These handicaps to a scientific approach are exacerbated by the host of features that characterise how a bulk material will behave within a particular storage and handling situation.

Basic information as to bulk density and wall friction are relatively simple to define and measure. Information as to the bulk strength and deformation behaviour of a bulk solid is inherently more complex. Fundamental to the problem is that there is no single or 'once only' flow property measurement for any given bulk material. Even if there were one for given conditions, the interaction of equipment geometry, materials of construction, and variables of operation and environmental conditions, could lead to this generalised information being misleading. It is most important to appreciate that measurements of the flow properties of a bulk materials refer only to one specific condition of the material, and that any physical change in the material totally invalidates the design value of these measurements. Tests must be conducted for different product conditions if the material is likely to vary in any physical manner, such as particle size composition or moisture content, and measurements taken for the 'worst' condition used as a basis for relevant design features.

Also note that the characteristics of interest differ widely according to the application. Aspects of material quality and hygiene, safety and operator considerations, features of the site and preferred form of equipment are all application related. The supply consistency of the bulk material must also be taking into account. The effect of multiple sourced products, operation and process variables, constituent changes, homogeneity of the bulk and the stability of the material with time or ambient variables, are vitally important with respect to how the material will behave.

To address this awkward situation the Bulk Solids Handling Committee of the Institution of Mechanical Engineers produced a document, setting out a format for the specification of bulk solids for storage and handing purposes [I. Mech. E. 1996]. The publication includes basic fact sheets (Tables 1 and 2), supplemented with queries concerning differing features of possible interest.

Quantified values are essential for predictable designs and tend to figure increasingly with quality programmes and design verification procedures. There remain certain features that are judgement related and essentially must fall within the province of the user to rank as to their importance. For these it is necessary to utilise a grading system indicating the degree to which the suitability of equipment may be effected. Whereas some aspects may not be relevant in particular cases, a systematic review of this type prompts attention so that no feature is overlooked by default. Guide notes outlining test procedures and their relevance are also included in the publication.

Two comprehensive studies of plants that handle bulk solids have shown that their performance efficiency compares very unfavourably with plants handling liquids and gasses [Merrow 1981 and 1985]. More significantly, this discrepancy has shown little signs of improvement compared with plant built in the sixties, despite the advances made in bulk technology since that time. It is clear that steps taken to introduce the use of more quantified values for equipment design must progress alongside the diffusion of knowledge in particulate solids technology, for performance standards to improve in the ubiquitous industries that handle bulk solids. Hence, one reason for this article and the means adopted for its propagation.