Boiler Draft System

Draft (Draught): Difference between the atmospheric pressure and the static

pressure of combustion gases in a furnace, gas passage, flue or stack.Driving force for the gas movement.

Product of Combustion to be taken through a tortuous path It is to be dispersed in atmosphere according to the prevailing environmental regulations

Induction of sufficient quantity of air for complete combustion Removal of the product of combustion

Stack&

Fan

Boiler Draft SystemDraft SystemNatural Draft

Mechanical DraftForced Draft

Induced Draft

Balanced Draft

Natural Draft: No power needed, lesser control, for smaller system

Forced Draft: Leakage of gas through the system, less volume (mass) flow rate

Induced Draft: Infiltration of air, larger volume (mass) flow rate

Draft Draft : A flow of air and combustion gases in steam generating units is required to supply the combustion air and to remove the remove the gaseous combustion products. This flow, confined to ducts, boiler settings, heat exchangers, flues and stack is created and sustained by stacks and fans. Draft is a term commonly used to designate static pressure in a furnace, air or gas passage, or stack.Stacks seldom provide sufficient natural draft to cover the requirements of the modern boiler units. The 200 feet(60m) high stack with 490 F(250 C) average gas temperature will develop theoretical natural draft of approximately 1.15 in. of water, whereas resistance to gas and air flow may be as high as 50 inches. There higher draft loss systems require the use of mechanical draft equipment and a wide variety of fan designs and types is available to meet this need.Draft is controlled by ID fans by varying their control damper position or speed whereas, in implosion control the outlet dampers of ID fans are made to close to control the negative pressure excursion.

A typical stack or chimney

Conical Shape to increase velocity

Better structural stability

Stack alone is not sufficient to create draft. Fans are needed

A typical stack or chimney

DESIGN OF STACK

• STACK EFFECT:- difference in pressure caused by difference in elevation between two locations in a vertical ducts or passages conveying heated gases at zero gas flow.

2

2 2

1 1 stack draft effect driving pressure, N/m

of gravity, 9.8 m/s g 1 kgm/Ns

elevation between point 1 and 2 (m)

density of air

SE a g SEc c a g

c

a

g gP z z P

g g v v

g acceleration

z

3

3

3

3

at atmospheric pr. (kg/m )

density of flue gas (kg/m )

specific volume of air at atmospheric pr. (m / )

specific volume of flue gas (m / )

g

a

g

average

v kg

v average kg

• Stack flow loss

For natural draft units stack flow losses are typically less than 5%

6

2 2

2 2

2 2

stack flow loss, lb/ft (N/m )

= friction factor

= length of stack, ft (m)

= stack diameter, ft (m)

G= mass flux = m/A, lb/h.ft (kg/m )

A=stack cross sectional area, ft ( )

32

2 2

2 2

l

c

P

f

L

D

s

m

g

L G GP f v vl D g gc c

2 2.17 lbm ft/lbf s (1 kgm/Ns )

Stacks for Natural Draft units

• The required height and diameter of stacks for Natural Draft units depend on:-1. Draft loss through the boiler from the point of balanced draft to the stack

entrance2. Average temperature of the gases passing up the stack and the temperature

of the surrounding air3. Required gas flow from the stack 4. Barometric pressure• Important points to be considered to determine stack height and diameters:-1. Temperature of surrounding air and gases entering the stack2. Drop in temperature of the gases within the stack due to heat loss to

atmosphere and air infiltration.3. Stack draft loss associated with the gas flow rate.

7

Stacks for Natural Draft units

Leak tight connections- Cold air leak increases flow friction, reduces stack effect, erosion potential of the stack

Erosion due to particulate matter, acid corrosion

Effect of erosion and corrosion is more at the entrance and where the gas has to take a sharp turn

Abrasion resistant material and corrosion shields at those locations is a good maintenance practice

Stack operation and maintenance

Schematic representation of a balanced draft system

Balanced Draft System

FansA fan moves a quantity of air or gas by adding sufficient energy to the stream to initiate motion and overcome all resistance to flow. The fan consists of a bladed rotor, or impeller, which does the actual work, and usually a housing to collect and direct the air or gas discharged by the impeller. The power required depends upon the volume of the air or gas moved in unit time, the pressure difference across the fan and the efficiency of the fan and its drive. There are essentially two different kinds of fans , 1) The centrifugal fan in which gas or air accelerates radially outward in a rotor from heel to tip of blades, discharging into surrounding scroll casing. And 2) The axial flow fan in which the gas or air is accelerated parallel to the fan axis, similar to the ordinary desk fan, but with a casing added to develop static pressure. The axial fans can be a single stage or multiple stage fan. Both the types of fans are used in practice though axial fans with variable blade pitch control are preferred for their part load efficiency .

Forced Draft Fan, Induced Draft Fan, Primary air fan, Gas recirculation fan, Seal Air fans, Scanner Air fan, Igniter air fan.Types of fanCentrifugal Fan ( forward Curved, backward curved, radial, aerofoil shaped)Axial Fan –Axial reaction and axial impulse. Fans provided in 500 MW Trombay unit 5 (Typical example)· FD Fans :2 axial reactive single stage Variable pitch with 6.6 KV, 990 RPM. 2700 KW motors· PA Fans :2 axial reactive double stage Variable pitch with 6.6 KV, 1480 RPM. 2750 KW motors· ID Fans: 4 radial with 6.6 KV, 740 RPM, 2050 KW motors

Fans

Fans

The fans are called Primary Air(PA), Secondary Air(SA), Induced Draft(ID) fans , Gas Recirculation (GR) fans, Seal Air fans, Scanner Air fans and Igniter fans depending on the application of their use. FGD units are provided with booster fans to boost up the pressure.• Fan control : To meet varying requirements of the system, common methods of fan output control are damper control, variable-speed control and blade pitch control in case of axial flow fans. Variable speed control is the most efficient method of controlling fan output since it also reduces power consumption. From the relationships of speed to capacity, pressure and power it follows that by reducing speed by one half, fan output will drop one half, pressure one fourth, and horsepower one eighth.

Different options of Vane control

Fan PerformanceFan performance is best expressed in graphical form. These functional relations are the fan characteristic curves. In these curves capacity in cfm is shown horizontally as the independent variable and head (static pressure), shaft horse power and static efficiency are dependent variables plotted vertically. Fan speed is constant. Since fan operation for a given capacity must match single values of head and horsepower on the characteristic curves, a balance between fan static pressure and system resistance is required. If the system resistance for a given capacity is less than the head indicated on the fan characteristic curve, additional variable flow resistance, such control damper, must be added to the system. Varying the operating speed to yield a family of curves, will change the numerical performance values of the characteristics. However, the nature of the curves remains substantially unaltered. Performance at different speeds for the same efficiency can be related by the following :1. Capacity is directly proportional to speed.2. Head is directly proportional to speed squared.3. Power output is directly proportional to speed cubed.

Fan PerformanceSystem resistance along with the fan static pressure characteristics at various speeds, both as function of volumetric flow rate are plotted. If the fan operates at constant speed, any output less than that shown at the intersection of the system resistance and specified rpm curves must be obtained by throttling the excess fan head. This results in wastage in power that can be avoided by using a variable speed drive.Backwardly curved blade wheels are generally selected for forced draft service because the high speed is suitable for standard motor drive. The power demand is self limiting, and the static efficiency is high. These fans may be satisfactorily operated in parallel. Induced draft fans operate in gas of much higher temperature and may handle gases laden with dust. Forwardly curved blade wheels run at the lowest speed to develop a given pressure, hence are frequently chosen for induced draft service so that the centrifugal stresses in the wheels will be least. The forward curvature reduces the blade depth, but gives a large inlet opening for the gas. conditions.

Fan PerformanceFan Safety Factors : To make sure that the fans will not limit a boiler’s performance, margins of safety are added to the calculated or net fan requirements to arrive at satisfactory test block specification. These margins are intended to cover conditions encountered in operation that can be specifically evaluated. For example, variation in fuel ash characteristics or unusual operating conditions may foul heating surfaces. The unit then requires additional draft. A need for rapid load increase or a short emergency overload often calls for overcapacity of the fans. The customary margins to allow for such conditions are 15 to 20 % increase in the net weight flow of air or gas, 15to 20% increase in net head, and 25F increase in the air or gas temperature at the fan inlet.an be specifically evaluated.

Fan PerformanceGeneral performance requirements for force draft fans.Reliability : Modern boilers must operate continuously for long periods (up to 18 months) without shut down for repairs or maintenance. The fan must be well balanced, and the blades so shaped that they will not collect dirt and disturb this balance. Efficiency : High efficiency over a wide range of output is necessary because boilers operate under varying load conditions.Pressure : Fan pressure should vary uniformly with output over the capacity range. This facilitates damper control and assures minimum disturbance of air flow when minor adjustments to the fuel burning equipment change the system resistance.

Fan Performance

Capacity : When two or more fans operate in parallel, the pressure out-put curves should have characteristics similar to the straight blade or backward curve blade fans in order to share the load equally near the shut off point.Horsepower : Motor driven fans require self limiting horse power characteristics, so that driving motor cannot overload. This means that the horsepower should reach a peak and drop off near the full load fan output.ID fans : Induced draft fans has the same basic requirements as forced draft fan except that it handles high temperature gas which may contain erosive ash. Flat, forward curved and occasionally backward curved blades with less curvature are used. Excessive maintenance from erosion is sometimes avoided by protecting casing and blades with replaceable wear strips. Bearings, usually water cooled have radiation shields on the shaft between rotor and bearings to avoid overheating.

Typical Fan Characteristic Curves

Control options for centrifugal fansDamper control•Lowest capital cost •Ease of operation-automatic control•List expensive fan drive•Continuous, not step operation

Wastage of power

Performance control characteristics

•Areas of constant efficiencies run parallel to the boiler resistance line-high efficiency over a wide boiler load range

•Control range is very large both above and below the maximum efficiency

•The lines of constant blade angle are actually individual fan curves-as the curves are very steep, change in resistance produces very little volume change.

•As the blade angle can be adjusted from minimum to maximum flow change is nearly linear

•Variable blade pitch, together with the high cost of energy and the decreased amount of particulates in the gas streams, have increased in the axial fans in the power plant applications.

Axial fans

Stall Condition :The significant of the axial fan characteristic is the stall area to the left of the peak pressure point. This is caused by the fan blade stalling in much the same way as an aeroplane wing stalls. If the fan is operated in this region, because of the accidental blockage in the flow, it continues to pump energy into the gas/air without developing significant flow. The fan housing can overheat rapidly under such conditions. The angular relationship between the air flow impinging on the blade of a fan and the blade itself is known as “the angle of attack”. In axial flow fan, when this angel exceeds a certain limit, the air flow over the blade separates from the surface and centrifugal force then throws the air outwards, towards the rim of blades. This action causes a build up of pressure at the blade tip, and this pressure increases until it can be relived at the clearance between the tip and the casing. Under this condition the operation of the fan becomes unstable, vibration sets in and the flow starts to oscillate. The risk of stall increases if a fan is oversized or if the system resistance increase excessively.

Axial Fans

Stall is aerodynamic phenomenon which occurs when a fan operates beyond its performance limits and flow separation occurs around the blade. A typical problem with the stall region would occur with two axial fans operating in parallel. If one fan were operated first at low furnace load, it would be impossible to bring up the second fan in service through its stall region without reducing load on the first fan.

Stalling of Axial Fans

Axial Fan Stalling

Stall prevention

When axial fans are sized properly and the resistance curve is parabolic chances of stall is less

Possibility of stall increases when the fan is over sized compare to volume capacity, System resistances increases significantly or fans are operated improperly

Noise in a FanSingle tone noise is generated when the concentrated flow encounters a stationary object after leaving the rotating blade passage. The distance between the blades and the stationary objects affects the soundThe blade passing frequency and its first harmonic is most dominant.Broad band noise is produced by the fluid passing through the fan housing, contains a range of frequencies

Noise in a FanSingle tone noise is generated when the concentrated flow encounters a stationary object after leaving the rotating blade passage. The distance between the blades and the stationary objects affects the soundThe blade passing frequency and its first harmonic is most dominant.Broad band noise is produced by the fluid passing through the fan housing, contains a range of frequencies•Sound radiates from the inlet opening, the discharge duct and the fan housing. All three areas should be analyzed separately and treated in appropriate manner. Inlet sound level from the primary air fans and forced draft fans can be reduced by absorption silencer •Fan casing noise can be minimized by mineral wool insulation and acoustic lagging•Fan discharge noise needs more detail analysis to have cost effective solutions•For forced draft fans and primary air fans, absorption discharge silencer is sufficient•For induced draft fans additionally thermal insulation and lagging are needed. •Stack outlet noise can be reduced by discharge silencer.•However, it is not suitable for coal fired units as the panels gets plugged by fly ash

Noise Control

PA fan

Primary Air fan : The fan consist of the following components:a) Suction bend, with an inlet and an outlet side pipe for volume measurementsb)Fan housing with guide vanes (stage 1)c) Main bearings (anti-friction bearings)d) Rotor consisting of shaft, two impellers with adjustable blades and pitch control mechanism.e) Guide vane housing with guide vanes (stage 2)f) Diffuser with an outlet-side pipe for pressure measurements.Suction bend, fan housing and diffuser are welded structural steel fabrications, reinforced by flanges and gusets, resting on foundation on supporting feet. On its impeller side, the suction bend is designed as an inlet nozzle. Guide vanes of axial flow type are installed in the fan and guide vane housings, in order to guide the flow.Suction bend and diffuser are flexibly connected to the fan housing via expansion joints.

PA Fan

Fan and guide vane housing are horizontally split, so that the rotor can be removed without having to dismount the servomotor. The fan is driven from the inlet side. The main bearings are accommodated in the core of the fan housing. The impellers are fitted to the shaft in overhung position. The centrifugal and axial forces of the impeller blades are absorbed by the blade bearings. For this purpose the blade shaft is held in a combination, of radial and axial antifriction bearings. Each blade bearing is sealed off by means of seals.Blade pitch control unit : An oil hydraulic servomotor flanged to the impeller and rotating with it adjusts the blades during operation. The servomotor consists of piston, cylinder and control parts.At pitch control, the translational movement of the servomotor piston is converted into rotational movement of the blade shafts via adjusting levers, so that the blade angles are variable.

FansOil System : The main bearings and hydraulic servomotor are supplied with oil from a common oil tank. Two oil pumps are mounted on the tank. One is operated as a main pump, whereas the other one is used as standby pump. The latter is started via the pressure switch, in the event of control oil pressure declines.FD fans construction is similar to that of PA fan. But FD fan is a single stage fan.Induced draft fans : There are four ID fans provided per boiler, 3 operating and one standby. (some 500 MW boiler are provided with 3 ID fans, two operating and one standby). These fans are single stage double inlet centrifugal fans. The principal elements of the fan are :Housing, inlet dampers, rotor with bearings and shaft seal. Regulation : The capacity of ID is changed by varying the speed(either by VFD or hydraulic coupling) and also by adjustable inlet dampers arranged in front of impeller.

FansScanner Air Fan : The function of scanner air fan is to provide a continuous supply of clean air to purge and cool the flame scanners. Air for the system is drawn from the FD fan discharge ducts through a filter by one the two scanner fans then discharged through distribution pipe work to each flame scanner. One of the fans is provided with DC supply. In case of AC failure DC scanner fan gets started. An emergency damper is provided in the suction duct to facilitate suction from atmosphere. The discharge from each fan includes a pneumatically operated isolating damper which will open and close in response to signals from FSSS. A pressure switch is provided to initiate the automatic start up of the standby fan if scanner duct to furnace differential becomes less than 6 inches.Seal Air Fans : These fans take suction from cold PA header and boost up pressure for providing sealing air to coal mills/feeders.Igniter Air Fans : These fans take suction from FD fan discharge duct and provide air for igniters.

Air / draft system Operation (typical)System operation(start up of unit) : During unit start up all air and flue gas duct dampers should in start up position. 1. ID fan outlet shut off dampers open, inlet control dampers closed.2. FD Fan outlet shutoff dampers and blade pitch open ( FD fan blade

pitch should be closed before the fan is started).3. Primary air outlet shutoff dampers and blade closed.4. Gas recirculation fan outlet shut off and control dampers closed.5. Air heater gas inlet and outlet, air inlet and outlet dampers should

be open.6. Over fire dampers closed.7. Windbox (secondary) auxiliary air dampers open or modulating.8. One set of pulveriser seal air filter and booster fan shut off dampers

open.9. One igniter fan shut off damper open.

Air / draft system Operation (typical)It is assumed that the unit will be started two ID, two FD fans, both

airpreheaters and both PA fans in service. In case only one set of above equipment is available dampers associated with the idle equipment should be closed.

Caution : The ID fans may be capable of developing drafts in excess of furnace design pressure (Implosion). Therefore the operator must take care to establish and maintain air flow path through the unit, prior to starting ID fan and prior to opening the ID fan inlet control dampers by insuring that the other dampers in the system are in the start up positions. ID fan inlet control dampers must be kept closed until after the fan is started. In case of variable speed operation the speed is kept at minimum. This procedure will minimize the possibility of developing excessive negative pressure in the unit during starting procedure. Implosion : Condition of very high negative pressure arising during operation of the unit due to various reasons, such as loss of fuel, malfunction of draft and or fan controls, ash build up in boiler path etc. It is the phenomenon contrary to explosion.

Fuel Firing System

Fuel firing system: Fuel Oil System – The fuel oil system prepares fuel oil for use in burners (16 per boiler, 4 per elevation) to establish initial boiler light up of the main fuel(coal) and for sustaining boiler low load requirements up to 15% MCR load. Fuel oil system comprises of fuel oil pumps, oil heaters, filters, steam tracing lines. The system ensures proper pressure and temperature of oil(viscosity for atomization) to be burned in the burners. Coal System : The coal system prepares the main fuel(pulverized coal) for firing it in boiler furnace. The raw coal from coal bunker is fed to mill via feeder, where it is pulverized for optimum combustion efficiency. Primary air transports the pulverized coal to coal burners at each corner.Windbox Assembly : The fuel firing equipment consists of four windbox assemblies located in the furnace corners. Each windbox assembly is divided in its height into number of sections or compartment, the coal compartments (fuel air compartment) and intermediate air compartments (auxiliary air compartment). Some of the auxiliary air compartments between coal nozzles contain oil guns. Secondary air is supplied

WINDBOX ARRANGEMENT ALONG WITH OF DAMPERS

Close Coupled Over Fire Air SystemOVER FIRE AIR COMPARTMENTS

•OVERFIRE AIR IS INTRODUCED INTO THEFURNACE TANGENTIALLY THROUGH TWO ADDITIONAL AIR COMPARTMENTS, TERMED AS OVERFIRE AIR PORTS, DESIGNED AS VERTICAL EXTENSIONS OF THE CORNER WINDBOXES.

•THE OVERFIRE AIR PORTS ARE SIZED TOHANDLE 15 PERCENT OF TOTAL WINDBOX AIR FLOW

•AT DESIGN LEVELS OF OVERFIRE, A 20TO 30% REDUCTION IN NOX FORMATIONIS ACHIEVED

Wind Box (Scanners and Igniters)OF2

OF1

AA

A

AB

B

BC

C

CD

D

DE

E

EF

F

FG

G

GH

H

HH

Scanners Ignitors

Boiler and auxiliaries –secondary air distributionFD fans supply secondary air. Air from each fan passes over a SCAPH and then through RAPH to a wind box surrounding boiler furnace. At the sides of the furnace the ducts supply air to each burner/air nozzle elevations in the burner box. Each elevation is fitted with a pneumatically operated regulating damper which is controlled by Secondary Air Damper Control System (SADC) to maintain optimum secondary air distribution for combustion with varying fuels and firing conditions. Five basic types of burner dampers are used :a) Coal/air dampers which admit air immediately around the pulverized fuel nozzle (primary stage of combustion). b) Secondary air dampers, which admit air around the coal /air and P.F nozzles and hence are involved in the later stages of combustion. These dampers are controlled to maintain the desired differential pressure between the secondary air to burners and the furnace. c)Oil/secondary air dampers, when burning oil, the associated damper will modulate according to oil header pressure. d) Bottom tier secondary air dampers is utilised to maintain clear conditions in the lower furnace. e) Overfire damper which direct air over the coal flame to minimize Nox production.

ID FANA B C D

A FD FAN B PA 5BPA 5A

GR 5A GR 5B

ESP

RAPH-5A RAPH-5B

AIR FROM ATMOS

FD A

AIR FROM ATMOS

RAPHB

FD B

RAPHA

FROM GRF - B

FROM GRF - A

IGNITOR AIR FAN

• FURNACE

SECONDARY AIR CIRCUIT

35 DEG C

35 deg

350 DEG

-10 mm

SCAPHB

SCAPHA

SD- 2

SD

7.1

7.2

SD9.1 9.2

SD-1

SD-5

SD-6

SD

10.1

10.2

350 DEG

SD

8.1

8.2

Flue gas system

R

A

P

H A

R

A

P

H B

5 A

5 B

5 C

5 D

ESPGD- GD-

3.1

3.2

GD-7 GD

11A

11B

GD-15 GD-23

GD-

GD-19

GD-4.1

4.2 GD-10

GD

14A

14B GD-18 GD-26GD-22

12A

12B

13A

13B

GD 29.1 29.2

GD 30.1 30.2

GD 31.1 31.2

16

17

2024

21

25

GD 5.1 5.2

8

9

ID A

ID B

ID C

ID D

Stack

GD

27.1

27.2

GD

28.1

28.2

PA 5BPA 5A

RAPH-5A RAPH-5B

Primary Air System

In unit 5 furnace draft is controlled by inlet guide vane.In unit 6 furnace draft is controlled by speed of the ID fans through VFD, IGV always remains 100 % openStarting sequence .

FAN STARTING SEQUENCE

ID FAN

FD FAN

2ND ID FAN

2ND FD FAN

3RD ID FAN

4TH ID FAN

Permissive for Starting Fan

6.6 KV Breaker racked in.

Breaker in remote not on local.

Fan Lube oil / control oil pr. ok

ACW flow through LO cooler not low.

LO tank Level not low.

A Clear Path Logic satisfied.

Outlet Damper close

Inlet Damper Open

IGV / Blade pitch in minimum position.

Any bearing temp. not hi