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The Importance of Understanding Property Flow in Hopper Bottom Bins

November 20, 2012

Hopper bottom silos are prevalent in commercial and industrial storage. A hopper bottom silo has a coned top and bottom with wall sections between. Support structures or the legs of the silo, combined with a leg ring or structural skirt, allow the unit to be free standing. External options or accessories required for loading, venting, and safety may change the appearance slightly, but the outward look of hopper bottom silos is generally consistent. Although they may feature a common appearance, there is more that goes into the design of these silos than meets the eye.
    A variety of products are stored in hopper bottom silos. Variation in property flow characteristics must be considered and incorporated into the silo’s design accordingly to avoid any complications. The true design of a silo is based on how the product will be discharged. There are three different types of flow that impact its design: mass flow, funnel flow, and expanded flow. These three variations determine the degree, material, and design of the silo. The former discrepancies my not be noticeable “at a glance,” but are the reason for hopper bottom silos.
    Mass flow means all material being discharged is in motion. This is achieved by having a cone degree of 55 or greater. The product motion is transferred down the walls of the silo and out of the bottom opening. Materials such as cement, fly ash, and flour have the properties of mass flow and will require a silo with a steeper cone.
    Funnel flow has the center or core discharging before the product held against the wall of the silo. The center moving product rotates carrying the non-flowing material along with it. Again, this is achieved with the degree of the bottom cone. Funnel flow designs typically consist of a hopper ranging from 40 to 45 degrees. Products like sand, salt, and fertilizer will flow through in a funnel direction.
    Expanded flow is a combination of funnel flow and mass flow. The upper storage section of the silo has the funnel started with the bottom section emptying with mass flow. A silo designed for this application would utilize a staged hopper cone or oversized outlets, often rectangular. These sections are often referred to as chisel bottoms.
Silos are great means of storing small or large quantities of bulk materials. Reviewing the characteristics of the product being stored will determine what hopper is needed for your silo. Correctly designed hoppers will eliminate issues such as rat holing and bridging. Proper designs reduce inadequate emptying, promote proper storage, and protect your investments.
    Customers ordering storage units should be mindful of the characteristics of the product being stored and ask questions regarding how products will flow through a silo. A hopper bottom silo built to customer design specifications is the best way to ensure that the silo will meet and exceed your expectations. Awareness of product density, angle of repose, and consistency of material are points that should be reviewed with the silo manufacturer. If elements are unknown, ask the manufacturer to collect that information and present it.
    Jenna Pattison is marketing and communications coordinator at Meridian Manufacturing Group. She received her Bachelor of Arts degree in Communications from the University of Calgary. She can be reached at jpattison@meridianmfg.com.
 

Meridian Manufacturing Inc.