In such cases, the joint can be ground out and re-welded. Often during the inspection, we observe cracks in the attachment joint before the complete (catastrophic) separation of the conical hopper occurs. This unnecessary scenario ends in equipment damage, facility shutdown and sometimes the loss of life. We have investigated past catastrophic failures and identified this mechanism as the root cause. The movements of the hopper shell as the silo is filled and emptied can cold-work (fatigue) this joint to the point of failure. This reduced cross-section at the weld creates high tensile stresses in the joint. The weld joint is typically specified during the design phase to have a complete penetration fillet weld, but because of the limited access to the joint during the welding process, the weld metal does not penetrate the thickness of the hopper wall and can be as little as half the thickness (see photo). We at J&J have inspected many of these type weld joints and found some of them to be improperly installed when the silo was initially fabricated. The key issue here is the welded joint that is used to attach the hopper section (see figure at right). The thin hopper plate behaves as a tensile membrane structure that moves (strains) relative to the cylinder at the attachment point as the silo is filled and emptied. Most conical hoppers are generally thin plate type structures that are welded to the cylinder shells, which can also be constructed from a thin plate or something heavier like a built-up ring girder. The material in the cylinder above the hopper can press down with loads as high as 70% of the silo contents. I’m primarily talking about circular silos constructed of welded steel with conical hoppers for the purpose of this discussion, although it applies to the other types of hoppers as well.ĭepending on the size of the silo, the converging structure must safely hold up its contents along with the contents of the cylinder section that it is attached to. Flow problems aside, it is hard to think of silos without hoppers. These converging parts are necessary and provide a practical means of directing and concentrating the discharge of the silo contents to a specific point where it can be efficiently handled.
Hoppers can be conical, wedge, pyramidal or a combination of these shapes. My definition of “hopper” is a sloped wall converging structure suspended along its top edge from the vertical wall portion of the silo.
Bulk Handling Equipment Design & Supply.
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