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Does Your Bulk Storage Scheme Design Stack Up? – Part One

World Cement,

It is stating the obvious that if the storage and handling steps in cement manufacture processes do not function efficiently then the process will be directly impacted upon. However obvious this critical relationship between handling and processes might seem, it is still one that causes considerable frustration and production losses in many plants. In the majority of cases it is a lack of understanding of the bulk characteristics of the material being stored or handled that is the root cause of subsequent problems.

Many of the most popular methods used to obtain measurements of bulk characteristics have been in use for many decades and are based on ease of execution rather than any true application to the design process. Considering the issue of the design of silos or bunkers, the main requirements that are often listed in specifications are that the storage scheme should hold ‘x’ tonnes of product and have a capability to discharge at ‘y’ tonnes per hour. Obviously, physical constraints (such as head space limitations, or local planning limitations on height) will also feed into the thinking behind the design. In many situations, the discharge rate requirement will be used to define the dimensions of the feeding device – which, in turn, will dictate the sizing of the silo/bunker outlet. The selection of the angle for the convergence lead towards the outlet will typically be 45° or 30° (measured from the vertical), which will connect into a vertical section at a diameter appropriate to the desired storage capacity. This is fairly ‘standard’ for a gravity discharge arrangement – shallow floored stores with fully aerated discharge are a separate type, which this article will touch on later. Such gravity discharge designs (if they are steel silos) may come ready equipped with discharge aids, such as pneumatic hammers or air injection, whilst concrete construction stores may incorporate air cannons into their ‘as constructed’ specification. The need for such types of discharge aid can be a reflection of physical constraints that preclude the adoption of geometry conducive to reliable and consistent gravity discharge of bulk materials – but, unfortunately, in many cases these types of device are considered ‘de rigueur’ for any type of installation.

For bulk materials that are free-flowing, these ‘standard’ constructions are likely to be adequate for establishing a reliable discharge when operation of the process commences. However, even for an easily handled bulk material, achieving ‘reliable discharge’ can often be only part of the picture regarding the overall performance of a given storage scheme. The favourable flow behaviour of free-flowing bulk materials is considered highly desirable in many process plants. However, this same free-flowing behaviour can also have the effect of rendering a bulk material (or blend) susceptible to segregation. Thus is it possible to end up with a vessel that readily discharges either imbalanced blend or material exhibiting variation in particle size or density.

Read part two here.

This is an abridged excerpt from the article ‘Does Your Bulk Storage Scheme Design Stack Up?’ by Richard Farnish, which was published in the June issue of World Cement. To read the full article, subscribers can download the issue by logging in here.

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