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A technical ‘top 5’ of bulk solids handling mistakes

World Cement,

Designing silos without bulk solid properties
All too often, silos are designed using minimal flow properties information like bulk density and particle size. While the design procedure for a liquid pumping application requires properties of density and viscosity, the design procedure for a silo requires key flow properties, like cohesive strength, coefficient of sliding friction, and bulk density over a range of pressures, for specification of a hopper’s outlet size/shape and its slope.

It is imperative for an engineer to consider the factors that affect bridging and ratholing tendencies for a bulk solid including variables of moisture, particle size, and storage at rest. Knowing this information will allow proper selection of a silo discharge pattern, namely funnel or mass flow.

Funnel flow occurs when the sloping hopper walls of a silo are not sufficiently steep and low enough in friction for material to flow along the walls. Under these conditions, particles slide on themselves rather than on the hopper walls, and usually a small internal flow channel develops. Funnel flow is only suitable for solids that are coarse, free flowing, and not prone to caking or segregation.

Mass flow in a silo results when all of the material moves when any material is discharged. Mass flow prevents ratholing, provides usable (live) capacity equal to the silo’s design volume, provides a first-in-first-out flow sequence, eliminates stagnant material, and reduces sifting segregation. The mass flow silo in Figure 2 was designed using limestone properties. Limestone can vary tremendously from hard, coarse consistency to soft, fine particles surrounded by clay.

Using incorrect ‘rules of thumb’
Incorrect design rules, like ‘selecting a hopper angle at least as steep as the angle of repose,’ continue to be used for silo design. Fluid mechanics principles are frequently employed by engineers to estimate bulk solids flow behavior – this generally does not work because bulk solids are different than liquids, given they have internal friction and are usually compressible.

Pyramidal hoppers are actively selected because they theoretically have a greater volume than conical hoppers and are easier to build. Yet, from a flow perspective, pyramidal hoppers yield the most flow problems because the shallow valley angles discourage solids flow. Even though they may be the cheapest option to build, from an operational standpoint, they are usually the most costly.

Ignoring feeder/hopper integration
Mass flow requires that solids flow through the entire cross-sectional area of an outlet. A partially live outlet, such as due to a partially open slide gate, will result in funnel flow regardless of the hopper design. It is imperative that a feeder be capable of withdrawing solids from the entire outlet of the hopper.

Also, standard screws withdraw solids only from one end of the slotted hopper outlet. Since material cannot flow from the hopper outlet into the remaining sections, a narrow flow channel forms inside the hopper, which can lead to ratholing, flooding, and segregation. A mass flow screw feeder draws material uniformly from the entire hopper outlet, and is designed to provide a uniform increase in capacity along its length.

Underestimating material-induced loads
The first step for the proper structural design of a silo is to determine the loads exerted by the stored material under initial fill and flow conditions, as well as the external loads (e.g., wind, seismic). Bulk solids do not behave like liquids since they develop frictional forces against the wall in their static and sliding conditions; this considerably affects the loads on the silo structure.

Commonly, structural engineers will incorrectly assume a hydrostatic loading condition for a mass flow silo. This mistake yields the maximum load at the hopper outlet, whereas the largest lateral load is actually at the cylinder/hopper interface. If these loads are not considered, structural failure of the silo can occur, leading to loss-of-life, costly production loss, and high costs to repair/retrofit the structure. It is also important for engineers to consider eccentric withdrawal conditions in a silo, which can result in large bending moments in the structure’s walls.

Designing rock-box chutes for cohesive solids
The design of belt-to-belt transfer chutes in cement plants is often neglected since chutes are generally considered ‘low technology’ equipment. Unfortunately, if one cannot convey raw materials into the plant via the belt and chute systems, then the plant cannot produce cement! Given that many raw materials handled are abrasive, a rock-box arrangement in the chute head box is typically used to minimise wear. However, most raw materials are also cohesive (sticky), thus using a rock-box design that encourages material build up for protective reasons can actually lead to frequent chute plugging. A chute should be designed to gently transfer solids from one belt to another, while preventing pluggages, spillage, and excessive dust generation.

Author: Eric Maynard of Jenike & Johanson, Senior Consultant, Jenike & Johanson, Inc

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