MSA Guide to Flame Detection

8/10/2019 MSA Guide to Flame Detection

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INNOVATION INFLAME DETECTION

Industries involved inmanufacturing, processing,

storing or transportation offlammable material are constantlyin need of reliable and fastresponse fire detection systems.It is evident that the smaller thefire, when detected, the easier itis to extinguish. In this respect,

fire detection systems, especiallyoptical flame detectors, are themost powerful apparatus in firefighting due to their ability toremotely detect a small fire froma long distance.

It may seem simple and straightforward to design a sensitiveoptical flame detector by utilizingUltraviolet (UV), Infrared (IR) or acombination of UV/IR sensors.However, these detectors often

operate in industrial environmentswhich contain many radiationsources that could impair detectorperformance and even causefalse alarms. Moreover, manyapplications require flamedetectors to withstand harshenvironmental conditions and stillmaintain their entire envelope ofperformance.

Most applications for optical flamedetectors are "High Risk - High

Value", that require detectors tobe designed, qualified andmanufactured according tosophisticated and advancedmethods to ensure the installedproduct is reliable.

These requirements haveaccelerated the technology raceto research and develop newapproaches to fire detection

employing scientific disciplinessuch as physical chemistry,physics, electro-optics,

electromagnetic physics,electromagnetic spectral analysisand thermodynamics. This Guidedescribes an innovative approachto flame spectral analysis that has

led to the development of aunique multipurpose Infrared-type(IR) flame detector.

BACKGROUND FORFIRE DETECTION

Afire scenario can be analyzedby several approaches,

depending on the monitoredparameters such as fuel

consumption, oxygen/airconsumption, heat evolving orchemical reactions taking place inthe vaporized fuel zone. Figure 1describes the anatomy of a

hydrocarbon fire where the

vaporized fuel is dispersed in thesurrounding atmosphere where itimmediately reacts with oxygenand the flame chemical chain

reaction takes place to give offgaseous products such as CO2,H2O, HC - (unburnedhydrocarbon molecules), C(Soot), CO. Fire detection

technologies throughout the yearshave relied on these factors forthe developing detection devices.

OPTICAL FLAMEDETECTION

The energy radiated from a fireserves as a major factor in it's

detection analysis. 30% - 40% ofthis energy is dissipated in theform of electromagnetic radiation

at various spectral ranges, such asUltraviolet (UV), visible, infrared

(IR) bands. Figure 2 schematicallyshows a typical hydrocarbon fireemission spectrum where UV andIR spectral bands are "highlighted"to show the spectral ranges thatare usually selected for existingflame detectors.

The flame radiation spectralpattern, being unique, allowsseveral spectral ranges to beemployed in the various detectiondevices. Flame detectors usuallyutilize optical sensors working atspecific spectral ranges (usuallynarrow band) that record theincoming radiation at the selectedwavelengths. The signals recordedby the sensor are then analyzedaccording to a predeterminedtechnique that includes one ormore of the following:

1. Flickering frequencyanalysis

2. Threshold energy signal

comparison

3. Mathematical correlationbetween several signals

4. Comparison techniques(Ration, AND gate, OR gatetechniques)

5. Correlation to memorizedspectral analysis

Detection devices using several ofthe above-mentioned techniquespromise to be most reliable with

respect to detection sensitivityversus immunity to false alarms.

Four major families of opticaldetectors have emerged in thelast 20 years.

1. UV Detectors

2. IR Detectors

3. UV/IR Detectors

4. IR/IR Detectors

MSA’s Guide to Selectingthe Right Flame Detectorfor Your Application



Guide to Selecting the Right Flame Detector for Your Application

Anatomy of a FirAnatomy of a Firee

Figure 1.



Each of these detector familiesuses one or several of theparameter analyses listedpreviously, and employ the mostadvanced optical sensors at thespecific spectral wavelengths.

However, each family of detectorsis recommended for use only inspecific applications. Theseapplications are usuallydetermined by evaluating to whatextent false alarms caused byenvironmental stimulus couldcreate major problems.

UV FLAMEDETECTION

The UV spectral band, because

of shortwave characteristics,is absorbed in the surroundingatmosphere by air, smoke, dust,gases and various organicmaterials. Hence UV radiationdispersed in the atmosphere,especially at wavelengths shorterthan 300 nm (the solar blindspectral band), is being absorbedby the surrounding atmosphereand will not create false alarms onthese flame detectors. UVdetectors based on this technology

are detecting flames at high speed(3-4 milliseconds) due to the UVhigh energy radiation emitted byfires and explosions at the instantof their ignition.

The occurrence of random UVradiation from sources such aslightning, arc welding, radiationand solar radiation (which are notabsorbed by the atmosphere dueto holes in the ozone layer and

solar bursts) cause false alarmsin UV detectors.

IR FLAMEDETECTION

Infrared radiation is present inmost flames (as can be seen

from Fig 2). The flametemperature, its mass of hotgases (fire products), emit aspecific spectral pattern that canbe easily reorganized by

employing IR sensor technology.

However, the flames are not theonly source of IR radiation andany hot surface such as ovens,lamps, incandescent halogenlamps, furnaces, solar radiation,emit IR radiation which coincideswith the flame IR radiationwavelengths.

DUAL WAVELENGTHDETECTION

In order to discern the flames'"spectral signature" from other

IR source spectral signatures,various parameter analysis and

mathematical techniques areemployed. The most accepted areflickering analysis and narrowband IR threshold signalsprocessed in the IR 4.1um-4.6umwavelengths. These IR detectors

are still subject to false alarmscaused by blackbody radiation(heaters, incandescent lamps,halogen lamps plus others).

In order to minimize or eliminatefalse alarms, dual wavelengthtechnology has been adopted foroptical flame detection.

This dual wavelength technologyhas two major branches:

1. UV/IR Spectral Bands

2. IR/IR Spectral Bands

In recent years dual spectraldetection was considered themost advanced method to copewith false alarms.

UV/IR FLAMEDETECTION

The dual spectrum UV/IRtechnology employs a solar

blind UV sensor, with a high signal

to noise ratio and a narrow bandIR sensor. The UV sensor itself isa good fire detector but is easilyactivated by alarm stimuli such aswelding, lightning, Xrays and solar


Figure 2.



spikes. To prevent false alarmscaused by these sources, the IRsensing channel was added. TheIR spectral channel has a spectralsignature characteristic to fire inaddition to the fire's UV flame

detector spectral signature andtogether serve as a reliabledetector for most mid-rangeapplications. Even this advancedtechnology has limitations, sinceeach type of fire has its ownspecific ratio of UV to IR output.

For example, a hydrogen flamegenerates a high amount of UVradiation with very little IR, while acoal fire generates little UVradiation and a high amount of IRradiation. Since the dual UV/IRdetector combines both signals toan “AND-gate", there could be afire that will not be detected. Toensure the reliability of the firesignal, a discriminating circuitcompares the UV radiationthreshold signal, the IR thresholdsignal, and their ratio, as well astheir flickering mode.

The fire alarm is confirmed onlywhen all parameters satisfy thedetection mathematical algorithm.

In industrial environments thesources for false alarms arevariant, including UV radiatingsources such as: welding,electrical arcs, lightning (highvoltage coronas), torches (in thepetrochemical industry), solarspikes, and IR radiating sources(heaters, incandescent lamps,halogen lamps, etc). Since thesefalse alarms affect both UV and IRchannels, certain scenarios mayoccur where a false fire stimulus is

present, i.e.: when an IR source(sunlight) and a UV source(welding) are present at once.

In certain detectors, a seriousproblem may occur when a strongUV source (welding) is presentand a fire ignites. The strong UVsignal blocks the detector's logicfrom comparison with the IR

channel, thus impairing its abilityto detect a fire.

Further discrimination relating tothe percentage of time each signalis present using "windows" wherethe UV signals are countedcontinuously, enable elimination ofstrong signals not emitted byactual fires. Comparabletechniques using "AND-gate"methods, process the UV and IRsignals received by both sensorsin the detector, thus ensuring theaccuracy of these detectors.

IR/IR FLAMEDETECTION

Another dual wavelengthtechnology combines twonarrow spectral ranges in the nearIR spectral band. Since thehydrocarbon flames emit energy ofa continuous nature in the near IR(0.9um - 3.0um) and a uniquepeak at the 4.3um -4.5um (causedby the hot CO2 fire product) thesefeatures are the "heart" of mostdual IR detectors. Common dualIR flame detectors employ twonarrow bands 0.9um and 4.3um,

for fire signal analysis.

Another approach to dual IRdetection technology hasemerged in recent years, where afire's main spectral characteristicfeature at 4.3um -4.5um isanalyzed thoroughly.

The basis to this analysis is the"differential spectral" approachwhere two spectral ranges areanalyzed. One spectral range isemitted strongly by the fire while

the second spectral range isemitted weakly by the surrounding.The ratio between these twosignals provides a substantialmathematical tool for fire signalprocessing. This type of IRdetector senses the radiation atthese two channels and processesthe input signals based on thefollowing parameters:

• Flickering analysis

• Radiation intensityabove a certain threshold

• The ratio between both signalsreceived at the 2 sensors

Since most of these dual IRdetectors use the 4.3um sensor astheir main channel for firerecognition (where the CO2emission peak exists), they sufferfrom atmospheric attenuation,especially on long range detectionapplications.

ADVANCEDTECHNIQUES FOR

FLAME SPECTRALANALYSIS

Each of the previouslydescribed detection methods

has drawbacks. It is evident thatclassic fire analysis methods areinsufficient for some applications.The development of electro-optictechnology enables advancedtechniques for performing deeperand more comprehensive spectralanalysis.

The spectrum of flame radiationmeasured by the detector isinfluenced by the distancebetween the detector and the fireand by the concentration of theCO2 gas in the atmosphere.

Two factors limit the detectionrange of dual IR detectors:

1. The fires radiation intensitystrongly decreases as thedistance increases around

the 3.3um peak. The inputsignal received by the sensoris very weak (the more CO2in the atmosphere, the higherabsorption of thiswavelength and the lower thesignal received). This couldbe omitted and notrecognized as fire by dualIR/IR type detectors.




2. The ratio between the 4.3umspectral band and thesecond IR channel (thebackground 4.9um spectralband), approaches equality(1:1) and ceases to be

typical to the ratio existingin fires.

Once the ratio approaches 1:1, thealgorithm processing the firesignals gives a no-fire signal, eventhough a fire may occur at thatvery moment.

The first limiting factor may bereduced by choosing a sensor witha wide band spectral range. Thiswill enhance the input signal, but

will not solve the problemdiscussed in the second limitingfactor (2). The ratio between thetwo IR channels becomes equalfor a long distance fire in the caseof high concentration of CO2 in theatmosphere. This criteria, whenemployed in IR/IR fire detectors,makes the distinction betweenflames and false alarm sources(electrical heaters) virtuallyimpossible.

To address both limitations, theuse of a narrow-band, spectralfilter is suggested. The use of thisnarrow spectral band sensor inaddition to the second IR channel,provides a typical fire ratio atlonger distances. Once the properspectral band is selected, thelimiting flame detector factor forthe detection range is no longerthe atmospheric attenuation, butthe sensitivity of the specificsensor.

If the input signal is not significantlygreater than the internal noise ofthe sensor, the ratio and measuredintensity are not reliable as fireindicators. IR sensors currentlyavailable on the market have lowratios between input signal andinternal noise.

For these sensors, the signal from

a fire at a distance greater than afew meters is not significantlydistinguishable from their internalnoise,requiring sophisticatedmathematical techniques for propersignal recognition.

In summary, the dual IR firedetection technology, althoughsuitable in some indoor and limitedoutdoor short range applications,have serious limitations thatprevent the application of thistechnology to long-range firedetection.

To resolve some of these limitingfactors, a unique approach hasbeen introduced into the firedetector market. Its scientific

background can be described asfollows:

Most fire radiation is due to hotCO2 and H2O molecules that arethe main combustion products. Inthis novel approach, the fire isconsidered an alternating infraredsource that emits strongly at theCO2 emission band and weakly atthe background emission band.Most of the IR sources (consideredIR false-alarm stimuli) including

sun, incandescent and halogenlamps, arc discharge, electricalheaters, etc., do not possess thisunique spectral feature.

Three spectral wavelength bandshave been selected for this flamedetection technique:

The mathematical relation betweenthe three (or more) sensors,detecting the specific wavelengthsof IR radiation, is typical to each IR

source for distinguishing betweena fire scenario and interfering IRstimuli. Each IR source has its ownIR spectral signature and gives adifferent signal ratio at the threesources.

Taking into consideration the ratiobetween the three IR channels, afire can be singularly detected withalmost no false alarms. Further

improvement of this IR analysis

technique enables the accurate

detection of a hidden fire

(smoldering fire) where the

radiating flames are hidden, but

the hot mass of CO2 gases are

emitted and therefore detected.

Using correlation techniques where

each IR channel is auto-correlated

to a pre-determined value and

further, (using the ratio between

the specific IR channels),

discrimination between fire and

false-alarm stimuli is possible.

CONCLUSIONS

Optical flame detectors have

existed for over 20 years.

Through the years there have

been developments of these

detectors by combining various

sensors and employing new logic

and mathematical techniques.

MSA’s FlameGard ® IR3 Flame

Detector is the new generation in

flame detection, offering high

sensitivity as well as immunity to

false alarms.

With the introduction of the

FlameGard IR3 Flame Detector

with its extended range, fewer

detectors are required to cover an

area.

For example, when laying out

detection for an oil or gas loading

facility in the past, 4 or 5

detectors per bay were required.

With the FlameGard IR3 detector,

only 2 to 3 detectors per bay

would be required. This results insubstantial savings on the cost of

equipment, while at the same

time providing the same

protection.

In the case of train loading, the

number of detectors required

could also be cut substantially

with the new FlameGard IR3.





Maximum Distances from Detector to Potential Fires

Type of Fire 30FT. 40FT. 50FT. 100FT. 200FT.

1 SQ.FT. Gasoline All models All models All models U; I I1 SQ.FT. Diesel All models U; I U; I I I

11 SQ.FT. n-Heptane All models All models All models U; I I1 SQ.FT. Alcohol All models U; I U; I I I4 SQ.FT. JP4/JP8 All models All models All models All models I

Comparison Between Various Types of Flame Detectors

Technology Advantages Disadvantages Applications

Infrared (IR) High speed. Moderate Affected by temperature. Subject Indoorssensitivity. Manual self-test. to false alarms. IR sources.Low cost.

Ultraviolet (UV) Highest speed.High sensitivity. Affected by UV sources. Subject IndoorsAutomatic self-test. Low cost. to false alarms. Blinded by

thick smoke. Moderate cost.Dual Detector High speed. High sensitivity. Affected by specific UV/IR. Ratio Outdoors/(UV/IR) Low false alarm rate. Automatic created by false stimuli. Affected by Indoors

self-test. thick smoke. Moderate Cost.Dual Detector Moderate speed. Moderate Limited operation by temperature Outdoors/(IR/IR) sensitivity. Low false alarm rate. range. Affected by IR sources. Indoors

Moderate cost.Triple IR High speed. Highest sensitivity. Moderate cost. Outdoors/

Lowest false alarm rate. IndoorsAutomatic self-test.

Recommended Flame Detectors for Various Fire Scenarios

For These Types of Potential Fire Hazards...

Gasoline Paints Alcohol Hydrogen Plastic

JP4/JP8 Solvents Propane WoodOils Methane PaperDiesel CFC’s

Possible False AlarmSources Present Use These Detectors:

Arc welding UV/IR*, IR3 UV/IR*, IR3 UV/IR*, IR3 UV/IR*, IR3 UV/IR*, IR3

X-rays UV/IR* UV/IR* UV/IR* UV/IR* UV/IR*(for maintenance properties) IR3 IR3 IR3 IR3 IR3

Hot surfaces UV.UV/IR UV, UV/IR UV, UV/IR UV, UV/IR UV, UV/IRHigh temperatures IR3 IR3 IR3 IR3 IR3

Background

Incandescent light UV**, UV/IR UV**, UV/IR UV, UV/IR UV, UV/IR UV, UV/IRFluorescent light IR3 IR3 IR3 IR3

Halon lighting- w/glass UV**, UV/IR UV**, UV/IR UV**, UV/IR UV, UV/IR UV, UV/IRHalon lighting- w/o glass IR3 IR3 IR3 IR3

Mercury lamps UV/IR UV/IR UV/IR UV/IR UV/IRFlash lamps IR3 IR3 IR3 IR3 IR3

*Model UV/IR only.**For indoor applications only.

Charts and Graphs




FlameGard ® Models* and Characteristics

Detection Range1Ft. x 1FT.

Model Type Pan Fire Response Time Description

U UV 50FT. 0.5 second nominal Fast response UV detector

U UV 50FT. 0.5 second nominal As above with automatic or manual built-intest for verifying lens cleanliness andelectric operation. Indoor operations.

I UV/IR 50FT. 1 second nominal Dual UV/IR indoor and outdoor applications.

I UV/IR 50FT. 2 seconds nominal As above with automatic or manual test forverifying lens cleanliness and electricoperation. Indoor and outdoor applications.

IR3 IR3 200FT. 5 seconds Extended detection range alarms.Automatic or manual bit 4-20mA interface.

4 SQ.FT. JP4 1 second Optional RS85 interface. Adjustable timeFuel at 100FT. delay up to 30 seconds.

*All models are explosion-proof and approved by FM/CENELEC/CSA.

Typical Layouts

OverwingProtection

Hangar detector

coverage using 8FlameGard IR3 flamedetectors.Double coverage.

Sensitivity: 1’ x 1’ panfire at 200 feet.

Single PlaneHangar

Underwing coverageusing 4 FlameGard IR3

flame detectors.

Sensitivity: 10’ x 10’.

Industrial Application

Oil tank installation with floating roof.



Laying Out the Area to be Protected

1. Draw a floor plan of the area for which detection is to be provided. Use graph paper if possible and fill in the outline measurements.

2. Locate detectors within and around the potential hazard area so at least one detector has an unobstructed view of every location withinthe hazard area. Use the distance charts in this guide, taking into account the type of fire that is likely to occur.

3. For example, in an aircraft hangar, the most likely fire would occur in a spill underneath the aircraft. Therefore, detectors should be lookin

underneath the aircraft wings. In a warehouse application where chemicals are being stored in racks, the entire volume should be

covered, taking care not to have non-burning obstructions within the detector’s field of view.

4. If performance criteria have been established, define the minimum size fire to be detected (i.e.1FT. pan fire at 50FT.). Then use the chart

provided in this guide to determine the distance from the potential source of fire to where the detector(s) can be located. (Remember, the

larger the area the detector can cover, the fewer the detectors required for your application.)

Mine Safety Appliances CompanyIn U.S, 1-800-MSA-INST or FAX (724) 776-3280In Canada, 1-800-267-0672 or FAX (416) 663-5908Elsewhere, MSA International, (412) 967-3228 or FAX (412) 967-3373

I n s t r u m e n t D i v i s i o n : P. O . B o x 4 2 7 , P i t t s b u r g h , P A 1 5 2 3 0 U . S . A .

SCALE=1 FT.

Bulletin 07-2000 © MSA 1998 Printed in U.S.A. 9804 (L)

Note: This bulletin contains only a general description of the MSA FlameGardFlame Detectors. While uses and performance capabilities are described,under no circumstances should the product be used except by qualified,trained personnel, and not until the instructions, labels or other literatureaccompanying the product have been carefully read and understood and theprecautions therein set forth followed. Only they contain the complete anddetailed information concerning this product.

If you would like MSA to generate a suggested layout, please FAX us at (412) 776-3280 with the outline dimensions showing clearly any

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MSA Guide to Flame Detection