Successful flame detection is based on a number of properties including optical radiation emissions, smoke emission, and a rise in temperature. The most consistent and rapid method of identifying fire in outdoor or large spaces is Ultraviolet flame detection. Emission of visible UV or IR forms the basis of this kind of detection.
As you read through this article, you will learn what UV and UV/IR flame detectors are, their working principle, and applications.
Before we dive right into the nitty-gritty of this article, we are first going to be looking at the definition of a flame.
What is a flame?
The rapid combustion of hydrocarbon flame produces carbon (IV) oxide and water. The return of the elements that are involved in the reaction to their original state occurs through photon emission at a particular wavelength.
In the emission of flames, there are some bands that are characteristic of their signature. Below are a few of them.
A sudden rise in the UV radiation to about 200 nanometers, and a near visible part between 1 and 3 microns, which is sometimes more intense or less intense.
The second signature is a basically a temporal nature used in differentiating flame from background radiation in an industrial and environmental application.
A live flame will flicker randomly when it is burned naturally. Artificially combusted flames, on the other hand, will not flicker. The radiation that is emitted by flames is usually modulated from one to twenty HZ band which creates a room for the efficient pre-filtering of the signals received by the device.
What exactly are UV flame detectors
UV flame detectors are sensory devices that are designed to respond to Ultraviolet radiation that has a wavelength that is less than 300nm.
The sensor in the device is a photo-tube that includes a cathode and an anode placed in a large P of about 300 volts and sealed in a tube filled with inert gas. The photons are lit up or illuminated by ultraviolet radiation and hit the cathode which, in turn, releases electrons that move towards the anode. The electron ionizes the gas molecules in the bulb. Then an output signal is generated in the voltage pulse and sent by the detector.
Most times, UV flame detectors are used for combustible liquids like hydrocarbon fires, but their use is not exclusive to this. UV detectors are one of the few devices that can be used to detect metal fires, hydrogen, and ammonia.
The main reason why most people prefer UV flame sensors to others is due to their fast response time which is less than a millisecond. The sensitivity of the device can reach up to Pico watt/cm cube.
Granted, this detector is great, but its performance is reduced in the presence of smoke. Therefore, in the event of a large inferno in a closed environment, the device may fail to see the flame on time and trigger an alarm obviously because of the heavy smoke from the fire.
The sensitivity of the device can be altered by certain factors. For example, UV radiation that ought to read the device may be absorbed by oil films or other organic compounds that may be present on the window of the detector or present in the environment in which the device was installed.
The working principle of Ultraviolet detectors
UV detectors work by detecting the radiation emitted by a flame. They are capable of detecting explosions and fires within 4 milliseconds with a time delay of 3 to 4 seconds to minimize false alarms that can be triggered by other UV sources like sunlight, lightning, radiation and arc welding. UV detectors normally operate with wavelengths less than 300nm to reduce the effects of environmental background radiation. The effectiveness of UV detectors can be reduced by oily contaminants.
Advantages of Ultraviolet detectors
- Solar insensitive.
- Responds to metal, hydrocarbon and hydrogen fires.
- Quick response i.e. They respond in less than 10 milliseconds.
Disadvantages and limitation of Ultraviolet detectors.
- Certain vapor and gasses will reduce the sensitivity and performance of the device.
- May respond to welding at long range.
- Some UV sensors have a broad detection range that results in solar false alarms.
- May respond corona, X-rays, and sparks.
Ultraviolet detectors are sensitive to most fire, including metals (magnesium) hydrocarbons (solids, liquids, and gasses) and other compounds like sulfur, ammonia, hydrazine, and hydrogen. The UV detector is likely the most flexible optical fire detectors on the market, as it can be used for different purposes. They are reliable, fast and have a couple of false alarm sources.
UV/IR Flame detectors
UV/ IR Flame detectors consist of Infrared and ultraviolet sensors that are joined together in a single unit. UV sensors work by detecting the UV radiation that is emitted by flames and is very sensitive to many other flammable fuels like sulfur, ammonia, hydrocarbons, and hydrazine.
The major radiant emission band for hydrocarbons related fires ranges from 4.3 to 4.4 microns; it is an IR emission that Infrared sensors can detect with ease. What makes the emission to be detected by the Infrared detector is the energy released by the combustion of Carbon (IV) oxide at a frequency of 4.3 microns.
The most significant benefit of having IR and UV sensors combined in a single unit or device is that they share the same alarm source. As previously mentioned, each device operated separately, but adding a signal processing algorithm will enable the combination of both sensors to deliver an outstanding fire detector performance to a wide range of volatile solids, gases, and flammable liquids. Keep in mind that, the IR and UV sensors in the device can individually provide a reliable false alarm rejection.
Another benefit delivered of the IR and UV flame detectors includes their continuous self-testing and a wide range of diagnostic options and configuration.
Pros of UV/IR detectors
- Very high response speed- less than 500 milliseconds
- Insensitivity to welding, sunlight, sparks, lightning, corona, and arcs
Cons and limitation of UV/IR detectors
- Some vapors and gases will reduce their performance
- It is not suitable for detecting non-carbon fires.
Ultraviolet and infrared detectors are used in areas where heat or smoke detectors would be prone to false alarms.
Ultraviolet flame detectors are one of the most important safety devices in an industrial or commercial environment. When properly installed they can quickly detect a small flame before it intensifies and causes large-scale damage. To get the most out of them, it is best you install them in areas free of vapor, as it can alter the performance of the device.
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