An effective dust collection system is a proven method for eliminating or mitigating dust deflagration hazards. However, an ineffective dust collection system represents a significant combustible dust hazard and one with associated high risks of deflagration. All of the NFPA combustible dust standards--61 (Food/Agri), 484 (metals), 652 (Fundamentals), 654 (Chemical), and 664 (Wood)--have requirements that must be met for any dust collection system to be effective and “combustible dust compliant.”
A dust collection system that is “combustible-dust-compliant” is one that complies with the requirements found in the applicable NFPA standards. The best single source for this information is found in NFPA 652 Standard on the Fundamentals of Combustible Dust (2019) in section 9.3. The following provides a summary of the key portions of this standard.
First, whoever designs the dust collection system must know what they are doing. A true expert is needed, not a salesman, not an HVAC sheet metal fabricator, or a junior engineer, but an individual or company with proven experience.
Second, the system must be designed and maintained to ensure (author’s emphasis) that the airflow in the duct will meet or exceed the minimum required to keep the inside of the ducting free of accumulations under all normal operating conditions. The only effective and proven method to produce this result is the use of the “balanced-by-design” methodology and exclusion of the use of manual slide or “blast” gates for flow adjustment. The “balanced-by-design” method uses physics which inherently provides continuous proper air distribution throughout the system.
Third, each of the dust sources (i.e. dust collection hoods) of the system must be designed to achieve and maintain the minimum air flow required for capture, control, and containment of the generated dust source. This is the three C’s of successful dust collection hood design. If all three are successfully done the airborne generated dusts can be controlled at the source.
Fourth, each of the dust sources (i.e. hoods) must have a documented minimum air flow based upon the system design. This further emphasizes the need to use the “balanced-by-design” method for the design of the system. It is not possible to design a successful dust collection system without this basic information.
Fifth, existing dust collection systems must not be changed, either by removing or adding dust sources, without ensuring the required airflow (as per the original design) and air-balance is maintained. Changing the system without this approach--which basically means a re-design--virtually assures combustible dust accumulations somewhere in the system. A company with an effective management-of-change policy will keep this from becoming a problem.
Sixth, the fan (AMD or air-moving device) and dust collector (AMS or air material separator) must be designed for the application. The author has audited hundreds of existing systems and the vast majority will have improperly selected dust collectors and/or fans. Too often cost pressures drive vendors to make promises that prove invalid in actual application.
Seventh, it is highly likely the dust collector (AMS) will require explosion/deflagration protection and mitigation, along with isolating the ducting (i.e. dust sources) and collected material discharge from the effects of any explosion or deflagration event. As a result, it is best to locate the dust collector (AMS) outside, although it is feasible to locate the unit inside under specific conditions. It is also important to remember there may be dust sources that need isolation or other mitigation from the rest of the system, such as hammermills and spark-generating operations.
There are other design considerations for combustible dust compliance, such as proper manifolding, avoidance of the use of slide or “blast” gates, proper installation, bonding/grounding, training, system monitoring, etc.
If these standards are followed the result is a properly designed dust collection system that requires minimal maintenance, operates effectively, and is in full compliance for decades of use. However, if these standards are ignored the typical result is a system that is a maintenance headache and one that increases the likelihood of an unwanted explosion/deflagration, rather than reducing those hazards and risks to an acceptable level.
Jack Osborn is senior project engineer for Airdusco Engineering and Design Services LLC, Memphis, TN. Osborn has 46+ years of experience in the concept, design, project management, start-up, and operations/maintenance for an extensive range of mechanical equipment systems, including dust collection, ventilation, bulk handling systems (pneumatic and mechanical), storage, weighing, mixing, batching, etc. In addition he is also a member of all six NFPA committees on combustible dusts (61, 484, 652, 654, 664, and Correlating).