Silos flow problems

The science of soil mechanics was integrated with the related field of powder mechanics and reduced to industrial practice by Jenike [1] in 1964. Since then, it has been possible for industry to reliably measure the flowability of powders and relate the measurements, in engineering units, to the design requirements for silo flow. However, Jenike s publication was neither the first effort to quantify flowability nor the last. New testing methods continue to be introduced, with varying degrees of success. In many cases these alternative measurement methods are the result of an industrial necessity and reflect some shortcoming of the Jenike method. In other cases, they exist because the Jenike method is not known to the people involved or is not relevant to their problem. Business value can be derived from many different types of measurements. 

Knowing the flow properties of a bulk solid a silo can be designed in such a way that flow problems can be avoided... 

This is one of the most common causes of silo structural problems, since it is so often overlooked. It results when the withdrawal point from the hopper is not located on the vertical centerline of a circular silo [7,8], and is particularly common when using silos with multiple hoppers in which only one or two of the hopper outlets arc used at a lime. If the resulting flow channel intersects the silo wall, non-uniform pressures will develop around the circumference of the silo leading to horizontal and vertical bending moments. See Figure 1. Many silo designers incorrectly account for these non-uniform pressures by only increasing hoop tension. [9,10]... 

M.G. Jones, P. Marjanovic, The modification of material properties for improved flow characteristics, Proceedings of the Seminar on Solving Problems in Hopper and Silo Systems, IMechE, London, UK, June 1996. 

Particle flow could be driven by gravity or external forces. Particle discharge from silos or hoppers is an example of particle flow under gravity. Common problems in such plants are nonuniform and/or the interruption of flow from the discharge orifice in the hopper or converging section beneath a storage vessel for powders. DEM simulations have been extensively used to study such systems [61-66]. 

Silos and bins should be inspected on a routine basis to anticipate potential flow and structural problems before these become major. 

The doming - no doming criterion was explained qualitatively with help of Fig. 7. For predicting the critical height h quantitatively the dependencies Oc = f(cTi) and 0 = f(oi) have to be known. Since doming is mainly a problem in the hopper section, only this part of the silo has to be considered, 0 and Oi can be calculated, a and 0 both increase linearly with distance from tire hopper apex and both are equal to zero at the apex. Therefore, the ratio of both, called flow factor ff = oi / cti, is a constant in the hopper, ff depends on the parameters (pe, cpx and 0, already used for the mass flow/funnel flow-decision in Fig. 5, and can be read from equivalent graphs in the literature [1]. The dependence o, = f(oi) is a bulk solid property, called flow function. It can be measured with help of shear testers (see chapter 5). 

Very often a silo is build without knowing the flow properties and during operation a lot of problems occur. To solve these problems first the flow properties have to be measured and then the silo has to be adjusted to the measured flow properties in such a way that the problems disappear. 

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