Page 15 - OHS, September 2021

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COMBUSTIBLE DUST
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BY SCOTT CZARNECKI
To determine the most effective combustible dust hazard mitigation approach for new and existing dust collection system installations, a dust hazard analysis (DHA) conducted by a qualified person should be performed.
NFPA 652: Standard on the Fundamentals of Combustible Dust defines a qualified person as, “A person who, by possession of a recognized degree, certificate, professional standing, or skill, and who, by knowledge, training, and experience, has demonstrated the ability to deal with problems related to the subject matter, the work, or the project.”1 For a DHA to be effective, it must analyze all the potential fire, deflagration and explosion scenarios associated with the process and connected ventilation system. The hazards associated with combustible dusts are complex.
Managing Risk
All too often, combustible dust collection systems are purchased and installed without the completion of a thorough DHA. Failing to conduct a dust hazard analysis is a common oversight and can lead to installations with unmitigated risks.
Do not allow vendors or contractors to increase your risk and liability. Equipment manufacturers might design systems for a range of applications, but they cannot know the conditions specific to your facility, creating risk to life and property.
Often a dust collector’s operation and installation manual will contain a full page of text explaining why they are not responsible for the system’s design, operation or installation:
■ “Vendor does not provide engineering consulting services related to process or dust hazard analyses, or code or standard compliance.”
■ “The process owner’s final selection of dust collectors and risk mitigation strategies should be based on the outcome of a dust hazard analysis...”
Implementing a quality DHA is critical. How a dust collection system is designed, installed and operates—your risk mitigation— should be based on the results of a dust hazards analysis.
Testing for Dust Explosibility Characteristics
Determining if your dust is explosible is the first step to select an appropriate dust collection system.
Explosibility characteristic data is essential for the design of an effective system. Typically, the first two parameters used in characterizing dust and designing a dust collection system are the Kst and Pmax.
The Kst is a measurement of explosion severity. The higher the Kst value, the stronger the force the dust could generate during an explosion. Note that a low Kst does not mean the dust is only slightly combustible or nearly noncombustible. It represents the force of explosion, how fast an explosion may travel.
The Pmax or the maximum amount of pressure that the dust
Michal Zduniak/Shutterstock.com
could generate in an enclosed volume is the second test used in designing a system. Pmax data enables a system design to manage expected pressures through passive and active explosion controls and predict its severity.
Absent Kst and Pmax information, a supplier may use worst-case estimates of these values, which will raise your cost of equipment. A supplier might refuse to provide equipment without knowledge of dust explosibility characteristics.2
If a dust collector vendor does not ask for this information before selling you a system, you should consider a different vendor.
Selecting Equipment
With the explosibility characteristics of the dust identified, appropriate fire and explosion controls can be selected. Many devices and systems are available to comply with NFPA standards for the explosion protection of dust collection systems. Passive and active controls are the two general categories.
Passive systems do not require detection devices and react in response to deflagration pressure. An active system uses a sensor to detect the pressure rise during a deflagration event and activates the respective explosion protection device.
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SEPTEMBER 2021 | Occupational Health & Safety 11