Getting off the beaten track

While most process engineers have to grapple with systems that handle and process powders and bulk solids at some point in their career, few ever get any formal training in the subject. And, as every different powder has a different behaviour, experience of what works in one place with one powder, generally will result in a misjudgement if it is applied directly to another powder, or in a different context.

A familiar example is in hopper design how many hoppers or silos do not show outbreaks of “hammer rash” where they have been beaten to try to get material to flow, or have vibrators or air pads bolted on in an attempt to stop “arching” or “rat-holing” of the powder inside?

Around 40 years ago, a design procedure was put into the public domain that determines for a given powder or set of powders the slope angle, shape and outlet size that will give reliable discharge under gravity alone, with no need for beating, vibration or aeration.

Unfortunately, this design method is often not applied: many engineers do not even know that there is such a design procedure.

Hoppers are not the only problem area pneumatic conveyors, mechanical conveyors, feeders and other handling systems can be troublesome. Problems with the powder like segregation, lump formation, dust emission, particle breakage are all commonplace.

Whether you are buying, using or supplying solids handling kit, the application of a little knowledge helps greatly when selecting equipment for the job.

Training courses in the area can explain the unique nature of different powders, how the powder behaviour can be measured objectively, the potential problems, the design methods there are for selecting the right solution, and also methods for troubleshooting of equipment that doesn’t work correctly.

Both sides need to take some responsibility in this; the buyer knows about his process, and needs to apply enough knowledge to identify what key features the kit needs to embody, whereas the equipment supplier needs to know how he can measure the behaviour of the customer’s materials, identify potential problems arising from the context and apply the right design methods.

With this sort of knowledge, it is easier to make sure that the equipment works properly and the relationship between supplier and buyer does not turn sour.

Professor Mike Bradley is director of the Wolfson Centre for Bulk Solids Handling Technology