People all around the globe come into contact with bulk solids (bulk materials), starting from sugar and spices to cement and sand, in their day-to-day lives. These materials are used for food preservation, building/road construction, food seasoning, medicine production, and baking, the list is endless. It is safe to say that bulk solids are practically part of our everyday life. Bulk materials undergo various handling processes in their respective processing industries (e.g., food, plastics, cement, pharmaceutical, chemical etc.) and from time to time, processing industries may encounter material flow problems in storage vessels (bins, silos, or hoppers)  during handling processes.

Many different factors contribute to flow problems in storage vessels. One of the most notable contributing factors is improper equipment design. The architecture of the equipment must be in such a way that it does not interfere with the product quality or the product discharge. Most equipment design (storage vessels to be specific, regarding this blog) is done with little to no regard for the flow properties of the handled bulk material, and consequently, numerous material flow problems in storage vessels arise. Some of the common flow problems in storage vessels encountered during bulk material handling processes are discussed below.

Ratholing

Ratholing refers to the occurrence of a material flow channel that takes place directly above the storage vessel outlet. Instead of an even bulk material discharge, the material tends to accumulate along the sidewalls of the storage vessel, leaving a void or cavity in the center. Rathole typically results from a flow pattern referred to as funnel flow which does not exhibit a first-in, first-out (FIFO) flow sequence. If the material is cohesive, a stable rathole may form in the funnel flow type resulting in a discharge problem.

Bridging

Bridging also referred to as arching is a common phenomenon observed with cohesive bulk materials that have a tendency to agglomerate or interlock. The bulk material forms a stable bridge (arch) above the outlet of the storage vessels, thus obstructing material flow. The effectiveness of the bridge primarily depends on the cohesive strength and the internal friction of individual particles.

Segregation

Segregation of bulk materials refers to a condition in which particles separate primarily because of their differences in physical and/or chemical properties such as size, density, shape etc. Therefore, bulk materials having such differences may segregate during handling processes. During handling processes, lighter or smaller-sized particles are inclined to migrate to the top, while denser or large-sized particles sink to the bottom. Blended bulk materials tend to segregate, creating an inconsistent material discharge, thus affecting the product quality due to its non-uniformity.

Understanding the behaviour and nature of bulk materials is fundamental to engineering ideal storage vessels that guarantee optimal operations within a plant. With properly designed/engineered storage vessels, the aforementioned flow challenges can be avoided, thus improving safety, production rate, and product quality as well as reducing downtime.