1-What do you mean by fan & and what are the various fans used in power plant Boilers?
Fan is a Mechanical device , used to move volume air, vapour or flue gas at specific static pressure
Fans used are;
1-Induced draft fan (ID fan)
2-Forced draft fan (FD fan)
3-Secondary air fan (SA fan)
4-Cinder reinjection fan (CR fan)
2-What type of fans used in Boiler?
Generally centrifugal radial, forward & backward curved
fans
3-What are the function of Boiler fans in Boiler
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SL No.
|
Fan type
|
Functions
|
|
1
|
ID fan
|
1-To suck & expel the flue gas from Boiler
2-To maintain negative & balanced draft in furnace
|
|
2
|
FD fan
|
1-To provide combustion air for Boiler
2-To maintain balanced draft in Boiler
3-To maintain fluidisation on bed
|
|
3
|
SA fan/PA fan
|
1-To carry fuel into the furnace or bed
2-To assist fuel in suspension burning (OFA)
3-To spread the fuel in case of Biomass Boilers
|
|
4
|
CR fan
|
To re-inject unburnt fuel from Bank, Economiser & APH back into
the furnace
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know about POWERPLANT SAFETY
4-Which type of fan is more efficient & why?
Centrifugal backward curved is more efficient as they do not
over load & speed controlling is very easy. Fan operates beyond the maximum
efficiency point at lower in put power
5-Differentiate between forward & backward curved
centrifugal fans
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Sl No.
|
Backward curved
|
Forward curved
|
|
1
|
Less number of blades
|
More number of blades
|
|
2
|
For this type of impeller, flow is produced in a radial direction
because the impeller develops static pressure across the longer length of
blade. On the front side of the blade a positive pressure is generated
pushing the air outwards and on the reverse side of the blade a negative
pressure is generated
|
For this type of impeller, flow from the impeller is produced in a
tangential direction. The forward curve of the blade imparts kinetic
energy to the air requiring a scroll housing to convert the kinetic energy
into static pressure
|
|
3
|
Does not over load the motor at higher flow rates
|
Overloads the motor at higher flow rate
|
|
4
|
High pressure
|
Low pressure
|
|
5
|
Lower & medium flow rate
|
Higher flow
|
|
6
|
Higher efficiency
|
Lower efficiency
|
6-What are the various parts of Boiler fans (centrifugal type)?
- Parts of Boiler fans
- Casing
- Inlet cone
- Evase
- Inlet box
- Impeller blade
- Impeller shroud
- Impeller back plate
- Shaft & coupling
- Bearings
7-What are the material of composition (MOC) of Fan’s
various parts
|
SL No.
|
Particular Fan part
|
MOC
|
|
1
|
Casing
|
Mild steel IS 2062
|
|
2
|
Inlet cone
& inlet box
|
Mild steel IS 2062
|
|
3
|
Evase
|
Mild steel IS 2062
|
|
4
|
Impeller blade
|
SAILMA 350
|
|
5
|
Impeller shroud
|
SAILMA 350
|
|
6
|
Impeller back plate
|
SAILMA 350
|
|
7
|
Shaft
|
EN-8
|
8-What is thickness of fans casing
Casing of ID fans is 8mm & that of FD, SA & PA fans is 6mm
9-What
are factors considered for Boiler fans design?
Following factors are considered while designing the Boiler fans
- Required flow
- Inlet & outlet static pressure
- Total pressure
- Operating & design temperature
- Density of gas/air
- Dust load
- Relative humidity
- Nose level
- Fan speed
10-What
type of bearings used for Boiler fans?
Generally, spherical roller bearings for fans having speed up to 1500
TPM
11-What
are the various instruments used for fans?
Draft sensor (draft transmitter)
Flow meter (Aerofoil)
Temperature sensors
12-What
are the protections given for fans?
Start permissive,
Inlet Damper close
Bearing temperature normal
Bearing vibration normal
Protections:
High bearing vibrations trip > 7mm/sec
High bearing temperature trip>90 deg C (for journal & rolling
contact bearings)
High/low static pressure
Over load
13-
Explain the terms static pressure, total pressure and velocity pressure.
Static Pressure is
Resistance to flow
Equal in all directions
Can be Positive or Negative
Independent of air velocity
Measured by pressure tap perpendicular to airflow
Used for fan selection
Total Pressure is
A fluid in motion will exert a Total Pressure on an object in its path.
Total Pressure measured by pressure tap pointed directly into the air
stream.
Used to find velocity pressure.
Velocity Pressure
Cannot be measured directly.
A Pitot tube uses both Static pressure and Total pressure taps.
Used for measuring CFM in a system.
Total Pressure = Static Pressure + Velocity Pressure
17-
What is the differential static pressure?
It is the difference between the
static pressure at fan inlet and out let
18-
What are the flow control methods employed in fans?
Controlling
inlet guide vanes
Discharge damper control method
V belt
position change on pulley (If fan is stepped pulley driven type)
Incorporating Variable Frequency Drives (VFD)
to fan motors to control the speed
19-Calculate the differential static pressure of ID fan,
whose inlet static pressure is -300 mmwc & outlet 0 mmwc
We have,
Differential static pressure = Outlet static
pressure-Inlet static pressure = 0-(-300) = 300 mmwc
20- Calculate the differential static pressure of FD fan,
whose inlet static pressure is -10 mmwc & outlet 250 mmwc
Differential static pressure = Out let static
pressure-Inlet static pressure = 250-10= 240 mmwc
21-What size of FD air duct is required to get flow 70 m3
/sec. at 11 m/sec. speed?
We have,
Volumetric
flow = Area of duct (M2) X Velocity of air (m/sec.)
Area of duct = 70 / 11 = 6.36 M2
22- State the relation between fan’s flow, static
pressure, and speed and power consumption.
Following are the some affinity laws related to fans:
A. Fan speed is directly proportional to its flow.
(N1/N2) = (Q1/Q2)
Where N1 & N2 are speed RPM & Q1 & Q2
are flow in M3/hr
B. Static pressure is directly proportional to
square of the speed.
(SP1/SP2) = (N1/N2)2
Where, SP1 & SP2 are static pressure at two
different speed/RPM
C. Power consumption is directly proportional to
cube of speed.
(P1/P2) = (N1/N2)3
Where, P1&P2 are power consumption at speed N1 & N2
23-Calculate the power consumed by an ID fan if its speed
reduced from 900 RPM to 750 RPM, take 120 KW power consumption at 900 RPM
As per affinity law
(P1/P2) = (N1/N2)3
(120/P2) = (900/750)3
P2 = 69.44 KW
24-Calculate the anticipated static
pressure when FD fan RPM is increased from 1100 RPM to 1175 RPM, static
pressure at 1100 RPM is 220 mmwc
As per affinity law
(SP1/SP2) = (N1/N2)2
(220/SP2) = (1100/1175)2
SP2 = 268.3 mmwc
25-Boiler ID fan of flow capacity 55 m3/sec
having static inlet pressure & efficiency are is 300mmwc & 78% respectively, calculate
the rated power required to run the fan if motor of efficiency 96% is used to
drive the fan.
We have,
Motor rated power = Flow (M3/sec)
X Static differential pressure (mmwc) / (102 X Fan static efficiency X motor
efficiency)
Motor rated power P = 55 X 300 /
(102 X 0.78 X 0.96)
Motor rated power P =216.03 KW
26-A Boiler FD fan of capacity 24m3/sec
having total pressure 265 mmwc & Mechanical efficiency 82%, then calculate
the fan shaft input power
Fan shaft power = Flow (M3/sec) X
Total differential pressure (mmwc) / (102 X Fan mechanical efficiency)
Fan shaft power = 24 X 265 / (102
X 0.82) =76.04 KW
27-Calculate the rated capacity of Boiler
FD fans based on following data.
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SL No.
|
Particular
|
Value
|
|
1
|
Boiler capacity
|
100 TPH
|
|
2
|
Coal consumption
|
17 TPH
|
|
3
|
Coal to air ratio
|
1:6
|
|
4
|
Fan inlet static pressure
|
5 mmwc
|
|
5
|
Draft loss in ducts from Fan to APH
|
15 mmwc
|
|
6
|
Draft loss in APH
|
80 mmwc
|
|
|
Draft loss aerofoil flow meter
|
25 mmwc
|
|
7
|
Draft loss in ducts from APH to plenum
|
20 mmwc
|
|
8
|
Draft loss in Grate
|
50 mmwc
|
|
9
|
Static efficiency of the fan
|
75%
|
|
10
|
Motor efficiency
|
95%
|
|
11
|
Air temperature
|
28 deg C
|
Solution,
Total air required for burning the
coal in Boiler = 17 X 6 =102 TPH
Take 20% extra margin on air flow ,
then air flow = 102 X 120% = 122.4 = 123 TPH
Convert air flow from TPH to m3/sec
Density of air at temperature 28
deg C = 273 X 1.293 / (28+273) = 1.17 kg/m3
Where, 1.293 is density of air at 0
deg C
Air flow = 123000 kg/hr /1.17 =29.2
m3/sec
Based on above flow we can select,
two fans of capacity 29.2/2 = 14.6 or 15 m3/sec
Now calculate the total static
pressure required to over come the resistance in air flow path
Pressure/draft loss drop = Draft
loss in APH inlet ducts + Draft loss in flow element + Draft loss in APH +
Draft loss in APH to grate ducts + Draft loss in grate
= 15 + 80 +25 + 20 + 50 = 190 mmwc
Take 20% extra margin on draft
Discharge static pressure = 190 X
120% = 228 = 230 mmwc
Now calculate the motor rated
power, P = Flow (M3/sec) X Static differential pressure (mmwc) / (102 X Fan static
efficiency X motor efficiency)
P = 15 X (230-5) / (102 X 0.75 X
0.95) = 46.43 KW
Take 15% extra margin on motor
capacity
Then, compensated rated motor power
= 46.43 X 115% =53.4 KW
Select nearest standard size motor
i.e 55 KW
Therefore for boiler of capacity
100 TPH, FD fans capacity 55 KW X 2 are required
28-Why does ID fan trip during Boiler
operations?
Reasons are
1-Over load
2-High bearing vibrations
3-High bearing temperature
4-Electrical& Instrumentation
related issues like short circuit, mal functions etc
5-Low drum level
6-High Boiler pressure &
temperature (if interlocks are provided)
7-Low pressure (more negative
pressure) in furnace
29-Why does FD fans trip during Boiler
operations?
Reasons are
1-Over load
2-High bearing vibrations
3-High bearing temperature
4-Electrical& Instrumentation
related issues like short circuit, mal functions etc
5-High drum level
6-High Boiler pressure &
temperature (if interlocks are provided)
7-Low pressure (more negative
pressure) in furnace
8-Upon tripping of ID fan
30-What will happen if Boiler ID fan trips
(one fan out of two)?
1-Same stream FD fan trips
2-Same stream SA fan trips
3-If pressure drops fuel feeding
system trips
4-Boiler pressure becomes low &
eventually boiler load reduces
31-What will happen if Boiler FD fan trips
(one fan out of two)?
1-Same stream SA fan trips
2-If pressure drops fuel feeding
system trips
3-Boiler pressure becomes low &
eventually boiler load reduces
32-What are the maintenance activities that
you will carryout on Boiler fans
- Fans cleaning
- Casing thickness checking
- Impeller inspection, includes
Shroud plate, blade and back plate thickness checking
- Wear liner inspection and hard
facing
- Inlet cone inspection
- Drive end non drive end bearings
overhauling and grease replacement
- Shaft run out checking
- Coupling inspection
- Shaft ultrasonic continuity test
- Suction and discharge dampers and
pneumatic cylinders overhauling
- Motors servicing
- Realignment
- Manhole doors and inspection doors
proper sealing
- Fan trial and dynamic balancing at
site
33-Write down the start of steps of Boiler
fans
Start-up checks
- Ensure all the maintenance
activities are carried out on fans and equipments are normalized
- Ensure all the instruments like
bearing temperature sensor, vibration sensor etc are fitted & showing
readings in DCS
- Ensure none of the interlocks are
bypassed
- Ensure Fans suction & discharge
dampers are in closed condition
- Ensure coupling & bearing
cooling fan guards are fitted properly
- Ensure local emergency push button
is healthy & in good condition
- Give clearance to DCS or electrical
person to start at 10% initially
- Check abnormal sound from fan casing,
bearing & couplings, if found normal open discharge damper slowly &
subsequently suction damper
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