GB2097120A

GB2097120A - Flame detector

Info

Publication number: GB2097120A
Application number: GB8210663A
Authority: GB
Original assignee: EMI Ltd
Current assignee: EMI Ltd
Priority date: 1981-04-16
Filing date: 1982-04-13
Publication date: 1982-10-27

Abstract

A flame detector includes a non-microphonic infra-red sensor 7 having a broad band frequency response. To maximise sensitivity to radiation from flames in the 4.3 mu m solar absorption band a narrow band optical interference filter 6 is transmissive in a band having preferred limits of 4.54 mu m to 4.38 mu m, the filter being chosen such that the integrated solar energy transmitted thereby is less than an alarm threshold. A filter 9 prevents re-radiated heat from filter 6 reaching the sensor 7. A filter 10 prevents radiation of wavelengths less than 4.38 mu m from reaching the filters 6 and 9 reducing heating thereof and also provides a series of filters to stop direct broad band transmission to the sensor through pin holes in the filters. A signal processor 11 detects flames by virtue of the flame flicker frequencies and includes provision for compensating for the effects of low frequency components on the amplitude of radiation produced by flames. <IMAGE>

Description

SPECIFICATION Flame detector eke present invention relates to a ilame detector.

lt is konwn to detect flames by virtue of the infre- red radiation which they emit @ end by virtue ofthe flickering of the infra-red radiation produced by the flames Although the infra-red flame detectors which have been used exhibit high sensitivity to flames, th, also respond in varying degrees to infra-red soLrrces other than flames and particularly to solar radiation.Careful siting of such detectors can reduce false alarms, but in some circumstances they cannot be used because of spurious sources.

Consequently, flame detectors responsive to ultra-voilet radiation have been widely used, as they are less affected by the spurious sources which cause false alarms with the known infra-red detec tore. Ultre violet detectors are, however, rsery sensi- tire to arc-welding, an activity which is common place on oil rigs or in chemical processing plants and oil refineries, where flame detection is especially important. Thus ultra-violet detectors are very susceptible to false alarms generated by arc-welding.In addition they are troubled by window contamina- tion, particularly by-thin oil films and smoke, and are less sensitive to flames than infra-red detectors.

such disavantages can be reduced by techniques such as using two ultra violet detectors, one of which surveys an area to be protected and the other which surveys the background. The outputs of the two detectors are compared. Such a technique how- aver increases the cost of a flame detector and is also susceptible to false alarms due to reflections of arc-welding radiation.

Nevertheless, the konwn infra-red detectors are widely regarded as unsatisfactory where they are used in areas in which solar radiation is present, in particular outdoors, and many more ultra-voilet detectors have been installed, despite their disadvantages, on oil rigs, in oil refineries and in other plant where flame detection is important Practical experience of the use ofthe ultra-voilet detectors has shown they too are unsatisfactory due to the disadvantages specified above.

Brutish Patent 1550334 discloses aflame detection apparatus arranged to detect the presence of flames, the apparatus including detector means arranged to receive radiation form flames, the detector m means being responsive substantially only to radiation hav- ing a wavelength in the range from 4.19 m to 4.45 m inclusive.A specific example is sensitive in the range .25 m to 4.45,um. The wavelengths in the rangefrom 4.19 m are chosen because solar radiation in that range is strongly absorbed by the earths stmosphere. whereas infra-red radiation emitted from hydrocarbon flames exhibits a peak within the same range. Thus, underlying the specified range of wavelengths is the need to reliably avoid detecting radiation from the sun and the range is chosen to be well within the band of absorption of solar radiation by the atmosphere.

The resulting detector is workable and useful but its application is limited by its narrow field of view and temperature sensitivity.

According to the present invention there is provided a flame detector arranged to receive rediation from flames and responsive to radiation in a range having an upper wavelength of 4.48 m to 4.65 m at which detector response is substantially half peak detector response and having a lower wavelength within a band of absorption of solar radiation by the atmosphere, which hand is centred on a wavelength of about4,3 m. at which lower frequency the detector response is substantially halfthe peak detector response, and the detector being arranged to be unresponsive to the total solar radiation within and outside the said range.

According to another aspect, there is provided a flame detector comprising: a sensor sensitive to infra-red radiation over a broad band width to produce an electrical output; signal processing means connected to receive said output for producing therefrom an indication of the presence of flames; and a narrow band filter arranged to transmit to the sensor radiation having wavelengths only in a range having an upper wavelength of 4.46 mtc 4.65 Am at which detector response is substantially half peak detector response and having a lower wavelength, within a band of absorption of solar radiation by the atmosphere, which band is centred on a wavelength of about 4.3 m at which lower wavelength the detector response is substantially half the peak detector response, and the filter being chosen so thatthe total solar radiation transmitted to the sensor is insufficient to cause said signal processing means to indicate the presence of flames.

The said upper wavelength may be 4.5 m to 4.6 m and is preferably4.52 m to 4.56 m.

The chosen range allows an increased field of wiew by extending the upperwavelength of the range into the region of the spectrum where there is solar energy. The detector is still unresponsive to solar energy because the alarm threshold of the signal processing means is greaterthan the level of the integrated solar energy, when 180% modulated, transmitted to the sensor via the narrow band filter.

An embodiment of the flame detector comprises an infra-red sensor which produces an electrical output in response to infra-red radiation, and a narrow band filter transmissive to infra-red radiation in the said range. An example of such a filter may absorb radiation outside the range.Heat from the absorbed radiation may reach the sensor by radiation or conduction away from the filter, and cause a spurious electrical output Thus in a modification a further filter is provided betwesn the narrow band filter aid the sensor to prevent re radiation andlor heat transfer from the narrow band filter to the sensor. This filter may be of material transmissive to wavelengths less than 6.5 m; ;e.g.Sapphire.

In another modification another fitter is provided to absorb radiation of wavelengths shorter than the said lower wavelength of the said range to reduce the absorption of energy by the narrow band filter. A material such as Germanium which does not trans- mit radiation of wavelengths below which there is significant solar energy e.g. 1.5 !m ,fllrn or shorter, may be used as this filter Furthermore some types of filters tend to have very small pinholes which are fully transmissive.By providing several filters in series, t.e. the narrow band filter, the further filter and the another filter, the chance of the full transmission directly to the sensor is reduced to an insignificant level. A Germanium filter is useful in this context as it does not have pinholes, by its nature.

In order to detect flames, the electrical output of the transducer is processed to detect the presence of flicker in the received radiation. Conventionally, the processing detects the average amplitude, over a set time period, of signals having frequencies in the range 4Hz and above. If the average amplitude exceeds a threshold an indication of the presence of flames is produced. Such processing can be used in the present invention. However the inventors have found that due to low frequency components of the flicker spectrum of flames the amplitude of the electrical output of the transducer irregularly can become so small for such a period as to maintain the said average amplitude below the threshold. Thus no flame indication is produced.

Thus in an embodiment of the invention the signal processing means comprises means for producing a signal indicative of whether the rate at which the amplitude of the said electrical output crosses a threshold level exceeds a preset rate, the means being arranged to at least maintain its value after each said crossing for a preset time period depen dent on the time for which the said amplitude is likely to be less than the threshold level due to the low frequency components, and means for producing an indication of the presence of flames if the said signal indicates the said rate exceeds the preset rate for a predetermined time.

In a preferred em bodiment the signal processing means comprises filter means to select frequency components in a preset range, means for producing a first signal of value which varies in a first sense at a predetermined rate, means for producing a signal indicative of whether the rate at which the amplitude of the selected frequency components crosses a threshold level exceeds a preset rate, and means for resetting the value of the first signal to a preset level if the period between a said crossing and its succeeding crossing exceeds a predetermined amount dependent on the time for which the amplitude of the said selected frequency components is likely to be less than the threshold due to the low frequency components.

For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure lisa schematic representation of the spectrum of solar radiation subject to atmospheric absorption, Figure 2 is a schematic representation of the 4.3 jim absorption band, Figure 3 is a schematic representation of radiation from flames in the 4.3 pm band, Figure 4 is a schematic representation of an exam pie od a flame detector in accordance with the invention, Figures 5 is a schematic representation o;;- modifi- cation of the flame detector of Figure 4, Figure 6 is a schematic representation of another example of a flame detector, Figure 7 is a schematic representation of a preferred embodiment of a flame detector, Figure 8 is a schematic representation of the spectrum of flicker of flames, Figure 9 is a schematic circuit diagram of the flame detector of Figure 4,5,6 or7, and Figures 10A and 10B comprise idealised waveform diagrams illustrating the operation of the circuit of Figures.

Referring to Figure 1, the spectrum of radiation from the sun is subject to absorption in the atmosphere at several wavelengths only a few such wavelengths being shown in the Figure. In a band of wavelengths centred on about 2.7 jim absorption is caused by carbon dioxide (CO2) and water (H20) and in a band centred on about 4.3 jim, absorption is caused by carbon dioxide (CO2) in the atmosphere.

Flames associated with hydrocarbon sources emit radiation in the CO2 bands. As shown in more detail in Figure 2 the 4.3 jim absorption band has an abrupt transition from absorption to transmission at the upper (5 ,um) wavelength end of the band and a transition at the lower (4 jim) wavelength end.As shown in Figure 3, the intensities I of radiation from flames falling in the 4.3 jim absorption band have a small peak near the lower wavelength end of the band, with a trough in approximately mid-band due to absorption of the radation in the atmosphere, and a large peak close to the said transition at the upper 5 jim wavelength end of the band, the radiation from the flame overlapping to some extent the transition into the region where solar radiation is not absorbed.

Thus in accordance with the invention, a flame detector is arranged to receive radiation from flames and is responsive to radiation in a range having an upper wavelength of 4.46 jim to 4.65 jim at which the response is half the peak response and having a lower wavelength, at which the response is half the peak response, within the band of absorption of solar radiation by the atmosphere which band is centred on a wavelength of about 4.3 jim and arranged so that it is unresponsive to the total solar radiation within and outside the band.The upper wavelength may be 4.5 jim to 4.6 jim. In a preferred example of the flame detector, the said lower wavelength is 4.38 jim. An example of such a detector comprises a narrow band optical interference filter having an idealised filter response pattern as shown at 1 in Figure 3. The response 1 illustrates that the half peak transmission point 2 for the upper wavelength is at 4.54 ,am and the half peak transmission part 3 for the lower wavelength is at 4.38 jim.

Radiation is transmitted to an infra-red sensorthe electrical output of which is fed to a signal processor, which indicates the presence of flames if an alarm threshold (amongst otherthings) is exceeded.

All optical interference filters have a shift to shorterwavelength X1 with an angle of incidence deviat ing from the normal given by A,= (n2-sin9) where i= angle of incidence, Ao= wavelength at normal incidence, and n = effective refractive index of the filter.

The field of view is thus increased by extending the upper wavelength into the region where there is solar energy and a detector which does not respond to solar energy is still obtained, provided the integrated energy (100% modulated) transmitted to the sensor via the optical filter is less than the alarm threshold. The upper wavelength chosen depends upon: (a) The fire sensitivity required (b) The solar response required (This is related to the filter wavelength shift with angle characteristics) (c) The shape of the filter slope (d) The temperature range required (and howthe filter pass band shifts with temperature) and (e) The tolerances on the filter manufacture.

In the preferred embodiment4.54jim has been chosen as the upper wavelength half power point.

The lower wavelength half power point is chosen such that the wavelength shift with angle for a high angle of incidence will not shift the lower wavelength edge in the solar region below 4.17 jim.

In the preferred embodiment this is achieved by choosing the lower wavelength half power point of 4.38 jim.

In this way the flame detector has a significantly wider field of view than the flame detector of British Patent No. 1550334 and is less sensitive to temperature changes.

An example of a flame detector is shown in Figure 4. It comprises a sealed enclosure 4 having a broad band window 5 of e.g. Germanium, Silicon or Sapphire, which is transmissive to infra-red radiation at least in the 4.3 jim band. Within the box is the aforementioned narrow band optical interference filter 6 and a non-microphonic infra-red sensor7 which produces an electrical output in response to infra-red radiation over a broad bandwidth. An example of such a sensor comprises Lead Selenide, or a pyroelectricsensors such as Lithium -Tantalate. It comprises a box having a broad-band window 8 of material transparent to infra-red radiation, e.g. Germanium, Silicon or Sapphire. A signal processor 11 processes the output of the sensor 7 to produce a flame indication.

The narrow band filter6 istransmissive over the range specified above and blocks energy outside that range. Some ofthe energy however is absorbed, causing a variation in the temperature of the filter.

This variation can be sensed by the sensor 7 causing a spurious output from the processor 11 which responds to signal variation. In order to avoid this, filter6 could be massive in order to have a large thermal inertia and so not vary in temperature. However, in Figure 4 a further filter 9 is placed between the narrow band filter 6 and the sensor 7 to prevent radiation from the hot filter 6 reaching the sensor 7 and to prevent conduction from the filter 6 to the sensor 7. As the radiation re-radiated from the hot filter 6 is at long wavelengths the filter 9 has a filter response as shown at 9 in Figure 1, with a transition from transmission to absorption at for example 5 jim or 6 jzm.

A further problem which occurs is that the solar spectrum has a large amount of energy at wavelengths shorter than the 4.3 jim band. Also practical interference filters tend to have pin-holes which transmit radiation over the range of wavelengths shorter than the 4.3 jim band. Thus a filter 10 is provided having a response as shown at lOin Figure 1 to transmit only wavelengths longer than about 1.5 jim to prevent most of the energy reaching the detector 7.

A series of several filters 10, 6, 9 reduces the chance of radiation outside the desired transmission band of 4.36 jim to 4.54 jim reaching the detector through pin holes to an insignificant level.

It is in fact unnecessary to have separate windows 5 and 8 and filters 10,6 and 9. Furthermore, if filter 10 is provided so that filter 6 does not absorb a substantial amount of radiation, filter 9 could be left out.

Referring to Figure 5, the filter 10 is incorporated with the window 5 being formed by a coating on the window 5. Similarly the filter 9 is incorporated with window 7. In this example, the narrow band filter 6 remains a separate element.

A possibility shown in Figure 6 isto provide the narrow band filter 6 which provides fine control of the wavelengths passed to the sensor 7,8 and a further band pass filter BPF which is substantially transmissive only within a coarse range wholly within which the narrow band filter is transmissive.

The pass band filter PBF may be between the narrow band filter 6 and the sensor 7,8 or remote from the sensor 7, 8. The two dashed blocks in Figure 6 illustrate the latter case.

Figure 7 shows a preferred embodiment in which the window 5 is of sapphire for physical strength, and is coated with Germanium to form the filter 10 which blocks short wavelengths of 1.5 jim or shorter.

The window 8 of the sensor 7 is formed of a broadband filter 9' (e.g. 4.2 jim to 4.7 jim pass band) which prevents secondary radiation and conduction from the narrow band filter 6 affecting the sensor 7. The filter 6 is held in a tube 16 which is a light-tight fit on the box of the sensor being spaced from the box by a ring 17.

Other variations (not shown) are also possible. For instance it may not be essential to interpose filter 9 between the filter 6 and the sensor 7. Filter 6 could form the window 8 of the sensor, and filters 9 and 10 combined to form the window 5 filters 6 and 9 combined to form the window 8 of the sensor. Yet further filter 6 could form the window 5, filter 9 the window 8, and filter 10 could be a separate element between filters 6 and 9.

The electrical output of the sensor 7 isfedto a signal processor 11. The signal processor may pro cuss the electrical output in known manner. For example the processor detects the presence of flicker ering in the infra-red radiation at frequencies, typically 4Hz and above, associated with the flickering of flames. The average amplitude of the flicker frequencies over a set time is sensed and compared with a threshold. If the threshold is exceeded an indication of the presence of flames is produced.

This indication may be electrical, visual, and/or aud ible.

The inventors have found that the conventional processing is subject to errors due to low frequency components in the flicker spectrum.

Referring to Figure 10A, waveform A schemati cally illustrates an electrical signal produced by flame. It is apparent that the peaks of the signal exceed a threshold T, except over a period L, where the amplitude of the signal is very low due to the low frequency components. The periods such as L occur irregularly. Such periods cause the average amp litude of the flicker frequencies over the set time ta be less than the threshold value in the conventionat processing.

A preferred signal processor shown in Figure 9, is designed to reduce the problems caused by the low frequency components.

Referring to Figure 8, the general trend of the spectrum of flicker frequencies is schematically shown on a Log-Log scale. Much of the energy is concentrated around very low frequencies i.e. DC to 5 Hz.

In the circuit of Figure 9, the electrical output of the infrared sensor7 isfedto afiltercircuit 12 which outputs the frequency components in the range e.g.

1 Hz to 10Hz, (in contrast to the conventional 4Hz and above).

Referring to Figures 9 and 10A, the filtered output produced from waveform A is applied to a transistor TR4 which becomes conductive each time a threshold T is exceeded. A capacitor C16 is connected via resistors R21 and R20 to be charged from a positive rail +V at a rate defined principally by R20.C16 whilstTR4 is non conductive, then discharges very quickly through TR4 at a rate defined by C16.R21. Thus capacitorC16 charges up when TR4 is off and discharges when TR4 is on as shown on part C16 of Figure 10A. If the amplitude of the output of the filter t2 remains below the threshold T, e.g. for the period L due to the effects of wind, the capacitor C16 continues to be charged, because TR4 is off.

A capacitor C17 is also connected by resistors R29 and R22 to the positive rail +V to be charged at a rate defined principally by R22.C17, this rate being much, e.g. 10 times, slower than the rate of charging of Cl 6.

AtransistorTR5 is connected across C17, the base of TR5 being connected to C16 via a zener diode D2, so that C17 discharges through TR5 when the voltage on C16 exceeds a further threshold D2 set by diode D2.

Thus providing the voltage on C16 does not exceed the further threshold D2, C17 is continuously charged, despite periods such as L, until an alarm threshold FT is reached. This alarm threshold is set by a voltage V ref fed to one input of a comparator 1C3, the other input of which is connected to the junction of resistors R22 and R29 to sense the voltage on C17.

Referring to figure 10B if the output ofthe filter remains lowfora predetermined time, signifying no flames, the capacitor C16 charges until the further threshold D2 set by Zener diode D2 is exceeded. TR5 then turns on and capacitorC17 discharges through it. The capacitor C16 will charge up to the threshold D2 much more quickly than the capacitor C17 charges up to FT.

Thus in the absence of flames the output of the filter remains low for a long time, C16 charges up to greater than D2,TR5 remains on, and C17 remains discharged An indicator 13 responds to the comparator circuit IC3 to produce an electrical, visual and/or audible warning of flames.

The signal processor is powered via a regulator circuit 14 fromFa DC powersupply 15.

Claims

1. A flame cfetectorarrnnged to receive radiation from flames anclrnsponsive to radiation in a range having an upper wavelength of 4A6 jim to 4.65 jim at which detector response is substantially half peak detector response and having a lower wavelength within a band of absorption of solar radiation by the atmosphere, which band iscentredon awavelength of about 4.3 jim, at which lowerfrequencythe detector response is substantially half the peak detector response, and the detector being arranged to be unresponsive to the total solar radiation within and outside the said range.

2. A detector according to Claim 1, wherein the said upper wavelength is 4.5 jim to 4.6 jim.

3. A detector according to Claim 1 or 2, wherein the said upper wavelength is 4.52 jim to 4.56 jim.

4. Aflame detector comprising: a sensor sensitive to infra-red radiation over a broad band width to produce an electrical output; signal processing means connected to receive said output for producing therefrom an indication of the presence of flames; and a narrow band filter arranged to transmit to the sensor radiation having wavelengths only in a range having an upper wavelength of 4.46 ILm to 4.65 jim at which detector response is substantially half peak detector response and having a lower wavelength, within a band of absorption of solar radiation bythe atmosphere, which band is centred on a wavelength of about 4.3 jim, at which lower wavelength the detector response is substantially half the peak detector response, and the filter being chosen so that the total solar radiation transmitted to the sensor is insufficient to cause said signal processing means to indicate the presence of flames.

5. A detector according to Claim 4, wherein the said upper wavelength is 45 jim to 4.6 jim.

6. A detector according to Claim 4 or 5, wherein the said upper wavelength is 4.52 jim to 4.56 jim.

7. A detector according to any one of Claims 4,5 or6 wherein there is included a further filter means substantially transmissive to radiation over a broader range than the said range of the narrow band filter, but wholly including the said range of the narrow band filter, and substantially nontransmissive to radiation outside the broader range, the broader range being chosen to substantially reduce the amount of radiation reaching the said sensor having wavelengths outside the said range of the narrow band filter.

8. A detector according to Claim 7, wherein the further filter means is on that side of the narrow band filter remote from the-sensor.

9. A detector according to Claim 7, wherein the further filter means is between the sensor and the narrow band filter.

10. A detector according to any one of Claims 4 to 9 wherein there is further included a filter means arranged to prevent energy re-radiated from the narrow band filter from reaching the sensor.

11. A detector according to Claim 10, wherein the filter arranged to prevent energy re-radiated from the narrow band filter from reaching the sensor comprises material transmissive to wavelengths less than 6.5 jim.

12. A detector according to any one of Claims 4 to 11, wherein there is also included a filter arranged to prevent a substantial proportion of radiation having wavelengths shorter than the said lower wavelength from reaching the narrow band filter.

13. A detector according to Claim 12, wherein a filter arranged to prevent a substantial proportion of radiation having wavelengths shorter than the said lower wavelength from reaching the narrow band filter comprises material transmissive to wavelengths greater than 1.5 jim.

14. A detector according to any one of Claims 4 to 13, wherein the signal processing means includes filter means to select frequency components of the said electrical output having frequencies in the range 1Hzto 11Hz.

15. A detector according to any one of Claims 4 to 14, wherein the signal processing means includes means for producing a signal indicative of whether the rate at which the amplitude of the said electrical output crosses a threshold level exceeds a preset rate, the means being arranged to at least maintain its value after each said crossing for a preset time period dependent on the time for which the said amplitude is likely to be less than the threshold level due to lowfrequencycomponents, and means for producing an indication of the presence of flames if the said signal indicates the said rate exceeds the preset rate for a predetermined time.

16. A detector according to Claim 14, wherein the signal processing means comprises: means for producing a first signal of value which varies in a first sense at a predetermined rate, means for producing an indication of the presence of flames if the value of the first signal reaches a preset threshold value, means for producing a signal indicative of whether the rate at which the amplitude of the selected frequency components crosses a threshold level exceeds a preset rate, and means for resetting the value of the first signal to a preset level if the period between a said crossing and its succeeding crossing exceeds a predetermined amount dependent on the time for which the amplitude of the said selected frequency components is likely to be less than the threshold due to low frequency components.

17. A detector according to any preceding claim wherein the said range having the said upper wavelength is centred on 4.46 jim and the said lower wavelength is 4.36 jim to 4.40 jim.