case study 8 maintenance of boilers[1]

8/3/2019 Case Study 8 Maintenance of Boilers[1]

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Operation & Maintenance

of Boilers


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OPERATION, MAINTENANCEOF BOILERS


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Would like to touch upon someimportant factors

Operation Tuning combustion is the most important

aspect

Maintenance Preventive Predictive

RLA

Care of boilers

Performance monitoring Trouble shooting Reducing tube failures


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General Boiler Operation

Design & power production have becomesophisticated

Basic operating principles still apply

Combustion safety & proper steam /water cooling are essential Before firing no lingering combustible material

Purge is essential 25% of max air flow

Once combustion established air/fuel ratio tuning

Other parameters to be checked & maintained

General safety considerations Pressure parts failure remain major concern

CO2 NOx &SOx are to be optimised


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Boiler Operation

Pre-commissioning operation

Initial start-up operation first Commissioning operation

Alkali boil out

Chemical cleaning

Steam boiling

Safety valve floating

Trial runs Stabilization operation Regular operation

Cold start-up

Warm start-up

Hot start-up

Shutting down Normal

Emergency

Operation for guarantee run


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Boiler Operation (Cont)

Marginal difference in operationbased on type of boiler Natural circulation

Once through Fluidized bed

Bubbling bed

Circulating

Pressurized

Chemical recovery

Stoker fired


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Key operation functions

Burner adjustments Fuel air ratio

Carbon burn out

Pollutant optimization

Excess air adjustments Fuel conditions

Pressure & temperature liquid fuel

Particle size & primary air

Feed water & boiler water conditioning Soot blower operation

Primary air to secondary air ratio

Wind-box settings


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Typical operator walk down check list

Look for valve & valve packing leak

Discoloration, hotspots on casing / ductwork or vapor leak

Open all inspection door & check for slag accumulation

Listen for tube leak

Check for unusual noises, overheating & adequatelubrication on all motors

Look for leaks in gage glasses & water columns

Check that no soot blowers are stuck & no leak

Tilting tangential for uniformity in all corners

Check secondary air damper setting At firing floor check for fuel leak coal, oil & gas

Check all vertical coal pipe for plugging or over heating

Inspect ash pit for bridging

Check level of water in ash hopper& bottom seal trough


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Typical operator walk down check list(Cont)

At the pulverizers check for Gear case oil temp, flow & temperature

Excessive spillage or malfunction of pyrite system

Indications for mill fire

Unusual noise

Coal leaks

At air-heaters check Soot blower leakage

Drive motor, support & guide bearing lubrication &cooling water

Cleanliness of air side through observation port


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COMBUSTION AIR REGIMETUNING


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COMBUSTION AIR REGIME TUNINGHINTS

Indian high ash coals result in highprimary air requirements -primaryCombustion Dilution

Sec. Air distribution at requiredelevation is very important

Avoid / reduce all unwanted sec. Air at

any location And divert them to other needy

elevation.


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COMBUSTION AIR REGIME TUNINGACTION

Keep mill air flow just above settling velocity.

Do pitot traverse to check primary air flow

Keep reducing primary air - settling start

Slight furnace disturbance

Increase by primary air 1-2 t/hr

Keep total air flow - 20% excess air @ eco out

Close all fuel air dampers if VM less than 20 - 22% Lookflame front - decide for higher VM coal

Keep wind box pr. 100 - 150 mm - better distributionAcross elevation.


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COMBUSTION AIR REGIME TUNINGACTION - Cont

Wall Blower Optimisation

Change in SH spray without change inother parameters indicates furnacedeposit increase

SH spray increase above a particularlevel (to be determined for each boiler)operate wall blowers


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COMBUSTION AIR REGIME TUNINGOTHER TIPS

Check % VM in Coal by Hcl Leaching Once ina Month If VM > 20% and Flame Front AwayFrom Nozzle

FC/VM Ratio Vs % FA Comb. Is Very Good for500 Mw Unit - Establish This for Your Plant

+50 Retention for Bottom Ash


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COMBUSTION AIR REGIME TUNINGOTHER TIPS Cont

Wind Box Damper Setting for Completion ofCombustion and Bottom Ash Collection

Increase LRSB Operation for Fouling Type

Ash in Coal And Exit Gas Temp. Control Change in SH Spray Pattern Due to Change

in Radiation Heat Transfer Because ofChange Combustion Completion Elevation.

Check PC Analysis on Rieve Distribution.Chart / Roslin-Rammler Chart


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Energy Conservation


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Energy Economic DecesionMaking

Life Cycle Costing

Using the Payback Period Method

Using the Life Cycle Costing

The Time Value of Money

Investment Decesion Making

Making Decession for Alternate Investments

Depreciation, Taxes & Tax Credit

Computer Analysis


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boiler T

482.8 Mkcal/hr 420 Mkcal/hr 180.6 Mkcal/hr 172 Mkcal/hr

200 MW

120.7 t/h

4000 kcal/kg

87.0 %

37.4%

35.6 %

43.0 %2000 kcal/kwhr

2299 kcal/kwhr

2414 kcal/kwhr

auxpower 10 MW

PLANT EFFICIENCY & HEAT RATE

210


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Improved Availability/ Reliability

Reducing boiler outage occurrencespressure part failures & other causes(increasing MTBF)

Reducing forced outage duration - onaccount of boiler pressure partfailures + other causes (reducingMTTR)

Reducing occurrences of partial outage -(restricted loading) of boilers

Reducing the duration of operation underpartial outage mode

E C ti M i


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Energy Conservation Measures in

Boilers Improved Boiler Efficiency by Exit Gas

Temperature Reduction

Introduction of Variable Speed Drive Motors

SCAPH in By-Pass Duct of FD Fan Air Duct Low Excess Air Burners for Coal, Oil & Gas

Higher Pressure Cycle for Power Generation

On Line Water Chemistry Analyzers toReduce Continuous Blow Down

Micro Processor Controls to EnsureConsistency in Operating Parameter


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Energy Conservation Measures inBoilers (Cont..)

Two Out of Three Logic for ReducingSpurious Trips

Measures to Reduce Tube Failures

Acoustic Tube Leak Detection System toReduce MTTR

Retrofits for Availability & PLFImprovements

Intelligent Soot Blowing

Boiler Performance Optimization On-Line(Under Trial &error)


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High Energy Arc Igniters in Place of LDOIgniters

FBC & CFBC Technology for High Ash

Indian Coals & Difficult to Burn Fuels Combined Cycles & Co-generation Plants

Integrated Coal Gassification Combined

Cycle Plant ( Under Trial ) Once Through Super Critical Boiler for

Large Power Plants


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For Old and Aged Boilers Improvements in Availability/ Reliability

Lost Capacity Restoration

Predictive Maintenance of Pressure Parts Residual Life Assessment of Pressure Parts

Thermal Performance Testing & Evaluation

Tuning of Combustion & Optimization

State-of-the-art Up-Grades Micro Processor Controls

Introduction Steam Traps Where ever Possible


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WHERE RLA FIGURES IN

LIFE EXTENSION PROGRAMME (LEP)

PLANT CONDITION PERFORMANCE R L ADATA COLLECTION EVALUATION

PROBLEM AREAIDENTIFICATION

VARIOUS PROPOSALSFOR LIFE EXTENSION

F DONE IN PHASEDE FULL OUTEDBA

CK

PERFORMANCE EVALUATION


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Why RLA Study Ageing of boilers consequent to creep /

fatigue stresses in pressure parts of boiler

Material degradation

due to corrosion,erosion and oxidation


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Why RLA Study (Cont)

Operation of boiler at elevatedtemperatures more than design limit

Variations in coal quality

Deviations in Water Chemistry


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Scanning Electron Micro Scope


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Creep Cavity Level ClassificationSystem Proposed by Wedel & Neubauer


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Run Repair Replace

Evaluates For All Phases

RLA


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Trouble Shooting

Studying the problem, analyzing theproblem, finding the root cause of

the problem, taking the corrective &preventive action and takingpreventive to avoid recurrences infuture by pro - active action.


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Boiler Tube Failures

Boiler Tube Failures - main cause offorced outages in electric utility steamgenerating boilers

Single tube Failure in a 500 MW Rs. 5to 6 Crores (replacement powercharges for 3-4 days to repair) besides

affecting Plant Morale.


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Boiler Tube Failures (22 Primary Mechanisms)Stress Rupture Short Term Overheating

High Temperature Creep

Dissimilar Metal Welds

Fatigue Vibration

Thermal

Corrosion

Water-side Corrosion

Caustic Corrosion Hydrogen Damage

Pitting

Stress Corrosion Cracking

Erosion

Fly Ash Falling Slag

Soot Blower

Coal Particle

Fire-side Corrosion Low Temperature

Waterwall - Coal Ash - Oil Ash

Lack of Quality Control Maintenance cleanin

damage Chemical Excursion damage

Material Defects

Welding Defects - indicates that such problems have not been reported in India


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LONG TERM OVERHEATINGOVERHEATING, CREEPINCORRECT MATERIAL

OVERHEATING BULGING, SATELLITE SCALE CRACKINGOVERHEATING WATERSIDE DEPOSITS


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Tube Failures (Areawise)

Ceiling SH

1%

Div.Panel

2%

Reheater

9%

Wall RH

2%

Waterwall

33%

Economiser

16%

Platen SH

6%

LTSH

16%

SCW

15%

Tube Failures (Causewise)

Overheating

9%

Pitting

3%

Steam

Erosion

6%

Shop Weld

6%

Site Weld

16%

Matl. mix-up

3%

P.F.Erosion

4%

Mech.Rubbing

5%

Ash Erosion

28%

Attach.Weld

20 %

Based on NTPC 500 MW boiler tube failure data


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Design Improvements for ReducedTube Failure

Lower flue gas velocity over tube banks

Plain tube in-line arrangement of heat

transfer surface Optimum end caps to avoid preferential gas

flow

Erosion shields / cassette baffles Erosion allowance for leading tubes


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Design Improvements for ReducedTube Failure (Cont)

Higher flexibility in SH / RH nipples

Redesigned flexible connectors for pendanttype SH coils

Improved supports for LTSH / Eco. Coils

Improved seal plate connection for bottomhopper

Modified LTSH inlet tube connection


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Impact Of Coals On TubeFailures

&

Design Improvements for

Tube Failure Reduction


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DETERIORATION OF COAL QUALITY AVAILABLE FORPOWER GENERATION OVER THE PERIOD

1970s 1989s 1990s

PROXIMATE ANALYSIS

FIXED CARBON % 36.5 32.4 25.0VOLATILE MATTER % 25.5 21.6 18.0

MOISTURE % 10.0 16.0 12.0

ASH % 28.0 30.0 45.0

HHV kcal/kg 4750 4050 3000HGI 50 50 50


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COAL / ASH HANDLED

HHV kcal/kg 4750 4050 3000

UNIT RATING mw 210 500 210 500 210 500

ASH % 28 28 30 30 45 45

FUEL FIRED t/h 110 272 129 319 174 430

ASH PRODUCED t/h 30.8 76.2 38.7 95.7 78.3 193.5


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INDIAN COAL Vs USA COAL

PARAMETER INDIAN UNIT USA UNIT

210 MW 500 MW 210 MW 500MWHEAT DUTY mcal/h. 454 1070 454 1070

FUEL FIRED mkcal/h. 520 1215 520 1215

HHV kcal/kg. 3800 3800 6000 6000

QUANTITY OF FUEL t/h. 137 320 87 202.5

AVERAGE ASH CONTENT % 40 40 8 8

QUANITY OF ASH t/h. 55 128 7 16.2

Problems Associated With


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Problems Associated WithCoals

Deteriorating heating value of the coal

Inconsistent coal properties

Presence of extraneous matters in coal

High quantum of ash with high percentageof quartz

Highly abrasive nature of coal ash

Due to low sulphur content extremely highelectrical resistivity of ash

C S d


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Case Study

670 T/hr boiler, natural circulation and with tangential firingsystem

The furnace size is 1386 x10592 mm

6 bowl mills, with five mills catering to full load of boiler

Designed for an excess air operation of 20%

The design coal proximate analysis.

Fixed carbon : 25.0%

Volatile matter : 20.0%

Moisture : 15.0%

Ash : 40.0%

Grindability index : 80 HGI

High heating value : 3200 kcal/kg


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Case Study (Cont) The Proximate Analysis, Ultimate Analysis & Coal Ash Analysis Of

The 6 Samples Of Coal Fired During Trials

TRIAL NUMBER 1 2 3 4 5 6 7

PROXIMATE ANNALYSIS

MOISTURE % 6.08 7.06 7.02 6.86 8.37 6.54 7.1

VOLATILE MATTER % 21.14 22.14 22.05 21.85 22.04 20.93 21.14

ASH % 46.7 41.35 41.78 41.87 40.58 46.13 44

FIXED CARBON % 26.08 29.47 29.15 29.42 29.01 26.4 27.76

HHV Kcal/Kg 3086 3402 3396 3367 3416 3098 3172

ULTIMATE ANNALYSIS

CARBON % 33.65 37.86 37.17 37.28 37.69 34.05 35.43

HYDROGEN % 2.23 2.55 2.53 2.52 2.51 2.3 2.46

SULPHUR % 0.51 0.5 0.55 0.5 0.5 0.54 0.58

NITROGEN % 0.95 0.89 1.05 0.9 0.95 1.02 1.07

OXYGEN % 5.01 5.05 5.52 5.39 4.89 4.64 4.76

ASH ANALYSIS

SiO2 % 61.8 61 62.1 59.6 60.8 61.4 60.8

Fe2O3 % 17.2 16.9 17.4 14 15.9 15.2 13.7TiO2 % 0.7 0.9 0.9 0.8 0.8 0.7 0.7

Al2O3 % 8.6 9 8.1 10.7 9.8 10 9.6

CaO % 7.6 7.9 7.5 10.6 8 7.8 9.7

MgO % 3.4 3.7 3.2 3.3 3.9 4.1 4.8

Na2O % 0.1 0.2 0.1 0.2 0.1 0.2 0.1

K2O % 0.3 0.3 0.2 0.4 0.3 0.3 0.3


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Case Study (Cont)

The Flame Temperatures Taken During All The Six Trials

ELVATION TRIAL 1 TRIAL 2 TRIAL 3 TRIAL 4 TRIAL 5 TRIAL 6 TRIAL 7

METER TEMPERATUR IN DEGREE CENTIGRADE

14.2 800 815 850 780 813 823 825 NOTE: THE TRIALS WERE CARRIED OUT IN

17.1 1066 1086 1108 1073 1076 1054 1009 THE 670 T/Hr BOILER AT LOADS THAT

20.4 1120 1110 1150 1150 1175 1155 1070 WAS TO BE MAINTAINED BY THE TURBIN

23.2 1217 1213 1211 1263 1250 1225 1140

26 1180 1240 1210 1291 1275 1255 1215

29 1246 1254 1267 1291 1250 1235 1275

32.4 1211 1261 1251 1266 1232 1220 1282

35.7 1192 1243 1229 1212 1181 1195 1246

41.9 1172 1163 1148 1100 1117 1115 1210

44.5 1170 1150 1125 1025 1065 1055


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Case Study (Cont)

The Details Of The Winbox Auxiliary Air Dampers SettingFF FF AT 10% OPEN FF OPEN AT 80%

F

EF ALL FUEL AIR ALL FUEL AIR ALL FUEL AIR ALL FUEL AIR

E DAMPER AT 5% OPEN DAMPER AT 5% OPEN DAMPER AT 5% OPEN DAMPER AT 5% OPEN

DE

D ALL AUX. AIR DAMPER

CD AT 30% OPEN AUX. AIR DAMPER AUX. AIR DAMPER AUX. AIR DAMPER

C GRADUALY REDUCED GRADUALLY OPENED GRADUALLY OPENEDBC FROM 60% AT AA FROM 10% AT AA FROM 10 % AT AA

B TO 12% AT FF TO 50% AT FF TO 50% AT EF

AB

A AA AT 60% OPEN

AA

TRIALS TRIAL 1 TRIAL 2 TRIAL 3 & TRIAL 4 TRIAL 5 & TRIAL 6

MILS IN A B D E F A B D E F A B D E F ( 3 ) B C D E F (5)

OPERATION A B C D E ( 4 ) A B C E F (6)

BOILER LOAD 496 T/Hr 496 T/Hr 480 T/Hr & 596 T/Hr 608 T/Hr & 604 T/Hr

SH SPARY 70 T/Hr 75 T/Hr 85 T/Hr & 60 T/Hr 75 T/Hr & 77T/Hr

RH SPARY 21 T/Hr 21 T/Hr 21 T/Hr & NIL T/Hr NIL T/Hr


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Case Study (Cont)

The Flame Temperature Profile

PARAMETER IN THE X- AXIS IS FLAME TEMPERATURE IN DEGREE CELSIUS

PARAMETER IN THE Y- AXIS IS BOILER PEEP HOLE ELEVATION FROM WHERE THE TEMPERATURE IS MEASURED

FIGURE:4

TRIAL 1

0

10

20

30

40

50

0 500 1000 1500

TRIAL 2

0

10

20

30

40

50

0 500 1000 1500

TRIAL 4

0

10

20

30

40

50

0 500 1000 1500

TRIAL 5

0

10

20

30

40

50

0 500 1000 1500

TRIAL 6

0

10

20

30

40

50

0 500 1000 1500

0

10

20

30

40

50

0 500 1000 1500

TRIAL 3


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Conclusion The combustion behaviour of coals can very widely hence

combustion optimisationis a must for boilers It can be concluded from the flame temperature study thatfor low reactive coals the air distribution plays a veryimportant roll

The height at which the maximum quantity of the

hydrocarbon to be burnt will depend upon the reactivity,the petrographic characteristic and the burning profile ofthe coal being fired

Understanding the type of coal being fired andcorrespondingly making proper operational adjustments /modification will help in combustion optimisation and

reduction of unburned carbon in bottom ash / fly ash. The high ash coals are found to give large variation in

properties due to its virtue of formation which also affectsthe carbon loss in the boiler and needs a regular watchand tuning of the operational regime


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Boiler Losses Operator ControllableEfficiency

Dry gas Loss

Excess air

Exit gas temperature

Air ingress

Fouling

Tempering air

Tramp air

Carbon loss

Excess air

Air regime

Mill fine ness

Factors Affecting Dry Gas Loss


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1 Coal

Moisture

Carbon

Gross CV

2 Air temperature entering AH

Ambient

SCAPH

3 Gas temperature

AH leakage

AH entering air temperature

AH entering gas temperature

Boiler load

FW temperature

X ratio of AH

Tempering airAir ingress

4 Gas quantity

Excess air

AH leakage


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The various factors (coal property, design,

operating condition) that influence carbon loss

Coal rank and quality Coal Petrographic characteristics Characteristics and quantum of carbonaceous shale Presence of low melting inorganic in coal ash Residence time available for combustion in furnace

Type of burners and numbers Type of milling system and primary air control system Fineness of pulverised coal - Percentage of coarser particles Primary air to secondary air ratios Excess air at the burner/furnace and distribution of air into the

burner/furnace Burner Tilt position

case study 8 maintenance of boilers[1]

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