Btl Uprvunlpresent i

7/29/2019 Btl Uprvunlpresent i

1/65


2/65

BOILER TUBE FAILUREESTIMATED OVER 1500-2000 BTF

TAKE PLACE EVERY YEAR ININDIAN POWER INDUSTRY.

BTL IS RESPOSIBLE FOR ABOUT8% OF LOSS IN AVAILABILITY

ANTICIPATED ANNUAL LOSS 8000 MW


3/65


4/65

UPRVUNL-PIE STATIONSBTF LOSSES AV. ONE BTF EVERY 1.5 DAY

EST. GENERATION LOSS: 6.25 MU PER DAY

AV. DAILY AVAILABILITY LOSS: 14.65%

EST. REVENUE LOSS: Rs 1.25 Cr per day@Rs 2/- per Kwh

EST. LOSS IN PROFIT: Rs 12.5 Lakhs per day@ 10%


5/65

BOILER TUBE FAILURE

WATER TOUCHED TUBES e.g. WATER WALL, ECONOMISER etc.

IF OPERATED TO DESIGN REGIME SHALL

HAVE INFINITE LIFE EXCEPT FOR NORMAL

WEAR

STEAM TOUCHED TUBES e.g. SH/RH etc.

DUE TO HIGH PRESSURE & TEMPERATURE

STEAM CONDITION ARE SUBJECTED TOCREEP INITIATED PHENOMENON.

LIFE OF SUCH TUBES IS LIMITED TO 200 000

Hrs.


6/65


7/65

3.0 OVERHEATING & CREEP Short Term Overheating due to Starvation (Blockage)

Long Term Overheating Creep

Graphitization

4.0 POOR WATER CHEMISTRY Hydrogen Damage/ Embrittlement Caustic Gouging Pitting Chemical Cleaning/ Excursion Damage

BTL CAUSE


8/65

5.0 POOR MAINTENECE Material Mix-up

Weld Defect

Improper Inspection Non Availability of Material Mechanical Rubbing/Attrition or Fretting due to inadequate fix up/ support6.0 OTHERS Ageing of DMW Joints

Ageing of Shop Weld Joints with allowable defects

BTL CAUSE


9/65

BTL CAUSE IMPACT ANALYSIS

1.0 EROSION: 40% Ash Erosion: 28% Steam Erosion: 8% PF/Coal Erosion: 4%

2.0 CORROSION & PITTING: 3%3.0 OVERHEATING: 7%

4.0 POOR MAINTENANCE: 48%

Attachment Weld Failure: 18%

Site Weld Failure: 16% Stub joint/DMW Failure: 6% Material Mix-up: 3% Mech. Rubbing/Attrition: 5%

5.0 OTHERS: 2%


10/65

MAJOR AREA OF CONCERNIN

PRESSURE PARTS

1.0 WATER WALLS: 35%

2.0 SECOND PASS (ECO/LTSH/SCW): 43%

3.0 SUPERHEATERS: 11%

4.0 REHEATERS: 11%


11/65

BTF MANAGEMENT

UNDERSTANDING

&PREVENTION


12/65

EROSION

Flue Gas erosion is caused by the abrasive particles mainly alphaquartz and pyrites in ash and is dependant on the amount and

velocity of ash particles. Alpha quartz in ash in Indian conditions

varies from 10-20%.

FG erosion causes polishing of the tubes. This form of erosionoccurs mainly in Economizer and LTSH.

Erosion by steam is severe if it is wet e.g. by soot blowers.

Falling slag results in abrasion of the tube surface.

Coal particle erosion results from damaged coal burners.


13/65

EXTREME END COILS ON BOTH LHS AND RHS OF ECONOMIZER & LTSH ASWELL AS OTHER COILS TO BE LIFTED/LOWERED FOR A THORUGH INSPECTION AND

THICKNESS SURVEY.

TUBES IN EROSION PRONE AREAS TO BE INSPECTED FOR HEALTHINESS AT

EVERY OPPORTUNITY & SHIELDED OR SHIELDS RESTORED.

COIL BANKS BENDS TO BE PROTECTED BY CASSETTE BAFFLES & INSPECTEDAT EACH OPPORTUNITY.

VISUAL INSPECTION AND THICKNESS SURVEY TO BE CARRIED OUT AT EVERY

UNIT OVERHAUL AS WELL AS EVERY AVAILABLE OPORTUNITY. TUBE THICKNESS IN

EROSION PRONE AREAS TO BE MEASURED AND RECORDED.

TUBES WITH 15 -20 % LOSS IN THICKNESS ARE TO BE REPLACED.

INSTALLATION OF VELOCITY DEFLECTORS/ EROSION CONTROL DEVICES TO

REDUCE HIGH FLUE GAS VELOCITIES AND INEQUITABLE FLOW DISTRIBUTION

COILS TO BE INSPECTED TO ENSURE THEIR ALIGNMENT AND PITCH. HANGERS

& HOLDING CLAMPS AREA TO BE INSPECTED FOR EROSION AROUND THE AREA

APPLICATION OF REFRACTORY IN HIGHLY EROSION PRONE AREAS

EROSION MANAGEMENT


14/65

OVERHAULING OF WALL DESLAGGERS. ENSURE PROPER NOZZLEALIGNMENT IN EVERY UNIT OVERHAUL TO AVOID STEAM IMPINGEMENT

STEAM BLOWING PRESSURE TO BE MONITORED AND SET AT THE WALLBLOWERS THE SET PRESSURE TO BE CHECKED QUARTERLY FOR ANYDEVIATIONS AND CORELATED WITH THE EROSION RATE IN SUBSEQUENTSHUTDOWNS

COAL COMPARTMENT ASSEMBLIES TO BE REPLACED AT THE END OFTHEIR LIFE OR DURING UNIT OVERHAUL, AS REQUIRED

COAL BURNERS AND NOZZLE TIPS TO BE INSPECTED AT EVERYOPPORTUNITY AND DAMAGED BUFFER PLATES AND TOP PLATES ARE

REPLACED

PLATES WITH WEAR RESISTANT COATING CAN BE USED TO ENHANCECOAL NOZZLE TIP LIFE AND REDUCE PF EROSION

COAL COMPARTMENT CAN BE HARD FACED AT EROSION PRONE AREASTO REDUCE THE RATE OF EROSION

EROSION MANAGEMENT


15/65

INSPECTION AND REPAIR /REPLACEMENT OF AIR NOZZLES AND

TIPS IN EVERY CAPITAL OVERHAUL TO ENSURE ADEQUATE AIRFLOW

BURNER TRANSITION TUBES AND CORNER TUBES TO BE

INSPECTED DURING OVERHAULS THOROUGHLY

WEAR BARS CAN BE WELDED ON CORNER TUBES OF S-PANEL

REFRACTORY CAN BE APPLIED AT UPPER AND LOWERTRANSITION TUBE BENDS TO REDUCE EROSION

THE INTERFACE OF S-PANEL TUBES WITH SIDE WATER WALLS TOBE SEALED WITH REFRACTORY

THICKNESS SURVEY OF S-PANEL ZONE TO BE DONE DURING EACH

AVAILABLE OPPORTUNITY

EROSION MANAGEMENT


16/65

OVERHEATING

Operation of tubes at temperatures higher than the design temperatures causesreduction in tube material strength. Overheating results in deterioration inmicrostructure of tube material and enhanced oxidation occurs.

Short term overheating failures are characterized by fish mouth bursts withvery thin burst lips which display a partially or fully hardened structuredepending on the temperature.

Such failures are caused by loss of circulation either because of choking orbecause of loss of water level in the drum, flame impingement on tubes, andsecondary combustion. These failures are common in high heat flux areas likewater walls.

Choking may be caused by foreign materials or pre boiler deposits due toimproper water treatment or condenser leakage which get deposited at highheat flux areas or bends. High dissolved gases in feed water, condenser leakageall give rise to pre boiler deposits.

Flame impingements are caused by damage to the burner system.


17/65


18/65

When deposits are suspected diameter measurement of other tubes at the sameelevation as failed tubes should be taken for indications of bulging & swelling.Bends in the tube circuit also need to be examined.

Chemical examination of the deposits to carried out to determine their source.

Boiler water chemistry to be monitored strictly to prevent long term

overheating. Maintenance procedures to be followed to prevent tools, cutting debris and

weld spatter from entering tube circuits & blockage.

Operating procedures should limit tube metal temperatures to design values. Ifthis is not possible then possibilities to upgrade tube material can be explored.

Metallographic examination of tube samples in high heat flux zones gives agood indication.

Avoid slagging and wall fouling by proper operation of soot blowers,combustion controls through proper burner & SADC management & avoidingoperation of the unit at low or high operational regime.

OVERHEATING MANAGEMENT


19/65

CORROSION

Corrosion accounts for about 15-30% of tube failures. Useful life of tube steels is affected by a number of internal corrosion

mechanisms that are controlled by the chemistry of water. Dosants andcontaminants in water have a major influence on corrosion.

Under normal operating conditions there is a protective magnetic oxide scale

on the inside surface of the boiler tubes. This protective magnetite has low solubility in water under slightly alkaline

conditions.This solubility increases rapidly at pH values below 5 and above 12causing risk of corrosion. If this protective layer is disturbed the tubes corrodeat a faster rate causing tube failures.

On load corrosion (H2 damage and caustic gouging) is caused by concentrationof impurities in water.

Caustic corrosion develops from deposition of feed water corrosion productswherein NaOH can concentrate to high pH levels. At high pH the protectivelayer becomes soluble leading to rapid corrosion, irregular wall thinning or

gouging on the water side of the tube .


20/65

Hydrogen damage results from fouled heat transfer surfaces and acidic

condition of boiler water e.g. where sea water is used in condensers. Hydrogencombines with carbon to form methane at grain boundaries. Large methanegas molecules become trapped between the grain boundaries and cause anetwork of discontinuous internal cracks.

Pitting is caused by attack of dissolved oxygen in boiler water on tube internalsurface.

Stress corrosion cracking (SCC) results from a combination of tensile stress,corrosive environment and susceptible material like austenitic stainless steels.

The problem is common with stainless steel cold bends when heat treatmentsare not done or are not proper.

For austenitic stainless steels chlorides are mainly responsible for SSC. AsChromium provides the corrosion resistance, formation of ChromiumCarbide makes the tube susceptible to SCC. Hence stabilized grades of SS

which have alloy elements like Titanium and Columbium, that are strongcarbide formers are preferred.

CORROSION


21/65

Monitoring and control of boiler water chemistry to prevent high caustic/acidic levelsand restoration to design values.

Maintenance of condensers & feed water heaters and chemical treatment andmonitoring to be carried out without deviations.

Pitting can be controlled by rigorous monitoring, identification and control ofcondenser leaks and others causing air ingress. During boiler operation pH of boiler

water should be maintained between 9.5-10.5. Residual dissolved oxygen should be lessthan 0.005ppm. During boiler shutdown boiler circuit should be completely filled withDM water at a pH of 9.5 and containing minimum 200ppm hydrazine. The hydrazinecontent should be checked everyday and extra quantity should be added if the valuefalls below 50ppm. Alternatively in dry preservation the boiler circuit is filled with drynitrogen and maintained at a positive pressure.

Corrosion damage can be reduced by minimizing ingress of feed water corrosion and

condenser tube leakage.

Periodic Tube samples to be taken to verify the amount of deposition in the tube. As itresults in wall thinning extent of damage can be estimated by NDE methods.

Carry out periodic inspection of water wall tubes in high heat flux areas for internaldeposits and analyzing the need for chemical cleaning.

Sophisticated UT techniques can identify locations where H2 damage has embrittled the

steel.

CORROSION MANAGEMENT


22/65

Cyclical stresses or strains imposed on boiler tube lead to fatiguefailures. Fatigue failures are mainly caused by the restraintoffered to free expansion of tubes at attachment welds or wheretubes are fixed to a drum, header, wall, seal or support

attachment.

They can also occur at butt welds if they are not made properlyi.e. incomplete penetration.

Fatigue cracks occur at stress concentration regions such as rootof notch or locations near sudden change of cross section.

Fatigue Failures are easily identified as they have relativelysmooth fracture surface and the cracks appear straight with a

blunt edge.

FATIGUE


23/65


24/65

DIS-SIMILAR METAL WELD FAILURE

OCCURANCE

It occurs in steam cooled tubing at the joint of ferritic and austenitic steel.

Degradation in DMW joint is related to service time, operating temperature,temperature cycling and system stress.

DMW failure results in circumferential fracture in the joint which is parallel tothe weld fusion line.

ACTION PLAN

Sample selected DMW joint for visual and metallurgical analysis.

Using Ni based filler metal welds instead of SS based gives a service life of 3-4times.

Scaffolding load should not fall on the DMW joint areas. Similarly DMW jointshould be positioned as far as possible from the fixed loads.

QUALITY IN MAINTENANCE


25/65

WELD DEFECTSOCCURANCE

Statistical data shows that about 5% of tube failures are attributed to weld andmaterial defects. These failures occur particularly during the initial periods ofoperation.

Weld defects occur due to lack of quality control over the essential weldingvariables. Defects in the welds occur due to inadequate stress relief, incompletefusion, lack of penetration, inclusions, porosity etc.

ACTION PLAN

Quality control procedures like qualification of welder, carrying out welder test

are essential. This minimizes chances of common welding process defects likeLP, porosity, excess penetration, incomplete fusion, undercuts, etc.

Testing of weld joints by radiography methods. 100% of joints to be radiographed. .

Gas cutting to be avoided especially in vertical tube sections. At unavoidableconditions it must be done in presence of competent boiler maintenance

engineer. A plan can be made for NDT of old weld joints in a phased manner.

QUALITY IN MAINTENANCE


26/65

BTL FAILURES

PATTERN & EXAMPLES


27/65


28/65


29/65


30/65


31/65


32/65


33/65

steam errosion platten SH tube-1


34/65


35/65


36/65


37/65


38/65


39/65

corrosion fatigue of the cold

bend of SH tubes


40/65


41/65

fire side corrosion on ww tubes


42/65

oil ash corrosion of furnace ww tubes


43/65

stress corrosion cracking


44/65

Water wall tube overheating due tointernal oxide layer deposition


45/65


46/65


47/65


48/65


49/65

hydrogen damagethick wall longitudinal burst


50/65

Hydrogen damage-blow out of tube


51/65


52/65


53/65


54/65

hydrogen embrittlement-window failure


55/65


56/65


57/65

over heating failureof down comer tubes


58/65


59/65


60/65


61/65


62/65


63/65


64/65


65/65

Btl Uprvunlpresent i

Documents

Transcript of Btl Uprvunlpresent i