Boilers Operation

7/30/2019 Boilers Operation

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Operation Of Boilers

BOILER START-UP

Thelload control range

The steady regime off boiler operation

The allowable boiler range

The unsteady regime of boiler operation

BOILER START-UP:

A. Boiler start up follows after all systems have been properly checked (visually,hands on, and electronic systems checks) for proper operation and assures thatsafety devices are in proper working order.

1. Check water level in sight glass and assure water supply to boiler, fill theproper level(s) as required.

2. Boiler types water level sight glass

a. Steam boiler water should be to center of sight glass.

b. Water boiler water should completely fill sight glass.

NOTE:

On both steam and water boilers a vent or test valve is supplied to vent excess airwhen filling the boiler. Leave the valve open on a steam boiler until steam appears,then close, with a water boiler leave valve open until water begins to discharge, then

close the valve.

3. Check all settings on operating controls.

4. Check all reset and lock out mechanisms.

5. Close supply valve to distribution header.


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6. On combination fuel units, set fuel selector switch to primary fuel to be used(in this case gas).

7. Turn burner switch to on position

1. Blower motor will energize to purge combustion chamber in the pre-purgeperiod and continues to run

2. Damper closes.

3. Automatic igniter lights off boiler in low fire. Boiler continues to run inlow fire until properly warmed up before burner is allowed to go into highfire.

The Load Control Range

The automatic control system of a boiler responds quickly to the loadwithout the interference of the operating personnel. The lowest limit isfrom 40-50% of rated load. Smaller boilers not used for power stationshave much lower control range.

The steady regime of boilerOperation

The steam parameters vary iinsignificantly at any lload..Thellowest llimitiis from 30-40% of ratedlload.

The Allowable Load Ranges

The allowable load ranges include loads from limit of control range to thelowest load at which the boiler can function steadily.


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The Unsteady Regime of Boiler Operation

Load variation and fluctuation of steam Parameters occur due to internalor external disturbances.

Internal disturbances are variations in::

Flow rate

Temperature

Fuel consumption rate

Combustion air flow rate

External disturbances are variations in:

Steam pressure

Load of the turbo-alternator

The degree of opening of start-up and shut down device.

Bringing a Boiler on Load

In bringing a boiler on load the key parameter to be maintained is

temperature.

In this case it is not maintained constant but is changed accordingto a predetermined pattern.


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This a pattern is a compromise between the desire to bring theboiler on-load as quikly as possible and the risk of boiler damageby thermal stresses arising from uneven.

There are ffour stages in bringing aBoiler onload:

1. Warming up before circulation is established.

2. Warming up after circulation is Established.

3. Stage when significant quantities of steam are being taken.

4. Bringing the boiler on load.

1.Warming up before circulation isEstablished.

During this phase the limit on the system is the temperature ofboiler tubes.

Until circulation is established there is a risk of local overheating inregions of pockets of trapped steam and of serious uneven heatingbetween adjacent tubes.

Limited input of energy into the system is needed and light oilburners are used.

The provision of boiler circulation pumps removes this stage fromprocedure.


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2.Warming up after circulation isEstablished.

During this phase the main concern is stresses on the boiler drumarising from uneven heating along its length or through thethickness of its metal.

These limitations are met by restricting the permissible rate of riseof drum pressure and hence of drum temperatures.

Progressively more energy is taken from the system by steam flowto drains and slightly tighter input is needed.

Careful control off the energy input is most important at this stageand this is achieved by varying the number of oil burners ifapplicable.

3.Stage when significant quantities of steamare being taken:

This stage is present when boilers used for industrial processes arealso used to generate power, and then turbine conditions mustmatch with the steam output conditions.

The limit now passes to the maximum permissible value in thesuper heater tube metal temperatures.

The steam and fuel flows are increasing since appreciable energy isbeing taken from the system, but the steam flows are not yetadequate to ensure super heater cooling.

Considerable drainage on the super heater may still be necessary.


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4.Bringing the boiler on load

Here the limit is still the super heater temperature.

During this section there must be smooth change from the light oilused in the initial stages of boiler operation to the heavy fuel oilused under spontaneous operation.

The sequential ignition of these burners provides the fine control ofthe energy needed.

As each burner is put into service, care must be taken to see that itignites properly,, and that it burns with bright smokeless flame anddoesnot subsequently go out.

Water and Boiler

Water is the raw material converted in the boiler into the end productsteam. The quality or purity of steam is only as good as the quality ofinput FW and its conditioning in the boiler. In its passage through theboiler, water

Is heated

Undergoes phase modification from liquid to vapor

Is superheated after becoming steam

Effects of Water on Boilers

Water, although adequately treated, harms the boilers in three ways,

unless it is conditioned suitably:

1. Corrosion2. Scaling3. Carryover


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Water Treatment

The objective of water treatment, combining the external treatment andinternal conditioning, in one word is cleanlinesscleanliness of thewetted parts. This, in turn, facilitates the production of clean steam,which keeps the boiler, piping, and turbine protected. External watertreatment is done before water is fed into the boiler and is differentiatedfor a better clarity from the internal water conditioning within the boilerisland.

Water treatment consists of the following stages:

1. Clarification (sedimentation followed by filtration) toremove suspended solids

2. Softening or demineralizationto remove hardness anddissolved solids

3. Degasificationto eliminate CO2 and other dissolved gases

Deaeration and O2 Scavenging

Deaeration:

Deaeration is done primarily by heating the incoming water, consistingusually of condensate and makeup (and at times certain waste streams inprocess plants), by low-pressure steam to its saturation temperature whenaround 98% of dissolved gases separate from water and vent out. Figure4.2 gives the solubility of O2 in water. The solubility levels decreasedramatically as the saturation temperature is approached. As even small

traces of O2 are exceedingly corrosive to the feed lines and economizer(ECON), a thorough scrubbing of water is necessary to make itcompletely free of O2. So the deaerators provide a combination ofheating and scrubbing and manage to remove all the dissolved O2.Scrubbing action is performed inside a deaerator by any of the following,with progressively increasing scrubbing efficiency and reducing steamconsumption:


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1. Spray2. Tray3. Spray and tray arrangements

In scrubbing action, the following two factors are at work:

Water droplets are reduced in size so that the trapped gas has to travelsmaller distance to reach the periphery.

Surface tension and viscosity are lowered to make it easier for the gas toescape.

Solubility levels of oxygen in water.

Failure due to oxygen pitting.


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Spray- and tray-type vertical deaerator without feed tank.

Spray- and tray-type horizontal deaerator mounted on feed tank.


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Schematic arrangement of a deaerator.

O2 Scavenging

Removal of last traces of oxygen is done by chemical scavengers such assodium sulfi te (Na2SO3) or hydrazine (N2H4).

Sodium Sulfite (Na2SO3 )

For boilers operating at pressures


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Na2SO3 with or without the catalyst is an effi cient and fast O2scavenger even at low temperatures. But at higher temperatures like100C, the reaction is really rapid. For every rise of 10C the speed ofreaction doubles. The reaction proceeds rapidly at pH values between 9and 10. Na2SO3 added to the solids in boiler water increases the

carryover, unlike hydrazine, which turns eventually into N2 and H2O.This addition is unsuitable where spray attemperation is to be done onsteam unless it can be fed beyond the point from which FW fordesuperheating is taken. Theoretically, 7.88 ppm of pure Na2SO3 isrequired for each ppm of dissolved O2. But for technical-grade catalyzedNa2SO3, it is appropriate to consider 10 ppm or 10 kg/1 kg of O2 presentin FW. Na2SO3 should be dosed only on a continuous basis to achievecomplete O2 removal. Intermittent feeding is not recommended exceptfor low-pressure systems. Fe, Cu, Co, Ni, and Mn are among the mosteffective materials for acting as catalysts for Na2SO3. Typically

catalyzed Na2SO3 can reduce O2 nearly completely in 10 s whereas plainNa2SO3 can take even 10 min to reduce O2 from 9.8 to 6.6 ppm.Where ECONs are used, sulfi te residuals of 1015 ppm with pH >8.3are recommended for protection against O2 attack.

Hydrazine (N2H4 )

For boilers operating at pressures >70 bar, hydrazine is preferred tosulfite as

1. Hydrazine adds no solids to the boiler water.

2. Na2SO3 can decompose at higher pressures to form H2S and SO2that can cause corrosion of return condensate system.

As pure hydrazine has low fl ash point, a 35% solution is used.Theoretically, 1 ppm of hydrazine is required to remove 1 ppm ofdissolved O2 but in reality is between 1 and 1.5 ppm.

With higher water temperatures and pressures, hydrazine ispreferred to Na2SO3 although it is much slower because it adds nosolids to boiler water. Hence it is well suited to spray attemperatorapplication.

An added advantage is its ability to passivate Fe- and Cu-bearingSurfaces.


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Effectiveness of O2 scavengers.

Organically catalyzed NH4, with reaction times speeded 10 to 100times and with passivating properties also enhanced, is in aposition to extend application to a medium pressure of 45 bar.

Concern about cancer-producing properties dictates extremely

careful handling.

Another concern is the breakdown of hydrazine into ammonia,which is highly corrosive to Cu and Cu-bearing alloys, and a verycareful control of the dosage is required.

Reaction with O2 depends on the water temperature, pH, andimpurities. Figure 4.6 compares the speeds of reaction of Na2SO3and N2H4 and the effect of catalyzed N2H4.

Substitutes for O2 scavengers available in the market must be examinedclosely before using.


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Major Impurities in Water and Their Effects and Removal

Abbreviations:

S, softener; DM, demineralizer; Z, zeolite; A, aeration; Da, deaeration; F,filtration; AX, anion exchanger; CX, cation exchanger; TS, total solids;DS, dissolved solids; SS, suspended solids; B, boiler; T, turbine; HX,heat exchangers.

a Adds to solids.


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Operation Control for Boilers

Controls are those items which carry out the function of regulating the

various quantities indicated by the instruments and which can bearranged, with interlocks, to shut the plant down if any values passoutside the allowable operating range. Control systems can vary insophistication from local manual operation of the various valves anddampers to a fully computerized system with little manual interventiononce the system is programmed and verified. It is worth reflecting on the

statement, Before you can control you must measure. This applies tomanual as well as to automatic control

Manual control, however, is tedious, it is prevailing in small capacity

boilers. It requires continuous watch on all the instruments to ensure thatsafe conditions exist. It is also necessary to include alarms to alert theoperator to the fact that corrective action is required.

To control a boiler, the following quantities require to be regulated asapplicable to a particular system:

1. The heat input to the boiler to match the required heatoutput;

2. The fuel/air ratio to maintain optimum combustionconditions (combustion control);

3. In the case of steam boilers the water flow to match thesteam flow from the boiler;

4. Combustion chamber pressure in the case of balanced-draught boilers to maintain a small negative pressure on thegas side;

5. Where high degrees of superheat are generated, the steamtemperature may be controlled to protect the super heater,

steam pipe work, and the device using the steam againstoverheating; and

6. Combustion safety (burner management).


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The scope and complexity of automatic controls andinstrumentation can vary enormously from the simple on/offschemes as applied to small fire-tube boilers to the more complex

modulating schemes with extensive visual-display and computerdata storage facilities used on some of the larger boilers.

and ReliefBoiler Pressure Measurement, IndicationLocal Pressure Indication

This, along with the corresponding temperature measurement and controlis perhaps the most basic function required. First, the display (which mustbe easily seen and read by the operator) is necessary to ensure the safetyof the plant, a pressure rising above a clear mark indicating the working

pressure on the dial signals that the heat input must be reducedimmediately. A falling pressure means that the demand for heat isexceeding the heat input and therefore that the firing rate must beincreased. The indicating instrument is the well known Bourdon gauge,which consists of a flat tube bent to a curve. This tends to straighten outas the internal pressure increases and is arranged to drive a pointer over acircular scale.

Safety Valves

Boilers are designed to withstand certain pressures only, and on noaccount must be subjected to greater pressures. In most cases themeasuring and control devices described suffice to avoid an overpressurecondition but it is mandatory, on both steam and hot water boilers, to fitsafety valves, which lift and relieve the pressure.

bustion ControlCom

This incorporates both the control of the boiler heat input and that of fuelto air ratio. Combustion control systems must ensure that at all timesadequate quantities of air are available to meet the fuel requirements, soas to burn the fuel efficiently without smoke and with minimum harmfulemissions discharge from the stack.


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The main source of signal for the operation of a combustion controlsystem is the steam pressure at the boiler outlet, in the case of steamgenerators, and the water outlet temperature, in the case of hot-waterboilers. Combustion controls therefore also control the boiler pressure asa stage in controlling the heat input.

Combustion Control Schemes:

There are three basic control schemes used for regulating multiplevariables such as fuel and airflow in a combustion control system. Theseare:

Series, in which a variation of the master control signal,steam pressure, causes a change to take place in the

combustion airflow, which, in turn, causes a change in fuelflow,

Parallel control, in which a variation of the master controlsignal adjusts the fuel and air flows simultaneously andrepresents a typical positional control system, and

Series/parallel control, in which a variation of the mastercontrol signal adjusts the fuel flow and, as steam flow isapproximately proportional to air flow, variations of steamflow resulting from a change of load are measured and used

to adjust the air flow.


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Basic Control Schemes

(a) Series control, (b) Series/parallel control, (c) Parallel control

Types of Combustion Control System

There are three basic types of automatic combustion control. :

a) On/off Control SystemsOn a steam boiler, using an on/off system, the fuel andair are shut off as the steam pressure rises to a presetvalue. The steam pressure then falls gradually as the

demand continues, until it reaches a preset low valueat which the fuel and air are turned on again.

With hot-water boilers, high and low watertemperatures are used as the initiating signals. Atypical example of the on/off control is the systemused with a gas-fired domestic heating system. This


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method of control results in a fluctuating steampressure. Its use tends to be restricted to very smallunits generating hot water or saturated steam. Itcannot be used when generating superheated steambecause, during the off periods, there are no gases

flowing over the super heater from which the steamcan receive its superheat. A variation of the on/offsystem is high/low/off, where there are three controlsettings instead of two.

b) Positioning Control SystemsWith positioning systems, the fuel and combustion aircontrollers (the fuel valve in the case of oil or gasfiring, and dampers or fan speed in the case ofcombustion air) are interconnected mechanically in

such a way that for a given fuel valve position the airdamper will always be in the same position. Suchsystems are called open-loop and assume that theflow through the valve or damper will always be thesame for a given valve or damper position. Theinterconnecting linkage usually incorporates someform of cam, the shape of which is determined duringcommissioning by manual adjustment of the fuel andair controllers to give optimum conditions over theload range of the boiler.

On a typical positioning system applied to fire-tubeboilers the pressure control signal is generated byseparate sensors, two of which are generally used. Thefirst is to signal an overpressure condition to the fuel-feed regulator, which in turn is linked to thecombustion air supply. Should an overpressurecondition occur, the firing appliance is shut down,generally accompanied by visual and audible alarms,and needing manual reset. This control is mandatory

for automatic boilers. The second sends an electricalsignal, which is proportional to the change of pressurefrom the set point to a servomotor connected to thefuel regulator and to the air-regulating dampers (or tothe fan-motor speed controls). These are thus adjustedto restore the pressure to the set value.


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c) Metering/Modulating Control Systems

With metering systems, the fuel and air are regulatedby the master signal from the steam pressure, a fall in

pressure indicating that an increase in fuel and airinputs is required. The fuel and airflows are measured,the two signals are compared in a ratio controller(feedback) and one of them is adjusted by operatingthe flow controller until the correct ratio or set point isachieved. The combustion conditions are thereforemaintained at the optimum irrespective of any changesthat may occur to the system resistance orcharacteristics of the controller. Such systems arecalled closed-loop. The ratio controller is arranged

so that the set point can easily be adjusted manuallywhile the boiler is in operation should there be anychange in the fuel characteristics and hence in the heatinput to the boiler for a given fuel flow signal.Metering systems require a flow-measuring device inthe fuel and air systems.

Soot Blowing

To ensure that the performance and thermal efficiency of a boilerare maintained, it is essential that the heated surfaces are keptclean. On the gas-swept surfaces, this necessitates removal ofmaterial deposited on the tubes from the flue gases. If this is notdone, the rate of heat transfer from the gases will be reduced andthe gas temperatures will rise. On most solid-fuel fired boilers and(depending upon fuel properties), on some gas- and oil-fired andwaste-heat boilers, soot blowers are installed to enable the boilersurfaces to be cleaned while the boiler is operating.

A soot blower is a device that directs a jet of steam or compressedair to blow across tube surfaces in contact with the flue gases. Thistechnique is used to remove material deposited on the tubes. Sootblowers can be of the multi-nozzle or multi-jet rotary type, or of theretractable type.


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A multi-nozzle soot blower (Fig 10.2) consists of a steel tube of50-64 mm diameter which is inserted through the wall of the boiler,which has been equipped with nozzles, which project a blowingmedium (steam). The nozzles are positioned to coincide with thespaces between the tubes to enable the steam to blow down the gas

passages between the tubes. The blower can be rotated through anyangle up to about 280, to cover the greatest amount of heatedsurfaces. Where it is required to blow around a full 360, two rowsof diametrically opposite nozzles are used and the blower is rotatedthrough 180. The effective radius of cleaning from the centerlineof tube is about 2 meters. Multi-nozzle blowers which remain inthe gas stream can only be used in gas temperatures up to about1000 C due to the lack of suitable materials of construction forhigher gas temperatures. Their use is, therefore, mainly restricted tothe evaporative convection, economizer, and air heated surfaces.

Multi-Nozzle Rotary Soot blower


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Where gas temperatures exceed those for which fixed blowers aresuitable, the retractable type has to be used. These can be either short orlong. With the short type, see fig (10.3), the nozzle projects just beyondthe boiler wall and can be used to blow either the combustion chamberwall tubes or the convection heating surfaces on narrow boilers. With

retractable blowers, the tube is withdrawn from the gas stream when notin use, and there are nozzles only at the end of the tube. When soot-blowing is being carried out with a long retractable blower, it is rotatedand moved in such a way to traverse the gas stream and cover the fullwidth of the boiler. Consequently, the steam jet will follow a helical path.

The full cycle includes blowing while the tube traverses back across theboiler and withdrawn. The steam issues from the nozzles immediately asthey enter the gas stream to ensure that the tube is always adequatelycooled. Long retractable blowers have opposing nozzles at the end to

ensure that the reaction of the steam jets is balanced, so as to reducedeflection of the tube.

The tube is available in lengths up to about 15 m as required by the widthof the boiler. Blowers can be fitted in both sides if required, to reduce thelength of the tube. For wide boilers, allowance has to be made in thelayout of the heated surfaces for the deflection of the tube due to its ownweight.


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ShortRetractableSootblower

Burners are the devices responsible for:

1. Proper mixing of fuel and air in the correct proportions, forefficient and complete combustion.

2. Determining the shape and direction of the flame.

---------------------------------------------------------Burner turndown

-An important function of burners is turndown.

-This is usually expressed as a ratio and is based on the maximum firingrate divided by the minimum controllable firing rate.

-The turndown rate is not simply a matter of forcing differing amounts offuel into a boiler, it is increasingly important from an economic andlegislative perspective that the burner provides efficient and proper


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combustion, and satisfies increasingly stringent emission regulations overits entire operating range.

-As has already been mentioned, coal as a boiler fuel tends to berestricted to specialized applications such as water-tube boilers in power

stations.

-The following Sections within this Tutorial will review the mostcommon fuels for shell boilers.

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Heat losses in the flue gases

-The losses are attributable to the temperature of the gases leavingthe furnace.

-Clearly, the hotter the gases in the stack, the less efficient theboiler.

The gases may be too hot for one of two reasons:

1.The burner is producing more heat than is required for a specificload on the boiler:

This means that the burner(s) and damper mechanisms require

maintenance and re-calibration.

2.The heat transfer surfaces within the boiler are not functioningcorrectly, and the heat is not being transferred to the water:

This means that the heat transfer surfaces are contaminated, andrequire cleaning.

Too much cooling of the flue gases

Too much cooling of the flue gases may result in temperatures fallingbelow the 'dew point' and the potential for corrosion is increased by theformation of:

1. Nitric acid (from the nitrogen in the air used for combustion).


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2. Sulphuric acid (if the fuel has a sulphur content).

3. Water.

Radiation losses

Because the boiler is hotter than its environment, some heat will betransferred to the surroundings.

Damaged installed insulation will greatly increase the potentialheat losses.

A reasonably well-insulated shell or water-tube boiler of 5 MW ormore will lose between 0.3 and 0.5% of its energy to thesurroundings.

This may not appear to be a large amount, but it must beremembered that this is 0.3 to 0.5% of the boiler's full-load rating,and this loss will remain constant, even if the boiler is notexporting steam to the plant, and is simply on stand-by.

This indicates that to operate more efficiently, a boiler plant shouldbe operated towards its maximum capacity. This, in turn, mayrequire close co- operation between the boiler house personnel andthe production departments.

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Valve Trains

Piping and valve trains control the supply of gas fuels, liquid fuels andatomizing media to burners.

Trains can be mounted on a free-standing pipe rack.

All electrical components are pre-wired to numbered terminals in ajunction box.

Designs to meet hazardous-area classifications are also available.

Piping trains are solvent-cleaned and painted with one coat of oil-resistantenamel.

Requirements for efficient and environment friendly combustion system:


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Low excess air requirement, coupled with fine controls, permit efficient,safe and flexible operation.

The burner is designed to match the flame shape to the furnace

configuration.

Single Throat Swirl type burner are offered for larger capacities andCircular Register type burners for smaller capacities.

A stand by auxiliary oil atomizer permits cleaning of the main oil burnerwithout affecting rated steam output.

Bi-fuel or tri-fuel burner designs are available to fire gaseous or liquidfuel either by themselves or in combination to take advantage of lowest

fuel costs.

Special design are offered to fire lean gases such as blast furnace gas, offgas, carbon monoxide etc.

Conversion from one fuel to another can be accomplished quickly andconveniently.

Burner start up cycle incorporates a purge cycle to clear the furnace ofresidual combustible gases.

In the event of a boiler stoppage, the fuel lines are cleaned with atomizingsteam.

The Main flame is started by a pilot flame that shuts off after the mainflame is established.

Types of burners:

-Oil burners. Gas burners -Duel burners.


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1-Oil burners

The burning fuel oil efficiently requires a high fuel surface area-to-

volume ratio.

Experience has shown that oil particles in the range 20 and 40 m are themost successful.

Because. Particles which are:

o Bigger than 40 m tend to be carried through the flamewithout completing the combustion process.

o Smaller than 20m may travel so fast that they are carriedthrough the flame without burning at all.

A very important aspect of oil firing is viscosity.

The viscosity of oil varies with temperature: the hotter oil, the easily itflows.

Indeed, most people are aware that heavy fuel oils need to be heated inorder to flow freely.

What is not so obvious is that a variation in temperature, and henceviscosity, will have an effect on the size of the oil particle produced at theburner nozzle.

For this reason the temperature needs to be accuratelycontrolled to give consistent conditions at the nozzle.

Oil Lances and atomizers


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- The energy consumption in many process industries is the singlelargest factor influencing production costs.

- Optimizing the efficiency of fuel utilization provides immediatesavings in operating costs, and reduces pollution.

- Incomplete combustion caused by poor air / fuel mixing, andatomization results in extra fuel usage and unacceptableatmospheric emissions.

- When firing on diesel or heavy fuel oil, the lance should provide agenerous 8:1 turndown, allowing you the control required whenwarming the kiln.

- The choice of atomizer design is crucial for maximizing fuel

efficiency.

- It must be pointed out that to obtain the best performance with anyatomizer design it is essential that the steam and oil are supplied inthe correct condition.

- This is normally 138 to 140 C for heavy oil and steam in a dry andlightly superheated condition.

- Diesel oil does not require heating.

- There is also a link between flame stability and the atomizationquality.

- If the droplet size distribution is too coarse then there is a need forthe flame ignition point to move away from the end of the burnerfiring pipe.

- In the extreme case, the ignition point can move a considerabledistance away from the burner and lead to flame extinction.

- This can sometimes be witnessed in a cold chamber during lightup.


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- This is a potentially dangerous situation, particularly during thenext ignition attempt, as the chamber may contain unburned fuelvapor that will readily ignite/explode when a source of ignition isintroduced.

- Full ranges of atomiser lance assemblies are available.

- Available in a full range of liberations from 3 - 85 MW.

- Whether for warm-up or full production firing, we have theatomizer lance you require.

Atomizer Assemblies

The correct choice of atomizer is essential to running a cost efficientoperation.

Fuel costs is the major expense in any minerals processing plant andgetting the most out of that fuel is critical.

Poor atomization can lead to increased fuel costs due to unburned fuel(High CO levels), and unacceptable emissions.


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For those applications where the fuel oil is contaminated with particulate,Or wet steam is used, causing premature atomizer life, providingmanufactured in a cobalt base alloy, which is very wear resistantextending the expected life by several times.

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Some types of oil burners

1-Rotary cup burner for fire tube boiler

The working principle of rotary cup burners:

- It is based on atomizing by centrifugal force.

- The atomizing cup is driven at high speed via a heavy-duty belt drive.

- The oil is gently positioned at low pressure into the spinning cup wheregradually forced by the centrifugal action of the cup.

- It moves forward until it is thrown off the cup rim as a very fine, uniform film.

The high-velocity primary air discharged around the cup strikes the oil film,breaks it up and converts it into a mist of fine particles which are introduced

into the combustion zone and burner.

- The secondary air necessary for complete combustion is supplied by a forced-draught fan through the wind box and burner air register.

- Normally, atomizing is effected at a viscosity of approx. 45 cSt. which ensuresa particle size small enough to burn quickly and completely.


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-

The advantages of rotary cup atomizer:

1. Reliable operation.

2. Easy maintenance.

3. Minimum installation requirements.

4.There is no concern of fuel oil stuck during heavy fuel oil burningand the Rotary Cup burner can obtain stable combustion for longperiod.

5. Wider range of viscosity can be accepted for the fuel oil applyingto the Rotary Cup Burner.

6. No stuck of high-viscosity oil or dust due to nozzle-less structure.

7. No flame failure due to stable atomizing.

8. Only minimum adjustment is required when you switch two totaldifferent fuel oils due to its wider application of fuel viscosity.

9. Rare splash accident due to Low oil pressure (0.3-0.5MPa).

10.Efficient combustion at any range due to even atomizing particle.

11.Fuel saving due to lower value of excess air ratio which aggravatesboiler efficiency.

12.No steam is required to assist the atomizing of high viscosity oil.


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13.Because the atomisation is produced by the rotating cup, ratherthan by some function of the fuel oil (e.g. pressure), the turndownratio is much greater than the pressure jet burner.

2-Pressure jet burners

A pressure jet burner is simply an orifice at the end of a pressurized tube.

Typically the fuel oil pressure is in the range 7 to 15 bar.In the operating range, the substantial pressure drop created over theorifice when the fuel is discharged into the furnace results in atomizationof the fuel. Putting a thumb over the end of a gar hosepipe creates thesame effect.

Varying the pressure of the fuel oil immediately before the orifice(nozzle) controls the flow rate of fuel from the burner.

However, the relationship between pressure (P) and flow (F) has a squareroot characteristic.

For example if:F2 =0.5 F1

P2 =(0.5)2

P1P2 =0.25 P1

So. If the fuel flow is reduced to 50%, the energy for atomization isreduced to 25%.

This means that the turndown available is limited to approximately 2:1for a particular nozzle.


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To overcome this limitation, pressure jet burners are supplied with arange of interchangeable nozzles to accommodate different boiler loads.

Advantages of pressure jet burners:

1-Relatively low cost.

2-Simple to maintain.

Disadvantages of pressure jet burners:

If the plant operating characteristics vary considerably over the course ofa day, then the boiler will have to be taken off-line to change the nozzle.

Easily blocked by debris. This means that well maintained, fine meshstrainers are essential.

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3-Standard Oil-Fired Duct Burner


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Description of performance

1. When firing fuel oils, oil-fired duct burner delivers superior

performance.

2. Unique design produces the lowest emissions in the industry and itachieves lower NOx and CO emission levels.

3. Oil atomizers are mounted externally, allowing them to beremoved and cleaned without turbine or boiler shutdown.

4.The patented flame shield ensures uniform heat distribution whenfiring gas or oil.

5.Turbine Exhaust Gas (TEG) is supplied to the windbox byindividual ducts taking a slip stream of TEG upstream of theburner.

6.The oil atomizer, capable of firing a range of fuels from Naphtha toNo. 6 fuel oil, can be safely removed for maintenance.

Advantages of Oil-Fired Duct Burner

1-Uniform heat distribution.

2-Low CO and low particulate.

3-Built-in gas firing capability.

4-Side Fired Atomizer guns for on-line maintenance and cleaning.

5-Turndown: 5:1 with all elements firing.

6-TEG oxygen levels down to 11.5% vol., wet.

7-Heavy oil, light oil, Naptha.

8-Steam or air atomization.


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Flame Monitoring System

There are many types of flame control systems each is used for a kind offuel flame which includes ultraviolet Viewing Head and Infrared ViewingHead IR and other

.

1- Ultraviolet Viewing Head

- The ultraviolet viewing head is recommended for gas and oilflames.

- It consists of a gas discharge type sensor with a spectral responseonly in the ultraviolet region, approximately 185 to 300nanometers with a peak response at 200 nanometers.

- The highest ultraviolet intensity occurs near the flame root (first30% of the flame) but this zone of higher ultraviolet intensity doesnot overlap the same zones of adjacent or opposing burners so that,with proper sighting, discrimination is predictable.

-


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-2-Infrared Viewing Head IR

- The infrared viewing head is recommended for pulverised coal andoil flames.

- The IR viewing head uses an extended range, 200 - 1200nanometers, silicon photodiode which is operated in thephotovolatic mode.

- The IR system takes advantage of the fact that all flames pulsatewithin two bands of the visible and near infrared spectral regions.

- This device incorporates an automatic gain control that operates onthe brightness of the flame signal.

- This automatic gain control action is provided to overcome theproblems associated with monitoring pulverized coal flames, whichvary in brightness from low to high firing rates in addition tovariations caused by ash and inconsistent fuel flow.

- The flame signal, after this first stage of amplification is ACcoupled to the next stage.

- This AC component of the signal is flame flicker which covers arange from zero to over 1000 Hz. The IRIS infrared viewing headaccomplishes this by incorporating a variable high pass filter stageafter the second stage of amplification.

- This variable filter has four positions that can be switched at theviewing head to optimize the discrimination ratio between flameON and flame OFF.

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3-Infrared Viewing Head IRGS

- The IRGS viewing head is specially designed for the detection ofoil and gas flames.

- The operation of the IRGS is identical to the IR head, the exceptionbeing the germanium photodiode flame sensor, which operates, inthe spectral range 750nm - 1900nm.

- As with the IR head, good discrimination is achieved by anautomatic gain control and a four position high pass filter switch.

- The IRGS has been specifically designed for operation on multiburner oil and gas burner applications with oil and gas being firedindividually or together.

------------------------------------4-Parallel Viewing Heads

- Parallel operation of viewing heads with one monitor board andamplifier is possible with this system.

- Two of the same combination of viewing heads can be wired inparallel.

- The self-checking characteristics are still operational because theshutters are driven together in unison.

- The flame signals will be additive possibly needing a lowersensitivity setting.

- Two infrared viewing heads can be connected in parallel to thesame flame signal amplifier and still provide independentsensitivity adjustment.

- This capability is particularly useful for multi-burner, multi-fuelapplications.

Shifting flame patterns, commonly encountered on burners withwide turndown ratios, may require parallel viewing heads to provethe flame at the highest and lowest firing rates.


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In this case, one viewing head supervises the pilot (interrupted) and bothdetectors supervise the main burner flame. During the main burner "run"period, either viewing head is capable of maintaining system operation.

In addition to assuring more reliable flame detection, parallel viewing

heads facilitate maintenance during burner operation.

A viewing head can be removed in turn without shutting down thesupervised burner.

-----------------------------------------------------5-Redundant Flame Detection System

- Two viewing heads connected to two flame safeguard controls with

their outputs wired in parallel comprise a redundant flamedetection.

- In addition to the features of parallel flame detectors, a redundantsystem decreases nuisance shutdowns and is thereforerecommended for critical burner applications.

- Flame signal loss, or flame simulating failure occurring in eithercontrol or viewing head, will cause an alarm only, and allowingcorrective action to avert a shutdown.

Gas burners

- At present, gas is probably the most common fuel used in a lot ofcountries.

- Being a gas, atomization is not an issue, and proper mixing of gaswith the appropriate amount of air is all that is required forcombustion.

Two types of gas burner are in use 'Low pressure' and 'Highpressure'.

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A type of gas burners

1-Low pressure burner

- These operate at low pressure, usually between 2.5 and 10 mbar.

- The burner is a simple venturi device with gas introduced in thethroat area, and combustion air being drawn in from around theoutside.

- Output is limited to approximately 1 MW.

----------------------------------------

2-High pressure burner

These operate at higher pressures, usually between 12 and 175 mbar, andmay include a number of nozzles to produce a particular flame shape.

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3-Propane and Natural Gas Pilot Burners

1-Over view on Natural Gas Pilot Burners

- Pilot burners are miniature gas burners installed in, or close to, themain burner to provide a small proven flame as the ignition sourcefor the principle fuel(s).

- At the front of the pilot are situated the ignition and flame rodelectrodes, gas/air mixing chamber and firing nozzle.

- A small air blower and gas supply are connected at the rear of thewhole assembly to provide the correct flammable ratio.

- Pilot flame detection is via the fail-safe ionisation rod, whichensures that if there is no pilot flame present you will be unable toproceed to open the main fuel block valves.

- Prior to igniting the pilot burner the combustion chamber must firstbe purged with air to minimize the risk of a flammable vapor beingpresent.

- Conventional pilot burners and their associated ignition systemsare notoriously unreliable.

- Anyone concerned with plant operation will have experienced thefrustrating and time consuming delays common with manysystems.

- Most of these delays can be traced to failure of the spark ignitor,incorrect air / gas ratio or poor flame detection.


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- The pilot system is a self-contained complete unit with heatreleases ranging from 2 kW to 120 kW.

- Firing on propane or natural gas, all the controls are mounted at therear of the pilot.

- This enclosure contains a built-in transformer for ignition, and pilotflame detection relay.

- A control box fitted in the control panel ensures the correctsequence for successful ignition.

- A single cable from the pilot to the control panel is all that isrequired.

2-The principle causes of delay combustionproblem are:

- Narrow air / gas ratio limits make air and gas settings critical forcorrect operation.

- Poor design of the ignition electrodes that make them vulnerable toelectrical short circuit and, once this occurs, the ignitor fails andthe burner cannot be lit.

- Wet plant air causes electrical short circuits with high voltageelectrodes.

- Optical flame scanners incorrectly sighted or obscured by dust etc.

- Poor design and fragile construction of pilot burner makes it proneto damage and difficult to maintain correctly.


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The advantages of pilot burner :

- Rugged durable design suitable for heavy process plant.

- Simple, reliable, easily maintained flame detection system.

- Suitable for use on many types of pilot burner.

- In service with pulverized coal, oil, gas and multi-fuel burners.

- Simple to operate and maintain.

- Safe for use by operators without supervision.

- Proven in service on thousands of kilns and furnaces world-wide.

Construction of pilot burners

1.The pilot consists of an outer steel pipe that contains the gassupply to the main jet, a high temperature electrode and a flameionization rod.

2.The gas jets and electrodes terminate in an Ioniclloy firing tube

where the pilot flame is stabilized.

3.The internal electrodes are stainless steel rods that are supportedalong the pilot with ceramic spacers every 0.5 meters.

4.The high temperature electrode engages directly into the secondarywinding of the high temperature transformer, which in turn, isdirectly mounted on the rear of the pilot.


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5. By constructing the pilot in this way, any cabling between the

transformer and high temperature electrode is avoided.

6. The ionization rod is connected in a similar manner to the flameionization detector.

7. There are no serviceable cables within this pilot design.

Combustion and cooling air for the pilot:

1. Combustion and cooling air for the pilot is supplied by a dedicated,directly driven 3-phase 415-volt blower.

2.This type of blower removes the need for serviceable drive belts.

3.The blower provides a filtered air supply to the to the pilot at thecorrect pressure, removing the need for any operator adjustments.

4.The supply of cooling air must be maintained whenever the plant isin service to protect the pilot internals and prevent any ingress ofdust.

5. An air filter is mounted on the inlet of the blower to prevent andlong term build up of dust on the pilot internals.

6.The pilot requires a gas supply at a pressure of 150 mbar at thepilot's pressure test point, and will produce a flame with amaximum rated output of 120kW.

7. Usually, propane is supplied to the inlet of the valve train at 2.0barg and this is regulated to the desired pressure on the valve train.


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Controlling the pilot burner

- The pilot is operated from a plug-in Flame Safeguard Control unitthat is mounted within the control panel.

- This unit manages the entire pilot start up and shut down operation.In case the pilot fails to ignite, a reset button is mounted on thefront of the enclosure.

- Also, on the front of the safeguard unit is a series of LED's forindicating the sequence steps, this function is useful for diagnosingproblems as it will indicate the last up successful event.

- Thus, the next sequence step not illuminated is the one that isfaulty.

- This facility can save many hours of wasted time investigatingsystems that are in fact working correctly.

The reasons Surge Controls prefer this pilot is due to thefollowing:-

Few valve train components.

Reliable technology.


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Less sensitive to gas and air pressure fluctuations.

Filtered air supply via direct driven blower.

Separate flame monitoring

Ionization Flame Detection

- The small explosions resulting from burning cause the atmospheresurrounding them to become ionised.

- When ionisation is present, the atmosphere becomes conductive.

- This characteristic is used with flame rods on both conduction andrectification flame sensing systems.

- For reasons of inherent safety, the latter method is preferred forflame detection.

- Flame phenomena centres in the ionisation characteristics, whichpermit a current to flow through the flame when a voltage isapplied between two lectrodes immersed in the flame.

- Tests have shown that the impedance of a flame is about 1 millionohms.

Rectification System

- The rectification system uses electrodes in the flame.

- The area of the two electrodes must be designed to immerse agreater area of one in the flame than the other.

- The "flame" electrode or flame rod in rectification equipment musthave the least exposure to flame.

- The other electrode identified as the groundside is much larger.The area ratio will exceed four to one if satisfactory results are tobe obtained.


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- When the electrodes are different sizes as recommended above,more current would flow from the smaller flame rod to the largerground area.

- To better visualize the reason for this rectifying action, imagine a

man with a shotgun standing by a fence post, and shooting at abarn.

- He will manage to get a lot of pellets into the barn, even if he isn't avery good shot.

- Next, the man walks over to the barn and blazes away (1 shot only)at the fence post he was just standing at moments earlier.

- To give him the benefit of the doubt, assume he is a good shot and

the pellet pattern is centred on the post.

- As a disinterested party in the shooting procedure, but as ascientific observer he decides to examine the results by countingthe pellets that hit the barn and compere that number with thenumber that hit the post.

Dual fuel burners


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- The usual arrangement is to have a fuel oil supply available onsite, and to use this to fire the boiler when gas is not available.

- This led to the development of 'dual fuel' burners.

- These burners are designed with gas as the main fuel, but have anadditional facility for burning fuel oil.

- The dual fuel burner oil firing operation procedure being:

Isolate the gas supply line.

Open the oil supply line and switch on the fuel pump.

On the burner control panel, select 'oil firing'.(This will change the air settings for the different fuel).

- Purge and re-fire the boiler:

This operation can be carried out in quite a short period.

- In some organizations the change over may be carried out as partof a periodic drill to ensure that operators are familiar with theprocedure, and any necessary equipment is available.

- However, because fuel oil is only 'stand-by', and probably onlyused for short periods, the oil firing facility may be basic.

On more sophisticated plants, with highly rated boiler plant, thegas burner(s) may be withdrawn and oil burners substituted.

Burner control systems

- The burner control system cannot be viewed in isolation.

- The burner, the burner control system, and the level control systemshould be compatible and work in a complementary manner tosatisfy the steam demands of the plant in an efficient manner.


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Types of Burner control systems:

1-On / off control system

1.This is the simplest control system, and it means that either theburner is firing at full rate, or it is off.

2.The disadvantage to this method of control is that the boiler isapplied to large and often frequent thermal shocks every time theboiler fires.

3. Its use should therefore be limited to small boilers up to 500 kg / h.

Advantages of an on / off control system:

1-Simple.2-Least expensive.

Disadvantages of an on / off control system:

1. If a large load comes on to the boiler just after the burner hasswitched off, the amount of steam available is reduced.

2. In the worst cases this may lead to the boiler priming and locking

out.

3.Thermal cycling.--------------------------------------------


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2-High / low / off control system

1.This is more complex system where the burner has two firing rates.

2. The burner operates first at the lower firing rate and then switchesto full firing as needed, resulting reduce the worst of the thermalshock.

3.The burner can also revert to the low fire position at reduced loads,again limiting thermal stresses within the boiler.

4.This system is usually used to boilers with an output of up to 5000kg / h.

Advantages of a high / low / off control:

- The boiler is better able to respond to large loads as the 'lowfire' position will ensure that there is more stored energy inthe boiler.

- If the large load is applied when the burner is on 'low fire', it

can immediately respond by increasing the firing rate to'high fire', for example the purge cycle can be omitted.

Disadvantages of a high / low / off control system:

- More complex than on-off control.

- More expensive than on-off control.----------------------------------------------------------------------------


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3-Modulating control system

1. A modulating burner control will alter the firing rate to match theboiler load over the whol e turndown ratio.

2. Every time the burner shuts down and re-starts, the system must bepurged by blowing cold air through the boiler passages.

3.This wastes energy and reduces efficiency.

4. Full modulation, however, means that the boiler keeps firing overthe whole range to maximize thermal efficiency and minimizethermal stresses.

5.This type of control can be fitted to any size boiler, but should

always be fitted to boilers rated at over 10 000 kg / h.

Advantages of a modulating control system:

1.The boiler is even more able to tolerate large and fluctuating loads.

2.This is because:

- The boiler pressure is maintained at the top of its control band,

and the level of stored energy is at its greatest.

- Should more energy be required at short notice, the controlsystem can immediately respond by increasing the firing rate,without pausing for a purge cycle.

Disadvantages of a modulating control system:

1. Most expensive.

2. Most complex

3. Burners with a high turndown capability are required.

Boilers Operation

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