment. Factories are required by law to comply with limitations on the amount of VOCs that can be released by location over a period of time.
So just as we turn to “green” alternatives for other things, it’s clear that numerous choices also exist when it comes to selecting a storage tank or silo. Thankfully, environmentally friendly options do exist. So please consider the environmental factors when making your decision, and drive carefully.
Keith McGuire, PE, is manager of engineering design and development, Columbian TecTank, Kansas City, KS.