SHAPA Handling the impossible
Keeping material on the move takes more than just good design. Material knowledge is a must
It is obvious that a materials handling system should be designed to cope with the materials to be handled. In reality, however, this may be easy to say but rather more difficult to achieve. One of the presentations at the SHAPA Knowledge Conference in 2010 pursued this topic, leaving the delegates with many points to consider, not just in the design of vessels, discharge and conveying equipment but also how to assess the abilities of the prospective supplier and installer. Generally, free flowing material presents few problems, but many, perhaps with cohesive properties cause difficulties. Even dry coarse materials can cause arching at outlets preventing flow. Hammering the silo to encourage flow is not the ideal solution, but attention to discharge diameters and vessel valley angle at the outset will ultimately prove to be far more satisfactory. Cohesive materials may also cause arching, or “rat-holing”, where a core flow pattern results in only a central core of material flowing through the silo, leading to uneven discharge quality. Indeed some material may remain in the silo indefinitely – not good if the material has a limited shelf life. If mass flow – even discharge across the entire material surface in a silo – is not achieved, then segregation may occur. “Mass flow” ensures a first in / first out regime, where the entire contents of the vessel steadily and uniformly progresses towards the discharge, with no latent or “dead” areas. The equipment supplier must glean from the client the exact processing qualities of his material. This may be well documented to enable the system designer to work form established data; this information may, however, be in doubt for various reasons, including mixture proportions, temperature, humidity, friability, to mention but a few. If in doubt sample testing should be suggested and implemented. Reputation, not to mention profitability of both vendor and client, may well hang on materials knowledge being properly applied.
These principles apply to the whole system, with flow rate compatibility between all elements being essential. Other material characteristics such as bulk density, shear
24 Solids & Bulk Handling • March 2011
www.solidsandbulk.co.uk
strength and the actual nature and shape of the particles within the bulk mix all play a part in the correct selection of feeders, silos, discharge devices, conveyors and processing plant.
The SHAPA membership includes leading solids handling companies, assisted by academic institutions who continuously research and solve flow problems. The annual SHAPA Knowledge conference held in early autumn is open to all – not just members – providing an invaluable concise forum. The SHAPA annual general meeting will be at the “SS Great Britain” centre in Bristol on April 6th, with former Notts Forest, Leeds, Everton and Chelsea player Duncan McKenzie as guest speaker, together with an opportunity to visit the ship, plus of course an in depth update on SHAPA activities. If you would like to participate please visit:
www.shapa.co.uk.
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