Congratulations on completing your dust hazards analysis (DHA) prior to the September 2020 deadline. Now that you’re aware that a dust collection system for combustible dusts can create hazards as well as mitigate them, let’s go over your DHA results. In your case, it appears that your systems suffer from improper design, especially the dust collection hoods. This situation is typical of the hundreds of dust collection systems I’ve observed in my 44 years of experience.
Hood design is indeed important. A dust collection system can have properly selected main equipment from the best manufacturers and, even properly designed and installed ducting, but if the hoods aren’t designed properly, the result is a colossal waste of financial resources. Improper hood design is a major factor in determining whether a dust collection system is successful or unsuccessful in controlling dust emissions. This is especially critical when combustible dusts are involved.
Proper hood design involves the three Cs of hood design: containment, capture, and control (or conveying). For a dust collection hood to be successful in controlling airborne dust emissions, it’s necessary for the hood to provide these three design elements in relation to dust emissions. When all three functions are achieved, then a hood is effective in controlling dust emissions.
Containment of dust emissions is the first requirement for proper hood design. In some cases, containment is predetermined by your process. For example, when venting an enclosed hopper or large bin, the dust emissions are already contained in the vessel. But for most hoods, this function involves providing a fabricated steel partial enclosure. The enclosure should be shaped to both limit the volume where airborne dusts can travel and force the induced airflow (from the dust collection system) to come from a direction where it can achieve capture and control.
However, there can also be either too little containment or too much containment. If there’s insufficient containment, the airflow provided may not be able to capture and control the airborne dusts allowing them to escape into the surrounding area. Conversely, too much containment can result in restrictions that don’t allow for consistent and continuous airflow into the hood. This restriction allows dust to accumulate on the surfaces within the enclosure, which is a significant combustible dust hazard.
Capture is the second requirement necessary to correctly collect dust emissions. Assuming the dust collection system has the ability to produce the desired airflow, the key then is to properly capture the dust emissions to ensure the hood design forces the induced airflow to pass through the airborne dusts as it moves toward the hood’s face or front side. If the induced airflow doesn’t pass through the dust in sufficient volume (or mass) and velocity, it won’t capture the dust and move it toward the hood’s face.
Control (or conveying) of the contained and captured dust emissions is the third requirement. Achieving control of the dust particles within the airflow is directly related to the method of transition from the hood’s face to the ducting connection and the volume (or mass) of induced airflow. Both must be done properly to maintain control over the captured dust. Proper design for control is also critical to minimizing the system’s energy requirements and ensuring that the airborne dust is contained and captured.
Proper hood design isn’t a simple requirement. Both experience and understanding of how the dust emissions are created are critical to implementing the three Cs. Without proper hood design more problems and hazards are created than solved.