Nao Manual Draft

NAO Inc. — Property of NAO Inc. 1-1

NAO Inc.

Quality Manufacturer ofCombustion, Pollution, and Safety Equipment

NAO has been designing, manufacturing, and servicingcombustion and pollution control equipment since 1912.Our fully engineered systems are installed throughout theworld for oil & gas exploration, for oil & gas production,in refineries, in LNG & LPG plants, in loading terminals,and in chemical & petrochemical processing facilities.NAO’s products, systems and services are also widelyused in the power generation, pulp & paper,pharmaceutical, and basic metals industries.

Our goal is to serve our customers by remaining theworldwide technological leader for industrial burners,thermal incinerators, flares, and other pollution controlsystems, as well as associated fail-safe and safetyequipment.

With the continuity of athree-generation family-management team andwith manufacturingfacilities in Philadelphia,Pennsylvania, Houston,Texas, Milan, Italy, andTokyo, Japan, we havepursued this primaryobjective for more than80 years.

With the widest range of process flares, including self-contained, trailer-mounted portable flaring systems,energy-conservation pilots, flame-front and electronic-ignition systems, subsea ignition systems, flame arrestors,liquid seals, knock-out drums, and the patented, no-moving-parts Fluidic Seal,TM this uncommonorganization provides practical, cost-effective solutionsto environmental, safety, and energy-control problemsthroughout the world.

NAO Inc.Regional offices in Asia, Europe, and North America, Representatives inmajor offices around the world


BurnersOver 500,000 Installations

NAO offers the widest range of industrial burner designs,types, sizes, and fuel-firing options. In some developingnations, burners built by the company before 1920 arestill in service because NAO is committed to providingspare parts and service for the company’s “vintage”burners, which, by the way, are low NOx.

NAO’s new Low NOx FGITM burners comply with themost stringent worldwide environmental regulations(including California rules 1109 & 1146) by controllingCO and NOx without sacrificing combustion efficiencyor flame stability.

In addition to burners, NAO designs, manufactures andservices a variety of burner packages and accessories.These include: Air Heaters, Burner Control Systems,Flame and Detonation Arrestors, Furnace Accessories,Access Ports & Sightports, Ignitors & Pilots,Incinerators, Inert Gas Generators (a combustor wherethe products of combustion—carbon dioxide andnitrogen—are used to purge tanks and ships), ProcessEquipment, Pumping Systems, Specialty Valves, andHigh-Temperature Castable Burner Tiles.

Flaring SystemsLong-Life, Low-Maintenance

NAO’s high-performance flaring systems haveestablished the standards for reliability and trouble-freeoperation for onshore and offshore operations. Ourworldwide “standards” are not and cannot be emulatedby our competition. For example: The first enclosedground flare installed in a petrochemical complex inTaiwan achieved only 5% capacity before it failed. NAOwas called in to evaluate the defective system, designedand manufactured by another American Company.

Subsequently, NAO supplied a total of eight enclosedflares (over $17 million) to several petrochemicalcomplexes that are surrounded by urban development.All of them continue to operate safely, efficiently,quietly, and dependably.


As part of the flaring systems, NAO designs,manufactures, and services a variety of flare accessories.These include: Energy-Conservation Controls, Flameand Detonation Arrestors, many types of Ignitors &Pilots for NAO and competitor flaring systems.

In addition to “flaring” systems, NAO designs,manufactures and services all combustion solutionsystems including Incinerators (today we use the wordsafterburner or thermal oxidizer—incinerator is a “dirtyword”) and Vapor Control Units.

Flame & Detonation ArrestorsEssential Safety Devices

There can be no margin for error with a flame arrestor ora detonation arrestor. It must work right the first time,and every time. When a subsonic deflagration orsupersonic detonation reaches an arrestor, there is noopportunity to change variables to save your plant orpersonnel.

In 1979, in response to an urgent request for full-scaletests for a nuclear power plant, NAO was asked todesign, manufacture and test special detonation arrestorsfor an explosive mixture of pure hydrogen and oxygen.To accomplish that top-priority project, NAO purchaseda 110-acre tract of land near Houston, Texas, which hassubsequently been fully equipped and staffed to serve asNAO’s Environmental Research & Service Center.

In the past 15 years, extensive full-scale deflagration anddetonation tests at the ER&S Center, supplemented byempirical calculations and tests at other NAO facilities,have enabled NAO to design and manufacture the widestrange of state-of-the-art deflagration and detonationarrestors, including U. S. Coast Guard accepted units.


Fully Engineered Pollution Solutions IncludeThermal Oxidizers and the World’s LargestFleet of Emergency Rental Equipment.

For efficient control of volatile organic compounds(VOC’s) and other waste gases and liquids, NAO offersa complete line of thermal oxidizers with optional heatrecovery sub-systems, liquid & slurry burners, landfillflares, sewage digester flares, sour gas flares, andpopulated area combustors (enclosed flares). Allincorporate state-of-the-art designs for unsurpassedsafety, efficiency, and reliability with minimalmaintenance.

Identified under services, we provide consulation,engineering design, and contractor services. Weprovide: Emergency Services, such as flare rentals andservice (we also rent incinerators and thermal oxidizersin addition to flares), Engineering Services, wheresomeone wants to buy the design package and build itlocally, and Field Services, which can include inspection,maintenance and modeling—where we build a model of aburner or a furnace to measure flow rates, heat release,and emissions, and to determine if uniform and properheat distribution is maintained. We have also developedtechnical reports on a variety of combustion related topicfor customers and for publication.

We perform testing on all of our products—burners,flares, pilots, flame arrestors, and detonation arrestors—to demonstrate exactly how they work. With burners,we are actually measuring emission levels of NOx,carbon monoxide, and hydrocarbons so we can providethe customer with accurate information to present to theEPA or their local air control board.


Manufacturing ExcellenceKey to Quality

As an ISO-9001 manufacturer with a worldwidereputation for dependability that rests on more than 80years of proven performance, NAO is convinced:Quality keeps customers.

NAO always bids to customer specifications. Nothing is“accidently” omitted. Nothing is more frustrating thanthe loss of an order when a competitor ignores sectionsof a customer’s specifications; purposely undersizes aburner, flare, or thermal incinerator; omits state-of-the-art safety devices; substitutes lighter gage metals; usessimple butt welds instead of full-depth fillet welds; andcuts other corners to submit a low bid only to invoice thecustomer for “extras” when completing the order.

Keeping Customers Fully Informed

As the leader in pioneering innovative, dependable, long-term solutions to the most difficult combustion, safety,and environmental control problems, we have anobligation to keep our customers fully informed.

To satisfy this obligation, NAO participates in tradeshows and seminars, conducts formal classroom andhands-on training sessions at the “NAO University” nearHouston, Texas, and schedules on-site technicalpresentations throughout the world.

For information about upcoming trade shows, seminars,flare schools, burner schools, and technical presentationsscheduled in locations around the world, send a fax to“NAO Training Administrator” at 215.743.3018.

NAO engineers also author dozens of informative reportsevery year, and reprints are available. For a list of thesereprints, send a fax to “NAO Reprint Administrator” at215.743.3020.

Principles of Combustion — Property of NAO Inc. 2-1

Principles ofCombustion

What is Combustion?

Combustion, or burning, is a chemical reaction betweenfuel and oxygen resulting in a release of heat. There arethree components to combustion: Fuel, oxygen, andignition.

Oxygen comes from air, with is about 21% oxygen and78% nitrogen by volume. Most fuels contain carbon,hydrogen, and, sometimes, sulfur. For the purposes ofprocess burners, “fuel” means hydrocarbons—oil or gas.For the purposes of flares and thermal oxidizers, the fuelcan be any combustible liquid or vapor.

Combustion does not (generally) take place just becausefuel and air are brought together; they must be intimatelymixed, in the right proportions, and energy must besupplied to start the reaction.

The success or failure of combustion equipment inindustry depends on the designer’s understanding ofcombustion processes and how to apply thatunderstanding to solve customer challenges.

Combustion Chemistry

The chemical elements in common fuel substances arecarbon, hydrogen, sulphur, and oxygen. Among these,the true fuel elements are carbon, hydrogen, andsulphur—the presence of oxygen in a fuel moleculealways lowers the heat of combustion. For example,methane (CH

4) has a heating value of 23,800 Btu/lb

(55,350 kJ/kg), while methyl alcohol (CH4O) has a

heating value of 10,260 Btu/lb (24,860 kJ/kg). The

Composition of Air*

0% rh 100% rh

Oxygen, O2

20.99† 20.6223.20‡ 22.94

Nitrogen, N2

78.03 76.6775.46 74.63

Argon, Ar 0.94 0.921.30 1.29

Water, H20 0.00 1.75

0.00 1.10

Other** 0.04 0.040.04 0.04

Equiv. MW 28.96 28.77Density, lb/ft3 0.07632 0.07581

* At 60 °F dry bulb** CO

2, H

2, He, Ne, Kr, Xe

† % by volume‡ % by weight


presence of oxygen is an indication that part of theoriginal heat of combustion has been used up.

With the exception of carbon, all fuels burn as a gas.Liquid fuels must be vaporized prior to combustion.Hydrocarbons burn with blue and yellow flames.Hydrogen burns extremely rapidly, producing a pale blueflame that is difficult to see (except against a darkbackground). Carbon burns slowly, producing a brightyellow flame. A hydrocarbon burns yellow due to thedissociation, or cracking, of the hydrocarbon into itscarbon and hydrogen constituents followed by separateand rapid burning of carbon. If the same fuel burns witha blue flame, it is due to progressive oxidation withoutdissociation.

Time – Temperature – Turbulence

All combustion equipment is designed around the three“T’s” of combustion: Time, Temperature, andTurbulence.

Time is designed into enclosed flares, incinerators,heaters, and furnaces through the space in thecombustion chamber. The volume of the chamber ismade large enough to retain the gas flow long enough toallow for complete combustion of the volatile gases. Forinstance, thermal oxidizer chambers are generallydesigned for flow velocities of 40 ft/second, so thechamber height is 20 feet for ½ second residence time.

Temperature is the most important “T” of combustion.Heat is used as the driving force to sustain combustion—it excites molecules and speeds the reaction. If theheating value of a waste gas is high enough, thecombustion will sustain itself, allowing for an open flare.In other instances, supplemental heat is supplied throughan auxiliary burner, as in a thermal oxidizer, or throughan assist gas ring on an open flare. To reduce acustomer’s operating expenses, NAO has developedseveral flares and incinerators that maintain hightemperature combustion of low heat value gases withoutthe use of assist gas.


Turbulence is designed into all of NAO’s combustionequipment with baffles, restrictions, retainers, andpatented nozzle designs. Changes in direction andvelocity thoroughly mix the combustibles with the air(oxygen) necessary for combustion. Turbulenceguarantees effective mixing of fuel and air for completecombustion. If turbulence is not part of the design,separation or stratification of the combustion gaseswould occur and some of the volatile gases would gothrough the equipment unburned.

Gas Volume Relationships

The volume of any gas that is free to expand increaseswith temperature and decreases with pressure. Whenconsidering gas volume, it is necessary to have in mindthe temperature and pressure concerned. Whencomparing gas volumes, it is always advisable toconsider them at the same temperature and pressure.The universally recognized Standard Temperature andPressure is 60 °F and 1 atmosphere pressure(30 inches Hg).

The increase or decrease in the volume of a gas causedby variations in temperature and pressure is easy tocalculate by simple proportion. For practical purposes,the volume increases exactly in proportion to theabsolute temperature and decreases exactly in proportionto the pressure. This can be summarized by the relation:

V1

P0·T

1—— = ———V

0P

1·T

0

Another important fact about gases is that under thesame conditions of pressure and temperature, equalvolumes of all gases contain the same number ofmolecules. (The number is fantastically large, about 2.7x 1019 per cubic centimeter.)

Once we know this, it follows that the weight of a givenvolume of any gas will be in proportion to the weights ofthe molecules it contains. As a comparison, consider abasket containing 1,000 ping-pong balls weighing 1/100th of a pound, the total weight is 10 pounds. If theping-pong balls are removed and replaced with 1,000

Gas Volumes†

MW

Hydrogen, H2

2Methane, CH

416

Water, H20 18

Nitrogen, N2

28Carbon monoxide, CO 28Oxygen, O

232

Carbon dioxide, CO2

44Sulphur Dioxide, SO

264

Sulphure Trioxide, SO3

80

†The weight of these gases inpounds will occupy 380standard cubic feet,i.e. 2 pounds of H

2 at 1 atm

and 60 °F.


golf balls weighing 1/10th of a pound, the total weightincreases to 100 pounds. The volume of the basket isrelatively unchanged and the number of balls is the same,but the weight increases by a factor of 10, the same ratioas the weights of the individual balls.

Knowing this relationship, it can be shownexperimentally that the molecular weight (MW) of allgases occupy the same fixed volume at STP (1 atm,60 °F). For instance, the MW of any gas in pounds willoccupy 380 standard cubic feet. If we look at air, forinstance, with a molecular weight of 29, we know that29 pounds of air at STP occupies 380 scf. One pound ofair, then, occupies about 13.1 scf (380 ft3/29).

In practice, it is rare to get air that is free frommoisture—the normal atmosphere contains a significantquantity. Since the total number of molecules of allgases in any volume is always the same, it follows that ifthere are water molecules present, there must be fewernitrogen and oxygen molecules.

In cases where the climate changes regularly from warm,humid conditions to cold, dry conditions and vise versa,

Heat of Combustion†

——————————Reaction Gross Net Btu Max. flame

per temp.‡

C + ½ O2 → CO 3,964 3,964 lb

C + O2 → CO2 14,155 14,155 lb 3,820

CO + ½ O2 → CO2 323 323 ft3 3,960

H2 + ½ O2 → H20 325 275 ft3 3,960

CH4 + 2 O2 → CO2 + 2 H20 1,011 910 ft3 3,640

C2H6 + 3½ O2 → 2 CO2 + 3 H20 1,783 1,630 ft3 3,710

C3H8 + 5 O2 → 3 CO2 + 4 H20 2,572 2,365 ft3 3,770

C4H10iso + 6½ O2 → 4 CO2 + 5 H20 3,353 3,093 ft3 3,780

C5H12n + 8 O2 → 5 CO2 + 6 H20 3,987 3,685 ft3 3,720

C2H4 + 3 O2 → 2 CO2 + 2 H20 1,606 1,505 ft3 3,910

C3H6 + 4½ O2 → 3 CO2 + 3 H20 2,332 2,181 ft3 3,830

C4H8iso + 6 O2 → 4 CO2 + 4 H20 3,077 2,876 ft3 3,810

C2H2 + 2½ O2 → 2 CO2 + H20 1,498 1,447 ft3 4,250

C6H6 + 7½ O2 → 6 CO2 + 3 H20 3,744 3,593 ft3 3,860

C6H6liquid + 7½ O2 → 6 CO2 + 3 H20 17,980 17,250 lb 3,840

C7H8liquid + 9 O2 → 7 CO2 + 4 H20 18,300 17,470 lb 3,850

C10H8solid + 12 O2 → 10 CO2 + 4 H20 17,290 16,690 lb

†All gases are dry at 60 °F and 1 atmosphere.‡Maximum flame temperature when burned with theoretical air.


this becomes very significant. For example, consider aplant that is set to operate at 10% excess air at 32 °Fwith nearly zero humidity. One pound of air, then,occupies 12.4 ft3. If the plant requires 1,000 lbs/hr forstoichiometric combustion, the air dampers will be set topass 110% air, about 13,600 ft3/hr.

Now consider the situation with atmospheric air at 85 °Fand saturated with moisture. The volume of one poundof (dry) air is now 13.7 lbs/ft3 and the associatedmoisture occupies about 0.6 ft3, so the actual volume ofair saturated with water at 85 °F is 14.3 ft3. If the burnerreceives 13,640 ft3 of this air in one hour, the weight ofactual (dry) air will be 954 pounds. Instead of 10%excess air, the plant will be operating at 5% deficientair—they are dumping fuel to the atmosphere!

The Primary Refinery Process

Composition of Petroleum Oil

Crude petroleum is a liquid of varying viscosity. Itranges in color from light brown to black depending onits origin. Crude petroleum is a mixture of threeprinciple classes of hydrocarbons: The paraffins, thenaphthenes, and aromatics.

Paraffins: These are all hydrocarbons of the formulaC

nH

2n+2. Paraffins can be gases such as methane (CH

4) or

butane (C4H

10), liquids such as hexane (C

6H

14), or even

solids such as Hexadecane (C16

H34

). The lightestmembers of the paraffin group come from the natural gasassociated with the oil. The heavier members aredistilled from the oil in the refinery.

Naphthenes: Napthenes have the general formula CnH

2n.

Aromatics: Aromatic compounds are all derivatives ofbenzene, C6H6.

In addition to these organic materials, there is alsodissolved nitrogen, other compounds of oxygen andsulphur, and come compound of mineral origin such asvanadium.


Processing Crude Oil

The typical refinery process involves fractionaldistillation of the petroleum oil into its constituentproducts. These products are often used in secondaryprocesses.

To distill the petroleum fractionally, it must first beheated in a fired heater. The types of heaters will bediscussed later. In the crude heater, the oil is heated to atemperature of about 660 °F by passing it through tubesunder a pressure of 150-200 psig.

The oil is pressurized to prevent it from flashing in theheater tubes. Without the high pressure, bubbles of gaswould form that can cause the tube to overheat and,consequently, to coke up. The coking can plug thetubes, causing a rupture and a huge fire. Also, onheaters with short-radius turns, bubbles erode the metalas they stream past.

While still under pressure, the heated oil is passed to thefractioning column or flash tower. In this column, thehot, pressurized oil flows through an orifice to flash it asa vapor into the low pressure column.

The column consists of a large number of trays designedto filter the hot oil vapors. As the hot vapors rise, theycool and condense. The heavy constituents with thehighest boiling point condense first, the lightconstituent—the lighter fractions—will rise to a higherlevel before they cool sufficiently and condense.

When the various oils condense inside the column, theyfill up their respective trays until enough of the liquid hascondensed to cause it to overflow. These overflows arecalled sidestreams.

In theory, you can remove as many different sidestreamsas there are plates, because the oil on each plate hascondensed at a slightly different temperature. From apractical point of view, this is not necessary. A numberof sidestreams are grouped together in a fraction, the oilswhich condense over a given temperature range. Anexample of this is tractor vaporizing oil, all of whichdistills between 300 °F and 530 °F, with 40% distilling at390 °F.


To keep the fractions within specifications, it isimportant to maintain tight control over the distillationranges of the sidestreams, and it is necessary to removethe more volatile products that are entrained on trayslower than those on which they would have distilled.This operation is carried out in a sidestream stripper, avessel that is really only a small, secondary columnconsisting of a few plates.

The sidestream enters the stripper near the top and filtersdown. Steam is introduced at the base, reducing thevapor pressure and causing the volatile constituents tovaporize. This re-vaporized portion in returned to thecolumn higher up while the sidestream runs out of thebase of the stripper.

Secondary Refinery Processes

Cracking

With the popularity of the internal combustion engine,the demand for light fractions exceeds the limit naturallypresent in petroleum oil. The process used to producewhat does not come naturally is cracking. Cracking isthe process of breaking large oil molecules into smallerones.

Thermal Cracking

Thermal cracking offers an inexpensive way to upgrade amajor portion of crude oil. In thermal cracking, heavypetroleum distillates are typically subjected totemperatures of 800-930 °F in fired heaters underpressures as high as 400 psig. At this temperature, thelong chain paraffinic hydrocarbon molecules break intosmaller chains, forming unstable compounds that can bestabilized in pre-determined directions, depending uponthe product required. For the gasoline engine, theproduct is a fuel rich in naphthenes and aromatics; thesegive good ignition characteristics.

Visbreaking: The visbreaking process is a mildthermal-cracking process that produces low-viscosity


oils. It is called “visbreaking” because the processbreaks the viscosity of a heavy oil.

Visbreaking is used to upgrade a heavy, residual fuel oiland make it marketable. Upgrading is often achieved by“cutting” the residual oil with a middle distillate.Visbreaking reduces the consumption of these distillatesby about 20%.

The basic equipment of soaker visbreaking consists of afired heater and a fractionating column with a “soakingdrum” placed between them. The soaking drum is areactor in which the cracking takes place. The feedstockis heated to 895 °F and kept at this temperature in thesoaker for a pre-determined amount of time before beingfractionated. The use of the soaker provides for longresidence time, which permits low cracking temperatureand saves operational and capital costs. Lowertemperatures mean smaller heat exchangers, less coking,and extended tube life.

Coker Cracking: A severe form of cracking convertsoil residues into additional yields of naptha forsubsequent processing. The residuals are heated torelease lighter fractions. If the resulting coke is low insulphur, the coke can be used in metallurgical processes.

Steam Cracking: Steam cracking is designed toproduce raw materials for the petroleum chemicalindustry. In this case, heat is used to crack a feed of rawgasoline. The feedstock is mixed with steam and quicklyheated to a very high temperature. The molecules ofgasoline break down or change shape to form light gases(such as ethylene, propylene, butylene, and butadiene)and high-quality gasoline fuel.

The cracked products are compressed, refrigerated, andpassed through a series of fractionating columns toseparate the individual compounds.

Catalytic Cracking

In the catalytic cracking process, petroleum oil contactsthe hot catalyst where it is vaporized and reformed intogasoline-rich fractions. The process is sometimes calledfluid catalytic cracking. It is “fluid” because the catalyst,


in the form of a powder, behaves like a fluid when blownwith air or oil vapor.

In the process, the oil is sprayed into a stream of red-hotcatalyst. The catalyst cracks the oil, and the oil fluidizesthe catalyst. In doing so, however, coke is deposited onthe surface and in the pores of the catalyst so thatperiodically the catalyst must be regenerated.

Regeneration is achieved by blowing air over thecatalyst. This causes an exothermic reaction that reheatsthe catalyst for another pass with the oil. Theregenerator gas has a high carbon monoxide content andis often used as a fuel for a CO-fired furnace. With theefficiency of modern catalysts, less coke is deposited, soregeneration produces less heat and an external heatsource must sometimes be used.

Hydrocracking

Hydrocracking is a combination of catalytic cracking andhydrogenation. The process involves hydrogen atpressures from 100 to 2,000 psig. Olefinic materialscreated by cracking are saturated with the hydrogenbefore they can contribute to coke formation.

Hydrocracker units costs more to build and operate thanthe fluid catalytic cracker, so it is usually used only tosupplement the cat cracker process. The hydrocrackerwill handle heavier fractions better than the cat cracker,and it can be tailored for varying ratios of gas, gasoline,and middle distillate.

Polymerization

Polymerization is the reverse of cracking. It is thecombination of small molecules to produce larger ones.Light gases produced in the cat cracker are combined toproduce heavier gases, like butane, and a further supplyof gasoline.

Polymerization competes with alkylation for olefinicfeedstocks. In polymerization, about 1.4 barrels of olefinfeed is necessary to produce 1 barrel of gasoline.


Alkylation produces gasoline more efficiently, sopolymerization of olefins is primarily reserved for theproduction of petrochemical materials.

Reforming

Reforming changes the shape, rather than the size, ofhydrocarbon molecules. The need to increase the octanerating of gasoline blends is the major reason forreforming the molecular structure of a petroleumfraction.

For example, reformers make high-quality gasoline fromthe raw gasoline produced by primary distillation. Theraw-gas feedstock is passed through a series of furnacesand reactors that use high pressures, specialatmospheres, and catalysts to convert straight-chainmolecules into aromatic forms.

Most feedstocks for reforming are first hydrotreated toremove arsenic, sulfur, and nitrogen compounds that canpoison the reforming catalysts.

Hydrogen is a by-product of catalytic reforming. Someof this hydrogen is recycled to sustain reactor pressureand to suppress coke formation. The remainder isavailable for other processes, such as hydrotreating,hydrocracking, isomerization, and the manufacture ofpetrochemicals.

Isomerization: The process of isomerization reformsnormal paraffins into their isomers. The conversionincreases octane ratings of the feed significantly. Forexample, normal pentane with an unleaded octane ratingof around 60 can yield its isomer having a rating over 90.

Another route to higher octane uses isomerization toconvert normal butane into iso-butane needed foralkylation.

Alkylation

Alkylation is second to reforming as a means of gettingeconomical octane ratings from processed fractions.


Alkylation improves the gasoline yield of a refinery bycombining gaseous feeds to produce liquids. Theprocess combines an iso-paraffin, usually iso-butane,with olefins such as propylene, butylene, or amylene.The resulting product is a stable gasoline with a highoctane rating.

The catalysts used for alkylation are sulfuric acid,hydrofluoric acid, and aluminum chloride withhydrochloric acid. Alkylation units have recycle systemsto recover these acids for re-use.

Alkylation has replaced the polymerization process forthe production of gasoline from olefinic feedstocks. Inpolymerization, 1 barrel of gasoline is produced from1.4 barrels of olefin feed. When the 1.4 barrels arecombined with iso-butane and fed into an alkylation unit,2.5 barrels of gasoline. The alkylation process alsoproduces a gasoline with a higher octane rating.Polymerization is now used primarily for petrochemicalmanufacturing. Though, alkylation is used in theproduction of some petrochemicals. Benzene andethylene are combined by alkylation to produceethylbenzene, which is used to form styrene and syntheticrubber.

Hydrotreating

Hydrotreaters are used to treat the by-products of olefinmanufacture. The by-product liquid are high in aromaticcontent, but they are contaminated with acetylene anddiolefins that polymerize and form sludge. Hydrotreatingselectively saturates these molecules without destroyingthe aromatics.

Hydrotreating is used to pre-treat the feedstock ofcatalytic reformers. Hydrotreatment reduces thenitrogen, sulfur, and other compounds that wouldotherwise poison the catalysts.

For most hydrotreating processes, the pressures rangefrom 100 to 3,000 psig. Temperatures range from350 °F to 850 °F, though most processing is done in therange 600-800 °F. Hydrogen consumption rates arearound 200 scf per barrel of feedstock.


Solvent Refining

Though most separations are made by distillation,solvents can also be used to isolate fractions on the basisof their relative solubility. Solvent refining methods areused primarily to manufacture lube oil fractions and toprepare feedstocks for subsequent processing.

Crude oil de-salting is a type of solvent refining. Wateris used to dissolve salts and other solid contaminatesfrom the oil before it is subjected to any other refiningprocess. In some cases, other chemicals are added to aidin the de-salting process. In other cases, an electrostaticfield is used to improve the separation of the water andsolids from the oil.

Gas Treating

Gas treating is used to clean up hydrocarbon gases byremoving the acidic gases of carbon dioxide, carbonylsulfide, hydrogen sulfide, and mercaptans. The greatestuse of gas treating is in natural gas processing.


References

Cross, Frank L. Handbook on Incineration. TechnomicPublishing, Westport, Connecticut: 1972.NAO Book #373

For Additional Reading

For copies of these and other related technical papers,please mail or fax your inquiry to: LiteratureDepartment, NAO Inc., 1284 East Sedgley Avenue,Philadelphia PA 19134, Fax 215.743.3020.

Burners — Property of NAO Inc. 3-1

Burners

Established in 1912 as a burner manufacturer, NAO hasbeen making burners and combustion systems longer thanany of our competition. NAO uses the best and the latesttechnology to build the best burners on the market.

We provide every type of standard burner on the market,and we custom engineer specialty burners for specific clientapplications.

Our burners have been field proven in over 500,000installations worldwide to have superior turndown andlonger life than our strongest competitors.

If the customer will provide us with proper specifications,we can supply the best burner for their application. Properspecifications include: Flame Shape, Heat Capacity,Available Fuel, Required Turndown, and Required Draft.

Firing the Burners

NAO supplies all types of burners for any fired-fuelapplication. These burners can usually be fired in anydirection, and they can be designed to produced a wide-range of flame patterns.

The direction which the burner fires will depend on the typeof furnace, heater, smelter, or other equipment. Once thedirection is known—down fired, up fired, horizontally fired,or angle fired—the acceptable flame shape will be known.The shape of the flame is very important becauseimpingement of the flame on the furnace wall or heat-exchanger tubes must be avoided.

If the flames impinge upon the tubes, carbon will form insideof the tubes: that will change the heat transfer rate in thetube, actually causing more carbon to build up. The tubeswill eventually plug, burn up, and crack, which could lead toa rupture of the tubes and a very large fire, so flame

3-2 Burners — Property of NAO Inc.

impingement is very bad and must be minimized. Therefore,in order to generate a quotation for new or replacementburners, we must obtain some kind of a drawing of theinside of the heater or furnace showing the tubes and wallsso our engineers will know what kind of space there is to firein. We can determine if we need a long or short cylindricalflame or a flat flame.

Another important characteristic is burner turndown.Turndown is the ratio of the maximum rate of firing to theminimum rate. NAO burners are known for their excellentturndown, which gives them a wide range of performancethat allows customers to fine-tune their fired equipment.

When you need good turndown—where the burner isoperating at a very low firing rate—the pressures might beonly inches of water column, so don’t be too concernedabout the minimum pressure. What you want to consider isthe maximum pressure available, so that we can analyze thisand design a unit with the greatest operating range.

Controlling the Fuel

Inspirating Burners

An inspirating burner is also known as a pre-mix gas burner(inspirating burners only use gas fuels). Inspirating burnersrely on a high-velocity jet of gas to pull in the proper amountair so you have a stoichiometric gas and air mixture.Inspirating burners are unique among gas burners in that thegas and air mix before combustion starts to take place.NAO makes many types of pre-mix burners.

One of NAO’s original inspirating burners is the Rams HornInspirating Gas Burner. The Rams Horn is a burner thatsplits into two burner heads. Normally, these are mountedunderneath the furnace on a track that allows them to slidein and out for easy removal and maintenance. They fire upfrom the floor of the heater, and usually at a slight angle.The flame bounces off a wall, from which the heat radiates.


Raw Gas Burners

Raw Gas Burners are also known as Nozzle Mix GasBurners. With the Raw Gas Burners, the gas andcombustion air do not mix until the gas leaves the ports ofthe burner. The key to stable combustion in a Raw GasBurner is turbulence. NAO burners use the patented JetMix burner tip to deliver a turbulent vortex of raw gas intothe combustion air stream. The gas and air mix very well forcomplete and stable combustion of the burner flame.

Controlling the Air

Natural Draft Burners

A natural draft burner works due to buoyancy of the hotgases in the stack. The fact that you have hot air and it haslower density it will tend to float up, creating a negativepressure in the furnace. Natural draft is a function of thechange in density of the expanding gas and the height of theflue stack.

Induced Draft Burners

Induced draft is very similar to natural draft. Natural draft iswhere you have a negative pressure pulling the air throughthe burner. Induced draft is created by a fan in the fluestack. It is usually a high temperature fan because theproducts of combustion in the stack are very hot.

Forced Draft Burners

Forced draft burners have a fan pushing air through them, soit is under a positive pressure. By pushing more air throughthe burner, you can push more fuel through it and increasethe its heat release.

Forced Draft Burners can also come with natural draftback-up. This is usually important when a heater or furnacemust remain in operation if the air blower that supplies theforced draft fails. There is a panel or a trap door on the air


plenum, so that if the forced draft fan fails the door willautomatically open. These burners usually have some kindof electronic catch or a pneumatic cylinder so when the fanfails the door pops open. Air is sucked in by the chimneyeffect of the furnace. At this point, heat release may drop to50% of the normal forced-draft rate.

High Intensity Burners

“High intensity” means that the combustion is very intense.You have a very short, very well knit flame. These are aswirling type of injection burner. They are also very stableand are used mostly for boilers, dryers, kilns, and otherprocess furnaces.

High intensity burners were developed to improveefficiencies of cabin-fired heaters and furnaces. Most burnerflames are typically about one-million Btu’s per foot, so ifyou have a 10-million-Btu burner, you have roughly a 10foot flame. When you have a high intensity burner, 10-million Btu’s, it would only be a two-foot long flame. Or, inother words, about 5-million Btu’s per foot. Now this isimportant for certain process units if it is a very-small, very-tight type of unit. Then you can obtain a lot of heat release ina very small space.

In the center of some NAO burners, in place of the oil gun,is a gas burner. The holes in this type of gas burner aredrilled in such a fashion that, as the gas comes out, it swirlsin a vortex. This way you can get very good flame stabilityand also a very short, intense, and clear blue flame. NAOhas a patent on this type of burner.

NAO High Intensity Burners perform very well, but at aprice. The hotter flame of a high intensity burner generates alot of NOx. To meet new requirements, we haveincorporated the latest technology into NAO burners tosatisfy the most stringent NOx emission requirements.


Controlling Flame Direction

Vertically Fired Burners

Vertical fired usually means the burner flame is directedvertically upward or downward. A vertically-fired burnercan be either a radiant wall burner or a cylindrical flameburner. Air heaters and furnaces generally use vertically up-fired burners.

Down-fired burners are very common in AmmoniaReformers where ammonia or fertilizer is made. Theburners are mounted in the roof of the furnace and all theflames are downward.

Horizontally Fired Burners

Horizontally Fired Burners are mounted horizontally. It ismounted in the wall of a heater and fire out along the radiantfloor to heat it up or they might fire above the floor so that itis just a cylindrical flame. This information will be specifiedon the drawing or sketch provided by the customer.

Angle Fired Burners

Angle firing means the burner is at any angle between down-fired and up-fired. Angle fired burners are often directed ata furnace floor or a radiant wall, like the NAO Rams HornBurner (an inspirating burner).


Controlling Flame Shape

Cylindrical Flame Burners

Most conventional burners have a cylindrical flame. Acylindrical flame is either a pencil flame where the cylinder islong and thin, or it is (on a high-intensity burner) a short andwide flame.

To reduce pumping costs, the cylindrical burners are usuallydesigned for very low pressures, like the NAO LowPressure Cylindrical Gas Burner designed for fuel pressuresof 2 to 5 psig.

Flat Flame Burners

Flat flames look like a Japanese fan. Broad side, the flamehas a “V” shape; turn it 90° and it is wide but not very high.The flat flame can either be fired up along a wall, orhorizontally along a floor. Flat Flame Burners are usually putin heaters or furnaces that are long and thin and that requirea wide flame with a very uniform heat distribution. Asalways, it is very important to remember that you do notwant to have the flames from the burner to impinge upon thetubes inside the heater.

Flat Flame Burners are often inspirating gas burners, but theterm “flat flame” can also apply to oil fired burners. Oncombination oil & gas burners, the Hexad gas tips areusually drilled to produce a flat flame.

Spider Burners

A Spider Burner, for example, is a burner that looks verymuch like an old fashioned turnstile or a star. There is a hubwith eight arms radiating from it, like your gas stove at home.This produces a very short, intense blue flame.

Typical Flame Lengthfor Cylindrical Burners

Natural Draft 1 foot—–————

1 MM Btu

Forced Draft 1 foot—–————

2 MM Btu

High Intensity 1 foot—–————10 MM Btu

Typical Flame Lengthfor Flat Flame Burners

Natural Draft 1 foot—–————1¼ MM Btu

Forced Draft Not standard

High Intensity Not standard


Radiant Wall Burners

A radiant wall burner is usually a pre-mixed or inspiratingburner that functions by firing into or across against a flooror wall.

There are three types of radiant wall burners: Wall-mountedburners, floor-mounted burners, and side-mounted burners.Wall-mounted burners fire back against refractory-linedwalls. Wall-mounted burners have a flame that looks like aflower or a daisy that is pressed against the wall. Floor-mounted burners fire up along refractory-lined walls. Side-mounted burners are wall-mounted, up-fired burners that areup-fired out of and along refractory-lined walls. Therefractory lining absorbs and radiates heat throughout theburner chamber. Because the chamber is heated byradiation from the wall, slight variations in burner heat outputare eliminated and heating is very uniform.

Radiant Wall Burners

Wall-Mounted Floor-Mounted Side-Mounted


Burner Fuels

NAO can supply a burner for almost any fuel. It is notuncommon for NAO to supply a burner that uses threedifferent fuels. In the gas category, almost anything that iscombustible, including hydrogen and low-heat-value gasessuch as carbon monoxide (CO), or other combustible gasesdiluted with inert carbon dioxide (CO

2), nitrogen, or water

vapor. Typically, though, NAO burners use methane(natural gas), propane and refinery off-gases. Refinerygases are a little rough, because you don’t know exactlywhat those waste gases include. Natural gas is a commonconstituent, but other gases, including hydrogen can bepresent. These off-gases come from pressure relief valvesthat dump into the header system. One moment it is a veryheavy fuel with a lot of butane in it, the next moment maybring high hydrogen content. The sudden change in fuel willaffect the flame shape, and with pre-mix burners the suddenhigh hydrogen content could cause flashback.

In the past, the refineries vented many of these gases, butwith concern about smog and the greenhouse effect, ventingof volatile organic compounds (VOCs) isn’t acceptablethese days. These VOC’s must be collected and incineratedin an afterburner or thermal oxidizer, or put into a boiler orfurnace to produce heat. In this way, refineries now recoversome of the money they used to “throw away.”

Oil Fired Burners

Under the category of oils, there are various liquid fuels andalcohols. In the pharmaceutical industry, they use alcoholsto manufacture pills and medicines. The contaminatedalcohols must then be burned off, so this liquid waste couldbe just about anything. Sometimes the liquid waste burns toproduce secondary pollutants like chlorine gas or sulphurdioxide. In these instances, a scrubbing system is requiredto get rid of the chlorine or the sulphur dioxide.

Oil Guns are used for firing Naptha and refined & crudeoils. Naptha is similar to lighter fluid. Number 2 Oil is dieselfuel. Number 5 Oil is a heavy oil, and Number 6 is theheaviest standard grade of oil. Usually, #5 and #6 oil areused in heating very large buildings. Bunker C is a very


heavy oil. It is nearly the lowest grade and it is used mostlyin refineries for process heat, otherwise, it would be wasted.

Naptha can be a safety hazard if the burners are notdesigned properly or if the users are careless. Since it is avery light fluid, if it gets only little bit hot, it will flash and turnto gas; but it doesn’t burn well as a gas. NAO’s special gunfor Naptha burners utilize two separate pipes to keep theNaptha cool to prevent flash-over.

NAO oil burners can also fire an oil-water emulsion. This iswhere you have water mixed in with the oil—usually in therange of 10 to 20%. When you get more that 30% water, itis not going to burn well and the flame will be unstable.Such an emulsion may be used if you have very dirty oil. Itmakes the dirty oil burn cleaner, with less particulate andless NOx.

Besides refined oils, our units also burn various tars andcrude oil. When selecting burners to fire these heavy“fluids,” you must know their viscosity. You can think ofviscosity as the opposite of fluidity. If the viscosity is toohigh, fluidity is low, and the tar or oil must be heated beforeit flows through the burner. Tars are hard to burn becausethey have to be heated to a very high temperature beforethey flow through the pipes and to burn. Crude oil is usuallyrelatively easy because it is light weight, though this can be asafety problems because crude can contain a lot of lightgases that flash readily; therefore, crude oil must remainunder pressure until it is fired at the burners.

Oil guns can also be used to incinerate solids, coal dust, andcoal particles, either in a solvent or in an oil. SolventRefined Coal (SFC) is a way of having coal in a liquid form,so it can be pumped for use in burners. The coal is crushedto a very fine powder and mixed in oil. This type of slurrywas fairly common in the 1970’s during the energy crises.NAO has not handled many jobs like this recently, but westill maintain the capability.

Oil Atomizers

To make oil burn efficiently, the liquid must be broken up(atomized) into very small droplets that can vaporize quickly.The smaller the droplet, the greater the ratio of surface areato volume; so, more heat is absorbed per unit volume in a


given amount of time, and the oil vaporizes more quickly.NAO makes different types of oil atomizers: Steam, Air,Gas, Mechanical, and Dual-Stage (mechanical/steam or gas/steam).

Steam atomization is the most common oil atomizingmedium. When we use the word steam, remember that youcan substitute compressed air, high-pressure natural gas(around 100 psig minimum), or any other high pressuregas—even nitrogen and carbon dioxide. Though, normallyyou would not want to use an inert gas because they dilutecombustion.

In some cases, where you don’t need much turndown, youcan use pure mechanical atomization. However, this is rare,and this is where the dual stage comes into play.Mechanical atomization works good at high rates, but atlower turndowns it starts to fall off, so steam atomization willcomplete the job. This is dual-stage atomization.

Dual-stage atomization using inert gases and steam is oftenused on ships. On a ship there is no start-up steam to start aboiler burner with steam atomization. Ships do have bottledCO

2, however, that can atomize oil to get some heat into the

boiler. When the boiler is operating and making steam, theatomizing medium is switched from CO

2 to steam.

In special cases like these, you may be able to employprocess gases, such as with the Low Pressure AirAtomizer. This is actually uses a mechanical atomizer forthe oil at first, then air hits the oil spray and breaks it upfurther.

Oil Gun Safety

If you look at the bottom of an oil burner, at the very lowestpoint, you see the part of the oil gun known as detachinggear—something similar to a horseshoe that fits around theburner gun. A handle allows you to unscrew the gun andpull it out for cleaning. If an operator is careless, the oil gunmay be removed while it is under pressure. If thepressurized line contains crude oil with dissolved light gases,these can flash suddenly, ignite from surrounding burners,and cause a tremendous fire. To protect personnel againstsuch accidents, NAO can supply oil guns with safetyinterlocks.


Oil Guns often come with Safety Interlocks when used withvery light fuels—alcohol, naptha, or waste gasoline—youhave to be very careful. Any leaks could flash and cause amajor fire. Safety interlocks prevent the operator fromremoving a gun until everything is shut off and all valves areclosed. The interlocks are two valves that, when closed,shield the oil gun. Closing the valves moves the shields,allowing for the removal of the oil gun for service. Theseinterlocks prevent an unsuspecting plant operator fromopening a pressurized line of volatile products and causing amajor fire.

Gas Fired Burners

Low Pressure Gas Burners

Low Pressure Gas Burners are generally can be Multi-JetBurners or Multi-Tip Burners that rely on low pressure gas(less than 2 psig).

Hexad Gas Burners

Generally surrounding the Oil Guns or Low Pressure GasBurners are four to eight Gas Burners. These are NAOHexad Gas Burners. The Hexad is designed for pressuresof about 10 psig as a minimum and with pressures to 50 psigat the maximum rate.

Hexads are mounted on two semi-circular manifolds thatsurround the main oil and gas burner. The reason for thisdesign is so you can remove these burners for easymaintenance or cleaning. Sometimes refineries have verydirty gases with a lot of condensate (liquids like propane orbutane) that cause problems because they tend to flash andcoke inside the burner standpipes, eventually clogging theburner. To remove the Hexad Burners, you normallyincrease flow to the oil burner or the low pressure gasburner so the unit is still operating, then you remove theHexad manifold with the standpipes and tips to perform therequired maintenance. Many times the oil gun will alsorequire maintenance. Oil is very dirty stuff, and it is verylikely to drip if you do not have the atomization properlyadjusted. So, the total unit is designed either to fire the oilcylindrical gas burner (the Low Pressure Burner) while


working on the Hexad or to fire the Hexad while working onthe oil or cylindrical-gas burner. You can always keep anNAO burner in operation and firing while doing maintenancework.

NAO manufactures different types of gas burners, oilburners and combination burners. The burner can be dualgas, such as waste gas and fuel gas; it can be tri-fuel, such asgas, waste gas and oil.

One very important design feature of burners is low excessair, which affects the burner efficiency. If you allow a lot ofextra air into the burner, it is going to waste a lot of energybecause it has to heat up this extra air. The air is going alongfor a “free” ride. The way you control the amount of excessair is with very tight air control. This means that the airdampers must be able to shut tightly and the furnace wallsand floor must be sealed. If an NAO burner is out ofservice because it needs cleaning or maintenance, you canclose the damper off fully so no extra air will leak into theheater and degrade your efficiency.


Burner Controls

NAO supplies packaged control systems with infrared (IR)and ultraviolet (UV) flame scanners to control the pilot gas,the fuel oil, and the fuel gas of our burners. We also supplya number of air damper designs, manual and automatic, toregulate the amount of air going into the burner. With over80 years of burner design experience, we can provide acomplete burner package at a great price to satisfy anycustomer. These package systems can range anywherefrom $5,000 to $150,000. NAO provided a large, AirHeater package to Fina in Houston for $400,000.

NAO also supplies various types of oil and steam indicatingvalves to regulate steam and oil for various oil burners. Theyactually have a dial right around the top so you can see thefuel setting at the burner.

Packaged Systems

Package burner systems are burners with the blower, thewind box (the duct work where the air enters the burner),and all of the controls necessary for efficient operation.

A common packaged system is the Oil Patch Burner. Thisburner is used in heater treaters in oil field production. Formore information, see the applications section.


Burner Pilots

NAO makes several types of burner pilots. The mostsimple is an inspirating pilot with manual ignition. We usethese on natural draft burners. The pilot gas is injected intoa bell mouth inspirater to draw in combustion air. Manualignition saves money because operators can use one hand-held ignitor to light many burners.

Burner Pilot Ignitors

NAO has several Hand-Held Ignitors. One is actually apilot attached to a long rubber hose with a pressureregulator from a propane bottle. It is used to light off all theburner pilots within reach of the hose.

Another Hand-Held Ignitor is the NAO Portable Ignitor. Ithas a propane bottle, a sparker, and a battery. We alsohave a newer version—the NAO Portable Burner Ignitor,NPBI—that does not need a battery. These are cost-effective and safe alternatives to the “traditional” method ofusing a burning rag to light pilots.

NAO also makes a Spark-Ignited Burner Pilot. It has aspark generator near the inspirater with a long electrode thatextends to the nozzle where it produces a spark.

NAO has a High Energy Electronic Ignitor, used on airheaters or reactors. Often we sell them to customers forchemical reactors. It generates sparks to ignite the burner.This is a retractable ignitor, so once we prove the flame,either with a UV scanner or some other means, then it pullsout to prevent burn up.

Burner Pilot Controls

Burner pilot controls are used to sense if the pilot isoperating. When the pilot is burning, the controls aresatisfied, if not, the controls may try to re-light the pilots orlockout the main fuel and sound an alarm. The response ofthe controls will depend on the design requirements.


Two common methods of sensing the pilot flame are flameionization rods and ultraviolet (UV) scanners. With theionization rod, the flame hits the rod, and the rod senses thatthe pilot is running. The flame rod passes a very smallcurrent, just a thousandth of an amp, but enough to senseions formed in the pilot flame. The UV scanner looks forUV rays generated in the pilot flame. These are very goodflame sensors because they can focus on the pilot andinstantly detect if the flame goes out.

Before the development of UV sensors, infrared (IR)detectors were commonly used to monitor pilot flames inburners. (IR detectors are still commonly used to detectpilot flames on flares.) IR sensors do not work well onburner furnaces, however, because, in the heat of thefurnace, the IR sensors cannot detect the difference betweenthe heat of the pilot and the heat of the furnace floor andwalls. UV sensors offer the advantage of detecting flame,not just heat.


Burner Efficiency

Air Pre-Heat

An air pre-heating system reclaims heat from the flue gasand adds that heat to the air required for combustion. Airpre-heaters are often used on forced-draft systems toincrease burner efficiency.

With standard oil and gas burners, the efficiency of theoverall performance of the burner system will increase about2% for every 100 °F increase in air temperature.

Pre-heating air creates two potential problems. First,heating the combustion air raises the temperature of thecombustion products in the heater, which increases theformation of NOx. There is a trade-off between efficiencyand pollution control.

Second, pre-heating the combustion air with the stack gascan reduce the gas temperature below the dew point of thesulfur compounds in the products of combustion. If thisoccurs, H

2SOx can forms and collect on the walls of the

stack, accelerating corrosion of the metal.

Combustion SystemCheck List

for energy-cost reduction

√ Multi-fuel capability

√ Unconventional fuels

√ Combustion efficiency

√ Low excess air

√ High swirl

√ Heat recovery

√ Minimize losses in wallsand stack

√ Controls for optimizedoperation


Burner Noise

Burners tend to create noise—almost like a rocket takingoff. This is a big concern, particularly since the early 1980’swhen OSHA required that we have various means to quitedown the burners. We use either individual noise shrouds orplenums. A plenum is a large box you put around theburners. This will be illustrated in some of the later photos.

We have Acoustically Shrouded versions of all NAOburners. Basically, the shroud is a metal box with anacoustical inner lining to absorb combustion noise and thehigh-frequency noise of steam and gas injection.

The acoustical plenums can serve additional purposes. Theyare insulated, so they reduce heat loss through the bottom ofthe burner. Sometimes, the plenums are designed tocirculate water and thermal fluids to preheat air to improveburner efficiency. However, this usually raises NOx levels,so you have to balance air pollution versus efficiency.


Low NOx Burner Technology

Since the introduction of high intensity burners in the 1960’s,NAO has developed the technology to reduce the formationof NOx in the combustion flame. We have incorporated thelatest NOx technology into NAO burners to satisfy the moststringent emission requirements.

Interestingly, NAO’s original burner designs like theRams Horn burners have proven to be some of the bestLow NOx burners on the market.

NOx formation is a function of the temperature of the flame.Cool flames do not produce NOx, but cool flames are notas efficient as the high-intensity flame of modern burners. Asa compromise, NAO has developed staged-combustionburners and burners that re-circulate the flue gas to reduceNOx while maintaining hot flames.

Before exploring the design methods to limit NOx, we canexplore the factors that either cause, or increase, NOxproduction. NOx includes all oxides of nitrogen, this can beNO, NO

2, or NO

3. The factors mentioned are not

necessarily in order of importance.

Factors Leading to NOx Formation

1. Flame Temperature2. Furnace Temperature3. Residence Time at High Temperature4. Bonded N

2 in the fuel.

5. Hydrogen in the fuel6. High Molecular Weight (MW) of fuel7. Un-saturated fuel8. Temperature of combustion air

Oxidation of part of the free nitrogen in the combustion air isthe source of NOx, except in Item 4. Chemically bondednitrogen oxidizes to NOx in higher proportions than free N

2

in air.

Hydrogen in the fuel (Item 5) increases NOx for tworeasons. First, the flame temperature is higher, and second,NOx formation increases because the quantity of flue gas


per heat generated is lower. Hydrogen releases atremendous amount of heat per unit mass (51,600 Btu/lb)compared to other gaseous hydrocarbons (~21,000 Btu/lb)

It is obvious that among items 1-8, some factors are noteasily changed. The one pertinent, changeable factor isflame temperature. For a given fuel, it is reduced by anumber of methods, which are:

A. Staging Air, in which the initial (primary) combustionis starved for air (approximately 75% of theoretical),with second stage air making up the completedcombustion.

High-intensity burners with very hot flames generatefairly high NOx levels. By using staged air, we canreduce these NOx levels. NAO burners have aseries of slots around the outside of the burner toprovide staged air (also known as secondary air).The slots surround a large main opening where themain flame emerges.

B. Staging Fuel, in which the primary stage fuel(approximately 60% of total) is given all thecombustion air needed (100% of total), reducing itsflame temperature, with the remaining fuel injected ata higher level, maintaining the lower flametemperature. This method is superior by some 20parts-per-million-by-volume (ppmv) to Staged Air.

This is a good time to insert the observation that,among standard unstaged burners, the pre-mix orbi-mix types are superior by several parts-per-million to raw gas types; but both are inferior tostaged fuel burners. It may also be noted thatstringing out the combustion process results inlonger, lazier flames than industry has grown todemand.

C. Injection of Steam into the combustion air.

D. Injection of Inerts (CO2, H

2O, but not N

2) into the

fuel stream.

E. Recirculated Flue Gas (20-30%) into thecombustion air. This is the ultimate procedure,producing NOx in the 20-ppmv range, but it is far


Technique

Low Excess Air

Off-StoichiometricCombustiona. Burners out of

serviceb. Overfire Airc. Air lances

Low-NOx Burners

Flue GasRecirculation(FGR)

Water/SteamInjection

Reduced AirPreheat

Selective CatalyticReduction

SelectiveNoncatalyticReduction)–UreaInjection

SelectiveNoncatalyticReduction–Ammonia Injection

Descriptions

Reduces availableoxygen

Staged combustioncreates fuel-rich orfuel-lean zonesreducing flametemperatures

Provides internalstaged combustionand/or recirculation,thus reducing peakflame temperatures

As much as 20-30%of the flue gas ismixed withcombustion air, thusdecreasing peakflame temperatures

Injection of steamor water at theburner to lowerpeak flametemperature

Reducing air preheatreduces flametemperature

Catalyst located influe gas stream(usually upstream ofair heater)promotes reactionof ammonia (NH3)with NOx toproduce H20 and N2

Injection of ureainto furnace toreact with NOx toform nitrogen andwater

Injection ofammonia intofurnace to reactwith NOx to formnitrogen and water

Advantages

Simple modificationby closing airdampers

Low operating cost;No capital cost forburners out ofservice

Low operating cost;Compatible withFGR as acombinationtechnology tomaximize NOxreduction

High NOx reductionfor natural gas andlow-nitrogen fuels

Moderate capitalcost; NOx reductionsimilar to FGR

High NOx reductionpotential

High NOx removal

Low capital cost;Simple system;Moderate NOxremoval; Non-toxicchemical

Low operating cost;Moderate NOxremoval

Disadvantages

Low NOx reductionpotential

a. Typically requireshigh air flow tocontrol COemissions

b. Relatively highcapital cost foradditional blowers

c. Moderate capitalcost

Moderate to highcapital cost; Notapplicable to allburners and fuels

Moderate to highcapital andoperating costs;Will affect heattransfer and systempressures

Efficiency penaltywith additionalwater vapor lossand fan powerrequirements forincreased mass flow

Significantefficiency loss (1%drop per 40 °Freduction in airpreheat)

Very high capitalcost; Highoperating cost;Extensive ductworkrequired; Increasedpressure drop mayrequire induced-draft fan or largerforced-draft fan;Reduced efficiency;Ammonia sulfateremoval equipmentrequired; Watertreatment of airheater wash required

Temperaturedependent; Designmust consideroperatingconditions; NOxreduction maydecrease at lowloads

Moderate to highcapital cost;Ammonia handling,storage,vaporization, andinjection equipmentrequired

Impacts to Consider

High CO emissions;Flame length;Flame stability

Flame length;Forced-draft fancapacity; Burnerheader pressure

Forced-draft fancapacity; Flamelength; Designcompatibility;Flame stability athigh turndown

Forced-draft fancapacity; Furnacepressure; Burnerpressure drop;Flame stability athigh turndown

Flame stability;Efficiency penalty

Forced-draft fancapacity; Efficiencypenalty

Space requirements;Ammonia slip;hazardous-wastedisposal

Furnace geometryand residence time;Temperatureprofile

Furnace geometryand residence time;Temperatureprofile

Applicability

All fuels

All fuels;Multiple-burnerdevices

All fuels

Gas fuels andlow-nitrogenfuels



Gas fuels andlow-sulfur fuels(liquid and solid)

All fuels

All fuels

NOx Reduction

1-15%

30-60%

30-50%

40-80%

40-70%

25-65%

70-90%

25-50%

25-50%

NOx Control Technologies


and away the most expensive, requiring a stack exitnozzle, piping to an induced draft fan by way of aheat exchanger to reduce temperature to a leveltolerated by the fan. At the same time, a forceddraft fan brings new combustion air in at a pressureequal to the inducer’s discharge pressure.Recirculation will add hundreds of thousand dollarsto a large heater installation.

For purposes of developing low-NOx burners in thereasonable-price category, NAO has concentrated onburner design using the staged-fuel format.

In early designs, burners were built in round and rectangularshapes, with pre-mix and raw gas first stages, and raw gassecond stages, in all proportions of distribution ranging from90% primary and 10% secondary to 10% primary and 90%secondary. Two salient observations came to light: LowerBtu burners were superior to higher, and rectangular burnerswere superior to round.

With the exception of one round, floor-mounted, vertically-fired burner with recirculating features resulting in NOxemissions around 35 ppmv, all the tested burners arecapable of being fired vertically up or down, or horizontallywith equal results.


Burner Applications

Oil Patch Burners

Oil Patch Burners are used in oil production fields to heatcrude so its pumps more easily. Oil Patch Burners areusually very simple. They come with a jet mix burner, pilot,damper and flame arrestor. At the bottom of thehousing—the register for the burner--you can see a flamearrestor sandwiched between two square flanges. Thisflame arrestor grid prevents flashbacks and explosions ifthere happens to be any gas in the air that gets drawn intothe burner.

Fuel Pumping Station

We have built a number of packaged burner units to runFuel Pumping Stations. Tars and other very heavy oils mustbe heated to be pumped. Sometimes you must heat them to300 or 400 °F before pumping is possible.

Submerged Combustion Units

Submerged Combustion Units are burners that fire downinto a water seal or pot of water. Submerged CombustionUnits are used to evaporate or heat water, particularly whenlarge quantities of hot water are required. The submergedburner is very efficient because all the heat of combustiongoes into the water stream. Machine shops use theseburners to clean water-base cutting oils. The submergedburner is a way to evaporate most of the water and reducethe cutting oil to a very low volume so it can then belandfilled—with volume reductions of 90 to 95%, there aresubstantial savings in disposal expense.

Inert Gas & Nitrogen Generators

Inert Gas Generators are usually fired by diesel, natural gas,or propane, to produce water, nitrogen and carbon dioxide.Most of the time, a dryer is used to remove the water. Thedesired product is nitrogen and carbon dioxide. The Inert


Gas Generator is a way of providing inert gas for tanks andvessels.

Another recent focus of NAO activity is nitrogen generators.These use carbon beds or membrane-filter technology toextract nitrogen from compressed-air or forced-draft feeds.We hope to see these used on flare stacks to purge the lineand prevent air ingress. Customers now rely on natural gasor refinery gas to purge their lines, but this can be expensive,and it is not an environmentally friendly procedure. WithNAO’s next-generation, mini Inert Gas Generator,customers will be able to provide an inert purge gas to theirflares at a very low operating cost.

Duct Heater

Duct heaters are a type of air heater. They are used onturbines that are used for standby power. In cities where thepower plants are not large enough to handle the peak powerrequirements, for example, gas-fired turbine generatorscover peak electric loads. These turbine generators tend tobe very inefficient, so they use a duct burner on the turbine’sexhaust, which has a large air volume at around 500 °F. Byinstalling an NAO duct burner, the exhaust is heated andpushed through a boiler to recover the heat as steam tomake more electricity.

Fire Simulation Systems

NAO has developed a number of packaged, real-world-conflagration test systems for firefighters to use for skillstraining. NAO’s simulation systems incorporate uniqueburner heads, reliable pilots, high-energy ignitors, fail-safevalves, fire-proof wiring, and optional fire-snuffing systemsto deliver safe simulations of conflagration disasters atprofessional firefighter schools.

Specialty Burners

NAO custom engineers many specialty burners. We oncemade burners for the incineration of disposable paperclothes that people wear at a nuclear facility. When they


come out, the paper clothes are burned and the radioactiveash and dust are accumulated and stored.

We are also currently developing a burner system for theincineration of High Explosive Dust, a contaminant generatedduring the production (and de-commissioning) of nuclearwarheads. Our burners will be used to “sterilize” equipmentas it is removed from service and before it is landfilled.


Common Problems with Burners

No pilot spark

Shorted electrode gap: Moisture, corrosion, or otherparticulate from the pilot gas stream can collect between thespark-ignition electrode.

Improper grounding: If the electrode is not grounded,there is no potential difference between the electrode andthe pilot wall that causes an electric arc to ignite the pilotgas.

Ignition transformer: The ignition transformer may not bereceiving power, or the transformer is burned up and notallowing for passage of electric current.

No pilot flame

Blocked jet: The pilot jet orifice is blocked due toparticulate from the gas or from corrosion. If the problem ispersistent, consider installing pilot gas filters.

Low (or high) gas pressure: Low gas pressure creates agas mixture with too much air for ignition. High gas pressurecreates a mixture with too much gas for ignition.

High pilot-air flow: If the air inspirater is open too far, thepilot jet inspirates too much air for the pilot to light.

High combustion air flow: Burner combustion air affectsthe pilot. On forced-draft and induced-draft units, the airblowers should be at their low-fire settings to avoid dilutingthe pilot gas mixture.

UV Scanner detects spark: If the scanner detects the pilotspark, it thinks the pilot is lit and it stops the ignitionsequence. The UV scanner can be aimed at the top of thepilot nozzle rather than directly into the nozzle.


Small pilot flame

Partially blocked jet: The pilot jet orifice is blocked due toparticulate from the gas or from corrosion. If the problem ispersistent, consider installing pilot gas filters.

Plugged filter: A plugged pilot gas filter will allow forcorrect pressure settings but will prevent gas flow.

Low gas pressure: Low pressure means low gas flow.Check for obstructions in the gas line/header and forpartially closed valves.

High pilot-air flow: Allowing the pilot to inspirate toomuch air can cause a small (generally unstable) flame.

High combustion air flow: High combustion air flow willpush the pilot flame into the pilot nozzle.

Unstable pilot

Wet gas: Water condensate in the pilot gas absorbs heatenergy from the pilot flame and leads to instability.

Low (high) gas pressure: Low gas pressure allows theflame to be diluted with too much pilot and burnercombustion air. High gas pressure can cause the flame tolift-off from the pilot nozzle.

Too much air: Improper pilot air adjustment allows toomuch air in the pilot. This should be evident by an intenseblue pilot flame.

Not enough air: Improper pilot air adjustment allows toolittle air in the pilot. This should be evident by a lazy yellowflame

Partially blocked jet: The pilot jet orifice is blocked due toparticulate from the gas or from corrosion. If the problem ispersistent, consider installing pilot gas filters.

High chamber draft: High combustion air flow can blowout the pilots.


Gas burner will not light

Too much air: Too much air dilutes the gas beyond itsupper explosive limit. Check damper and fan settings.

Too little fuel: Not enough fuel in the air/gas mixture onforced-draft and induced-draft units can be caused by low-gas pressure or by obstructions in the gas header.

Low fuel pressure: Low pressure leads to low gas flow.

Plugged strainers: Plugged gas strainers block flow evenwhen supply pressure is adequate.

Small pilot flame: Short pilot flames can miss the air/gasmixture of the burner.

Interlocks improperly set: Interlocking valves not fullyopen restrict flow to the burner.

Strainers in wrong position:

Oil burner will not light

Poor atomization: Oil must be properly atomized forstable combustion. Causes of poor atomization are listed inthe next section.

Oil and steam connections reversed: Crossing theconnections always causes poor atomization.

Low combustion air: Improper air settings or restrictions inthe air header lead to a mixture too rich in oil too burn.

Plugged strainers: Plugged strainers block flow.

Small pilot flame: Short pilot flames can miss the air/oilmixture of the burner.

Interlocks improperly set: Improper setting of interlockingpurge valves will allow steam or inert gas in the main fuelline.

Gun positioned improperly: Positioning the oil gunincorrectly may place it out of range of the pilot.


Poor atomization

Low oil temperature: Heavy oils—#5, #6, Bunker C—and tars must be pre-heated before they flow properlythrough pipes and before they can be atomized properly.

Low steam flow: Low steam (or compressed air or high-pressure gas) flow can be caused by low pressure or byobstructions in the header piping.

Low oil pressure: Low oil pressure can be caused bypartially blocked valves or plugged strainers.

Low steam pressure: High pressure steam (or compressedair or gas) provides the energy to atomize the oil. Ifpressure falls off, its atomizing effectiveness is reduced.

Oil and steam connections reversed: Crossing theconnections always causes poor atomization.

Distorted burner flames

Incorrect burner positioning: The refractory tiles ofburners stabilize the burner flames. If the burner isincorrectly positioned, it will be close to some tiles and farfrom others. The tiles close to the burners will absorb andradiate a dis-proportionate amount heat, distorting theburner flame.

Tips turned wrong direction: Burners are designed to firesymmetrically. Check the burner design for the proper anglefor tip alignment. Some burners have the tips firing at anangle to cause a swirling and turbulent cylindrical flame.

Incorrect burner tips: Burner tips are drilled for specificgas compositions and pressures. Mis-matching replacementtips with burners will cause firing problems.

Incorrect tip drilling: Burner tips are drilled for specificgas compositions and pressures.

Corrosion of tips: Tips can be plugged due to corrosion ofthe heading piping, or the tips can be worn away byparticulate in the gas stream. If tip wear is persistent, tipscan be made of extremely-hard, exotic alloys.


Uneven air flow: Improper design of air boxes can causeuneven air flow. This will be seen most often with forced-draft burners.

Warped floor: Heat damage to the floor of fired heatersand furnaces (or to walls of wall-mounted burners) lead toincorrect burner positioning.

Header piping not sized properly: Under-sized gas headerpiping causes a large pressure drop that reduces gas or airflow to the burner.

Broken or missing refractory: Refractory tiles of burnersstabilize the burner flames. If tiles are missing or broken andout of position, they will not absorb and radiate heatproperly, which can distort the burner flame.

Sparklers in flame

Water in gas: Condensation of water in the gas header willbe indicated by sparks in the flame.

Wet steam: Condensate in steam lines on atomizing oilburners will cause small, bright sparks in the flame.

Poor atomization: The larger the oil droplet, the slower itburns (if it burns).

Unstable gas flame

High air flow: Too much air tends to blow out the burnerflame.

Mis-positioned tips: Incorrect tip placement due toincorrect burner mounting or warped floors will affect theability of burner tiles to stabilize the burner flame.

Liquid carry-over: Slugs of liquid will bring too much fuelto the burner. If the liquid burns on the tip, it will greatlyaccelerate corrosion of the burner.

Improperly installed or designed flame retention device:Some burner designs use baffles to prevent air flow fromblowing out the burner flame. Others, like most NAO


burners, use burner block to absorb and radiate heat tostabilize the combustion flame.

Missing flame stabilizers: Raw gas burners have flameretention devices to prevent the flame from blowing off ofthe burner tip. Some burner designs have flame retainersthat are metal baffles that can corrode and break off, leadingto unstable gas flames.

Unstable oil flame

High or low oil temperature: Burners are designed forspecific fuel conditions. If these conditions are not met,combustion may not be stable.

Overheated oil gun: Overheated oil guns cause pulsatingflames.

High steam flow:

Oil and steam connections reversed:

Water in oil: Slugs of water displace fuel and snuff theflame.

Improperly installed or designed flame retention device:Some burner designs use baffles to prevent air flow fromblowing out the burner flame. Others, like most NAOburners, use burner block to absorb and radiate heat tostabilize the combustion flame.

Pump cavitation: Pulsating fuel flow causes unstablecombustion.

Pulsating pressure pump: Pulsating fuel flow causesunstable combustion.

Leaky purge (by-pass) valve: Steam or inert gas in the oilline can be caused by leaking or open purge valves.

Improper forced-draft fan turndown: Over-sized airblowers surge at low flow rates. Under-sized blowerscannot provide adequate air at high fire rates.


Carbon build-up on oil gun

Incorrect positioning of gun: Mis-alignment of the oil guncan cause the oil jet to touch the burner block. The blockprevents the proper atomization of the oil where it burns(partially) and forms carbon.

Poor atomization: Poor atomization causes oil to dribbleon the oil gun where it burns and forms carbon. See thesection on poor atomization for more information.

Corrosion & Erosion

Vanadium, sulfur, or sodium in oil: They corrode themetal of the burner tips, standpipes, and housing, and theycorrode the refractory lining and metal support structure ofthe heater. If the problem is persistent, exotic alloys can beused on the burners that are resistant to the corrosive effectsof vanadium and sulfur, but this does not protect the furnacetubes. The adverse effect of vanadium on the furnace tubescan be partially counteracted by lower the tube metaltemperatures. Except for furnaces operating withexceptionally high tube metal temperatures, vanadiumcontent of 20 ppmv or less are normally not of concern.Sodium is an equally serious problem in heaters, though de-salting the crude will generally eliminate this concern.

Solids (sand, grit) in fuel: Over time, solid particles wearaway the burner tip. This is the result of missing strainers orstrainers that are too coarse.

Poor material selection: Among other things, H2S reacts

with carbon steel piping to form a yellow powder that canplug tips and other orifices.

Burner shuts off automatically

UV detector doesn't see flame: The UV detector may beimproperly aligned, or the lens of the scanner is blocked.

UV detector reads pilot spark as flame: The UV scannerdoes not need to target the pilot nozzle. The burner pilotflame should be several inches taller than the pilot nozzle.


Solenoid valves backwards: If the fuel gas to someburners shuts down automatically as the firing rate isincreased, check the direction of the solenoid valves. If thevalve is backwards, it may function under some flowconditions and shut down suddenly under others.

Low heat release

Change in fuel gas: If the new fuel gas has a lower heatingvalue, the overall heat release will be lower under the samegas flow rates.

Too much or too little air: Too much air takes additionalheat as it passes through the furnace. Too little air allowssome fuel to pass through the furnace without burning.

Low gas flow: Restrictions at the burner will result in lowflow at proper pressures. This may only be evident by lowheat release.

High burner heat release

Burners over-fired: High gas pressure results in high gasflow and excessive heat release that can damage a heater orfurnace.

Oil and steam connections reversed: Pushing oil throughthe steam nozzle will allow for higher oil flow, yielding ahigher heat release.

Worn tips: Tip wear increases the drilling size, whichincreases the fuel flow through the tips.

Low excess air: The hottest combustion flame is atstoichiometric conditions. Excess air ensures completecombustion, but it also reduces flame temperature.

Burner block or tips out of position: Mis-alignment of theburner creates distorted flames that can cause pockets ofhigh heat in the furnace or heater.

Over-pressured furnace: High furnace pressure holdsproducts of combustion in the furnace longer and can reduceexcess air flow.


Insufficient draft

Stack damper (partially) closed: This causes a restrictionin the stack that reduces the flow of combustion air into theheater.

Blower wired backwards: Forced-draft fans only operateat a fraction of their rated capacity when they operatebackwards.

Excessive furnace leaks: Tramp air, excess air that entersthrough leaks in the furnace floor and walls, cools theproducts of combustion and reduces their buoyancy. Thedraft caused by the chimney effect is then reduced.

Too much draft

Stack damper wide open: Stack dampers restrict flow outof the furnace.

Air registers close improperly: Air registers may be stuckfully open.

Under-firing forced-draft burner: Under firing the burnermeans that the blower controls are not reducing air flow tothe burner to match a reduction in fuel flow.

Flashback

Gas flow too low (on pre-mix burner): Low gas flow on apre-mix burner slows the velocity of the air/gas mixturethrough the burner nozzle. If the velocity drops below theflame speed, flashback occurs.

Pockets of hydrogen in refinery gas: The flame speed ofhydrogen is significantly greater than methane or propane,the primary constituents of refinery gas.

Venturi (of pre-mix burner) overheats: Heating theventuri or other piping of a pre-mix burner due to damaged


insulation or due to absence of coolant may raise the gas/airmixture above its auto-ignition temperature.

Smoke and CO formation

Too little (or too much) air: Lack of oxygen prevents thecomplete combustion of fuel. Too much air can quench theflame before combustion is complete.

Over-fired oil gun: Too much fuel makes completecombustion impossible.

Low oil temperature: Improper pre-heating of oil reducescombustion efficiency that can lead to smoke and CO.

Oil and steam connections reversed: Oil is improperlyatomized so combustion is incomplete. Also, oil flowthrough the steam side of the oil gun is excessively high.

High NOx formation

High furnace temperature: NOx formation is a function oftemperature. The higher the flame temperature, the higherthe NOx formation.

High fuel-bound nitrogen: Firing burners at low excess aireliminates most of the nitrogen that can form NOx, howevernitrogen entering burners through fuel cannot be eliminated.

High hydrogen content of fuel gas: Hydrogen burnshotter than other gases.

Switch fuel from gas to oil: Oil produces more NOx thangas, in part because of fuel-bound nitrogen.

High intensity combustion: High intensity combustioncaused by flow swirling and vorticity increases the heatdensity and temperature of the flame.

Noise (resonance)

Unstable combustion flame: Distorted and unstable flamesgenerate low frequency noise.


Resonance of piping: Resonance of nozzle-mix burners orof steam injection can generate high frequency noise. Over-sized air dampers that are almost completely closed maycause cavitation of air flow past the damper.


References

Chemical Engineering Progress. January 1994


Bitterlich, G. M. The NOx Enigma discusses the factorscontributing to NOx formation using emperical datacollected from NAO burner testing. Ask forRP77-14.

“Burner Noise and Its Suppression” is a technical report byNAO that analyses and relates the effects of fuels,firing rates, and burner design on noise.

Combustion Tips, Volume 1, Number 4, “Lo-NOxBurners reduce nitrogen oxides to acceptable levels”reviews the newest feasible technology for reducingNOx in burners.

“Fire Simulation Systems” is a four-page piece of NAOliterature with photographs and descriptions of themany fire simulation systems we have provided inthe past few years.

For copies of these and other related technical papers,please mail or fax your inquiry to: Literature Department,NAO Inc., 1284 East Sedgley Avenue, Philadelphia PA19134, Fax 215.743.3020.

Air Heaters — Property of NAO Inc. 4-1

Air Heaters

NAO has designed and fabricated Air Heaters for manyyears. The first NAO unit was made around 1933 for acompany in Chicago called UOP (Universal Oil Products),which is involved with various process designs using airheaters. Currently, we have a large air heater for a UOPprocess for Foster Wheeler in New Jersey; the final user isFina in Port Arthur, Texas. This unit is 9 feet in diameter, 42feet long and has an operating pressure of around 150 psig.The Fina unit fires about 80 MMBtu’s of heat. We havemade a larger one for a company called Llanoven inVenezula that had a firing rate of about 150 MMBtu’s.

Operating Temperatures

Normally, air heaters operate at fairly low temperatures,1,400 to 1,500 °F. We have produced air heaters fortemperature ranges from 200 to 2,800 °F. However, fewair heaters get as hot as 2,800 °F, though high temperatureheaters are easier to make. Low temperature air heatersrequire a lot of bypass air. Bypass air dilutes the extremelyhot combustion products of the air heater burner. It alsotends to make the burner flame unstable, sometimesquenching the flame, unless the heater is designed properly.

We recently built a High-temperature Air Heater for ValeroPetrochemical in Corpus Christi, Texas. It was designed for2,700 °F—very hot air. Valero’s application was to heat aflue stack so that they would get more lift (from thebuoyancy of the gas inside) and to prevent the formation ofcondensate. They had scrubbers on their stacks to reduceemissions of sulfur compounds that form acid rain, but thescrubbers reduced the flue-gas temperature, producingcondensate. Injecting very hot air from our air heater raisedthe temperature and eliminated the condensate. Our unitreplaced an existing Brand-X Air Heater that had corrosionproblems: The moisture that the heater was supposed toeliminate was being thrown into the air heater, where itdestroyed the metal integrity of the housing.

4-2 Air Heaters — Property of NAO Inc.

Lower temperatures are used for drying grains andtobacco. We recently sold an induced draft unit to FMCCorporation to be used as a dryer. This unit operated at avery low temperature—300 °F to 400 °F. The lowertemperature air heaters can also be used for drying cementand for freeze prevention. Customers use them for airplanesin hangers and to dry out boilers and furnaces after repairs.

Air Heater Pressures

Air Heaters generally operate in one of three ways: ForcedDraft, Natural Draft, and Pressurized.

Forced-draft Air Heaters rely on fans or blowers to supplyair to the burner and bypass. The fans push the heated airthrough the Heater to the vent stack or drying area where itis needed. Forced-draft Heaters operate at positivepressures up to 2 psig.

Natural-draft Air Heaters are preferable to forced draft, ifthey are applicable. Natural-draft heaters do not require theadditional capital or operating expense of fans and blowers.An natural-draft air heater will have two openings withdampers: One opening will supply air to the burner, theother will supply air as a bypass. Natural draft relies on thebuoyancy of the hot combustion products to draw air intothe heater. These heaters operate at slightly negativepressure—from -2 to -6 inches of water column (-5 to -15mbar).

Pressurized Air Heaters are less common than natural orforced draft heaters. Pressurized Air Heaters can be madeto supply heated air at up to 150 psig. With pressurized airheaters, the ignition system must be designed particularlywell. Standard spark ignitors for burners do not work atpressures over 3 psig. NAO has a High-intensity SparkIgnitor that has been tested at pressures up to 3,000 psig.

Portable, Rental Air Heaters

There are a number of regulations developing in the UnitedStates for ships and tank trucks, requiring them to becleaned after each use. Portable air heaters would be idealfor blow-drying them so that the ship or truck can return to

Air Heaters — Property of NAO Inc. 4-3

service as quickly as possible. These can be skid-mountedor trailer-mounted units. NAO has the capability rent thesetypes of portable air heaters. They actually haven’t beenbuilt yet, but they can be quickly assembled to meet acustomer need.

Excess Air vs. Heater Temperature

This chart approximates the effect of excess air on the hotmix temperature for any burner. The actual value will varyslightly for differing fuel compositions.

4-4 Air Heaters — Property of NAO Inc.



Sprayers — Property of NAO Inc. 5-1

Sprayers

NAO has a very large market share of all the ProcessSprayers. We probably sell about 90 to 95% of them—anice piece of business! The sales per year fluctuate up anddown, but we often sell as much as a half-a-million dollarsof sprayers each year. We have only one main competitor,Brand X, but they have only a small part of the market.

Process Sprayers are used mostly to spray water forquenching or scrubbing. They also spray oil for cat crackerstart-ups, and they spray various chemicals for scrubbing orfor process units. These sprayers handle up to 3,500gallons per hour, even at very high pressures. In oneapplication, we supplied a sprayer for coal gasification at1500-psi reactor pressure—we had to have a winch on thegun to prevent it from flying out and causing injury ordamage.)

Many times, sprayers handle very heavy chemicals thathave sand and other solids in them, so the tips will tend toerode. There is quite a bit of replacement business here. Tohelp reduce wear, we often make the tips out of exoticmaterials such as Hastelloy or Stellite. Stellite is a very hardmaterial. It is almost impossible to drill or machine, so weessentially have to cast the tip with the holes or the slots rightin it.

If a sprayer must be removed for cleaning or repair, youmust loosen the packing gland, so the sprayer gun willretract. When you’ve opened the packing gland, you canclose the sprayer valve and remove the sprayer forinspection, maintenance, or cleaning.

5-2 Sprayers — Property of NAO Inc.

Sprayer Applications

Production and processing

• Spray drying of dairy products, coffee and tea,starch pharmaceuticals, soaps and detergents,pigments, and other products

• Spray cooling• Spray reactions, such as absorption and roasting.• Atomized suspension liquids such as effluents, waste

liquors, etc.

Treatment

• Evaporation and aeration• Cooling of spray ponds, towers, and reactors• Humidification and misting• Air and gas washing and scrubbing• Industrial washing and cleaning

Coating

• Surface treatment• Spray painting• Flame spraying• Insulation, fibers, and undercoating materials• Particle coating and encapsulation

Combustion

• Oil burners in furnaces and heaters• Diesel fuel injection• Fuel injection on gas turbines• Rocket fuel injection

Miscellaneous

• Dispersion of chemical agents• Agricultural spraying of insecticides, herbicides,

fertilizer solutions, etc.• Foam and fog suppression• Printing• Acid etching

Sprayers — Property of NAO Inc. 5-3



Refractory — Property of NAO Inc. 6-1

Refractory

Refractory materials serve one of two purposes. In someapplications, the purpose of refractory is to act as aninsulator. Insulating refractories shield the metal walls ofprocess equipment from the corrosive effects of products ofcombustion at elevated temperatures. In other applications,the purpose of refractory is to collect and radiate heat. Infired heaters, furnaces, thermal oxidizers, and othercombustion equipment, the ability of the refractory to storeand radiate heat insures a stable, uniform flame. In mostapplications, refractories serve the dual purpose of insulationand flame stabilization. Every application has specificrequirements that can be met by a wide variety of refractoryproducts.

There are a wide variety of refractory and insulation productsavailable. Their significant properties include:

• Ability to maintain strength under high-temperatureconditions

• Cold crushing strength—impact strength at low-temperature operations and the ability to withstandhandling and shipping without damage

• Modulus of rupture—a standard measure ofstructural strength provided by load testing

• Insulating value—the ability to provide resistance tothe flow of heat

• Porosity—the susceptibility to penetration by slagsor gases

• Erosion—the physical deterioration of material due tocontact with gases, liquids, an solids on the hot face

• Corrosion—the chemical process that destroys therefractory bond or the chemical integrity of theinsulation. Of particular interest is the operation ofrefractory in an oxidizing or reducing atmosphere. Areducing atmosphere, one deficient in oxygen, willtend to degrade refractory material containing iron orsilicon components.

• Thermal expansion—the reversible change in lineardimensions under heating and cooling. Whererefractory is used as a liner in a flue, the expansion ofthe refractory must be evaluated with respect tothermal expansion of the flue.

6-2 Refractory — Property of NAO Inc.

Castable Refractory

Castable refractory, sometimes called refractory concrete, issupplied dry, to be mixed with water before installation. It isinstalled by pouring (casting in place), troweling, pneumaticgunning, or ramming. A castable refractory provides asmooth, continuous, monolithic mass. Castable materials arenormally placed in a mold or in an area prepared with pins,mesh, or other anchor devices to hold the refractory duringplacement and curing. Mesh, grid, studs, or needles mayalso be used to enhance the strength of the refractoryinstallation.

Castable refractories are classified as dense or lightweight(insulating). Dense castable have excellent mechanicalstrength and low permeability. Their insulating properties,however, are relatively poor. As dense materials, over100 lb/ft3, they offer good resistance in wet service such asquencher linings.

Lightweight castables are excellent insulators. All castablematerials have the advantage of placement in irregular areassuch as furnace transitions and burner openings. They arecast in place. Poured castables require a form. Plasticcastables must be rammed in place. Gunned castable isblown in place and forms are not required. The use ofgunned material, however, is often dictated by the size of thejob in question. Gunning requires installation equipmentincluding air compressors and gunning applicators that wouldbe un-economical to provide when compared to therelatively small amount of equipment needed for poured orplastic castable installation. Normally, gunning is noteconomical for jobs less than a week in duration.

The quality of a castable installation is a function of mixing,curing, and application. Mixing must be performed to therefractory manufacturer’s instructions. If the amount ofwater added is not within the product specifications, thecastable refractory will be weak when curing and when inservice. A dry mix will not flow into its form properly andvoids will appear. A wet mix will promote segregation ofthe mix as it solidifies and cures, providing inconsistent andpoor refractory quality. Note that poured refractory is oftenvibrated to aid its flow, similar to the placement of concrete.Vibrating eliminates voids, but excessive vibration (longerthan 30 seconds) will promote segregation of the mix.


Curing refractory is necessary to set a ceramic bond,creating the strength of the refractory matrix. Withoutproper curing, the refractory will not develop mechanicalstrength or chemical resistance (to corrosion). Refractory isusually either air-set or heat-cured. Air setting requires onlya quiescent period where the refractory is not subject tomechanical changes (motion) or heating effects. Heat curingrequires the refractory to be heated at a controlled rate toestablish the ceramic bond. This bond may require thattemperatures of 1,000 to 3,000 °F be reached andmaintained. Often incinerator temperatures are not highenough to cure heat-setting refractories, and specialoperating procedures are necessary to bring the equipmentto the curing temperature. Most castables used in furnacesand heaters today are air set.

Castable refractories are manufactured at NAO’sEnvironmental Research & Service Center in Texas. NAOhas a large inventory of burner block and encased blocktiles.

Burner Block, also know as Muffle Block, are the “bricks”that are in the furnace floor. Gas burners fire through a ringof block tiles and heat them up. The block radiates heat andstabilizes the burner flame.

Encased Blocks are a variation of the Muffle Block usedwith oil guns. Rather than sit in the furnace floor,Encasements have a metal shroud that allows them to bolt tothe burner housing. Encasements surround and stabilize theoil flame, and they are surrounded by Hexad gas burners.

With our huge stock of molds, we can provide cast burnerblocks for all NAO units, and, if provided a drawing or asketch, NAO can easily make molds and produce thevarious blocks for any competitors unit. If necessary, wehave a kiln to cure and dry the cast block prior to shipment,if the customer so chooses. We have also worked withthird-party testing services to qualify our castable productsprior to shipment.

Plastic

Plastic is sometimes referred to as a castable materialbecause is can be formed to any shape. Plastic refractory is


a soft refractory that can be hammered into place. It is oftenplaced and packed on the walls of a heater a fired undercontrolled conditions when refurbishing old equipment. Thisis a cost-effective substitution to gunning a castable whenjobs are small.

Firebrick

Brick is formed by pressing castable refractory into a mold.It is slightly more dense than plastic, and it is pre-fired andshipped in a ready to use form.

Conventional firebrick is kiln-baked to uniform, controlledconsistency and quality. The term firebrick refers to densbrick, over 100 lb/ft3, normally placed in direct contact withthe hot gas stream. It has relatively poor insulating quality.Insulating firebrick (IFB) is a lightweight, porous brick,normally less than 50 lb/ft3, that can be placed in directcontact with the gas stream and that provides good insulatingcharacteristics. IFB is machined to its final shape, providingexcellent dimensional control as compared to firebrick whichis used as cast.

IFB is lower in strength than firebrick and, because of itsporosity, is a soft material not effective with erosive gasstreams—gas streams with high particulate components.IFB’s low abrasion resistance limits the maximum velocitiesallowed adjacent to it.

Where refractory brick is required to have high abrasiveresistance and insulating value, firebrick will often beprovided with insulation block as back-up between thefirebrick and the furnace-flue wall.

Anchors are normally provided to hold brick in place.Firebrick and IFB are both self-supporting. However,because of the relative structural strengths of these materials,IFB requires a more elaborate anchoring system thanfirebrick. Brick manufacturers each have their own uniqueanchor and anchor system design.


Ceramic Fiber

NAO uses ceramic fiber blanket as an insulating material onthermal oxidizers and ground flares. This is significantlycheaper than fire brick (which we once used) and it worksbetter. Ceramic blanket has a density of about 5 lbs/ft3,about 20 times less dense than castable refractory and firebrick. This allows the oxidizer to heat up quickly on start-upand to react quickly to changes in the flow stream. Thisreduces assist gas demand and saves money. Firebrickinsulation, on the other hand, requires a lot of time to heatup, and it reacts slowly to process changes. If the flow ofvapors to a thermal oxidizer increases suddenly, a ceramic-insulated combustion chamber will quickly heat up andcompensate for the change, reducing fuel costs andimproving combustion efficiency. A firebrick chamber willrequire more time and more assist gas to heat up; in themean time, harmful vapors are moving through the chamberun-combusted.

Ceramic fiber also comes in vacuum-formed shapes. It’scharacteristics are similar to ceramic blanket, only it isslightly more dense. The ceramic fibers are pressed into ashape, usually a hollow cylinder for our purposes. NAOuses vacuum formed ceramic fiber in small thermal oxidizers.


References

Brunner, Calvin R. Handbook of Incineration Systems.McGraw-Hill, Inc. 1991.



Flares — Property of NAO Inc. 7-1

Flares

Flaring is a high-temperature oxidation process for disposalof waste gases and vapors during normal operations andemergencies. Flares are used for the quick relief andeffective elimination of excess process streams due tounexpected equipment failures or major plant emergencies,such as instrument malfunction, power failure, or plant fires.

Prior to the utilization of process flares, gaseous wastestreams were vented directly to the atmosphere. Ventingcreated health hazards and safety hazards. Health hazardsoccurred when toxic vapors reached the ground where plantpersonnel worked. Safety hazards occurred whencombustible vapors reached the ground where they could beignited. The resulting explosions and fires had devastatingconsequences for operating personnel, the surroundingneighborhood, and the plant itself.

Flaring is a solution to these hazards (though flaring doespresent its own problems of heat, smoke, light, and noise).Burning the waste vapors in a flare converts the pollutantsinto less-noxious components. With proper combustiondesign, all of the components of the waste stream will beconverted to carbon dioxide and water vapor with fewsecondary pollutants.

Flares are primarily used in the oil and petrochemicalindustry, from oil production through transportation, storage,refining, processing, and distribution. Flares are seen onoffshore platforms, at propane (LPG) and natural gas(LNG) terminals, along hydrocarbon pipelines, in refineries,at storage terminals, at steel mills, and at petrochemicalplants. However, their use and importance is not limited tothese applications. Flares are used for sewage digesters,coal gasification, rocket engine testing, nuclear power plants,heavy water plants, and ammonia terminals.

These flaring applications require three general types offlares to insure high operating performance and efficiency.The least complex and most limited design is the non-smokeless flare, sometimes called the utility flare. It is used


for hydrocarbon and vapor streams that burn readily and donot produce smoke.

Non-Smokeless Flare Applications

• Methane• Hydrogen• Coke-Oven Gas• Carbon Monoxide• Ammonia

Heavier hydrocarbons can be flared using a non-smokelessdesign, but only with considerable smoking and lowercombustion efficiency. This may be tolerated if flaringoccurs only infrequently during short, emergency upsets.

A second type of flare, the smokeless flare, is often usedfor heavy hydrocarbons for clean, efficient disposal of theentire waste stream. Smokeless flares are required for anyparaffin above methane and for all olefins, di-olefins, andaromatics. They are also used for coke-oven and coal-gasstreams where tar particles or tar-laden vapors are present.These flares use outside momentum sources—steam, high-pressure fuel gas, water injection, high-velocity vortexaction, or air blowers—to provide efficient gas and airmixing and turbulence for smokeless operation andimproved combustion performance.

A third type of flare, the endothermic flare, is used for low-heat-content waste streams and toxic vapors. This unit iscalled an endothermic, or fired, flare because it providesadditional energy to the waste stream for completeoxidation. As a rule of thumb, whenever the heat content ofthe waste stream is below 100 Btu/ft3, then a fired flare withan assist gas must be used to insure complete oxidation ofthe waste. (Recently, NAO developed a waste gascombustion unit that was field tested to flare waste gas at 55Btu/ft3 with no assist gas.)

In the design of any flare, many critical factors—e.g.radiation, noise, and dispersion—must be considered andresolved in order for the flare to be operated safely. Lesscritical, but equally important, operational design factors—e.g. combustion efficiency and flame stability—must be


considered for the system to be considered environmentallyacceptable.

Critical Design Factors of a Flare System

• Thermal Radiation• Liquid Carry-Over• Noise• Dispersion• Pressure Drop• Visible Light• Explosion Hazard

Operational Design Factors of a Flare System

• Combustion Efficiency• Flame Stability• Positive Piloting• Reliable Ignition• Effective Air Entrainment and Mixing• Maintenance

Resolving the many design factors is not sufficient to insure asafe and reliable flare. The possibility of mechanical andoperational problems must be anticipated. A competentflare designer will be able to handle factors such as flamestability, but the designer must also be able to cope with thehigh-temperature problems associated with the closeproximity of the large flare flame. Maintaining flame stabilityfor varying flow rates and compositions is essential if theflare is to serve its principal purpose of waste gascombustion.

The Fluidic Flare

Some basic rules for proper flare mechanical design shouldalways be observed: No moving parts, No burning insidethe tip, and No small openings for steam or gas injectors.The NAO Fluidic Flare satisfies these requirements.


Among other things, the Fluidic Flare has the patented no-moving-parts multi-baffle Fluidic Seal to prevent burninginside the tip, and it has the patented VorTuSwirl Vanes atthe tip to retain and stabilize the flame.

NAO’s VorTuSwirl take approximately 30% of the gasflow and swirl it in a vortex. (The remaining 70% goesstraight up and that makes your main flame.) With theswirling effect around the outside, the flame is stabilized atvelocities approaching sonic velocity. With natural gas,which is mostly methane, sonic is a velocity is approximately1200 feet per second, or about 900 miles per hour. Withmost conventional flare tips, on the other hand, less than halfof this performance can be obtained; with no flame retainerat all, the exit velocity is restricted to two-tenths of sonic, orabout 250 feet per second for methane. If you have anolder type of flame retainer such as a bluff body, then youcan go to half of sonic, around 600 feet per second. Higherexit velocities all for more stable flare flames during highwind conditions, making the NAO Fluidic Flare the mostreliable flare tip on the market. With NAO, you have thebest technology currently available for flaring.

Located around the outside of the Fluidic Flare is acombination wind-and-heat shield—the conical windshield.The pilots actually protrude through the wind shield, so thatthey are protected from the combustion flame. This isespecially important for the small piping of the pilots which issusceptible to being burned up and destroyed on aconventional flare.

This next photo depicts an NAO Clear Glow Fluidic Flare(NFF-CG). This flare was supplied to BP for an LPGterminal—the Solem Vough installation. It replaced severaldifferent flares. At this terminal, they have a difficult situationwith a maximum flow of 130 metric tons an hour and a totalpressure drop of about 100-mm (4") of water column duringemergency situations. Most of the time, however, they flareonly about 5 tons an hour, a very small percentage (3%) ofthe overall waste gas with a pressure drop of just a fractionof a millimeter. As a result, the normal exit velocity is only afew feet per second.

At the Solem Vough installation, a small island on thenorthern tip of Scotland, they have very severe winds all thetime. It is common to have winds of 50 or 60 mph at gradelevel. Since this flare tip is on a 200 foot stack, it


experiences hundred-mile-an-hour winds all the time!Before our NFF-CG was installed, BP tried severalvendors, none of whom could supply a tip that would lastlonger than three or four months. They had a flaremanufactured by Company X that failed within three months.They tried a flare from Company D, and it also quicklyfailed. They tried a flare from Company I that had a windshield with slats that failed within four months. And theytried an old-type NAO flare, an NCG, with a picket-fencewind shield around the outside. It failed within four months.

We thought the best way to really test the new technology ofthe Fluidic Flare was to give BP one to try. It has been inservice now for almost 10 years and it is still going strong!The conical wind & heat shield is now a little bit warpedbecause it has seen a lot of heat, but the shield isn’t astructural member so it doesn’t really matter if it gets bentand distorted. Without the NAO conical wind shield, theflare body warps under the heat—even refractory-lined flarebodies will warp—until, inevitably, the metal of the flarebody completely disintegrates and you are left with a“burned-up” flare tip that must be replaced.

We gave BP a flare tip back in 1979 because we had onlytwo NFF Fluidic Flares installed at the very southern tip ofArgentina, and we had no real feedback back then on howthey were working. We decided the best approach was togive BP a Fluidic Flare for this worst-case installation; and ifit worked there it would work anywhere. That has provento be the case.

The NAO Fluidic Seal is covered by two patents: The firstcovered a multi-baffle seal down inside the body of a flaretip (the NCG). A more recent patent is for the Fluidic Sealat the top (the NFF). Both patents cover one or morebaffles, so if a competitor puts even one baffle near the topof a flare they will be infringing on NAO’s patented design.Also, if they put more than one baffle inside a flare tip,anywhere, they will also be infringing.

NAO now makes several types of Fluidic Seals, includinginsert types or segment types that fit into customer’s existingflares.

The Fluidic Seal works by using baffles of the seal and theflow of the purge gas to push air out of the tip. Gas flowsunrestricted in the “upward” direction. Flow through the


Fluidic Seal is about five times more difficult when moving inthe “downward” direction, so air tends not to move downinto the stack. NAO has physical models you can use thatreally illustrate how the unit works. Bulletin #38-A providesmany more details about the NAO Fluidic Seal.

The NAO Fluidic Flare has been used in many applicationsaround the world in horizontal, vertical, and angledinstallations, onshore and offshore. For more information onflare applications, see the section titled Applications in thissection.


10 Reasons for Selecting the NAO Fluidic Flare

1. Patented no-moving-parts Fluidic Seal2. Reduced Purge Gas Consumption3. Fluidic Seal eliminates burning inside tip4. Conical Windshield Prevents Flame Lick5. Flame Retainers keep Flame Erect, insure

Turbulence, and Reduce Smoke6. Higher Exit Velocity of gas insures Stable Flame in

Mixing Zone7. Minimum Profile reduces Wind Loading, which

reduces capital cost8. Energy Conservation Pilots save Money9. Conical & Pilot Windshield Protects Pilot Flame

under any wind condition10. NAO Fluidic Flare Systems are field proven to

Outperform the Competition

The Molecular Seal

To the right is a conventional flare tip with a molecular seal.The molecular seal works by allowing heavy gas moleculesto sink to the low point and light gas molecules to rise to thehigh point. These layers of molecules “seal” the stack fromair ingress.

Unfortunately the molecular seal does not become effectiveuntil the first bend. This allows burning inside the molecularseal, because the ambient air is as heavy or heavier than thepurge gas. The purge gas tends to flow out, the air tends tosink back in, and combustion flames move up and downinside the molecular seal. Depending on the location of theseal on the flare stack, you can actually get burning, almost30 feet down inside of the flare. Keep in mind that amolecular seal is made of carbon steel which tends to“burn.” We call this phenomenon “cancer.” This cancer ishidden, and it is gradual deterioration; but eventually a flamewill shoot out through the side of the molecular seal or theinternals of the seal will corrode away. Hence, themolecular seal is a major problem.

The only advantage of a molecular seal is that, because it is agravity-type of seal, if you lose purge gas (which is veryunlikely) a molecular seal will continue to function. Unless,


of course, the gas in the flare stack is hot, in which case itwill contract due to temperature changes, thus drawing airpast the seal. Most plants around the world have acceptedthe Fluidic Seal because they realize it is very unlikely that allpurge gas will be lost. More often, there is on-going leakageof gases to the flare system, so some purge flow neverstops. Remember, the purge rate for a Fluidic Seal istypically about three-hundredths of a foot per second (0.03fps, 0.2 mph, 1 cm/s). With no seal at all, the purge rateincreases to about a half-foot per second. Otherwise,burning occurs inside the tip and ruins the flare.

To satisfy customers that want all the benefits of the FluidicSeal with the additional security of the gravity seal, NAOhas developed a MoleFluidic Flare (Molecular Fluidic) inwhich the molecular seal is built into the flare tip. Since themolecular seal is located at the exit of the flare tip, it is actsas a Fluidic Seal by preventing air from coming down insidethe tip.


Sizing a Flare

It never pays to oversize a flare. An oversize flare requiresmore pilots, more pilot gas, and more purge gas, and anoversized flare receives more heat exposure because theflare flame at low flow is closer to the tip which overheatsthe metal and reduces flare life.

Surprisingly, flares are still being “oversized” to handle futurecapacity of a plant expansion. For example, the ultimateplant capacity may require a 54-inch flare, though at presentthe plant is limited to only half of capacity—requiring a36-inch flare. By building a 54-inch stack and installing areducing cone and 36-inch flare tip with a gas seal, one flarepilot will be eliminated, the purge gas requirement will besignificantly reduced, and the flare tip life will be extended.Depending on the size of the flare, this concept should beconsidered whenever a complete flaring system isengineered for future capacity requirements.

Normal, continuous flow rates to most process flares areusually quite low or non-existent. Many times, the normalflow to the flare is a purge flow to keep air from movingdown the stack. Although the normal, continuous purgeflow may not produce a visible flame, they can consumesubstantially more energy than the spectacular (and rare)major-emergency flare flows. Methods of controlling andreducing such continuous energy requirements should beconsidered for cost-efficient performance and overallreliability. Proper operating procedures are also importantin flare-energy conservation.

Flare Design Factors

• Tip Diameter and Pressure Drop• Thermal Radiation• Liquid Carry-Over• Air Entry• Combustion Efficiency• Reliable Pilot and Ignition

General factors influencing a safe flare design are sizing,pressure drop, radiation, liquid carry-over, combustionefficiency, air entry, and reliable pilots. All of these factors


but radiation and carry-over also relate to the cost-efficientperformance of a flare.

Tip Size and Pressure Drop

Accurate flare sizing is required to ensure safe operation.Sizing is dependent on flow rate, gas composition, gastemperature, and available pressure drop. Of these, flowrate and pressure drop are most critical for flareperformance. A reasonable estimate of these two criteriamust be determined.

If the flow rate is greatly overestimated and an oversizedflare is specified, burning inside the flare tip will occur andthe tip life will be greatly reduced. The flame of an oversizedflare is very lazy and is easily affected by cross-wind. Windwill cause the flame to tilt, which results in higher radiationlevels and reduced flare life due to flame lick.

Serious underestimation of the flow rate or pressure drop ofthe flare will result in poor flame stability. The flame may liftoff and blow out, releasing unburned hydrocarbons into theatmosphere. For typical open-ended pipe flares, themaximum exit velocity of the gas must be limited to 20%sonic velocity to prevent flame-out. Modern commercialflares use flame-retention devices to stabilize the flame atvelocities up to 50% sonic velocity. A typical deviceconsists of an annular ring with many small piloting holessurrounding a large central orifice.

Current flare design technology permits the use of smallerflare diameters with higher exit velocities and shorter stacksand booms. In this way, flaring is accomplished with lessflame tilt and thermal radiation, and the tip life is extended.

Thermal Radiation

The emergency burning of large quantities of hydrocarbonsin an open flare is a source of extreme thermal radiationmany times that of solar radiation. This radiation level mustbe determined to ensure the safety of operating personneland surrounding equipment. Sometimes, environmentaleffects of thermal radiation must also be considered—flareinstallations located on permafrost, for instance. For a


complete summary of thermal radiation, see the relatedsection in this chapter.

Liquid Carry-Over

Since flares are designed only for gas service, the dumpingof liquid into a flare can be catastrophic. The amount andthe extent of damage depends on the quantity of liquidhydrocarbon going to the flare, the wind speed, the winddirection, the flare-gas exit velocity, and the proximity of theflare to personnel and equipment.

Small liquid-hydrocarbon particles can be burned in a flare,but their size is critical. Particles with sizes up to 90 micronswill be burned within the flare flame and go unnoticed exceptfor a slight change in flame color and an increased tendencyto smoke. When the particle size approaches 150 microns,burning droplets outside the flame will be clearly visible,even in daylight. These particles will not normally reachgrade level, except for short flare stacks (heights under10 meters).

The exact trajectory of the droplets depends strongly onwind speed, and should be calculated for critical locations.Droplets larger than 150 microns will result in burning rainreaching grade level or the platform work area. This burningrain can injure operating personnel, scorch steel equipment,destroy instrumentation and electrical wiring, an igniteanything combustible at grade level.

A liquid spill-out of a flare tip will usually result in somehydrocarbon running down the side of the flare. Alloycomponents of the flare will not be seriously affected, but thecarbon-steel stack can be destroyed.

Knock-out drums, or disentrainment drums, are used toeliminate liquid carry-over. Knock-out drums use thesettling effect (for low flow rates) and cyclone action (forhigh flow rates) to separate liquid particles from gas flowstreams. For more information on knock-out drums, therelated section in this chapter.


Air Entry

Air entry into the hydrocarbon-filled stack could result in anexplosion of the flare stack. There are two paths of air entryinto a stack. One is through leakage in the system at lowflow; the other and most direct path is down the flare stackthrough the tip.

A continuous flow of purge gas is used to prevent airpenetration down the stack. If the flare stack contains alow-density waste gas or purge gas at low flow rates, thenthe frictional pressure drop of the flare stack is less than thebuoyant draft of the gas. The result is negative pressure atthe base of the flare stack. Any leakage of connections,flanges, or manways could then produce an explosivemixture in the stack that can be ignited by the flare pilots.

The best solution to the air leakage problem is a tight flareheader and stack with a reliable oxygen monitoring andalarm system. A continuous oxygen monitor will indicateoxygen (air) levels in the stack. This automatic system willeliminate the need for maintaining high purge rates tomaintain back pressure and keep the stack pressurepositive.

The result is significant energy savings, even for small-diameter flare systems. Exact energy savings will dependupon the diameter & height of the stack, the molecularweight & temperature of the purge gas, and the explosivelimits of the purge gas. Amazingly, some flare lines are stillcontinuously maintained under positive pressure! Withproper flare monitors and effective controls, the need forexcessive purge-gas flow rates is eliminated.

To further reduce purge gas requirements, consider the useof a Fluidic Seal and/or purge-control systems. A FluidicSeal can reduce the purge rate by 25-95%.

Gas Seals

To reduce the likelihood of air moving down the tip into thestack, gas seals such as the molecular seal or the FluidicSeal are used. The NAO Fluidic Seal uses cone-shapedbaffles to reduce the depth of air penetration at the tip of theflare. The baffles increase the velocity of the purge gas andpush air out of the tip. The Fluidic Seal and molecular sealare described in detail previously in this chapter.


Inert Gas Purge

With the present trend toward bigger flares, inert-gasgenerators can offer a fast pay-back and significant yearlysavings. For purging applications, expensive water-cooledgenerators are not required. The inert gas can be air-cooledto warm temperatures within stack design tolerances, or itcan be circulated to a water seal (to prevent freezing) or aknock-out drum (to prevent condensate).

NAO has developed a complete line of inert-gas generatorsspecifically designed for flaring applications. Typicalcomposition of the inert gas is 87% N

2, 12% CO

2, and less

than 1% oxidizing or reducing residuals.

Typically, one cubic foot of natural gas can be converted tonine cubic feet of inert gas. With proper installation of aneffective gas seal and automatic start-up and shut-downcontrol of the inert-gas generator, the total energy savingscan be substantial.

NAO also has developed an inert gas generator using thelatest in membrane technology. These rely on membranefilters to separate atmospheric nitrogen and oxygen. Theseunits efficiently generate 97% pure N

2 inert gas. These can

be installed remotely and only require electric utilities. Thesecan easily be supplied with storage pressure vessels tomaintain purge flows in case of power failure.

Combustion Efficiency

In refineries and petrochemical plants, smokeless operationis necessary to ensure complete combustion of the heavyand unsaturated hydrocarbon vapors. Generally speaking,complete combustion means that flares must operate at 98%combustion efficiency, or better. This applies to continuousflaring applications required to satisfy governmentregulations.

In the United States, if an open flare has a waste gas with aheating value of 300 Btu/ft3, and exit velocity not exceeding60 ft/sec, and no smoke is visible, it is assumed thatcombustion efficiency is 98%. Unfortunately, there is notcurrently a method for continuous monitoring of open-flarecombustion efficiency.


For enclosed ground flares, the question of combustionefficiency is easily answered by installing and emissionsmonitor near the exit of the flare stack.

For more on combustion efficiency, read the section onSmokeless Flaring.

Reliable Pilot and Ignition

Reliable pilot operation under all wind and weatherconditions is essential to the proper performance of a flare.The flare must be instantly available for emergency duty toprevent any possibility of a hazardous, or environmentallyoffensive, discharge to the atmosphere. Windshields andflame retainer are used to ensure continuous piloting underthe most adverse conditions.

Current flaring standards use flame-front ignition or sparkignition. Flame-front ignition is a remote system that shootsa ball of fire through a piping system to the pilot. Spark-ignited pilots use the latest in technology to provide reliableignition directly at the pilot. NAO uses a high-intensityignitor located in the pilot tube to ignite the pilot gas. Thespark-ignited pilot allows for faster response and lessmaintenance than flame-front ignition; the only drawback isthat when maintenance is required on a spark-ignitor, theflare systems must be off-line. Flame-front generators maybe serviced at any time because they are located away fromthe flare.


Smokeless Flaring

Today, smokeless flaring is required in most industrialapplications onshore, and in many places, offshoreregulations are tightening so that offshore flares also must besmokeless.

Refineries are one of the areas where smokeless flaring isrequired. The most common method for achievingsmokeless combustion is steam injection. Because of theavailability of steam, most refineries use steam injection.There are some places, however, where steam isn’tavailable. If the flare is in a remote part of the refinery or itis in a gas plant where they don’t have steam, then youshould consider other types of smokeless flares, includingwater-injection, multi-tip, and air-blower flares.

Jet Mix Flaring for Smokeless Operation

The most cost effective way to achieve smokeless operationis with a Jet Mix Multi-Tip Flare. With the Jet Mix Tips,pressure is raised because the tips actually restrict someflow. This pressure is used to entrain combustion air as thegas exits the tip. The photographs show a comparison offlaring with and without the Jet Mix Tips. Without the tips,there was a pressure of about a pound or two. It wasincreased by the tips to about 15 to 20 psig. Usually for aJet Mix Tip, you will need about 2 psig to assure goodoperation.

For large turndowns, to maintain that minimum pressure youneed Staged Flares. Essentially it is a main flare and a sideflare—or as many as two or three side flares, all built on thesame structure and the same flare tip. The different partsare activated as the flow increases. Normally this is bymeans of a very simple pressure switch and a staging valve.What we do is arrange all these valves in such a way that ifyou get an instrument failure or electrical failure, they willpop open. The flare might smoke at that time, but at least itwill function. If that isn’t feasible, then we use in-line rupturediscs or pressure-relief valves. Most of the time we use thefail-safe valves unless a customer wants it other way.


Benefits of NAO Jet Mix Flares

• No Assist Steam or Gas necessary• Best Turndown in the industry• Efficient Staging Controls

Steam Assist Flare

NAO has variations on the steam assisted flare. The mostbasic is the NAO Fluidic Flare with Steam Ring (NFF-SR).It has a ring on the outside with a series of jets at the top.They pull in outside air and generate turbulence in mixing tomake the flare smokeless. For large diameter tips and forsome very smoky gases, we use a Ring and Center SteamInjection Flare (NFF-RC). The center injects steam with thehydrocarbon to insure complete combustion and smokelessoperation. The center nozzle is especially important for alarge diameter flare. The outer steam ring is able to injectsteam and combustion air a distance of about 12 inches.Therefore, once we get up to a 24-inch flare, you begin toget a center core of gas that is not being mixed, which tendsto produce some smoking. If the flare tip is more than 24-

inch in diameter, there is aneed for a Center SteamFlare.

Another advantage of aCenter Steam Flare is itshigher turndown with theability to control the steamindependently in both thecenter pipe and the outerring. This provides higherturndown by operating eachone in sequence. Anexample would be if we hada 150 psig steam. As weturn that down, about four-to-one, the four is squaredto produce a 16-to-onepressure turndown—from150 to about eight pounds!However, at eight pounds,the steam from the jets is so


Combustion Efficiency of Steam-Assisted Flares


slow it won’t entrain any air or generate enough turbulenceto do anything. With the option of reducing flow to thesteam ring, you can increase the flow and pressure to thecenter steam and eliminate smoke at very low flows.

To ensure that we have sufficient mixing and turbulence, wewant to keep steam pressure at the tip above 10 psig. Thatisn’t practical if you have only one source of steam. But,with the Ring and Center Flare, you can control each oneindependently. When you reach the very low rates, youshut off the ring and just operate the center to provide theopportunity for additional turndown.

Advantages of NAO Steam-Assisted Flares

1. Efficient, Reliable Smokeless Combustion2. Large Turndown3. Maximum Air Entrainment with wide drilling pattern

of steam tips4. Small Steam Jets improve Turbulence and Reduce

Steam Consumption5. No Plugging of Steam Tips because steam jets

drilled large enough to pass most particulate6. More efficient than Water Injection7. Lower Operating Expense than air assist or water

injection8. Lower Capital Cost than Enclosed Flares

The Fluidic Flare with a steam ring can be used vertically,horizontally or at any angle. Normally, flares are tippedhorizontally or vertically. Very rarely is a flare tipped downand angled into a pit, but sometimes it is done. In pitinstallations, you have to prevent the flame from folding backover the flare tip and destroying or shortening the life of theflare—steam or water injection can help do this. Horizontaland angled flares are common offshore, though steam flaresare not used offshore because steam is scarce, so air-assisted or high-velocity Jet-Mix flares are recommended.

Water Injection Flare

If steam is not available and Air-Assist or High-PressureFlaring is not practical, you may choose the NAO Water


Injection Flare. Offshore flares commonly use waterinjection because of the availability of seawater; it is pumpedfrom below the platform and sprayed into the flare.

Water injection flares are usually not very efficient. A typicalsteam flare consumes about a quarter of a pound to as muchas a half pound of steam for each pound of hydrocarbon. Awater spray flare could take as much a five pounds of waterfor each pound of hydrocarbon! That’s a lot of mass topump. The key to the successful operation of a WaterSpray Flare is very fine atomization of the water and gettingit into the flame.

Benefits of NAO Water Injection Flares

• Better Efficiency with very fine atomization• Fluidic Flare less susceptible to cross wind—this

improves water injection performance

With water injection, it is very important to spread the waterthoroughly into that flame. The biggest problem with waterinjection is cross wind. When the wind blows, the flametends to blow out of the water-mixing region, then the unitstarts to smoke and malfunction. With some applications,we have used something like a shower rack with manydifferent nozzles for smokeless operation.

Offshore, customers are not so concerned with smokelessoperation. The main thing they are trying to do offshore isreduce the radiation from the flare; and water injection is anexcellent way to accomplish that objective. It will reduceradiation by as much as 50%! Tipping the flame out awayfrom the offshore platform with an angled Fluidic Flare alsoreduces radiation on the platform.

Air Assisted Flare

If steam is not available and Jet Mix Flaring is not feasible,the next method of smokeless operation is an Air BlowerFlare. Rather than use steam injection or high pressure toentrain air, you can push air into the gas stream using ablower. By providing adequate combustion air and


additional turbulence, the flare combustion efficiencyimproves and smoke disappears.

Typically, on an Air Blower Flare, it takes roughly fivepounds of air for each pound of hydrocarbon. If you wantto make one pound an hour smokeless, you need onestandard cubic foot per minute of air. Hence, if you need50,000 pounds an hour smokeless you need 50,000standard cubic feet per minute of air. Another good rule ofthumb is for each 1,000 pounds per hour smokeless, youneed one horsepower of steam.

Remember, when you have an air flare there is no mixing ofthe air except at the very end of the flare tip. If there is anymixing inside, the combustible mixture will burn inside theflare and destroy it.

Compressed Air is another type of Air Assist Flare; thoughit is commonly mentioned as a type of steam assist becauseit is operationally similar to steam injection. The compressedair entrains additional air and generates turbulent mixing ofthe air and the waste gas. The cooling effect of compressedair on the flare flame is less than with steam and, therefore,an additional 10% mass flow of air is required to match theeffectiveness of steam. Due to the high cost of compressionand the high pressure drop in the piping and injection systemwhen using air, compressed air is rarely used on assist flares.


Flare Systems

NAO has manufactured complete flaring systems sin the1970’s. These systems include the flare tips, the controls,and the flare structure. NAO Flare Systems includeelevated flares, enclosed flares, ground flares, and pit flares.

Elevated Flare

NAO makes a wide range of elevated flare stacks. Thestack structures can be guyed, derrick, self-supporting, or acombination of these. The stacks can include knock-outdrums, water seals, ladders, platforms, and aircraft lights.

There are two important reasons for elevating a flare. Oneis to reduce the radiation at grade level to protect plantpersonnel. Plant personnel can safely work an 8-hour shiftexposed to 400-500 Btu/ft2 of radiant heat (including solar).Plant personnel can safely evacuate an area during anemergency if radiation does not exceed 1,500 Btu/ft2. Ifradiation exceeds 1,500 Btu/ft2, personnel may receiveburns before moving to a safe distance from the flare.Besides personal safety, flares should be elevated so thatother equipment does not receive more than 3,000-5,000Btu/ft2.

The second reason for elevating a flare, and sometimes themost important, is to safely disperse toxic gases if the flareflame goes out. Though we design our flares for reliabilityduring any weather condition, there is always a possibility ofa pilot-gas failure or conditions that may snuff the pilots. Ifan emergency release occurs without an ignition source,lethal gases will be released to the atmosphere. If the flare isnot properly elevated, these gases can collect in the plant..

Guyed stacks consist of elevated piping to the flare tip withwire-rope cables holding the piping erect. Guyed stacks arerecommended where space is allowable because of theirlow-cost: As a rule of thumb, guy supported cable are at a45° angle to the ground, so the system requires an area ofland with a radius equal to the stack height. Guy cablesmust be maintained on a periodic basis (every 6 months) bygreasing the cables and checking that they are properlytensioned (they stretch over time).

Typical Air Duct Sizes†

Capacity Pipe Size(MMscfd) (inch)

1 62 83 104 125 14

10 1615 1820 2025 2430 2835 2840 3050 3260 3670 3680 3690 40

100 42

†Low-pressure, boom-mountedair flare, 200 foot height


Self-supported stacks are generally the next option whenchoosing an elevated flare. Self-supported stacks rely on awide base that narrows as it nears the flare tip. When acustomer requires a water seal, knock-out drum, or both,we usually try to use these accessories as the self-supportingbase to save money.

The third type of elevated flare is the derrick flare. Theseflares have a simple pipe as the flare stack with a derrickstructure supporting the it. The expense of purchasing anderecting the derrick structure that provides no benefit to theflaring system usually eliminates this type of flare structurefrom consideration. NAO has provided Articulated, orSelf-Lowering, Derrick Flare Stacks. Articulated stackshave the sections of the stack ride up and down on a rail, sothe entire stack can be raised and lowered from grade. Thisdesign is quite common in Europe where customers havemultiple flares on a single derrick structure. The self-lowering option allows customers to remove, repair, orreplace a flare tip without disturbing the operation of otherflares on the derrick.

Enclosed Flare

In some applications, a customer may not have the landavailable for an elevated or a ground flare. Urbandevelopment commonly prevents companies from usingopen flares. In this case, an enclosed flare is a goodsolution. The Enclosed Ground Flare—or Populated AreaCombustor (NPAC)—provides the closest control on theflare combustion process, with close temperature and aircontrol. The emissions of the flare can be easily monitoredfrom an attached platform since there is no thermal radiationhazard.

If you were to look inside an Enclosed Flare you will seethat there are a series of refractory blankets two-feet widethat line the entire chamber. These blankets are low densityceramic-fiber insulation and are sound absorbent. Becausethey are very low density they heat up very quickly andrespond to changes in the flow stream very well. Thedensity of NAO’s refractory lining is just over 5 pounds percubic foot. Most other manufacturers use castablerefractory very much like cement that is poured or gunited inplace. The castable refractory is between 60 and 140

NAO Guy Supported Flare

NAO Self Supported Flare


pounds per cubic foot. Because of its mass, it does not heatup quickly and it does not respond to changes in the flowstream. Also, the hard castable tends to reflect, not absorb,noise. Ceramic blanket, on the other hand, is very soft, so itactually absorbs the sound. Another benefit of the blanket isits flexibility. If a rain strikes and cools a small portion of thelining, the blanket can expand and contract, and it will not bedestroyed. When rain hits a castable refractory, it will spallor crack.

Advantages of Enclosed Flares

1. No Assist Steam or Assist Gas needed forsmokeless operation

2. Higher Combustion Efficiency—retained heat andresidence time eliminate more of the harmful vapors

3. Combustion Air Control—enclosure eliminateseffects of high cross winds

4. Low Noise with acoustical insulation5. No Visible Flame6. Low Radiation at grade7. Easy Maintenance because nothing is elevated8. Capability for Emissions Sampling

The NPAC operates using the chimney effect. A silo orchimney induces air by natural draft; the buoyancy of the hotcombustion gases sucks in ambient air. Usually we alsoinclude a windshield or a fence around the outside so thatwind variations do not disrupt the natural draft flow. Therefractory lining inside the steel shell maintains thetemperature inside the chamber, typically around 2,000 °F.The amount of heat emerging as hot gases is substantial atthe top, but it is all air, carbon dioxide and water vapor.This unit operates at about 300% excess air, so the“neighborhood heat” is not excessive. (Carbon dioxide andwater vapor radiate some heat, but they are only a verysmall percentage of the total flow, so the total heat radiatedis very small.)

In some operations, an enclosed flare operates inconjunction with an elevated flare. Daily and weekly low-flow rates are handled by the enclosed flare. When a majorupset occurs, the elevated flare comes into service. On theleft side of this illustration, there is a Knock Out Drum to

NAO Derrick Flare

NAO Tripod Flare


remove any liquids and two Water Seals. One seal is to theleft of the Ground Flare, the other is in the base of theelevated flare system. The water seal for the ground flare isvery shallow, and the water seal on the elevated flare is verydeep. For more information on water seals and knock-outdrums, see the sections titled Flame Arresting Systems andKnock-Out Drums, respectively.

For special applications, NAO has supplied EnclosedFlares for Production Ships and Roof Mounted Units.We once provide an enclosed flare for a municipal researchfacility where an open flame was not acceptable. It wasnecessary to eliminate radiation from the flame and toprevent light and noise from bothering the neighbors. Theflare is rectangular, rather than circular, so it would blend-inmore with other buildings and look like a cooling tower oran air conditioning unit.

We also make very small units for landfills and sewage gas,and we have developed other small units for pilot plants,including Arco Research in Plano, Texas, and UnionCarbide Institute in Charleston, West Virginia. The largestNPAC we have made is in a Polypropylene Plant in Taiwan.It is roughly 60 feet across and 100 feet tall, and it burnsabout 180,000 pounds per hour of propylene, which willburn very dirty and smoky if not handled properly.

The biggest advantage of enclosed flares is that they don’tneed steam or assist gas. Their smokeless operation is dueto the large volume of entrained air and to the fact that itemerges from many tips. The flame is spread out so it getsplenty of combustion air. However, they can beexpensive—the unit in Taiwan cost about two-million dollarsplus another half-million dollars to install. This money isoften recovered during the first two or three years ofoperation with the money saved on assist gas or steam.

Ground Flares

If you have plenty of space, you can use an Iso-FluidicOxidizer (NIFO). Ground flares are generally staged. Thisallows the customer to use the pressure of the gas stream toensure smokeless operation. Without staging, at low flowrates, the velocity of the gas will not be sufficient to entrainenough air for efficient combustion. If required, the NIFO


can have an acoustical fence around it for very low noiseattenuation.

NAO makes enclosed ground flares such as the VaporControl Unit (NVCU) and the Populated Area Combustor(NPAC). The open ground flare equivalent is called theVapor Control Flare (NVCF). The enclosed unit is morepopular today because there is a way of test emissions andprove that it works. With an open flare, there is no way tosample combustion efficiency. Our Vapor Control Units areused mostly for gasoline loading, either from trucks orbarges.

Vapor Disposal Units

• No Smoke• No Visible Flame• Low Noise• Complete Destruction of vapors• Quick Response to changes in flow conditions• Safety Interlocks for smooth start-up/shut-down

For remote, open-space applications, such as productionplants in deserts, NAO has designed the Mini Flare. Theheader piping is buried to prevent heat damage from theextremely high radiation. What you see are only the flaretips. In some applications, such as the desert, the ground isan insufficient heat shield, so we place a layer of gravel toact as a heat shield. The gravel is very coarse and irregularwith a lot of air gaps. It acts like a closed-cell acousticblanket to prevent heat from conducting through the soil tothe flare header.

Pit Flares

When flaring liquid-gas mixtures, pit flares are required. Theflare fires horizontally into a shielded pit so that liquids canfall out and burn on the ground. Many times, customershave placed the flare tips through shielding walls that allowthe flames to come up and destroy them. The flare must beshielded from flame impingement. They must be recessedinto the wall without openings for the flame to enter. Bypacking the opening with a ceramic blanket, NAO provides


flexibility between the flare and the wall without allowing theflame to get back into the flare.

The pilots of a pit flare are usually remote. In other words,there is no venturi mounted at the bottom of the flare.Instead, there is up to 100 feet of piping (usually installed bythe customer). All critical parts, including thermocouple, andventuri, are remote. Remote placement is to protect theequipment from flame lick and extend their life.

Very rarely is a flare tipped down and angled into a pit, butsometimes it is done. In those installations, you have toprevent the flame from folding back over the flare tip anddestroying or shortening the life of the flare. NAO hasworked with customer to provide flare designs that reducethe likelihood of flame lick on down-fired flares.

Offshore Flares

Because of the harsh environment and minimum space ofoffshore platform, NAO is one of the few flaremanufacturers in the world that can supply flares to meet thespecifications of offshore service. All of the flare designsused onshore are adaptable for use offshore. Because ofconstant high wind conditions offshore, as a minimum weextend the length of our conical windshields to reduce thedamage caused by flame lick. Generally we supply flareswith greater corrosion allowances and all-stainless-steelconstruction to extend their life.

There are three types of flares for use with an offshoreexploration or production drilling platform. Since steam isnot available, other methods must be employed to assuresmokeless flaring of the hydrocarbons. The tremendousproblem of space confinement--personnel cannot runaway--must also be considered in designing an efficient,reliable offshore flare.

With an offshore platform, space is at a premium. The“packed-in” design, the proximity of work boats, and thedifficulty (or impossibility) of personnel evacuation during anemergency, intensify the problems of thermal radiationassociated with flaring for safety and environmentalprotection.


When designing on offshore flare, the essential pre-requisitesare: Experience, field-proven design, and a thorough,intimate knowledge of all factors involved in the mechanismof turbulent flow and momentum transfer for efficient airentrainment into gas masses.

Specialty Flares

NAO has un-matched experience when it comes todesigning portable flares. We were the first manufacturer tooffer portable flares for rental applications. We have hadfour 8" x 80’ flare systems mounted on 40’ trailers inoperation for over 10 years. These have been usedthroughout North America for emergency flare service andturnaround service on pipelines and at refineries,pharmaceutical plants, and other production plants.

We have recently completed testing on a first-generationflare tip to burn low-Btu (50 Btu/ft3) waste gas without theaddition of assist gas. Equipment using this new technologywill pay for itself in less than one year.


Flare Pilots

NAO has many variations of the basic Flare Pilot, each withbenefits for specific applications. NAO pilots come in twodifferent sizes. A 2-inch-diameter pilot is the older design.It burns about 200 standard cubic feet per hour of naturalgas at 10 psig. If you examine the illustration, you can seethe thermocouple guide tube coming on the right side nearthe bottom. On the left side is the flame-front tube. Theventuri and jet orifice are at the bottom. Our newer pilot iscalled an Energy Conservation Pilot. It is a 1¼-inch-diameter pilots that burns only 45 standard cubic feet perhour of natural gas at 10 psig.

Flame-Front Pilots

The traditional flare pilot uses a 1-inch-diameter flame-fronttube to send a ball of flame to ignite the pilot. These systemsare common on flares around the world, particularly onelevated flares where the ease of maintenance of the flamefront generator is recommended.

Common problems of flame front pilots are water in theignitor tube and changes of ignitor gas. Water will alwayscollect in ignitor tubes. The ignitor combustion flamegenerates water vapor that, over time, will condense andcollect at the low points of the ignitor tube. It is absolutelynecessary for customers to provide drainage for their ignitorlines at the low points of the system.

Often, customers rely on refinery gas to operate their pilots.The composition of this gas can vary from time to time,which means that the regulators of the flame front generatormay not be correctly set. This requires that the flame frontgenerator be manually adjusted when lighting the pilots.

Spark-Ignited Pilots

The NAO High-Energy Spark-Ignited Pilot providesultra-reliable ignition without compressed air or itsassociated moisture problems. The Spark-Ignited Pilot usesa 3,500-Watt spark to ignited the pilot gas while it is in thepilot tube. This eliminates the need for hundreds of feet of


flame front ignitor piping and its associated problems. Thespark ignitors have been successfully completed millionscycles in our in-house testing, and they have been fieldproven in ten years of installations.

Because Spark-Ignited Pilots do not rely on compressed airfor ignition, they rarely need manual adjustment for reliablepilot ignition—even as the composition of the refinery gaschanges from day to day. This makes the spark pilotpreferable to the flame-front pilot when designing automaticignition/re-ignition systems.

Benefits of the NAO Spark-Ignited Pilot

1. No Compressed Air required2. No Time Delay when lighting pilot3. “Self-Cleaning” High-Intensity Spark blows off rust,

water, ice, and dirt4. No Water Drainage of ignitor line is required5. No Re-adjustment required with variations in

refinery gas6. Low Pilot Gas Consumption and Operating Costs7. Pilot Flame Stability under any weather condition

The only concern to have when selecting the Spark-IgnitedPilot is when designing elevated flare systems. If the sparkexciter of the pilot should fail to work, the flare system mustbe shut down to service the elevated pilot. This maintenanceconcern can be corrected by specifying the Dual-IgnitedPilot.

Dual-Ignited Pilots

Dual ignition pilots are becoming quite common today,especially in critical operations such as chemical plants. Thedual-ignited pilots provide the benefits of quick, reliable pilotignition with the spark ignitors and the ease-of-maintenanceof the ground-level flame front system.


Specialty Pilots

The NAO standard is a straight pilot, but we also have our“Z” pilot, which are used as replacement pilots on brand Xflares.

To satisfy the need for stable pilots during nitrogen purges offlare systems, NAO now has the Air Eductor Pilot that usesa small flow of instrument air to draw pilot combustion airfrom the base of the pilot to the pilot nozzle; this prevents thenitrogen purge from snuffing the pilots.

In some very special cases it’s not possible to use aninspirating type of pilot with a venturi. It could, for example,suck in sand with the air. Or, as in the arctic, snow and icemight plug up the air intake and freeze. For these situations,NAO has developed the Raw Gas Pilot with a little JetMix Tip inside the pilot shield. This is ideal where you haveproblems with plugging, such as from sand or freezing rains.The disadvantage of this type of pilot is that is it sucks up alot of gas. It uses approximately 1,000 cubic feet per hour,but its perpetual flame is highly reliable. Some of theseNAO units are used by BP in a pit in Alaska, and they havebeen burning since 1990 without any kind of interruption orchange. The raw gas pilots can have flame front ignition ormanual ignition with NAO portable ignitor, see the sectionon Pilots in the Burner section.

At Sun Oil, Puerto Rico, they had a Brand X flare with fourpilots, only one of which worked. They were concernedbecause if that one pilot went out, they would have to shutdown the entire plant. Sun hired NAO to provide newpilots for the flare and to install them with the flare inoperation. We used a very large mounting hook and put iton the windshield so we could attach the flare by using acrane, lifting it up, and attaching it to the Brand X flare tip.At the bottom of the pilot we put on two legs, so it would bekept away from the flare tip so that it wouldn’t get tangledinto the existing pilots and piping; this worked very well.We had an individual, stainless-steel flexible gas line comingdown for each one of the pilots. Tied to each hose was athermocouple extension wire a 24-volt ignitor wire. We hadan automatic ignitor at the flare base where we tied in withthe thermocouple, electric and gas. For more information onhow new pilots were installed on an operating flare, as forNAO Paper RP93-1.


Pilot Ignition Systems

NAO makes many different types of ignition systems,including those with air blowers, either electric or gasolinedriven. One shows a small system, which was acombination ignitor and purge system, that we installed in thedesert; the top is the ignitor; the bottom is the purge. Weused solar cells for power and a little gasoline motor with ablower for combustion air.

Our basic Flame Front Generator is the NAO IgnitionPanel (NIP). The one shown here has a three-way valve onthe right and two three-way valves on the left for a total ofthree outlets. The number of valves needed is always oneless than the number of pilots.

The standard Automatic Ignitor Panel comes withstrainers on left side, pressure regulators and solenoid valvesthat automatically control the gas and air. There’s a by-passaround each of the solenoid valves, in case they malfunction.

The other approach to using an ignition manifold is the aFlame Splitter. The Flame Splitter is a unit which has nothree-way valves on the panel. The gas comes out of theignition panel and up to the flare tip where it splits and ignitesall the pilots, simultaneously. This works very well. NAOhas tested a mile-long flame front ignitor pipe that lights allthe pilots simultaneously.

The benefit of the Flame Splitter is that it is very easy to usewith an automatic ignition panel. With the three-way valves,you need a large, motorized valve to position each of thepilots sequentially. Large motorized valves are expensiveand they are a problem because you have to coordinate theindexing and feedback to the controller. We’ve done itoften but the better approach is to use a flame splitterwithout valves. It is important to note that if an NAO pilot isalready burning, nothing will happen to it—if you shoot upanother ball of fire. This is because of the way we mountour flame front tube. If the flame front tube is not positionedproperly, the “pop” of the ball of flame can snuff the pilot.


Specialty Ignition Systems

Many plant use “refinery gas” to supply their pilots. Thisrefinery gas has varying levels of purity and can be all thecats-and-dogs in the gas system. One minute you mighthave all-hydrogen. Another minute, you might have all-propane, or something in between. This is very hard on theignition panel because the gas/air ratio changes for differentgases. With an Automatic Ignition Panel, when the gaschanges the settings are no longer correct. You have to goout and make manual changes. To compensate for this,NAO can custom engineer the automatic ignition panel toautomatically adjust for different gases.

If you don’t have compressed air, NAO has various othermeans of producing ignition. We have Inspirating Ignitors.We also have other electronic ignitors, such as the OffshoreSub-Sea Ignitor. If you tried to shoot a ball of fireunderneath the sea inside a tube, you would get a lot ofcondensate—and a very quick failure! NAO uses asubmarine cable which carries gas over to the ignitor. Then,an electrical signal produces the spark remotely. Inside thatsame cable may be thermocouple wires for automaticcontrol of the ignition system.

NAO was the first to develop the retractable pilot. TheNAO Electronic “Elevator” System raises the pilot toignite the waste stream, then lowers the pilot to protect itfrom the heat or to perform repairs.

We also have Portable Spark Generators. These units usea 24-volt battery box to produce the spark. They also canbe used for an inexpensive battery back-up in the event of apower failure. We can also supply battery back-up largeenough to control the solenoid valves, temperature board,and spark ignitor of a fully-automatic spark-ignited pilotsystem.

If you’re out in the field and don’t have any electricavailable, but do have compressed air, then you could use aNon-Electric Flare-Ignition System which uses apiezoelectric crystal. Or you can do the same basic thingwhen there is no compressed air or electric utility availablewith an inspirating ignitor.


Liquid Knockout Drum

If liquid condensate in the flare header is a problem, knock-out drums, or disentrainment drums are necessary. Withouta KO drum, liquid hydrocarbons will carryover to and burnon the flare tip. Since liquids burn hotter than the gases andsince they burn directly on the metal, the metal degradesextremely quickly. A flare with liquid carryover can burn upin a matter of months. If a KO drum is used properly, thisflare life can be extended to decades.

If you were to look inside the drum you will see a baffle.The NAO drum is designed for a cyclone effect. The gascomes in tangentially so that it swirls around. Any liquid isthrown to the outside of the drum, falls down due to gravity,and is collected underneath the baffle. If you look carefullyyou will notice that the baffle does not extend out all the wayto the wall. There is actually a gap to allow liquidaccumulating on the walls to run down underneath the baffle.As the gas comes along and spins, it actually ducks downunderneath that inner cylinder and then goes up the middle.That is a settling effect; when there is low flow that’s whatactually knocks out the liquid. The center of the baffle isperforated, allowing the liquid to drop down. The reasonfor the baffle is to prevent surging and waves. If the devicedid not have the baffle, the liquid sitting at the bottom wouldact like the ocean in a hurricane. You would have wavesand a lot of the liquid would be re-entrained—actuallypicked up by the gas flow.

Information on flare accessories like Liquid Seals and FlameArrestors can be found in the section titled Flame ArrestingEquipment.

Knock-out Plot for Particulate Disentrainment


Flare Radiation

The main factors affecting thermal radiation in a flare are:Flame temperature, flame emissivity, and flame tilt. A flameradiates heat from carbon particles, carbon dioxide, andwater vapor. Nitrogen, though a large part of thecombustion product volume, radiates very little heat due toits diatomic structure.

Most of the radiation emitted from a flare, about 80%,comes from carbon particles. The energy radiated from aflame is proportional to the fourth power of the absolutetemperature (≈T4) and is related inversely to the square ofthe distance from the flame (∝R2)—see the appendix forradiation calculations. Hence, for proper flare design, wehave three objectives: Keep the flame as cool as possible,keep it free of unburned carbon particles, and keep it erectto maximize the effective height of the stack or boom.

Reducing Flare Radiation

• Keep the Flame Cool• Eliminate Carbon Particles• Keep the Flame Erect

Flame emissivity—the ability of combustion products toradiate energy—is a property of the fuel being burned and ofthe fluid-dynamic pattern of the flaring system. The fluid-


dynamic pattern of a flare is the turbulence in the flare flamegenerated by external energy sources. This turbulence isextremely beneficial. Turbulence increases combustionefficiency, decreases the emission of unburned gases, andsubstantially reduces the concentration of carbon particles inthe flame. The end result is a substantial reduction in thermalradiation.

Exposure Limits

NAO has conducted intensive research and test programs todetermine flare performance and to measure actual radiationlevels. Included in these programs are investigations of theactual skin-cooling effects of various ambient temperatures,different wind velocities, and different levels of relativehumidity. By measuring actual values, we have shown thatmany established standards for maximum radiation exposuremust play an important role in designing effective flares.

These standard values indicate safe limits for radiationexposure. For instance, 1,500 Btu/hr·ft2 is generally takenas the maximum allowable radiation exposure at any time forpersonnel areas. If an emergency flare goes off, personnelwill have about 30 seconds to evacuate an area beforereceiving painful burns on bare skin. For continuousoperation, personnel can work in areas so long as radiationlevels do not exceed 440 Btu/hr·ft2.

It should be noted that these figures are very conservative.There is no provision for wind, ambient temperature, or

clothing. The run-clear value of 1,500 Btu/hr·ft2, forexample, is for bare skin. Many plants now have fireresistant clothing requirements and use 2,000 Btu/hr·ft2. Thewind effect and ambient temperature can be even moresignificant than clothing. With a radiation intensity of1,500 Btu/hr·ft2, 70 °F ambient temperature and no wind, aperson will experience severe pain on bare skin within 30seconds. With a radiation intensity of 3,000 Btu/hr·ft2 and a30 °F, 30-mph wind, a person can continuously withstandthe radiation exposure with any (or no) clothing.

In cold climates, low solar radiation is generally combinedwith windy and frequently dry weather conditions, so peopleworking around a flare will wear heavy clothing in order tobe more comfortable. In hot climates, on the other hand,


people will wear light clothing to be more comfortable withhigh solar radiation, high humidity, and little, if any, wind.

We cannot design an efficient flare at low cost by usingaverage values. Obviously, a design which is satisfactory forone set of parameters may be completely inadequate foranother set of climate conditions. It is essential that flareengineering be handled by experienced people. They musthave a wealth of background experience and not just relyupon a few outdated procedures, assumptions, andcalculations.

Shielding

Common sense must prevail when designing flare systems toreduce radiaiton exposure. The use of extremely longbooms or other means of keeping the flame as far awayfrom the offshore platform or personnel area must berestricted to practical mechanical desing applications.Shielding can effectively reduce the need for extra-longbooms.

Experience with water shields or water curtains has not beensuccessful. Such shields can be valuable for fighting fires,but they cannot be made large enough to protect a relativelylarge area on an offshore platform. The corrosive effects ofa continuous sea-water spray also rule out the use of watershields for normal day-to-day activities.

Corrugated aluminum plates and screens shields arecommonly used to shield the three levels of a derrick and toprotect the sides of ladders and handrails on the flare side ofa rig. This also serves as a noise barrier. Flare noise isdiscussed in the next section.

Radiation Pain Threshold

Radiation Intensity Time to painBtu/hr•ft2 kW/m2 seconds

550 1.74 60740 2.33 40920 2.90 30

1,500 4.73 162,200 6.94 93,000 9.46 63,700 11.67 46,300 19.87 2


Flare Noise

A big problem with a flare is the noise it generates. Youhave two sources of noise. One is the combustion noise, thelow-frequency rumbling type. The other one is the high-frequency steam jet.

Low frequency combustion noise is the sound of combustionand cannot be entirely eliminated. Combustion noise usuallybecomes a problem when a customer oversteams a flare.When too much steam is injected through a steam ring, aflame pulsation will occur—you will see the flame flare upevery 1-2 seconds.

To counter the effects of high-frequency steam noise withelevated flares, NAO has developed the Soft Mix Flare.This actually is an acoustically shrouded flare. It has adouble-wall design with a mixing star at the very top. Steamis injected in the inner cylinder where is entrains air andpushes it out the top. This cylinder is shrouded, so thesteam cannot be seen or heard. The steam-air mixture exitsthe inner cylinder through a mixing star at the flare tip whereit mixes with the flare gas. Note: One caution with theSoft Mix Flare: If a turbulence level is too high, you willhear a lot of rumbling.

From API Recommend Practice 521

Recommended Design Flare Radiation Levels Excluding Solar Radiation

Permissible Design LimitBtu/hr•ft2 kW/m2 Conditions

5,000 15.77 Heat intensity on structures and in areas where operators are not likely to beperforming duties and where shelter from radiant heat is available, for ex-ample, behind equipment.

3,000 9.46 Permissible limit at design flare release at any location to which people haveaccess, for example, at grade below the flare or on a service platform of anearby tower. Exposure must be limited to a few seconds, sufficient forescape only. On towers or other elevated structures where rapid escape isnot possible, ladders must be provided on provided on the side away from theflare, so the structure can provide some shielding when the design level isgreater than 2,000 Btu/hr•ft2 (6.31 kW/m2).

2,000 6.31 Heat intensity in areas where emergency actions lasting up to 1 minute maybe required by personnel without shielding but with appropriate clothing.

1,500 4.73 Heat intensity in areas where emergency actions lasting several minutesmay be required by personnel without shielding but with appropriate clothing.

500 1.58 Permissible limit at design flare release at any location where personnel arecontinuously exposed.


Soft Mix Flares come with external steam rings forsmokeless flaring at high rates—at that point, it’s usually anemergency—and the noise attenuation is not the primaryproblem. During normal operation, all of the required steamis enclosed.

If a customer want to eliminate some of the noise on anexisting flare, NAO can offer to replace their existing steamtips with the NAO Steam Dog tips. These are longer thanour standard steam tips, and they inject the steam over awider area of the combustion flame. This makes combustionmore efficient (reducing low-frequency noise) and itincreases the number of steam jets (reducing high-frequencynoise). The Steam Dogs can be included on any new NAOSteam-Assisted Flare at a nominal cost.

The most effective way to eliminate high-frequency steamnoise is to eliminate the steam. This can be done by using anNAO Enclosed Ground Flare (the NVCU or NPAC). Formore information, see the section on Enclosed Flares.

Radiation Allowancefor Ambient Temperature

Radiation Allowancefor Wind

To be added

To be added

Permissible Noise Exposure†

Duration Sound Levelhours/day dBA

8 906 924 953 972 1001 105½ 110¼ 115

† From 29 CFR 1910.95

Typical Flare Noise Spectra


Flare Controls

All flares require some kind of control system. The simplestare manual valves with manual push-buttons that requireoperations personal to physically monitor. With newregulations, however, these fully manual systems are rarelyallowed. Most areas now require a monitor on the flarepilots to ensure that flame is always present.

Pilot Monitoring

NAO recommends thermocouples to monitor pilot flames.We provide the option of complete pilot control packageswith flares tip purchases and with pilot replacements on flaresystem refurbishments.

On all NAO pilots with thermocouples, the thermocouplesare enclosed in stainless steel guide tube that are enclosedinside the pilot tubes. This allows the pilot gas to cool thethermocouple and extend its life. In addition to the guidetube, NAO uses a thermowell in the pilot nozzle to preventthe pilot flame from directly impinging on the thermocouple.With the addition of this thermowell, we have extendedthermocouple life in some applications from a few months toyears and years.

Besides thermocouples, NAO also has infrared pilotmonitoring systems. These involve infrared detectorslocated at grade level that look for the heat of the pilotflame. Infrared monitors have one advantage overthermocouples in that these systems can be maintained whilethe flare is in operation; unless a customer has selectedretractable thermocouples—an option on NAO pilots—theflare must be off-line to replace a damaged thermocouple.The problem with infrared monitors is that they cannotdistinguish between pilot flames and flare flames. If the flarehas two of three pilots out, the monitor may not detect thefailure of the first two. This can have hazardousconsequences if the third pilot fails as the plant dumps gas tothe flare. With thermocouples, the status of each pilot isknown individually.

For more information on NAO Flare Pilots, see the sectionon Flare Pilots which follows.


Steam Control

NAO has several Automatic Control Systems for steamcontrol. Generally, customers opt for advanced steamcontrol technology to reduce the cost of operating theirsteam assisted flare. Sometimes, however, advanced steamcontrol is necessary to prevent the noise associated withoversteaming the flare during low-flow operation.

We have many different control systems today. The oldersystem is a combination flow and radiation control system.It works by having a flow sensor in the flare header andradiation sensors at the top which provide a fine tuning.Today the trend is more towards optical control, which willbe described later. That’s where you have an infrared pickup and it operates the entire process. That process is a littlebit easier and simpler. The older control is ideal if you havejust a very simple type of a flow, such as only propane oronly butane. It’s also very good if you have a low Btu flare.A flare that will not burn by itself, one for which you have toadd supplemental fuel. Under such conditions we canmeasure the flow of the waste gas coming along and thenproportion in the amount of assist gas we need for completecombustion.

The trend today, though, is towards optical systems. Anoptical sensor is mounted at the base of the flare to monitorand control the amount of steam going to the flare forsmokeless operation. These controls look for molecularintermediates that form during combustion. If theseintermediates increase, combustion efficiency is degrading,so steam flow is increased to keep combustion complete.

Other Controls

Thermal Radiation Monitors, which are pyroheliometersor radiation-of-the-sun meters are used to monitor radiationlevels of the flare. The are used as warnings, mostlyoffshore where knowing the level radiation is critical topersonnel. These monitors are also used when highways orother roads pass nearby a flare.

NAO also makes oxygen analyzers for flares. TheAutomatic Oxygen-Monitoring System continuouslymonitors oxygen levels in flare stacks and vents to assure


safe flaring and venting of combustible gases. It calibratesitself every day.

To reduce operations expenses, NAO has developedEnergy-Conservation Control Systems to save energy bycontrolling the pilot and purge gases and turning them ononly when necessary for flare relief. A popular conservationsystem is Purge Gas Control. This system will correct fortemperature and flow and conserve purge gas. This is reallynot a critical type of service, but it is popular. NAO is nowdeveloping an Inert Gas Generator to use at the base of aflare stack to purge the flare stack using only instrument air.NAO has eliminated the need for control systems for pilotsby developing the ultra-reliable conservation pilot.

Most customers don’t favor automatic energy conservationsystems because they have critical components. If thecontroller fails to operate, then they have to “dump the flare”because there’s no pilot ignition. By using ConservationPilots, they have continuous, dependable pilots. That iswhy the control systems have fallen out of favor. Mostpeople feel a lot safer by having a continuous running pilot.


Flaring Applications

NAO has decades of experience providing complete flaringpackages to industry. This section discusses some typicalflaring challenges that you might see.

In addition to the sections that follow, keep in mind thatNAO enjoys tackling new combustion challenges that ourcompetition will not or cannot accept. We have theexperience and the equipment to solve any feasible challengea customer can present.

NAO has truly been the worldwide leader when it comesto providing flares to industry. We were the first todevelop flares for coke ovens and steel mills. Recently,we developed the first flare for the carbon blackindustry to operate without the necessity of assist gas.

Ammonia Terminals

Next, let’s consider some of the different applications forflares. The photo to the right shows an ammonia terminal.Please note the small pipe, projecting above the tank.That’s actually the flare. Usually the flare is mounted on topof the access staircase going up the side of the tank. Theseflares are fairly small. They may be only 6, 8 or 10" indiameter. Operating at very low pressure, they resemble anNFF-CG flare. Ammonia burns well with only 6 or 7%unburned ash, but that doesn’t really matter because it’s anextremely rare event when one of these tanks will vent. Themain reason for tank venting is failure of the ammoniarefrigeration system, or loss of electric power which causesthe refrigeration to fail.

NAO uses a pressure switch on the top of the ammoniatank to activate the pilots. It is important to remember thatthe ignitor must have a battery backup, because when it isusually needed during a power failure. With our batterybackup, the ignition system will work and the flare will burnthe ammonia. With ammonia terminals, we usually don’tworry about complete combustion, because we get a fairlyhigh efficiency rate.


Landfill & Digester Flares

With new regulations, landfills and wastewater treatmentplants have been required to install vapor recovery systemsor flares to destroy methane and other VOC’s that form asorganic matter decomposes.

The landfill, or digester, flare is enclosed by a shroud.Combustion occurs inside the shroud which hides part of theflame and insures a good stable incineration. There is apartially visible flame because this is not a fully enclosedflare. The flame may emerge two or three feet above theshroud. The gas burned in a landfill or digester flare istypically about half inert (nitrogen and carbon dioxide).Since the heating value is 400-500 Btu’s, it burns well.Sometimes, however, there are lower methaneconcentrations, and that’s why you need the shroud.

Steel Mills

NAO has pioneered the use of smokeless flares for steelmills and coke ovens. During the late 1980’s, NAO didextensive testing at coke plants to develop the best designfor their smokeless flaring of the dirty, low-heating-valuegas. Our steam-assisted Fluidic Flare is considered by theEPA to be the Maximum Available Control Technology(MACT) for safe and reliable emergency flaring of coke-oven waste gas.

Portable Flares

With over a decade of experience with portable flares,NAO has the expertise to provide reliable, long lasting flaresystems that can be moved from site to site. These systemssave customers money by allowing them to purchase oneportable flare to use at many sites during turnarounds orduring temporary flaring applications.


Common Problemswith Flare Systems

Pilot does not light

Wrong pilot gas pressure: Check the original designspecifications for proper gas pressures. In general, NAOConservation Pilots require 5-psig of propane and 10-psigof methane or natural gas, but this can change for specialapplications.

Wet pilot gas: Condensate in the pilot gas collects in lowpoints of the pilot gas line near the pilot and gets pushedalong the along the pilot tube wall when the gas is turned on.This is more of a problem when using spark-ignited pilots.

Plugged pilot tube, venturi, air adjustor, or pilotwindshield: Corrosion, water, insects, and bats haveplugged gas and air flow through pilots.

Pilot gas filter is plugged: If the pilot gas filter plugsfrequently, install a primary filter at grade that is convenientto replace. The filter on the pilot should remain in place. Inspite of what operators may think, the filter will not plugnearly as quickly as the tiny orifice of modern (conservation)pilots.

Flame-front ignition failure

Plugged ignitor tube: On start-up, if opening thecompressed air causes back pressure on gas side, the ignitorand pilot gas tubes have been crossed or the ignitor tube isplugged. If the pressure gauges are slow to return to zero,the ignitor tube is partially blocked.

No flame: The generator can fail due to improper air/gasmixture, wet gas or air, or power failure.

Flame quenched: The flame front probably will not reachthe pilot if there is condensate in ignitor tube, if an ignitortube drain valve is open, if flex hose connections are used onthe ignitor tube, if a strainer is in the ignitor tube, and if theignitor tube is anything other than 1-inch pipe.


Flame front blows out pilot flame: Ignitor tube smallerthan 1-inch pipe or the ignitor tube length less than 100 feet.Also, if the air flow in the ignitor tube is too high, the mixturereaches stoichiometric proportions and detonates in theignitor tube; this can damage light-gauge pilot windshields.

Spark ignition failure

Power failure: Improper grounding of the 24-volt powersupply or a broken wire prevents the ignition capacitor fromcharging.

Water in exciter housing: Worn seals on the weatherproofhousing can allow water to collect inside and short theignition board.

Spark ignitor failure: Excessive tar-like carbon build-upand long-term water immersion are the only knownenvironments that have cause ignitor failure. Given time,though, the stainless steel spark ignitor can corrode.

Radiation damage: Exceeding the design limits of the flarecan damage the ignition board. If the problem is persistent,consider moving the spark-ignition components further fromthe pilot tip.

Lightening strikes: The spark-ignitors have a small ignitionboard mounted on the pilot.

Pilots shut off automatically

Pilot monitor failure: Pilot monitor does not detect flamedue to monitor failure or low pilot temperature.

Pilot gas solenoid valve failure: Most solenoid valves onpilot gas lines are normally closed. If power fails, the gas isshut off.

Pilot monitor failure

UV/IR failure: UV/IR scanners cannot always distinguishbetween flare flame and pilot flame. If this occurs, move the


monitors closer to flare and narrow their range of vision (i.e.don't target the entire flare tip from 3,000 feet, target a pilotfrom 500 feet).

Thermocouple corrosion: Thermocouples placed in theflare flame can be exposed to excessive (H

2S) corrosion

attack. Thermocouples should be located below flare tipand out of the flare flame. Exposure to corrosion attack canbe minimized or eliminated by using thermocouple wells andby locating thermocouple outside of flare flame.

Thermocouple short: Water in Mineral Insulated sheathshorts thermocouple when corrosion destroys sheath orwhen epoxy potting head cracks due to vibration or duringinstallation. This can be avoided by using thermocouplewells, by locating thermocouple outside of flare flame, andby using heavy-duty potting head at base of thermocouple.

Low pilot temperature

Fuel gas composition not per design specifications:Using low heat value gas can lead to low pilot temperatures.

Fuel gas pressure low: Low fuel pressure yields low gasflow.

Pilot jet plugged or restricted: If the flame is small andlazy but pressures are correct, there is probably anobstruction in the line that is restricting gas flow.

Pilot gas filter is plugged: If the pilot gas filter plugsfrequently, install a primary filter at grade that is convenientto replace. The filter on the pilot should remain in place. Inspite of what operators may think, the filter will not plugnearly as quickly as the tiny orifice of modern (conservation)pilots.

Air adjustor restricted: The air adjustor needs to beopened or it is partially (or completely) blocked bycorrosion. The pilot gas doesn't burn until it exits pilotwindshield and mixes with additional air. The flame lifts offof the pilot nozzle and moves away from thermocouples.This can lead to pilot flame instability. Opening the airadjustor a little pulls the flame into the pilot and increases thetemperature seen by the thermocouple.


Pilot flame instability

Low gas pressure: Check filters or strainers, provide aprimary filter at grade level for ease of maintenance, ifnecessary.

High gas pressure: The pilot gas velocity is so high that theflame lifts off of the pilot flame retainer. This makes the pilotsusceptible to cross wind.

Air adjuster improperly set: Inadequate air enters atventuri due to improper setting, corrosion, or accumulationof dirt.

Corrosion of pilot windshield: Large holes in the pilotwindshield short-circuit the windshield ability to block strongcross winds.

Changing pilot gas composition: Variations in refinery gascomposition result in different flame speeds. If the gas flowrate is constant, the pilot flame will move up and down insidethe windshield.

Snuffing during rain: Water can get into the air adjustor ifit does not have a rainhood on the air adjuster. Thisrainhood is now standard on NAO pilots. Older systemsmay not have them.

Inert gas purge: Using high nitrogen or other inert-gaspurges on a flare system can starve the pilot of combustionair. This can be overcome by using eductor pilots thatinspirate all of the required air ten feet or more from theflare tip.

Flare Smoking

Plugged steam lines or steam tips: Particulate in dirtysteam can collect at sharp turns in the steam line, of whichthere are many at the tip. Dirty steam comes from dirtywater, from corrosion of the steam header piping, and fromwelding slag, gravel, and other junk in the header piping.When starting up a flare for the first time, make sure that thesteam line is blown out prior to installing the tip.


Low steam flow: Flow at less than design specificationscan allow vapors to pass under steam tip of ring. This canbe caused by the failure of steam control valves to open.

Missing tips or cracked ring: Damaged steam rings entrainair inefficiently and can allow steam to miss flame zone.

Poor turndown: Smoking of high-pressure Jet-Mix Flare atlow flow. Jet-Mix Flares are designed to use the highpressure of the gas flow to entrain it own air for smokelesscombustion. If design flow pressures change from originaldesign specifications, a new tip or a staged system may berequired.

Flare Noise

Combustion noise: Some combustion noise is unavoidablewhen flaring. Efficient, stable combustion generates the leastamount of noise.

Pulsating, low-frequency noise: Caused by Oversteamingflare (better steam controls will provide proper amount ofsteam for a given gas flow rate) or excessive water-sealpressure drop (flow stops for a second at seal until pressurebuilds and a slug of vapor passes). Continuous combustionrumble can be minimized by using NAO Steam-Dog steamtips.

High-frequency steam noise: Some steam noise isunavoidable. If required, the steam injection for normal flareflow can be shrouded to minimize the continuous flare noise.

Flare Flow Restricted

Molecular seal plugged: Mole seals collect looserefractory and water. These must be cleaned on a regularbasis.

Water seal plugged: In cold climates, the water freezes.The addition of glycol is a common solution.

Flame arrestor plugged: Flame arrestors must beperiodically removed from service and cleaned. Use asolvent or high-pressure steam or air to clean the arrestor.


Knock-out drum plugged: Check for build-up ofparticulate or corrosion. Any trash in the flare header willeventually make its way into the knock-out drum.

Flashback in Flare Header

Leakage in header piping: Flange leaks allow air to enter.Any flare gas that is lighter than air (mole weight = 29) willcause a slight negative pressure in the flare header as it risesby its own buoyancy. This is particularly dangerous at lowflow conditions.

Poor gas seal at tip: Allow for air ingress at low flow.

Failure of water seal: Evaporation of water makes sealineffective, the magnitude of this problem increases with flaregas temperature. Check level meters and the automatic fillequipment.

Failure of flame arrestor: This is generally due toimproper application or to a damaged grid. Flame arrestorsmust be periodically removed from service for inspection. Ifgas compositions change, the flame arrestor selection mustbe reviewed. A given flame arrestor does not work for allapplications.

Cracked or Warped Flare Tip

Low flow leads to flame lick: Use a solid conicalwindshield to protect flare body. Damage to the windshieldis irrelevant compared to the problems associated withdamage to the flare body.

Liquid carry-over: Condensate in the waste gas is carriedby the motion of the gas to the flare tip where it burnsdirectly on the metal at a higher temperature than gas. Lookfor carbon build-up at tip (carbon residue in gas seal ortiger-stripes on outside of tip). Use a knock-out drum thatfunctions properly to remove liquids from the gas stream.


Cracked Steam Ring or Tips

Lack of expansion joints: U-joints or bellows will allowfor heat expansion and prevent cracks in the steam ring. Onderrick flares where the flare stack moves relative to thestructure and where the steam header is mounted to thestructure, make sure the growth of the stack due to heatexpansion does not pull the tip away from the steam line.

Water hammer: Slugs of water in the steam strike thesteam ring at high velocity. The water slams into the suddenrestriction at the steam tip and puts large mechanical stresseson the steam ring. Installing good steam traps (bucket traps)to knock-out condensate should prevent this problem.

Overheating: Operating a steam-assisted flare with nocooling steam then turning on steam can cause thermal stressand cracks in ring and tips.

Structural Damage to Stack

Corrosion: Due to liquid carry-over (high H2S content).

Sometimes liquids condense in the flare stack and collectinside the base; this quickly corrodes the metal and cancause the stack to lean; this also allows air to enter at thebase of the flare and increase the potential for flashback.

Loose guy wires: Cables should be inspected, tensioned,and greased every 6-12 months.

Staged Flare Problems

Rupture disks: Rupture disks are prone to prematurefailure, particularly from downstream pressure. They mustbe replaced annually (this requires flare to be shut down). Ifthis problem is persistent, switch to rupture pin valves (theyare more expensive). Rupture pin valves are not prone topremature failure, they are not susceptible to back pressure,and they can be easily replaced without shutting down theflare.



“Combustion Tips,” Volume 1, Number 4, Safe Loading ofGasoline and other Flammables reviewsapplications for the Vapor Disposal Unit (NVDU),an enclosed ground flare.

“Flare Technology Safety” is a general review of manyflaring applications.

“Flares: Design and Operation” is a six page review of flaredesign with calculations for thermal radiation.Included are schematics for refinery, pipeline, LPGstorage, ammonia storage, and offshore flares.

“Flaring for Gaseous Control in the Petroleum Industry,”NAO Paper #78-58.8, gives a brief introduction tosmokeless flaring and describes in detail a four-month study done at NAO on 2-inch, 3-inch, and 6-inch smokeless flares.

“High Performance Offshore Flares” a presentation at theFourth International Flare System Seminar discussessafety and engineering design criteria for offshoreflares.

“Solving Flare-Noise Problems” describes many goodsolutions for reducing decibel levels of smokelessflares.

"Specifications for Flare System" provides a basis fordeveloping a flare system specification for,particularly, a self-supported emergency flare.Ask for NAO Specification ELF-1.


Thermal Oxidizers — Property of NAO Inc. 8-1

Thermal Oxidizers

Thermal oxidation offers the safest, most effective andreliable solution to the problem of eliminating objectionablewaste gases while complying with stringent regulations on airpollution control. The fuel for a thermal oxidizer may be thecombustible waste product itself, or it may be augmented bya conventional fuel if the heating value of the waste stream istoo low or too dilute.

NAO designs, manufactures and services all combustionsolution systems including Incinerators (today we use thewords afterburner or thermal oxidizer–incinerator is a “dirtyword”) and Vapor Control Units.

NAO makes incinerators for gases, liquids, slurries andvapors–any kind of stream that can be pumped. A slurry,for example, would be pumped through one of our AtomizerGuns. For vapors, we use our patented Flame ArrestorVapor Burner. If requested, we have thermal oxidizers withheat recovery like the NAO Liquid Incinerator. We recentlysupplied one to a pharmaceutical plant to generate 150-psisteam by burning spent alcohol.

NAO’s portable thermal oxidizers include theVOCWagon,TM the VOCSkid,TM the VOCBox,TM and thelarge-capacity trailer-mounted PTOTM units available forlease, sale, or rental. These efficient fume incinerators arewidely used in the oil, gas and petrochemical industries tocontrol hazardous vapors and objectionable odors duringtank clean-outs and other plant shutdowns. They are alsoused for soil remediation projects involving leaks or spills ofgasoline, benzene, and other VOC’s.

Volatile Organic Compounds

Volatile organic compounds (VOC’s) may be released tothe atmosphere during routine processing in a refinery or in achemical, petrochemical, or pharmaceutical manufacturingplant. More often, those dangerous VOC’s are vented tothe atmosphere during emergency overloads, maintenance

8-2 Thermal Oxidizers — Property of NAO Inc.

outages, equipment failures, liquid spills, or other accidentalreleases of carcinogenic, malodorous or highly flammablecompounds.

Because large clouds of hazardous vapors can be generated,the uncontrolled emission of VOC’s is detrimental to theenvironment on a global scale. It also creates serious safetyproblems in the immediate vicinity of a liquid spill orflammable gas cloud.

Before the Clean Air Act was passed, many chemical andhydrocarbon vapors were routinely vented to theatmosphere. National regulations now limit VOC emissionsfrom loading operations, including gasoline loading terminalsfor ships, barges, railroad tankcars and trucks, to less than35 milligrams per liter of the loaded product.

Some states are more stringent and demand betterperformance from new loading terminals before installationpermits are granted. Other states are enacting new lawsregarding emergency emissions of VOC’s caused byprocess upsets. There are also new regulations and pendinglegislation regarding the degassing of storage tanks.

Florida now requires a total shutdown in the event of anyprocess upset that created emissions in excess of 35 mg/liter. In California, there are stringent controls on thedegassing of storage tanks. Tank farm operators are beingpressed to collect and oxidize the vapors released duringperiodic venting of their tanks.

The Environmental Protection Agency (EPA) is enactingnew regulations. Compliance with those new EPAregulations will be required before permits are issued bystate and local Air Quality Control Boards.

Thermal Oxidation Systems

For routine and intermittent venting of hazardous waste, athermal oxidizer can be employed in conjunction with avapor collection system. Permanently installed on-site, thethermal oxidation system will include burners, pumps, fail-safe operating controls, and a water seal or flame arrestoron the waste-gas inlet. The fuel for a thermal oxidizer maybe the combustible waste product itself, or it may be


augmented by a conventional fuel if the heating value of thewaste product is too low.

To insure complete incineration of hazardous or foul-smellingwastes, those wastes are subjected to temperatures between1,400 °F and 1,800 °F for at least ½ second. Theobjectionable wastes are thus converted to innocuous ashes,carbon dioxide, and harmless water vapors.

When halogenated compounds are incinerated, secondarypollutants are generated. For those applications, NAO’sthermal oxidizers incorporate integral scrubbers orparticulate-collection subsystems to capture evolvedhydrochloric acid, hydrofluoric acid, and other secondarypollutants.

For permanent installations with optional gas scrubbers, theeffluent gases are spray cooled in a quench chamber toremove non-combustible particulates and absorb theundesirable secondary pollutants. The particulates are

Destruction Efficiency and Emission Profile†


filtered and the scrubbing liquor is pH adjusted andrecycled. The exhaust gases, which are primarily carbondioxide and water vapor, comply with the most stringentEPA and individual state regulations for air pollution.

NAO Thermal Oxidizers

• High-efficiency, low-NOx burners• Capacities up to 100,000 scfm• Combustion efficiencies 99+% standard• Residence time of ½+ second at 1,400-1,800 °F

standard• Field-proven low operational costs and low

maintenance costs• Complete packaged systems with flame arrestors,

water seals, and booster blowers

Horizontal Thermal Oxidizer (NHTO)• Optional scrubbing and particulate collection

systems• Optional waste heat recovery boiler or air preheater

Vertical Thermal Oxidizer (NVTO)• Standard sizing reduces capital cost• Vertical design allows for natural draft firing–no air

blower required–forced draft available

Portable Thermal Oxidizers

By employing state-of-the-art designs for thermal oxidationand manufacturing self-contained, portable systems, NAOoffers economic solutions to VOC problems where the costof permanently installed equipment cannot be justified.

These portable units, trademarked VOCWagon, VOCSkid,and NPTO provide solutions-to-go for serious liquid and airpollution problems. Designed for use in the event of a plantmalfunction, equipment failure, or whenever an emissioncontrol system has been rendered inoperative due to a fireor mechanical or electrical shutdown, these units aredesigned to be set-up in a few hours with fully automaticcontrols to keep a plant on-line. These new, portable unitshave also been used for emergency spills, maintenance

Sizing Thermal Oxidizers

Tflame

+ 4601. ————— = GF

Tgas

+ 460

2. FR x GF = FRact

FRact3. ——— = TO

area40

⎡ TOarea

⎤½4. ⎢———— ⎥ = TO

ID⎣ 0.7854 ⎦

5. TOID

+ 0.56 = TOOD

6. RT x 40 = TOSH

FR Flow rate, scfmFRact Flow rate, acfmGF Growth factor by temperature riseRT Residence time at Tflame, (½) secondsTflame Oxidizer temperature, (1,600) °FTgas Vapor temperature entering unit, °FTOarea Area of chamber, square feetTOID Chamber inside diameter, feetTOOD Chamber outside diameter, feetTOSH Chamber height, feet


outages, and during the cleaning of pipelines, storage tanks,and process vessels.

Available in three sizes to incinerate up to 39.950 cubic feetper minute of dangerous vapors, each trailer or skid-mounted unit is completely self-contained. Positiveflashback protection is assured by a unique flame arrestorhead, a liquid seal, and automatic lockout controls.

$10,000

$5,000

$15,000

$20,000

$25,000

$30,000

$35,000

$40,000

1,000 3,000 5,000 7,000 9,000 11,000

$60/hour

$80/hour

$40/hour

$20/hour

Waste Vapor Flow Rate, scfm

Thermal Oxidizer Cost Estimate

Capital Cost

Operating Cost


NAO Portable VOCSkid & VOCWagon

• 1,200 scfm capacity• Proven 99+% combustion efficiency–complies with

all federal, state, and local regulations• No visible flame, no smoke, very low noise• Packaged with complete controls and interlocks for

automatic, un-attended operation• Easily transportable–on-line in a matter of hours• Flame arrestor burners handle any vapor

composition• Adjustable natural-draft or forced-draft firing• Flame arrestors, water seals, booster blowers, and

eductors are available• Factory technicians available for start-up and

operations assistance

All of the portable units are ready for immediate, emergencyservice. They are available for short-term rental, for long-term lease, or, of course, for sale. Customers are welcometo inspect the portable rental fleet at any time. Equipment isstored and maintained at the home office in Philadelphia,Pennsylvania, and at the Environmental Research andService Center northwest of Houston, Texas.


NAO Portable Thermal Oxidizer (NPTO)

• 39,950 scfm capacity–only NAO has a temporary,portable unit with this capacity

• Proven 99+% combustion efficiency–complies withall federal, state, and local regulations

• No visible flame, no smoke, very low noise• Easily transportable–on-line in a matter of hours• Flame arrestor burners handle any vapor

composition• Adjustable natural-draft firing• Flame arrestors, water seals, booster blowers, and

eductors are available• Factory technicians available for start-up and

operations assistance

NAO Thermal Oxidizers have been used: In plantturnarounds; for soil remediation and emergency clean-upsof leaks and spills; and to clean and maintain pipelines,storage tanks, process vessels, railcars, and tank farms.

Case In Point

An NAO skid-mounted Thermal Oxidizer was used duringthe emergency clean-up of a million-gallon gasoline leak at aloading terminal on Long Island, New York. In addition tocontaminating nearby streams, that massive leak formed avast underground pool of gasoline. Volatile hydrocarbonvapors and carcinogenic benzene vapors from theunderground pool threatened nearby neighbors.

A standard pick-up truck towed a trailer with a VOCSkidoxidation unit to the site. Clean-up operations commencedafter a relatively quick and easy hook-up. Since thehydrocarbon and benzene vapor stream was extremelyvolatile and rich enough to burn on its own, no assist gaswas needed to incinerate that waste stream.

The volatile vapors were sucked in by an explosion-proof,hermetically-sealed booster blower, which in this casehandle 500 cubic feet per minute of the contaminated air/vapor stream.


Operating at approximately 1,800 °F, the skid-mounted unitprovided a combustion efficiency of 99.9%, effectivelydestroying the rich fumes and preventing a potentiallycatastrophic explosion.

Applications

EPA Testing & SuperFund Sites

To determine the best method for controlling NOx levelsfrom various waste streams, including strong ammoniastreams, NAO has set-up portable thermal oxidizers for theEPA to test the effects of oxidation temperature, residencetime, excess air, and recirculation. EPA tests concluded thatNAO’s thermal oxidizer was “the best of four systemstested” before approving it for a SuperFund clean-up site inRhode Island.

Chemical Plants

Portable Thermal Oxidizers are used for controlling VOC’sfrom emergency upsets, liquid spills, off-spec products,storage tank blow-down, and process system turn-arounds.

Gas & Oil Storage and Transport

Portable Thermal Oxidizers have been used for ventinghydrocarbon and chemical storage tanks and for ventingtransport pipelines.

Pulp & Paper Mills

Thermal oxidizers are used to destroy odoriferous sulfurcompounds and non-condensable gases.


Common Problemswith Thermal Oxidizers

Failure on start-up

Pilot failure: Gas pressure may be low, spark-ignitor mayfail, or the pilot gas line or pilot tube may be plugged withcorrosion or other particulate.

Thermocouple failure: Thermocouples are directlyexposed to combustion products and susceptible tochemical attack. If problem is persistent, consider exoticalloys for sheath material. Thermocouple wells will slow theresponse of the thermal oxidizer to changes in gascompostion, which should be avoided.

Low assist gas pressure: The thermal oxidizer will not beable to achieve and maintain minimum oxidationtemperatures without an adequate assist-gas supply.

Low waste vapor pressure:

UV scanner failure: The scanner may be out of alignmentso that it does not see the pilot flame. The UV scanner canbe blocked by corrosion, carbon particulate, or watercondensate.

Gas valves fail to open: After the thermal oxidizer reachesoperating temperature, waste gas valves will be signaled toopen on autmated systems. The valves may not be receivingthis signal, or the valves may be stuck and failing to respondto the signal.

Flashback

Burner manifold overheats: Flames may be curling underthe burner tips and heating the piping over the auto-ignitiontemperature for the vapors. Combustible vapor mixturesrequire flame arrestors or water seals upstream of theburner.

Flame arrestor failure: If gas compositions change, flamearrestors may also have to change. Arrestors must beperiodically removed from service and inspected for


damage. Cracked or shifted grids may compromise thearrestors effectiveness.

Temperature not maintained

Too much combustion air: The oxidizer will not be able tomaintain minimum temperatures if too much air enters theunit. Close natural-draft dampers or scale back the forced-draft blower.

Not enough assist gas: Low assist-gas flow can be a resultof un-open valves or undersized header piping that results inlow pressure at the oxidizer.

Change in waste vapor composition or flow rate: Ifchanges are frequent and severe, the thermal oxidizer maynot react fast enough to compensate. Properly designedequipment should minimize or eliminate this problem.

Thermocouple failure: The equipment may be capable ofreaching the set-point temperature, but failure to receive asignal from the thermocouple interpreted as lowtemperature.

Unit overheating

Too much assist gas: Results when the modulating valvedoes not close or when gas pressure is too high. This isindicated by flames shooting out of the top of the thermaloxidizer. Continuous operation at excessive heat willdamage the unit.

Waste vapor flow rate too high: The flow rate of thecombustible vapor mixture may exceed the design capacityof the thermal oxidizer.

Change in waste vapor composition: If the heating valueof the vapor increases more quickly than the response of thethermocouple and assist-gas modulating valve, the unit willquickly overheat.


Unit shuts down automatically

Pilot failure: If the UV scanner does not prove the pilotflame on start-up, the unit will shut down. The UV scannercan be blocked by corrosion, carbon particulate, or watercondensate.

Low assist-gas pressure: Low assist-gas pressure willpreclude low temperature in the oxidizer. The unit anticipatethis and shuts down the waste gas flow.

Low waste vapor pressure: This can be caused bybooster-blower failure, undersized header piping, orrestriction at the flame arrestor or water seal.

High temperature set-point too high: If the hightemperature set point is close the set-point operatingtemperature, the unit will quickly reach maximumtemperature every time the waste vapor concentration orflow rate increases. Try to balance the operating set-pointbetween the high and low set-points.



Capture Heat From Air-Pollution Control Systems is anNAO article that describes in technical detail thebenefits of heat recovery systems for thermaloxidizers.

Straitz, John F. III, “Flares & Thermal Oxidizers: Marineand Vapor Control” presents the combustion andoxidation techniques used to solve marine-terminalvapor-control and safety problems. Ask for NAOReport RP92-12.

Straitz, John F. III, “Thermal incineration of VOC’s:Collection and control of volatile organiccompounds” is a two-page report on the availablesystems for incinerating VOC’s. Ask for NAOReport RP94-2.


Flame Arresting Equipment — Property of NAO Inc. 9-1

Flame ArrestingEquipment

There can be no margin for error with a Flame ArrestingEquipment—Flame Arrestors, Detonation Arrestors, &Water Seals. It must work right the first time, and everytime. When a subsonic deflagration or supersonicdetonation reaches an arrestor, there is no opportunity tochange variables.

In the past 15 years, extensive full-scale deflagration anddetonation tests at the Environmental Research & ServiceCenter in Houston, Texas, and at our research facility inPhiladelphia, Pennsylvania, supplemented by empiricalcalculations and tests at these and other NAO facilities, haveenabled NAO to design and manufacture the widest rangeof state-of-the-art deflagration and detonation arrestors,including U. S. Coast Guard accepted and UL approvedunits.

Every NAO flame arrestor is tested specifically forcustomer’s operating conditions prior to the sale of thearrestor. We do not sell an arrestor unless we areabsolutely confident that it will quench a flame and preventdisaster.

Flame & Detonation Arrestors

The flame arrestor is designed to stop a flame from passingthrough the gas source. The basic idea behind the flamearrestor design is that it must stop a gas/air mixture whichhas been ignited. A flame arrestor is placed betweenpossible ignition sources and the gas supply. When theflame front reaches the flame arrestor grid, it is dispersedacross the grid element and the heat is dissipated. If theflame arrestor is successful the flame is then quenched. Ifnot, a catastrophe may occur.

9-2 Flame Arresting Equipment — Property of NAO Inc.

Arrestor Selection

The flame arrestor selection process is important andsometimes time consuming. The selection of the correctflame arrestor will prevent a catastrophe from happeningduring usage.

Critical Flame Arrestor Design Information

1. Gas Composition2. Gas Temperature3. Gas Pressure4. Distance from all Ignition Sources5. Allowable Pressure Drop6. Maximum Gas Flow Rate7. Emergency Conditions8. Any Unusual Performance Characteristics

The arrestor selection process requires a minimum ofinformation such as: Gas Composition, Gas Temperature,Gas Pressure, Distance from all Ignition Sources, AllowablePressure Drop through the flame arrestor, Maximum GasFlow Rate, and Any Other Information that makes theapplication out of the ordinary. Please note: If emergencyconditions are known, they should be the basis of thearrestor design—i.e. at an emergency condition, thetemperature is 595 °F, the pressure may rise to 25 psig, anda different gas composition may be present.

Flame Arrestor Type

There are three types of arrestors. A Vent Arrestor, orEnd-of-Line Arrestor is located at the very end of a vaporpipe line. This arrestor vents gases to atmosphere. Some ofthe common sources of ignition may be static discharge,lightning, hazardous area or hazardous people.

The next type of arrestor is the In-Line flame arrestor. Thisflame arrestor is generally not placed further then ten (10)pipe diameters from an ignition source.

The last type of arrestor is the Detonation Arrestor. Itcovers in-line arrestors which cannot be placed within ten

NAO Vent Arrestorwith rainhood


(10) pipe diameter of the ignition source. These arrestorsmay be situated in a pipeline or at the base of a tall flaresystem.

Of these three types of arrestors, we manufacture a verywide range of flame arrestor configurations. Standardarrestors come flanged, threaded, and plain for placementbetween flanges. Often, customers avoid flanges to reducecosts; on large units, flanges can double the cost of anarrestor.

Arrestors come in any size over ¾-inch diameter. Largecustom units have been built up to 12 feet in diameter. Anyof our arrestors can be custom engineered to satisfy anypiping requirements.

Some In-line arrestors actually have a water spray to washoff the grids. Vent arrestors often have a “Chinese hat” orflapper hood on it to keep rain and snow off of the grid.Usually this is put on a tank as a vent, so the tank can breathand emit the various fumes.

If pressure drop is a main concern, we can make anexpansion arrestor. For instance, you might have a 4 inchpipe with a 6-inch or 8-inch flame arrestor grid. Using thewider grid element, we can often reduce pressure drop to 1-2 inches of water column.

Flame & Detonation Arrestor Approval

NAO has UL 525 approval for the Vent Arrestor. NAOalso has United States Coast Guard acceptance for thedetonation arrestor. These approval processes help insurethat the arrestor is tested and operates efficiently with theworst possible cases that can be present in an application.However, we have tested UL and FM approved arrestorsmanufactured by our competition that have failed our testingprogram. We do not sell any safety device that does notmeet and surpass industry standards or our standards.

Water Seals

Last, but not least, is the Liquid Seal. We make a widerange of sizes of these units, from 1 foot in diameter and 3feet high to units that are 40-feet long and 10-feet in

In-line Flame Arrestor

In-line Flame Arrestorwith expanded grid

for low pressure drop

Large capacity in-line FlameArrestor


diameter—these are for very-large flare loads. With a waterseal, the gas comes in through a bubble plate that breaks itup into many small bubbles. You can see distinct orseparate bubbles rising through the liquid. If one of thebubbles catches fire and explodes, it can’t get to the nextone, so the flame is quenched.

NAO did a lot of testing on water seals for a nuclear facility,called Three Mile Island in the early 1980’s. There they hada mixture of hydrogen and oxygen, which is a very fastburning gas. The flame speed—15,000 mph—was a majorconcern. We developed standards for bubble patterns andbubble diameters to effectively quench this flame. We thendesigned a special unit that had twice the area of a standardseal because the gas mixture exploded so violently that, in astandard seal, one bubble would have ignited the next.

For staged flare flows, NAO makes a Dual StageHorizontal Water Seal. It is actually a drum, divided intotwo different parts. By dual stage we mean that there aredifferent levels of water in it. One is shallow and one isdeep. As flow and pressure increase, the second stagecomes on automatically.

Many times, water seals are used as the base of a self-supporting flare because they serve as an economicalfoundation and structure. Separate drums can be expensivebecause you have to have separate foundations, supports,and piping for them. If required, knock-out drums are alsoincorporated with water seals to reduce the cost of theequipment.

NAO Research & Development

NAO performs testing for various applications and forspecific customer requests. NAO has two testing facilities.One facility is located in Philadelphia, Pennsylvania, and theother is in Plantersville, Texas, northwest of Houston. Thesmaller and quieter tests are performed at the Philadelphia

NAO Water Seal

NAO Disentrainment Drumwith integrated Water Seal


facility. The larger and louder tests are conducted at thePlantersville facility. Between the two facilities, we have thecapability for full-scale testing of any flame or detonationarrestor.

Common Problemswith Flame Arrestors

Flame passage

Improper arrestor location: Flame arrestors should belocated no more than 5-10 pipe diameters from the sourceof ignition.

Mis-application of arrestor: Arrestors should be designedfor the most dangerous gas in a vapor mixture at the worstpossible conditions—highest temperature, highest pressure.

Continuous burning: Flame arrestors should havethermocouples to indicate when continuous burning is takingplace on the grid. Without flame detection, the temperatureof the grid can quickly exceed the auto-ignition temperatureof the vapor, particularly at low flow conditions.

High vapor temperature: The higher the temperature ofthe waste vapor, the more dangerous the condition.

Changing gas compositions: Arrestors should be designedfor the most dangerous gas in a vapor mixture at the worstpossible conditions—highest temperature, highest pressure.

Changing gas pressure: High pressure is difficult to arrest.The highest possible pressure must be specified whenselecting an arrestor.

Cracked or shifting grid: Arrestors should be periodicallyremoved from service and inspected for damage. Arrestorsmust be inspected after a known flashback to determine ifthe shock of the explosion shifted or cracked the grid.

NAO 18" Flame Arrestor Test


High Pressure Drop

Plugging of grid: The pressure drop across the grid shouldbe monitored. Over time the pressure loss will increase asthe arrestor begins to plug from particulate in the vaporstream. If the problem is persistent, two arrestors should beused in parallel so that one can be removed with the vaporline in service, or the customer should consider using a waterwash to clean the grid while it is in service.

Grid size too small: The smaller the grid, the safer thedesign; but the smaller the grid, the higher the pressur drop.If necessary, surface area of the grid can be increased toreduce the pressure drop.

Not enough surface area of grid: An expanded arrestordesign—for example, using a 4-inch grid on a 2-inch line—can provide safety and a low pressure drop.

Plugging of Grid

Matting of dirt on surface: Particulate in the vapor streamcollect in the small openings of the flame arrestor grid.Clean the grid by using solvent or high-pressure steam or air.Never pick at or drill out the debri.

Freezing of condensate in grid: The flame arrestor gridacts as a perfect condenser. If ambient temperature isbelow freezing, condensate in the vapor stream will freezeand plug the grid. Heat tracing or a glycol spray can preventfreezing of the grid element.

Mis-Application of Arrestor

Location of arrestor: The further from the ignition source,the more difficult it is to arrest the flame. As rule of thumb, ifthe arrestor is located more than 5-10 pipe diameters fromignition source or if it is located at entrance to furnace orheater, a detonation arrestor may be required (depending ongas composition).

No universal arrestors: Customer must specify all possiblegases that may be present in an application. The arrestormust be designed for the most dangerous gas in the mixture.


If the gas composition changes, the arrestor design must bereviewed.

Group A & B gases: Group A & B gases are the mostdifficult to arrest. Group A gases include any mixturecontaining acetylen. Group B gases include any mixturecontaining gases like hydrogen, ethylene oxide, andbutadiene†.

Group C & D gases: Group C & D gases are easier toarrest. Group C gases include any mixture containingethylene or carbon monoxide. Group D gases include anymixture containing gases like methane, propane, andbutadiene†.

† Butadiene behaves as Group B gas at sometemperatures and pressures, and as Group D at others.All gases can produce intermediate products during anexplosion. These intermediate products can be moredangerous than the original gases.



Mendoza, Smolensky, & Straitz, “Flame Arrestors:Selection, Design and Testing” is an eight pagereport on NAO criteria used when designing andtesting flame arrestors and water seals. Ask forNAO Report RP93-7.

Straitz, John F. III, “Arrestors Guard Against Explosions” isa two page article describing the essential criteria offlame and detonation arrestors. Ask for NAOReport RP94-5.


Vents — Property of NAO Inc. 10-1

Vents

Vents are used on-shore and off-shore to releasecombustible and inert vapors to atmosphere. Two criticalfactors must be considered when designing a vent system:The vent must prevent air from entering the line be vented,and the vent must disperse gases to prevent dangerousvapor clouds from forming around equipment and personnel.

When storing fuels under atmospheric pressure, customersmust leave an opening in a storage tank to relieve pressure.This can be hazardous, however. Lightening or staticdischarge can ignite vapors that mix with air near the tank. Ifair gets into the tank, explosions and tremendous fires canresult.

Depending on the nature of the gas being vented, there arethree types of vent tips: Gas seals, vent arrestors, and Jet-Mix™ vents.

Gas Seals

There are two types of gas-seal vents. The first, and themost popular, is the Fluidic Vent. The Fluidic Vent is anapplication of NAO's patented Fluidic Seal. The secondtype of gas-seal vent is the molecular seal. Both protectvent lines by preventing air entry. The Fluidic Vent has anumber of advantages over the molecular seal.

At a given flow rate, the Fluidic Vent is more effective atdiffusing air than the molecular seal. For the Fluidic Vent,the pressure drop for reverse flow through the vent is four(4) times greater than normal flow of the same rate. For amolecular seal, the pressure drop is the same for normal andreverse flow. It is more difficult for air to move in thereverse direction with the Fluidic Vent than with themolecular seal.

The Fluidic Vent can be installed in any direction. The no-moving-parts venturi design of the Fluidic vent do not limit itto vertical applications. The molecular seal is a gravity seal.

10-2 Vents — Property of NAO Inc.

If installed at an angle or horizontally, it ceases to function.The Fluidic Vent does not depend on gravity action, so itcan be used effectively in horizontal angled applications.

In addition to better performance, the Fluidic Vent alsorequires less maintenance than the molecular seal. TheFluidic Vent is manufactured from high-temperature stainlesssteel and it does not collect condensate. The molecular sealis usually fabricated from carbon steel, which corrodes, andit collects condensate; this condensate accelerates corrosionand restricts flow through the seal. In some locations, themolecular seal must be heat traced to prevent freezing of thecondensate.

Another advantage of the Fluidic Vent is its low profile andlight weight. The molecular seal is, typically, twenty times asheavy as the equivalent Fluidic Vent and two times itsdiameter; this adds to the structural load of the vent andrequires larger support structures.

Advantages of the Fluidic Vent

• Simple, straight-path, wall-less venturi flow path forwaste gas

• No plugging• Light-weight• Low profile reduces wind loading• Works in any direction—vertically, horizontally, and

any angle in between• No moving parts• No drainage or corrosion problems

To prevent static electricity from discharging in the vent pipeand igniting vapors, we offer the option of electrostatic ringson the vent. You must be very careful with high gasvelocities or if you have any liquid particles which cause astatic electric buildup that could cause ignition. With thestatic rings, there are no sharp edges, and this insures nobuildup in static electricity.

If a vent does ignite, NAO makes various Automatic FireSuppression Systems that use either dry chemical, CO

2, or

Halon to extinguish a fire. If you want to extinguish a fire ona very large vent, it is often cost effective to use a Jet Mix

Vents — Property of NAO Inc. 10-3

Fire Suppression System. This sucks in a lot of air with thecombustible gas so that it will just barely burn. In this case,it doesn’t take much dry chemical or CO

2 to snuff out a fire.

Flame Arrestor Vents

When venting vapors at low flow rates, dispersion will bevery low when the wind velocity is low. The resultingvapors will be a combustion hazard. To eliminate thepossibility of flashback under these conditions, flamearrestors designed to quench the flame can be used on anend-of-line vent.

Flame arrestor vents are often used as pressure relief ventson storage tanks where vapor volumes change asatmospheric temperature rises and falls. Often, these ventscome with rainhoods to prevent the collection of condensatein the vent line.

For more information on flame arrestor vents, see thechapter on Flame Arresting Systems.

Jet-Mix Vents

Jet-Mix Vents are used where toxic and highly combustiblevapors are vented. Jet-Mix Vents allow for very gooddispersion of the gases by spreading the vent tip over awider area and by increasing the velocity of the gases beingvented.

10-4 Vents — Property of NAO Inc.



Property of NAO Inc. Appendix A-1

Unit Conversion

Basic and Derived Standard UnitsU.S. Customary SI

Unit Symbol Formula Unit Symbol FormulaLength foot ft * meter m *Mass slug slug lbf·s

2/ft kilogram kg *Time second s * second s *Temperature Rankine R * Kelvin K *

Force pound lbf * Newton N kg·m/s2

Pressure lbf/ft2 Pascal Pa N/m2

Energy, work ft·lbf Joule J N·mPower ft·lbf/s Watt W J/s

*base unit

Unit PrefixesFactor Prefix Symbol Factor Prefix Symbol10-18 atto a 101 deka da10-15 femto f 102 hecto h10-12 pico p 103 kilo k10-9 nano n 106 mega M10-6 micro µ 109 giga G10-3 milli m 1012 tera T10-2 centi c 1015 peta P10-1 deci d 1018 exa E

Lengthinches feet km meter mile* yard

1 inch 1 0.0833 2.540E-5 0.02540 1.578E-5 0.02781 foot 12 1 3.048E-4 0.3048 1.894E-4 0.33331 kilometer 39 370 3 281 1 1 000 0.6214 1 0941 meter 39.37 3.281 0.001000 1 6.214E-4 1.09361 mile 63 360 5 280 1.609 1 609 1 1 7601 yard 36 3 9.144E-4 0.9144 5.682E-4 1

*1 mile = 0.8688 nautical mile1 fathom = 6 feet1 furlong = 10 chains = 40 rods = 220 yards


Unit Conversion

Areaacre* cm2 ft2 hectare in2 km2 mile2 m2

1 acre 1 4.047E7 43 560 0.4047 6.273E6 0.004047 0.001563 4 0471 cm2 1.941E-8 1 0.001076 1.000E-8 0.1550 1.000E-10 3.861E-11 1.000E-41 ft2 2.296E-5 929.0 1 9.290E-6 144.0 9.290E-8 3.587E-8 0.092901 hectare 2.471 1.000E8 107 639 1 1.550E7 0.01000 0.003861 10 0001 in2 1.594E-7 6.452 0.006944 6.452E-8 1 6.452E-10 2.491E-10 6.452E-41 km2 247.1 1.00E10 1.076E7 100.0 1.550E9 1 0.3861 1.000E61 mile2 640.0 2.590E10 2.788E7 259.0 4.014E9 2.590 1 2.590E61 m2 2.471E-4 10 000 10.76 1.000E-4 1 550 1.000E-6 3.861E-7 1

1 acre = 4 roods = 160 square rods = 4 840 square yards

Volumeinches3 feet3 meter3 liter fluid oz US gallon UK gallon US barrel

1 inch3 1 5.787E-4 1.639E-5 0.0164 0.5543 0.004329 0.003604 1.031E-41 foot3 1 728 1 0.02832 28.32 0.001044 7.481 6.229 0.17811 meter3 61 024 35.32 1 1,000 33 810 264.2 220.0 6.2901 liter 61.024 5.787E-4 0.001000 1 33.81 0.2642 0.2200 0.0062901 fluid ounce 1.804 957.9 2.958E-5 0.02957 1 0.007812 0.006505 1.860E-41 US gallon 231.0 0.1337 0.003785 3.785 128.0 1 0.8327 0.023811 UK gallon 277.4 0.1605 0.004546 4.546 153.7 1.201 1 0.028591 US barrel 9 702 5.615 0.1590 159.0 5 376 42.00 34.97 11 acre-foot 7.527E7 43 560 1 233.5 1.233E6 4.171E7 325 85 2.713E5 7 7581 US pint 28.88 0.01671 4.732E-4 0.4732 16.00 0.1250 0.1041 0.0029761 US quart 57.75 0.03342 9.464E-4 0.9464 32.00 0.2500 0.2082 0.005852

Massg kg oz lbm short ton long ton metric ton slug

1 gram 1 0.001 0.03527 0.002205 11.02E-7 9.842E-7 10.0E-7 6.852E--51 kilogram 1 000 1 35.27 2.2046 11.02E-4 9.842E-4 10.0E-4 0.068521 ounce 28.35 0.02835 1 0.0625 3.125E-5 2.790E-5 2.835E-5 0.0019431 pound 453.6 0.4536 16.00 1 0.00050 4.464E-4 4.536E-4 0.031081 short ton 9.07E5 907.2 32 000 2 000 1 0.8929 0.9072 62.161 long ton 1.02E6 1 016 35 840 2 240 1.120 1 1.0160 69.621 metric ton 1.00E6 1 000 35 274 2 205 1.102 0.9842 1 68.521 slug 14 594 14.59 514.8 32.17 0.01609 0.01436 0.01459 11 dram 1.772 0.00177 0.06250 0.003906 1.953E-6 1.744E-6 1.772E-6 1.214E-4

1 slug = 1 lbf·s2/ft


Unit Conversion

Forcedyn* kip lbf poundal** N

1 dyn 1 2.248E-9 2.248E-6 7.233E-5 1.000E-51 kip 4.448E8 1 1 000 32 174 4 4481 lbf 444 822 0.001000 1 32.174 4.4481 poundal 13 826 3.108E-5 0.03108 1 0.13831 N 100 000 2.248E-4 0.2248 7.233 1

*1 dyn = g·cm/s2

**1 poundal = 1 lbm·ft/s2

Pressureatm bar kg/cm2 kPa* inchHg** mmHg** inchH20*** lbf/in

2

1 atm 1 1.013 1.033 101.3 29.92 760.0 407.2 14.701 bar 0.9870 1 1.020 100.0 29.53 750.1 401.9 14.501 kg/cm2 0.9678 0.9807 1 98.07 28.96 735.6 393.1 14.221 kPa 0.009869 0.01000 0.01020 1 0.2953 7.501 4.019 0.14501 inch Hg 0.03342 0.03386 0.03453 3.386 1 25.40 13.61 0.49111 mm Hg 0.001316 0.001333 0.001360 1.333E-5 0.03937 1 0.5358 0.019341 inch H20 0.002456 0.002488 0.002537 0.2488 0.07347 1.866 1 0.036091 N/m2 9.869E-6 100 000 101 971 0.0010 2.953E-4 0.007501 0.004019 1.450E-41 psi (lbf/in

2) 0.06805 0.06895 0.07031 6.895 2.036 51.71 27.71 11 dyn/cm2 9.869E-7 1.000E-6 1.020E-6 1.000E-4 7.501-4 7.501E-4 4.019E-4 1.450E-51 micron 1.32E-6 3.94E-5 1.000E-3 1.95E-51 mm H20 9.668E-5 9.796E-5 9.989E-5 0.009796 0.002893 0.07347 0.03937 0.0014211 osi 0.00425 0.00431 0.004394 0.430 0.1272 3.233 1.732 0.06251 torr 0.001316 0.001333 0.001360 1.333E-5 0.03937 1 0.5358 0.01934

*1 Pa = 1 N/m2

**32°F, 0°C, 1 mmHg = 1 torr***60°F, 15.6°C

Velocityft/min ft/s in/s km/hr knot m/s mile/hr mile/s

1 ft/min 1 0.01667 0.2000 0.01829 0.009874 0.005080 0.01136 3.157E-61 ft/s 60.00 1 12 1.097 0.5925 0.3048 0.6818 1.89E-41 inch/s 5.000 0.0833 1 0.0914 0.0494 0.0254 0.0568 1.58E-51 km/hr 54.68 0.9113 10.94 1 0.540 0.2778 0.6214 1.73E-41 knot 101.3 1.688 20.25 1.852 1 0.5144 1.151 3.20E-41 m/s 196.9 3.281 31 557 3.600 1.944 1 2.237 6.21E-41 mile/hr 88.00 1.467 17.60 1.609 0.869 0.4470 1 2.78E-41 mile/s 316 800 5 280 63 360 5 793 3 128 1 609 3 600 1

Unit Conversion

Accelerationft/min2 ft/s2 km/hr2 m/s2 mile/hr2

free fall 115 027 32.17 127 094 9.807 6.094E-61 ft/min2 1 2.778E-4 1.097 8.467E-5 0.68181 ft/s2 3 600 1 3 950 0.3048 2 4551 km/hr2 9.113E-4 2.532E-7 1 7.716E-5 0.62141 m/s2 11 811 3.281 12 960 1 8 0531 mile/hr2 1.467 4.074E-4 1.609 1.242E-4 1

Volume Flow Rateft3/min ft3/hr US-gal/min l/min m3/s

1 ft3/min 1 60 7.481 28.32 4.72E-41 ft3/hr 0.0167 1 0.1247 0.472 7.87E-61 US-gal/min 0.1337 8.021 1 3.785 6.31E-51 l/min 0.0353 2.119 0.2642 1 1.67E-51 m3/s 2 118.9 1.27E5 15 850 6.0E4 1

Mass Flow Rateg/s kg/hr lbm/hr lbm/min ton(metric)/day ton(short)/day

1 g/s 1 3.600 7.937 0.1323 0.08640 0.095241 kg/hr 0.2778 1 2.205 0.03674 0.02400 0.026461 lbm/hr 0.1260 0.4536 1 0.01667 0.01089 0.012001 lbm/min 7.560 27.22 60.00 1 0.6532 0.72001 ton(metric)/day 11.57 41.67 91.86 1.531 1 1.1021 ton(short)/day 10.50 37.80 83.33 1.389 0.9072 1

EnergyBtu erg* ft·lbf hp·hr kcal kJ** kW·hr

1 Btu 1 1.055E10 778.2 3.930E-4 0.2521 1.055 2.931E-41 erg 9.478E-11 1 7.376E-8 3.725E-14 2.388E-11 1.000E-10 2.778E-141 ft·lbf 0.001285 1.356E7 1 5.051E-7 0.3238 0.001356 3.766E-71 hp·hr 2 544 2.685E13 1.980E6 1 641.2 2 685 0.74571 kcal 3.968 4.187E10 3 088 0.001560 1 4.187 0.0011631 kJ 0.9478 1.000E10 737.6 3.725E-4 0.2388 1 2.778E-41 kW·hr 3 412 3.600E13 2.655E6 1.341 859.8 3 600 11 cooling-ton·hr 12 000

*1 erg = 1 dyn·cm**1 kJ = 1 000 W·s = 1 000 N·m


Specific EnergyBtu/lbm cal/g kW·hr/kg kJ/kg*

1 Btu/lbm 1 0.5556 6.461E-4 2.3261 cal/g 1.800 1 0.001163 4.1871 kW·hr/kg 1 548 859.8 1 3 6001 kJ/kg 0.4299 0.2308 2.778E-4 1

1 kJ/kg = kW·s/kg

Energy DensityBtu/ft3 kcal/m3 kJ/m3

1 Btu/ft3 1 8.904 37.261 kcal/m3 0.1123 1 4.1871 kJ/m3 0.02684 0.2388 1

Heat Release — PowerBtu/hr ft·lbf/min hp kcal/hr kJ/hr kW*

1 Btu/hr 1 12.97 3.930E-4 0.2520 1.055 2.931E-41 ft·lbf/min 0.07710 1 3.030E-5 0.01943 0.08135 2.260E-51 hp 2 544 33 000 1 641.2 2 685 0 74571 kcal/hr 3.968 51.47 0.001560 1 4.187 0.0011631 kJ/hr 0.9478 12.29 3.725E-4 0.2388 1 2.778E-41 kW 3 412 44 253 1.341 859.8 3 600 1

*1 kW = 1 kJ/s

Radiant Heat Flux — Energy FluxBtu/hr·ft2 ft·lbf/hr·ft2 hp/ft2 kcal/hr·m2 kJ/hr·m2 Langley/hr W/m2

1 Btu/hr·ft2 1 778.2 3.930E-4 2.71 11.4 0.271 3.1551 ft·lbf/hr·ft2 0.001285 1 5.051E-7 0.003486 0.01459 3.485E-4 0.0040541 hp/ft2 2 544 1.980E6 1 6 902 28 896 690.1 8 0271 kcal/hr·m2 0.369 286.9 1.449E-4 1 4.187 0.09993 1.1631 kJ/hr·m2 0.088 68.52 3.461E-5 0.2388 1 0.02388 0.27781 Langley/hr 3.687 2 869 0.001449 10.01 41.87 1 0.085981 W/m2 0.3170 246.7 1.246E-4 0.8598 3.600 11.63 1

Thermal ConductivityBtu/hr·ft·°F Btu·in/hr·ft2·°F cal/s·cm·°C kcal/hr·m·°C W/m·°C

1 Btu/hr·ft·°F 1 12.00 4.134E-5 1.488 1.7311 Btu·in/hr·ft2·°F 0.08333 1 3.445E-6 0.1240 0.14421 cal/s·cm·°C 24 181 290 291 1 36 000 41 8681 kcal/hr·m·°C 0.6720 8.064 2.778E-5 1 1.1631 W/m·°C 0.5778 6.933 2.388E-5 0.8598 1

Unit Conversion


Heat Transfer CoefficientBtu/ft2·hr·°F cal/cm2·s·°C W/m2·°C

1 Btu/ft2·hr·°F 1 1.356E-4 5.6781 cal/cm2·s·°C 7 373 1 41 8681 W/m2·°C 0.1761 2.388E-5 1

Specific Heat — Specific Gas ConstantBtu/lbm·°F cal/g·°C ft·lbf/lbm·°R kJ/kg·°C

1 Btu/lbm·°F 1 1.000 783.6 4.1871 cal/g·°C 1.000 1 783.6 4.1871 ft·lbf/lbm·°R 0.001276 0.001276 1 0.0053431 kJ/kg·°C 0.2388 0.2388 0.01872 1

Unit Conversion


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1.5

Natural Gas Fuel Oil Coal

ppm corrected to 3% dry O at 68 °F, 1 atm†2

Unit Conversion: NOx ppm to lb/MMBtu†


Gas Data

Property of NAO Inc. Appendix B-1

Water Air (moist) Natural Gas——————— ——————— ———————

Specific weight, lb/ft3; kg/m3 62.35 1000 0.0763 1.225 0.046 0.738Specific heat, Btu/lb; cal/g·°C 1.0 1.0 0.240 0.240 0.55 0.55Thermal conductivity, Btu·ft/hr·ft2·°F; Watt/m·°K 0.344 0.595 0.0148 0.0256 0.02 0.03Absolute viscosity, lbm/ft·hr; kg/m·hr 2.72 4.05 0.0434 0.0646 0.0266 0.0396Kinematic viscosity, ft2/hr; 106·m2/sec 0.0436 1.126 0.567 14.68 0.578 14.93

Gross heat release with 1 ft3 air + stoichiometric amount of natural gas or oil = about 100 BtuGross heat release with 1 m3 air + stoichiometric amount of natural gas or oil = about 900 kcal

Stefan-Boltzmann constant = 0.1713 x 10-8 Btu/ft2·hr·°R4 = 4.88 x 10-8 kcal/m2·hr·°K4

Universal gas constant, (MR) in pv=(MR)T/M, M=mole weight: (MR) = 1,544 ft·lbm/°R·lb-mole(MR) = 84.7 m·kg/°K·kg-mole(MR) = 8.314 Joule/°K·g-mole

Velocity of sound in air at 68 °F (20 °C) and 1 atm = 1,129 ft/sec = 769.5 mph = 344.1 m/sec

Volume of 1 lb-mole of any gas at 60 °F (15.6 °C) and 1 atm = 379 ft3

Volume of 1 kg-mole of any gas at 60 °F (15.6 °C) and 1 atm = 23.7 m3

Material Specifications

Property of NAO Inc. Appendix C

Periodic Chart of the Elements

Property of NAO Inc. Appendix D

Pipe Specifications

Property of NAO Inc. Appendix E

Nomograms for Flare Tip Sizing

Property of NAO Inc. Appendix F

Flare Radiation Calculations

Property of NAO Inc. Appendix G

Fluidic Flare Tip Cost Estimate

Property of NAO Inc. Appendix H

Hazardous Electrical Classifications, Groupings, Divisions†

Class I. Locations in which flammable gases or vapors are, or may be, present in the air in quantities sufficient to produceexplosive or ignitable mixtures.Division 1 locations

a. Hazardous concentrations exist continuously, intermittently, or periodically under normal operating conditions.b. Hazardous concentration may exist frequently because of repair or maintenance operations or because of leakage.c. Where breakage or faulty operation of equipment or processes which might release hazardous concentrations of

flammable gases or vapors might also cause simultaneous failure of electrical equipment.Division 2 locations

a. Where hazardous volatile liquids, vapors, or gases are normally confined within closed containers or closedsystems from which they can escape only in case of accidental rupture or breakdown of such containers orsystems, or in case of abnormal operation of equipment.

b. Hazardous concentrations are normally prevented by positive mechanical ventilation but might become hazardousthrough failure or abnormal operation of the ventilating system.

c. Areas adjacent to Division 1 locations to which hazardous concentrations of gases or vapors might occasionallybe communicated.

Group A: Atmospheres containing acetylene.Group B: Atmospheres containing hydrogen or gases or vapors of equivalent hazard such as a manufactured gas.Group C: Atmospheres containing ethyl ether vapors, ethylene, or cyclopropane.Group D: Atmospheres containing gasoline, hexane, naphtha, benzine, butane, propane, alcohol, acetone, benzol,

lacquer solvent vapors, or natural gas.

Class II. Locations which are hazardous because of the presence of combustible dust.Division 1 locations

a. Areas in which combustible dust is or may be in suspension in the air continuously, intermittently, or periodicallyunder normal operating conditions, in quantities sufficient to produce explosive or ignitable mixtures.

b. Where mechanical failure or abnormal operation of machinery or equipment might cause a combustible dust mixtureto be produced, and might also provide a source of ignition through simultaneous failure of electrical equipment,operation of protective devices, or from other causes.

c. Areas where dusts of an electrically conducting nature may be present.Division 2 locations

a. Areas where dangerous concentration of suspended dust are not likely, but where dust accumulations might formon, or in the vicinity of, electrical equipment in sufficient quantities to produce explosive or ignitable mixturesexcept under conditions:

1. Where deposits or accumulation of dust may interfere with the safe dissipation of heat from electricalequipment or apparatus.

2. Where deposits or accumulation of dust on, in, or in the vicinity of electrical equipment might be ignited byarcs, sparks, or burning material from such equipment.

Group E: Atmospheres containing metal dust (magnesium, aluminum, bronze powder, etc.).Group F: Atmospheres containing carbon black, coal, and coke dust.Group G: Atmospheres containing grain dust (flour, starch, pulverized sugar and cocoa, dairy powders, dried fat, etc.).

Class III. Locations where easily ignitable fibers or flyings are present but not likely to be in suspension in quantities sufficient toproduce ignitable mixtures.Division 1 locations

a. Areas where easily ignitable fibers or materials producing combustible flyings are handled, manufactured, or used.1. Such locations include some parts of rayon, cotton, combustible-fiber manufacturing and processing plants;

cotton gins and cottonseed mills; flax processing plants; clothing manufacturing plants; woodworkingshops; and establishments and industries involving similar hazardous processes or conditions.

Division 2 locationsa. Areas where easily ignitable fibers are stored or handled (except in the process of manufacture).

1. Such fibers include rayon, cotton (cotton linters and waste), hemp, kapok, excelsior, sisal or henequen, istle,jute, tow, cocoa fiber, oakum, baled waste, Spanish moss, and other materials of similar nature.

†Essential information defining limitations as given in NEC Article 500

Note: Some plant areas in the manufacture, handling, and storage of explosives or ammunition and nitrocellulose products suchas celluloid photographic films, etc., involve conditions that are not covered by NEC classifications. This is particularlytrue where black powders, smokeless powder, dust from TNT, and other explosives are present. These areas requirespecial equipment and installation methods.

Property of NAO Inc. Appendix I

40 CFR 60.18a. Introduction. This section contains requirements for control devices used to comply with applicable subparts of parts 60

and 61. The requirements are placed here for administrative convenience and only apply to facilities covered by subpartsreferring to this section.

b. Flares. Paragraphs (c) through (f) apply to flares.(c) (1) Flares shall be designed for and operated with no visible emissions as determined by the methods specified in

paragraph (f), except for periods not to exceed a total of 5 minutes during any 2 consecutive hours.(2) Flares shall be operated with a flame present at all times, as determined by the methods specified with paragraph (f).(3) Flares shall be used only with the net heating value of the gas being combusted being 11.2 MJ/scm (300 Btu/scf) or

greater if the flare is steam-assisted or air-assisted; or with the net heating value of the gas being combusted being7.45 MJ/scm (200 Btu/scf) or greater if the flare is non-assisted. The net heating value of the gas being combustedshall be determined by the methods specified in paragraph (f).

(4) (i) Steam-assisted and non-assisted flares shall be designed for and operated with an exit velocity, as determinedby the methods specified in paragraph (f)(4), less than 18.3 m/sec (60 ft/sec), except as provided in paragraphs(b)(4) (ii) and (iii).

(ii) Steam-assisted and non-assisted flares designed for and operated with an exit velocity, as determined by themethods specified in paragraph (f)(4), equal to or greater than 18.3 m/sec (60 ft/sec) but less than 122 m/sec(400 ft/sec) are allowed if the net heating value of the gas being combusted is greater than 37.3 MJ/scm(1,000 Btu/scf).

(iii) Steam-assisted and non-assisted flares designed for and operated with an exit velocity, as determined by themethods specified in paragraph (f)(4), less than the velocity, V

max, as determined by the methods specified in

paragraph (f)(5), and less than 122 m/sec (400 ft/sec) are allowed.(5) Air-assisted flares shall be designed and operated with an exit velocity less than the velocity, V

max, as determined

by the method specified in paragraph (f)(6).(6) Flares used to comply with this section shall be steam-assisted, air-assisted, or non-assisted.

(d) Owners or operators of flares used to comply with the provisions of this subpart shall monitor these control devices toensure that they are operated and maintained in conformance with their designs. Applicable subparts will provideprovisions stating how owners or operators of flares shall monitor these control devices.

(e) Flares used to comply with provisions of this subpart shall be operated at all times when emissions may be vented tothem.

(f) (1) Reference Method 22 shall be used to determine the compliance of flares with the visible emission provisions of thissubpart. The observation period is 2 hours and shall be used according to Method 22.

(2) The presence of a flare pilot flame shall be monitored using a thermocouple or any other equivalent device to detectthe presence of a flame.

(3) The net heating value of the gas being combusted in a flare shall be calculated using the following equation:n

HT = K • Σ C

iH

ii=1where: H

T=Net heating value of the sample, MJ/scm; where the net enthalpy per mole of off-gas is based on

combustion at 25 °C and 760 mm Hg, but the standard temperature for determining the volumecorresponding to one mole is 20 °C;K=Constant, 1.740 x 10-7 (1/ppm)(g mole/scm)(MJ/kcal) where the standard temperature for (g/mole scm) is20 °C;C

i=Concentration of sample component "i" in ppm on a wet basis, as measured for organics by Reference

Method 18 and measured for hydrogen and carbon monoxide by ASTM D1946-77 (Incorporated byreference as specified in Subpart 60.17); andH

i=Net heat of combustion of sample component "i" in kcal/g-mole at 25 °C and 760 mm Hg. The heats of

combustion may be determined using ASTM D2382-76 (incorporated by reference as specified in Subpart60.17) if published values are not available or cannot be calculated.

(4) The actual exit velocity of a flare shall be determined by dividing the volumetric flow rate (in units of standardtemperature and pressure), as determined by Reference Methods 2, 2A, 2C, or 2D as appropriate; by theunobstructed (free) cross-sectional area of the flare tip.

(5) The maximum permitted velocity, Vmax

, for flares complying with paragraph (c)(4)(iii) shall be determined by thefollowing equation: Log

10(V

max)=(H

T+28.8)/31.7, where V

max=Maximum permitted velocity (m/sec) and H

T=The net

heating value as determined in paragraph (f)(3).(6) The maximum permitted velocity, V

max, for air-assisted flares shall be determined by the following equation:

Vmax

=8.706+0.7084(HT), where V

max=Maximum permitted velocity (m/sec) and H

T=The net heating value as

determined in paragraph (f)(3).[January 21, 1986]

Property of NAO Inc. Appendix J

Burner Design Capacities

Property of NAO Inc. Appendix K

Nao Manual Draft

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