H EALTH & SAFETY detection Minimising false alarms Jonathan Saint , Marketing Manager, Emerson Process lenging. Examples of these false sources are heated emissions, moving lights, sig - Management, Net Safety Monitoring, outlines some of nals or combinations of non-modulating sources being altered by objects moving the common causes of false alarms in flame detector back and forth in front of them in between the source and the sensor (vehi - applications. cles, personnel, or fan blades, for example). This is overcome by the use of lame monitoring is one of several trum IR detectors are typically the best multi-bands which can distinguish the IR detection techniques that should choice. spectrum between and other Fassist the protection of life and sources of . property in industrial applications. Overcoming false alarms While UV detectors work well in Implementing an effective flame detec - A huge consideration in the selection of , other factors in outdoor appli - tion installation is not straightforward a flame detector is the potential for cations may negatively affect them. UV though. The key is to match the tech - false alarms. False alarms are generally sensors are designed to monitor solar- nology with the application and hazard; not the result of an issue with an instru - blind UV, the band of UV energy that is this greatly increases the overall perfor - ment, but rather its response to blocked by the layer in the upper mance and can reduce costly downtime. non-flame radiation sources that fall atmosphere. Powerful sources of this Optical systems such as within the field of view. There are two energy wavelength are commonly pro - (UV) and (IR) spectroscopy are basic types – natural and man-made. duced in industrial settings by halogen the methods that most flame detectors Natural sources include rain, lamps, arc welding and even lightning. use to perform their function. Almost all and sunlight, while man-made source Additional bands can be employed, or flames produce heat, carbon dioxide, examples are artificial light sources, combined UV/IR detectors will overcome carbon monoxide, water, carbon and welding, and radiation from heaters, almost all of these sources of inter- other products of combustion, which flare stacks and machinery. Falling into ference. emit visible and measurable UV and IR these two primary sources are four pri - Finally, window contamination will radiation. These spectral emission ‘by- mary types – solar-blind UV, window negatively affect the detector’s perfor - products’ are what flame detectors contaminates, non-modulated IR and mance and can cause the instrument to sense in order to quickly and accurately modulated IR sources. go into fault mode. Attenuating energy determine the presence of a . With non-modulated sources of radia - sources will hit or deposit on the However, these same emissions from tion, the energy is constant over time or window face of the detector as well as non-flame sources can cause nuisance varies at an extremely slow rate. accumulate on external reflectors used false alarms and plant shutdowns. Examples of these are IR energy emitted for automated visual integrity testing. from heaters, lamps and heat from the Water droplets, condensation, snow and Flame detector selection . Additionally, there’s a small amount ice are powerful absorbers of IR energy Today’s optical flame detector options of IR radiation emitted from all objects that can be delivered in random scales include single wavelengths of UV and IR, which is constantly present in any and intensity, and are a well-known integrated UV/IR sensors, and more detector’s field of view. To overcome source to trigger false alarms or faults advanced units that offer triple wave - this, the majority of flame detectors when combined with modulated energy length IR sensors. The performance and available on the market today are sources like direct sunlight. UV radiation advantages of each of these systems designed to only detect modulated IR is also easily absorbed by a range of oils, varies, so each must be examined in light radiation sources – a key characteristic of , carbon and specific gases. of various criteria and requirements. flames. With modulated sources, charac - Engineers need to be aware of the pres - Outdoor applications must contend terised by varied and sporadic energy or ence of vapours such as with the visible range of sunlight, which as a combination of non-modulated sulphide, benzene, ammonia, ethanol, covers 0.3 to 0.8 microns. A detector sources, identification can be very chal - acetone and others when selecting a that reacts to direct or reflected sunlight flame detector for their application. is clearly not appropriate for these appli - cations. UV detectors generally detect Flexible in the field energy below solar emissions (0.185 to There’s no perfect flame detection 0.260 microns) and can be a suitable system for every application – all have choice for outdoor applications because challenges. But understanding the type of their extremely fast response and of fire to be detected, the environ - wide field of view; but UV/IR and triple mental conditions surrounding the IR options offer higher immunity to installation, and the required perfor - potential false alarms from high-energy mance makes the choice of flame bursts from reflective surfaces. detection technology a much more Safety engineers must also consider manageable decision. the source of the fire when selecting a A detection solution that allows for detector. If the fuel could potentially be field sensitivity and time delay settings Triple infrared (IR) flame detectors offer hydrogen-based, for example, a the highest level of immunity and will help mitigate the more challenging specially tuned detector is required. For maintain a very fast response with wide false alarm sources by allowing users to -based from fuels such area coverage fine-tune their instruments in-situ for as methane and gasoline, multi-spec - Source: Net Safety Monitoring optimal performance. G

42 PETROLEUM REVIEW NOVEMBER 2012