Powder, powder, everywhere – the challenges of dust in bulk powder handling
Transferring bulk powders through factories and between machinery is a critical operation in many industries, including food processing, mineral processing, pharmaceuticals, and manufacturing of construction materials. However, the process is fraught with risks due to the presence of finely powdered dust. If not managed properly, these risks can soon compromise safety, operational efficiency, and product quality.
Key Dangers of Powder Dust
Explosion Risk
Fine powders can become combustible when suspended in the air in the right concentrations, even with materials not ordinally considered highly flammable. A single spark or static discharge can ignite the dust, leading to catastrophic explosions. For example, mishandling combustible powders such as wood dust, flour, or aluminum powder has caused industrial accidents with severe consequences.
A dry substance that might take a lot of sustained heat energy to burn when in its solid block form can be instantly ignited when suspended in air as dust. The surface area is vastly increased and the oxygen supply is optimised for rapid explosion.
Health Hazards
Prolonged exposure to powder dust can harm workers' respiratory systems, potentially leading to chronic conditions like silicosis or asthma. Dust particles that are too small to be filtered by the nose or throat can settle in the lungs, causing long-term damage.
Product Contamination and Equipment Damage
Uncontained dust can compromise product quality by introducing impurities, while also causing mechanical wear or clogging of nearby equipment that isn’t designed to cope with it. This can lead to inefficiencies and costly downtime, as well as early degradation of asset value.
Abrasive powders such as fly ash, glass dust, sand, aluminium oxide and baking soda require handling machinery made with special coatings or alloys – don’t allow it to contaminate more delicate equipment or other product batches.
Slip Hazards
Walking surfaces can become slippery when a layer of dust forms. Open-grating helps prevent dust buildup on gangways where there is a risk of settling dust, although all reasonable efforts should be taken to contain it in the first place.
Mitigating the Risks from Dust
Improve Dust Containment Systems
Install sealed equipment and enclosed transfer systems to minimise dust release. It helps to think of the entire system being like a hose pipe; just one pin-pick of a hole and you’ve got a high-pressure leak. Even the smallest gaps around panels and bolted joints can release powder where there’s a positive internal pressure, centrifugal force, or vibration. For this reason, vacuum suction is often used in preference to blowers and compressed air movement. A leak will then draw air in; reducing system efficiency but at least preventing large volumes of dust from escaping.
Whether vacuum and blower transfer is used, the drawn-through air will need filtering at its release point/ end of the system.
Where a negative pressure cannot be maintained, it is essential to seal any joins with gaskets, flexible sealant, or rubber flanges.
Shrouds and skirts are useful for equipment such as conveyor belts and dispensing hoppers, or other places where permanent sealing can’t be done. An example of this is rising hoppers which dispense bulk powder from quayside silos into ship holds.
In addition to primary containment, advanced suspended dust collection systems that filter the air are often essential to capture airborne particles that inevitably escape the system. For this reason, it is good to house powder handling equipment in its own closed-off section of the factory.
While keeping powder within the system, access will be necessary for service, inspection and repairs. Build access hatches with rubber sealings and include a vision panel to reduce the frequency of opening. A vision panel can also be useful to check internal conditions for air-borne dust before opening.
Proper Ventilation and Filtration
Adequate ventilation and high-efficiency particulate air (HEPA) filters can help maintain clean air and reduce dust exposure risk for workers.
When filling a closed system, it will be necessary to install filter vents to prevent pressure differentials from forming between internal and external environments. Even small pressure differences can cause the implosion or explosion of a sealed vessel.
As a final mitigation, consider if explosion vents are required in any dust-prone zone. These are designed to absorb and release shockwave pressure in a safer, controlled direction in the event of a catastrophic explosion caused by ignition.
Keep material agitation and flow-rate to a minimum
Slow and steady is best for dust reduction during powder movement. Minimise unnecessary vibration and aggressive stirring to keep dust from forming. Positive displacement techniques such as screw conveyors tend to produce less dust than high-energy blowers or gravity-fed systems.
Worker Training and Protective Equipment
Training workers to handle powders safely is crucial. Good system design should take into account human factors, making it easy for staff work safely and avoid accidentally causing a dust-related incident. Finally, what residual risks cannot be mitigated by design and training should be addressed by equipping staff with personal protective equipment (PPE) such as masks and goggles, monitoring working practices, and regular occupational health surveillance.
Explosion Prevention Measures
Mitigation strategies include grounding and earth bonding to eliminate static electricity, installing explosion-proof/ATEX rated equipment, and using inert gas blanketing to reduce oxygen levels in dust-prone areas.
Reduce open air transfer points
Wherever possible, consider the entire powder processing line as one system and avoid transfer points. Unavoidable transfers should be done using well-engineered connections, with shut-off valves. Sliding gate valves, and system to system compatible couplings are beneficial for containing dust at powder transfer points such as truck to silo.
