Flame monitoring method

Abstract: The presence of a burner flame, in a multiple burner installation, is monitored by sensing a signal indicative of the spectrum of the fluctuating component in the radiation of the flame over a range of frequencies. In the lower frequency range a measure (D1,D2) is obtained of the difference of signal strength at two predetermined frequency levels (L,LL). The signal strength is also measured at a higher frequency (H). At the higher frequency there is a significant difference in signal intensity between the flame-on and flame-off conditions, while the change of signal strength between the spectra of flame-on and flame-off conditions in the lower frequency range is sensitive to frequency. By processing the two measures together they can augment each other and produce an enhanced change of signal between flame-on and flame-off conditions, making detection easier.

Description: This invention relates to a method for monitoring the presence of a flame. It is known to use a monitor with radiation sensing means for the remote detection of a flame. The direct radiation from the flame, characterised by its flickering nature, is distinguished from the sensed radiation by filtering out of the signal any steady-state background radiation. An example of apparatus for this purpose is described in DE 1960218 which relates to a fire detection apparatus that employs two radiation receivers responsive to different ranges of radiation wavelength, representative of combustion gases and carbon particles respectively, the signals in those two ranges being processed to produce an output dependent on the difference or the quotient of the signals, that output actuating an alarm when said output is characteristic of flames or fires. It is also proposed to make the apparatus more responsive to presence of a fire by generating one of the signals from the drop of response between two different parts of the frequency range concerned. Further precautions may be needed for detecting a flame from a particular source where there may be multiple sources, as in the example of multi-burner equipment, to ensure that the radiation from another source does not influence the reading for the flame being monitored. In industrial boiler installations, for example, there may be a bank of closely spaced burners each of which has to be monitored individually. The space available may then be so limited that it is not possible to site each radiation detector where its line of sight will impinge on the combustion zone of only a single burner. Typically, the detector may be located to one side of the burner with its optical axis inclined towards the burner axis so as to enter the flame at a point along its length nearer the burner outlet than the tip of the flame. If the flame should be extinguished, the combustion zone of another flame could become visible along the line of sight. This problem is further complicated by the fact that the loss of a flame from one burner may allow the flames of adjacent burners to spread towards the space previously occupied by the extinguished flame. A solution to this problem is offered by an apparatus known from GB 1396384 in which there are two radiation sensors directed onto the flame from one side of the burner, as already mentioned, but aligned on axes that intersect near their point of entry into the flame. Signal processing means for the output signals from the sensor devices filter out any non-identical components from the two signals to give the processed signal that, in principle, is dependent on the fluctuating radiation from the zone of intersection of the two detector axes. Such an apparatus, while it offers an effective solution for the problem, is inherently both expensive and space-consuming because it requires the two radiation sensors and the joint processing of their outputs. It is an object of the present invention to provide a more cost-effective approach. According to the present invention, there is provided a flame monitoring method for a multiple burner arrangement method in which signals representing the strength of the fluctuating radiation of flames are derived at least at two different frequencies and said signals are employed to produce a first output signal dependent on the relative strength of said signals, a second output signal is derived dependent on thestrength of the fluctuating radiation at a higher frequency than said two frequencies, and said first and second output signals are processed to produce a resultant output signal exhibiting a change of relative signal strength between flame-on and flame-off conditions, a first flame in the multiple burner arrangement being monitored in a first zone occupied by it to derive a single detection signal related to the sensed fluctuation radiation over a range of frequencies and said signals at said two different frequencies as well as the second output signal at the higher frequency are derived from said detection signal, the magnitude of said change of relative signal strength being employed to indicate the presence or absence of said flame in said first zone independently of a flame in another zone of said multiple burner device. The invention is based upon the observation that the frequency characteristic of a flame varies over its extent. In the example of an obliquely aligned flame monitor in a multi-burner set-up, the line of sight from the intended zone of the flame to be monitored will extend to some other zone of a neighbouring flame that might be sensed if the intended flame is extinguished. The frequency spectrum of a signal from the monitor will thus differ, in dependence upon which flame is being sensed. A typical spectrum of the fluctuation frequency for the radiation near the base of the flame being monitored will show a progressive reduction in the signal intensity with increase of frequency, this being more marked in the lower frequency range. Although there may not be much difference in the magnitudes of the signals sensed at these lower frequencies from one flame or the other, a clearer difference emerges between the two flames by providing a measure of the change of amplitude between two frequencies in the lower frequency range. In this way it is possible, therefore, to discriminate between flame-on and flame-off conditions. In a preferred form of the invention, a third measurement of the radiation is made at a higher frequency. In conventional detection techniques it is a higher frequency component that provides the measurement signal because, by choosing an appropriate region of the flame for monitoring, the higher frequency component will have a greater magnitude when the flame is on. In known apparatus, however, the change of signal level this represents can only be used reliably if there is a high degree of discrimination in the signal processing means, which carries its own disadvantages. By comparing both the relative intensity changes in the lower frequency range, where the difference in intensity level at any particular frequency in the two conditions may be relatively small, and the different levels of high frequency signal in the flame-on and flame-off conditions, it becomes much easier to distinguish reliably the loss of an individual burner flame. For example, from a sensed fluctuating signal, the processing may produce an output signal related to the ratio of the high-frequency component to the difference between the two lower-frequency components of the sensed signal, although other processing algorithms are possible. By suitable choice of frequencies in particular cases, at least one of the two components of the lower-frequency difference signal may show a significant change of magnitude between flame-on and flame-off conditions; it would then be possible to perform a similar processing in which that one of the two components forming the frequency difference signal takes the place of the higher frequency component in the algorithm. As another example, it may be preferred in some cases to produce a ratio signal rather than a difference signal from the two lower frequency components and form a ratio of this with the high-frequency component. The invention will be described in more detail by way of example with reference to the accompanying schematic drawings wherein: Fig. 1 illustrates in plan a multiple burner arrangement with the sighting head of a flame monitor in place for one of the burners, Fig. 2 is a graph showing typical spectra of the fluctuation frequency that might reach the sighting head in Fig. 1, and Figs. 3 and 4 illustrate alternative means of processing the sensed signals in accordance with the invention. Fig. 1 is a horizontal section of a burner wall W in a boiler, showing a row of burners B1,B2... at the level of the section plane. The sighting head S of a flame monitoring device is illustrated only schematically because such equipment is well known, for example as supplied by Hamworthy Combustion Engineering Ltd of Poole, England. In such devices a sighting head is mounted obliquely in the wall so that its optical axis A impinges on the flame F2 of the burner B2 being monitored, about one third of the length of the flame from the burner. Fig. 1 also shows, as an example that the axis A may meet a more distant zone of a flame F' from one of a further row of burners at a lower level, although the presence of the flame intended to be monitored will normally mask this other flame from the sensor. The sighting head S comprises a transducer which senses a chosen optical spectrum (the spectrum range depending in known manner on the fuel being burnt) as a corresponding electrical signal. As already explained, the radiation from the flame F contains a flickering or fluctuating component and the sighting head and its processing circuits are arranged not to respond to any steady-state illumination. With the burner lit, therefore, a spectrum X of fluctuation frequencies is sensed which is shown in Fig. 2 as a plot of fluctuating signal level (L) against frequency (F). If the burner B2 is unlit, the sighting head still receives a fluctuating signal (spectrum Y) from the remaining burners, but as Fig. 2 illustrates, this is considerably weaker in the higher frequency range, such as at the frequency H. In known flame detectors, the fluctuating signal from the sighting head will be processed so as to detect the change of signal level (S) between XH and YH. The higher frequency band is a clear choice for measurement of the signal since it can be seen from Fig. 2 that there is highest signal ratio between the burner on and off conditions. Fig. 2 also shows that the two spectra sensed have significantly dissimilar profiles. In particular, in the low frequency range their rates of change of signal strength with frequency are very different. As a result although the difference in magnitude between the signals at any particular frequency in this range may be small, over a low frequency band such as L to LL the change between the differences (D1 and D2) of the signal strengths at the frequency values L and LL or the ratios of the strengths at those values will have very different magnitudes. By combining appropriately these changes at the higher and lower frequency regions of the spectrum, it is possible to enhance very considerably the sensed difference between the flame on and flame off conditions. For example, the difference between the signal strengths at the two lower frequencies L and LL is much greater when the flame is off. This difference value may be subtracted from the absolute signal value at the higher frequency H, and since a larger difference value is subtracted from a higher frequency signal that is already smaller when the flame is off, there is substantially improved discrimination between the on and off conditions. This process is operated by the apparatus in Fig. 3. The signal from the sighting head is input through terminal 10 to three variable gain amplifiers 12,14,16 in parallel having rectifier diodes 18 at their outputs. The amplifiers 12,14,16 have, respectively, high frequency, low frequency and very low frequency pass bands (H,L,LL). In fact, it may not be necessary for all the amplifiers to have specific top pass cut-off frequencies because of the fall-off of signal strength with frequency. The outputs from the amplifiers L and LL go to a differential amplifier 20 to produce a signal proportional to D1 or D2 which is arranged not to go negative, as it is subtracted from the high frequency signal in a further differential amplifier 22. The change of high frequency signal, proportional to the input strength drop S, which appears upon loss of the flame is thus augmented by the change of the lower frequency difference signal from D2 to D1 to give a greater resultant change in the output from the amplifier 22. As a numerical example, in adverse conditions, ie. when the sighting head receives a considerable amount of fluctuating illumination from other sources, the ratio between the flame-on and flame-off states of the detected signal at the higher frequency H may conceivably fall to 5:3. But at the lower frequencies L and LL, the signal differences in the two states might be 1 and 2 respectively. By subtraction, therefore, the ratio is changed from 5:3 to 4:1, which clearly provides a much greater discrimination between the two states. The two low frequencies are chosen in this case to be relatively close together in order to ensure that the signal strengths at those values will tend to fluctuate together. As a result there is a substantially steady difference signal, so that its influence on the high frequency value will be stable. In Fig. 4 there are transconductance amplifiers 26,28,30 operating on similar frequency bands to the three amplifiers of Fig. 3, and the amplifiers 28,30 similarly feed the differential amplifier 20. The difference signal is inverted in a further differential amplifier 32 and the inverted output provides a gain control signal for the higher frequency amplifier 26. The gain in that amplifier is therefore reduced when it is operating on the weaker higher frequency signal. In an analogous way it is possible to process the two lower frequency signals to produce an output that is a ratio of their strengths. As in the previous example, the change in the difference of the lower frequency signals augments the change of high frequency signal between the flame-on and flame-off conditions. In the case of Fig. 4, with the numerical input values given above as an example for the Fig. 3 circuit, the change in the gain ratio between flame-on and flame-off conditions would be 1:0.5. The ratio between the high frequency signals of 5:3 is thereby modified to 5:1.5 It is to be understood that the frequency values chosen for the pass bands will depend upon the particular installation and more particularly upon the type of fuel being used. It is, however, very simple to establish empirically from the spectra the frequency values that will determine the optimum values.

Claims: Flame monitoring method in which: signals representing the strength of the fluctuating radiation of flames are derived at least at two different frequencies and said signals are employed to produce a first output signal dependent on the relative strength of said signals, a second output signal is derived dependent on the strength of the fluctuating radiation at a higher frequency than said two frequencies, and said first and second output signals are processed to produce a resultant output signal exhibiting a change of relative signal strength between flame-on and flame-off conditions, characterised in that in a multiple burner arrangement a first flame is monitored in a first zone occupied by it to derive a single detection signal related to the sensed fluctuation radiation over a range of frequencies and that said signals at said two different frequencies as well as the second output signal at the higher frequency are derived from said detection signal, and that the magnitude of said change of relative signal strength is employed to indicate the presence or absence of said flame in said first zone independently of a flame in another zone of said multiple burner device. A method according to claim 1 wherein the resultant output signal is produced from the ratio of said first and second output signals. A method according to claim 1 wherein the resultant output signal is produced from the difference between said first and second output signals. A method according to any one of claims 1 to 3 wherein the first output signal is obtained from the difference between said two different frequency signals. A method according to any one of claims 1 to 3 wherein the first output signal is obtained from the ratio of said two different frequency signals.

Inventors: INNES DAVID CHARLES KENNETH GO, INNES, DAVID CHARLES KENNETH GORDON
Applicants: HAMWORTHY COMBUSTION ENG LTD, HAMWORTHY COMBUSTION ENGINEERING LIMITED
Application year: 1991
Main classification: F23N5
IPC codes: F23N5