2. Under the esteemed guidance of
SRI.G.SATHISH
ASST. PROFESSOR IN MECHANICAL DEPARTMENT
3. The power generation plays a vital role for any country
development. One of such power generation unit is
thermal power plant which is the main source for large-
scale production of electrical energy. The cycle used is
“MODIFEID RANKINE CYCLE” which includes super
heated steam, regenerating feed water and reheated
steam. Raw materials used in thermal power plant are
coal, water, oil and air.
The main objective behind my project is to study about
the thermal plant, how power is generated, what are its
sources, working of boilers and boiler tube failures.
This work concentrates much about boiler
tubes, different type of tube failures and main cause for
failure of tubes and preventive measures in boiler tubes.
4. Principle of power generation
Boiler
Boiler accessories
Boiler tube failures
Case study of boiler tube failures
Conclusion
5. It is known fact that when coal is burnt releases heat energy. The same
phenomenon when chemically represented.
C + O2 = CO2 + Heat Energy (395 KJ/Mole)
Energy Transfer: -
In the boiler chemical energy of fuel is converted into thermal energy by
heating water and converting it into steam. The steam produced in the
boiler is expanded in the turbine where thermal energy is converted to
kinetic energy.
As the steam expands it rotates the turbine this motion of the
turbine is transmitted to generator in which mechanical energy is
converted into electrical energy, which is transmitted to various load
centers through transmission lines .
6. The basic theoretical working cycle of a steam power plant on which
it works is “ RANKINE CYCLE” the modern steam power plant uses
modified Rankin cycle which includes reheating, super heating and
regenerative feed water heating.
RANKINE CYCLE:-
7. Chemical Energy BOILER Thermal Energy TURBINE
Kinetic Energy GENERATOR Electrical Energy
8. Boiler is the main part in the power generation.
Boiler is a sealed vessel in which water is converted into
steam.
Boiler acts as a medium in which water is converted into
steam by using the heat released in the process of
combustion of coal in the presence of oxygen.
Before combustion process the coal is collected from
bunkers into millers.
This coal is converted as powder (pulverized) in the millers
During this process if their is any wet coal ,it is converted
into dry coal.
And finally It sent to the boiler by using the air.
9. BOILER DRUM
It is the main part of the boiler, in which steam and
water separates.
Inside the boiler drum, saturated steam is separated from
water and then directed in to super heating tubes for further
temperature increase.
Water separated from steam will combine with the incoming
boiler feed water and then re-enter the down comers to
repeat the cycle.
Thus the boiler drum acts as a steam separator.
10. DOWNCOMERS:-
There are six down comers in (500 MW) which carry water
from boiler drum to the ring header.
They are installed from outside the furnace to keep density
difference for natural circulation of water & steam.
DRUM INTERNALS:-
PRIMARY SEPERATORS – Consists of baffle arrangement
devices which change the direction of flow of steam and
water mixture
SECONDARY SEPERATORS- Separators employing spinning
action
11. Almost all modern power boilers are equipped with water walls .
In large boilers water walls completely cover the interior surfaces of
the furnace.
These tubes receive water from the boiler drum by means of down
comers connected between drum and water walls lower headers.
In a boiler the water walls absorbed approximate it 50%of heat
released by the combustion of the fuel in the furnace.
Heat so absorbed by the water walls used in the evaporation of
water supplied to boiler.
12. The purpose of economizer is to preheat the boiler feed
water before it is introduced into the steam drum by
recovering heat from the flue gases leaving the boiler.
Feed water after heated goes in to the boiler drum.
The economizer is used for improving the efficiency of the
boiler and it is the one of the important for the accessories
for the boiler.
ADVANTAGES OF ECONOMISER:
220 C reduction in flue gas temperature increases boiler
efficiency by 1%.
6oC raise in feed water temperature, by economizers
corresponds to 1% saving in fuel consumption .
13. A super heater is a device which heats steam to high
temperatures.
A super heater consists of group of tubes made of special
alloys such as chromium –molybdenum.
The wet steam from the boiler drum enters in to the super
heater and gets heated up to a temperature of 535 deg cen
and pressure of 170kg/sq cm by gaining heat from the flue
gases leaving the furnace .
The steam entering the super heater and then the
secondary super heater to the tertiary super heater.
14. RE-HEATER:
The function of re-heater is to increase the
temperature of the steam leaving the HPT again to
540 deg C and the pressure up to the 42kg/cm2.
BURNERS:
500 MW boiler is a tangential corner fired boiler having
32 burner assemblies i.e. 8 burner assemblies at 4
different elevations.
16. Overview:
Boiler tube failures are the leading cause of forced outages
of fossil-fired power plants.
Boiler tubes are subjected to various failure mechanisms
driven by temperature, stress and environmental
hazard, depending on their designs and functions.
Super heater tubes, re heater tubes & furnace water wall
tubes always experienced high temperature ductile failures
such as stress-rupture failure, initiated by creep process or
sudden overheating due to steam starvation or flue gas
disruption.
Suspended tubes with a lack of support or exposed to
vibration / cyclic loading would bear a risk of mechanical
fatigue.
17. Major reasons contributing for boiler tube
failures are :
Short Term Overheating
Long term overheating
Dissimilar Metal Welds
Hydrogen Damage
Pitting
Falling Slag Erosion
Soot Blower
Material Defects
Welding Defects
18. Short term overheating in water cooled tubes occurs because of
abnormal coolant flow are excessive combustion gas temperature.
As a result, the tubes are subjected to excessively high
temperature often hundreds of degrees which results in rapid
failure.
A considerable increase (> 5%) in the inside or outside diameter of
affected tubes.
Once flow conditions are interrupted as described above the normal
cooling effects of the water no longer occur and tube metal
temperature rises rapidly.
MAIN CAUSES:
Partial blockage caused by maintenance
activities.
Poor control of drum level.
CORRECTIVE ACTIONS:
The deposits can be blown off which is very time consuming, or
The unit can be chemically cleaned.
19. Long term overheating can caused by deposits attached on inner
surface of tubes.
Long term overheating can occurs in all of pressure part in steam
boiler such as water wall tubes, header, super heater, re heater,
economizer, steam drum, water drum
Almost 90% failure can occurs in super heater
Definition of long term overheating is a condition in which pressure
parts have metal temperature higher than temperature design in
long term
CORRECTIVE ACTIONS:
Temporary pad welds should not be used because of the
uncertainty associated with base metal condition.
Tube shielding.
20. As the elevation of the boiler is very much higher we
have to join the different tubes to that much of higher
elevation according to the temperatures inside the
boiler.
Quality control needs to be applied to assure sound
weld.
Results from application of high temperature and
stress beyond design value
CORRECTIVE ACTIONS:
Replacement
Frequent inspection.
21. Hydrogen damage in boiler tubes is caused by a corrosive reaction
between steam and steel
Shown below:
Fe +H2o =Fe304 + H2
The hydrogen that is released reacts with carbides to decarburize the
steel and forms methane gas at the boundaries.
Results in wall loss due to corrosion and decreases the strength in
material attacked by hydrogen.
The damage usually occurs in areas of high heat flux and flow
disturbances includes tubes opposite to burners and tube bends
Corrective Action:
Control Boiler Water Chemistry
Check corrosion product ingress
Chemical cleaning
Replace affected tubes
22. The formation of small pits in a surface as a consequence of
corrosion.
Pitting corrosion may takes place in the environment of
stagnant, oxygenated water formed during shutdown.
Most familiar form of oxygen attack in boiler and
condensate system.
Dissolved oxygen is essential to corrosion of copper alloy
tubes.
At higher temperatures oxygen weakens steel.
CORRECTIVE ACTIONS:
If D.O in feed water is >10ppb,its source needs to be identified and
arrested may be mechanically or chemically.
Proper operation of deareator,L.P.Feed water Heater.
23. Typical Locations:
Lower furnace sloping wall near bottom opening
Probable Root Cause
Slagging of coal and massive clinker fall
Coal properties and boiler design
Corrective Action
Change in fuel
24. A soot blower is a system for removing the soot that is
deposited on the furnace tubes of a blower during combustion.
Soot deposited on the heating surfaces of a boiler acts as a heat
insulator. The result is that less heat is transferred to the water
to raise steam and more heat is wasted up the chimney. This
leads to higher fuel consumption and/or poor steaming.
Various types of soot blowers such as wall blowers, long
retractable blowers and air heater blowers are used for
cleaning.
Steam is normally used as a medium for blowing away the soot.
PROBLEMS CAUSED BY SOOT
Reduce efficiency
25. FLY ASH EROSSION:-
UNIT NO: 7
Location: Z- Panel Corner 1 Tube No. 1
Failure Date: 30.01.2012
Reason of failure: Falling slag erosion.
Action: All the 7 tubes replaced. Radiography taken
found OK.
Action proposed: Plastic refractory will be provided in this
location during next overhaul.
Unit-VII
11.10.2011
BACK
Unit-VII30.01.2012
BACK
26. Long term over heating:
UNIT NO: 5
Location: Final Re heater Coil No. 17 Tube No. 19
.Failure Date: 31.01.2012
Reason of failure: Long term overheating.
Action: All tubes replaced. Total no. of joints 14
radiography done and cleared. Vacuum pulling
done and found OK.
Action proposed: Oxide scale will be measured in this area in next
overhaul.
Other action plan:
will be decided based on R&D report.
BACK
29mtr.Elevation RHBank-I Coil 56
T12shop weldIPWjoint
circumferentialcrack
27. MECHANICAL RUBBING:-
UNIT NO: 5
Location: Spacer Tube of Final Super heater near Coil No. 21
Reason of failure: Erosion due to mechanical rubbing of Super
heater cleat with spacer tube.
Failure Date: 13.12.2011
Action taken: All the three tubes replaced.
Total number of joints 6. All joints radiography
done and cleared.
Action proposed: Spacer tube locking and
alignment to be ensured in all overhauls.
Unit-V28.10.2011
Unit-VI03.04.2011
& 31.08.2011
BACK
BACK
28. The boiler efficiency for stage(3) in NTPC
Ramagundam is 87.145 % with out any “FAILURES” in
boiler tubes.
If there was any failures in boiler tube then
its efficiency will be lower than required one.
Daily NTPC Ramagundam supplies 62.4MU
If Stage(3) overhaul for few hours.
i.e24hrs then power loss will be 12MU
In order to overcome the efficiency and power
generation. Boiler tubes will be in good condition .
To check over them vacuum pulling test and
radiography test will be done
29. RADIOGRAPHY TEST:-
It is a NDT Method.
1. It is a simplest weld examination technique.
2. It passes Gamma radiation through the component
using photographic means.
3. Radiation is produced by using Radioactive source.
4. Presently used Isotope- Iridium-192/ Cobalt-60.
Thus we can identify the failures in the water tubes by
using the radiography test
30. Boiler tube failures therefore form a very important
component of a plant outage.
Following are the best practices for boiler tube failure
reduction:-
Intensive visual examination and NDT inspection during unit overhauls
Root cause analysis of every BTF and adherence to action plan
Learning more from another NTPC plant experiences
Monitoring operational parameters like metal temperature excursion and
water chemistry through PI server
workmen for quality of work by involving them in root cause analysis
Formation of area specific team consisting of FQA and BM-PP for
inspection and execution of boiler haul works
