FLAME SAFEGUARD & COMBUSTION CONTROLS

FLAME SAFEGUARD & COMBUSTION CONTROLSThe above three products of combustion arechemical compounds. They are made up ofmolecules in which elements are combined incertain fixed proportions.As per the law ofscience, matter is neither created nor destroyedin the process of combustion, and the heat givenoff in any combustion process is merely excessenergy which the molecules are forced toliberate because of their internal make-up.Stoichiometric combustion is obtained when nofuel or air goes unused during the combustionprocess. Mixing and burning exactly the rightproportions of fuel and oxygen so nothing is leftover does this. Combustion with too much(excess) air is said to be lean or oxidizing. Theexcess air or oxygen plays no part in thecombustion process. In fact it reduces theefficiency. The visual effect is a short and clearflame. Combustion with too much fuel is saidto be rich or reducing, producing incompletecombustion. The visual effect is a long andsometimes smoky flame.The oxygen supplyfor combustion generally comes from theambient air. Because air contains primarily(78%) nitrogen, the required volume of air isgenerally much larger then the required volumeof fuel. Primary air is air that is mixed with thefuel before or within the burner's fuel deliverysystem. Secondary air is usually brought inaround the burner's fuel delivery system andspun through a diffuser or turning vane systemin order to optimize air-fuel mixing. Tertiary air isair brought in downstream of the secondary airand is sometimes used to control the shape ofthe flame envelope, and/or to control flametemperature on low-NOx burners.achieving optimum results in any of thesefunctions. The following lists some variations ofburner types found:BURNERSFig. 3 Principle parts of a forced draft burner.a)a)b)c)d)e)Gaseous fuel fired:Natural draft burnerInspirating burnerBalanced draft burnerInduced draft burnerForced draft burnerLiquid fuel fired: (forced or balanced draft)a) Mechanically atomized.b) Air atomizedc) Steam atomizedFinal fuel delivery and combustion-air & fuelmixing varies dependent on the burner types asper the following examples:a) Gun type (gas or oil)b) Cane (spud) type (gas)c) Ring type (gas)d) Rotary cup type (oil)Burners are a simple device to convert fossilfuels into useable heat energy. The primaryfunctions of burners are:a) Controlled fuel delivery.b) Controlled combustion-air deliveryc) Controlled fuel and air mixing.d) Controlled and reliable ignition.e) Evacuation of products of combustion.f) Controlled emissions.Regardless of fuels fired, burners must reliablyperform all functions. Choices of fuels burnedand type of burner affect the difficulties inFig. 4 Forced draft burner.Page 5

FLAME SAFEGUARD & COMBUSTION CONTROLSA burner must be equipped with a monitoringand control system to assure safe and reliableoperation throughout itsintended use.Complexity of this system is in relation tocomplexity of the process at hand and can varyfrom a single burner firing a single fuel, to a multiburner environment where many burners areoperating into a common combustion chamberand a multiple choice of fuels are burned. Thelarger the burner input, or heat release of aburner, does not necessarily mean the morecomplex the monitoring and control systemneeds to be.Conditions effecting complexity of controlsystems for burners are generally stipulated by:a) Type of process.b) Type of burner.c) Multi or single burner environment.d) Multi or single fuel operation.e) Safety hazard of fuel burnedf) Local codes and standards.g) Redundancy and reliability factors.h) Continuous or intermittent burner operation.Technological advances in recent years inparticular microprocessor based hardware, hasmade it important that only qualified ation hardware used in today's burneroperating and safety systems. Componentsmaking up the system for monitoring and controlof burners are subject to standards set by localauthorities.FUELSMost fuels are mixtures of chemical compoundscalled hydrocarbons. When burning these fuels,the final products contain carbon dioxide andwater vapor unless a shortage of oxygen existsin which case the products of combustion maycontain carbon monoxide, hydrogen, unburnedhydrocarbons and free carbon. Heat availablefrom fuels is measured in Btu/lb or Kcal/kg(Btu/gal or Kcal/l for fuel-oil). Natural gas fuel isthe most straightforward fuel to use. It requiresno special handling in filtering, drying, heating,etc.On the other hand, the efficiency inutilization of fuel oils depends to a large extentupon the ability of the burner system to atomizethe oil and mix it with combustion-air in thecorrect proportions. Heavy fuel oils are usuallypre-heated with steam. Tank heaters may raiseheavy fuel-oil temperatures sufficiently to reduceits viscosity in order to facilitate pumping Coke l (8,583)Oil 9(9,720)14,03012,9003,5003,100CoalTable 1 Comparative heating values for typicalfuels.In order to burn a liquid fuel, most burnersatomize the liquid. Atomization is the formationof the smallest possible droplets. This isrequired in order to expose as much surfacearea of the fuel as possible within the flameenvelope.Steamatomizationcanbeaccomplished by projecting steam tangentiallyacross jets of oil at the oil nozzle, resulting in aconical spray of finely divided oil after themixture leaves the nozzle. Air atomization isaccomplished by using air as an atomizing agentin an arrangement such as a proportioninginside-mixing-type oil burner using low-pressureair as an atomizing agent. Large capacity oilburners use two steps to get the oil combustible:atomization and vaporization. Vaporizationconverts oil from the liquid to the Vapor State byapplication of heat at the flame-front.By firstatomizing the oil into millions of tiny droplets, theexposed surface area is increased many timesand the oil can vaporize at its highest rate. Forgood atomization and vaporization, a largevolume of air must be initially mixed with the oilparticles. Mechanical atomization - atomizationwithout the use of either air or steam - issynonymous with pressure atomizing.Thenozzle used in mechanical atomizing consists ofPage 6

FLAME SAFEGUARD & COMBUSTION CONTROLSa system of slots tangential to a small inner whirlchamber followed by a small orifice. In passingthrough the slots, the liquid volume is increased.The high velocity prevailing in the whirl chamberin a tangential direction imparts a centrifugaleffect that forces the oil against the walls of thenozzle. It then passes through the orifices in thenozzle tip and into the combustion chamber,fanning out into a cone shaped spray of verysmall particles.FLAMEA flame is merely a zone within which thecombustion reaction takes place at a rate thatproduces visible radiation. A flame-front is thecontour along which the combustion starts (thedividing line between the fuel-air mixture and thecombustion process). In stable flames, theflame front appears to be stationary. The flameis actually moving towards the burner-nozzle(s)at the same speed that the fuel-air mixture isleaving the burner. Wide ranges of feed rangesexist in a wide range of burner designs. In orderto select the most suitable flame detectionhardware, it is necessary to know the basiccharacteristics of flames.The combustionprocess actually is surprisingly complicated, yetit is only required to understand a few of thegeneral characteristics of flames to assist you inselecting the best detector for the job. Commonflame characteristics are:a) Production of heat energy.b) Expansion of gases.c) By-product production.d) Radiation emission.e) Ionization within the flame envelope.FLAME DETECTIONHeat energy from a flame has not been found tobe suitable for flame detection. Sensors used todetect the presence or absence of heat given offby the flame do not respond fast enough.Additionally, the need to directly insert a sensingdevice into the flame to detect the targetedflames' heat energy would make such a systemsubject to high maintenance.Expansion of gases as a fuel-air mixture burnscan be detected and used as flame detection. Itis not a useful system for main burner flamedetection. In a multi-burner environment it issometimes seen in igniter-flame detection. Asthis system requires the detection of relativelyminute changes in pressures at the burnernozzle, it deals with tubing which run from theburner nozzle back to delicate pressuremeasuring devices, these systems also aresubject to high maintenance to keep themoperative.Production of by-products, chemically is areliable source to detect whether or notcombustion is taking place. But, as with heatenergy, response time would be too slow, andthe detection of an individual flame in a multiburner furnace extremely unlikely.Emission of radiation by the flame, andionization within the flame-envelope are the twomost commonly used flame characteristics usedin FSG flame detection hardware. In industrialFSG systems, emission of radiation is used formain flame detection by means of optical flamedetectors. Ionization, by means of a flame rod, isgenerally used for detectionof a gas-igniter flame. Commercial and lightindustrial FSG systems may at times apply asingle detector to detect both main and igniterflames.FLAME RODFig 5Flame envelopFlame Ionization is the process of heat in theflame causing the molecules in and around theflame envelope to collide with one another withsufficient force to free some of the outerelectrons of the atoms that make up themolecules. In this way, free electrons andpositive ions have been created.Thisionization process allows a very small current tobe conducted through the flame.Flameconductivity is low. Resistance can vary from100,000 to 100,000,000 ohms.Currentconducted through the flame (flame current) isgenerally in the range of 2 - 4 micro-amps. If twoPage 7

FLAME SAFEGUARD & COMBUSTION CONTROLSelectrodes were placed in a flame and a voltagewas applied, a current would be conductedbetween the two rods (flame rods). Naturally thepositively charged ions would flow to thenegatively charged rod. In order to use thisprocess to determine presence of flame and toprevent the potential hazard of a high resistanceshort to ground effectively simulating flamesignal, the flame current is rectified.The large grounding electrode generally formspart of the burner’s fuel-nozzle, helping tostabilize and hold the flame firmly in place.Flame rods are simply small diameter metal rodssupported by an insulator in order to be able tomount it such that the tip-end of the rod canproject into the flame. Flame rods typically aremade of a material called Kanthol, a hightemperature alloy capable of operating intemperatures of up to 2400 degree F. or 1300degree C.Other materials for even highertemperature ratings such as Globar, a ceramicmaterial, are also available.Requirements for successful flame rodapplications are:a) Stable flame. (no movement from flame rod)b) Gas burners only, premixed were possible.c) Adequateflame-rod-to-groundingareaproportioning. (4 to 1 minimum).d) Proper placement of flame rod in flames(short as possible, yet adequate contact).e) Proper rectifying flame current andassociated circuitry.Fig. 6 Electron flow through ionization withinthe flame envelop.Generally referred to as a Flame RectificationSystem, it is achieved by placing a groundingelectrode in the flame that is several times(generally 4 times) larger than the flame rod orelectrode. An AC supply voltage is appliedacross the electrodes. During half of the ACcycle the flame rod is positive and the groundrod is negative. The positively charged ions willflow to the negatively charged grounding area.As the grounding area is larger, its capacity tohold electrons is increased, resulting in a relativehigh flame current flowing through the flameduring this first half cycle. During the secondhalf cycle, the reverse process will take place.However, the capacity of the flame rod to holdelectrons is less than the grounding area andthe resulting flame current is smaller. Thegreater the ratio of grounding area to flame rodarea, the greater the rectified current. The onlyaccepted type of current by the system is thisrectified flame current. Any high resistance typeshort circuit will result in an AC type flamecurrent, which is rejected by its FSG control.Fig. 7 Typical pilot burnerflame-rod flame detection.assemblywithPage 8

FLAME SAFEGUARD & COMBUSTION CONTROLSFig. 8 Flame-rod system operational diagram.Advantages of flame rod applications:a) Location sensitive. Flame rod detectorsprove the flame with extreme locationsensitivity. The flame must be where therelative small rod can be in contact with it.This is most useful in controlling forexample, that a pilot flame is detected onlywhen it is at the optimum location to reliablyignite the main burner. Flexibility in placingthe rod can assure this. Also, this locationsensitivity will detect flame lift-off and assuch can be used to prevent unstable flameconditions for both pilot and main flames.b) Fast response time. The relative low costflame rod system has been put to usereplacing many, and sometimes working asan auxiliary to, slow responding bi-metal andthermocouple based systems.c) Fail-safe system. Using electronics, thissystem responds safely to abnormalconditions such as open or short circuits,leakage to ground, all causing the system tofail-safe.the rod directly above the burner assembly isleast favored as this may allow it to prove a lazyupward bending inadequate flame.Some atmospheric burner assemblies use arunner type pilot, a pilot that is used to light offmore than one burner. It may consist out of atube with holes drilled along its side throughwhich fuel gas is burned. An assembly such asthis requires it to be proven at its ignition sourceand also at the extreme end of the runner pilot.A thermocouple may be used to prove the pilotat the ignition source and a flame rod may beused to prove it at its extreme, with the flame rodsystem in charge of main fuel.On some direct fired air handling units burnersare installed which are of significant lengthrequiring the main flame to be detected at theend farthest away from the ignition source. Thisalso has proven to be an ideal job for flame rodsystems.INSTALLATION AND APPLICATIONIdeally, the flame rod is mounted verticallyalongside its vertical burner nozzle with only thetip bent in to make contact with the flameenvelope. When installing a flame rod on ahorizontally orientated burner assembly, caremust be given to favor installing the rod on eitherone of the sides of the burner or directly below,with the tip bent up to make contact. InstallingThe Fireye part number 69ND1 flame rod is aspark plug type unit consisting of ½” NPT mount,a Kanthal flame rod, a glazed porcelaininsulating rod holder and a spark plug connectorfor making the electrical connection. The 69ND1is available in 12”, 18” or 24” lengths.Page 9

FLAME SAFEGUARD & COMBUSTION CONTROLSThe flame rod may be located to monitor onlythe gas pilot flame or both the gas pilot and maingas flames. It is mounted on a ½” NPT coupling.The following installation instructions should beobserved:1. Keep flame rod as short as possible.2. Keep flame rod at least ½” from anyrefractory.3. Flame rod should enter the pilot flame fromthe side so as to safely prove an adequatepilot flame under all draft conditions.4. If the flame is nonluminous as with premixedburners, the flame rod tip should extend atleast ½” into the flame, but not more thanhalf way through.5. If the flame is partly luminous, the flame rodtip should extend only to the edge of theflame. It is not necessary to maintainabsolutely uninterrupted contact with theflame.6. It is preferable to angle the rod downward tominimize the effect of sagging and toprevent it from coming into contact with anyobject.7. An adequate grounding surface for the flamemust be provided. The grounding surface inactual contact with the flame must be atleast four times greater than the area of theportion of the flame rod in contact with theflame. It is essential to adjust the flame rodand ground area ratio to provide maximumflame signal reading.8. Interference from the ignition spark can alterthe true flame signal reading by adding to, orsubtracting from it.Interchanging theprimary wiring (line voltage) to the ignitiontransformer sometimes may reverse thistrend.This interference can also bereduced by the addition of groundedshielding between the flame rod and ignitionspark.9. Proven types of flame grounding adaptersmay be used to provide adequate groundingsurface. High temperature stainless steelshould be used to minimize the effect ofmetal oxidation. This assembly may welddirectly over the pilot or main burner nozzle.10. Use the proper wire and wiring techniques.No. 14 wire, rated at 90 C or higher isrecommended for control to flame rod.Actual wire size is not critical. Increasedresistance that occurs with smaller wire sizewill not be of significance. Type of insulationis important. Use wire with the highestleakage resistance to ground. In practicewire runs can be up to 200 feet long. Do notshare the flame rod wire with ignition wiresin same conduit.RADIATION PROPERTIES OF FLAMESEmission of radiation from withi

The content of this manual outlines the Fireye, flame safeguard (FSG) environments as well as the princi