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Flame Scanners 101: Your Go-To Guide

FlamScanners
In Info-Tec No. 1, only flame rod scanners were covered in any detail. Since scanners cause many problems, this Info-Tec will deal with the rest of the flame scanners. In order to apply the most suitable flame detector to a combustion chamber, it is necessary to know a little about the characteristics of flames.
All flames, whether they are from burning wood, oil, gas, coal, or even a candle, produce heat and light (radiant energy), and during the burning process, ions are formed. The only difference between the various flames is in the magnitude of each of these quantities. Some flames are blue, some are yellow, and some produce more heat than others do, but basically they are all the same. For instance, the atomized oil burner produces a characteristic yellow flame, but at the same time it is also radiating light in the infrared region and the ultraviolet region. You cannot see the ultraviolet or infrared light, but the warmth you feel on your face is a result of the infrared radiation. The ultraviolet radiation can only be detected by an ultraviolet sensitive photocell. The pale blue flame of a gas burner is familiar to us, and it is apparent how the visible light is present in both this and the oil flame. Both flames differ in brightness as seen by the eye as well as in color and in the amount of infrared and ultraviolet radiation they produce.
The table below shows relative amounts of radiation in the visible, infrared and ultraviolet regions for fuels normally used.
 
           Radiant Energy
Fuel
Infrared
Visible
Ultraviolet
Oil (Atomized)
High
High
Medium
Gas (Premix with Air)
Low
Low
High
Gas (Without Premix)
Medium
Medium
Medium
Coal (Pulverized)
High
High
Medium
 
The radiation from a flame in the visible region is easy to visualize because it is visible to the eye. Whenever a human eye can see a flame, it is radiating visible light. The infrared radiation from an oil flame is very similar in pattern to the visible radiation except that it will be present slightly beyond the flame envelope. In a gas flame, the infrared radiation will be strongest in the last two-thirds of the flame. The area of the flame that radiates ultraviolet is much more limited than either the visible or infrared areas.
 Practically all the ultraviolet radiation comes from the first one-third of the flame for all fuels that are premixed with air. For atmospheric flames, such as a candle or raw gas burning at the end of a pipe, the area of the flame radiating ultraviolet will be the outside surface of the flame.
The radiation from a flame constantly varies in amplitude at all times. That is, it is momentarily changing from brighter to dimmer to brighter and so on. We have all seen how some flames flicker at a low frequency, but even those flames that seem to be perfectly steady to the eye flicker. A photoelectric detector can pick up this flicker. The rate of flicker is usually too fast to be seen by the eye. The frequency of modulation depends to some extent on the type of flame, but it has been found by a large number of measurements that a substantial amount of all flames flicker with rates ranging from 10 to 50 Hz.
The most familiar property of a flame is, of course, the high temperature of the gases within the flame and in the gas products of the flame. The maximum temperature of the flame depends on the fuel being burned and the availability of oxygen.
Flame failure control can utilize another characteristic of flames; the light transmission of the flame’s combustion products. When a flame is burning without enough air for clean burning, there will be soot (free carbon) produced which causes a smoky flame and smoke in the combustion products. When the smoke is bad enough, it will be completely opaque in the visible region. In the infrared region, the loss is much less. When a flame is clean burning or air rich, the combustion products are transparent in both the infrared and visible regions. In the ultraviolet region, the combustion products of any flame, rich or lean, are opaque. These differences are important in applications involving more than one burner in a combustion chamber.
The following characteristics are those used for detecting a flame other than temperature.
• Visible light radiation
• Infrared light radiation
• Ultraviolet light radiation
• Ionized gas molecules in the flame
Visible Light Detectors
Detection of flame by seeing the visible light, is, of course, the way man has detected flame since it was first observed. Since the radiation travels at the speed of light, the detection of flame by this method is not limited in speed. There are two standard detectors used for sensing the flame in the visible region, the oldest of which is the photo-emissive photocell. This device is a glass-enclosed structure, which usually has been completely evacuated. There are two elements within the envelope, a thin metal plate that is the cathode, and a collector assembly, which is the anode. When light strikes the cathode, electrons are emitted which are drawn to the anode. The current in this type of tube is very small and requires considerable amplification before it can operate a relay.
The other type of detector in the visible region is usually a smaller device that is constructed from an insulating plate covered with a deposit of cadmium sulfide that is mounted in a protective enclosure with a glass window. This device acts like a resistor, which is sensitive to light. When the cadmium sulfide detector is in the dark, the resistance of the element will be many megohms. When it is exposed to a light from a flame, the resistance will drop to a few thousand ohms; and, in most devices, enough current will flow to energize a relay without further amplification. Flame detectors that operate in the visible region will also operate from other light in the visible region such as daylight or light from an incandescent of fluorescent lamp. It is therefore necessary to make sure that they are used only in locations where they cannot see other sources of light. Also, they will respond to the visual light signal from hot refractory in the combustion chamber.
Infrared Light Detectors
All flames radiate infrared energy, and since this radiation travels at the speed of light, the detection of flames by this method is not limited in speed. There is only one standard detector for use in the infrared region, and this is the lead sulfide detector. This device is constructed by depositing a thin film of semi-conductor material on an insulating surface. Two leads are attached to opposite ends of the film. The device operates like a resistor whose value of resistance changes with the amount of infrared energy falling on the surface of the detector. These devices have dark resistance values from a few thousand ohms to a few megohms. When these detectors are exposed to light, they decrease in resistance depending on the intensity of the light. Their percentage change in resistance is not as great as in the cadmium sulfide detectors since the resistance of the element in the dark is much lower than the cadmium sulfide detector. This requires a different type of amplifier for infrared detectors especially since the signal from a hot refractory is strong in the infrared region. An example of the infrared scanner is Type 48PT1 and 48PT2.
Ultraviolet Light Detectors
Ultraviolet radiation from a flame travels at the speed of light, therefore, the detection of flame by this method is not limited in speed.
The newer type of ultraviolet detector is constructed of two similar electrodes made from very clean wire of tungsten or molybdenum. These operate from UV light falling on the wire electrodes, which starts an avalanche of electrons. These tubes quench very fast. The fast quenching of these tubes allows them to be used on 50 or 60 cycle supply voltage, which provides automatic quenching.
Application of Photoelectric or Photocell Detectors in the Visible Region
Operation in the visible spectrum is limited to single burner applications because it is very difficult to differentiate between the burner being monitored and other burners which may be visible to the scanner since this would give a false indication of flame. The normal application is for the smaller oil burners where the field of view is limited to the combustion flame itself. A blue gas flame will not operate any of the detectors that operate only in the visible region.
Detectors for use in the visible region should be located so that their field of view will include the flame at all times. It is important to consider the size of the flame under the worst condition, especially on those burners where the firing rate is variable. Usually, if the scanner can view the low fire flame, it will be satisfactory for the high fire. The best scanner position for the larger burner is on the end of a sight pipe mounted to the burner front. This should be located as near the burner as possible, and the line of sight should be as near parallel to the burner centerline as possible.
In some burners, it is possible to mount the scanner in the blast tube provided the swirl-inducing blades at the front of the blast tube do not obstruct the view.
NOTE: The ambient temperature at the scanner should not exceed the rated temperature of the scanner.
In most cases, this will be not be a problem, but if the temperature of the scanner becomes too high, provisions should be made for connecting purge air to the scanner. This not only keeps the scanner cool, but also prevents the accumulation of dirt on the lens.
The sight pipe should be located so that the scanner is above the centerline of the flame so that the sight pipe will be angled downward to prevent dust and dirt from accumulating. The leads to the scanner should be arranged with a service loop so that the scanner can be removed for servicing. There should be clearance necessary for cleaning lenses or windows. The field of view must include enough of the flame to provide a signal with a good margin of safety so that changes in the flame and accumulation of some dust and dirt will not reduce the signal to the point where the control will shut down the burner. It is important that the field of view should not include hot refractory, which will hold in the control flame relay without a flame present.
Application of Infrared Detectors
Infrared detectors can be used with almost any single burner application. Some difficulty may be encountered with small gas pilots that have a limited infrared output. The infrared radiation passes readily through normal combustion products and dirty surfaces which makes them very dependable in a single burner application, but with multiple burners it is relatively easy for the scanner to see the adjacent burner. Discrimination then becomes a problem of limiting the field of view of each scanner in the installation. Infrared detectors are ideally suited to large oil burner installations.
When a single scanner is to be used for detecting both pilot and main flame, the field of view of the scanner must be aimed at the intersection of the pilot and the main flame. This is to insure that a satisfactory signal from the scanner indicates that there is a flame at a point, which will be sure to ignite the main flame.
If two scanners are used to monitor the pilot and the main flame, then the scanner for the main flame must be aimed so that it will not detect the pilot. The scanner for the pilot should be aimed so that it views the intersection of the pilot and the main flame exactly as above in the case of a single scanner for pilot and main flame. The field of view of the scanners should be arranged as nearly as possible to avoid sighting hot refractory since this will reduce the sensitivity of the scanner.
The ambient temperature at the scanner should not be allowed to exceed 125 degrees F. If temperature becomes a problem because of heat conducted up the sight pipe, it may be controlled by using insulating tubes or by providing purge air to the sight pipe. Providing purge air has the additional advantage of keeping dirt and dust out of the line of sight. If purge air is used or if the combustion chamber operates under higher than atmospheric pressure, it will be necessary to use a sealing union which has a pressure-tight window. The scanner sighting tube should be aimed in a downward direction so that dirt will not accumulate in the sight pipe and leads should be arranged with a service loop so that the scanner can be removed for cleaning or servicing.
Application of UV Detectors
All flames produce sufficient UV for ultraviolet detection. Even those flames completely invisible to the eye are easily seen with a UV detector. Since the combustion products of all flames are opaque in the UV region, this detector is well suited to multiple burner installations. A scanner will not detect the ultraviolet radiation from an adjacent flame because its combustion products will block the radiation. The ultra-violet detector is ideally suited to all gas burners or combination gas-oil burners as well as all multiple burners including powdered coal.
The area to scan with an ultraviolet detector is within the first 1/3 length of the flame since this is the major source of UV and also since, as pointed out earlier, the combustion products of the flame are opaque in the ultraviolet region. Do not try to scan at the outer fringes of the flame. The field of view should include the intersection of the pilot and the main flame so that a signal from the scanner will insure that the pilot is in the proper position to ignite the main flame.
Since the radiated energy from an electric spark igniter is very rich in the UV region, the field of view should be aimed so that it does not see an electric spark igniter nor any reflector that is close to the spark. Another very effective method of avoiding the signal from an electric ignition spark is to disconnect power from the ignition transformer before proving main flame. This not only eliminates the source of the interfering radiation but also insures a good stable pilot flame before turning on the main fuel valve.
The ambient temperature where the scanner is located must not be higher than 212 degrees F. Scanner cooling is possible with an insulated coupling or nipple or cooling air supplied to the sight pipe or by a sealing window. The sealing window or lens assembly prevents leakage of hot gases when the combustion chamber is operated at higher pressure than atmospheric. The window is used when normal signal level is produced by the flame lens assembly when the UV level is too low for reliable operation.
Note:   It should be kept in mind that any windows in the UV scanner assembly must be made of special UV transmitting glass or quartz. Ordinary window glass or heat resisting glass will not transmit any UV radiation.
The sight pipe should be arranged to slant downward so that dirt and dust will not collect in the sight pipe. The leads to the scanner should be arranged with a service loop so that the scanner may be removed for cleaning or servicing.
The field of view should be large enough so that the signal produced by the scanner has a margin of safety to allow for changes in the flame and some accumulation of dust on the scanner window or lens.
Applications Requiring Self Checking
Ordinary burners used for heating and industrial processes normally cycle on and off many times during a day or week and this sequence are used to test the condition of the flame sensor and its amplifier.
A Failure that causes the flame relay to energize before the fuel valve is opened will prevent the burner from being started. This important safety check does not occur if the burner operates continuously. In those applications where a burner operates 24 hours a day for periods of seven days or more, it is recommended that either the flame failure control be tested daily or some form of self-checking control should be used.
For ultraviolet sensors, this is accomplished by placing a shutter in front of the detector. When the shutter is closed, the sensor and amplifier must show a no-flame condition and when the shutter is open a flame-on condition must be detected. This checking operation is repeated every six seconds. When reading the flame signal of a self-checking UV system, it will be noted that the signal level drops when the shutter is closed. The signal actually drops to zero, but a time delay in the meter circuit prevents the meter from dropping to zero.
For infrared sensors, the natural fluctuation of the flame radiation is used to actuate a special Autocheck control unit. The fluctuations are used to charge a capacitor network that operates a flame relay. Thus, a short circuit or open circuit in the scanner or aiming the scanner at a steady light source will not energize the flame relay. Also, any failure in the flame amplifier that prevents the fluctuating signal from charging the capacitor network will de-energize the flame relay.
Optical Principles
An understanding of basic optical principles will help solve three types of application problems.
1. Avoiding an unwanted signal such as:
a.      An adjacent flame from another burner
 
b.      Electric spark igniter (UV detector)
 
c.      Hot refractory (visible light detector)
 
d.      Light (visible light detector)
 
2. Defining the exact area of a flame being monitored, such as the junction of the pilot and the main                                                                  flame. This is necessary in order to insure that detection of the pilot will show that at least the minimum size required to light the main flame.
 
 
3.                                          Increasing the amount of signal from a flame to insure reliable hold-in of the flame relay.
Problems 1 and 2 are generally handled by controlling the field of view and problem 3 is handled either by increasing the field of view controlling the line of sight or by gathering more light with a lens arrangement. The different methods that can be used are described in this section.
This section covers the basic principles of optics as they apply to the use of flame detectors that operate from radiant energy. We use the term radiant energy because we are covering not only the visible light that we can see but also light in the infrared and ultraviolet regions. This radiant energy travels at the speed of light, which is 186,000 miles per second. The radiation also travels in straight lines and will pass through transparent materials.
Anything that is obviously transparent to the eye will be transparent to detectors, which operate in the visible region. The normal material for lenses and windows in the visible region is glass. Many transparent materials, such as ordinary glass and most plastics will not transmit in the ultraviolet regions. The only two common materials used in the ultraviolet region are fused quartz and a special UV transmitting glass. In the infrared region, some materials are transparent to the infrared but not in the visible region. The normal optical material used for flame detectors in the infrared region is heat-resisting glass.
The amount of light received by a detector varies with the intensity of the light source and with the distance from the light source. Since the signal from the detector in most cases depends on the amount of light received by the detector, it will be necessary to get more light on the detector if an installation does not provide sufficient margin of signal for trouble-free operation. In the case of ultraviolet detection, as has been shown, an increased amount of light falling on the detector will not necessarily increase the signal. This is because most of the UV is generated in the first one-third of the flame and because of light absorption by the combustion products. In this case and in other cases involving discrimination or background interference, it will be necessary to control the field of view of the scanner.
The amount of light received by the detector varies with the distance from the source of light by a ratio, which depends on the optical arrangement. If the source of light is of a very small size, such as a candle or small gas pilot, and the detector is mounted at a relatively great distance, the light received by the detector will follow the "square law". The optical term for a small light source is a point source. The light from a point source radiates in all directions. Only the narrow bundle of light rays that falls on the sensitive area of the receiver will generate a signal. The light received varies inversely as the square of the distance according to the formula
Where                   L is the light intensity received
And                        d is the distance from the source to the detector
Notice that a change in position of the detector will affect the amount of light falling on the detector to a great extent; for instance, if d is doubled, the light received will be ¼ the intensity of the original light at the original distance.
The detector is frequently mounted in a sight pipe, but in this case this does not affect the signal received by the detector. Neither the length of the sight pipe nor its diameter affects the signal received. The "line of sight" is the direction in which the scanner is looking, or the centerline of its field of view.
The field of view of the scanner may be considered either the area, which the scanner can see, or the solid angle of the unobstructed viewing of the detector. The end of the opening of the sight pipe limits the field of view so that the detector sees everything within this solid angle and nothing outside of the field of view. An aperture that limits the field of view is called the field stop.
If the detector were tilted so that the light falling on the sensitive surface arrives at an angle other than perpendicular, the amount of light received will decrease. If the angle of tilt is measured as the angle away from perpendicular, it will be found that the amount of light received is proportional to the cosine of the angle. Therefore, when the angle becomes 90 degrees, the amount of light seen by the detector will be zero.
In systems without lenses, there are two methods for increasing the amount of light on a detector or the signal from it. These are: 1) Decrease the length of the sight pipe and 2) Increase the field of view.
Flame detectors operating from radiant energy can operate either intentionally or accidentally from reflected light, such as polished metal or a mirror.
Rough surfaces are poor reflectors because they absorb some of the light and the rest is reflected in all different directions. The reflecting ability of materials varies in each of the spectral ranges. Infrared light reflects better from most materials than visible light, and the visible in turn reflects better than ultraviolet. In the case of ultraviolet, the reflection of an unwanted source such as an electrical spark is very poor, but since the spark is such a powerful light source in the ultraviolet region, even a small percentage of the reflected light can cause an unwanted signal. Reflecting devices that are intended to help pick up a difficult signal should be considered under the worst operating condition. This would normally be with accumulation of dust and dirt and oily films, which will decrease the efficiency of reflection.
Lenses are frequently used with optical scanners to narrow the field of view or sometimes to increase the sensitivity of a scanner. A lens would, of course, have to be constructed from a material that would transmit the light intended.
Accessories
Unusual operating conditions may require special scanner mounting accessories to meet the requirements of accurate scanning, avoiding excessive operating temperatures or preventing a scanner from seeing a flame except at the proper time. Heat insulators are available for infrared and ultraviolet scanners.
In some installations, it is difficult to aim the scanner at the junction of the pilot and main flame burners because the mounting pipe for the scanner must go through the front burner plate at an odd angle which is not available in the usually available pipe fittings. Swivel mounting adapters are available so that the scanner can be mounted at the approximate angle. The scanner can be precisely aimed by means of a ball joint swivel. The swivel mounting can then be welded into position to insure that the aiming will not be changed.
When powered and active, Honeywell UV scanners glow orange. Fireye UV scanners have a blue glow.
One cannot mix control and scanner manufacturers when using infrared or UV.  Honeywell controls take Honeywell scanners, etc.
When upgrading a Fireye mechanical control that used an infrared detector to the D or E series of solid-state controls, the infrared cell should be changed to the new "green" cell, the 4-263-1. The 4-263 is more sensitive than the 4-128 cell.
Every effort should be made to keep scanner wire runs as short as possible and to run the wires all alone, not in conduits with other wires. In the typical boiler room, there is an abundance of electrical noise due to relays, motors, transformers, etc. This noise will interfere with the scanner signals. Every effort should be made to shield the scanner wires to avoid noise-related problems. Often the manufacturer’s literature will say it is OK to run scanner wires with other wires, but why take the chance of inducing unwanted noise? One major source of electric noise is the ignition transformer. Keep scanner wires as far away as possible from ignition transformers.
The signal strength that a scanner is sending can be measured in various ways. Depending on the manufacturer, signals can be measured in micro-amps, DC voltage, or an arbitrary numbering system.
Honeywell’s mechanical controls have signal currents in the microampere range. Therefore, a micro ammeter will be required to measure the signals. A steady, minimum signal of 2 microamperes or more is required. The new solid-state 7800 series now use DC voltage as the signal strength measure, with 1.25 VDC the minimum. Fireye uses volts DC for signal strength measure, varying according to the control and scanner application. So many variations exist in Fireye that it is best to refer to Fireye literature for minimum required signal strengths. The E series uses an arbitrary numbering system from 1 to 100 to represent signal strength and displays the signal on the message center. 10 is minimum, with 20 to 80 a normal signal for the E series.
One of the most important measurements to be taken when servicing or troubleshooting flame guard controls is the signal strength. In all cases, it should be a steady non-fluctuating reading, as high as can be obtained. Minimum recommended signals are fine for testing such as minimum pilot test or checking controls on testers, but minimum flame signals should be considered unacceptable for actual operation. Any fluctuation of a minimum signal probably will cause nuisance shutdowns, and every effort should be made to increase signal strengths over the minimum requiremenIn Info-Tec No. 1, only flame rod scanners were covered in any detail. Since scanners cause many problems, this Info-Tec will deal with the rest of the flame scanners. In order to apply the most suitable flame detector to a combustion chamber, it is necessary to know a little about the characteristics of flames.

All flames, whether they are from burning wood, oil, gas, coal, or even a candle, produce heat and light (radiant energy), and during the burning process, ions are formed. The only difference between the various flames is in the magnitude of each of these quantities. Some flames are blue, some are yellow, and some produce more heat than others do, but basically they are all the same. For instance, the atomized oil burner produces a characteristic yellow flame, but at the same time it is also radiating light in the infrared region and the ultraviolet region. You cannot see the ultraviolet or infrared light, but the warmth you feel on your face is a result of the infrared radiation. The ultraviolet radiation can only be detected by an ultraviolet sensitive photocell. The pale blue flame of a gas burner is familiar to us, and it is apparent how the visible light is present in both this and the oil flame. Both flames differ in brightness as seen by the eye as well as in color and in the amount of infrared and ultraviolet radiation they produce.

The table below shows relative amounts of radiation in the visible, infrared and ultraviolet regions for fuels normally used.

  Radiant Energy
Fuel Infrared Visible Ultraviolet
Oil (Atomized) High High Medium
Gas (Premix with Air) Low Low High
Gas (Without Premix) Medium Medium Medium
Coal (Pulverized) High High Mediu

 
 
The radiation from a flame in the visible region is easy to visualize because it is visible to the eye. Whenever a human eye can see a flame, it is radiating visible light. The infrared radiation from an oil flame is very similar in pattern to the visible radiation except that it will be present slightly beyond the flame envelope. In a gas flame, the infrared radiation will be strongest in the last two-thirds of the flame. The area of the flame that radiates ultraviolet is much more limited than either the visible or infrared areas.

Practically all the ultraviolet radiation comes from the first one-third of the flame for all fuels that are premixed with air. For atmospheric flames, such as a candle or raw gas burning at the end of a pipe, the area of the flame radiating ultraviolet will be the outside surface of the flame.

The radiation from a flame constantly varies in amplitude at all times. That is, it is momentarily changing from brighter to dimmer to brighter and so on. We have all seen how some flames flicker at a low frequency, but even those flames that seem to be perfectly steady to the eye flicker. A photoelectric detector can pick up this flicker. The rate of flicker is usually too fast to be seen by the eye. The frequency of modulation depends to some extent on the type of flame, but it has been found by a large number of measurements that a substantial amount of all flames flicker with rates ranging from 10 to 50 Hz.

The most familiar property of a flame is, of course, the high temperature of the gases within the flame and in the gas products of the flame. The maximum temperature of the flame depends on the fuel being burned and the availability of oxygen.

Flame failure control can utilize another characteristic of flames; the light transmission of the flame’s combustion products. When a flame is burning without enough air for clean burning, there will be soot (free carbon) produced which causes a smoky flame and smoke in the combustion products. When the smoke is bad enough, it will be completely opaque in the visible region. In the infrared region, the loss is much less. When a flame is clean burning or air rich, the combustion products are transparent in both the infrared and visible regions. In the ultraviolet region, the combustion products of any flame, rich or lean, are opaque. These differences are important in applications involving more than one burner in a combustion chamber.

The following characteristics are those used for detecting a flame other than temperature.
• Visible light radiation
• Infrared light radiation
• Ultraviolet light radiation
• Ionized gas molecules in the flame

Visible Light Detectors

Detection of flame by seeing the visible light, is, of course, the way man has detected flame since it was first observed. Since the radiation travels at the speed of light, the detection of flame by this method is not limited in speed. There are two standard detectors used for sensing the flame in the visible region, the oldest of which is the photo-emissive photocell. This device is a glass-enclosed structure, which usually has been completely evacuated. There are two elements within the envelope, a thin metal plate that is the cathode, and a collector assembly, which is the anode. When light strikes the cathode, electrons are emitted which are drawn to the anode. The current in this type of tube is very small and requires considerable amplification before it can operate a relay.

The other type of detector in the visible region is usually a smaller device that is constructed from an insulating plate covered with a deposit of cadmium sulfide that is mounted in a protective enclosure with a glass window. This device acts like a resistor, which is sensitive to light. When the cadmium sulfide detector is in the dark, the resistance of the element will be many megohms. When it is exposed to a light from a flame, the resistance will drop to a few thousand ohms; and, in most devices, enough current will flow to energize a relay without further amplification. Flame detectors that operate in the visible region will also operate from other light in the visible region such as daylight or light from an incandescent of fluorescent lamp. It is therefore necessary to make sure that they are used only in locations where they cannot see other sources of light. Also, they will respond to the visual light signal from hot refractory in the combustion chamber.

Infrared Light Detectors

All flames radiate infrared energy, and since this radiation travels at the speed of light, the detection of flames by this method is not limited in speed. There is only one standard detector for use in the infrared region, and this is the lead sulfide detector. This device is constructed by depositing a thin film of semi-conductor material on an insulating surface. Two leads are attached to opposite ends of the film. The device operates like a resistor whose value of resistance changes with the amount of infrared energy falling on the surface of the detector. These devices have dark resistance values from a few thousand ohms to a few megohms. When these detectors are exposed to light, they decrease in resistance depending on the intensity of the light. Their percentage change in resistance is not as great as in the cadmium sulfide detectors since the resistance of the element in the dark is much lower than the cadmium sulfide detector. This requires a different type of amplifier for infrared detectors especially since the signal from a hot refractory is strong in the infrared region. An example of the infrared scanner is Type 48PT1 and 48PT2.

Ultraviolet Light Detectors

Ultraviolet radiation from a flame travels at the speed of light, therefore, the detection of flame by this method is not limited in speed.

The newer type of ultraviolet detector is constructed of two similar electrodes made from very clean wire of tungsten or molybdenum. These operate from UV light falling on the wire electrodes, which starts an avalanche of electrons. These tubes quench very fast. The fast quenching of these tubes allows them to be used on 50 or 60 cycle supply voltage, which provides automatic quenching.

Application of Photoelectric or Photocell Detectors in the Visible Region

Operation in the visible spectrum is limited to single burner applications because it is very difficult to differentiate between the burner being monitored and other burners which may be visible to the scanner since this would give a false indication of flame. The normal application is for the smaller oil burners where the field of view is limited to the combustion flame itself. A blue gas flame will not operate any of the detectors that operate only in the visible region.

Detectors for use in the visible region should be located so that their field of view will include the flame at all times. It is important to consider the size of the flame under the worst condition, especially on those burners where the firing rate is variable. Usually, if the scanner can view the low fire flame, it will be satisfactory for the high fire. The best scanner position for the larger burner is on the end of a sight pipe mounted to the burner front. This should be located as near the burner as possible, and the line of sight should be as near parallel to the burner centerline as possible.

In some burners, it is possible to mount the scanner in the blast tube provided the swirl-inducing blades at the front of the blast tube do not obstruct the view.

NOTE: The ambient temperature at the scanner should not exceed the rated temperature of the scanner.

In most cases, this will be not be a problem, but if the temperature of the scanner becomes too high, provisions should be made for connecting purge air to the scanner. This not only keeps the scanner cool, but also prevents the accumulation of dirt on the lens.

The sight pipe should be located so that the scanner is above the centerline of the flame so that the sight pipe will be angled downward to prevent dust and dirt from accumulating. The leads to the scanner should be arranged with a service loop so that the scanner can be removed for servicing. There should be clearance necessary for cleaning lenses or windows. The field of view must include enough of the flame to provide a signal with a good margin of safety so that changes in the flame and accumulation of some dust and dirt will not reduce the signal to the point where the control will shut down the burner. It is important that the field of view should not include hot refractory, which will hold in the control flame relay without a flame present.

Application of Infrared Detectors

Infrared detectors can be used with almost any single burner application. Some difficulty may be encountered with small gas pilots that have a limited infrared output. The infrared radiation passes readily through normal combustion products and dirty surfaces which makes them very dependable in a single burner application, but with multiple burners it is relatively easy for the scanner to see the adjacent burner. Discrimination then becomes a problem of limiting the field of view of each scanner in the installation. Infrared detectors are ideally suited to large oil burner installations.

When a single scanner is to be used for detecting both pilot and main flame, the field of view of the scanner must be aimed at the intersection of the pilot and the main flame. This is to insure that a satisfactory signal from the scanner indicates that there is a flame at a point, which will be sure to ignite the main flame.

If two scanners are used to monitor the pilot and the main flame, then the scanner for the main flame must be aimed so that it will not detect the pilot. The scanner for the pilot should be aimed so that it views the intersection of the pilot and the main flame exactly as above in the case of a single scanner for pilot and main flame. The field of view of the scanners should be arranged as nearly as possible to avoid sighting hot refractory since this will reduce the sensitivity of the scanner.

The ambient temperature at the scanner should not be allowed to exceed 125 degrees F. If temperature becomes a problem because of heat conducted up the sight pipe, it may be controlled by using insulating tubes or by providing purge air to the sight pipe. Providing purge air has the additional advantage of keeping dirt and dust out of the line of sight. If purge air is used or if the combustion chamber operates under higher than atmospheric pressure, it will be necessary to use a sealing union which has a pressure-tight window. The scanner sighting tube should be aimed in a downward direction so that dirt will not accumulate in the sight pipe and leads should be arranged with a service loop so that the scanner can be removed for cleaning or servicing.

Application of UV Detectors

All flames produce sufficient UV for ultraviolet detection. Even those flames completely invisible to the eye are easily seen with a UV detector. Since the combustion products of all flames are opaque in the UV region, this detector is well suited to multiple burner installations. A scanner will not detect the ultraviolet radiation from an adjacent flame because its combustion products will block the radiation. The ultra-violet detector is ideally suited to all gas burners or combination gas-oil burners as well as all multiple burners including powdered coal.

The area to scan with an ultraviolet detector is within the first 1/3 length of the flame since this is the major source of UV and also since, as pointed out earlier, the combustion products of the flame are opaque in the ultraviolet region. Do not try to scan at the outer fringes of the flame. The field of view should include the intersection of the pilot and the main flame so that a signal from the scanner will insure that the pilot is in the proper position to ignite the main flame.

Since the radiated energy from an electric spark igniter is very rich in the UV region, the field of view should be aimed so that it does not see an electric spark igniter nor any reflector that is close to the spark. Another very effective method of avoiding the signal from an electric ignition spark is to disconnect power from the ignition transformer before proving main flame. This not only eliminates the source of the interfering radiation but also insures a good stable pilot flame before turning on the main fuel valve.

The ambient temperature where the scanner is located must not be higher than 212 degrees F. Scanner cooling is possible with an insulated coupling or nipple or cooling air supplied to the sight pipe or by a sealing window. The sealing window or lens assembly prevents leakage of hot gases when the combustion chamber is operated at higher pressure than atmospheric. The window is used when normal signal level is produced by the flame lens assembly when the UV level is too low for reliable operation.

Note:   It should be kept in mind that any windows in the UV scanner assembly must be made of special UV transmitting glass or quartz. Ordinary window glass or heat resisting glass will not transmit any UV radiation.

The sight pipe should be arranged to slant downward so that dirt and dust will not collect in the sight pipe. The leads to the scanner should be arranged with a service loop so that the scanner may be removed for cleaning or servicing.

The field of view should be large enough so that the signal produced by the scanner has a margin of safety to allow for changes in the flame and some accumulation of dust on the scanner window or lens.

Applications Requiring Self Checking

Ordinary burners used for heating and industrial processes normally cycle on and off many times during a day or week and this sequence are used to test the condition of the flame sensor and its amplifier.

A Failure that causes the flame relay to energize before the fuel valve is opened will prevent the burner from being started. This important safety check does not occur if the burner operates continuously. In those applications where a burner operates 24 hours a day for periods of seven days or more, it is recommended that either the flame failure control be tested daily or some form of self-checking control should be used.

For ultraviolet sensors, this is accomplished by placing a shutter in front of the detector. When the shutter is closed, the sensor and amplifier must show a no-flame condition and when the shutter is open a flame-on condition must be detected. This checking operation is repeated every six seconds. When reading the flame signal of a self-checking UV system, it will be noted that the signal level drops when the shutter is closed. The signal actually drops to zero, but a time delay in the meter circuit prevents the meter from dropping to zero.

For infrared sensors, the natural fluctuation of the flame radiation is used to actuate a special Autocheck control unit. The fluctuations are used to charge a capacitor network that operates a flame relay. Thus, a short circuit or open circuit in the scanner or aiming the scanner at a steady light source will not energize the flame relay. Also, any failure in the flame amplifier that prevents the fluctuating signal from charging the capacitor network will de-energize the flame relay.

Optical Principles

An understanding of basic optical principles will help solve three types of application problems.

 

1. Avoiding an unwanted signal such as:

a.      An adjacent flame from another burner
b.      Electric spark igniter (UV detector)
c.      Hot refractory (visible light detector)
d.      Light (visible light detector)
 
2. Defining the exact area of a flame being monitored, such as the junction of the pilot and the main flame. This is necessary in order to insure that detection of the pilot will show that at least the minimum size required to light the main flame.
 
 
3. Increasing the amount of signal from a flame to insure reliable hold-in of the flame relay.

Problems 1 and 2 are generally handled by controlling the field of view and problem 3 is handled either by increasing the field of view controlling the line of sight or by gathering more light with a lens arrangement. The different methods that can be used are described in this section.

This section covers the basic principles of optics as they apply to the use of flame detectors that operate from radiant energy. We use the term radiant energy because we are covering not only the visible light that we can see but also light in the infrared and ultraviolet regions. This radiant energy travels at the speed of light, which is 186,000 miles per second. The radiation also travels in straight lines and will pass through transparent materials.

Anything that is obviously transparent to the eye will be transparent to detectors, which operate in the visible region. The normal material for lenses and windows in the visible region is glass. Many transparent materials, such as ordinary glass and most plastics will not transmit in the ultraviolet regions. The only two common materials used in the ultraviolet region are fused quartz and a special UV transmitting glass. In the infrared region, some materials are transparent to the infrared but not in the visible region. The normal optical material used for flame detectors in the infrared region is heat-resisting glass.

The amount of light received by a detector varies with the intensity of the light source and with the distance from the light source. Since the signal from the detector in most cases depends on the amount of light received by the detector, it will be necessary to get more light on the detector if an installation does not provide sufficient margin of signal for trouble-free operation. In the case of ultraviolet detection, as has been shown, an increased amount of light falling on the detector will not necessarily increase the signal. This is because most of the UV is generated in the first one-third of the flame and because of light absorption by the combustion products. In this case and in other cases involving discrimination or background interference, it will be necessary to control the field of view of the scanner.

The amount of light received by the detector varies with the distance from the source of light by a ratio, which depends on the optical arrangement. If the source of light is of a very small size, such as a candle or small gas pilot, and the detector is mounted at a relatively great distance, the light received by the detector will follow the "square law". The optical term for a small light source is a point source. The light from a point source radiates in all directions. Only the narrow bundle of light rays that falls on the sensitive area of the receiver will generate a signal. The light received varies inversely as the square of the distance according to the formula:

 image002_1.gif

Where L is the light intensity received 

And d is the distance from the source to the detector

Notice that a change in position of the detector will affect the amount of light falling on the detector to a great extent; for instance, if d is doubled, the light received will be ¼ the intensity of the original light at the original distance.

The detector is frequently mounted in a sight pipe, but in this case this does not affect the signal received by the detector. Neither the length of the sight pipe nor its diameter affects the signal received. The "line of sight" is the direction in which the scanner is looking, or the centerline of its field of view.

The field of view of the scanner may be considered either the area, which the scanner can see, or the solid angle of the unobstructed viewing of the detector. The end of the opening of the sight pipe limits the field of view so that the detector sees everything within this solid angle and nothing outside of the field of view. An aperture that limits the field of view is called the field stop.

If the detector were tilted so that the light falling on the sensitive surface arrives at an angle other than perpendicular, the amount of light received will decrease. If the angle of tilt is measured as the angle away from perpendicular, it will be found that the amount of light received is proportional to the cosine of the angle. Therefore, when the angle becomes 90 degrees, the amount of light seen by the detector will be zero.

In systems without lenses, there are two methods for increasing the amount of light on a detector or the signal from it. These are: 1) Decrease the length of the sight pipe and 2) Increase the field of view.

Flame detectors operating from radiant energy can operate either intentionally or accidentally from reflected light, such as polished metal or a mirror.

Rough surfaces are poor reflectors because they absorb some of the light and the rest is reflected in all different directions. The reflecting ability of materials varies in each of the spectral ranges. Infrared light reflects better from most materials than visible light, and the visible in turn reflects better than ultraviolet. In the case of ultraviolet, the reflection of an unwanted source such as an electrical spark is very poor, but since the spark is such a powerful light source in the ultraviolet region, even a small percentage of the reflected light can cause an unwanted signal. Reflecting devices that are intended to help pick up a difficult signal should be considered under the worst operating condition. This would normally be with accumulation of dust and dirt and oily films, which will decrease the efficiency of reflection.

Lenses are frequently used with optical scanners to narrow the field of view or sometimes to increase the sensitivity of a scanner. A lens would, of course, have to be constructed from a material that would transmit the light intended.

Accessories

Unusual operating conditions may require special scanner mounting accessories to meet the requirements of accurate scanning, avoiding excessive operating temperatures or preventing a scanner from seeing a flame except at the proper time. Heat insulators are available for infrared and ultraviolet scanners.

In some installations, it is difficult to aim the scanner at the junction of the pilot and main flame burners because the mounting pipe for the scanner must go through the front burner plate at an odd angle which is not available in the usually available pipe fittings. Swivel mounting adapters are available so that the scanner can be mounted at the approximate angle. The scanner can be precisely aimed by means of a ball joint swivel. The swivel mounting can then be welded into position to insure that the aiming will not be changed.

When powered and active, Honeywell UV scanners glow orange. Fireye UV scanners have a blue glow.

One cannot mix control and scanner manufacturers when using infrared or UV.  Honeywell controls take Honeywell scanners, etc.

When upgrading a Fireye mechanical control that used an infrared detector to the D or E series of solid-state controls, the infrared cell should be changed to the new "green" cell, the 4-263-1. The 4-263 is more sensitive than the 4-128 cell.

Every effort should be made to keep scanner wire runs as short as possible and to run the wires all alone, not in conduits with other wires. In the typical boiler room, there is an abundance of electrical noise due to relays, motors, transformers, etc. This noise will interfere with the scanner signals. Every effort should be made to shield the scanner wires to avoid noise-related problems. Often the manufacturer’s literature will say it is OK to run scanner wires with other wires, but why take the chance of inducing unwanted noise? One major source of electric noise is the ignition transformer. Keep scanner wires as far away as possible from ignition transformers.

The signal strength that a scanner is sending can be measured in various ways. Depending on the manufacturer, signals can be measured in micro-amps, DC voltage, or an arbitrary numbering system.

Honeywell’s mechanical controls have signal currents in the microampere range. Therefore, a micro ammeter will be required to measure the signals. A steady, minimum signal of 2 microamperes or more is required. The new solid-state 7800 series now use DC voltage as the signal strength measure, with 1.25 VDC the minimum. Fireye uses volts DC for signal strength measure, varying according to the control and scanner application. So many variations exist in Fireye that it is best to refer to Fireye literature for minimum required signal strengths. The E series uses an arbitrary numbering system from 1 to 100 to represent signal strength and displays the signal on the message center. 10 is minimum, with 20 to 80 a normal signal for the E series.

One of the most important measurements to be taken when servicing or troubleshooting flame guard controls is the signal strength. In all cases, it should be a steady non-fluctuating reading, as high as can be obtained. Minimum recommended signals are fine for testing such as minimum pilot test or checking controls on testers, but minimum flame signals should be considered unacceptable for actual operation. Any fluctuation of a minimum signal probably will cause nuisance shutdowns, and every effort should be made to increase signal strengths over the minimum requirements.

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