Flame detection
Encyclopedia
Flame detection is the technology for detecting flames, using a flame detector
. Flame detectors are optical equipment for the detection of flame phenomena of a fire. There are two categories of flame detection:
Several different methods of flame detection are possible.
(UV) sensor is often sensitive for radiation
in the 185 to 260 nm range. This frequency range is the least sensitive for natural background radiation
sources like cosmic radiation and especially sunlight. The sunlight is, in the higher frequencies, absorbed by almost all vapours and gases; especially by ozone
and smoke but also by an oil or grease film on the window of a flame detector. Almost every fire radiates UV light, and the UV sensor is a good all round flame detector. A disadvantage is that quite a few artificial false-alarm sources occur; like halogen
and quartz
lighting (without regular glass), electrical welding
, corona and static arcs.
information, which in this case is made visible. The corona camera
is an example of this equipment. In this equipment the information of an UV camera mixed with visible image information. It is used for tracing defects in high voltage
equipment and fire detection over high distances.
(IR) sensor (0.7 to 1.1 µm) is especially able to monitor flame phenomena, without too much hindrance from water and water vapour. Pyroelectric
sensors operating at this wavelength can be relatively cheap. Multiple channel or pixel
array sensors monitoring flames in the near IR band are arguably the most reliable technologies available for detection of fires. Light emission from a fire forms an image of the flame at a particular instant. Digital image processing
can be utilized to recognize flames through analysis of the video
created from the near IR images.
(for example, wood or fossil fuels such as oil and natural gas) much heat and CO2 is released. The hot CO2 emits much energy in its resonance frequency of 4.3 µm. This causes a peak in the total radiation emission and can be well detected. Moreover, the "cold" CO2 in the air is taking care that the sunlight and other IR radiation is filtered. This makes the sensor in this frequency "Solar blind", however sensitivity is reduced by sunlight. By observing the flicker frequency of a fire (1 to 20 Hz) the detector is made less sensitive to false alarms, caused by heat radiation, for example caused by hot machinery. Multi-Infrared detectors make use of algorithms to suppress the effects of background radiation (blackbody radiation), again sensitivity is reduced by this radiation.
A severe disadvantage is that almost all radiation can be absorbed by water or water vapour, but particularly this is valid for infrared flame detection in the 4.3 to 4.4 µm region. From approx. 3.5 µm and higher the absorption by water or ice is practically 100%. This makes the infrared sensor for use in outdoor applications very unresponsive to fires. The biggest problem is our ignorance, some infrared detectors have an (automatic) detector window self test, but this self test only monitors the occurrence of water or ice on the detector window.
A salt film is also harmful, because salt absorbs water. However, water vapour, fog or light rain also makes the sensor almost blind, without the user knowing. The cause is similar to what a fire fighter does if he approaches a hot fire: he protects himself by means of a water vapour screen against the enormous infrared heat radiation. The presence of water vapor, fog, or light rain will then also "protect" the monitor causing it to not see the fire. Visible light will, however be transmitted through the water vapour screen, as can easily been seen by the fact that a human can still see the flames through the water vapour screen.
A fire emits radiation, which human eye
experiences as the visible yellow red flames and heat. In fact, during a fire, relatively sparsely UV energy and visible light energy is emitted, as compared to the emission of Infrared radiation. A non-hydrocarbon fire, for example, one from hydrogen
, does not show a CO2 peak on 4.3 µm because during the burning of hydrogen no CO2 is released. The 4.3 µm CO2 peak in the picture is exaggerated, and is in reality less than 2% of the total energy of the fire. A multi-frequency-detector with sensors for UV, visible light, near IR and/or wideband IR thus have much more "sensor data" to calculate with and therefore are able to detect more types of fires and to detect these types of fires better: hydrogen, methanol
, ether
or sulphur. It looks like a static picture, but in reality the energy fluctuates, or flickers. This flickering is caused by the fact that the aspirated oxygen and the present combustible are burning and concurrently aspirate new oxygen and new combustible material. These little explosions cause the flickering of the flame.
The sun
emits an enormous amount of energy, which could be harmful to human beings if not for the vapours and gases in the atmosphere, like water (clouds), ozone
, and other vapours and gases in the air, through which the sunlight is filtered. In the figure it can well be seen that "cold" CO2 filters the solar radiation around 4.3 µm. The Infrared detector which uses this frequency is therefore solar blind. Not all manufacturers of flame detectors use sharp filters for the 4.3 µm radiation and thus still pick up quite an amount of sunlight. These cheap flame detectors are hardly usable for outdoor applications. Between 0.7 µm and approx. 3 µm there is relatively large absorption of sunlight. Hence, this frequency range is used for flame detection by a few flame detector manufacturers (in combination with other sensors like ultraviolet, visible light, or near infrared. The big economical advantage is, that no expensive sapphire
detector windows but quartz
windows can be applied. These electro-optical sensor
combinations also enable the detection of none-hydrocarbons like hydrogen fires without the risk of false alarms, caused by artificial light or electrical welding.
Infrared flame detectors suffer from Infrared heat radiation which is not emitted by the possible fire. One could say, that the fire can be masked by other heat sources. All objects which have a temperature higher than the absolute minimum temperature (0 kelvin
s or −273.15 °C) emit energy and at room temperature (300 K) is this heat already a problem for the infrared flame detectors with the highest sensitivity. Sometimes a moving hand is sufficient to get the IR flame detector in an alarm. At 700 K a hot object (black body) already starts to emit visible light (glowing). Dual- or multi-infrared detectors suppress the effects of heat radiation by means of sensors which detect just off the CO2 peak; for example on 4.1 µm. Here it is necessary that there is a large difference in output between the applied sensors (for example sensor S1 and S2 in the picture above). A disadvantage is, that the radiation energy of a possible fire must be much bigger than the present background heat radiation. In other words, the flame detector becomes less sensitive. Every multi infrared flame detector is negatively influenced by this effect, regardless how expensive it is.
The cone of vision of a flame detector is determined by the shape and size of the window and the housing and the location of the sensor in the housing. For infrared sensors also the lamination of the sensor material plays a part; it limits the cone of vision of the flame detector. A wide cone of vision does not automatically mean that the flame detector is better. For some applications the flame detector needs to be aligned precisely to take care that it does not detect potential background radiation sources. The cone of vision of the flame detector is three dimensional and is not necessarily perfectly round. The horizontal angle of vision and the vertical angle of vision often differ; this is mostly caused by the shape of the housing and by mirroring parts (meant for the self test). Different combustibles can even have a different angle of vision in the same flame detector. Very important is the sensitivity at angles of 45°. Here at least 50% of the maximum sensitivity at the central axis must be achieved. Some flame detectors here achieve 70% or more. In fact these flame detectors have a total horizontal angle of vision of more than 90°, but most of the manufacturers do not mention this. A high sensitivity on the edges of the angle of vision provides advantages for the projection of a flame detector.
The range of a flame detector is highly determined by the mounting location. In fact, when making a projection, one should imagine in what the flame detector “sees”. A rule of thumb is, that the mounting height of the flame detector is twice as high as the highest object in the field of view. Also the accessibility of the flame detector must be taken into account, because of maintenance and/or repairs. A rigid light-mast with a pivot point is for this reason recommendable. A “roof” on top of the flame detector (30 x 30 cm, 1 x 1-foot) prevents quick pollution in outdoor applications. Also the shadow effect must be considered. The shadow effect can be minimized by mounting a second flame detector in the opposite of the first detector. A second advantage of this approach is, that the second flame detector is a redundant one, in case the first one is not working or is blinded. In general, when mounting several flame detectors, one should let them “look” to each other not let them look to the walls. Following this procedure blind spots (caused by the shadow effect) can be avoided and a better redundancy can be achieved than if the flame detectors would “look” from the central position into the to be protected area. The range of flame detectors to the 30 x 30 cm, 1 x 1-foot industry standard
fire is stated within the manufacturers data sheets and manuals, this range can be affected by the previously stated de-sensitizing effects of sunlight, water, fog, steam and blackbody radiation.
The square law applies to flame detection and relates the flame area and the distance from the flame to the flame detector: If a flame detector can detect a fire with an area A on a certain distance, then a 4 times bigger flame area is necessary if the distance between the flame detector and the fire is doubled. In short:
Double distance = four times bigger flame area (fire
).
This law is valid for all flame detectors, also for the ones, which are based on camera technique. It is a law, which is valid for all optical detectors. The maximum sensitivity can be determined by dividing the maximum flame area A by the square of the distance between the fire and the flame detector: c = A/d2. With this constant c can, for the same flame detector and the same type of fire, the maximum distance or the minimum fire area be calculated: A = cd2 and d = √(A/c). It must be emphasized, however, that the square root in reality is not valid anymore at very high distances. At long distances other parameters are playing a significant part; like the occurrence of water vapour and of cold CO2 in the air. In the case of a very small flame, on the other hand, the decreasing flickering of the flame will play an increasing part.
Flame detector
There are several types of flame detector. The optical flame detector is a detector that uses optical sensors to detect flames. There are also ionisation flame detectors, which use current flow in the flame to detect flame presence, and thermocouple flame detectors.-Ultraviolet:Ultraviolet ...
. Flame detectors are optical equipment for the detection of flame phenomena of a fire. There are two categories of flame detection:
- Flame detector for the detection of a fire in a fire alarm system
- Flame scanner for monitoring the condition of a flame in a burner
Several different methods of flame detection are possible.
Ultraviolet
An ultravioletUltraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
(UV) sensor is often sensitive for radiation
Radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...
in the 185 to 260 nm range. This frequency range is the least sensitive for natural background radiation
Background radiation
Background radiation is the ionizing radiation constantly present in the natural environment of the Earth, which is emitted by natural and artificial sources.-Overview:Both Natural and human-made background radiation varies by location....
sources like cosmic radiation and especially sunlight. The sunlight is, in the higher frequencies, absorbed by almost all vapours and gases; especially by ozone
Ozone
Ozone , or trioxygen, is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope...
and smoke but also by an oil or grease film on the window of a flame detector. Almost every fire radiates UV light, and the UV sensor is a good all round flame detector. A disadvantage is that quite a few artificial false-alarm sources occur; like halogen
Halogen
The halogens or halogen elements are a series of nonmetal elements from Group 17 IUPAC Style of the periodic table, comprising fluorine , chlorine , bromine , iodine , and astatine...
and quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
lighting (without regular glass), electrical welding
Welding
Welding is a fabrication or sculptural process that joins materials, usually metals or thermoplastics, by causing coalescence. This is often done by melting the workpieces and adding a filler material to form a pool of molten material that cools to become a strong joint, with pressure sometimes...
, corona and static arcs.
Visible light
A visible light sensor (for example a camera: 0.4 to 0.7 µm) is able to present an image, which can be understood by a human being. Furthermore complex image processing analysis can be executed by computers, which can recognize a flame or even smoke. Unfortunately, a camera can be blinded, like a human, by heavy smoke and by fog. It is also possible to mix visible light information (monitor) with UV or InfraredInfrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
information, which in this case is made visible. The corona camera
Camera
A camera is a device that records and stores images. These images may be still photographs or moving images such as videos or movies. The term camera comes from the camera obscura , an early mechanism for projecting images...
is an example of this equipment. In this equipment the information of an UV camera mixed with visible image information. It is used for tracing defects in high voltage
High voltage
The term high voltage characterizes electrical circuits in which the voltage used is the cause of particular safety concerns and insulation requirements...
equipment and fire detection over high distances.
Near Infrared
A near InfraredInfrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
(IR) sensor (0.7 to 1.1 µm) is especially able to monitor flame phenomena, without too much hindrance from water and water vapour. Pyroelectric
Pyroelectricity
Pyroelectricity is the ability of certain materials to generate a temporary voltage when they are heated or cooled. The change in temperature modifies the positions of the atoms slightly within the crystal structure, such that the polarization of the material changes. This polarization change...
sensors operating at this wavelength can be relatively cheap. Multiple channel or pixel
Pixel
In digital imaging, a pixel, or pel, is a single point in a raster image, or the smallest addressable screen element in a display device; it is the smallest unit of picture that can be represented or controlled....
array sensors monitoring flames in the near IR band are arguably the most reliable technologies available for detection of fires. Light emission from a fire forms an image of the flame at a particular instant. Digital image processing
Digital image processing
Digital image processing is the use of computer algorithms to perform image processing on digital images. As a subcategory or field of digital signal processing, digital image processing has many advantages over analog image processing...
can be utilized to recognize flames through analysis of the video
Video
Video is the technology of electronically capturing, recording, processing, storing, transmitting, and reconstructing a sequence of still images representing scenes in motion.- History :...
created from the near IR images.
Wideband infrared
A wideband infrared sensor (1.1 µm and higher) monitors especially the heat radiation of a fire. A special frequency range is 4.3 to 4.4 µm. This is a resonance frequency of CO2. During burning of a hydrocarbonHydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
(for example, wood or fossil fuels such as oil and natural gas) much heat and CO2 is released. The hot CO2 emits much energy in its resonance frequency of 4.3 µm. This causes a peak in the total radiation emission and can be well detected. Moreover, the "cold" CO2 in the air is taking care that the sunlight and other IR radiation is filtered. This makes the sensor in this frequency "Solar blind", however sensitivity is reduced by sunlight. By observing the flicker frequency of a fire (1 to 20 Hz) the detector is made less sensitive to false alarms, caused by heat radiation, for example caused by hot machinery. Multi-Infrared detectors make use of algorithms to suppress the effects of background radiation (blackbody radiation), again sensitivity is reduced by this radiation.
A severe disadvantage is that almost all radiation can be absorbed by water or water vapour, but particularly this is valid for infrared flame detection in the 4.3 to 4.4 µm region. From approx. 3.5 µm and higher the absorption by water or ice is practically 100%. This makes the infrared sensor for use in outdoor applications very unresponsive to fires. The biggest problem is our ignorance, some infrared detectors have an (automatic) detector window self test, but this self test only monitors the occurrence of water or ice on the detector window.
A salt film is also harmful, because salt absorbs water. However, water vapour, fog or light rain also makes the sensor almost blind, without the user knowing. The cause is similar to what a fire fighter does if he approaches a hot fire: he protects himself by means of a water vapour screen against the enormous infrared heat radiation. The presence of water vapor, fog, or light rain will then also "protect" the monitor causing it to not see the fire. Visible light will, however be transmitted through the water vapour screen, as can easily been seen by the fact that a human can still see the flames through the water vapour screen.
Emission of radiation
Human eye
The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...
experiences as the visible yellow red flames and heat. In fact, during a fire, relatively sparsely UV energy and visible light energy is emitted, as compared to the emission of Infrared radiation. A non-hydrocarbon fire, for example, one from hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
, does not show a CO2 peak on 4.3 µm because during the burning of hydrogen no CO2 is released. The 4.3 µm CO2 peak in the picture is exaggerated, and is in reality less than 2% of the total energy of the fire. A multi-frequency-detector with sensors for UV, visible light, near IR and/or wideband IR thus have much more "sensor data" to calculate with and therefore are able to detect more types of fires and to detect these types of fires better: hydrogen, methanol
Methanol
Methanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with the formula CH3OH . It is the simplest alcohol, and is a light, volatile, colorless, flammable liquid with a distinctive odor very similar to, but slightly sweeter than, ethanol...
, ether
Ether
Ethers are a class of organic compounds that contain an ether group — an oxygen atom connected to two alkyl or aryl groups — of general formula R–O–R'. A typical example is the solvent and anesthetic diethyl ether, commonly referred to simply as "ether"...
or sulphur. It looks like a static picture, but in reality the energy fluctuates, or flickers. This flickering is caused by the fact that the aspirated oxygen and the present combustible are burning and concurrently aspirate new oxygen and new combustible material. These little explosions cause the flickering of the flame.
Sunlight
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
emits an enormous amount of energy, which could be harmful to human beings if not for the vapours and gases in the atmosphere, like water (clouds), ozone
Ozone
Ozone , or trioxygen, is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope...
, and other vapours and gases in the air, through which the sunlight is filtered. In the figure it can well be seen that "cold" CO2 filters the solar radiation around 4.3 µm. The Infrared detector which uses this frequency is therefore solar blind. Not all manufacturers of flame detectors use sharp filters for the 4.3 µm radiation and thus still pick up quite an amount of sunlight. These cheap flame detectors are hardly usable for outdoor applications. Between 0.7 µm and approx. 3 µm there is relatively large absorption of sunlight. Hence, this frequency range is used for flame detection by a few flame detector manufacturers (in combination with other sensors like ultraviolet, visible light, or near infrared. The big economical advantage is, that no expensive sapphire
Sapphire
Sapphire is a gemstone variety of the mineral corundum, an aluminium oxide , when it is a color other than red or dark pink; in which case the gem would instead be called a ruby, considered to be a different gemstone. Trace amounts of other elements such as iron, titanium, or chromium can give...
detector windows but quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
windows can be applied. These electro-optical sensor
Electro-optical sensor
Electro-optical sensors are electronic detectors that convert light, or a change in light, into an electronic signal. They are used in many industrial and consumer applications, for example:* Lamps that turn on automatically in response to darkness...
combinations also enable the detection of none-hydrocarbons like hydrogen fires without the risk of false alarms, caused by artificial light or electrical welding.
Heat radiation
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
s or −273.15 °C) emit energy and at room temperature (300 K) is this heat already a problem for the infrared flame detectors with the highest sensitivity. Sometimes a moving hand is sufficient to get the IR flame detector in an alarm. At 700 K a hot object (black body) already starts to emit visible light (glowing). Dual- or multi-infrared detectors suppress the effects of heat radiation by means of sensors which detect just off the CO2 peak; for example on 4.1 µm. Here it is necessary that there is a large difference in output between the applied sensors (for example sensor S1 and S2 in the picture above). A disadvantage is, that the radiation energy of a possible fire must be much bigger than the present background heat radiation. In other words, the flame detector becomes less sensitive. Every multi infrared flame detector is negatively influenced by this effect, regardless how expensive it is.
Cone of vision
The detection range
Industry Standard
Industry Standard is a 1982 album by The Dregs. It is their only album featuring vocals and garnered the group their fourth Grammy nomination.-Track listing:All tracks are written by Steve Morse, except where noted.#"Assembly Line" – 4:25...
fire is stated within the manufacturers data sheets and manuals, this range can be affected by the previously stated de-sensitizing effects of sunlight, water, fog, steam and blackbody radiation.
The square law
Double distance = four times bigger flame area (fire
Fire
Fire is the rapid oxidation of a material in the chemical process of combustion, releasing heat, light, and various reaction products. Slower oxidative processes like rusting or digestion are not included by this definition....
).
This law is valid for all flame detectors, also for the ones, which are based on camera technique. It is a law, which is valid for all optical detectors. The maximum sensitivity can be determined by dividing the maximum flame area A by the square of the distance between the fire and the flame detector: c = A/d2. With this constant c can, for the same flame detector and the same type of fire, the maximum distance or the minimum fire area be calculated: A = cd2 and d = √(A/c). It must be emphasized, however, that the square root in reality is not valid anymore at very high distances. At long distances other parameters are playing a significant part; like the occurrence of water vapour and of cold CO2 in the air. In the case of a very small flame, on the other hand, the decreasing flickering of the flame will play an increasing part.