Safety IR Flame Detection – Single Sensor


IR radiation is present in most flames. The flame temperature and its mass of hot gases (fire products) emit a specific spectral pattern that can be easily recognized by IR sensor technology. However, flames are not the only source of IR radiation, in fact, any hot surface, e.g. ovens, incandescent lamps, halogen lamps, furnaces and solar radiation, emits IR radiation that coincides with flame IR radiation wavelengths.


Most single band IR detectors are based on pyroe-


lectric sensors with a 4.4 micron (µ) optical filter and a low frequency (1-10 Hz) electronic band pass filter. This type of detector will recognise a 1 sq.ft. Gasoline pan fire from a distance of 15m.


However, these IR detectors are still subject to false


alarms caused by blackbody radiation (heaters, incan- descent lamps, halogen lamps, etc.).


Single frequency IR detectors respond only to a


certain flicker and radiation intensity of 4.4µ. Under certain conditions, it is possible for flickering, caused by such things as shimmering water, rotating lights or interrupted thermal radiation, to be interpreted as fire by single frequency IR detectors.


In order to minimise or eliminate these false alarms in single UV and IR detectors, dual, triple and multiple wavelength technologies have been adopted for optical fire detection.


UV/IR Flame Detection – dual sensor


Dual spectrum UV/IR technology employs a solar blind UV sensor with a high signal-to-noise ratio and a narrow band IR sensor. The UV sensor itself is a good fire detector. However, it easily false alarms, thus the IR sensing channel


was added, working at the 2.7µ or the 4.1µ-4.6µ spectral ranges and serve as a reliable detector for many mid- range applications.


channels, certain scenarios may occur when a fire is present. A serious problem may occur when a strong UV source (welding) is present and a fire ignites. Here, two UV signals are produced (one strong, the other weak) thus blocking the detector’s logic from further comparison with the IR channel, and preventing fire detection.


Unwanted solar spikes in the UV (in the spectral band where fires emit most of their UV energy) combined with flickering IR sources (such as moving objects in front of hot sources) are liable to cause false alarms, even when a fire is not present.


Again, detection distance is limited to max 15m.


IR/IR Flame Detection – Dual Sensor Common dual IR flame detectors employ two narrow


bands (0.9µ and 4.3µ) for fire signal analysis or a combination of short wavelength 0.8-1.1µ and long wavelength 14-25µ IR channels. Some dual IR detectors include, in addition to one near IR channel for fire


detection, a channel for the background detection in the 4.7-16µ IR band.


However, more recently, the fire’s main spectral


characteristic feature at 4.3µ - 4.5µ is analysed thoroughly. This “differential spectral” approach is where two spectral ranges are analysed: one emitted strongly by the fire, while the second is emitted weakly by the surroundings, thus the ratio gives a substantial mathematical tool for fire signal processing.


However, since most dual IR detectors use the 4.3µ sensor as their main channel for fire recognition, they


suffer from atmospheric attenuation, especially at long range detection applications. Again, detection distance is limited to max 15m.


Triple Infrared (IR3) Flame Detection


TRIPLE IR (IR3) technology is a major breakthrough in fire detection, which detects by concurrently monitoring with three IR sensors.These signals are further analysed mathematically with respect to their ratios and correlations


IR3 detectors will not false alarm to any continuous, modulated or pulsating radiation sources other than fire (including sources like black or gray body radiation). The high sensitivity of the Triple IR technology, coupled with its inherent immunity to false alarms, enables another major benefit of this technology - substantially longer detection ranges than previously obtained with standard detectors – 65m compared to 15m for the same test fire.


Figure 2: UV/IR Flame Detection


However, even this advanced technology has its limitations, since each type of fire has its own specific ratio of UV to IR output. For example, a hydrogen flame


generates a lot of UV with very little IR (in the 2.7µ band), while a coal fire will generate little UV and a high amount


of IR (in the 4.1µ-4.6µ bands). Hence, specific dual UV/IR detectors must combine both signals and compare them accordingly to distinguish a fire signature from false alarm stimuli.


To ensure the reliability, a discriminating circuit compares the UV and IR thresholds, their ratio as well as their flickering mode. Only when all parameters satisfy the detection algorithm is a fire signal alarm confirmed. However, UV radiating sources are sources for false alarms.


Since false alarms can affect both UV and IR Figure 3: IR3 Flame Detection


Conventional IR3 flame detectors may have a limited ability for discriminating between distant fires and fires


Multispectral Flame Detection A major concern in optical flame detection is IR radiation with spectra, at least superficially, similar to those emitted by flames, which may be produced by many non-flame sources, including but not limited to warm objects (including, under some circumstances, people or animals), sunlight (direct or reflected) and various forms of artificial lighting. Such IR radiation may be mis- interpreted as a flame. However, simply ignoring or filtering this radiation may result in actual flames being masked.


Analysing multiple spectral bands, identifying the absence of a strong peak or other well-defined marker, eliminating spectra resembling a blackbody curve, employing wide band and narrow band filters, are some of the modern ‘tools’ in flame detection.


The use of multiple IR sensors is the best technology, provided the selection of sensors and filters covers most of the flammables spectra (including hydrocarbons and hydrogen flames) and eliminate all the false alarm spectra in the monitored area. Such detectors can simultaneously detect a hydrocarbon fire at 65m and a hydrogen flame at 30m.


The increased activity in LNG and LPG processing and storage also requires the use of flame detectors, and recent improvements in the effective detection range for such gas type flames (e.g. methane, propane, etc.) means that fewer detectors are required to properly protect any given area than was previously the case.


Due to the increased reliability, durability, high quality


and performance, SharpEye 40/40 Series Flame Detectors are approved to IEC 61508 - SIL2 (TUV) for safety integrity; performance approved to EN54-10, FM3260 and DNV Marine as well as Ex Zone 1 hazardous area approved by ATEX, IECEx, FM, CSA and GOST with a resultant extension in the warranty period to 5 years. This represents a major investment in the development, design and manufacture processes along with the high costs of these many 3rd party approvals. Most 3rd party approvals also entail an initial and continuing annual factory assessment to ensure that standards, processes and performance are being maintained.


Summary


Flame detection technologies have advanced signifi- cantly since the first UV detector, primarily ‘pushed’ by the ever-growing demands of modern industries for reliable and cost-effective detection equipment for their expensive high-risk, high-end facilities and processes. Smaller in size, larger in brains (with their miniature microprocessors), modern optical flame detectors provide enhanced flame detection reliability and longer detection ranges with minimal (or no) false alarms, backed by independent confirmation of their perfor- mance and integrity.


within the area they are tasked to protect. For example, petroleum drilling and processing facilities often have large flares that burn off hydrocarbon gas. Typically, flares emit IR radiation characteristic of hot CO2


. Stack


flares typically represent known phenomena, and generally are not considered a legitimate alarm source. However, flares are often visible for miles. If within the field of view of a conventional triple IR flame detector, an alarm may be triggered.


To eliminate this, advanced IR3 detectors include special filters for the hot CO2


distinguish between near and far fires. peak that enable them to


23


CCTV Flame Detector is Certified as SIL 2 Capable


Ideal for use in environments where functional safety is critical, the Draeger (Germany) Flame 5000 colour imaging based CCTV Flame Detector has been certified as SIL* 2 capable. Offering innovative, reliable flame detection technology for oil and gas installations as well as chemical and pharmaceutical plants, this explosion-proof system is easy to install in any industrial application where a potential fire source exists.


Believed to be unique, the Draeger Flame 5000 is different to traditional radiation, or combined radiation and CCTV cameras because it uses the camera to detect the flame. Designed as a stand-alone system and housed within a single unit, it combines colour imaging with advanced digital signal processing and software algorithms to process live video images and interpret the characteristics of a flame. As a result, it can eliminate false alarms caused by day-to-day workplace influences such as hot processes, flare reflections and hot CO2 emissions.


The Draeger Flame 5000 can also be used to provide live video images, and can be fully integrated with a control system or fire panel to provide fault and fire signalling using normal 0-20mA or relay outputs. As well as the surveillance benefits, this capability removes the need to despatch operators to investigate alarms, reducing the risk of injury whilst improving response time to hazardous situations to around 4 seconds. Able to detect fires of 0.1m² or more at 44m within a 90º horizontal field of view, it can be operated in temperatures ranging from -60 to +85ºC. Simple to install with a stainless steel mounting bracket that can be rotated to ensure optimum positioning, this accurate detector is also fitted with automatic optical verification. This advanced facility checks the window for contamination and ensures that the field of view is not compromised by obstructions placed immediately in front of the detector.


Reader Reply Card No 89 Annual Buyers’ Guide 2010


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