Tips To Select The Right Flame Detector

Consider a number of factors to determine the most appropriate option

By Edward Naranjo, Emerson Automation Solutions

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Besides considering performance variables, check that the flame detectors are listed or approved for use in hazardous or classified areas and for the specific environments in which they will operate. Certification under regulatory performance standards such as FM 3260 [4], EN 54-10 [2] and ULC/ORD-C386 [5] also attests to product fitness because these norms subject detection systems to a set of reproducible tests to certify their performance on deployment.

Detection Technologies

Let’s now look at the different types of instruments available, their applicability and limitations, as well as factors to consider in making a choice.

• IR flame detectors. These devices find broad use in process plants. They are popular because the fixed emission wavelengths of flame in the IR spectrum can be separated from most non-flame sources and analyzed in various domains [6,7].

IR flame detectors can detect hydrocarbon or hydrogen flames. Hydrocarbon configurations suit facilities that process hydrocarbon gas or liquids — but are wholly unsuitable for detecting hydrogen flames or flames from other inorganic materials. Modulated radiation from hot surfaces, hot exhaust gases, sunlight, light from flares, and solar and flare reflections can affect certain IR flame detectors. The presence of these sources reduces flame response sensitivity and may cause false alarms. Don’t use IR flame detectors if flare radiation can be seen, either directly or reflected.

Multispectral IR flame detectors are recommended for many uses, including crude oil tanks, diesel storage facilities and enclosed gas compressor buildings. They are the most versatile of the detector types available and have the longest detection range as well as strong false alarm immunity. These detectors can characterize flames more fully due to their large number of sensors. Instead of relying on a single spectral span, they use several to establish the presence of a flame. As a result, the instruments don’t produce a false alarm when only one spectral span indicates a flame. Moreover, some sensors — called immunity sensors — monitor for the presence of false alarm sources.

• UV flame detectors. These devices suit facilities in which the only fuel sources are hydrogen and hydrocarbon gas. Because smoke scatters UV light to a far greater extent than it does IR, it’s best to locate UV detectors well below roof level as smoke produced by paint, cables or oils may accumulate in ceilings or roofs. To prevent false alarms, inhibit UV detectors during welding, radiography and exposed flame hot work. Just as for IR devices, don’t use UV detectors in areas where you can see direct or reflected flare radiation.

UV detectors are unsuitable for detecting hydrocarbon liquid fires or fires in highly congested spaces. Liquid fires produce little UV light and the dense smoke from these fires greatly reduces detection coverage. In addition, because UV light is most prevalent at the base of an open flame, the devices may not readily detect fires in congested spaces that block the line of sight to the base of the fire. Another consideration for UV detectors is the presence of oil or dust. Deposits of oil film or dust on detector windows can severely reduce flame response sensitivity; therefore don’t use these devices to monitor environments that contain airborne oil droplets or are dusty. UV detectors do best when detecting clean flames (i.e., those from natural gas, pure ethane or butane).

• UV/IR flame detectors. These devices, such as the one shown in Figure 1, combine the characteristics of UV and IR to provide a detector for general hydrocarbon or hydrogen fuel applications. They generate an alarm signal only if a flame is detected on both bands. UV/IR detectors best suit clean flames because few light sources emit as strongly in both spectral regions as clean flames. Such flames afford a shorter detection range than that of a fire from a liquid fuel (ethanol, for example).

• Closed-circuit-television (CCTV) flame detectors. These devices process video images and resolve flame characteristics. They best suit highly hazardous areas and normally unattended ones, where manual intervention could take a long time, making loss prevention more difficult. Because CCTV detectors transmit a video signal to the control room, they can allow quick assessment of the overall situation in the event of a fire alarm and confirm the presence and magnitude of the fire. Certain CCTV flame detectors can mask parts of their field of view, a feature useful for monitoring areas exposed to direct light from flares or flare reflections, such as top deck process areas on offshore platforms.

Don’t use CCTV detectors when you’re concerned about invisible or nearly invisible flames like those produced by hydrogen and alcohol fires.

Table 1 provides an overview of the principal characteristics of these flame detector types.

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