Flame detection, or the detection of thermal radiation, has been an effective form of hazard protection for many years. This article provides a review of the different forms of flame detection and covers some of the techniques developed to improve detection reliability and range, whilst reducing the susceptibility to false alarms.
Radiation
All objects emit thermal radiation and it is this radiation which forms the basis of the techniques used to detect flames. The wavelength of emitted light from a flame will depend on its temperature, which is governed by the type of combustible material and on how much oxygen is available.
Development of detectors has focused on sensors operating in the ultraviolet and the infrared, with flame imaging techniques in the visible spectrum. Unfortunately, a number of naturally and man-made radiation sources exist in this spectrum of radiation wavelength and much development has been undertaken to minimise the occurrence of potential false alarms from these sources.
Ultraviolet (UV) detectors use a sensor sensitive to wavelengths in the radiation band (0,185 to 0,245 µm) and virtually all fires emit wavelengths in this band. UV detectors are solar blind and will not false alarm in response to radiation from the sun. There is a dip in transmissivity of solar radiation in the infrared (IR) region at 4,4 µm which permits the use of 'solar blind' IR detection. Conveniently at this wavelength there is a peak in the radiated emission from hydrocarbon fuel fires making IR detectors ideal for this application.
Fundamentals of UV and IR detection technology
A UV flame detector is based on a sensor tube that detects UV radiation. The sensor tube contains an inert gas, which absorbs UV radiation. When an incident photon strikes a gas molecule, the molecule is sent to a higher energy state and loses this extra energy through the emittance of an electron (photoelectric effect). The emitted electrons produce conductivity between two electrodes housed in the tube, which generate a pulse, which is analysed by the sensing electronics.
UV flame detectors are sensitive to most fires including hydrocarbon (liquids, gases and solids), metals (magnesium), sulphur, hydrogen, hydrazine and ammonia. Potential false alarm sources include arc-welding, electrical arcs, lightning, X-rays used in nonradioactive materials. Time delays are normally incorporated in the detection electronics to ignore lightning and short duration electric arcs. Technology has been applied to compensate for extraneous UV radiation that may reach the detector (eg from distant arc welding) and also for the presence of X-rays and gamma rays.
In general, UV detectors are used indoors or in a tightly controlled environmental setting outdoors. UV detectors will typically provide the fastest speed of response to most open fires with a typical response time of three to five seconds. The sensors are capable of flame detection within 10 ms but time delays are built for false alarm rejection.
IR detectors are based on the use of photovoltaic or photoresistive cells. Thermopiles are devices whose circuits which consist of a number of thermocouples in series. Thermocouples generate a current when one side of the device is subject to a temperature change caused by the incident radiation.
To minimise false alarms from a stationary source of IR radiation, the electronic circuitry looks for a change in the signal representative of flicker from a flame. Flicker from continuous sources of IR can occur from, for example, regular pulsing (modulation) of black body radiation from a turbine or compressor. A technique known as time-domain signal analysis has been developed which analyses the input signal in realtime, requiring the IR signal to flicker randomly in order to recognise it as a fire condition. This method effectively ignores regularly chopped IR signals that could cause an alarm using standard signal processing methods.
A further interesting development has been a technique, which essentially looks at the rate of rise of the flame size. This is useful when detection of an event such as a fireball is required where there is insufficient time to detect flame flicker. Response in this mode can be as quick as under 50 ms. Typical response time outside of this mode is between 3 to 5 s.
IR detectors will not respond to sources such as arc welding, X-rays and gamma radiation. A build-up of ice or water film on the detector window will reduce IR detector sensitivity but IR detectors are less affected by smoke, oil and some of the gases or vapours than UV detectors. Single frequency IR detectors are suitable for reliable detection of hydrocarbon fires, in the presence of hot objects, and in atmospheres with high concentrations of airborne contaminants and/or where UV radiation sources may be present.
A dual frequency IR flame detector consists of two thermopile IR sensors equipped with different bandpass filters that only allow a specific radiation wavelength to reach the sensors. The IR radiation emitted by a typical hydrocarbon fire is more intense at the wavelength accepted by one sensor than the other.
Dual frequency IR detectors are sensitive to most hydrocarbon fires (liquids, gases and solids). Other fires such as burning metals, ammonia, hydrogen and sulphur do not emit significant amounts of IR radiation in the IR detector range of sensitivity. Dual frequency IR detectors are false alarm-resistant, achieving this at the cost of reduced sensitivity and reduction of application range.
Combination UV/IR detectors
The benefits of false alarm rejection from both a UV and IR detector can be combined into a single detection device requiring both UV and IR detectors to detect a fire before an alarm is generated.
Since the UV/IR detector pairs two sensor types it will typically only detect fires that emit both UV and flickering IR radiation, ie generally only hydrocarbon fires (liquids, gases and solids).
Industrial and commercial applications
Industrial applications of flame detection systems are wherever there is the danger of fire from highly combustible materials or where there is a need for an instantaneous response to flame. Examples include: gasoline transport loading terminals, offshore drilling and production platforms, refineries, marine engine rooms, storage facilities, hydrogen and ammonia production, munitions production, etc.
Industrial flame detectors are, in general, flameproof (explosion rated) and can typically detect a one square foot petrol fire at up to 27 m distance.
Flame detectors used for commercial applications do not have the same requirement to be flameproof and generally have shorter ranges of up to 8 to 10 m. Normally all commercial applications are indoors and use UV sensors.
For further details contact FDIA on tel: (011) 397 1618.
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