Issue link: https://www.e-digitaleditions.com/i/1366075
24 / May 2021 powderbulk.com before flame and smoke development. 2 Early detection can dramatically reduce the hazard response effort and minimize the explosion hazards' residual effects. "Seeing" electronically 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 pro- cess 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 process- ing facilities that recycle filtered exhaust air and other spark- and ember-generating processes. 4 Although conventional explosion prevention sys- tems 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 continu- Identifying ignition sources with smart sensors To successfully mitigate dust explosions, a deflagra- tion event's contributing elements must be controlled. Consider the five elements of a dust explosion — fuel (combustible dust), oxygen, ignition source, dust dis- persion, and containment, as shown in Figure 1. Four of the five elements (fuel, oxygen, dispersion, and containment) are typically normal parts of the pro- cess operation. This leaves ignition-source control as the most effective prevention method for process dust explosion hazards. Ignition sources can origi- nate 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 dif- ferent detection and control methods. With smart devices, ignition-source detection is facilitated by elec- tronic 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 pro- cess and existing systems, ranging from a pre-alarm warning for operator intervention to a water deluge and full process stoppage. "Smelling" electronically 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 burn- ing, 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 combus- tion-gas sensors analogous to a bloodhound's keen sense of smell. Early combustion-gas sensors, for exam- ple, 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 FIGURE 2 A pyrolysis gas detector can detect pyrolysis by-products such as carbon monoxide, hydrogen, or nitrogen oxide gases.