FLAME DETECTION

Abstract: A method of detecting the condition of a flame includes monitoring an emf generated by the flame, which emf is indicative of the condition of the flame. An electrically isolated electrical conductor (17, 39, 9'') may be utilized to monitor this emf. The conductor is conveniently in electrical contact with the flame. A furnace assembly adapted to incorporate the method includes a housing (12) forming a combustion chamber (14) and means (16) defining a flame position in the chamber. Insulation elements (35, 37, 40) electrically isolate the conductor (17, 39, 9'').

Description: "FLAME DETECTION" The present invention relates to the detection of the condition of a flame, for example a flame of a burner. The term "condition" in this context embraces the presence or absence of the flame, or more generally a state of the flame indicating the state of combustion at the flame. The unscheduled extinction of the flame of a burner results in a mixture of unburnt gases entering the combustion chamber. This is highly undesirable as any subsequent ignition of the unburnt mixture is potentially hazardous to both personnel and equipment. There are two methods commonly used for detecting flame failure in burner systems associated with furnaces. In general, such burner systems comprise a main burner and a pilot burner, the pilot burner being provided since it is an efficient method for igniting the fuel-air mixture from the main burner. The first method is based on the use of an alloy rod (usually a high nickel, chromium, iron alloy) known as a "flame rod" that is inserted into the front end of the main burner and extends into the combustion space. A voltage supply (typically 120 volts AC) is applied to the rod and the electrical conductivity to the earth potential via the flame is measured. Since the flame is capable of partially rectifying an alternating current, flame failure can be detected by the absence of rectification in the applied current between the flame rod and the earth potential. There are several- disadvantages associated with the use of flame rods and these may be summarized as follows: (a) Flame rods are subject to oxidation and corrosion in the high temperature environment existing within the furnace. Such deterioration is accelerated by the fact that the flame rod must be positioned to extend into the high temperature region of the flame. (b) Rectification measurements must be carried out accurately since electrical conductivity of the hot refractories between the flame rod and the earth generally is very significant. The extent of rectification is the component of a total signal which must be identified in order to positively identify that a flame connection exists in the high voltage circuit being monitored. (c) In situations where a furnace comprises a number of relatively closely spaced burners it can be difficult to be certain that : measurements relate to the burner near the location of the flame rod. (d) A power supply is necessary to drive the measuring circuit and an electronic circuit capable of detecting the extent of rectification is required. The second method for detecting flame failure in burner systems in furnaces is based on the use of an optical device to sense the presence of a flame. An entry port or sighting hole is provided in the main burner cowl and is fitted with an optical device which focuses the light emanating from the flame. The light is ~ focused onto a photo-sensitive element so that the light intensity can be monitored continuously. Light of wavelengths in the blue to ultra-violet range is measured by filtering in order to detect light from the flame rather than from the incandescent contents of the furnace. Light detection devices have the following limitations: (a) The devices do not sense some flames satisfactorily (in particular those fed by natural gas and other relatively non-luminous combustion mixtures) . (b) The devices are difficult to align with the correct area of the flame. (c) Often, it is necessary to turn off the pilot flame in order to ensure that the main burner flame is being sighted and therefore proved. (d) Vibration of the furnace and related equipment often causes difficulties in proper aligning of the devices. An object of the present invention is to provide a method, of flame detection that alleviates the disadvantages of the known methods and apparatus discussed in the preceding ' aragraphs. The present invention is based on the realization thatr. the natural electrical phenomena associated with chemical reactions and temperature differences within a flame. resuXts in an electromotive force (emf) in the flame, and that this emf can be monitored, for example, by means of an isolated electrical conductor in contact with the flame to provide an indication of the condition of the flame. The inventor has further realized that observation, of this emf not only allows detection of the presence or absence of the flame but further permits meaningful monitoring of the state of combustion at the flame.. This arises because the mean D.C. level of the emf- is observed to be dependent- on the stoichiometric (oxidant/fuel ratio) of the combustion. Fluctuations which are observed during monitoring this naturally occurring voltage are thought to arise from transient local phenomena in the flame and from random changes in contact conditions. The invention accordingly provides a method of detecting the condition of a flame comprising: monitoring an emf generated by the flame, which emf is indicative of the condition of the flame. The method preferably further comprises providing electrically isolated electrical conductor means and utilizing said conductor means to monitor said emf. The electrical conductor means may be provided in electrical contact with the flame. The invention also provides a furnace assembly comprising: a housing forming a combustion chamber; means defining a flame position in said chamber; electrical conductor means positioned to be in proximity to or in electrical contact with the flame during operation of the furnace assembly; and means electrically isolating said conductor means, which may thereby be utilized to monitor an emf generated by the flame; wherein said emf is indicative of the condition of said flame. A relatively high monitored emf (compared with background voltage levels associated with the furnace) will indicate that there is a flame, and a relatively low emf will indicate that the flame is extinguished. It is to be understood that monitoring of the emf may entail a direct measurement, e.g. of electric potential, or an indirect measurement, e.g. of electrical current or of an induced or other potential in a circuit including the flame. Where the flame is generated by a burner including a casing to which combustion components are fed, the electrical conductor means may conveniently comprise an elongate conductor projecting into the flame through an electrically insulated aperture in the rear of the casing. This conductor may. project a distance sufficient to electrically contact a cool part of the flame, but insufficient to reach the hotter parts of the flame and furnace interior during normal operation of the burner. Alternatively, and especially in the case of a pilot burner, the electrical conductor means may comprise the burner itself. In a still further alternative, very convenient where it is applicable, the electrical conductor means ma comprise a further flame in electrical contact with the flame whose condition is being detected. The emf may. then be monitored by simply measuring the voltage between the two burners. This technique is especially -applicable where the furnace includes a plurality of burners^ e.g. a main burner and a pilot burner, positioned such that the flames from the burners contact each other. The Invention additionally provides apparatus for detecting the condition of a flame comprising electrically isolated electrical conductor means positionable in electrical contact with the flame, and means to monitor an emf generated by the flame, which emf is indicative of the condition of the flame. As already foreshadowed, the present invention may he., employed in the control of oxidant-fuel ratio (stoichiometry) during the flame combustion process. It has been observed that the mean D.C. level of the emf being monitored at a given stoichiometry changes when the ratio of fuel to oxidant is altered. If both fuel and oxidant are altered to maintain a given relationship to each other the voltage does not change significantly. By monitoring the D.C. voltage level, the combustion of the burner gases, and therefore the furnace oxidation state, can be kept within desired limits. In most applications where air is the oxidant, close control of the air-fuel ratio is therefore possible by continuously monitoring the voltage level in accordance with the present invention and adjusting either the air supply or fuel supply so that the voltage level is maintained constant. A detailed description of preferred embodiments of the present invention will now be provided by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic sectioned view of a first embodiment of a furnace assembly in accordance with the invention, in which the states of the main burner and pilot flame are separately monitored; Figure 2 schematically depicts in greater detail the structure of the pilot burner of the furnace assembly shown in Figure 1; Figure 3 is a schematic sectioned view of a modified form of elongate probe for use with the main burner of the furnace assembly shown in Figure 1; Figure 4 is a schematic sectioned view of a second embodiment of furnace assembly in accordance with the invention, in which the pilot flame is utilized as electrical conductor means in electrical contact with the main burner flame; and Figure 5 is a block electrical circuit diagram of an arrangement for directly controlling fuel supply to the main burner of a furnace in response to the monitored flame emf. The furnace assembly 10 shown in Figures 1 and 2 includes a refractory brick wall housing 12 forming a combustion chamber 14; respective apertures 16, 18 in housing 12, defining main flame and pilot flame positions; a main burner 15 and pilot burner 17 mounted respectively in apertures 16, 18; and separate electrical leads 20, 22 for detecting the condition of each flame. The main burner 15 comprises a suitable metallic casing 24 formed with separate inlet ports, 26, 28 for delivering air and fuel gas to the interior of the casing. Similarly, the pilot burner 17 comprises a metallic casing 25 formed with separate air and gas inlet. orts 27, 29 coupled to respective supply pipes 31, 33. As best seen in Figure 2, pilot burner 17 is positioned towards the outer surface 11 of refractory walll 12 so that the space between the pilot burner 17 and. the inner surface 13 of the refractory wall 12 defines a-, port 30. As is. the usual practice the main burner 15 and housing 12 are electrically connected to ground. On the other hand, as can best be seen in Figure 2 , pilot burner- 1.7 is electrically isolated by separating the front section of casing 25 from housing 12 by means of a wrapping 35 of asbestos or glass fibre materials, and by positioning insulation 37 between the flanges 29 forming the- connection between the air and gas inlet ports 27, 29 and the respective air and gas supply pipes 31, 33. It will be understood that pilot burner 17 thereby constitutes electrically isolated electrical conductor means in electrical contact with the pilot flame. It is less practicable to similarly isolate the main burner and accordingly like means for the main burner flame comprises an elongate flame front conductor 39 that projects through an aperture 38 in the rear of the main burner 5 and is positioned to extend through the interiox of casing 24 into the combustion chamber to contact the flame from the main burner 15 when there is a flame. Conductor 39 is electrically isolated by insulation steeving 40 in aperture 38. The flame detection apparatus further comprises a first amplifier or a recording or observing voltmeter V. connected between ground and the flame front conductor 39 by way of lead 20, and a second amplifier or a recording or observing voltmeter V_ arranged such that one terminal is grounded while the other is connected via lead 22 to the casing 25 of pilot burner 17. in use and in the manner already explained, if the main burner is operating with a flame 8 extending into the interior of the furnace from the main burner the flame 8 will generate a randomly fluctuating D.C. emf which is indicated by a significant reading on amplifier or voltmeter V... Failure of the flame will be immediately reflected by at least a substantial fall in thie reading below a predetermined level: monitoring of the natural flame emf is thus an effective technique for detecting the presence or absence of the flame. Similarly, so long as the pilot flame 9 is alight the flame will generate an emf indicated by the voltmeter V_. In general, conductor 39 need only extend a distance sufficient to electrically contact a cool part of the flame 8 and need not reach the hotter parts of the flame during normal operation of the main burner. In this manner, it is possible to avoid the corrosion problem discussed earlier in connection with prior art flame probes. However, in some burners greater versatility may be desirable, especially where large changes are made to the total volume of combustion components entering the burner system. Figure 3 thus illustrates a modified conductor 39' provided with concentric passages 50 for circulating substantially non-conductive coolant fluid (e.g. fresh water) through the interior of the conductor from a supply pipe 52 to a drain pipe 54. An insulating gasket 40* is provided at burner casing aperture 38' under a flange 39a on the conductor 39, and further insulating gaskets 40a are sandwiched in flange mountings 56, 57 for pipes 52, 54. In situations where the main burner 15 and the pilot burner 17 are positioned so that the flames from the burners contact each other, an alternative method for detecting the presence or absence of the flames can be used and is depicted in Figure 4, which shows how the pilot flame 9' * provides a conductor in contact with the main flame 8' ' and thus completes a conductive path between the main burner 15' ' and the pilot burner 17''. The measurement of the voltage between these two points by a voltmeter or other device V, will thereby provide an indication as to whether or not the flames are alight, the extinction of one or other of the flames resulting in a relatively low voltage reading when compared with that arising from a situation where both flames are alight. As shown in Figure 4, the main burner 15* ' itself provides the electrical connection with the main flame and it must therefore be electrically isolated. As an alternative to this arrangement, an elongate conductor such as conductor 39 of Figures 1 to 3 may be used to provide the electrical connection between the main flame and the voltmeter V,. In a still further alternative arrangement, burner 15 is isolated and the pilot flame, or any other secondary flame, simply provides the required electrical conductor means in contact with the flame whose condition is being monitored. Figure 5 is a diagram of an electrical circuit for enabling control of the fuel supplied to the main burner 15 in response to the flame detection apparatus of Figures 1 to 3. in this arrangement, the lead 20 from the flame front conductor 39 is connected to a control circuit 43 which is earthed at 45 and which is capable of producing a signal indicative of mean DC value of the flame emf, which has been found to relate to the fuel-to-oxidant ratio. The fuel inlet port 28 is coupled to a fuel supply line 47 which is fitted in turn with a variable-flow valve 49 controllable by a solenoid 51. Circuit 43 compares the monitored D.C. emf level with respective set points and if necessary transmits a control signal on line 51a to the solenoid 51 to adjust the valve 49 and thereby the fuel to air ratio. Where the D.C. level falls below the predetermined value or by the predetermined change indicative of flame failure, the control circuit closes valve 49 to shut off the fuel supply. The or a second controlled valve may of course be provided in the air supply line. Table 1 sets forth the monitored voltage as a function of time as the oxygen pressure was altered in the feed to an acetylene-oxygen flame. The conductor in electrical contact with the flame was a propane-oxygen flame of diffusion type. TABLE 1 Period Press. Press. Ratio Voltage Comments °2 C 2 H 2 0 2 /C 2 H 2 A 350 50 0.935 18.0 Excess acetylene B 350 50 0.935 0 Input shorted to determine zero level C 350 50 0.935 18.0 Excess acetylene D 500 50 1.118 36.0 Excess oxygen E 450 50 1.060 30.9 Excess oxygen F 400 50 1.000 25.2 Stoichiometric G 350 50 0.935 18.5 Excess acetylene The advantages of the present invention may be summarized as follows:- 1. There is no need to include in the flame detection apparatus any external voltage source, as is the case with the flame rod of the prior art. As a consequence, the apparatus is significantly simplified. The life of the pilot burner is almost indefinite and therefore the method by which the pilot or another secondary flame is used to provide the electrically conductive contact with the main flame is not subject to deterioration of the detection equipment, as is the case with the conventional flame rod. 3. In the case of the elongate flame front conductor, -its exposure may be less than that of a conventional flame rod since it need be positioned to extend only a short distance into the flame and in such a way that significant cooling of the rod occurs by virtue of unburnt ambient temperature gases that are forced from the burner past the rodinto the interior of the furnace. This is in direct contrast to the conventional flame rod which is subject to extremely high flame temperatures. 4. If necessary, it is practicable, in the absence of a substantial applied voltage, to cool the elongate conductor, such cooling being impractical in conventional flame rod systems. 5. The apparatus may be used not only to detect the presence or absence of the flame but also to determine the fuel to oxidant ratio and therefore the stoichiometry of the flame.

Claims: CLAIMS-: 14 1. A method of detecting the condition of a flame comprising monitoring ari emf generated by the flame, which emf is indicative of the condition of the flame. 2. A method according to claim 1 further comprising providing electrically isolated electrical conductor means and utilizing said conductor means to monitor said emf. 3. A method according to claim 2 wherein said conductor means is provided in electrical contact with the flame. 4. A method according to claim 3 wherein said flame is generated by a burner including a casing to which conbustion components are fed, " the method comprising utilizing as said electrical conductor means an elongate conductor projecting into the flame through an electrically insulated aperture in said casing. 5. A method according to claim 3 comprising utilizing as said electrical conductor means burner means generating said flame, which burner means is electrically isolated from adjacent furnace walls and piping supplying combustion components to the burner means. 6. A method according to claim 2 or 3 comprising utilizing a further flame as said electrical conductor means. 7. A method according to any preceding claim further comprising shutting off the supply of a fuel for the flame in response to a predetermined change in said emf indicative of the absence of said flame. 8. A method according to any preceding claim wherein the flame is fed by a mixture of combustion components, and the method further comprises controlling the proportions of components in this mixture to sustain said monitored emf between predetermined limits. 9. A furnace assembly comprising: a housing forming a combustion chamber; means defining a flame position in said chamber; electrical conductor means positionable to be in proximity to or in electrical contact with the flame during operation of the furnace assembly; and means electrically isolating said conductor means, which may thereby be utilized to monitor an emf generated by the flame; wherein said emf is indicative of the condition of the flame. 10. A furnace assembly according to claim 9 wherein said means defining a flame position comprises an opening in said housing and said assembly further comprises burner means for generating said flame and including a casing adjacent said opening, said electrical conductor means comprising an elongate conductor projecting through an electrically insulated aperture in said casing a distance sufficient to electrically contact a cool part of the flame, but insufficient to reach the hotter parts of the flame during normal operation of the burner means. 11. A furnace assembly according to claim 9 wherein said means defining a flame position comprises an opening in said housing' and said assembly further comprises burner means for generating said flame and including a casing adjacent said opening, said electrical conductor means comprising an elongate conductor projecting through an electrically insulated aperture in said casing, which elongate conductor includes passages for circulating coolant fluid therethrough. 12. A furnace assembly according to claim 9 wherein said electrical conductor means comprises burner means for generating said flame, and said electrically isolating means isolates said burner means from said housing and from piping supplying combustion components to the burner means. 13. A furnace assembly according to claim 9 wherein said electrical conductor means comprises a second burner arranged to generate a further flame in electrical contact with the first-mentioned flame. 14. A furnace assembly according to any one of claims 9 to 13 further comprising valve means determining the supply of a fuel for the first mentioned flame, and means for controlling said valve means in response to said monitored emf in_accordance with one or more predetermined values for said emf. 15. Apparatus for detecting the condition of a flame comprising electrically isolated electrical conductor means positionable in electrical contact with the flame, and means to utilize said conductor means to monitor an emf generated by the flame, which emf is indicative of the condition of the flame. 16. Apparatus according to claim 15 wherein said electrical conductor means comprises a burner arranged to generate a further flame in electrical contact with the first-mentioned flame.

Inventors: KEMLO KENNETH GARRY, KEMLO, KENNETH, GARRY
Applicants: BROKEN HILL PTY CO LTD, THE BROKEN HILL PROPRIETARY COMPANY LIMITED
Application year: 1986
Main classification: F23N5
IPC codes: F23N5