Jeramy Slaunwhite | Rembe
The risk of combustible dust explosions in the powder and bulk solids industry is sometimes part of the job. However, steps can be taken to reduce those risks, including implementing explosion safety technology into your processing system. This article describes the smart technology available to help reduce the explosion risks in processes involving combustible dust.
When wood, flour, sugar, or other powder and bulk solids are included in the material handling process, there’s a chance that combustible dust will be produced, potentially leading to an explosion. Explosion hazards are an apparent reality for many manufacturing and processing facilities. Managing explosion hazards can include various concepts and magnitudes of risk identification and reduction. Conventional explosion protection systems rely on the initiation of a deflagration event to activate the protection system. Examples of deflagration indicators include rising internal pressure that opens deflagration vent panels or the electronic detection of a spark or pressure rise to activate a chemical explosion suppression system. In this way, conventional systems are reactive to an initiated explosive event rather than proactive. While there’s often no practical and reliable substitute for explosion protection systems, the ideal scenario is to prevent explosion conditions before reaching a critical hazardous state.
Explosion hazard management occurs at different stages, depending on where the hazard is in development. At each stage, a respective amount of cost and effort is required to control influencing conditions and revert to a normal operating state. This ranges from simple tasks such as housekeeping to manage dust accumulation to refurbishing a vessel following a vented deflagration. As we move into the Industry 4.0 era with the automation of traditional manufacturing and industrial practices and artificial intelligence and interconnected smart devices increasingly become part of our daily lives, the opportunity and practicality of implementing these technologies for fire and explosion safety is also growing.
We accept and rely on automation, sensors, and algorithm controls on a daily basis without a second thought. For example, when driving our cars, we rely on many passive and active safety and control systems including seatbelts, anti-collision systems, automatic breaking, and airbags. Each system has a hazard management role at various stages of risk and consequence. Some systems are fully automated, whereas others respond to driver input. The obvious primary objective in a car is to avoid a collision, with a secondary goal being to minimize injury to the occupants. Similarly, the primary objective of explosion safety is to prevent explosions, and the secondary goal is to minimize the consequences.
Identifying ignition sources with smart sensors
To successfully mitigate dust explosions, a deflagration event’s contributing elements must be controlled. Consider the five elements of a dust explosion — fuel (combustible dust), oxygen, ignition source, dust dispersion, and containment, as shown in Figure 1.
Four of the five elements (fuel, oxygen, dispersion, and containment) are typically normal parts of the process operation. This leaves ignition-source control as the most effective prevention method for process dust explosion hazards. Ignition sources can originate from dozens of causes, and their study is a field of research and analysis on its own. Some common ignition sources are smoldering material, open flames or sparks, hot surfaces, friction or mechanical impact, and electrostatic discharges.1 Different ignition sources have varying energy and longevity that require different detection and control methods. With smart devices, ignition-source detection is facilitated by electronic sensors that serve as the processing system’s electronic nose and eyes. Advanced sensors can be specifically configured for material types, processes, or both, including the detection of material-specific signature combustion gases and elevated temperatures well below the material flammability temperature and the exclusion of defined “safe regions” with respect to heat sources and normally occurring hot areas such as burners and heaters. Hazard mitigation response and process recovery can be tailored specifically to the process and existing systems, ranging from a pre-alarm warning for operator intervention to a water deluge and full process stoppage.
Conventional smoke detectors indicate the by-products of active combustion, and a smoke detector’s operation is comparable to someone noticing the smell of smoke in their home. At the point that smoke is detected by the smoke detector or a human nose, material is burning, and significant action must be deployed, such as water deluge or other fire extinguishing systems. The recovery effort from fire extinguishing systems can be significant depending on the suppressant’s invasive impact on the surrounding area, such as water flooding and smoke damage. Advanced technology has enabled the development of super-sensitive electronic combustion-gas sensors analogous to a bloodhound’s keen sense of smell. Early combustion-gas sensors, for example, a pyrolysis gas detector as shown in Figure 2, can detect pyrolysis by-products, such as carbon monoxide, hydrogen, or nitrogen oxide gases, in parts per million specific to the material in the early stages of smoldering before flame and smoke development.2 Early detection can dramatically reduce the hazard response effort and minimize the explosion hazards’ residual effects.
Electronic eyes are also persistent watchdogs for fire and sparks. Spark detectors can identify the infrared energy released by even the smallest spark from metal friction contact (such as from fast-moving metal parts in mixers or foreign metal in the material stream) or glowing embers (such as from wood or tobacco). Some proactive spark detection and extinguishing systems can activate on-the-fly ignition-source management systems, such as inline spark quenching, bypass diverters, or process stoppages.3 Commonly applied to spark-prone mechanical and bulk material pneumatic conveying systems, spark detection and extinguishing systems can, in some cases, pose little to no impact to the process, support a reasonably fast and easy process recovery, or do both. For example, in a pneumatic conveying system with inline spark detectors and water-misting nozzles, if a spark is detected, a short, downstream water spray is triggered in the duct to neutralize the spark. After the spark is neutralized, the system continues to operate normally. Spark detection systems are a prescriptive requirement for most dust collection systems in woodworking and wood processing facilities that recycle filtered exhaust air and other spark- and ember-generating processes.4
Although conventional explosion prevention systems have the ability to detect sparks, the detection technology is limited to detecting sparks versus the lack of sparks. This baseline technology is unlike smart optical and electronic technology, which has expanded the capability to digitally see more than sparks and embers. Thermographic imaging and integrated signal analysis electronically visualizes a wide spectrum of infrared heat energy and continuously evaluates the data against defined parameters such as equipment and material surface temperature, temperature differential, or rate of temperature rise. Additionally, as a true smart device, a thermographic imaging system can be configured to either target or ignore specific areas within the viewing field. These systems can also be specifically designed to detect embers, flames, and hot surfaces in harsh, demanding industrial environments.
Applied thermographic visualization technology examples include monitoring belt conveyor roller bearings and dryer processes, as shown in Figure 3. High temperatures are expected in defined areas of a dryer but hotspots can occur. A hotspot is an area within a system that has a noticeably higher temperature than its surroundings. If the thermographic visualization technology identifies a hotspot outside the defined area, or if the peak temperature exceeds the setpoint threshold value, it triggers a warning or alarm for operator intervention or an automated response. The setpoint threshold value is the temperature warning and alarm limit — with a safety margin included — in comparison to the material ignition temperature. Detecting the cause of a potential ignition source before the source reaches critical dust cloud ignition energy (also known as minimum ignition energy or the lowest temperature at which a combustible dust will ignite) can save time, cost, and resources compared to either later explosion prevention methods or explosion protection systems.
Sensing trouble with other methods
Other fire and ignition-source prevention control strategies that you won’t likely find in conventional explosion protection systems include vibration analysis and thermal monitoring on rotating equipment and process upset monitoring with material flow and plugging sensors. Equipment performance monitoring can include electrical current meters, mechanical speed sensors, pneumatic flow meters, and mechanical misalignment systems to name a few. All of these devices add additional degrees of safety and control to prevent hazardous events, such as a friction-based hotspot or fire, overloaded material, and equipment malfunction. In addition to preventing process interruption and material loss, these safety devices can prevent potentially hazardous suspended combustible dust and potential ignition sources. Specific performance monitoring equipment, such as monitors for bucket elevators, can also be a compliance requirement according to the National Fire Protection Association’s NFPA Standard 61: Standard for the Prevention of Fires and Dust Explosions in Agricultural and Food Processing Facilities.5
Explosion protection system response and effects
In the event an explosion protection system performs its job by intervening to prevent a dust explosion’s damaging effects, a series of actions and events follow. Halting the process and associated equipment immediately following an explosion event is important to mitigate fire spread, damage, and material loss. A rapid response is conducive to minimizing residual damaging or loss effects. Explosion safety systems can use data from deflagration burst sensors, isolation valve activation switches, and active system activation or trouble sensors for extended hazard control. Safety system controls can instantaneously analyze sensor data in milliseconds to initiate the appropriate automatic shutdowns, response actions, and process controls to contain and minimize explosion event residual effects.
Integrated automation and control for explosion safety
Sensors and monitoring devices can be simple or advanced as well as broad or precise. The commonality amongst all of these devices is that they collect data. The differentiation between a simple device and a smart device is how the collected data is used. We can apply the same analogy to sitting in our vehicle driver’s seat. Vehicle sensors collect and display potential hazard data such as an object in a blind spot, low tire air pressure, or potential collision objects. As the driver, we can interpret this data and make informed decisions to manage risk. With vehicle safety technology and artificial intelligence advancements, some vehicles have the ability to collect and interpret hazard data, evaluate scenarios, and physically respond. An example of this is automatic vehicle braking for collision avoidance and autonomous control with self-driving cars.
In most industrial facilities, human input and labor are critical resources to the process. In the event of a fire or explosion, people are typically required to step out of their process operator roles and into the hypothetical hazard-control driver’s seat. Here, these operators need to make time-sensitive decisions for hazard management and process recovery based on the data available to them. Making these decisions can be an extremely difficult and stressful undertaking, especially for a large or serious event. Decisions must be made in the critical moments, seconds to minutes following an event, often with a degree of panic and limited data available to the operator.
Comparable to the lightning speed of digital evaluation and a self-driving car’s response, a smart explosion safety controller, such as the safety cockpit technology shown in Figure 4, represents its namesake by taking control in the event of a fire or explosion.6 This smart safety controller can interpret field and hazard sensor data to initiate immediate alarm and fire response action. When configured with integrated networking, the controller can notify personnel on their smartphones regardless of the person’s location. The event data, including plant location, equipment, and sensors, is displayed on a local connected computer or accessed remotely via a mobile app. Connection with plant video cameras can allow on-site and off-site operators immediate observation of the event so they can make quick decisions to initiate plant fire protection systems or services, dispatch the local fire department, or announce a complete facility evacuation with the push of an on-screen button. An operator’s rapid response to an explosion event can reduce the potential for injury, damage, and extended process downtime. An automated and controlled process is advantageous because the process is always ready and available with instantaneous action. Automated control removes the reliance on human input and assists with immediate decision-making based on real-time, relevant data.
Explosion event aftermath
Following a protected explosion event, the primary objectives are typically to resume process operation as soon as possible and determine the event’s cause to prevent future repeat events. Smart explosion safety systems can support both of these tasks. Following the explosion protection systems’ activation, field sensors can indicate to the smart controller that deflagration vents were ruptured or chemical systems were discharged. While plant personnel focuses on equipment assessment and cleanup, the networked controller can send an automated notice to the explosion protection equipment manufacturer to initiate the process of a rush equipment replacement order or service request. Considering explosion cause determination, in combination with process control data, an integrated smart explosion safety system stores time-stamped sensor data. This data can be used to create a precise sensor data timeline to investigate the sequence of events.
We rely on the design and function of our vehicles’ multiple layers of hazard mitigation safety sensors and systems to keep us safe and informed. The existence of anti-collision sensors in our car doesn’t mean we can or should ignore airbags or seat belts. All of the systems have a role for different hazards and recovery levels. Likewise, an integrated smart explosion safety system provides a wider spectrum of monitoring, security, and response than simple prevention or protection alone. Predefined actions serve as the autopilot in hazard scenarios to relieve critical pressure on operator interaction and implement the important immediate response actions.
- Rolf K. Eckhoff, Dust Explosions in the Process Industries, 3rd edition, Gulf Professional Publishing, 2003, page 10.
- “Explosion Prevention with GSME-X20 and Hotspot-X20,” REMBE Explosion Protection, accessed February 1, 2021, https://www.rembe.us/products/explosion-protection/gsme-and-hotspot/.
- “Spark Detection and Extinguishing Systems, GreCon BS7,” Fagus GreCon Fire Protection, accessed February 1, 2021, https://fagus-grecon.us/en/fire-prevention/prevention-concepts/spark-detection-system-grecon-bs7/.
- NFPA 664: Standard for the Prevention of Fires and Explosions in Wood Processing and Woodworking Facilities, 2020 Edition, National Fire Protection Association (NFPA), 1 Batterymarch Park, Quincy, MA 02169-7471; 800-344-3555, fax 800-593-6372 (www.nfpa.org).
- NFPA 61: Standard for the Prevention of Fires and Dust Explosions in Agricultural and Food Processing Facilities, 2020 Edition, National Fire Protection Association (NFPA), 1 Batterymarch Park, Quincy, MA 02169-7471; 800-344-3555, fax 800-593-6372 (www.nfpa.org).
- “Rembe iQ Safety Cockpit, Product Information,” Rembe, accessed February 2, 2021, https://www.rembe.us/fileadmin/produkte/explosionsschutz/REMBE_iQ/Product_Information_iQ_Safety_Cockpit.pdf.
For further reading
Jeramy Slaunwhite, P.Eng. (902-220-6396), is the explosion safety consultant at Rembe located in Halifax, Nova Scotia. He graduated with a mechanical engineering degree from Dalhousie University in Halifax, Nova Scotia, and has more than 15 years of applied engineering experience. He’s a member of the NFPA combustible dust safety technical committees for the food & agriculture and wood materials processing industries.
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