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Considerations for Choosing a Storage Tank

November 13, 2012
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If you’re in the market for a new storage tank, you may find yourself faced with several questions: What are the differences between welded silos and bolted tanks? Why is a tank’s coating so important? What is the average construction timeline? Should maintenance be considered in the cost?
    Reviewing key design considerations will help to determine what you need (and don’t need) in a storage tank and ensure you get the best dry bulk storage vessel for your budget.
    It is most important to remember that dry bulk materials such as fly ash, cement, lime, food, chemicals, and minerals each have their own specific storage requirements. Those requirements will affect the tank’s size, materials of construction, design, and coating. To determine what type of tank you need, start first with a basic consideration – the construction material.

Materials of Construction
What is the volume and type of material to be stored? The material of construction for a tank often will be determined by the type and volume of material to be stored. Carbon steel, stainless steel, and aluminum are common construction materials available for field welded, factory-welded, and bolted tanks. Plastic, fiberglass, and factory-welded tanks work well for small volumes. Exceptionally large volumes may require field welded steel and concrete tanks. Steel bolted tanks are versatile, accommodating a wide range of dry bulk materials and volumes.
    Concrete tanks must be poured and cured. Their construction is reliant on good weather and environmental conditions. Welded steel tanks are often shipped to the site in panels and erected by certified welders. Before use, the welds must undergo third-party inspections. Environmental conditions and weather play a role in a welded tank’s construction cycle.
    Bolted tanks come in two primary designs: flange panel and flat panel. A flanged-panel design takes advantage of gravity to form a compression seal. The weight of the tank panels and product loads compresses the gaskets to form and maintain a leak-tight seal. Flange-panel bolted tanks are erected with scaffolding while flat-panel bolted tanks are erected with jacks. Both designs of bolted tanks are shipped to the site in panels and enjoy quick turnaround due to their construction with hardware and gaskets.
    Factory-welded steel tanks are constructed prior to shipment. They are transported to the site in one unit and stood upright. Their erection is quick, but they are somewhat limited in the volume of material they are able to accommodate.

Size and Location
Will the tank and its construction be limited by space or height requirements? A limited space may require the tank to be of smaller diameter and taller height. Unlimited space or spaces with height restrictions will allow the tank to be of wider diameter and shorter height. Both designs – tall and thin as well as short and wide – can work well to store a variety of materials when properly designed.

Proper Design
Have you considered flow testing? Whether dry foods, plastics, chemicals, wood products, or minerals, each material has its own unique requirements. Consider your product’s abrasiveness, the bulk density of the product, and importantly, its flow requirement. Is your product free flowing or non-free flowing? If you are not familiar with your product’s flow characteristics, you should have it flow tested before buying a tank. Flow testing will determine the proper material of construction, hopper slope and outlet size needed for reliable flow.  

Components Installed at the Factory
If you’re choosing a factory-welded tank, would it save time to have components installed at the factory? Some manufacturers of factory-welded tanks offer the ability to install customers’ components prior to shipment, cutting down on time and cost for field-installation. Common factory-installed components include bin activators, gates, valves, feeders, pumps, control panels, fans, sensors, insulation, and the associated electrical connections.

Industrial Use Versus Commercial Storage
What is the frequency of filling and emptying your silo? For long-term storage that may require filling and emptying only a few times a year, corrugated tanks may be a good choice. Bolted tanks, along with welded or concrete, may be filled and emptied day in and day out. Another consideration will be the need for a cone-hopper bottom versus a flat bottom.

The Environment
How might the environment affect the tank? Wind loads, seismic conditions, and roof loads (such as snow loads) are important considerations in the design of a storage tank. Tanks of wider diameter and shorter height are better suited for areas that experience greater seismic activity and higher wind loads. In addition, upgraded exterior coating systems should be considered in corrosive, coastal and industrial environments.

Think About Expansion or Moving
Will your growing business require expansion or moving of your storage vessel a few years from now? Expansion (think upwards) is an important consideration. Expansion is most practical when the original foundation and lower portion of the initial tank are built with the future expansion in mind. More often than not, vertical expansion of an existing tank is more cost effective than constructing a second tank.
    Concrete tanks and field-welded tanks are not easily moved and difficult to expand. Factory-welded tanks are moveable, but not easily expandable. Bolted tanks are the best choice for moving and expansion because new panels may be installed upward with hardware and gaskets.

Coatings – Corrosion Control
What type of coating is best? Corrosion is the principal enemy of the storage tank. So the coating is one of the most important factors you will consider. But taken one step further, the process in which the coating is applied will be the best determination of the quality of the coating you’re getting.
    Most tank vendors offer a technologically advanced coating designed for a variety of dry bulk materials which provide exceptional corrosion and abrasion resistance. Some coatings are available to meet specific design criteria, such as for extreme temperatures. Commonly used are air-cured liquid epoxy coatings. However, powder epoxy coatings offer better impact, abrasion-, and corrosion-resistance properties than air-cured liquid epoxy coatings. On the exterior, powder and liquid epoxy coatings often are combined with urethane topcoats to provide protection against environmental elements.

Coating Application Process: the Most Important Factor
How is the coating applied? How is the coating tested? Corrosion takes advantage of “missed spots” in the coating application to wreak havoc. Therefore, the key difference in coatings lies in the application process. The highest quality coatings are applied and cured in the factory under environmentally controlled circumstances to ensure the most consistent application. Some manufacturers apply the coatings in the factory and 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 altogether. The optimal coating solution is one that is applied and thermally cured in controlled factory conditions, before the tank is shipped and erected in the field. High quality bolted and factory welded tanks offer this feature.
    Field-welded and concrete tanks usually receive their coatings onsite once the tank has been erected. Be aware that once a tank has been erected in the field, there are often areas of the tank that are extremely difficult to sand blast and/or fully prepare for coating. The thickness of coating applied in the field cannot be controlled as closely as is possible under ideal factory conditions, which may result in uneven application. If you are reviewing field-welded or concrete tanks, make sure there are adequate quality control measures listed in the specification, and consider third-party inspections. Finally, weather and the environment – dust, humidity, temperature and wind – will affect the quality and timeliness of the curing process in the field.

“Green” Tanks
How good is your tank for the environment? If environmental concerns are important to your company, consider that some tanks are more “green” than others. The process of field welding presents challenges to the environment. Sandblasting to prepare the surface as well as the field application of coatings includes the possibility of overspray and the release of VOCs into the environment. Safety measures must be taken to capture the VOCs. Factory-welded and steel-bolted tanks are most green because their coatings are applied under controlled conditions at the factory. In addition, bolted steel tanks require less steel in the manufacturing process.

Testing the Coating
How is the coating tested? Because the coating is such an important consideration, make sure the quality control tests used are up to standard. A wet sponge detector is commonly used by tank manufacturers. A wet sponge with an electrical charge is moved across the surface of the cured coating. The metal behind the coating also receives a charge. Wherever the wet sponge touches a “missed spot” or bare metal, a current is completed and the holiday is made known by either an audible or visual indicator. However, this test is accurate only for bare spots. It does not test for thinly applied coating. A more accurate test, the high-voltage spark detector, also shows where coating does not meet minimum thickness requirements. Both a wand and the tank wall receive a charge and where the coating is bare or less than minimal thickness required, a spark appears showing the exact location of the holiday. Determining thin spots in the coating is vital because thinly applied coating may allow for premature corrosion of the tank wall.

The Safety Factor
Is the tank safe? Coatings play a role in safety. It is unsafe to use an old storage tank for a product for which it was not designed and tested. If you are considering recycling a used tank for storing a new product, be sure to first have the tank professionally evaluated for safety and corrosion resistance. Incorrect use of a tank could result in premature failure of the tank, harm to the product, or serious injury to personnel. Also keep in mind that the coating for tanks used for storage of food should meet FDA guidelines.

Construction Cycle
How quickly do you need the tank in service? Small tanks that ship in one piece such as factory-welded tanks may be put in service quickly from the time of order placement. Bolted tanks also require shorter construction timeframes because weather and the environment play little role in erection (they may even be erected in the snow). They also require no weld inspections or time for curing. Field-welded and concrete tanks require the longest timeframes (up to 4-5 times longer than bolted or factory welded) due to their reliance on weather, the environment, on-site coating preparation and application, and weld inspections.

Tank Features
What features are available for my specific application? A variety of features are available for dry bulk storage vessels. These include spiral stairways, access platforms, guardrails, safety ladders, drive-through designs (suitable for trucks and railcars), support structures, custom components, stacks, deck filter flanges, level indicator openings and nozzles, manways, pressure relief devices, etc. Talk to your tank manufacturer for available options.

Cost Review: Compare Total Life Cycles
When reviewing bids from tank manufacturers, make sure you compare apples to apples. Up front cost is important, but maintenance requirements are also an important consideration. For example, field-applied coatings may require recoating after 10 years. A coating applied under stringent conditions in the factory may have a higher up front cost, but offer a more durable cure and longer life cycle. A high-quality coating often provides less maintenance and recoating requirements overall, which saves time and money in the long run.

Locating an Experienced Vendor
How long has your vendor been in the tank business? Experience is key factor for choosing a vendor so ask them specific questions about their experience. How long have they been manufacturing tanks? What is their coating application and testing process? Are their coatings applied at the factory or in the field? How many tanks have they sold for applications such as yours? In your market? In your country? What are their quality control measures? Do they offer third party accreditation, such as API 12B and ISO 9001 quality certifications? Lastly, beware of manufacturers that do not have quality certifications, but suggest they build to those standards. If they are not audited and certified, then the work they do is not guaranteed to those standards.
    David Wheat is director of inside sales for CST Storage, a division of CST Industries Inc. CST Storage specializes in the design, fabrication, factory coating and construction of tanks and covers for dry bulk storage applications. For more information, call 620-421-0200 or visit www.cst-storage.com.

CST Storage