Power plant and calculations

Power plant and calculation site basically includes the detailed study of power plant operation and maintenance, its related all calculations and thumb rules. It also involves detailed troubleshooting guides for operation and maintenance of power plant system/equipments like Boiler, fans, compressors, belt conveyors, ash handling system, ESP, steam turbine, cooling tower, heat exchangers, steam ejectors, condensers WTP. etc. Heat rate, efficiency

Showing posts with label pumps/compressors/fans. Show all posts

Interview questions and answers on Boiler fans

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

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

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

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.

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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Boiler feed pumps start up & shutdown procedure

Start-up Pre-check lists

Ensure Boiler feed pump is not in permit for maintenance works
Ensure all the maintenance activities on pumps are completed
Ensure equipment are normalized from electrical & mechanical side
Ensure Boiler feed pump is free from obstacles
Ensure enough lighting or illumination is available there
Ensure pump coupling is well guarded by robust coupling guard
Ensure Pumps local field instruments like pressure gauges, switches, vibration sensors, DP gauges are healthy
Ensure coupling is free to rotate

Starting procedure:

Start permissive (interlocks)

Ensure deaerator level is normal
Ensure feed pumps suction valves is open
Ensure cooling water line & outlet valves to bearings & mechanical seals are open and also water is circulating through the lines.
Ensure bearing temperatures are normal
Ensure bearing vibration showing DCS is zero
Ensure there is no alarm exists for strainer DP (differential pressure)
Ensure suction pressure is normal
Ensure discharge valve of the pump is closed
Ensure minimum recirculation valve or ARC valve is in open condition
Ensure balance leak off line valve to deaerator is open (In some valves are not provided to balance leak off line)

Start-up sequence

Take clearance from electrical & field person to start the pump
After getting the clearance from both the team, start the pump by giving minimum set point 80% if it is VFD driven.
Once the pump started, ensure pump will attain minimum 40% RPM in just 10 seconds (in case of journal bearing pumps).For non-journal bearing pumps speed ramp up time is not recommended specifically.
During speed ramp up check bearing vibrations & temperatures are normal.
Ensure pressure in the discharge pressure gauge is increasing slowly & has reached rated pressure at rated speed.
Then check the vibration, bearing temperatures & sound from pump.
Check & ensure the correctness of balance leak off line pressure
Wait for stabilization of the pump, once the pump attained rated parameters, check the motor current, temperature & vibrations & ensure they are within permissive limit.

Also recheck pump bearings vibration, temperatures & any abnormal sound from pump.

Pump normal shutdown procedure

For VFD driven pumps reduce the pump speed to 80% (for start delta driven pumps speed cannot be reduced)
Close the pump’s discharge valve
Then stop the pump

Do not’s for Boiler feed pumps

Do not rotate the pump in reverse direction, reverse direction leads the seize of pump
Don not run the pump if its suction water temperature is more than design/required
Do not operate the pump at less than 80<% load for long time to avoid damage to the bearings
Do not close the auto recirculation line valve
Do not close the balance leak off line valve (if provided)
Do not stop the pump with discharge valve open
Do not throttle the suction valve of the pump at lower load, can throttle discharge valve

For related articles read

powerplant & calculations

32-differences between centrifugal pumps and reciprocating pumps

COMPARISON BETWEEN CENTRIFUGAL PUMPS & RECIPROCATING PUMPS
Sl No.	Centrifugal pump	Reciprocating pump
1	It is dynamic type pump	It is a positive displacement pump
2	It produces more flow	It produces lesser flow
3	It produces less head (pressure) for same size of pump	It produces more head (pressure) for the same size of pump
4	Work is done by rotating the impeller	Work is done by back & forth movement of piston or plunger or diaphragm
5	Simple construction	Complex construction
6	Simple operation	Complicated operation
7	Smoother operation no vibrations	Due to reciprocating movement vibrations are always there
8	Continuous supply of fluid	No continuous supply of fluid
9	Pressure increases by decreasing the flow	Flow is independent on pressure
10	High viscosity fluid cannot be pumped easily	High viscosity fluids can be p[umped very easily
11	Low viscosity fluids can be pumped very easily	Cannot be pumped very easily
12	Centrifugal pumps are low costlier	More costlier than centrifugal pumps
13	Maintenance is easy	Maintenance is difficult
14	Less maintenance cost	More maintenance cost as there are more rotating & moving parts in pump
15	Grease & oil lubricated	Most of the pumps are oil lubricated only
16	Cab ne installed easily	Complicated installation
17	Require less space for same size pump	Require more space for same size pump
18	Less accurate in flow measurement	More accurate for flow measurement
19	Not suitable for small quantity dosing system like chemical plants or water treatment plants	Best for small & accurate quantity dosage like in water treatment plants to maintain desired water parameters
20	Priming is required for centrifugal pumps	Priming is not required
21	Pumps can be started with discharge valve closed condition	Pumps cannot be started with discharge valve closed condition
22	In consists of impeller to create head & flow	In consists of plunger, pistons & diaphragms to create head & flow
23	Centrifugal pumps might be of positive as well as negative suction	Most of the reciprocating pumps are of positive suction
24	PRV is not required	PRV is required to protect the pump during discharge line blockage
25	Minimum recirculation system is there to protect pump at lower discharge flow	Recirculation line is not required as head is independent of flow
26	Pump major internal parts are impeller, wear ring, shaft sleeve, stuffing box, lantern ring, balance & counter balance disc etc	Pump major internal parts are, connecting rod, crank, piston, plunger, diaphragm, gears etc
27	Reverse rotation of the pump affects pump performance & capacity	Reverse rotation of the pump does not affect much
28	Pipe lines have fittings like eccentric & concentric reducer	Eccentric & concentric reducers are not used
29	Generally air vessels are not used in reciprocating pumps	Air vessels are used in reciprocating pumps
30	Thrust balancing is required in high pressure centrifugal pumps	Thrust balancing is not required
31	Cavitation can damage pump internals & related piping system	Cavitation does not affect much
32	For same flow foundation required is not much robust & strong	Strong & robust foundation is required