Uncontrolled dust release is a major industrial hazard associated with a number of health, safety and environmental (HSE) concerns. Dust release during daily operations, such as bulk outloading, can present an inhalation risk to personnel, inhibit visibility or even form an explosive atmosphere. Furthermore, dust spreading through an operating area is lost product that has an associated economic value.
In this article we review the problems associated with dust release and examine the technology available for dust suppression, focusing on the Dust Suppression Hopper, a device that eliminates dust release at source. A case study demonstrates the benefits that this technology can deliver in bulk outloading operations.
The generation and hazards of dust
During the transfer of bulk materials there is a tendency for any fine particles present to escape from the bulk, spreading into the surrounding environment in the form of dust. In harvested crops, such particles may be fine dusty dirt while in a manufactured product they may simply be the finest fraction of the particle size distribution. Air currents induced by the bulk flow and any prevailing wind help to spread the dust, which contaminates the working area and can form large clouds that present a significant hazard.
A routine operation often associated with dust release is bulk material intake or outloading during receipt of a feedstock or release of a product. This process involves the transfer of material from a lorry into a storage building, or vice versa, and is a daily task at many sites. Poor visibility, respiratory protection for operational personnel, excessive clean-up, machinery damage and the risk of an explosive atmosphere, may all be on-site concerns but beyond this, dust release has the potential for wider environmental damage.
Air contamination is a growing problem as population centres expand around facilities that may, in the past, have been some distance from a community of any size. In addition, fugitive dust emissions are also a potential source of watercourse degradation. Tackling environmental issues is becoming critical for facilities anxious to secure good community relationships, or indeed an on-going licence to operate in the face of tightening legislation. Controlling or eliminating dust release is therefore becoming vital.
When it comes to tackling a potential dust control problem there are three possible strategies: prevention, isolation and/or control. Conventional technology such as telescopic bellows typically employs the second and third approaches and can be effective for confining dust release within the factory limits. However, keeping dust to a contained area around the operation does not prevent damage to local machinery (or maintenance to avoid such damage) or the creation of a dusty work environment that requires regular clean-up. Eliminating dust release at source is a more elegant and efficient solution.
Introducing the Dust Suppression Hopper
The Dust Suppression Hopper (DSH) is an innovative piece of equipment that eliminates dust release at source, during bulk material transfer. Economically competitive with telescopic bellows, when lifetime costs are rigorously compared, it can be used for a wide range of granular materials including grains, minerals, wood pellets, fertilisers and foodstuffs such as salt and sugar.
The essential aspect of the DSH, and where it differs from conventional telescopic bellows, is that it discharges material in a dense, essentially de-aerated column of material. This ‘condensed’ flow of powder is produced by the unique operating principle of the system. A conical outer hopper is suspended from the top frame of the unit by a number of springs, and a safety chain. Within the hopper is a central plug which remains stationary at all times.
As material flows into the DSH the weight of the material causes extension of the springs and the outer hopper lowers relative to the stationary central plug. This opens up an annulus, allowing the material to discharge. However, flow is maintained only if there is a sufficient head of material to provide the downward force necessary to extend the springs. If not then the annulus will close. In practice the outer cone gently oscillates up and down during discharge, as the balance of forces between the non-linear springs and weight of material fluctuates. This action maintains the characteristic solid-like stream of product.
With regard to installation, a DSH is positioned directly beneath a feed point, and can be suspended at some height over the required transport, as the flowing column of exiting material is highly tolerant of drop. The position of the hopper is maintained, apart from it’s contained up and down motion, throughout a transfer operation. This eliminates the manoeuvring required with conventional bellows, the lowering and lifting that is responsible for the complexity of bellows design and many of the problems associated with their use. Furthermore, the squeezing action to which the material is subjected, as it is pushed out of the annulus, reduces excess air in the materials, largely eliminating segregation. During transfer, any dust present is entrained and drawn down into the material column so there is minimal dust emission when the product hits the ground or floor of the receiving vehicle or vessel.
This elegant design eliminates many of the issues associated with the use of conventional technology. The DSH has no moving parts and maintenance requirements are minimal. Dust release is almost completely eliminated while discharge times are generally maintained. Economic benefits accrue in the form of:
• Reduced clean up costs
• Less product loss
• Lower maintenance and spares requirements
These economic gains offset the initial capital investment associated with installation of a DSH system which, of course, may be primarily installed to address HSE concerns. The following case study shows what can be achieved.
A port loading facility in New South Wales Australia undertook a trial to assess the DSH and compare its performance to their current standard unloader. With a parallel loading system already in place the facility was able to replace one of its standard unloaders with a DSH hopper to carry out back-to-back tests, unloading the same material.
In a first trial a 28,000 tonne vessel load of soya bean meal was discharged to a series of trucks. The original unloader filled a truck in less than 3 minutes but in a first trial the DSH took longer, around 6 minutes. Soya bean is a dusty product so this trial presented an ideal opportunity to directly compare the dust release associated with each discharge solution. The DSH performed well in this respect but a doubling of discharge rate was not acceptable. The hopper height was lowered to increase the spout opening size and adjustments were carried out to the loader slide gate that controls the flow rate into the hopper. A repeat trial with this optimised set-up gave an acceptable lorry filling time of roughly 3 minutes with no spillage.
Building on this success, tests were carried out with three other materials, all fertilisers. Load and discharge times for these materials are shown in table 1.
Fertiliser Name Total Load Size Truck filling rate achieved with optimised DSH
DAP 500 mt 1 min
MAP 5,422 mt 1 min
ME510 2,350 mt 2 min
The results of this second trial showed that the DSH was most successful for the discharge of soya bean meal, where it maintained discharge rates and largely eliminated dust release. In initial trials with the fertilisers, the DSH resulted in slower discharge rates, however as with the soya bean meal, these were improved by optimising hopper set-up to account for differences in bulk density between the two products. The height of the DSH gave perfect clearance for the truck trailers, easing the logistics of loading operations.
As a result of these successful trials, this port loading facility has installed two DSH hoppers, one on each unloader. With the new installation, all products that pass through the facility are now being unloaded at the same speed as with the old discharge technology but with appreciable dust reductions, marking a major improvement in facility operation.
With a foundation of 50 years of experience, Guttridge is a flourishing company, globally respected for delivering well-engineered, reliable materials handling solutions that continue to anticipate dramatically changing industrial needs.