By Udi Tzuri, Director of Product Management, Flame & Gas Detection, Emerson Automation Solutions
There’s no perfect flame detection system for every application. Matching optical flame detector options – including single wavelengths of UV and IR, integrated UV/IR sensors, and more advanced units that offer triple wavelength IR sensors – to your requirements is everything. If you understand the type of flame to be detected, the environmental conditions surrounding the installation, and the required performance, the choice of flame detection technology becomes easier and the potential for false alarms is decreased.
Almost all flames produce heat, carbon dioxide, carbon monoxide, water, carbon, and other products of combustion, which emit visible and measurable UV and IR radiation. These same emissions from non-flame sources cause nuisance false alarms and plant shutdowns. There are two basic types of these emissions: natural sources including rain, lightning, and sunlight; and man-made sources including artificial light sources, welding, and radiation from heaters and machinery. All types include solar-blind UV; window contaminates; non-modulated IR; and modulated IR sources.
Energy that is constant over time or varies at an extremely slow rate like the IR energy emitted from heaters, lamps, and heat from the sun are described as non-modulated sources of radiation. Additionally, there’s a small amount of IR radiation emitted from all objects which is constantly present in any detector’s field of view. As a result, the majority of flame detectors are designed to only detect modulated IR radiation sources – a key characteristic of flames. Still, the detection isn’t straightforward. False sources include heated emissions, moving lights, signals, or combinations of non-modulating sources being altered by objects moving back and forth in front of them in between the source and the sensor (e.g., vehicles, personnel, or fan blades). This is overcome by the use of multi-bands which can distinguish on the IR spectrum between flames and other sources of radiation.
Outdoor applications must contend with the visible range of sunlight, which covers 0.3 to 0.8 microns. UV detectors generally detect energy below solar emissions (0.185 to 0.260 microns) and can be a suitable choice for outdoor applications because of their extremely fast response and wide field of view; but UV/IR and triple IR options offer higher immunity to potential false alarms from high-energy bursts from reflective surfaces. Safety engineers must also consider the source of the fire when selecting a detector. If the fuel could potentially be hydrogen-based, for example, a specially tuned detector is required. For hydrocarbon-based fires from fuels such as methane and gasoline, multi-spectrum IR detectors are typically the best choice.
Window contamination will negatively affect the detector’s performance and can cause the instrument to go into fault mode. Water droplets, condensation, snow, and ice are powerful absorbers of IR energy that can be delivered in random scales and intensity and are a well-known source to trigger false alarms or faults when combined with modulated energy sources like direct sunlight. UV radiation is also easily absorbed by a range of oils, smoke, carbon, and specific gases. Engineers need to be aware of the presence of vapors such as hydrogen sulfide, benzene, ammonia, ethanol, acetone, and others when selecting a flame detector for their application.
By analyzing your application for these types of potential false alarm triggers, you can let your flame detection expert know all the parameters for an optimal detector selection. If you’re experiencing a lot of false alarms, this may be a good time to review your choice of flame detection technology.
What kinds of problems do you experience with false alarms?
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