fuel burner & firebox operation & control

Note: The source of the technical material in this volume is the ProfessionalEngineering Development Program (PEDP) of Engineering Services.

Warning: The material contained in this document was developed for SaudiAramco and is intended for the exclusive use of Saudi Aramcos employees.Any material contained in this document which is not already in the publicdomain may not be copied, reproduced, sold, given, or disclosed to thirdparties, or otherwise used in whole, or in part, without the written permissionof the Vice President, Engineering Services, Saudi Aramco.

Chapter : Vessels For additional information on this subject, contactFile Reference: MEX10405 R.K. Khanna

Engineering EncyclopediaSaudi Aramco DeskTop Standards

Fuel Burner And Firebox Operation And Control

Engineering Encyclopedia Vessels


Saudi Aramco DeskTop Standards

Contents Pages

FUELS AND BURNERS.....................................................................................................1

Fuel Options..............................................................................................................1

Burners.....................................................................................................................1

Burner Components.......................................................................................2

Gas Burners...................................................................................................4

Oil Burners....................................................................................................8

Combination Gas and Oil Burners..................................................................9

Forced Draft Burners.....................................................................................9

Emissions Control.........................................................................................10

FUEL SYSTEMS................................................................................................................16

Fuel Gas Systems.....................................................................................................16

Components..................................................................................................16

Controls........................................................................................................20

Fuel Oil Systems.......................................................................................................21

Components..................................................................................................21

Controls........................................................................................................26

Effects on Boiler and Process Heater (Furnace) Design...........................................26

MAJOR CONCERNS AND GUIDELINES FOR FIREBOX OPERATION........................27

Major Concerns........................................................................................................27

Flame Characteristics and Patterns................................................................27

Typical Draft Profile.....................................................................................28

Optimum Operation for Excess Air...............................................................29

Operating Guidelines for Natural Draft Furnaces......................................................29

GLOSSARY........................................................................................................................30



Saudi Aramco DeskTop Standards

Table of Figures Pages

Figure 1. Typical Natural-Draft Oil/Gas Burners.......................................................2

Figure 2. Pilot Burner...............................................................................................4

Figure 3. John Zink Burner Capacities......................................................................7

Figure 4. Typical Fuel Oil Burner.............................................................................8

Figure 5. Forced-Draft Boiler Combination Burner..................................................10

Figure 6. Typical LowNOx Burner..........................................................................12

Figure 7. AVC LowNOx Burner..............................................................................13

Figure 8. LowNOx Burner with Flue Gas Recirculation...........................................14

Figure 9. Flue/Fuel Gas Mixture..............................................................................15

Figure 10. Typical Gas Burner System (Automatic Startup).....................................17

Figure 11. Typical Gas Burner System (Supervised Manual Startup)........................18

Figure 12. Pilot Gas System.....................................................................................19

Figure 13. Orifice Flow...........................................................................................20

Figure 14. Fuel Oil System......................................................................................22

Figure 15. RT Liquid Fuel System...........................................................................23

Figure 16. Typical Oil Burner System (Automatic Startup)......................................24

Figure 17. Typical Oil Burner System (Supervised Manual).....................................25

Figure 18. Furnace Natural Draft Profile..................................................................28

Figure 19. Optimum Excess Air for a Fired Heater..................................................29



Saudi Aramco DeskTop Standards 1

FUELS AND BURNERS

Fuel Options

The fuel options include gas and oil. Gas can include natural gas and/or a refinery or gas plantfuel gas. The refinery or gas plant fuel gas includes natural gas and make gas from the processes,and usually has a higher heating value than dry natural gas. Oil can be No. 6 fuel oil, bunker fueloil, resids, or crude oil. It is almost always more economical to burn fuel oil and resids than toburn crude oil.

Burners

Burners are mechanical devices for mixing fuel and air for combustion. The main functions ofburners are:

To provide and mix the proper quantities of fuel and air.

To provide a stable flame.

To release heat in the desired pattern.

Burners and their combustion control systems must produce satisfactory combustion over therange of expected operating conditions (fuel compositions and firing rates). Burners are expectedto be reliable and to meet these requirements with reasonable initial cost and maintenanceexpense.

Failure of burners to perform any of these functions adequately can lead to inefficient combustionand/or poor flame patterns. This can lead to localized overheating and damage to furnace orboiler components, resulting in increased maintenance costs. This damage can also cause apremature shutdown due to failure (or impending failure) of critical components such as tubes,tube supports, or the refractory lining.

Burners are available in two general types: natural-draft and forced-draft. Either type can be usedfor burning gas or liquid fuels alone, or both fuels in combination.

Natural-draft burners are used in all Saudi Aramco process heaters.

Forced-draft burners are used in all boilers. Forced-draft burners may also be used inprocess heaters (furnaces). At this time, none of Saudi Aramco's process heaters useforced-draft burners.




Burner Components

Burners consist of the following main components illustrated in Figure 1. Figure 1 shows typicalnatural-draft burners.

Spider Head

SecondaryAir Register

Pilot

Fuel/PrimaryAir Mixer

Fuel Gas Connectiona b

c

With permission from the John Zink Co.

Figure 1. Typical Natural-Draft Oil/Gas Burners

Air Register - Air enters the burner through the air register. Air flow can be controlled byadjusting the size of the openings in the register. For natural-draft burners, the most commontype of air register consists of fixed and moveable concentric cylinders, each with slots.




Plenum Chamber - The air register in some burners is located inside a plenum chamber, whichusually serves all the burners in the furnace or boiler. In boilers, the plenum chamber is called awindbox. The plenum chamber can serve several purposes:

Reduces noise emissions from the burners. Reduces wind effects on natural-draft burners. Permits combustion air supply from a single source, such as a fan. Enables total air flow to the furnace or boiler to be controlled at one point.

Fuel System - This consists of manifolds and piping to deliver the fuel to the burner tips. Thefuel passes through one or more openings in the tips, which act as restriction orifices. The fuel isinjected into, and mixes with, the airstream. Various types of burner tips are used, dependingupon the type of fuel and the flame pattern desired.

Refractory Burner Tile - This is located at the burner exit. The tile helps stabilize combustionand shape the flame. The burner tile is usually shaped so that one section has a minimum cross-sectional flow area. This is called the throat and acts as a venturi in the airstream.

The opening in most burner tiles is circular. Together with the air register and fuel tip designs,this produces a circular-shaped flame. In some cases, the tile and other components are designedto produce a non-circular flame shape. A rectangular-shaped tile opening can be used to producea flat flame, which is needed in some furnace arrangements.

Pilot Burner - This small burner is used for ignition of the main burner flame. In some burners,it also acts as a stabilizer for the main flame. In boilers, the pilot flame itself is usually ignited byan electric igniter. Pilots in process heater burners usually operate continuously, while pilots inboilers are usually shut down after the main flame is ignited. Individual pilots should beremovable for maintenance while the boiler or process heater remains in operation. A typical pilotburner is shown in Figure 2.




Figure 2. Pilot Burner

Gas Burners

Natural-draft burners rely on the draft (negative pressure) in the furnaces radiant box to inducethe air required for combustion. The efficiency of fuel/air mixing is strongly affected by thekinetic energy available in the air (i.e., the velocity of the air as it passes through the burnermixing zone). Since the draft available is usually only about 0.3-0.5 in. water at the burner, theenergy for mixing is relatively low. Consequently, natural-draft burners are limited to a maximumcapacity of about 15 M Btu/hr. Flame lengths can be excessive in large natural-draft burners.

Many types of natural-draft burners have much lower capacities, resulting in some large furnaceshaving many burners. For example, the Ras Tanura Crude Unit 15 F-100 A&B Atmosphericfurnaces each have 56 burners with a maximum firing capacity of 6.25 MBtu/hr each.

There are two principal types of gas-fired burners: raw gas and pre-mix.




Raw Gas Burners - Raw gas burners (Figure 1a) are used for most applications. In theseburners, the fuel gas passes through orifices in the gas tip and is injected directly into thecombustion zone where it mixes with air. A stabilizer cone is located just below the gas toimprove combustion stability. Raw gas burners have a high turndown ratio (about 5 to 1). Theyare suitable for mounting in plenum chambers and can be used with preheated combustion air.

Raw gas burners of this type (John Zink VYD) are used in Ras Tanura Rheniformer furnace 493-F-301/2/3/4.

Pre-mix Burners - Pre-mix burners (Figure 1b) are sometimes used in special applications. Inthese burners, the kinetic energy made available by the expansion of the fuel gas through the fuelgas orifice inspirates about half of the combustion air (called primary air) into the venturi mixer.This mixture exits through a large burner tip, where it is mixed with the balance of the combustionair (secondary air). This secondary air flow enters the burner through the outer, secondary airregister. Primary air flow through pre-mix burners varies with fuel flow, giving these burners adegree of excess air control. Pre-mix burners require less furnace draft than raw gas burners.

Pre-mix burners can produce a wide range of flame shapes. Use of a "spider" shaped tip, asshown in Figure 1b, produces a short flame, which may be suited to a furnace with a short radiantbox. Other burner tips are used to produce other flame shapes (such as a long, thin flame).

One major disadvantage of pre-mix burners is their susceptibility to flashback. Normally the flamefront is stationary at or immediately above the burner tip, where the fuel/air mixture slows downafter emerging from the holes in the tip. At reduced firing rates and with a high flame velocity,the flame can "flash back" through the tip and burn inside the mixer, just downstream of the fuelgas orifice. Flashback will damage the mixer and burner tip if left unchecked.

Pre-mix burners are generally noisier than raw gas burners, and a muffler is almost alwaysrequired for the primary air inlet. Pre-mix burners also have less flexibility than raw gas burnersfor fuel composition changes.

Natural Draft Burner-Selection Factors - Burner sizes are based mainly on their air flowcapacities. Air flow through a natural-draft burner is determined by the available draft at theburner and by the size of the burners air register and burner tiles. The required air flowdetermines the required number and size of burners. The required air flow is calculated at 120%of maximum heat release and design excess air.

Fuel flow capacity is mainly a function of the size of the openings in the burner tips and theavailable fuel pressure.




The design maximum firing capacity of furnace burners includes some extra capacity above thenormal firing rate. This permits burners to be occasionally taken out of service for maintenancewithout reducing the total furnace firing rate. However, burners should not be excessivelyoversized. An oversized burner has reduced ability to operate efficiently at normal design andturndown conditions. Most of the air-side pressure drop in an oversized burner is taken by the airregister to control air flow. Very little pressure drop is taken by the low-velocity air flow throughthe burner throat, resulting in poor mixing of air and fuel.

The following table gives recommended maximum burner design capacities as a function ofnormal design capacity (furnace design firing divided by the number of burners), based on APIStandard 560, Par. 10.1.3:

Percent of NormalNumber of Burners Heat Release

Up to 5 burners 1256 or 7 burners 1208 or more burners 115

Burner design excess air rates are a function of the fuel fired, per API Standard 560,Par. 2.2.2, 32-SAMSS-029 for heaters, or 32-SAMSS-021 for boilers:

Percent Excess Air 32-SAMSS-029 32-SAMSS-021

Primary Fuel API Std 560 Heaters Boilers

Gas 20 10 5Oil (combination) 25 20 10

Capacity curves for typical natural-draft burners are shown in Figure 3. These curves are forrepresentative burners manufactured by the John Zink Company. Many of Saudi Aramco'sburners are manufactured by the John Zink Company. Similar curves are available from othermanufacturers. In these curves, burner capacity at a given excess air rate is shown as a functionof the air pressure drop through the burner (draft). The design maximum capacity of a natural-draft burner should be as close as practical to the manufacturer's rated capacity, withoutexceeding it.




With permission from the John Zink Co.

Figure 3. John Zink Burner Capacities




Oil Burners

Steam is always added to assist in the atomization of liquid fuels. The steam and oil are mixed inspecially designed fuel atomizers, where the kinetic energy of the steam jets breaks up the fuelinto small droplets. The resulting mixture of steam and finely dispersed oil is then released intothe air stream through a number of orifices in the burner tip. An efficient atomizer may use as littleas 0.1 pound of steam per pound of fuel but a typical design requirement is 0.3 pound of steamper pound of fuel. A typical atomizer is shown in Figure 4.

Tip

SteamOrifices

Fuel OilOrifice

Steam SteamOil

Figure 4. Typical Fuel Oil Burner

For good atomization and combustion of liquid fuels, the steam must be perfectly dry. If there ismoisture in the steam, this moisture will flash when it mixes with the oil, causing erratic oil flow.The atomizing steam should be superheated about 50F.

Mechanical atomization can be used when steam is not available. The kinetic energy in the oilitself is used for atomization by releasing the oil through the tip under very high pressure.Mechanical atomization is usually used only in large burners or with very clean fuels, since thevery small orifices required in smaller burners can become plugged by small dirt or coke particlesin the fuel.




Combination Gas and Oil Burners

A typical combination gas and oil burner is shown in Figure 1c. This burner can be used to fireliquid, gas, or a combination of liquid and gas fuels, depending upon the fuel systems suppliedwith the burner.

Liquid fuel is fired through a centrally located oil gun. The oil and atomizing steam are fedthrough separate pipes in the feed tube to the atomizer and burner tip. The fuel/steam mixture isinjected through orifices in the burner tip into the primary airstream, where combustion begins.The primary air flow is limited to a small percentage of total air flow, so that the fuel will not becooled before combustion begins. The spray angle of the burner tip is designed so that thefuel/steam spray just fills, but does not hit the primary refractory tile. This tile helps stabilize theinitial combustion of the oil spray. The balance of combustion air enters the burner through thesecondary air register.

Fuel gas is fired through a series of gas tips that are located in the secondary air flow path, aroundthe center oil gun. This type of burner is often used in gas-fired furnaces when provisions aremade for future oil firing.

Forced Draft Burners

Forced draft burners are used in all boilers. They rely on fans to supply the combustion air underpressure. In well designed forced-draft burners, the kinetic energy of the airstream is used toachieve much more efficient mixing of the fuel and air than is possible with natural-draft burners.As a result of this improved mixing, smaller flame volumes are obtained. Higher burner capacitiesand lower excess air (5% - 10%) are possible. Thus, fewer forced-draft burners are required forthe same total heat release than would be required with natural-draft burners. For example, theRas Tanura HP Boiler No. 8 has only six burners, each with a capacity of 108 M Btu/hr.

Boiler burners are normally sized for the maximum capacity of the boiler. These burners are notusually oversized in the same manner as process heater burners.

A typical dual-fuel forced-draft burner is shown in Figure 5. This burner is a combination oil andgas burner. The inner burner is an oil burner. The outer burner is a gas burner. Since mostboilers operate with an internal positive pressure, special interlocks are required to seal the oil gunconnection when the gun is removed (32-SAMSS-021). Otherwise, high-temperature flue gaswould escape through this opening, causing a safety hazard.

All boiler burners have pre-mix gas electric ignitor pilot burner. The pilot burner is used forignition only and is automatically tripped (shutoff) after flame ignition.




Figure 5. Forced-Draft Boiler Combination Burner

Emissions Control

There is a worldwide trend toward more stringent and comprehensive control of the emissionsfrom combustion equipment. Emissions of concern are noise, nitrogen and sulfur oxides, andunburned hydrocarbons and other particulates.




Noise - This is caused by the fuel and airstreams entering the burner, and by the combustionprocess. Noise emissions are most often reduced by installing mufflers on the air inlets. Limitingthe maximum fuel gas pressure also helps to reduce noise emissions.

Sulfur - Emissions are directly related to the sulfur content of the fuel. On large, power plant sizeboilers, flue gas cleanup processes can be used to reduce sulfur emissions. In other boilers andfurnaces, sulfur emissions are mainly reduced by using low sulfur fuels.

Incomplete Combustion - Can produce CO and particulate emissions. These can be minimizedby providing good combustion conditions (good atomization of liquid fuels, proper combustionair flow, proper combustion control, etc.). The ash content of the fuel, if any, will add directly tothese particulate emissions.

NOx Emissions - NOx formation is a function of the combustion temperature and the excess airlevel. Liquid fuels may contain a small amount of nitrogen, which can be converted to NOx in thecombustion process.

NOx emissions can be reduced by modifications to the combustion process. Two types of lowNOx burners are illustrated in Figure 6. These burners generally stage the combustion of the fuelto reduce the combustion temperatures and oxygen concentration in the combustion zone. Peaktemperatures are reduced because some of the heat from the first stage of combustion is radiatedto the radiant section tubes before the second stage begins. Since the combustion is staged, thetotal flame envelope is usually larger than that of conventional burners. This must be consideredwhen specifying the burners.

Staged air burners. Combustion begins in a fuel-rich zone. This is followed byinjection of the balance of the combustion air and completion of combustion in aburnout zone.

Staged fuel burners. A portion of the fuel is burned at high excess air levels, loweringthe combustion temperature and NOx production. After some radiation to thesurroundings, a second stage of combustion follows where the balance of the fuel isinjected into the flue gas from the first stage as shown in Figure 6. Staged combustionis more efficient and allows operation at lower percent excess air.




High Fuel-to-Air Ratioin Primary Zone

Staged Air

Primary Air

Fuel ConnectionPilot Gas

A. Staged Air Burner

B. Staged Fuel Burner

High Air-to-Fuel Ratio

in Primary Zone

Secondary Fuel

CombustionAir

SecondaryFuel ConnectionPrimary

Fuel Connection

"Combustors: Applications and Design Considerations", by W. Bartok, R.K. Lyon, A.D. McIntyre, L.A. Ruth &R.E. Sommerlad, Chemical Engineering Progress, Vol. 84, No. 3, pp. 54-71 (1988). Reproduced by permission ofthe American Institute of Chemical Engineers 1988 AlChE.

Figure 6. Typical LowNOx Burner




Figure 7 is a forced draft combustion LowNOx burner incorporating staged air and staged fuel.

Figure 7. AVC LowNOx Burner




Figure 8 is a forced draft LowNOx gas burner in the staged air, staged fuel and flue gasrecirculation. Flue gas recirculation dilutes the combustion mixture and reduces the combustiontemperature.

Figure 8. LowNOx Burner with Flue Gas Recirculation




Figure 9 shows how flue gas is mixed with fuel gas in the burner on Figure 8.

Figure 9. Flue/Fuel Gas Mixture




FUEL SYSTEMS

Fuel Gas Systems

Gas fuels vary from clean, dry, relatively constant molecular weight streams to dirty, wet mixturesof process waste gases that can fluctuate greatly in molecular weight and composition. In the firstcase, the fuel should burn easily and cause few problems. However, in the latter case, serioussafety and maintenance problems can occur unless the gas is thoroughly cleaned and dried. Bothsolids and condensed liquids can plug the fuel ports in the burner tip. This can restrict the burnercapacity and cause poor combustion due to maldistribution of the fuel in the burner. Condensatepassing through the fuel ports can also interrupt steady combustion and cause a flame failure.

To protect against wet or dirty gas, knockout drums should be provided in all fuel gas systems.These should be located as close to the furnace or boiler as possible, but no closer than 50 feet forsafety considerations. Piping downstream of the knockout drum may have to be heated andinsulated if condensation of the gas is a problem.

Filters should be provided in the fuel line downstream of the knockout drum, to remove scale ordirt entrained in the gas stream not removed in the knockout drum. Care should be taken toensure that the fuel gas lines downstream of the filter are clean and are not corroded.

When sour gas fuels containing more than 0.5% H2S are burned, additional fouling (corrosionand scaling) problems may be encountered in the fuel gas piping and burner tips. If foulingbecomes a serious problem, corrosion-resistant (stainless steel) piping may be necessarydownstream of the gas filter.

Components

Fuel gas systems include both the fuel to the main burners and the fuel to the pilot gas burners.Figure 10 shows the minimum fuel gas system as specified by SAES-J-602 for an automatic boilerstartup and includes a knock-out drum (KO pot) to remove solids and liquids from the fuel gas, apressure reducing station, a flow meter, automatic double block and bleed shut-offs, anemergency isolation valve and a firing control valve with a minimum pressure bypass. Many fuelgas systems have an additional KO pot after the emergency isolation valve, and steam trace andinsulate the fuel gas line from the KO pot to the burners to prevent liquids from condensing in thefuel gas lines. The emergency isolation valves provide a tight shutoff and will not reopen withoutbeing reset.




A - Manual Block Valve F - Pressure RegulatorB - BMS Operated Block Valve H - Manual Vent ValveC - BMS Operated Vent Valve FT - Flow TransmitterD - Flow Control Valve PI - Pressure Indicator (Gauge)E - Minimum Flow Regulator PS - Pressure Switch

Figure 10. Typical Gas Burner System (Automatic Startup)

Fuel gas systems without automatic startup provide the same functions as in Figure 10 usingmanual instead of automatic valves as shown in Figure 11.




A - Manual Block Valve G -Manual Block SupervisoryB - BMS Operated Block Valve H - Manual Vent ValveC - BMS Operated Vent Valve FT - Flow TransmitterD - Flow Control Valve PI - Pressure Indicator (Gauge)E - Minimum Flow Regulator PS - Pressure Switch

Figure 11. Typical Gas Burner System (Supervised Manual Startup)




Figure 12 shows the minimum pilot gas system specified by SAES-J-602 for an automatic boilerstartup and includes a filter, double block and bleeds shutoffs, a pressure reducing station, and anemergency shutoff valve. The pilot gas system does not have a firing control valve since it isalways at full flow when operating. Pilot gas systems without automatic startup provide the samefor three items as Figure 12 with manual valves and blinds.

A - Manual Block Valve F - FilterB - BMS Operated Block Valve G - StrainerC - BMS Operated Vent Valve H - Manual Vent ValveD - Pressure Control Valve PI - Pressure Indicator (Gauge)E - Bypass Valve PS - Pressure Switch

PC - Pressure Controller

Figure 12. Pilot Gas System




Controls

The required maximum fuel gas pressure to a burner is largely determined by the required rangeof burner firing rates. Gas burners typically have a turndown capability (ratio of maximum tominimum firing rates) of about 5 to 1, provided that the maximum fuel gas pressure at the burneris 30 psig or greater. 32-SAMSS-021 specifies that boiler burners should have a turn down ratioof 3:1 for normal operations but should be capable of 6:1 turn down ratio with stable burneroperation over this range.

Fuel gas flow through a burner tip is equivalent to flow through an orifice. The flow rate (and thecorresponding firing rate) is proportional to the square root of the fuel gas pressure. Thisrelationship, illustrated in Figure 13, is valid up to about 15 psig, and is very close to about 30psig. Thus, for a 5:1 burner turndown capability, the ratio of maximum to minimum fuel gaspressure will be 25:1.

Source: R.O. Reed, Furnace Operations.

Figure 13. Orifice Flow




The range of fuel gas pressures (and firing rate) is limited by the following constraints:

Maximum fuel gas pressure for satisfactory burner operations is about 40 psig. Higherpressures can cause excessive noise. Also, very high gas velocities passing through theburner tip can cause the flame to lift off the tip, which is an unstable firing condition.

Minimum fuel gas pressure is about 1-2 psig. Lower pressures are difficult to control

with standard instrumentation.

In cases where the fuel gas can vary considerably in molecular weight, burner turndown may belimited. Low molecular weight fuel gases (low density and low heating value) will require thehighest gas pressure to achieve the maximum firing rate. Conversely, high molecular weight fuelgases (high density and high heating valve) will require the lowest gas pressure at minimum firingrate. This range of gas pressures may be beyond the burner turndown capability, andcompromises may be required. The usual solution is to size the burner orifices for the maximumfiring rate with the lowest molecular weight fuel. At low firing rates, it may be necessary to turnsome burners off to keep gas pressures above the minimum.

For very low pressure gases (maximum available pressure at the burner of about 3 psig), specialburners can be used. These use a steam eductor to pull the gas through the burner. This typeburner is often used to combust waste gas streams, such as crude vacuum columnnoncondensibles.

Fuel Oil Systems

Components

Liquid fuels must be free of solids that may plug small atomizer holes in the burners. Liquid fuelsmust be atomized (broken up) into micron-sized droplets before they can be mixed with air andburned efficiently. The fuel oil system must be designed to control the pressure and viscosity ofthe oil to the range for which the oil burner was designed. The viscosity of the oil is controlled toabout 26 centistokes (cSt) by controlling the temperature. Steam is always added to assist in theatomization of liquid fuels.

Figure 14 shows a typical fuel oil system which includes an inline oil heater and a recirculationsystem to make sure the oil does not cool if the oil flow is shut off at the burners. Thetemperature control may be either in the day tank or at the inline heater shown in Figure 14.




Figure 14. Fuel Oil System




Figure 15 shows the Ras Tanura liquid fuel system. The chemical additive reduces fouling due tosoot and ash.

Figure 15. RT Liquid Fuel System

Figure 16 shows the minimum fuel oil system specified by SAES-J-602 for an automatic startupboiler. Figure 17 shows a similar system for a supervised manual startup boiler. The oil systemincludes a strainer (filter), a flow meter, an emergency isolation valve, and a firing control valvewith a minimum pressure bypass. The steam supply has a strainer (filter), and a differentialpressure control valve to provide steam at the pressure required for proper atomization.




A - Manual Block Valve G - StrainerB - BMS Operated Block Valve H - Manual Bleed ValveC - Steam/Oil Pressure Regulator BMS - Burner Management SystemD - Flow Control Valve CCS - Combustion Control SystemE - Minimum Flow Regulator PDS - Differential Pressure Switch

TS - Temperature Switch

Figure 16. Typical Oil Burner System (Automatic Startup)




A - Manual Block Valve G - StrainerB - BMS Operated Block Valve H - Manual Bleed ValveC - Steam/Oil Pressure Regulator BMS - Burner Management SystemD - Flow Control Valve CCS - Combustion Control SystemE - Minimum Flow Regulator PDS - Differential Pressure SwitchF Manual Block Valve, Supervisory TS - Temperature Switch

Figure 17. Typical Oil Burner System (Supervised Manual)




Controls

Maximum liquid fuel pressure at the burner is typically 60-100 psig, depending on the particularburner design. The pressure of the atomizing steam at the burner is usually 20-30 psi above thefuel pressure.

Effects on Boiler and Process Heater (Furnace) Design

Gas fuels are capable of rapid mixing with combustion air, resulting in rapid combustion and shortflame sizes.

Larger combustion volumes (Firebox) and clearances must be provided when liquid fuels arefired. Liquid fuels are introduced into the combustion zone as fine droplets that must first bevaporized before combustion takes place. Droplets of heavy liquid fuels burn from the outsidebefore vaporization is completed. As a result, the length of time required for completecombustion is much greater than for gas fuels. Consequently, flame sizes for liquid fuels arelarger and longer.

When both gas and liquid fuels are fired, the furnace or boiler design is based on the liquid fuels.If a furnace or boiler is designed for gas firing only, its capacity with liquid fuel firing may belimited.

For boilers, Saudi Aramco Standard 32-SAMSS-021 (Par. 5.2.2) requires a 33% largercombustion volume when fuels of 15API or heavier are burned (60,000 vs 80,000 Btu/hr ft3).The 15API corresponds to a specific gravity of 0.966. No. 6 fuel oil and resid are in thiscategory.

For furnaces, API Standard 560 (Par. 10.1.2) requires increased clearances between burners andtubes or refractory walls when any liquid fuels are fired.




MAJOR CONCERNS AND GUIDELINES FOR FIREBOX OPERATION

Major Concerns

Flame Characteristics and Patterns

Flame characteristics are primarily determined by the burner design and the fuel-to-air ratio.

The burner design determines how well the fuel and air are mixed. The better the mixing thequicker the combustion reaction can occur and the shorter the flames will be. The burner designalso determines the shape and stability of the flame during various operating conditions.

Over-Firing -Over-firing the burner (overdesign fuel rate) will result in longer flames andunstable flames. Flames can tend to lift off the burner with the possibility of flame failure. Under-firing a burner can also result in flame instability and the possibility of flame failure. Burnersshould be operated within the design limits of the 5:1 turndown specified for Saudi Aramcoburners.

Air-to- Fuel Ratio/Draft - The air-to-fuel ratio is critical to proper burner operation. Too lowan air-to-fuel ratio will result in long flames and can result in an unstable flame. Too high an air-to-fuel ratio will result in short flames but can result in an unstable flame. Too high an air-to-fuelratio is an uneconomic operation.

The air-to-fuel ratio in a forced draft boiler is controlled by the air-to-fuel ratio controller. Theratio is dependent to some degree on the heating value of the fuel. Higher heating value fuelsrequire a higher air-to-fuel ratio for combustion.

The fuel-to-air ratio in a natural draft furnace is controlled by the furnace draft and burner airregisters. The furnace draft also provides some kinetic energy for mixing in a natural draftfurnace. Inadequate draft will result in a low air-to-fuel ratio, long flames, and an unstable flame.Too high a draft will result in a high air-to-fuel ratio, short flames, and could result in an unstableflame. Too high a draft is an uneconomic operation.

Burner Distribution - All burners should normally be operating in a firebox. At low loads, itmay be necessary to shut off some burners in order to have stable flames. For reduced loadoperation, the number of burners operating in a firebox should be set such that the load for eachburner is approximately equal and about midway in the turndown ratio for the burners. Theoperating burners should provide an even flame distribution over the firebox. Uneven loading ofburners and/or uneven distribution of burners can result in overheating of tubes.




Typical Draft Profile

Figure 8 shows a typical draft profile for a natural draft furnace. The fire box has a positive draft(negative pressure). The lowest draft is at the top of the firebox.

Damper

Hood

Shield tubes

Radiant section

Burners

Secondaryair registers

Stack

Corbel

Convectionsection

Open Damper

Closed Damper

.4 .2 +.2 +.4

Pressure in inchesof Water Relative toAtmospheric PressureTypical fired heater is shown in cross-section

0.6

0

With permission from the Gas Processors Suppliers Association. Source: Engineering Data Book.

Figure 18. Furnace Natural Draft Profile




Optimum Operation for Excess Air

The optimum operation for excess air is dependent on the fuel and the burner design. Fuel oilsrequire a higher optimum excess air for proper operation. Each burner design will have a differentoptimum excess air. Forced draft burners have a lower optimum excess air than natural draftburners. Staged burners such as low NOX burners have a lower optimum excess air than normalburners. The optimum excess air for any firebox can be determined by reducing the air ratio untilthe CO content of the flue gas starts to rise. Then increase the air ratio until the flue gas COreturns to normal to find the optimal air ratio. Since the optimum is an air ratio the firebox willhave to be rebalanced when the fuel rate changes significantly as in a load change.

Operating Guidelines for Natural Draft Furnaces

Proper control of draft and excess air require control of both burner air registers and the stackdamper. The guide lines below show which one should be used in extreme situations.

Low Draft High Draft

Low Excess Air (O2) Open Damper Open Burner Air

High Excess Air (O2) Close Burner Air Close Damper

Figure 19. Optimum Excess Air for a Fired Heater




GLOSSARY

flashback Combustion in a pre-mix burner taking place inside the venturi-mixer, upstream of the normal combustion zone.

flashing The sudden vaporization of a liquid due to a change in conditions,such as a reduction in pressure.

knockout drum A drum used to separate any liquid in the fuel gas stream from thefuel gas vapor.

pulsing Intermittent, noncontinuous combustion.

purge A flow of air through a furnace or boiler and associated flues andducts, which will effectively remove any gaseous combustibles andreplace with air. Purging in furnaces is often accomplished withsteam instead of air.

pre-mix burner A burner in which the fuel gas and some of the air are mixed aheadof the burner combustion zone.

raw gas burner A burner in which the fuel gas and air are mixed in the combustionzone of the burner.

sour gas fuel Fuel gas containing more than about 1/2% H2S.

stack effect The difference in densities between the hot gases inside a furnaceand the ambient air outside the furnace.

turndown capability The ratio of maximum to minimum burner firing capability.

fuel burner & firebox operation & control

Documents

oil burners

draft burners

firebox operation

gas systems

oil systems

technical material

major concerns

draft profile