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FIELD OF INVENTION
The invention relates to an apparatus for detecting and discriminating flames of
different fuels (mainly oil/coal) fired in the furnace of a boiler and in other similar
applications.
BACKGROUND OF INVENTION
Boilers are employed worldwide by thermal power plants, refineries and many
other industries to produce steam for their operation. Many of the boilers are
designed for firing fuel oil and/or coal. After ensuring proper conditions for firing
the fuel, the fuel is admitted into the furnace of the boiler and ignited. The
detection of the flame is an important safety requirement for the operation of the
boiler. Fuel admitted without ensuring continuous combustion will lead to
accumulation of unburnt fuels, in turn leading to hazardous conditions like
explosions. In order to ensure the safety, the detection of the flame for the
burner and shutting off fuel to the burner in case of flame failure is an important
safety requirement.
Prior art flame detectors employ independent or separate flame detectors/
sensing electronics for different fuels. The flame detectors employ a collimator
and head assembly to focus the light from the flame on to an optical fibre. The
fibre optic cable guides the light from the flame to an electronic circuitry wherein

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a light-sensing device converts this light signal into an electrical signal for further
processing. The flexible light cable is employed for two purposes, namely:
I. To keep the sensing electronics away from the hot zone and to guide the
light from the flame.
II. The burners employed are of tiltable type in many boilers, necessitating a
flexible light guide for proper monitoring of the flame.
In the prior art flame detectors, proper discrimination of flames of different fuels,
when both the flames are present or when either one of the flame is absent, is a
complicated phenomenon due to availability of only one light sensing device.
Patent CA1058302, describes a flame scanner which uses a single light sensing
device to detect the presence/ absence of individual burners. This patent does
not talk about discriminating the flame signals in case of multiple fuels.
Patent US4368031 describes of a stationary flame scanner for tilting burners,
where a single scanner head consists of 3-transmission tubes- two to view the
flame from an acute angle facing above and below, and one for viewing the
flame horizontally, along with a separate water wall monitor. This invention talks
about viewing ports in a single head and also three independent light sensing
devices.

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Patent US7280891 describes a signal processing technique for improved flame
scanner discrimination, which employs a single light sensing element and
bifurcating the signal and processing the two signals parallely. This method does
not employ flame characteristic wavelength discrimination but only a method
based on flicker frequency.
Generally, in steam generating fossil- fuel fired boilers, during start-up and
during load changes there will be situations during which two different types of
fuels including their flames will be operating. Therefore there is a need for
detecting and discriminating the presence/ absence of the flame signals of
different fuels (mainly coal and/or oil), for which the method disclosed in this
invention provides a reliable solution.
OBJECTS OF INVENTION
It is therefore, an object of the invention to propose an apparatus for detecting
and discriminating flames for different fuels fired in the furnace of a boiler.
Another object of the invention is to propose an apparatus for detecting and
discriminating flames for different fuels fired in the furnace of a boiler, which
employs only one flame viewing head.
Still another object of the invention is to propose an apparatus for detecting and
discriminating flames for different fuels fired in the furnace of a boiler, which is

capable of outputting independent flame detection signals representing presence
or absence of flames of different fuels.
SUMMARY OF INVENTION
According to the invention, there is provided an improved flame detection
apparatus having an electronic circuitry for producing outputs representing
presence or absence of flame comprising: a lens barrel device, which collects and
focuses the incoming light signal; a fibre optic light guide which receives the
incoming light signal channel from the lens barrel device and divides the light
signal into two separate channels at a split end for detecting the presence of any
type of fuel; at least two light sensing device having different sensitivity in the
two channels, the first light sensing device being more sensitive to the
wavelength range corresponding to that of coal flame in the first channel, and
the second light sensing device more sensitive in the second channel to oil flame
light wavelength range; a buffer amplifier assembly, for separately amplifying the
two voltage signals; a voltage to - current converter assembly for converting the
amplified voltage signals to current outputs corresponding to the first flame and
second flame thereby detecting and discriminating the presence or absence of
the coal and /or oil flames.
The apparatus employs a split fibre optic light guide and independent light
sensing circuitry to detect and discriminate flames of different fuels based on the
characteristic radiation spectrum of the fuels. In particular, a technique of three-
stage discrimination based on characteristic spectrum, intensity level, and flicker
frequency is employed to detect and discriminate the flames. The flame
detection circuitry described herein will provide digital outputs for the presence
or absence of individual flames and also gives analog outputs proportional to the
intensity of the flames.

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The flame signals are independently processed in separate channels consisting of
light sensing devices that are sensitive to specific radiation spectrum of the fuel
and are then compared for the flicker frequency range of the flame to produce a
"flame ON" signal. Thus the method of the provides reliable output
corresponding to the presence/ absence of the flames.
The present invention employs a fiber optic light guide with split outputs at the
receiving end and employing two light sensing devices to detect two or more
different ranges of wavelengths of the light specific to the combustion
characteristics of different fuels. The invention provides a light sensing electronic
circuitry to discriminate the presence and absence of flames.
The present invention employs multiple flame sensing electronics tuned for
detecting different flames operating with different fuels.
The invention disclosed herein employs two individual photodiodes having
independent wavelength characteristics.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig. 1 - shows a schematic block diagram of the apparatus according to the
invention.
Fig. 2 - shows a light sensing electronic circuitry of the apparatus of fig. 1.

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DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF
THE INVENTION
The preferred embodiment of the proposed invention is schematically
represented in Figures 1 & 2. As shown in figure 1, the apparatus comprises a
lens barrel device (1), which collects and focuses the incoming light signal on to
a fibre optic light guide (3). The lens barrel device (1) is housed within a
collimator assembly (2), the later comprising a metallic tube (10), constituting a
quartz glass window (11), for serving the purpose of collecting and focusing the
light on to the lens barrel assembly (1). The light falling onto the fibre optic light
guide (3) is divided into two separate channels at a split end (4) for further
signal processing. The concept is to process the same incoming light from a
single viewing head (100) to detect the presence of any type of fuel (mainly coal
and oil) instead of using two different flame viewing heads. At least two light
sensing device (5) are used to detect the nature of the flame signal based on the
characteristic wavelength range of the fuel used. For example, in the case of coal
and oil fuels, the first light sensing device in a first channel (12) is more sensitive
to the wavelength range corresponding to that of coal flame (infra- red), while
the second light sensing device in a second channel (13) is more sensitive to oil
flame (visible) wavelength range. Thus the discrimination between coal and oil
flame is achieved at this stage and the two separate signals are then amplified
by a buffer amplifier (6). The two separately amplified voltages are converted
into current outputs using a voltage- to- current- converter (7), the current
outputs provide the current outputs from the two channels (12,13)
corresponding to a first flame (8) and a second flame (9) which can be

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transmitted with minimum attenuation for further processing. This improved
method of detection is advantageous, in that, it can detect and discriminate the
presence/absence of both coal and/or oil flames using a single flame viewing
head (100). Thus the same principle can be extended further, with multiple
channels, for the detection of any type of fuel using wavelength characteristics of
the light-sensing device that is employed.
Studies have shown that the relative radiation intensity of coal flame is maximum
in the infra- red region of the light spectrum, while that of oil flame is maximum
in the visible region. This aspect is also used in this invention for the second
stage in detecting the corresponding signals from coal and oil flames. The
current signal outputs from corresponding to the first flame (8) and the second
flame (9) are fed as inputs to the electronic circuitry (110) shown in Figure-2.
This circuitry (110) consists of two independent channels for processing signals
corresponding to the first and second flame (8,9) outputs respectively. An first
input flame signal (14) to the first channel (12) is in turn split into two, where
one portion is fed to a first window comparator (34) with a first hysterisis (17).
The first window comparator (34) is provided with a first pull- in value (15)- for
example, an upper threshold value and a first drop out value (16) for example, a
lower threshold using potentiometer arrangements, set at values higher than the
expected signal level for the second flame in the spectrum corresponding to the
first flame. For example, if the first and second flames (8,9) correspond to coal
and oil flame signals respectively, then the first pull-in value (15) and the first
drop- out value (16) for the coal flame detector will correspond to higher levels
than the expected signal from radiation of the oil flame in the infra- red region.
Thus if the output from the first comparator (34) is high, it indicates presence of

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the first flame (8), while a low output indicates a no flame condition. The other
portion of the input flamel signal is fed through a first capacitor (18) to a first
AC amplifier with an adjustable gain (19). The output is fed to a first pulse
shaper circuit (20). The pulse output is then fed to a first frequency discriminator
(22), fed with a frequency corresponding to the flicker frequency of the first
flame (8) through a first BCD switch (21). If the output of this first stage (35) is
high, it indicates the presence of the first flame signal. The two outputs (34 &
35) are given to a first AND gate (23) to get the final output " flamel ON".
The second channel (13) is symmetrical to the first channel (12), but is used for
indicating the presence of the second flame. An input second flame signal (24) is
similarly split into two and one is fed to a second window comparator (36) with a
second hysterisis (27). It is also provided with a second pull- in value (25) and a
second drop- out value (26), set at a value higher than the radiation intensity of
the first flame (8). For example, if the first and second flames (8,9) correspond
to coal and oil flame signals respectively, then the second pull-in and drop- out
values (25, 26) for the oil flame detector will correspond to higher levels than the
expected signal from radiation of the coal flame in the visible region. Thus if the
output from the second comparator (36) is high, it indicates the presence of the
second flame, while a low output indicates a no flame condition. The other
portion of the input second flame signal (24) is fed through a second capacitor
(28) to a second AC amplifier with an adjustable gain (29). The output is fed to a
second pulse shaper circuit (30). The pulse output is then fed to a second
frequency discriminator (32), which is fed with a frequency corresponding to the
flicker frequency of the second flame fed through a second BCD switch (31). If
the output of this second stage (37) is high, it indicates the presence of the

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second flame signal. The first and second outputs (36 & 37) are the given to a
second AND gate (33) to provide the final output " flame2 ON".
Thus the present invention provides a three- stage detection and discrimination
method to provide reliable output corresponding to the presence/ absence of the
flames.
It is to be understood that the description of the preferred embodiment (s)
herein is (are) to be considered only illustrative, rather than exhaustive, of the
present invention. Those of ordinary skill will be able to make certain additions,
deletions, and/ or modifications to the embodiment (s) of the disclosed subject
matter without departing from the spirit of the invention or its scope, as defined
by the appended claims and those modifications are not eligible to be depicted
as new ideas/ invention described herein.

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WE CLAIM
1. An improved flame detection apparatus having an electronic circuitry for
producing outputs representing presence or absence of flame comprising:
a lens barrel device (1), which collects and focuses the incoming
light signal;
a fibre optic light guide (3) which receives the incoming light
signal channel (12,13) from the lens barrel device (1) and divides
the light signal into two separate channels at a split end (4) for
detecting the presence of any type of fuel;
at least two light sensing device (5) having different sensitivity in
the two channels (12,13), the first light sensing device (5) being
more sensitive to the wavelength range corresponding to that of
coal flame in the first channel (12), and the second light sensing
device (5) more sensitive in the second channel (13) to oil flame
light wavelength range;
a buffer amplifier assembly (6), for separately amplifying the two
voltage signals;

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a voltage to - current converter assembly (7) for converting the
amplified voltage signals to current outputs corresponding to the
first flame (8) and second flame (9) thereby detecting and
discriminating the presence or absence of the coal and /or oil
flames.
2. The apparatus as claimed in claim 1, wherein when the flame produced by
multiple fuels, the apparatus further comprising :
an electronic circuitry (110) having two independent channels
corresponding to the outputs of said first and second flame (8,9),
the circuitry (110) receiving as the input from the voltage to
current converter (7), the inputted first flame signal (14) being
splitted into two components.
a first window comparator (34) with a first hysterisis (17)
provided with a first pull-in value (15) including a first drop-out
value (16) for receiving the first component of said first flame
signal (14), and outputting a value indicative of presence or
absence of said first flame (8);
a first A.C. amplifier with an adjustable gain (19) preceded by a
first capacitor (18) for receiving said second component of said
first flame signal (14), the output signal being fed to a first pulse
shape circuit (20);

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a first frequency discriminator (22) for receiving the pulse output
from the first pulse shaper circuit (20) which being
simultaneously fed with a frequency corresponding to the flicker
frequency corresponding to the flicker frequency of the first flame
(8) via a first BCD-switch (12); and
a first AND gate (23) for receiving the output from the first
window comparator (34) and further from the first frequency
discriminator (22) and outputting a presence or absence of the
first flame (8).
3. The apparatus as claimed in claim 1 or 2, wherein an inputted second
signal output (24) from said second channel when processed via said
electronic circuitry (110), the apparatus comprising :
a second window comparator (36) with a second hysterisis (27)
for receiving and processing a first splitted component of said
second signal output (24), the comparator (36) being further
provided with a second pull - in - value (25) including a second
drop - out - value (26);
a second AC amplifier with an adjustable gain (29) preceded by a
second capacitor (28) for receivcing a second splitted component
of said second signal output (24) and processing the input;

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a second pulse shaper circuit (30) for receiving the output from
the amplifier and feeding to a second frequency discriminator
(32), the second frequency discriminator (32) being
simultaneously fed with a frequency corresponding to a flicker -
frequency of said second flame (9) via a second BCD-switch (31);
and
a second AND gate (33) receiving the output from the second
window comparator (36) and further from the second frequency
discriminator (32), and indicating the presence or absence of the
second flame (9).
4. The apparatus as claimed in claims 1 to 3, wherein the first pull - in -
value (15) including the first drop-out value (16) is provided to the first
window - comparator (34) by adapting a potentiometer set at a value
higher than the radiation intensity of the second flame (9).

This invention relates to an improved flame detection apparatus having an
electronic circuitry for producing outputs representing presence or absence of
flame comprising: a lens barrel device (1), which collects and focuses the
incoming light signal; a fibre optic light guide (3) which receives the incoming
light signal channel (12,13) from the lens barrel device (1) and divides the light
signal into two separate channels at a split end (4) for detecting the presence of
any type of fuel; at least two light sensing device (5) having different sensitivity
in the two channels (12,13), the first light sensing device (5) being more
sensitive to the wavelength range corresponding to that of coal flame in the first
channel (12), and the second light sensing device (5) more sensitive in the
second channel (13) to oil flame light wavelength range; a buffer amplifier
assembly (6), for separately amplifying the two voltage signals; a voltage to -
current converter assembly (7) for converting the amplified voltage signals to
current outputs corresponding to the first flame (8) and second flame (9)
thereby detecting and discriminating the presence or absence of the coal and /or
oil flames.