Dust generated during industrial processes is often combustible. While a dust collector is supposed to address this hazard, it can present a danger of its own if it isn’t designed, configured, or protected properly.

When a dust collector explodes, it fragments the metal housing and sends flames, heat, and projectiles into the workplace. This outcome can be devastating and even fatal for workers, equipment, and structures, especially when the dust collector is located inside a plant. This article outlines four questions that can assist owners and managers in identifying the right explosion protection for their facility.

Question #1 – What is the K_ST and P_MAX?

K_ST and P_MAX are the explosibility parameters for dust cloud reactivity. ASTM E1226: Standard Test Method for Explosibility of Dust Clouds provides an indication of the severity of the dust explosion by determining the deflagration parameters. In general, the larger the value of K_ST, the more rapid the pressure development will occur in a confined explosion condition and the more severe the explosion may be.

Certain products have K_ST limitations. For example, there are approval restrictions for passive devices such as flameless vents and flap valves. Passive isolation is also not suitable for gas or hybrid applications. Even active systems may require manufacturer’s third-party approval. If the facility produces or handles metal dust, there are additional challenges.

Question #2 – Where is the Dust Collector Located?

This question addresses whether explosion venting is a viable option. For example, if the dust or vapor is toxic, it may not safely release to the atmosphere. Another consideration is whether the fireball can be directed to a safe space, as fireballs can be eight times the volume of the vessel (or more). Flame ejections can sometimes exceed 50’, with the thermal effects going even further. The location of the dust collector is very important in establishing whether standard explosion venting will be best for the company implementing the system, or other explosion protection solutions should be considered.

Question #3 – What is the Duct Size?

Duct size can indicate whether you use an active or passive system.

Passive isolation has limitations with larger duct sizes while chemical isolation can accommodate a wider range of sizes. Other considerations include:

• Flap valve isolation devices may require horizontal ducting at the device location while chemical isolation is suitable for both horizontal or vertical duct orientation.
• There are restrictions on allowable dust loading for passive devices. No such restrictions exist for active isolation.
• There are airflow velocity restrictions on passive devices. With active devices, this velocity can affect the active isolation barrier’s allowable distances from the vessel.
• With passive devices, there can be a greater risk of material buildup.

Question #4 – Where Does the Clean Air Go?

While it’s rare for flame to propagate via the clean air exhaust, if it does, results can be catastrophic. Passive protection options are more limited in this regard.

If the clean air exhausts outside the building, NFPA does not require additional protection. If it exhausts inside the building, additional protection is necessary. While passive options are limited, dry chemical isolation can be easily retrofitted for complex geometry ducting.

Additional Considerations

While Kst, dust collector location, duct size, and clean air exhausting are key criteria when determining the best explosion protection solution, there are other considerations. They include:

• Initial and Ongoing Costs: Passive systems typically have a lower installation cost unless it involves a retrofit, in which case the cost can be higher. However, once installed, ongoing maintenance is usually lower for passive systems.
• Vessel Strength: This factor will affect reduced pressure required after venting or suppression (P_RED). With small volume vessels with a high Kst, it can be difficult to keep P_RED low with active suppression.

Conclusion

Manufacturing and processing industries face a number of challenges when it comes to dust control. In facilities that create combustible dust, specifying the most appropriate, NFPA-compliant explosion protection solution for your dust collector can reduce the risk of injury to employees and damage to equipment and the facility.

The time required to identify the correct option is minimal when compared to the outcome, which is a safer workplace.

Burke Desautels - President