Operation Of Boilers BOI BOI L ER STA START-UP RT-UP
The The lload control range
The The steady regime of f boiler operation
The The allow llowable boiler iler range
The The unsteady regime of boiler iler operation ion
BOILER START-UP: A. Boiler start up follows after all systems have been properly checked (visually, hands hands on, and el electronic ectronic systems checks) for f or proper operation operation and assures that that safety devices are in proper working order.
1. Check water level in sight glass and assure water supply to boiler, fill the proper level(s) as required. 2. Boiler types – water level sight glass a. Steam boil boiler – water should be to cen cente terr of sight sight gl glass. ass. b. Water boiler – water should completely fill sight glass. NOTE: On both steam and and water water boil boi lers a vent or test valve valve is is suppl supplied to vent vent excess excess ai air when when fi filling the the boile boil er. L eave the valve open on a steam boil boiler until until steam appe appea ars, then close close, wi with a wate waterr boil boiler lea leave val valve open until water begins gins to discha discharge, then then close close the valve. valve. 3. Check all settings on operating controls. 4. Check all reset and and lock lock out mechanisms. chanisms. 5. Close supply valve to distribution header.
6. On combina binati tion on fuel units, set fuel sel select ector or switch switch to pri prim mary fue fuel to be used (in this case gas). 7. Tur Turn burner switc itch to “on” positio ition n
1. Blower motor motor wil wi ll energi nergize ze to purge combusti bustion on cham chamber ber in i n the pre-purge period period and and continue continues to run 2. Damper per closes closes. 3. Automatic igniter lights off boiler in low fire. Boiler continues to run in low fire until properly warmed up before burner is allowed to go into high fire.
Th T he L oad Con Control rol Ran Range The The automatic co control system of a boiler iler responds qu quick ickly to to th the loa load withou withoutt the the interfe rference rence of the operating rating personne rsonnel. The lowest owest li limit is is from 40 40--50 50% % of rated loa load. d. Sma Smaller boil boilers not us use ed for power stati stations ons have much lower lower control control range range.
Th T he steady reg regime ime of boiler iler Operation The The steam parameters vary iinsignificantly at any lload. load..The llowest llimit iis from 30-40% of ratedl ratedlload.
Th T he Allow Allowable L oad Ran Ranges The The allow llowable loa load ranges inc includ lude loa loads fro from limit of control range to the lowest load at which the boiler can function steadily.
6. On combina binati tion on fuel units, set fuel sel select ector or switch switch to pri prim mary fue fuel to be used (in this case gas). 7. Tur Turn burner switc itch to “on” positio ition n
1. Blower motor motor wil wi ll energi nergize ze to purge combusti bustion on cham chamber ber in i n the pre-purge period period and and continue continues to run 2. Damper per closes closes. 3. Automatic igniter lights off boiler in low fire. Boiler continues to run in low fire until properly warmed up before burner is allowed to go into high fire.
Th T he L oad Con Control rol Ran Range The The automatic co control system of a boiler iler responds qu quick ickly to to th the loa load withou withoutt the the interfe rference rence of the operating rating personne rsonnel. The lowest owest li limit is is from 40 40--50 50% % of rated loa load. d. Sma Smaller boil boilers not us use ed for power stati stations ons have much lower lower control control range range.
Th T he steady reg regime ime of boiler iler Operation The The steam parameters vary iinsignificantly at any lload. load..The llowest llimit iis from 30-40% of ratedl ratedlload.
Th T he Allow Allowable L oad Ran Ranges The The allow llowable loa load ranges inc includ lude loa loads fro from limit of control range to the lowest load at which the boiler can function steadily.
Th T he Uns Unsteady Reg Regime ime of Boile Boilerr Op Operat ration ion L oad variati variation on and and fluctua fl uctuation tion of steam Param arameters occur due to inte internal or external disturbances.
I nter nter nal nal di dis sturba tur banc nce es ar e var var iatio ations in::
Flow rate Tem Temperature
Fuel consumption rate
Combustion air flow rate
External disturbances are variations in:
Stea Steam pressure
L oad oad of the turboturbo-a alterna ternator tor
The The degree of opening ing of start-up -up and shut down device ice.
Bri Br ingi nging a Boile Boil er on L Lo oad
I n bringi bringing ng a boil boiler on load load the key param parameter ter to be mainta aintaiined iis s temperature. I n this this case case it is is not mainta aintaiined constant but but is is change changed accor accordi ding ng to a predeter predetermined pattern. pattern.
This This a pa pattern is a co compromise ise be between th the de desire ire to to br bring ing th the boiler on-load as quikly as possible and the risk of boiler damage by thermal stresses arisi ri sing ng fro from m uneven.
Th T here are ffo ffour st stages in bring ringin ing g aBoile Boilerr on on load:
1. Warming up before circulation is established. 2. Warming up after circulation is Established. 3. Stagewhen when signi signifficant cant quanti quantiti tie es of steamare be being taken. taken. 4. Bringing the boiler on load.
1. Warming up Established.
before
circulation
is
During uri ng this this phase the limit on the system system is the temperature of boiler tubes. Until circulation is established there is a risk of local overheating in regions regions of pockets pockets of trapped trapped steam and of seri serious ous uneven hea heating ting between adjacent adjacent tubes. L imited ted inpu inputt of energy into i nto the system system is needed and li light oil oil burners are used. used. The The provisio ision n of bo boiler circulation pumps removes this stage from procedure. procedure.
2. Warming up Established.
after
circulation
is
During uring this this phase the main ain concern concern is i s stresse stresses on the boil boiler drum arising from uneven heating along its length or through the thickness of its metal. The These limita itation ions are met by restrict icting ing the permiss issible ible rate of ris rise e of drum pressure and hence of drum temperatures. peratures. Progressi rogressively vely more energy is taken taken from from the system system by steam steam flow to drains and slightly tighter input is needed. Careful control off the energy input is most important at this stage and this this is is achie chieved by varying varyi ng the the number of oil oil burners if if applicable.
3. Stage when significant quantities of steam are being taken:
This This stage is present when boiler ilers used for for ind industrial ial pr processes are also also used used to gene generate rate power, power, and then then turbine turbine conditi conditions ons must match with with the steam output conditi conditions. ons. The The limit no now pa passes tto o th the maximum permiss issible ible va value lue in th the super heater tube tube metal etal temperatures. The The steam and fue fuel flow flows are inc increasing ing sinc ince appreciab iable energy is being taken from rom the system system, but the steam flows are not yet adeq adequa uate te to ensure ensure super heater cooli cool ing. Conside onsiderable rable drai drainag nage e on the super heater heater may stil still be necessary. ry.
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 oil used in the initial stages of boiler operation to the heavy fuel oil used under spontaneous operation. The sequential ignition of these burners provides the fine control of the energy needed. As each burner is put into service, care must be taken to see that it ignites properly,, and that it burns with bright smokeless flame and does not subsequently go out.
Water and Boiler Water is the raw material converted in the boiler into the end product— steam. The quality or purity of steam is only as good as the quality of input FW and its conditioning in the boiler. In its passage through the boiler, 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. Corrosion 2. Scaling 3. Carryover
Water Treatment The objective of water treatment, combining the external treatment and internal conditioning, in one word is cleanliness—cleanliness of the wetted parts. This, in turn, facilitates the production of clean steam, which keeps the boiler, piping, and turbine protected. External water treatment is done before water is fed into the boiler and is differentiated for a better clarity from the internal water conditioning within the boiler island.
Water treatment consists of the following stages: 1. Clarification (sedimentation followed by filtration) — to remove suspended solids 2. Softening or demineralization—to remove hardness and dissolved solids 3. Degasification—to eliminate CO2 and other dissolved gases
Deaeration and O2 Scavenging Deaeration: Deaeration is done primarily by heating the incoming water, consisting usually of condensate and makeup (and at times certain waste streams in process plants), by low-pressure steam to its saturation temperature when around 98% of dissolved gases separate from water and vent out. Figure 4.2 gives the solubility of O2 in water. The solubility levels decrease dramatically 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 it completely free of O2. So the deaerators provide a combination of heating 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 steam consumption:
1. Spray 2. Tray 3. 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 travel smaller distance to reach the periphery. Surface tension and viscosity are lowered to make it easier for the gas to escape.
Solubility levels of oxygen in water.
Failure due to oxygen pitting.
Spray- and tray-type vertical deaerator without feed tank.
Spray- and tray-type horizontal deaerator mounted on feed tank.
Schematic arrangement of a deaerator.
O2 Scavenging Removal of last traces of oxygen is done by chemical scavengers such as sodium sulfi te (Na2SO3) or hydrazine (N2H4).
Sodiu m Sulfite (Na2SO3 ) For boilers operating at pressures <70 bar, catalyzed Na2SO3 is the most common O2 scavenger due to its following features: 1. Low cost 2. Ease of handling 3. Nonscaling properties
Na2SO3 with or without the catalyst is an effi cient and fast O2 scavenger even at low temperatures. But at higher temperatures like 100°C, the reaction is really rapid. For every rise of 10°C the speed of reaction doubles. The reaction proceeds rapidly at pH values between 9 and 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 on steam unless it can be fed beyond the point from which FW for desuperheating is taken. Theoretically, 7.88 ppm of pure Na2SO3 is required for each ppm of dissolved O2. But for technical-grade catalyzed Na2SO3, it is appropriate to consider 10 ppm or 10 kg/1 kg of O2 present in FW. Na2SO3 should be dosed only on a continuous basis to achieve complete O2 removal. Intermittent feeding is not recommended except for low-pressure systems. Fe, Cu, Co, Ni, and Mn are among the most effective materials for acting as catalysts for Na2SO3. Typically catalyzed Na2SO3 can reduce O2 nearly completely in 10 s whereas plain Na2SO3 can take even 10 min to reduce O2 from 9.8 to 6.6 ppm. Where ECONs are used, sulfi te residuals of 10–15 ppm with pH >8.3 are recommended for protection against O2 attack.
Hydr azine (N2H4 )
For boilers operating at pressures >70 bar, hydrazine is preferred to sulfite as 1. Hydrazine adds no solids to the boiler water. 2. Na2SO3 can decompose at higher pressures to form H2S and SO2 that 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 of dissolved O2 but in reality is between 1 and 1.5 ppm.
With higher water temperatures and pressures, hydrazine is preferred to Na2SO3 although it is much slower because it adds no solids to boiler water. Hence it is well suited to spray attemperator application.
An added advantage is its ability to passivate Fe- and Cu-bearing Surfaces.
Effectiveness of O2 scavengers.
Organically catalyzed NH4, with reaction times speeded 10 to 100 times and with passivating properties also enhanced, is in a position to extend application to a mediumpressure 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 very careful control of the dosage is required. Reaction with O2 depends on the water temperature, pH, and impurities. Figure 4.6 compares the speeds of reaction of Na2SO3 and N2H4 and the effect of catalyzed N2H4.
Substitutes for O2 scavengers available in the market must be examined closely before using.
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.
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 be arranged, with interlocks, to shut the plant down if any values pass outside the allowable operating range. Control systems can vary in sophistication from local manual operation of the various valves and dampers to a fully computerized system with little manual intervention once the system is programmed and verified. It is worth reflecting on the statement, “Before you can control you must measure”. This applies to manual 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 that safe conditions exist. It is also necessary to include alarms to alert the operator to the fact that corrective action is required. To control a boiler, the following quantities require to be regulated as applicable to a particular system: 1.
The heat input to the boiler to match the required heat output;
2.
The fuel/air ratio to maintain optimum combustion conditions (combustion control);
3.
In the case of steam boilers the water flow to match the steam flow from the boiler;
4.
Combustion chamber pressure in the case of balanceddraught boilers to maintain a small negative pressure on the gas side;
5.
Where high degrees of superheat are generated, the steam temperature may be controlled to protect the super heater, steam pipe work, and the device using the steam against overheating; and
6.
Combustion safety (burner management).
The scope and complexity of automatic controls and instrumentation can vary enormously from the simple on/off schemes as applied to small fire-tube boilers to the more complex modulating schemes with extensive visual-display and computer data storage facilities used on some of the larger boilers. Boiler Pressure Measurement, I ndication and Relief Local Pressure Indication This, along with the corresponding temperature measurement and control is perhaps the most basic function required. First, the display (which must be easily seen and read by the operator) is necessary to ensure the safety of the plant, a pressure rising above a clear mark indicating the working pressure on the dial signals that the heat input must be reduced immediately. A falling pressure means that the demand for heat is exceeding the heat input and therefore that the firing rate must be increased. The indicating instrument is the well known Bourdon gauge, which consists of a flat tube bent to a curve. This tends to straighten out as the internal pressure increases and is arranged to drive a pointer over a circular scale.
Safety Valves Boilers are designed to withstand certain pressures only, and on no account must be subjected to greater pressures. In most cases the measuring and control devices described suffice to avoid an overpressure condition but it is mandatory, on both steam and hot water boilers, to fit safety valves, which lift and relieve the pressure. Combustion Control This incorporates both the control of the boiler heat input and that of fuel to air ratio. Combustion control systems must ensure that at all times adequate quantities of air are available to meet the fuel requirements, so as to burn the fuel efficiently without smoke and with minimum harmful emissions discharge from the stack.
The main source of signal for the operation of a combustion control system is the steam pressure at the boiler outlet, in the case of steam generators, and the water outlet temperature, in the case of hot-water boilers. Combustion controls therefore also control the boiler pressure as a stage in controlling the heat input. Combustion Control Schemes: There are three basic control schemes used for regulating multiple variables such as fuel and airflow in a combustion control system. These are:
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 fuel flow, Parallel control, in which a variation of the master control signal adjusts the fuel and air flows simultaneously and represents a typical positional control system, and Series/parallel control, in which a variation of the master control signal adjusts the fuel flow and, as steam flow is approximately proportional to air flow, variations of steam flow resulting from a change of load are measured and used to adjust the air flow.
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 Systems
On a steamboiler, using an on/off system, the fuel and air are shut off as the steam pressure rises to a preset value. The steam pressure then falls gradually as the demand continues, until it reaches a preset low value at which the fuel and air are turned on again. With hot-water boilers, high and low water temperatures are used as the initiating signals. A typical example of the on/off control is the system used with a gas-fired domestic heating system. This
method of control results in a fluctuating steam pressure. Its use tends to be restricted to very small units generating hot water or saturated steam. It cannot be used when generating superheated steam because, during the off periods, there are no gases flowing over the super heater from which the steam can receive its superheat. A variation of the on/off system is “high/low/off”, where there are three control settings instead of two. b)
Positioning Control Systems
With positioning systems, the fuel and combustion air controllers (the fuel valve in the case of oil or gas firing, and dampers or fan speed in the case of combustion air) are interconnected mechanically in such a way that for a given fuel valve position the air damper will always be in the same position. Such systems are called “open-loop” and assume that the flow through the valve or damper will always be the same for a given valve or damper position. The interconnecting linkage usually incorporates some form of cam, the shape of which is determined during commissioning by manual adjustment of the fuel and air controllers to give optimum conditions over the load range of the boiler. On a typical positioning system applied to fire-tube boilers the pressure control signal is generated by separate sensors, two of which are generally used. The first is to signal an overpressure condition to the fuelfeed regulator, which in turn is linked to the combustion air supply. Should an overpressure condition 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 electrical signal, which is proportional to the change of pressure from the set point to a servomotor connected to the fuel regulator and to the air-regulating dampers (or to the fan-motor speed controls). These are thus adjusted to restore the pressure to the set value.
c)
Metering/Modulating Control Systems
With metering systems, the fuel and air are regulated by the master signal from the steam pressure, a fall in pressure indicating that an increase in fuel and air inputs 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 operating the flow controller until the correct ratio or set point is achieved. The combustion conditions are therefore maintained at the optimum irrespective of any changes that may occur to the system resistance or characteristics of the controller. Such systems are called “closed-loop”. The ratio controller is arranged so that the set point can easily be adjusted manually while the boiler is in operation should there be any change in the fuel characteristics and hence in the heat input to the boiler for a given fuel flow signal. Metering systems require a flow-measuring device in the fuel and air systems.
Soot Blowing To ensure that the performance and thermal efficiency of a boiler are maintained, it is essential that the heated surfaces are kept clean. On the gas-swept surfaces, this necessitates removal of material deposited on the tubes from the flue gases. If this is not done, the rate of heat transfer from the gases will be reduced and the gas temperatures will rise. On most solid-fuel fired boilers and (depending upon fuel properties), on some gas- and oil-fired and waste-heat boilers, soot blowers are installed to enable the boiler surfaces to be cleaned while the boiler is operating. A soot blower is a device that directs a jet of steam or compressed air to blow across tube surfaces in contact with the flue gases. This technique is used to remove material deposited on the tubes. Soot blowers can be of the multi-nozzle or multi-jet rotary type, or of the retractable type.
A multi-nozzle soot blower (Fig 10.2) consists of a steel tube of 50-64 mm diameter which is inserted through the wall of the boiler, which has been equipped with nozzles, which project a blowing medium (steam). The nozzles are positioned to coincide with the spaces between the tubes to enable the steam to blow down the gas passages between the tubes. The blower can be rotated through any angle up to about 280°, to cover the greatest amount of heated surfaces. Where it is required to blow around a full 360°, two rows of diametrically opposite nozzles are used and the blower is rotated through 180°. The effective radius of cleaning from the centerline of tube is about 2 meters. Multi-nozzle blowers which remain in the gas stream can only be used in gas temperatures up to about 1000 °C due to the lack of suitable materials of construction for higher gas temperatures. Their use is, therefore, mainly restricted to the evaporative convection, economizer, and air heated surfaces.
Multi-Nozzle Rotary Soot blower
Where gas temperatures exceed those for which fixed blowers are suitable, the retractable type has to be used. These can be either short or long. With the short type, see fig (10.3), the nozzle projects just beyond the boiler wall and can be used to blow either the combustion chamber wall tubes or the convection heating surfaces on narrow boilers. With retractable blowers, the tube is withdrawn from the gas stream when not in use, and there are nozzles only at the end of the tube. When sootblowing is being carried out with a long retractable blower, it is rotated and moved in such a way to traverse the gas stream and cover the full width of the boiler. Consequently, the steam jet will follow a helical path. The full cycle includes blowing while the tube traverses back across the boiler and withdrawn. The steam issues from the nozzles immediately as they enter the gas stream to ensure that the tube is always adequately cooled. Long retractable blowers have opposing nozzles at the end to ensure that the reaction of the steam jets is balanced, so as to reduce deflection of the tube. The tube is available in lengths up to about 15 m as required by the width of the boiler. Blowers can be fitted in both sides if required, to reduce the length of the tube. For wide boilers, allowance has to be made in the layout of the heated surfaces for the deflection of the tube due to its own weight.
Short Retractable Soot blower
Burners are the devices responsible for: 1. Proper mixing of fuel and air in the correct proportions, for efficient and complete combustion. 2. Determining theshape 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 firing rate divided by the minimumcontrollable firing rate. -The turndown rate is not simply a matter of forcing differing amounts of fuel into a boiler, it is increasingly important from an economic and legislative perspective that the burner provides efficient and proper
combustion, and satisfies increasingly stringent emission regulations over its entire operating range. -As has already been mentioned, coal as a boiler fuel tends to be restricted to specialized applications such as water-tube boilers in power stations. -The following Sections within this Tutorial will review the most common fuels for shell boilers. ------------------------------------------------------------
Heat losses in the flue gases -The losses are attributable to the temperature of the gases leaving the furnace. -Clearly, the hotter the gases in the stack, the less efficient the boiler. The gases may be too hot for one of two reasons: 1. The burner is producing more heat than is required for a specific load 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 functioning correctly, and the heat is not being transferred to the water: This means that the heat transfer surfaces are contaminated, and require cleaning.
Too much cooling of the flue gases Too much cooling of the flue gases may result in temperatures falling below the 'dew point' and the potential for corrosion is increased by the formation of: 1. Nitric acid (from the nitrogen in the air used for combustion).
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 be transferred to the surroundings. Damaged installed insulation will greatly increase the potential heat losses. A reasonably well-insulated shell or water-tube boiler of 5 MW or more will lose between 0.3 and 0.5% of its energy to the surroundings. This may not appear to be a large amount, but it must be remembered 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 not exporting steam to the plant, and is simply on stand-by. This indicates that to operate more efficiently, a boiler plant should be operated towards its maximum capacity. This, in turn, may require close co- operation between the boiler house personnel and the production departments. ---------------------------
Valve Trains
Piping and valve trains control the supply of gas fuels, liquid fuels and atomizing media to burners. Trains can be mounted on a free-standing pipe rack. All electrical components are pre-wired to numbered terminals in a junction box. Designs to meet hazardous-area classifications are also available. Piping trains are solvent-cleaned and painted with one coat of oil-resistant enamel. Requirements for efficient and environment friendly combustion system:
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 and Circular Register type burners for smaller capacities. A stand by auxiliary oil atomizer permits cleaning of the main oil burner without affecting rated steamoutput. Bi-fuel or tri-fuel burner designs are available to fire gaseous or liquid fuel 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, off gas, carbon monoxide etc. Conversion from one fuel to another can be accomplished quickly and conveniently. Burner start up cycle incorporates a purge cycle to clear the furnace of residual combustible gases. In theevent of a boiler stoppage, the fuel lines are cleaned with atomizing steam. The Main flame is started by a pilot flame that shuts off after the main flame is established.
Types of burners: -Oil burners.
–Gas burners
-Duel burners.
1-Oil burners
The burning fuel oil efficiently requires a high fuel surface area-tovolume ratio. Experience has shown that oil particles in the range 20 and 40 µm are the most successful. Because. Particles which are: o
Bigger than 40 µm tend to be carried through the flame without completing the combustion process.
o
Smaller than 20 µm may travel so fast that they are carried through 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 it flows. Indeed, most people are aware that heavy fuel oils need to be heated in order to flow freely. What is not so obvious is that a variation in temperature, and hence viscosity, will have an effect on the size of the oil particle produced at the burner nozzle. For this reason the temperature needs to controlled to give consistent conditions at the nozzle. Oil Lances and atomizers
be
accurately
- The energy consumption in many process industries is the single largest factor influencing production costs. - Optimizing the efficiency of fuel utilization provides immediate savings in operating costs, and reduces pollution. - Incomplete combustion caused by poor air / fuel mixing, and atomization results in extra fuel usage and unacceptable atmospheric emissions. - When firing on diesel or heavy fuel oil, the lance should provide a generous 8:1 turndown, allowing you the control required when warming 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 any atomizer design it is essential that the steamand oil are supplied in the correct condition. - This is normally 138 to 140 C for heavy oil and steam in a dry and lightly superheated condition. - Diesel oil does not require heating. - There is also a link between flame stability and the atomization quality. - If the droplet size distribution is too coarse then there is a need for the flame ignition point to move away from the end of the burner firing pipe. -
In the extreme case, the ignition point can move a considerable distance away from theburner and lead to flame extinction.
- This can sometimes be witnessed in a cold chamber during light up.
- This is a potentially dangerous situation, particularly during the next ignition attempt, as the chamber may contain unburned fuel vapor that will readily ignite/explode when a source of ignition is introduced. -
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 the atomizer lance you require.
Atomizer Assemblies
The correct choice of atomizer is essential to running a cost efficient operation. Fuel costs is the major expense in any minerals processing plant and getting 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.
For those applications where the fuel oil is contaminated with particulate, Or wet steam is used, causing premature atomizer life, providing manufactured in a cobalt base alloy, which is very wear resistant extending theexpected life by several times.
------------------------------------------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 where gradually 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 forceddraught fan through the wind box and burner air register. -
Normally, atomizing is effected at a viscosity of approx. 45 cSt. which ensures a particle size small enough to burn quickly and completely.
-
The advantages of rotary cup atomizer: 1. Reliable operation. 2. Easy maintenance. 3. Minimuminstallation requirements. 4. There is no concern of fuel oil stuck during heavy fuel oil burning and the Rotary Cup burner can obtain stable combustion for long period. 5. Wider range of viscosity can be accepted for the fuel oil applying to 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 total different 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 rangedue to even atomizing particle. 11.Fuel saving due to lower value of excess air ratio which aggravates boiler efficiency. 12.No steam is required to assist the atomizing of high viscosity oil.
13.Because the atomisation is produced by the rotating cup, rather than by some function of the fuel oil (e.g. pressure), the turndown ratio 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 the orifice when the fuel is discharged into the furnace results in atomization of the fuel. Putting a thumb over the end of a gar hosepipe creates the same 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 square root characteristic. For example if: F2 =0.5 F1 P2 =(0.5)2 P1 P2 =0.25 P1 So. If the fuel flow is reduced to 50%, the energy for atomization is reduced to 25%. This means that the turndown available is limited to approximately 2:1 for a particular nozzle.
To overcome this limitation, pressure jet burners are supplied with a range 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 of a day, then the boiler will have to be taken off-line to changethe nozzle. Easily blocked by debris. This means that well maintained, fine mesh strainers are essential.
--------------------------------------------------------3-Standard Oil-Fired Duct Burner
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 it achieves lower NOx and CO emission levels. 3. Oil atomizers are mounted externally, allowing them to be removed and cleaned without turbine or boiler shutdown. 4. The patented flame shield ensures uniform heat distribution when firing gas or oil. 5. Turbine Exhaust Gas (TEG) is supplied to the windbox by individual ducts taking a slip stream of TEG upstream of the burner. 6. The oil atomizer, capable of firing a range of fuels from Naphtha to No. 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.
Flame Monitoring System There are many types of flame control systems each is used for a kind of fuel flame which includes ultraviolet Viewing Head and Infrared Viewing Head IR and other .
1- Ultraviolet Viewing Head
- The ultraviolet viewing head is recommended for gas and oil flames. - It consists of a gas discharge type sensor with a spectral response only in the ultraviolet region, approximately 185 to 300 nanometers with a peak response at 200 nanometers. - The highest ultraviolet intensity occurs near the flame root (first 30% of the flame) but this zone of higher ultraviolet intensity does not overlap the same zones of adjacent or opposing burners so that, with proper sighting, discrimination is predictable. -
2-Infrared Viewing Head IR-
- The infrared viewing head is recommended for pulverised coal and oil flames. - The IR viewing head uses an extended range, 200 - 1200 nanometers, silicon photodiode which is operated in the photovolatic mode. - The IR system takes advantage of the fact that all flames pulsate within two bands of the visible and near infrared spectral regions. - This device incorporates an automatic gain control that operates on the brightness of the flame signal. - This automatic gain control action is provided to overcome the problems associated with monitoring pulverized coal flames, which vary in brightness from low to high firing rates in addition to variations caused by ash and inconsistent fuel flow. - The flame signal, after this first stage of amplification is AC coupled to the next stage. - This AC component of the signal is flame flicker which covers a range from zero to over 1000 Hz. The IRIS infrared viewing head accomplishes this by incorporating a variable high pass filter stage after the second stage of amplification. - This variable filter has four positions that can be switched at the viewing head to optimize the discrimination ratio between flame ON and flame OFF.
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3-Infrared Viewing Head IRGS - The IRGS viewing head is specially designed for the detection of oil and gas flames. - The operation of the IRGS is identical to the IR head, the exception being the germanium photodiode flame sensor, which operates, in the spectral range750nm - 1900nm. - As with the IR head, good discrimination is achieved by an automatic gain control and a four position high pass filter switch. - The IRGS has been specifically designed for operation on multi burner oil and gas burner applications with oil and gas being fired individually or together.
-----------------------------------4-Parallel Viewing Heads - Parallel operation of viewing heads with one monitor board and amplifier is possible with this system. - Two of the same combination of viewing heads can be wired in parallel. - The self-checking characteristics are still operational because the shutters are driven together in unison. - The flame signals will be additive possibly needing a lower sensitivity setting. - Two infrared viewing heads can be connected in parallel to the same flame signal amplifier and still provide independent sensitivity adjustment. - This capability is particularly useful for multi-burner, multi-fuel applications. Shifting flame patterns, commonly encountered on burners with wide turndown ratios, may require parallel viewing heads to prove the flame at the highest and lowest firing rates.
In this case, one viewing head supervises the pilot (interrupted) and both detectors 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 the supervised burner.
----------------------------------------------------5-Redundant Flame Detection System - Two viewing heads connected to two flame safeguard controls with their outputs wired in parallel comprise a redundant flame detection. - In addition to the features of parallel flame detectors, a redundant system decreases nuisance shutdowns and is therefore recommended for critical burner applications. - Flame signal loss, or flame simulating failure occurring in either control or viewing head, will cause an alarm only, and allowing corrective action to avert a shutdown.
Gas burners - At present, gas is probably the most common fuel used in a lot of countries. - Being a gas, atomization is not an issue, and proper mixing of gas with the appropriate amount of air is all that is required for combustion. Two types of gas burner are in use 'Low pressure' and 'High pressure'.
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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 the throat area, and combustion air being drawn in from around the outside. - Output is limited to approximately 1 MW.
---------------------------------------2-High pressure burner These operate at higher pressures, usually between 12 and 175 mbar, and may 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, the main burner to provide a small proven flame as the ignition source for the principle fuel(s). - At the front of the pilot are situated the ignition and flame rod electrodes, gas/air mixing chamber and firing nozzle. - A small air blower and gas supply are connected at the rear of the whole assembly to provide the correct flammable ratio. - Pilot flame detection is via the fail-safe ionisation rod, which ensures that if there is no pilot flame present you will be unable to proceed to open the main fuel block valves. - Prior to igniting the pilot burner the combustion chamber must first be purged with air to minimize the risk of a flammable vapor being present. - Conventional pilot burners and their associated ignition systems are notoriously unreliable. - Anyone concerned with plant operation will have experienced the frustrating and time consuming delays common with many systems. - Most of these delays can be traced to failure of the spark ignitor, incorrect air / gas ratio or poor flame detection.
- The pilot system is a self-contained complete unit with heat releases ranging from 2 kW to 120 kW. - Firing on propane or natural gas, all the controls are mounted at the rear of the pilot. - This enclosure contains a built-in transformer for ignition, and pilot flame detection relay. - A control box fitted in the control panel ensures the correct sequence for successful ignition. -
A single cable from the pilot to the control panel is all that is required.
2-The principle causes of delay combustion problemare:
- Narrow air / gas ratio limits make air and gas settings critical for correct operation. - Poor design of the ignition electrodes that make them vulnerable to electrical short circuit and, once this occurs, the ignitor fails and the burner cannot be lit. - Wet plant air causes electrical short circuits with high voltage electrodes. - Optical flame scanners incorrectly sighted or obscured by dust etc. - Poor design and fragile construction of pilot burner makes it prone to damage and difficult to maintain correctly.
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 gas supply to the main jet, a high temperature electrode and a flame ionization 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 supported along thepilot with ceramic spacers every 0.5 meters.
4. The high temperature electrode engages directly into the secondary winding of the high temperature transformer, which in turn, is directly mounted on the rear of the pilot.
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 flame ionization 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 the correct pressure, removing the need for any operator adjustments. 4. The supply of cooling air must be maintained whenever the plant is in service to protect the pilot internals and prevent any ingress of dust. 5. An air filter is mounted on the inlet of the blower to prevent and long term build up of dust on the pilot internals. 6. The pilot requires a gas supply at a pressure of 150 mbar at the pilot's pressure test point, and will produce a flame with a maximum rated output of 120kW. 7. Usually, propane is supplied to the inlet of the valve train at 2.0 barg and this is regulated to the desired pressure on the valve train.
Controlling the pilot burner - The pilot is operated from a plug-in Flame Safeguard Control unit that 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 the front of the enclosure. - Also, on the front of the safeguard unit is a series of LED's for indicating the sequence steps, this function is useful for diagnosing problems as it will indicate the last up successful event. - Thus, the next sequence step not illuminated is the one that is faulty. - This facility can save many hours of wasted time investigating systems that are in fact working correctly.
The reasons Surge Controls prefer this pilot is due to the following:•Few valve train components. • Reliable technology.
•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 atmosphere surrounding them to become ionised. -
When ionisation is present, the atmosphere becomes conductive.
- This characteristic is used with flame rods on both conduction and rectification flame sensing systems. - For reasons of inherent safety, the latter method is preferred for flame detection. - Flame phenomena centres in the ionisation characteristics, which permit a current to flow through the flame when a voltage is applied between two lectrodes immersed in the flame. - Tests have shown that the impedance of a flame is about 1 million ohms.
Rectification System - The rectification system uses electrodes in the flame. - The area of the two electrodes must be designed to immerse a greater area of one in the flame than the other. - The "flame" electrode or flame rod in rectification equipment must have 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 to be obtained.
- When the electrodes are different sizes as recommended above, more current would flow from the smaller flame rod to the larger ground area. - To better visualize the reason for this rectifying action, imagine a man with a shotgun standing by a fence post, and shooting at a barn. - He will manage to get a lot of pellets into the barn, even if he isn't a very 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 a scientific observer he decides to examine the results by counting the pellets that hit the barn and compere that number with the number that hit the post.
Dual fuel burners
- The usual arrangement is to have a fuel oil supply available on site, 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 an
additional 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 part of a periodic drill to ensure that operators are familiar with the procedure, and any necessary equipment is available. - However, because fuel oil is only 'stand-by', and probably only used for short periods, the oil firing facility may be basic. On more sophisticated plants, with highly rated boiler plant, the gas 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 system should be compatible and work in a complementary manner to satisfy the steam demands of the plant in an efficient manner.
Types of Burner control systems: 1-On / off control system 1. This is the simplest control system, and it means that either the burner is firing at full rate, or it is off. 2. The disadvantage to this method of control is that the boiler is applied to large and often frequent thermal shocks every time the boiler 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 has switched off, the amount of steamavailable is reduced. 2. In the worst cases this may lead to the boiler priming and locking out. 3. Thermal cycling. --------------------------------------------
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 switches to full firing as needed, resulting reduce the worst of the thermal shock. 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 5000 kg / h.
Advantages of a high / low / off control: - The boiler is better able to respond to large loads as the 'low fire' position will ensure that there is more stored energy in the 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. ----------------------------------------------------------------------------
