The cost effective manufacture of products is usually considered to be based upon the efficiency of the (readily perceived) value adding steps that the powdered or granular materials pass through. This is a concept that most engineers can identify with without too much trouble. However the actual efficiency of a given production line is frequently governed by the ability of the non value adding elements of the process to store, discharge, meter and convey the bulk materials. Such non value adding elements are represented by silos, hoppers, pneumatic conveyors, etc.
Unfortunately these aspects of plant design are often defined technically by the budget available rather than their suitability for the bulk material(s) to be handled through the process.
Phrases such as “clumpy”, “not very free flowing”, “sticky” may be used to give an indication of how the bulk material is perceived to be by the client – but how really useful is this? And more to the point how misleading could this be? Focussing on “not very free-flowing” this could be construed as any one of the following:
● The material exhibits a degree of cohesion, and thus a large arching dimension, meaning that flow through small outlets is intermittent or irregular
● The material flows well if discharged within one hour of filling – after which time the likelihood of obtaining reliable discharge diminishes as residence time increases
● The material will not discharge by gravity unless a discharge aid is applied, after which flow is predictable.
Each one of these examples would require a different technical approach or changed plant operating condition to circumvent, and more importantly each would carry quite different price tags!
Clearly, vague descriptive of the bulk solid to be handled combined with the “de rigueur” inclusion of (often inaccurate) definitions of bulk density and angle of repose –leaves equipment designers in something of a wilderness when it comes to meaningful technical information. Unfortunately, because the technical understanding of bulk solids handling is nowhere near as widespread as it should be, the technical analysis of bulk solids is seldom undertaken prior to compiling potentially contractually binding specifications. Shear tests to determine the bulk behaviour of a given material can be used to provide a benchmark against existing (known to the plant) materials or used for equipment design purposes. (fig 1)
Ultimately, whether these tests are undertaken by the company writing the specification or the equipment designer, the end result will be plant that can be designed confidently and commissioned with the minimum of down time. University of Greenwich, The Wolfson Centre
