Bokaro Tube Leakage

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Presentation toChairman - DVC

onBoiler Tube Failure

at BTPS-B (3x210MW)Venue – DVC Towers, Kolkata

Date: 13.09.2004

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BACKGROUND

BTPS-B (3x210 MW) has balanced draft, double arch down shot fired ABL made

boilers with the following features:

S.N. Description BMCR

1 Evaporation 700 T/hr

2 SH O/L Press 137 kg/sqcm

3 SH O/L Temp 540 deg C

4 FW I/L Temp at ECO 227 deg C5 Excess air at ECO O/L 23 %

The boiler has been designed for the following coal characteristics:

GCV – 4000-4400 Kcal/Kg

VM - 11.25 % to 14 %

Total Moisture – 1 % to 10 %

Ash – 38 % to 45 %

FC – 33% to 47 %

Mills – 8.5 E 10 mills. Boiler has been designed for 5 mill operation with both worst and best coal with

two mills as standby. Each mill has one hot PA fan.

APH - Tubular

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Major Observations- Maintenance

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1. Boiler tube thickness measurement not done on a regular basis.

2. Testing of tube samples to establish the root case of failure not done.

3. Repetitive failure experienced.

4. Very few samples of primary failure found at site.

5. Most of the failed samples were of secondary failure.

6. On visual inspection of economizer coil replacement work in Unit 1, no erosion or

thinning of coil tubes was observed in both the rejected coils and the coils not

replaced.

7. In case of eroded bends, the complete replacement of the coil is not called for.

8. Inspection of straight tube lengths of rejected coils showed no signs of thinning or

damage.

9. No standard quality plan in practice for welding and NDT.

10. No standard clause existing in BTF rectification contracts for failure of joints.



Major Observations- Operation

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1. Control bed height vs. speed in coal feeders not maintained as per OEM

guidelines.

2. Mill primary air DP always kept maximum irrespective of feeder position.

3. Mill out let temperature maintained between 85 to 90 Deg C as against

recommended value of 121 Deg C.

4. Furnace to wind box DP is found to be low (40 mm wc against 65 mm wc as

recommended by OEM).

5. Tertiary air provision given in the furnace is abandoned.

6. Air registers openings not coal specific.

7. Enormous leakage from furnace, furnace roof, ducts and expansion joints.

8. Wall blowing not done

9. Units allowed to run with prolonged tube leakage.

10. Station Chemistry not complying with OEM’s norms.

11. Unburnt carbon in BA and FA very high – 23-27 % & 11-13 % respectively.



Major Operational FeaturesS.N. Parameter Design value Actual value

at 89% of BMCR

1 Load 186 1852 Flow 623 661

3 SH O/L press 137 120

4 Pri Air flow 223.2 x

5 Sec Air flow 488.7 x

6 Tertiary Air Flow 99.0 Nil

7 Total Air Flow 790.9 893

8 Pri Air as part of total air 28.2 % x

9 WB Press 44 40

10 FG temp at Eco O/L 334/323 286

11 APH Air O/L temp 273 215

12 Mill air out temp 121 85-90

13 Coal fineness thro’ 200 mesh 80% 58%

14 Un burnt Carbon – BA+ FA 7-8% 37-40%

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Tube leakages from April’03 to August’ 04

BTPS-BS.N. Area Unit1 Unit 2 Unit 3 NTPC ( Cum for all 65 units)

1. Economizer 19 13 10

2. Platen SH coils 3 3 2

3. WW bends 0 12 2

4. Pl. SH O/L header 3 5 0

5. Final SH 7 5 06. Re heater 2 1 0

7. Baffle wall/LTSH 3 4 3

8. TOTAL 37 43 17 150

• NTPC failure data is the cumulative of all 65 thermal units of NTPC.• No failure is on account of boiler water chemistry, water side or fire side corrosion,

fatigue and lack of quality control. All failures are on account of ash erosion and wallblower erosion. The alpha quartz in coal received in Singrauli Area ( i.e. for 17 units)is very high.

• The 150 leakages amount to 2% loss in availability against a global best of 1%. The

loss of availability target for the current financial year in NTPC has been kept as0.5%.

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Details of boiler tube failures experienced

S.N. Area Units Primary reason

1. Economizer 1,2 & 3 Erosion & Thermal Fatigue

2. Horizontal RH 1,2&3 Erosion and overheating3. LTSH / Baffel wall 1,2&3 Erosion

4. Pendant Reheater 1,2&3 Long term and short term OH

5. Platen SH coils 1,2 & 3 Long term & short term OH

6. WW bends 2 Caustic Corrosion & Erosion

7. Pl. SH O/L header 1&2 Corrosion fatigue

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BTF – Detail Analysis

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A) Economiser:

1. Economizer coil tube failure:

a) Solely because of erosion and thinning of tubes near end walls.

b) Observed mostly on the SH side

b) The erosion is as a result of increased flue gas velocities due to increased flow because of

change in gas biasing dampers position w.r.t designer’s recommendations.

2. Economizer inlet header stub joint failure:

a) This is on account of Thermal fatigue.

b) Thermal fatigue caused by wetting of tube surface at high temperature by water.

c) Wetting produces high bi axial tensile stresses on the wetted surface until the water evaporates.

d) On cooling contraction of metal is prevented portion of metal at high temperature.

e) This restriction in metal’s contraction results in high tensile stresses on the surface and

produces multiple cracks.



Economiser FailureAPDRP

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Bend near cage

wall

Second pass sidecage wall




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B) Platen SH coils:

a) Tube samples of platen SH coils show signs of swelling and bulging due to long term and shortterm overheating.

b) Long term overheating is only because of choking of the under drilled orifices in header.

c) A fibroscopic/ borosonic test of the orifice openings would establish the extent of choking.

d) A chemical analysis of the deposits would reveal the departure from standard of water and steam

chemistry norms in regular operation.e) Short term overheating is because of suspected improper combustion as resulting in high FEGT,

disturbed flow and temperature pattern as compared to design value.

C) Water Wall bends near burner:

a) This failure is suspected due to caustic corrosion.

b) Caustic corrosion causes thinning of tubes beneath the deposits, which also forms pin holes and

deep pits at the deposit zone.

c) Pinhole in water wall bends near the burner can also result out of external erosion and corrosion

due to ash particles in flue gas taking a bend to move upward.



Boiler firing zone detailAPDRP

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Combustion Zone

Tertiary

Air

PrimaryAir + Coal

Secondary

Air

Double down shot firing

Coal

Nozzle

WW bend leakage

area




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D) Cracks in Platen SH O/L header stub:

a) This failure is a perfect example of Corrosion Fatigue. b) Thermal cycling and expansion constraints are the primary strain loads causing corrosion

fatigue.

c) Cracks that originate on the outside surface are the result of adverse stresses from restricted

thermal expansion.

d) Caused due to large difference in temperature between tubes of the same header.e) Hot tubes are constrained from free expansion while cooler tubes are stretched to expand.

f) This temperature differential along with a large no. of start/shut down cycles results in corroison

fatigue.

g) Differential expansion between Headers and roof tubes also is also a prime reason for corrosion

stress.h) The tubes at the ends of the headers are mostly likely to be cracked since the expansion

deflection is the greatest at this location as observed at BTPS.

i) Vibration fatigue induced by gas also causes external cracks.



Platen O/L Header Failure

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Cold Hot

Expanded header

Roof differential

expansionRoof tubes



Platen O/L Header failure

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Element terminal

tubes

Header

Location of

stub and

ligament

cracks

Roof tubes

A




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Cracks in Platen SH O/L header stub:

Actions to confirm:

1. Visual examination for distortion or bending in adjacent tubes

2. Strain gauging of suspect locations to evaluate strains during starts and cyclic operation

3. LVDT measurements to monitor the relative movement of the header/tube during transients:

particularly whether header support allows for expansion.

4. Determine the extent of residual stresses in tight hair pin bends

5. Metallurgical examination to determine high cycle loading

6. Estimate natural and forcing frequencies.7. Visual and microscopic examination of weld quality.

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Recommendations : Short term

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i) Economiser:

a) Stainless steel shields to be provided in the bend at Economiser Inlet header

b) Economiser coil bends near side wall to be covered with SS wire mesh.

c) Tube thickness of vulnerable areas of the second pass to be measured and tubes having less

than 80% of thickness only to be replaced.

d) Quenching by spraying jet of cold water on hot metal surface should be avoided. Natural

cooling should only be allowed.

e) Alignment of coils and position of hangers to be checked. Gaps between coil and sidewalls

to be maintained as per design recommendations.

f) Failed tube samples should be tested to ascertain the cause of failure.

ii) Platen SH Coils:

a) Fibroscopic / Borosonic test of header to be done to identify choked header orifices.

b) A chemical analysis of the deposits should also be done to establish the deviation from

standard boiler chemistry norms.

c) Measurement of furnace temperature with the help of a pyrometer be done at different zones

d) Combustion to be optimised.

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iii ) Platen Super heater outlet header:

a) Metallography and testing of failed samples to be done. b) Combustion to be optimised

c) Penthouse sealing to be insured

iv) Waterwall tube failure:

a) Burner panels of Unit # 2 to be replaced.

b)Present WW bends to be replaced with bends of of higher thickness near burners for all units.

c) In addition to the above a hard face coating to be provided on the bend surface.

d) The bends to be then covered with a refractory coating 50 mm thick.

v) OTHER CRITICAL ASPECTS:

a) All pressure part failures to be tested and analysed for root cause.

b) Unit should not be allowed to run with tube leakage to avoid secondary damage.

c) During any shutdown thorough inspection of the boiler to be done.

d) Standard FQP for welding and NDT to be put into practice.

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Recommendations : Short termAPDRP

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e) Cation conductivity testing arrangement not existing to detect condenser tube leakage. The same

to be installed and DM water quality to be maintained.

f) Optimisation of flow in second pass to be done adjustment of gas biasing dampers by trial anderror method.

g) Wall blowing should be made mandatory and all wall blowers should be operated at least once

a shift.

h) A comprehensive program to OPTIMIZE COMBUSTION must be implemented. This should

necessarily include accurate adjustments to ensure proper air/ fuel regimes. The following must

be done:

1. Operating mechanism of feeder to maintain coal bed height and speed vs Load as per

manufacturer’s recommendations.

2. Replace/renovate/ modify the mill components like classifier and throat to achieve higher

fineness.

3. Correct Hot PA flow to mill to be maintained.

4. Maintain the designed PA: SA ratio

4. Restoration of tertiary air openings.

5. Maintain the higher value of Mill out let temperature between 100 to 120 Deg C for better

combustion.

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6. Clean air flow test of coal flow to be done. Flow in each pipe to be checked and equalized using

dirty pitot tube traverse.

7. Differential pressure across PA discharge damper to be calibrated with PA flow.

8. Furnace to WB Diff Press to be maintained as per design value.

9. Air registers to be made to modulate as per boiler load and should be coal specific.

10. Good flame conditions to be ensured by frequent visual inspection, fine tune up of air register

vanes etc. to obtain the best possible flame shape.

11. Continuous monitoring to be done to ensure no undue excursions in furnace draft, super-heater

metal temperatures, spray flows etc.

12. The following parameters to be monitored and maintained:

i) Oxygen in flue gas (ECO OUTLET) : 3.0% to 3.5%.

ii) Unburnt carbon in bottom ash: 4%- 6%.

iii) Coal fineness (sampling and measurements as per ASME): 80% thru 200 mesh 2 %

retention thru 50 mesh.

13. Additional oxygen probe to be retrofitted at ECO outlet (@ 2 per boiler) for better oxygen

monitoring.

14. All instruments to be calibrated both in the UCB and local equipments.

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i) Air tightness of duct and furnace to be done. Large amount of ash accumulation has been observed

in pent house and from ducting joints. All leakages to be rectified and tightness to be ensured.

j) Air pre heater performance to be evaluated from time to time and necessary corrective actions to

be taken to ensure maximum efficiency.

k) All LP heaters and HP heaters to be kept in service and heater efficiency to be monitored closely

so as to achieve the designed Economiser feed water inlet temperature.

l) Undertake comprehensive overhauls of all the burners, air registers & wind box so as to ensure i)rated coal throw put from each burner ii) Operability of the air registers & iii) Zero leakage from

wind box. The following have to be necessarily done.

a) Service burner internal and use impeller with correct material as recommended by OEM.

b) Maintain proper impeller clearances

c) Introduce monitoring systems to ensure healthiness of impellers at all times.

d) Service air register and vanes to ensure operability of the vanes in complete range.

e) Check and ensure ZERO leakage from the wind box.

f) Service oil guns to eliminate blockage and leakage.

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Recommendations : Long termAPDRP

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1) A thermal design audit of the boiler is required and modifications required if any in the following

areas to be evaluated and incorporated:

i) Reduction of thickness of refractory on water wall surface below the burners.

ii) Reduction in Horizontal RH surface in second pass

2) Mills to be modified specially at throat and classifier to improve fineness.

3) Erosion potential of present coal to be established and shields to be provided at required locations

without disturbing the gas flow pattern.

4) Calculation of FEGT and flue gas velocities in the present firing conditions and modifications

required to achieve the designed FEGT and velocity pattern to be evaluated and incorporated.

5) Platen SH O/L header absolute expansion and differential expansion to be evaluated w.r.t. present

operating conditions. Modification of header arrangement by incorporation of a collector header as

provided in all modified designs of ABL boiler.

6) Modification of mills for the following:•Different classifier arrangement to improve fineness in mills.

•Modification of throat to achieve high fineness.

•Reduce the velocity of pulverized coal at burner inlet.

•Resizing of coal entry port at burner, SA ports around coal port and other additional ports of

secondary air including tertiary air ports.

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THANK YOU

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Bokaro Tube Leakage