Managing Combustible Dust and Risk Mitigation

Managing Combustible Dust and Risk Mitigation

Failing to conduct a dust hazard analysis is a common oversight and can lead to installations with unmitigated risks. 

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." 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 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.

If a dust collector vendor does not ask for this information before selling you a system, 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.

Explosion Control

Explosion control can be provided using one or a combination of methods to protect an enclosure from rupture. Passive controls for dust collection systems include deflagration venting and flameless venting. Deflagration venting to an unoccupied area is permissible when a dust collector is installed outdoors. If the dust collector is located indoors, special consideration must be provided to protect occupants. Flameless venting may be an option to protect occupants from the effects of a vented deflagration.

Active controls to prevent an explosion in a dust collector include oxidant concentration reduction or chemical suppression. Oxidant concentration reduction reduces and maintains a system's oxygen concentration below which combustion is not possible.

Chemical suppression is more commonly used as an active control measure in dust collection systems. Such systems detect the onset of a deflagration and impending pressure rise within milliseconds and deploy a chemical agent to extinguish the flame before an explosion can happen.

Explosion Isolation

Once appropriate explosion protection controls have been determined for your vessel or dust collector, the next step is to isolate the hazard.

Why is Isolation Important?

Often the most destructive effects of a dust explosion are caused by flame propagation and pressure piling leading to the potential for a secondary deflagration or explosion. Isolation methods prevent flame and pressure propagation from one part of the system to another.

Passive isolation systems include float valves and flow-actuated flap valves. Both are mechanical barriers that isolate pressure and the flame front from propagating further through the ducting. Such devices can be a cost-effective addition to your mitigation plan.

Active isolation systems commonly use pressure or optical sensing devices that monitor conditions and quickly react by activating an isolation device. Chemical suppression, fast-acting knife gates or high-speed abort gates are active isolation systems that can be designed to respond in milliseconds.

System Certification

Has a recognized third-party organization tested the equipment you have selected for your dust collection system and the devices to isolate and protect upstream processes? NFPA 69: Explosion Prevention Systems states, "System design methodology and application range shall be supported by appropriate testing and certified by a recognized testing organization acceptable to the authority having jurisdiction."

Third-party testing and certification provide a performance demonstration of the safe application and operation or limitations of your explosion control systems.  This ensures that equipment has been tested and proven effective under real-world conditions. Products are sold on the market today without this extensive validation process. Prescribed design and application requirements vary depending on the equipment and may include, but not limited to:

*Approved dust types and explosibility characteristic ranges (Kst, Pmax)

*Minimum (Lmin) and maximum (Lmax) location placement distances from the expected ignition source

*Maximum number of flow direction changes

*Detection specification of activation pressure or rate of pressure rise After you have purchased validated equipment, who will conduct the installation? Is it your local general contractor? Is your contractor well versed in the application and installation requirements of combustible dust explosion controls?

Conclusion

Selecting and installing explosion controls requires alignment with the performance demonstration criteria in order to be most effective. For new installations, a preliminary design review can be utilized to better understand the extensive NFPA requirements, life safety objectives and installation requirements. The design and installation of these systems are complex and should be included in your dust hazard analysis.

This can be done internally or conducted by an outside resource, but it should be performed by a qualified person. An outside consultant or engineer prevents bias and conflicts of interest when evaluating combustible dust hazards and risk mitigation controls and strategies. Experienced consultants also provide an objective perspective and offer insights into industry best practices. Expert guidance and the selection of qualified vendors and contractors help maintain risk mitigation reliability and protects you from start to finish.

This article originally appeared in the September 2021 issue of Occupational Health & Safety.

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