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Safety High Performance Industrial Flame Detection
Ian Buchanan, European Manager, Spectrex Inc 218 Little Falls Road, Cedar Grove, 07009 NJ, USA Tel: HQ +1 973 239 9398 • Europe +44 141 578 0693 • Web:
www.spectrex-inc.com
Optical flame detection has progressed to meet the ever-growing demands for maximum reliability, availability and minimal false alarm events and is widely employed in many high risk industries, such as those in oil & gas (onshore & offshore), petrochemicals, hazardous material handling and storage, etc., to protect both high-value plant and personnel.
Flame Detectors are the favored solution for high risk areas and outdoors where smoke and heat detectors are not effective. Unlike smoke and heat detectors, the fire/products of fire (smoke/heat) do not have to reach the optical detector to be recognised as it can ‘see’ the fire (flame) radiation from distances up to 65 meters, within a 100º ‘cone of vision’ in all directions – and raise an alarm within 5 seconds. Detection is taken to the fire rather than waiting for the fire to reach the detector.
Optical flame detectors provide the fastest detection of a fuel fire in the early ignition stage. This capability, adjustable field of view and programmability make them extremely well-suited for this critical duty
About Flames
Optical flame detection is based on detecting the unique characteristics of the electromagnetic energy emitted by a fire, including its ‘spectral signature’ and frequency pattern and distinguishing it from the other myriad of heat radiation emitters and black-body radiation spectral signatures in the surrounding atmosphere.
Flames emit electromagnetic radiation at a wide range of wavelengths, which vary depending on the fuel being burned and environmental conditions that affect the radiation transmission in the atmosphere. Optical flame detectors operate by sensing one or more of these wavelengths.
non-organic fuels. In their combustion process, they generate a lot of hot water vapor that has a characteristic IR emission spectrum with a relatively strong peak around 2.7 microns.
In addition to these two major fire products (CO2
O), other intermediate radicals, and ions and by- products created in the fire process (such as CO, CHOH, COOH, CH3
and H2
, OH, etc.), emit electromagnetic radiation that can be detected either in the ultraviolet (UV) solar blind spectrum or in the wide IR band 0.8 - 3 micron.
One of the problems in detecting fire conditions, particularly small fires or at long range, is the potential for a high false alarm rate. False alarms can be generated by other electromagnetic radiation sources which are “friendly fires”(like flares in the petrochemical industry) or by spurious radiation sources such as direct and reflected sunlight, artificial light, welding, electrical heaters, ovens, and other sources of noise. Such spurious radiation sources might not be large enough to activate short-range detectors, but may activate detectors whose sensitivity has been increased to maximise their detection distance. A false alarm may result in a costly discharge of the fire extinguisher and its replacement and/or plant shutdown.
Several generations of flame detectors have been developed over the years to address the various fire and explosion hazards, particularly in today’s high-risk industries and on the ever-changing military battlefield where soldiers and their protective vehicles are faced with new incendiary weapons and explosives.
Flame Detection Technologies
Flame Detectors usually employ several optical sensors, working in specific spectral ranges (usually narrow bands) that detect simultaneously the incoming radiation at the selected wavelengths. The signals detected by each sensor are analysed according to a pre-determined technique that includes one or more of the following:
1. Comparator techniques (and-gate techniques). 2. Flickering frequency analysis. 3. Threshold energy signal comparison.
4. Mathematical ratios and correlations between various signals.
5. Correlation to memorized spectral analysis.
Modern Flame Detectors employ several of the above-mentioned techniques using multiple sensors to provide enhanced reliability and accuracy. The spectral bands selected for each type of detector determine the detector’s sensitivity, detection range, speed of response and immunity to false alarms. The flame radiation spectral pattern, being unique, allows several spectral ranges to be employed simultaneously in the various detection devices.
The following, in chronological order, is a brief review of the technologies, their limitations and the solutions that have been developed and incorporated into modern flame detectors. All are still in use today although early types tend to be restricted to very specific applications.
UV Flame Detection – Single Sensor Figure 1: Flames Spectral Analysis
Many combustible materials include carbon, and combustion of such hydrocarbon fuels, typically generate hot carbon dioxide (CO2
) gas. Hot CO2 has a
characteristic infrared (IR) emission spectrum, with a relatively strong and well-defined peak at wavelengths from approximately 4.2 to 4.5 microns, and relatively little intensity at wavelengths immediately on either side of the peak. In the presence of an actual fire, the radiation intensity in the peak band is generally high, while little or no radiation is received in the side bands. Thus, high radiation intensity in the peak band as compared to that in the non-peak side bands is used to determine whether a flame is present.
Some other combustibles lack carbon, for example hydrogen, ammonia, metal oxides, silane and other
The earliest flame detector utilised the UV spectral band. The UV spectral signature of some flames has a pattern that can be readily recognised over the background radiation. UV detectors based on this technology are detecting flames at high speed (3-4 milliseconds) due to the high-energy UV radiation emitted by fires and explosions at the instant of their ignition. However, this discernible UV radiation emitted by a flame from a distance (several meters) in outdoor applications can be attenuated by atmospheric pollutants, such as smoke, smog, hydrocarbon vapors, organic material accumulated on lenses or detector windows. In addition, the occurrence of random UV radiation from stimuli such as lighting, arc welding and radiation, X-rays, solar radiation (not absorbed by the atmosphere, due to holes in the ozone layer and solar bursts), cause false alarms in UV detectors.
A new generation of better performing and more reliable UV detectors now exist but are still susceptible to false alarms and limited to approx 15m detection distance. Today, they tend to be used indoors where other interfering radiation is not present and where very fast response is necessary, e.g. munitions manufacture.
Annual Buyers’ Guide 2010
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