Page 16 - OHS, September 2021

COMBUSTIBLE DUST
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.
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 Certifification
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.”3
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.
Scott Czarnecki, CSP, CHMM is a Process Safety Consultant with the SEAM Group.
REFERENCES
1. https://www.nfpa.org/codes-and-standards/all-codes-and-standards/ list-of-codes-and-standards/detail?code=652
2. https://ohsonline.com/Articles/2014/03/01/Combustible-Dust- Compliance.aspx
3. https://blog.ansi.org/2019/06/nfpa-69-2019-standard-explosion- prevention/#gref
12 Occupational Health & Safety | SEPTEMBER 2021
www.ohsonline.com