Damage to Bottom Ash Handling Sys

7/28/2019 Damage to Bottom Ash Handling Sys

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SUBJECT : TALCHER UNIT 3 (500 MW)

PROJECT : DAMAGE TO BOTTOM ASH HANDLING

SYSTEM & BOILER PRESSURE PARTS INBOTTOM S PANEL.

OCCURRENCE:On 29.03.03 cracks were noticed in grouting of few columns

in bottom ash hopper. Also foundation bolts (3 nos.) in column A1

& A2 were found sheared and the balance bolts (5 nos.) in these

two columns had bent.

Further in the early morning hours on 05.04.03, severe

damage was noticed in rear side column B2 (I-600). This columnhad buckled and its top edge had displaced by 100 mm with

respect to its base. On close examination, it was identified that

the other columns also have displacement at the top, affectingthe verticality to the extent varying from 5 mm to 35 mm.

Adjacent to the buckled column, SS seal plate in bottomwater through seal, connected to rear water wall had bowed down

by approximately 100 mm to a length of 3 mtrs. and few bolts,

fixing the seal plate to the scalloped bar, had sheared off.

Insulation on water wall, adjacent to the damaged seal plate areahad been pushed outside along with outer sheeting.

Cracks had developed in the seal through bottom plates incorner 2 to a length of approximately 150 200 mm.

Further, when the unit was shut down on 21.04.03, detailedinspection inside the bottom ash hoppers and bottom S panel area


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was organized. It was noticed that the horizontal section of waterwall in the bottom S panel had bulged in to the Dog house to an

extent of 250 mm for a length of 2.5 to 3 meters, covering 60

tubes. Two tubes were found to have dent marks near the bottomof the furnace support beam near S panel.

Additionally, the castable refractories applied on the left side

ash hopper, had also damaged substantially.

ANALYSIS:

The damage on structure of the bottom ash handling hopper and

the pressure parts was analysed for its cause. It is suspected tohave occurred due to sudden localized pressurization caused by

evolution of gas/vapour from the entrapped water in the falling

heavy porous lumps of slag/clinkers while immersing in bottom ashhopper water bath. The undue unidirectional waves which are likely

to generate under these circumstances, have exerted additional

force on the pressure parts in S-Panel and the structures of thebottom ash hopper and caused distortional damages to the above

mentioned components.OBSERVATIONS:There was no cross bracing in bottom ash hopper between the

buckled column B2 and the adjacent columns. Hence any additional

sudden load coming on these columns will not be distributed to

other structural members and making them vulnerable for suchdamages. Further, it was identified that one set of horizontal

bracings which were supposed to act as lateral tie between

columns, were not erected due to certain site constraints /obstructions.


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RECTIFICATION:

The following rectification works were carried out in bottom ash

hopper and pressure parts in bottom S Panel during unit shutdown from 21.04.03 to 08.05.03

WORKS IN BOTTOM ASH HOPPER

1. The buckled portion of the column B2 was removed for alength of 4 meters approximately and substituted by

welding a new section of same size using a splice plate.

2. Further, to improve the total stability and loaddistribution of the structural members, additional bracings

in between column 2 and 3 were erected both in front and

rear side of the hopper.

3. The damaged portion of castable refractory was relaid.RECTIFICATION ON PRESSURE PARTS:

1. The bow in horizontal portion of the S panel was rectifiedas per the hot correction procedure, given by Trichy as

follows:

a. The horizontal floor beam in the bow area is to be cutat either end (where the bow starts)

b. The fin between the water wall tubes in the bowportion is to be slit at 3 or 4 locations across thewidth of about 3 to 3.5 M.


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c. Bow to be corrected by carrying out the hot

correction method. The maximum temperature for

heating should be restricted to 650 deg. C. withneutral flame and the temperature verified bythermal chalk.

d. The correction can be done after heating by usingjack arrangement. The support for the jack can be

taken from the buckstay beam welded to inclined

protion of the water wall tube. After correction of

the horizontal floor panel, the support beam is to bere-welded with splice plate arrangement. Then the

fins between tubes are to be welded. After

correcting the bow, the seal plate is to be fixed tothe scalloped bar of the plate to maintain the

required clearance in the seal trough to take care of

the three dimensional expansion of the Boiler.

2. 2 Nos. dented water wall tubes were removed and newspool pieces were welded in position.

3. Since the channel below the SS through has deflectedby 90 to 100 mm for a length of 3 to 4 mtrs., new

scalloped bars were erected in horizontal portion of theS panel water wall tubes to suit the new profile of SS

trough. The dislocated SS seal plate was repositioned on

the newly erected scalloped bars.


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CLINKERING IN THE FURNACE:

a. Due to poor quality of coal being fired in the recentpast, clinker formation was experienced in Boiler.Most of the clinkers were found to be highly porous

and breakable by crushing with hands. But few

pieces were black in colour and hard to break.

These black colour clinkers were analysed at NTPC

lab and found that it consisted of more than 99%ash only and no unburnt in the clinker. The reason

for black colour was reported to be due to the

presence of complex iron oxides and sulphatescaused by pyrites in raw coal.

CONTROL ON CLINKERING.

The following operational maneuver have been adopted as

precautionary steps to control clinkering in the furnace.

1. At loads more than 300 MW, oil firing is totally avoidedto eliminate combination firing, which restricts elevation

heat loading.

2. Excess air is maintained at O2 level around 4%.3. SADC dampers position is periodically checked and any

deviation found are attended immediately to maintain thewind box to furnace differential as per the design.


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4. Wall blowers have been commissioned and being operatedonce in a day, which will be increased to twice in a dayshortly after establishing remote operation of wallblowers.

5. HP heaters will be stabilized shortly and put into regularoperation to restrict the overall furnace heat loading.

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Damage to Bottom Ash Handling Sys