In today’s episode of the Dust Safety Science podcast, Brian Edwards, Global Explosion Protection Consultancy Manager at Fike Corporation, about the pros and cons of various dust explosion isolation systems.
Dust explosions present a significant risk in industries handling combustible dust, and the consequences of not having proper isolation measures can be devastating. In this discussion, Brian shares insights on why isolation systems are crucial, the differences between passive and active systems, and the key factors to consider when selecting the best solution.
What Is Explosion Isolation, and Why Is It Important?
Explosion isolation is designed to stop the spread of a deflagration (a rapid combustion event) beyond its origin point, typically a vessel or piece of equipment. When combustible dust becomes suspended in an oxygen-rich environment with ignition sources present, a dust explosion can occur. Once started, this explosion may travel through connecting systems such as ducts or chutes, potentially reaching other equipment or even worker areas.
Isolation systems prevent flames from moving between equipment or reaching occupied spaces, thereby reducing risk to both people and equipment. Brian underscored that without isolation, a small explosion can easily propagate, causing damage far beyond the initial site. He talked about one instance of an explosion traveling through ductwork that reached a connected vessel not designed to withstand the increased pressure, leading to severe damage and injuries. Such incidents highlight why explosion isolation systems are crucial for safety and compliance.
Why Is Isolation Needed in Both Directions?
Isolation systems protect not only downstream equipment but also the workplace itself from fire hazards. When explosions spread into occupied spaces, the potential for injury or loss of life increases significantly. Brian explained that it’s vital to isolate in both directions, protecting upstream and downstream equipment and ensuring that flames don’t reach the workplace. He recalled cases where companies have implemented isolation systems to avoid incidents like the 2014 Kunshan, China explosion, where flames escaped through ductwork, leading to extensive workplace damage and fatalities. Having isolation that functions in both directions reduces such risks, keeping the workplace safer.
What Are the Differences Between Passive and Active Isolation Systems?
Brian discusses the two main categories of explosion isolation: passive and active systems, as outlined in the NFPA 69 standard.
Passive Isolation Systems
Passive isolation systems operate without any required action or monitoring; they are self-triggering when a deflagration occurs. These systems rely on their mechanical properties to block the fire’s path, so they require minimal maintenance and are generally simpler to install.
Active Isolation Systems
Active systems, on the other hand, depend on sensors and detectors to detect the deflagration and then activate the isolation mechanism. These systems are more complex and require careful installation and regular maintenance to ensure they function as intended. Active systems are often chosen for applications where the fire risk is high, as they provide a responsive barrier.
What Are the Passive Isolation Options?
Brian went over the main passive isolation options: flow-actuated float valves, flap valves, rotary valves, flame front diverters, and dust flame arresting devices. Each of these systems has unique features, pros, and cons.
Float Valves
Float valves are passive devices that isolate flames by using a float mechanism that automatically closes upon detecting a deflagration. Brian noted that float valves are often bidirectional, allowing for protection in both directions. However, these systems can be costly and aren’t suitable for lines with heavy dust loading or sticky materials, as buildup can prevent proper sealing. Float valves are most commonly used in industries that handle low-dust concentrations, such as some pharmaceutical and chemical manufacturing applications, where dust particles remain low, and clean airflow is vital.
Flap Valves
Flap valves are also kept open during normal operations and close when they detect a change in airflow, such as in a deflagration. These valves are less expensive than float valves and have lower maintenance requirements, making them popular in manufacturing facilities with straightforward dust control needs. However, flap valves typically only provide one-directional protection, so they’re limited in applications where bidirectional protection is needed.
Another consideration with flap valves is their inability to handle systems with heavy dust concentrations. In industries where significant dust particles are involved, material can build up on the valve, compromising its ability to close completely and posing a risk for fire spread.
Rotary Valves
Rotary valves serve as isolators when designed per NFPA 69 standards, which often includes creating a material plug above the valve. If a rotary valve is already part of the process design, it can be an economical and effective isolation solution. However, these valves are limited to specific configurations and may not function well in high-temperature processes due to thermal expansion. For example, industries using thermal processing, like food or metal recycling, may encounter issues with rotary valves due to temperature-related expansion, affecting the valve’s effectiveness.
Flame Front Diverters
Flame front diverters relieve pressure by redirecting flames away from the equipment and reducing explosion propagation risk. However, they do not stop the flame itself, so they aren’t suitable for all isolation needs. These devices are commonly used in applications where equipment pressure needs to be contained, but flame isolation is not the primary concern.
Dust Flame Arresting Devices
Newly added in the 2024 NFPA 69 standard, dust flame arresting devices use metal mesh to absorb heat and extinguish flames. These devices are suitable for clean air intakes or exhaust systems, preventing flames from escaping into occupied spaces. Brian highlighted that dust flame arresting devices are particularly useful in industries like wood processing or pharmaceutical manufacturing, where exhaust systems often interact with occupied areas.
What Are the Active Isolation Options?
Active isolation systems use monitoring equipment to detect a deflagration and then deploy mechanisms to isolate the flame. These systems include chemical isolation, fast-acting mechanical valves, and pinch valves. Brian and Chris discuss the pros and cons of each.
Chemical Isolation
Chemical isolation systems inject a suppressant, typically sodium bicarbonate, into the duct when a deflagration is detected. This disrupts the flame, preventing it from spreading to other areas. Chemical isolation systems are versatile, easy to install, and can be added to existing suppression systems without much effort. For facilities with existing explosion suppression setups, adding chemical isolation is often cost-effective and practical. Industries handling highly combustible materials, like grain processing, often use chemical isolation for its adaptability.
Fast-Acting Mechanical Valves
Mechanical valves, such as knife or gate valves, close a duct using a sensor-activated metal barrier, creating a robust seal that blocks flame movement. These valves prevent contamination and provide durable, dependable isolation. However, they are more expensive, require regular maintenance, and may need support structures to remain in place. Despite these drawbacks, mechanical valves are a popular choice in heavy manufacturing or metal processing industries, where contamination control and robust flame isolation are critical.
Pinch Valves
Pinch valves use compressed air to squeeze a rubber sleeve, sealing the duct and stopping flame spread. They’re generally less expensive than mechanical valves and easier to reset after deployment. However, pinch valves can wear out over time, particularly with abrasive or corrosive dusts, and they may not be suitable for handling highly combustible materials. Industries dealing with mild dusts, such as certain food processing applications, may find pinch valves a suitable solution for flame isolation.
How Do You Choose the Right Isolation System?
When selecting an explosion isolation system, it’s essential to consider the process requirements and the primary explosion protection system. Brian advised starting with the type of conveying system in place: for active conveying lines, options may be limited to rotary valves or active isolation. Matching the isolation system with the primary protection system is also practical; for instance, if the primary protection is chemical suppression, adding chemical isolation is cost-effective and efficient.
The dust characteristics also play a role. Highly combustible dusts require stricter isolation methods, while materials with lower combustibility may be suited to more economical solutions. The specific properties of the dust, such as particle size, stickiness, and combustibility, all influence the choice. Additionally, factors such as equipment location, regulatory requirements, and budget constraints are also vital.
Brian emphasized that it’s crucial to follow the installation requirements provided by manufacturers, including the minimum and maximum distances for active systems to work effectively. These guidelines ensure that systems can detect and activate quickly enough to provide proper protection.
Importance of Maintenance and Monitoring
Regular maintenance is essential for both passive and active systems. Active systems, in particular, require regular inspection of detectors and control panels to ensure proper functionality. Failing to maintain these systems can lead to ineffective isolation, which could mean the difference between a controlled deflagration and a catastrophic event. Facilities should establish a maintenance schedule that includes testing isolation mechanisms, verifying detector accuracy, and replacing components as needed.
Conclusion
Explosion isolation systems are a critical part of combustible dust safety. Whether using a passive solution like flap or float valves or an active solution such as chemical isolation or fast-acting mechanical valves, facilities must choose systems that align with their unique needs. Regular maintenance and adherence to installation requirements are key to ensuring these systems function as intended. By understanding the properties and demands of each process, facilities can make informed choices to protect their operations and safeguard personnel.
If you would like to discuss further, leave your thoughts in the comments section below. You can also reach Brian Edwards directly:
Website: https://www.fike.com/
LinkedIn: https://www.linkedin.com/in/brian-edwards-pe-gatech/
Email: [email protected]
If you have questions about the contents of this or any other podcast episode, you can go to our ‘Questions from the Community’ page and submit a text message or video recording. We will then bring someone on to answer these questions in a future episode.
Resources mentioned
The resources mentioned in this episode are listed below.
Dust Safety Science
Combustible Dust Incident Database
Dust Safety Science Podcast
Questions from the Community
Companies
Fike Corporation
Standards
NFPA 69
Articles
‘Pattern of negligence’: a chemical plant fire in Georgia forces tens of thousands to take shelter
Download the episode
DSS288: Weighing Pros And Cons Of Different Dust Explosion Isolation Systems With Brian Edwards
