Cc Power Plants

7/31/2019 Cc Power Plants

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COMBINED CYCLE POWER

PLANTS

IMPORATANT ASPECTS

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COMBINED CYCLE POWER

PLANTS

IMPORATANT ASPECTS

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HEAT RECOVERY STEAM

GENERATORS (HRSG) ARE BOILERS WHICH USE THE HEAT ENERGY OF

EXHAUST GAS OF GAS TURBINE TO GENERATESTEAM WHICH CAN BE USED IN THE PROCESSPLANT OR STEAM TURBINE PLANT TO GENERATE

ADDITIONAL POWER. GAS TURBINE COUPLED WITH HRSG FOR STEAM

GENERATION IS CALLED CO-GENERATION PLANT.

THEY CAN HAVE FACILITY FOR AUXILIARY FUELFIRING TO (1) ENSURE FULL GENERATION OFSTEAM EVEN WHEN GT IS ON PART LOAD, OR (2)TO ENSURE STEAM GENERATION EVEN WHEN GTIS SHUT DOWN.

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GASTURBINECOMP

CC

DIFFUSER

ST

ACK

HR

SG

INTERFACE OF GT WITH HRSG

GUILOTINE DAMPER

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COMBINED CYCLE EFFICIENCY CC

= (NET GT WORK + NET ST WORK) / (HEAT INPUT IN GT)

OPEN CYCLE GT EFFICIENCY IS LOW. WITH HIGH COSTOF FUEL, OPEN CYCLE MODE FOR GT OPERATION ISUNACCEPTABLE.

GT EXHAUST TEMPERATURE IN ADVANCE CLASS GTSUCH AS FRAME 9FA IS IN EXCESS OF 600C HENCE ISNEVER RUN IN OPEN CYCLE MODE. THIS, PERHAPS, IS THEREASON THAT AT COMBINED CYCLE PLANT SUCH AS AT

VEMAGIRI WE DO NOT HAVE BYPASS STACK HIGH GT EXHAUST TEMPERATURE PUTS VERY SEVEREOPERATING CONDITIONS ON HRSG. OUTAGE OF HRSGTHEREFORE FORCES SHUT-DOWN OF THE GT.

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M

AIN

STACK

HPSH HPEVA HPEC LPSH LPEV LPEC

TYPICAL HORIZONTAL

CONFIGURATION

INLET DUCT

PERFORATED PLATE

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TURNINGVANES

SH

EVA

VERTICAL CONFIGURATION OF HRSG

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INDICATOR OF EFFICIENCY OFSTEAM GENERATION

STACK TEMPERATURE

FOR SULFUR BEARING FUELS DESIGN STACK

TEMPERATURE IS 140-160 C DEPENDING UP ON SULFUR

CONTENT AND ACID DEW POINT TEMPERATURE (100 120

C)

FOR LOW SULFUR FUELS DESIGN STACK TEMPERATURE

CAN BE 120 130 C AND FOR VERY CLEAN FUELS SUCH AS

NATURAL GAS, IT CAN BE AS LOW AS 100 110 C.

ACTUAL STACK TEMPERATURE VERY NEAR DESIGNTEMPERATURE INDICATES VERY STRONG POSSIBLITY OF

EXCELLENT HEAT TRANSFER IN THE HEAT EXCHANGER

TRAINS IN THE FLUE GAS PATH.

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IMPORTANT CONSTRUCTIONAL

ISSUES FROM LONGETIVITY POINT

OF VIEW

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INSULATION BOTH INLET DUCT AS WELL AS BOILER CASING

REQUIRES INSULATION AS THE GASTEMPERATURES ARE VERY HIGH ( OF THE ORDEROF 600+C)

THE INSULATION CAN BE INTERNAL OREXTERNAL

FOR FLUE GAS TEMPERAURE > 570C, USE OFEXTERNAL INSULATION WILL REQUIRE USE OFAUSTENITIC STEELS WHICH ARE QUITEEXPENSIVE AND ALSO DIFFICULT TO WELD. THEYARE ALSO LIABLE TO STRESS RELIEF CRACKING.

ALSO ALLUMINIUM CLADDING IS REQUIRED TOMINIMIZE HEAT LOSS AS WELL AS FORPROTECTION FROM WEATHER EFFECTS.APPARENTLY EXTERNAL INSULATION IS MOREPROBLEMATIC.

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INSULATION .. DURING ITS PASSAGE, THE FLUE GAS

COOLS DOWN. THUS IN EXTERNALLYINSULATED HRSG, CHEAPER ALLOYSTEELS CAN BE USED IN LOWTEMPERATURE ZONE. HOWEVER THISNECESSIATES USE OF EXPANSION JOINT

TO TAKE CARE OF DIFFERENTIALCOEFFICIENTS OF EXPANSION EXPANSION JOINTS PROVIDED FOR ABOVE

MENTIONED PURPOSE AT TIMES, IS ASOURCE OF SEVERAL COMPLEX

PROBLEMS PERSONALLY RECOMMEND INTERNAL

INSULATION ONLY.

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INTERNAL INSULATION

USE OF CHEAPER MATERIAL OF CONSTUCTION OF INLET DUCT ANDBOILER CASING

NO NECESSITY OF LARGE FLEXIBLE JOINTS IN BOILER CASING AS AREREQUIRED FOR EXTERNAL INSULATION

TUBE LEAKS IN EVAPORATOR CAN CAUSE IMPINGEMENT OF HIGHVELOCITY JETS ON ROOF INSULATION.

GAPS IN INSULATION CAN CAUSE HOT SPOTS AND DISTORTIONS

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LAYOUT PLAYS A VERY IMPORTANT ROLE FROM FLOW

DISTRIBUTION POINT OF VIEW.

IN CASE OF CONSTRAINTS IN THE PLOT SIZE, THEGT EXHAUST CAN BE AT RIGHT ANGLE TO THEDIFFUSER. CONSEQUENTLY FLOW CORRECTORSSUCH AS TURNING VANES ARE REQUIRED TO BEINSTALLED. ANY ERRORS IN DESIGN & ERECTION

OF THESE CAN CAUSE SEVERE DISTORTION OFBOILER DUCT.

IN CASE IT IS NOT POSSIBLE TO MAKE DIFFUSERAND THE BOILER INLET DUCT CO-LINEAR, FLOWSTABILIZERS SUCH AS PERFORATED PLATE OR

TURNING VANES MUST BE PROVIDED. A NON-UNIFORM FLOW DISTRIBUTION WILL OFTEN

CAUSE STRUCTURAL DEFORMATION ANDFAILURES OF BOILER CASING AND ALSOPRESSURE PARTS. THIS GREATLY REDUCESEFFECTIVE LIFE OF HRSG.

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IDEAL LAYOUT

DIFFUSER

BOILERINLETDUCT

DAMPER

BYEPASS STACK

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OFFSET LAYOUT RESULTS IN DISTORTION OF INLET DUCT

DIFFUSER

BOILERINLETDUCT

DAMPER

BYEPASS STACK

Hotter top

Cold bottom

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HOT TOP SIDE

COLD BOTTOM SIDE

FAILURES OF JOINT

EFFECT OF OFFSET IS TO CAUSE DUCT TODISTORT AND ALSO MAY CAUSE FAILURE OF THEJOINT

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HOT TOP SIDE

COLD BOTTOM SIDE

FAILURES OF JOINT

AUSTENITICSTEEL

ALLOY STEEL

EXPANSIONJOINT

CASE STUDY : OFFSETBOILER INLET DUCTAND USE OF

DIFFERENT MATERIALSFOR BOILER CASING

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OFFSET LAYOUT CORRECTION BY TURNING VANE

DIFFUSER

BOILERINLETDUCT

DAMPER

BYEPASS STACK

TURNING VANE

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OFFSET LAYOUT CORRECTION BY PERFORATED PLATE

DIFFUSERBOILER

INLETDUCT

DAMPER

BYEPASS STACK

TURNING VANE

PERFORATEDPLATE

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USE OF TURNING VANESTO STREAMLINE THEFLOW

HRSG DUCT

TURNING VANES

APPROACHDUCT

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THERMO-MECHANICALFACTORS AFFECTING LIFE

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PRIME CONSIDERATIONS IN LARGEHRSG BEHIND LARGE GT

HIGH EFFICIENCY

HIGH RELIABILITY

HIGH DURABILITY

LOWEST INSTALLED COST

LOW LIFE TIME COST

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REQUIREMENT OF HIGHEFFICIENCY HAS RESULTED INTO

HIGHER FIRING TEMPERATURE IN GASTURBINE

VERY LARGE MASS FLOW RATE OF FLUE

GASES ENTERING HRSG VERY HIGH TEMPERATURE OF EXHAUST

GASES ENTERING HRSG. FOR VEMAGIRIPROJECT, IT IS + 600C.

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COMBINED CYCLE PLANTS AREREQUIRED TO MEET

VERY AGGRESSIVE START UPDURATION

VERY STEEP LOAD RAMP RATESPROMISED BY GT MANUFACTURER

VERY RAPID AND LARGE CHANGES INTHE GT EXHAUST TEMPERATURE

VERY HIGH RELIABILITY ANDAVAILABILITY OF HRSG

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GASTURBINECOMP

CC

DIFFUSER

ST

ACK

HR

SG

GUILOTINE DAMPER

STEAM TO STG

TIME

TEMPERATURE

~18 min.11 min

422C

60C DROP 155 MIN

GT SYNCRONIZED

COLD START GT EXHAUST TEMP PROFILE. THE HRSGPARTS WILL EXPERIENCE THIS IN EVERY COLD START

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DURING THIS TRANSIENT TUBES IN THE FLOW PATH GET HEATED UP AT VERY HIGH RATE.

THE HEADERS BEING OF VERY HEAVY CROSS-SECTION WILL BE AT

MUCH LOWER TEMPERTURE AND WILL GET STRESSED BY TUBESWHICH ARE GETTING HEATED.

THE HEADERS BEING VERY HEAVY MASSES WILL NOT ALLOWTUBES TO EXPAND FREELY AND THUS THE TUBES ALSO GETSTRESSED.

AS TIME PROGRESSES, THE TEMPERATURE WILL BECOME MORE

UNIFORM AND HENCE THERMAL STRESSES WILL REDUCE. TILL NEXT TRIP OF THE SET, THE STRESS LEVELS WILL REMAIN

UNIFORM. WHEN THE SET TRIPS, TUBES WILL GET COOLED MUCHFASTER WHILE THE HEADER WILL TAKE LONGER TIME TO COOL.THIS WILL CAUSE REVERSAL OF STRESSES. AFTER LONG TIMETHESE WILL ALSO GET SMOOTHENED OUT.

IT IS CLEAR THAT EACH COLD START IS ASSOCIATED WITHALTERNATING STRESSES DURING START UP AND SHUT DOWN.THIS CAUSES WHAT IS CALLED LOW CYCLE FATIGUE.

SIMILAR EFFECTS ARE SEEN IN BOTH WARM AND HOT START. ALARGE NUMBER OF STARTS IS THEREFORE VERY DAMAGING.

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THE CONSEQUENCES OFSUCH TRANSIENTS ARE

RAPID CONSUMPTION OF LIFE BYTHERMAL FATIGUE ADDED TO HIGHTEMPERATURE CREEP LIFE CONSUMPTION

IT IS NECESSARY TO KNOW THE ENTIREMECHANISM OF THESE FAILUREPROCESSES. THIS REQUIRES THAT WECLEARLY UNDERSTAND HEAT TRANSFER

PROCESS IN HRSG

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CONCLUSIONS HRSG IS A CRITICAL CONSTITUENT OF COMBINED CYCLE

POWER PLANTS SINCE ITS OUTAGE MEANS ATREMENDOUS DROP IN THE HEAT RATE OF THE OVERALLPLANT AND GENERATION COST BECOME PROHIBITIVLYHIGH

IT IS NECESSARY TO GIVE TOP PRIORITY TO LAY OUTRELATED ISSUES. IN CASE IT IS NOT POSSIBLE TO KEEP GT

AND HRSG CO-LINEAR, FLOW STRAIGHTENING DEVICESMUST BE INSTALLED.

MAJORITY OF THE PROBLEMS SUCH AS BUCKLING OFDUCT/BOILER CASING ARE DUE TO FLOW IMBALANCES.DEVICES SUCH AS TURNING VANES, PERFORATED PLATES

MUST BE USED TO REMOVE THESE IMBALANCES. HEAT TRANSFER AND TEMPERATURE DISTRIBUTION IN THE

FLUE GAS PATH ARE THE MOST IMPORTANT PARAMETERSTHAT DECIDE RELIABILITY AND AVAILABILITY OF HRSG

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GT EXH. TEMPTG

STACKTEMP T2

ECO IN t1

ECO OUT t2SAT. TEMP TS

SH OUT TSHPINCH POINT

APPROACH TEMP.

SUPPOSE 140000 lb/hr GAS AT TEMP.980F ENTERS HRSG GENERATINGSTEAM AT 200 psi. FEED WATER TEMP. IS 230 F , BLOW DOWN 5%.ASUME SP.HEAT OF GAS AT EVAPORATOR 0.27 AND AT ECONOMIZER0.253. LET US SIMULATE THERMAL DESIGN.

NORMALLY PINCH POINT AND APPROACH TEMP. IS IN THE RANGE15-30 F.LET US CHOOSE 20 FOR PINCH AND 15 FOR APPROACH.SATURATION TEMP. =388. THERFORE GAS TEMP LEAVINGEVAPORATOR = 388+20=408.TEMP OF WATER ENTERING EVA.=388-15=373 F.

ASSUMING 1% LOSS, EVAPORATORDUTY = 140000.0.99.0.27.(980-408)=21.4MMBTU/hr.ENTHALPY ABOSRBEDBY STEAM =(1199-345)+0.05.(362-345)

=855 BTU/lb. STEAM GENERATED =21.106/855 = 25000lb/hr. ECONOMIZERDUTY =25000.1.05.(345-198) =3.84.106BTU/hr.

GAS TEMPERATURE DROP = 3840000 /(140000.0.253.0.99) = 109F.TEMPERATURE OF GAS LEAVING THESTACK =408 -109 = 299 F ~ 150C.

WE THUS HAVE SIMULATED THEDESIGN.

KNOWING THE GAS TEMPERATUREPROFILE AND WATER/ STEAMPARAMETERS, WE CAN JUDGE THEPERFORMANCE OF HRSG.

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GT EXH. TEMPTG1

STACKTEMP Tg4

ECO IN tw1

ECO OUT tw2SAT. TEMP TS

SH OUT TSHPINCH POINT

APPROACH TEMP.

SUPPOSE 140000 lb/hr GAS AT TEMP.980F ENTERS HRSG GENERATINGSTEAM AT P. FEED WATER TEMP. IS 230 F. ASUME SP.HEAT OF GASAT EVAPORATOR 0.27 AND AT ECONOMIZER 0.253. LET US EVALUATETHE EXIT GAS TEMPERATURES FOR VARIOUS STEAM PRESSURES.

WgCPg(Tg1 Tg3) = WS(hsohw2) IN SH AND

EVAPORATOR ZONE. CONSIDERING NOW THEENTIRE HRSG

Tg3WgCPg(Tg1-Tg4) = WS(hso-hw1)

FROM THE ABOVE EQUATIONS WEOBTAIN (Tg1-Tg3) / (Tg1-Tg4) =

(hso-hw2) / (hso-hw1) = X. WE NOWEVLUATE EFFECT OF INCREASING

STEAM PRESSURE FROM 100 TO 600PSI ON STACK TEMPERAURE.

PRESS STEAM T SAT.T X EXIT T

100 SAT 338 0.904 300

250 SAT 406 0.833 313

400 SAT 448 0.790 353

400 600 448 0.800 367

600 SAT 490 0.740 373

600 750 490 0.773 398

THIS SHOWS CLEARLY THE REASONFOR MULTIPLE PRESSURE WHEN MAINSTEAM PRESSURE IS HIGH SO THATBEST HEAT EXTRACTION FROM FLUEGAS IS POSSIBLE

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A CASE STUDY

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HRSG BEHIND GE FRAME 6 GT

DESIGN EXHAUST TEMPERATURE 593C 136.5 KG/HR AIR

DESIGN STACK TEMPERATURE 160C

HP STEAM 97.6 BAR, 514C, 70.2 TPH

IP STEAM 15.8 BAR, 200.7C, 14 TPH

FORCED CIRCULATION RATED STEAM FLOW NOT AVAILABLE,

VERY HIGH ATTEMPERATION IN SH > 5.5 AGAINST 1.1 TPH

STEAMING IN IPECO DESPITE INCREASING IP FEED PRSSURE

HEAVY BUCKLING OF BOILER CASING ACTUAL STACK TEMPERATURE > 190C

PERFORMANCE DETERIORATING WITH PASSAGE OF TIME

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593CGAS TEMP PROFILE IN HRSG


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SH1 SH2 HPEV HPEC IPEV HP/IPECO

545.5

488.3

317

260

211

160

564

456

335

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GAS TEMP.PROFILE IN HRSG

DESIGN

ACTUALT SH 105ACTUAL 108

T EVA 172ACTUAL 121 ??

T HPEC 57ACTUAL 53

T IPEV 49ACTUAL 11 ??T HP/IP ECO 51ACTUAL 97??

SH1 ATTEMP. DESIGN 1.1ACTUAL >5.5 TPH

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THE TEMPERATURE


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SH2 SH2

SH1 SH1

HPEVA HPEVA

HPECO HPECO

IPEV IPEV

IPECO IPECOHPECOHPECO

194 189 181 207

333 350

322 292

361 352

482 452

560 564

467NA

368354

306

328287

307

THE TEMPERATUREDISTRIBUTION SHOWS AVERY HIGH BIAS OF FLUEGAS FLOW TOWARDSIPECO SIDE.

TEMPERATURE DROPACROSS IPEV NOTPROPER

IPECO CONTINUES TOSTEAM

HEAT PICK UP IN HPEVIS QUITE LESS ANDLESS STEAM ISGENERATED

NECESSARY TOINCREASE HP FEED FLOW

HEAVY ATTEMPERATIONIN SH TO CONTROL SHTEMPERATURE

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128,115,58.8 / 121,121,71SKB


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SH2 SH2

SH1 SH1

HPEVA HPEVA

HPECO HPECO

IPEV IPEV

IPECO IPECOHPECOHPECO

194 189 181 207

333 350

322 292

361 352

482 452

560 564

467NA

368354

306

328287

307

30 B,115C,13.3 / 23.5,121,14

29,250,12.2 /23.5,198,14

119,301,57

121,302,71

P ACROSS HPECO =9 BAR ??

HP/IP FEED TEMP.115 AGAINST 121. IP

STEAM TO PROCESSAT A TEMPARATUREOF 190 AGAINST200.7C. LOWDEAERATOR PRESSUREWHICH CAN INCREASE

HP STEAMPARAMETERS TO STARE 72 BAR, 494 CAND 57 TPH AGAINST98 B, 514 AND 71TPH

TO HPECO2

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SUGGESTIONS


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PERFORATED PLATE

FLOW

STRAIGHTNER

TURNING VANES

INSTALL FLOW STRAIGHTNER

INSTALLPERFORATED PLATE

INCREASE DEAERATOR PRESS.

INVESTIGATE AND CORRECTEXCESSIVE PRESSURE DROP INHP FEED LINE

INCREASE HPEVA AREA (10%)

SUGGESTIONS

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593CGAS TEMP PROFILE IN HRS


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SH1 SH2 HPEV HPEC IPEV HP/IPECO

545.5

488.3

317

260

211

160

564

456

335

282271

174

GAS TEMP.PROFILE IN HRS

DESIGNACTUAL

ACHIEVABLE

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CASE STUDY

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HRSG BEHIND GE FRAME 9E GT

DELTAK MAKE, UNFIRED, NATURAL CIRCULATION,THREE PRESSURE, HORIZONTALCONFIGURATION

HP STEAM 188 TPH, 59 BAR, 512 C

LP STEAM 21 TPH, 7.5 BAR, 223C

DEAERATOR BOILER 50 PSI

DA BOILER TUBE FAILURE

REDUCTION IN STEAM GENERATION ANDCONSEQUENT REDUCTION IN STG POWER

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HEAT EXCHANGER TRAIN

MODULE 1 SUPER HEATER 1

SUPER HEATER 2A (ROWS 1 TO 3)

SUPER HEATER 2B (ROWS 4 TO 6)

MODULE 2 HP BOILER 3 TRAINS HP ECONOMIZER 2 TRAINS

LP SUPER HEATER 1 TRAIN

MODULE 3 LP BOILER 2 TRAINS

HP ECONOMIZER 1 TRAIN DA BOILERS 4 TRAINS

FAILURES AT TOP BENDS IN THE LAST TRAIN OF DA BOILER

HIGH STACK TEMPERATURE

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DAB STORAGE TANK

OUTER MOST EXPERIENCESMINIMUM GAS TEMPERATURE.ADDITIONALLY THIS TUBERECEIVES MAXIMUM WATERFLOW. THE TUBE THERFORE HASLARGE WATER CONTENT. THIS

WATER CAUSES IMPINGEMENTAND THINNING OF THE TUBE BYEROSION. THE LAST ROW CANBE TOTALLY REMOVED SINCETHE REMINING THREE ROWSARE SUFFICIENT

IT IS BETTER TO REMOVEABOUT 10% TUBES IN SH1

7-10% INCREASE HPECO2SURFACE AREA

RECOMMENDATIONS

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FIRED HRSG

5 BURNERS SCREEN TUBES

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IMPORTANT ISSUES IN FIRED HRSG

LENGTH OF FIRING DUCT IS A VERY CRITICAL FACTORTO ENSURE PROPER RADIATION AND CONVECTIVE HEATTRANSFER.

THE BURNERS MUST ENSURE OPTIMUM PENETRATIONAND DIVERGENCE OF THE FLAMES. THE SCREEN BOILERMUST SEE A UNIFORM HEAT FLUX TO THE EXTENTPOSSIBLE. NON-UNIFORM HEAT FLUX DISTRIBUTIONMAY CAUSE SEVERE BUCKLING OF SCREEN TUBES

ALL THE BURNERS MUST BE TUNED TO GIVE THE SAMEFLAME SHAPE AND SIZE.

SELECTION OF FIRING ELEVATIONS DO PLAY ANIMPORTANT ROLE IN THE LONG TERM PERFORMANCE OFTHE HRSG

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FIRED HRSG CONFIGURATION

REQUIRES AUXILIARY AIR SUPPLY FOR COOLINGBURNER TIPS.

FORCED DRAFT FANS OR ID FANS. THE LATER ISNOT A POPULAR OPTION. RELIABILITY AND

AVAILABILITY OF FANS IS A CRITICAL ISSUE. DEPENDING UP ON THE CLEANLINESS OF FUEL,

IT IS ADVISABLE TO PROVIDE SOOT BLOWINGLANES IN FLUE GAS AND HEAT EXCHANGER

PATH TO ENSURE CLEANLINESS OF HEATTRANSFER SURFACES.

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CONCLUSIONS


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CONCLUSIONS COMPARED TO CONVENTIONAL COAL FIRED BOILER LIFE

EXPENDITURE IN HRSG IS VERY FAST DUE TO FACTORS

SUCH AS RAPID START UP AND VERY HIGH LOADINGRATES. HRSG DESIGNS FOR THIS PURPOSE ARE REQUIRED TO

BE TAILOR MADE SO THAT THE AGGRESSIVE START UPSAS WELL AS LOADING RATES DO NOT CAUSEPREMATURE FAILURES AND LIFE EXPIRY.

COMPARED TO CONVENTIONAL BOILER R&M ACTIVITIESFOR HRSG MUST START MUCH EARLIER.WE CANIDENTIFY THE PROBLEM AREAS IN HRSG BY STUDYINGTHE PRESENT FLUE GAS TEMPERATURE PROFILE, WATERAND STEAM PARAMETERS WITH REFERENCE TO THESAME AS GIVEN IN HMBD SUPPLIED BY OEM.

STACK TEMPERATURE IS ONE PARAMETER WHICH TO ALARGE EXTENT WOULD INDICATE THE QUALITY OFPERFORMANCE OF THE HRSG

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THANK

YOU

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