Key points
For many plants the consistency and repeatability of discharge or feed rates into a process are critical to the efficiency of the next process step. Inevitably this feeds into not only the quality of the final product but also the overall profitability of the exercise. A feeder exists to moderate the discharge rate achievable from a given bulk solids exiting a vessel into a process.
However, where flow limitations occur or transfer rates are unreliable, many operators tend to resort to the time honoured approach of beating the offending vessel into submission. Typically, despite short term intimidation of the equipment to an operating condition, unacceptable behaviour is often lurking just around the corner and soon returns.
The mistake made by operators and engineers tasked with ‘sorting out’ is that where irregular flow is occurring the true source of the problem is often not immediately obvious. The reality of the situation is typically that it is the interaction of the vessel or chute with the feeder/flow control device.
A common cause of flow rate irregularity is the flow channel that is propagated through the vessel from the flow control device. However even in situations where a vessel discharges in gravity flow the presence of geometries that are inappropriate for the process requirement is also a common issue.
If consistent and reliable flow is required (particularly where a high accuracy feed is sought) it is critical that during operation regions of non-flowing are minimised (or preferably eliminated) through careful equipment design with these objectives in mind.
Examples of non-flowing feeder regions
Examples of where non-flowing regions of material might develop include, above fixed capacity feeders (i.e. constant pitch screws, rotary valves), within vessels (i.e. core flow or poorly interfaced mass flow bins), through poor feeder interface design (i.e. direct onto vibratory trays, belts or drag link conveyors), or where gravity flow is throttled using slide gates, butterfly or iris valves. As you can see, there is plenty of scope for error.
In one recent industrial project pack weights were found to be varying by +/-140g on a 25kg sack fill, resulting in substantial losses through ‘give-away’ on a plant operating 24/7 over a year.
At an entirely different scale, a discharge accuracy of +/-2.5te on a 30te fill was occurring at wagon loading operation. In both cases the root cause was similar in that the flow channel that was established during discharge was encouraged to propagate through regions of non-moving material which were supported from the flow control device.
Relatively simple modifications (which could have been readily implemented into the original construction) resulted in an improvement to +/-45g in the first example and +/-500kg in the second example.
Common mistakes when designing feeder systems
The common mistake made when designing or modifying systems through which bulk materials will flow is to make the assumption that provided the granule (or powder) flows from the equipment, then all is well.
The reality is that most process problems relating to bulk particulates originate from a failure to take time to understand their bulk characteristics.
Most people that deal with bulk particulates will be of the opinion that cohesive (poorly flowing) materials are very difficult to work with – however, this is usually based on the usually frustrating experiences gained from the use of standard equipment types in conjunction with these types of powders.
The reality is that, following characterisation for flow, such powders can be handled very successfully provided that the equipment with which they are used incorporates geometry, interfacing and feeder specifications that are appropriate.
By contrast many believe that free flowing materials are much easier to work with – and to a certain extent they’d be correct. However, the presence of constrained flow channels can give rise to unacceptable flow variability even for ‘easy’ to handle bulk materials.
Key to controllable and efficient discharge
The key to controllable and efficient discharge of free flow or cohesive powders lies in the incorporation of an understanding of the bulk characteristics at an early stage in projects to modify or purchase new equipment.
Independent expertise: providing cost-effective solutions to industrial problems
The Wolfson Centre for Bulk Solids Handling Technology is internationally recognised for its expertise in fields associated with bulk particulate handling and in the science and practical application of technologies to deliver improved process efficiencies. Our mission is to help industry to get powders and bulk materials to behave predictably through processes.
Industry sectors benefiting from our services range widely and include power generation, chemicals, pharmaceuticals, foods, plastics, paper, minerals and cosmetics. Our clients are predominantly based in the UK, although many European and overseas companies take advantage of the independent and bespoke nature of our work.
Flow rate control courses
We have developed our portfolio of services over nearly four decades of interaction with industry, trade associations and professional bodies. We offer consultancy services to industry, conduct industrially focused research related to bulk solids handling issues and offer specialist short courses targeted at industry professionals.
For information about these courses go here . If you have a problem with your plant or process, we have the knowledge and experience to provide cost effective answers that you can rely on.
The centre is situated at the University of Greenwich's Medway Campus, where we are part of the School of Engineering. Our location offers excellent transport links within the UK and EU enabling us to provide a prompt and flexible response to site visit requests and client meetings.
Our experts have between them over one hundred man-years of experience of solving problems in the flow and processing of powders, granules and lump materials, in all industries from pharmaceuticals and food to mining and chemicals; from biomass waste and recycling to household goods and pet foods.
They concentrate on the needs of the industry and share their knowledge through the provision of technical research papers, articles and editorials through the media and at conference and exhibitions where they often present papers or hold workshops.
Our industrial-scale pilot plant is available to test a processing step, or mock up a whole handling process or logistics chain, at full scale or near full scale.
This can be used to find and solve the problems off-line, then when you transfer the same process or material to your own or your customers’ plant, you can be confident it will work!
Contact us now on Tel: 020 8331 8648 or E-mail: wolfson-enquiries@gre.ac.uk to find out how we can help you.
