Shop-Welded Tanks for Dry Bulk Storage: The New Extreme

During the last decade, the advances in dry bulk storage have been significant. In large-capacity applications (~500-5000 ton, ~3500-250,000 cu ft), bolted RTP (rolled, tapered panel) tank construction has grown in popularity. The advances and preference for bolted hopper tank designs is based on a variety of factors, including reliable field performance, low maintenance requirements, lower initial installed cost, and lower life cycle costs compared to concrete and field-welded tank construction.
    
During the same time period, the shop-welded hopper tank product line-up (with capacity range of ~20-600 ton, ~500-15000 cu ft) has also evolved to target specific bulk storage applications. The tank sizes continue to grow with many units in excess of 14 ft in diameter and 85 ft length being shipped as a single unit. Based on customer needs, each product maintains its service market, but also must incorporate design features, hopper/transition configurations, and coatings that will ensure long-term, trouble-free field performance.
    
Today’s shop-welded tank product line-up includes:
•    Standard skirted hopper tanks with walk-in door access
•    Standard hopper tanks supported on lugs or 4 legs
•    Commodity grain tanks supported on 6 legs
•    SITs (system installed tanks) – installation of the client’s system components at the tank manufacturing facility
•    Blend tanks
•    Mass flow, expanded flow, and funnel flow tank designs
•    XL shop-welded tanks uniquely designed for the power industry

    
Standard shop-welded tank designs have been utilized for years in a variety of markets. However, through the advancement of technology, along with improvements in coatings, design practices, and fabrication methods, existing limits are being surpassed and a new extreme has developed. The following four categories outline the advancement in capabilities that are now offered in the marketplace. The improvements made have broadened the dry bulk markets that are now served by a premier shop-welded storage tank.

XL Shop-Welded Capacities
XL shop-welded models represent a unique storage tank design utilized in the power industry. The units shown are factory built in two sections. At the jobsite, the sections are connected and the tanks are set in place. With this specific project, the units tower over 140 ft high. Based on their narrow configuration, wind strakes are also required. In comparison, field-welded units of the same size and configuration will cost two-and-a-half times the shop-welded counterpart.

System Installed Tanks
System installed tanks (SITs) have been around for years, but the new extreme today is pushing the limits on how much equipment you can install in the skirt of the tank. This includes installation of the customer’s components, insulation, control panels, elevated platforms, spiral stairways, electrical supply, etc. In essence, the client’s storage system is shipped completely installed inside the tank. At some shipping destinations, this is the preferred customer approach due to site limitations and restrictions.

Gravity Blend Tanks
Shop-welded gravity blend tanks are a common option selected for plastic resins to resolve problems associated with varying melt indices, color variation, and particle size, but they can also be utilized to provide a homogenous blend for aggregate storage. In essence, a blend tank discharges stored product from the tank that has been blended or mixed together. This is accomplished by material sampling from multiple graduated inlet ports inside the tank and channeling this material to a blend chamber located directly below the hopper. Gravity blend tanks are a problem solver at a lower cost than some alternatives.
 
Mass Flow Designs
By definition, a mass flow shop-welded hopper tank is one that is designed for first-in/first-out product storage. Basically, during tank discharge, every particle is in some degree of motion. A mass flow hopper tank design can resolve a variety of storage problems including material segregation issues. Some of the key design parameters required to achieve mass flow include the hopper slope, outlet size, and the hopper surface profile.

Shop-Welded Tanks That Resolve Customer Problems
Today’s shop-welded tank designs should be selected to resolve any storage problems identified in the past. If you haven’t experienced issues, then the selection process has been simplified. If you have encountered any of the storage problems noted below, then your selection should simply be narrowed to products that solve your storage problems.
    
The majority of storage tanks problems are not refurbish or retrofit situations, but rather shop-welded tanks that are not configured or coated correctly. Let’s look at some common design problems that should be addressed up front, not after the fact:

•    Material segregation during tank fill
•    Material segregation during tank discharge
•    Relative to plastic resins, color variation
•    Relative to plastic resins, variances in melt index
•    Relative to plastic resins, variances in particle size
•    Mass flow – the customer needed first-in, first-out material discharge and the product is discharging in a funnel flow pattern
•    Funnel flow – the customer’s product is mass flowing and the tank was designed for funnel flow discharge. Needless to say, the tank design loads are inadequate.
•    Material degradation – the material is degrading during transport, tank fill, and discharge.
•    Tank denting in sidewall – a preferential flow channel is present due to an improper hopper design slope, outlet size, surface profile, partially throttled discharge gate, and a variety of other factors.

Shop-Welded Tank Coatings
Paint failure is another concern that continues to surface. Why? There are certain products being stored that can attack a low-quality interior coating system, but this is not the norm in bulk storage. In most problem scenarios reviewed, the tank vendor did not provide a blasted surface profile prior to the coating being applied. Coating systems promoted by paint suppliers that do not require a blasted surface anchor profile are simply cheap systems and should be considered unacceptable for long-term performance in dry bulk storage applications. With the increased number of paint peeling issues, make sure to evaluate the coating system when selecting your new dry bulk tank.
    
Vendor-proposed coating systems should be tailored for the application and must be anchored to a blasted surface profile. Interior coating systems should always be designed to protect the stored material and the substrate surface. Additionally, interior coatings selected should provide corrosion protection and flow promotion. Exterior coatings should always be designed for environmental factors, including UV protection. A good one-coat system doesn’t exist for the dry bulk storage tank market. High-quality polyamide epoxies that incorporate rust inhibitors should be utilized for the primer/base coat, followed by an acrylic urethane topcoat system.

Summary
This article has focused on a small sampling of tank products that can solve storage problems and associated downstream material handling issues. Any of the problems outlined are preventable, and predictable, based on today’s industry knowledge of dry bulk storage applications. Today’s shop-welded tanks are available to offer commodity service or high-tech storage solutions. Always get the answers up front to your questions and then specify your shop-welded tank products for performance. With continued improvements in technology, coatings, design, and fabrication processes, the line-up of shop-welded tank products continues to expand. The new extreme for larger more complex designs that solve the storage problems of the past is the new benchmark of performance in dry bulk storage.

    Vince Horton is president, Tank Connection, a leading manufacturer of dry bulk storage systems. For more information, visit www.tankconnection.com.

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