Jeff Shelton, Integrated Global Services, explains how the ‘three Cs’ approach helped eliminate material flow issues at Panam Cement’s limestone silo.
Buildup problems in silos are a frequent occurrence. A buildup issue often referred to as ‘bridging’ or ‘ratholing’ is experienced in many different materials and industries. Often, this buildup issue is impacted by the environment. For example, high moisture content often makes material flow more difficult.
Bridging is the name given to a self-created arch that develops just above the outlet of the bulk material, silo, or hopper as it empties. A bridge forms when the wall friction holds up the ends of the arch. The material will compact in place and make an arch so strong that it is able to support the weight of the material. Often, it is easy to tell if the silo faces material flow issues, because operators frequently hammer the sides.
Ratholing occurs when the material flows in the channel directly above the outlet. This is common when the material is sticky. The material outside of the flow channel directly above the outlet will cake on the sides.
Common devices used to promote flow in silos are vibrators, sonic horns, and air cannons. The key to success with these products is their ability to fluidise the material and break the bond of the arch. Each device has been used successfully and has its benefits when used properly.
Air cannons were first installed in silos in the early 1970s. There are thousands installed successfully around the world. This is a proven technology for keeping the silo free from bridging and or ratholing. For air cannons to be successful, they must be installed correctly with the proper nozzles in the right operational sequence. Poorly installed air cannons can actually make a buildup problem worse.
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