performance improvement of infrared flame scanner card ... - Infraline

PERFORMANCE IMPROVEMENT OF INFRARED FLAME SCANNER 

CARD OF 500 MW UNITS AT NTPC LIMITED, FARAKKA 

SNEHASIS BHATTACHARYYA , Sr. Supdt. (C&I) , NTPC Ltd, Farakka 

ABDUL MAJEED KM , 

Sr. Engineer (C&I) , NTPC Ltd, Farakka 

BISWANATH MAJI , 

Sr. Engineer (C&I) , NTPC Ltd, Farakka 

ABSTRACT 

This paper describes a new method for performance improvement of infrared flame scanner card of 500 MW units at 

NTPC, Farakka. The front and rear pulverized coal fired furnace comprises of Infrared flame scanners for sensing 

& monitoring of hydrocarbon flame present in the furnace. Each scanner head consists of a card containing infrared 

detector and a high impedance matching amplifier to measure the inherent pulsation & quality of the flame. The 

newly designed and developed standard infrared source could replicate the boiler flame signal and compared the 

outputs of the insensitive cards against a standard healthy test card which greatly improved the reliability of flame 

sensing and safety of the boiler 

INTRODUCTION 

The 500 MW Boilers at NTPC / Farakka have a front and rear pulverized coal fired furnace of B&W design. The 

furnace, a unique of its kind in the country, comprises of eight elevations with 6 burners in each elevation and 48 

Infrared flame scanners per boiler for flame monitoring of these burners. This monitoring is done by SIE Systems, 

Italy make UR 600/IR Infrared Flame Scanner . 

RADIATION OF A (HYDROCARBON) FIRE 

A fire radiates an enormous amount of energy of which we humans only can see a very small part. As you can see in 

the picture-1 below most of the energy is invisible to us. The part that we can see is mostly red-yellow in colour 

caused by the Carbon in a fire. The invisible IR part of the fire we experience as heat. A non-Hydrocarbon such as 

Hydrogen burns light blue-transparent because there is no Carbon in the flame. It also doesn't have the CO2 peak at 

4.4µ and can therefore only be detected with a UV detector. 

The CO2 peak in the fire represents less then 2% of the total fire energy. A multi sensor Flame Detector that uses 

sensors such as UV, near IR, wide band IR etc. has much more sensor input and can therefore be more specific or 

less effected by false alarms. E.g. a "UV/visible/wideband IR detector" can detect burning Hydrogen and Metals 

besides common Hydrocarbons like Petrol, Wood or Natural Gas but is still very false alarm resistant. 

It looks rather static but in reality the fire energy fluctuates rapidly. The Fuel and Oxygen in the uncontrolled fire 

constantly burn as in small explosions and then sucks new Fuel and Oxygen to the flames. This process causes the 

flame flicker. The typical frequency is between 1 and 20 Hz. Most Infrared Flame Detectors use the flicker 

frequency as an extra criterion in order to make the detector more reliable.



CO2 

Near IR 

Wideband IR 

Energy, Kilowatt 

UV 

Visible 

WaveLength,(Micron) 4.4 

Picture-1:- Radiation from a hydrocarbon fire 

INFRA-RED FLAME SCANNERS 

Flame scanners are optical fire detection devices, which are able to detect infra-red and/or ultra violet radiation 

given off from a flaming fire. 

I.R. Flame Detectors respond to flaming fires emitting light in the infrared area of the spectrum (modulating at 5 to 

30 cycles per second). I.R. Flame Detectors can respond to a fire condition in typically less than 50 milliseconds and 

are designed to detect hydrocarbon fires whilst ignoring potential false alarm hazards such as arc welding, nuclear 

radiation and x-rays. 

The sensor usually incorporates a delayed response, selectable in the range 3-30 seconds, to minimise responses to 

non-fire sources of radiation. In this way alarms are only generated by a sustained, flickering sources of I.R. 

radiation - e.g. fire. 

The sensitivity of Infrared Flame Detectors is affected by the distance of the device from the fire source such that, if 

a distance is doubled the fire has to be four times as large to be detected. The trigger delay is therefore adjusted to 

suit the installation conditions shorter delays for lower mounting, longer delays for higher mounting. 

An infrared (IR) detector is basically composed of a filter and the system is used to screen out unwanted 

wavelengths and focus the incoming energy on a photovoltaic or photo resistive cell sensitive to infrared radiation. 

IR flame detectors can respond to the total IR component of the flame alone or in combination with flame flicker in 

the frequency range of 5 to 30 Hz.



IR detectors are sensitive to most hydrocarbon fires (liquid, gases and solids). Fires, such as burning metals, 

ammonia, hydrogen, and sulphur do not emit significant amounts of IR radiation in the 4.4 micron sensitivity range 

of most IR detectors. Thorough planning is required to apply IR fire detectors to non-hydrocarbon fuelled fire 

applications. 

CONE OF VISION 

The cone of vision of a flame detector depends on the shape and dimensions of the lens/housing and the position of 

the sensor. With IR sensors, lamination of the filter plays a role and can also limit the cone of vision. A wider cone 

doesn't automatically mean that the detector is better. It could even be a drawback if the detector has a wide cone of 

vision in case you want to avoid having false alarm sources in the field of view. In practice you don't need a angle of 

more than 90°. The cone is three-dimensional and doesn't need to be perfectly cylindrical. The horizontal and 

vertical field of view are mostly different due to the shape of the housing or the location of mirrors. (Self check). 

Very important is the sensitivity at the 45° edges. This must be minimum 50% of the maximum sensitivity on the 

central axe. Some detectors have a 70% performance here. In reality these detectors have a cone of vision that is 

wider than 90° but the manufacturer prefers not to tell you. (See picture below). 

Picture-2:- cone of vision



Figure 3: Example of typical flame flicker analysis for flame “on” and flame “off” (background) condition 

The curves in Figure 3 indicate the relative amplitude of this radiation at the corresponding flame flicker frequency. 

The “flame on” curve shows a relatively high amplitude in the 50 to 120 Hz range received from the ignition zone of 

the flame. The “flame off” (background) curve shows these 50 to 120 Hz frequencies at a much lower amplitude. 

The cause of this difference is that although the “flame off” condition receives nearly the same frequencies from 

adjacent flames in the background, they are further away from the detector. Therefore, there is less amplitude. This 

difference in amplitude at selected frequencies allows the flame detection system to discriminate. In a set-up such as 

this, where the detector is sighted at the ignition zone of the targeted flame, it is not uncommon to find that the 

lowest frequencies increase dramatically in a “flame off” condition. This happens because the ignition zone of the 

targeted flame “masks” the bright background low-frequency radiation while the targeted flame is on. When the 

targeted flame disappears, the background radiation comes into full view is shown in the curve. 

BACKGROUND GAIN CONTROL 

Infra-red scanners may feature Background Gain Control or BGC. BGC inversely adjusts the Flame Signal Gain 

based on flame brightness as sighted by the scanner. As flame brightness increases, automatic gain is decreased, 

thereby diminishing the detector’s flame signal. Some older detectors use an optical shutter for verification of valid 

flame presence data, in later models this electromechanical shutter is replaced by an electronic self-check circuitry. 

This periodically bypasses the photo resistor cell and checks the system for false or corrupted flame signal.



In spite of scanner tuning for better flame monitoring, healthiness of the scanner plays a vital role. Each scanner 

head assembly consists of a scanner card ( see picture-4) .The card consists of an infrared detector and a high 

impedance matching amplifier to measure the inherent pulsation and thus, the quality of the flame. The accurate 

measurement is essential for safe and reliable operation of the boiler. Any deterioration in performance of these 

scanner cards results in low flame sensing even when the flame condition in the furnace is good and causes 

unwarranted elevation trips. The subsequent disturbances in boiler flame leads to cascaded tripping of other 

elevations, unit emergencies and in extreme cases, even unit outages. Over a period of continuous use in the 

hazardous boiler environment, a lot many cards were found to be sensing low flame signals and needed immediate 

replacement with new cards. However, this called for a huge expenditure and was an uneconomical solution. 

Picture-4: Scanner head card and Newly designed test kit 

Earlier scanner cards were tested with the help of a candle light and the output waveform of the cards were seen in 

CRO. Qualitative and quantitative checking of the individual card was not possible as exact replication of of the 

boiler flame was not possible with the candle light. The output wave form was observed with candle light in on and 

off conditions with the help of obstacles. 

Then C&I laboratory has designed and developed a test kit with the help of infrared emitter source and an electronic 

circuit which simulates the boiler flame signal in boiler working condition. This test jig provides 250 hz square wave 

infrared pulses to the scanner head card. If the output of the card is above 15 db, the card is considered as healthy. 

The defective card and healthy card output responses were given below in figure 5. 

C&I Laboratory of NTPC, Farakka revamped the existing cards which otherwise would have had to be discarded. 

Also, the detailed component level diagram has been developed (see the figure 6.) in the laboratory as manufacturer 

had not provided the same. For comparison of performance The deteriorating components (FET 2N4340, IR 

detector IR/2305) have been replaced with new and standard ones. The cards were tested again in laboratory and 

found working all right . In total 192 flame scanners are getting revamped in every year in two 500MW units, which 

greatly improved the reliability of flame sensing and safety of the boiler. It has been observed that the average life 

span of the card is 2 years.



Figure 5. comparison of defective card reponse(left) versus 

healthy card response (right) with the help of test jig. 

Figure 6. Traced circuit diagram of the scanner head card



SAVINGS 

Total scanner head card population in stage 2 ( 2X 500 MW) = 96 X 2 = 192 no.s 

Average failure rate = 28 no.s per year. 

A) COST OF EACH CARD ASSEMBLY (AS PER LPP)= 1,080 Euros 

COST OF PROCUREMENT OF 28 CARDS = 32,040 Euros 

= RS 19.36 Lacs. 

B) COST OF REPAIR FOR 28 CARDS: = US $ 2042 

=RS.1.02 Lacs (COMPONENTS COST) 

DIRECT SAVINGS FROM REPAIR OF 28 CARDS : 

RS. 18.34 lacs=28,656 Euros approx. 

DIRECT FOREIGN EXCHANGE SAVINGS ON SPARES 

FROM THE REPAIR =28,656 Euros per year 

(Calculation are made taking 1 Euro=Rs.64/- & 1 US $ = Rs. 50/-) 

REFERRENCES 

1) B&W Boiler manuals for 500 MW Units.

performance improvement of infrared flame scanner card ... - Infraline

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