Diagnosis of a CFBC Boiler for Under Capacity Problem

8/17/2019 Diagnosis of a CFBC Boiler for Under Capacity Problem

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DIAGNOSIS OF UNDERCAPACITY PROBLEM IN A CHINESE MAKE CFBC BOILER

By K.K.Parthiban, Venus Energy Audit System – [email protected]

Problem

The CFBC boiler was recently commissioned and it was reported that the steam generation was less.

Altogether the CFBC boiler had been in operation for 15 days only. During this time, it was seen that

the boiler could not generate rated steam. The maximum generation as per log sheet was around 26

TPH only. The boiler design parameters were 75 TPH, 510 deg C, 66 kg/cm2 with feed water

temperature at 170 deg C. With deaerator it was possible to attain 127 deg C. Only after HP heater

commissioning, it would be possible to go to 170 deg C. The GCV of as fired coal, was in the range of

3300 – 3600 kcal/kg. The ID fan which was on VFD mode was at full rpm during the past operations.

The inlet damper was also full open. Due to the limitation of ID fan, the load on the boiler could not

be raised. The company contacted us to diagnose the under capacity problem.

Constructional details of the boiler

The CFBC boiler was arranged with two refractory lined cyclones. The second pass was with final

superheater, primary superheater and economiser. The secondary airpreheater was placed after the

economiser and the primary APH was placed as the last heat recovery device. The wing wall SH

served as the secondary SH. The second pass did not have any water cooled or steam cooled walls. Up

to economiser the second pass was with refractory brick walls backed up with MS casing.

Observations & modifications during shut down

The boiler was under shut down on the day of visit. The boiler was offered for inspection with almost

all the inspection doors under open condition but in hot condition. The following were the

observations.

1. The log readings showed very low gas side temperatures after the cyclone. The furnace outlet gas

temperature dropped from 850 deg C to nearly 450 deg C at SSH inlet. This was an indication of

large amount of air ingress, nearly to 64%.

2. Starting from the furnace up to ID fan inlet there were so many locations where the seal welding

of the boiler was not complete. The ID fan was run and leakages were checked with flame.

The cyclone roof plates were not fully welded.

The wing wall SH seal work was not complete. Lot of ash was seen on the furnace top. It wasadvised to apply insulation wool and cover with Plaster of Paris. This work was done

immediately. Yet the retainer plate of 100 mm height was recommended all around so that the

POP could serve its purpose. I advised to apply hessian cloth and black bitumen paint over the

POP mastic so that the rain water / furnace pressure pulsations cannot damage the insulation.

The cyclone outlet duct seal welding was found to be left out.

The sealbox around hanger tubes that come out of second pass roof was found to be leaky.

Dust was seen around. It was advised to remove the aluminium cladding around the header

and the insulation be covered with POP. The work was done immediately.

The entire second pass was made of refractory work and casing plates were to be welded to

structure. Welders were engaged to carry out the seal welding night & day. The work went on

for 20 more days.



The duct at outlet of APH to ESP inlet was found to be leaky at some places as inspected from

inside of the gas path.

The ESP inspection doors at roof top were found to be without any asbestos rope and

fasteners.

The ESP side casing inspection doors were found to be passing. The ESP inspection doors

should have been double door system to prevent condensation damage. Nowadays supplierswant to save some cost in this.

3. The ID fan impeller to suction cone clearance was found to be in order.

4. The ID fan suction damper was found to open 100% when the power cylinder was opened from

control room. However this damper should be 100% open when power cylinder is in closed

position. The damper has to be open when the air supply to damper fails. This is for safety reason.

5. The loop seal inspection doors were opened and inspected. There were castable refractory pieces

seen inside the loop seal. This could prevent the proper loop seal operation. If loop seal is not

operating, Cyclone can get filled and there will be additional pressure drop. ID fan can get loaded

due to this.

6.

The loop seal air fan was switched on and all nozzles were seen to be freely discharging air in both

loop seals. The nozzles were to be cleaned manually by a wire. The hole diameter is only 2 mm

and hence there is always chance of plugging if air flow is less.

Suggested parameters for CFBC operation

I advised the parameters for monitoring the CFBC:

1. Bottom bed temperature: 850 – 925 deg C – to be adjusted using PA air flow & adequate bed

height.2. Bed particles size control - Drain ash shales which settle at bottom.

3. Air box pressure & bed height – air box pressure shall be 850 – 1000 mmWC.

4. Upper furnace temperature control – adequate SA air to be given so that the gas temperature

increases no more.

5. Loop seal air shall be maximum so that positively the loop seal works.

6. Burner air dampers shall be kept open so that the PA comes through burner as well. This will

reduce the PA fan power consumption.

7. After all air leakages are arrested, O2 at eco outlet shall be 3.5 to 4 % with insitu O2 meter.

8.

The sieve analysis of APH ash / ESP 1 ash shall be checked for + 100 micron ash particles. If the+ 100 microns are increasing, it implies the loop seal is not working. Check the ash flows freely

from the loop seal.

9. A portable O2 cum CO analyser shall be purchased for monitoring the boiler. If CO is more than

250 ppm, it implies that the SA air has to be increased.

Improvements / modifications required in CFBC boiler.

1.

View holes are required in downleg with compressed air cooling so that the material flow is

checked easily.

2.

I recommended to have anubars for measurement of loop seal air flow for both loop seals. Any plugging of the loop seal can be easily identified by the difference in flow.

3.

I advised to conduct smoke bomb test so that the leakages can be further identified and rectified.



Conclusion in the first visit

It was clear that the ID fan was overloaded by the air infiltration and that caused less steam

generation. Welding was underway to make 100% leak proof casing. Later I came to know that the

steam generation improved from 26 TPH to 62 TPH. During the first visit, it was not possible to checkflue path leakages by inside inspection as the entire boiler was still warm.

Second visit

The second visit was made following the request from power plant in-charge with regard to vibration

experienced in secondary APH zone. The diagnosis of the vibration will appear in another paper. For

now I focus on the leakages in air & gas ducting seen inside the boiler. I wonder on the standard of

workmanship of boilers.

During the visit the following were noted.

1. During inspection inside the flue path, there was large amount of air leakage seen in the APH

through the lifting holes.

2.

The outside air is also seen to come through the damaged expansion bellow. At many places the

expansion joint was not welded to casing and the support beam.

3. There were cracks developed in duct due to flow induced vibration.

4. The air duct connections to APH were not fully sealed.

Again two days of seal welding went on before restarting the plant. The boiler touched its rated

capacity.

Conclusion

In fact the company contacted us for audit during construction. At the time the customer just availed a

day visit. Finally the money lost due to delayed generation was very high. Many project managers

believe they are capable of addressing quality of work along with project completion. Hiring

inspection agencies could eliminate most of mistakes in advance instead of suffering later and

undergoing a tremendous pressure from management. It sometimes leads to loss of confidence by

management on the concerned project manager. I see the design features are same for many CFBC

boilers of Chinese origin. I hope the readers take care of the installation defects / correct their boilers

in case it is already in operation.



Photo 1: The entire second pass is made of refractory backed up with steel casing plate. The

steel plates which are tack welded have not offered leak proof casing. No one had faith in my

statement that the air ingress was overloading the ID fan. But later it was seen that the leakage

was the culprit for under capacity.



Photo 2: The 2nd pass of the boiler is totally made of refractory up to APH inlet. It is seen that

there is considerable air ingress was present. This was proven by the boiler performance

improvement. Ash marks confirm that there was air ingress.

Photo 3: The loop seal was checked for sufficiency of the fluidizing air. The defluidization is

seen due to presence of castable refractory lumps. The refractory had spalled from the wall.



Photo 4: The view hole in the loop seal outlet chute to furnace shall be checked regularly. The

cooling air shall be such that the glass does not burst during service. Compressed air cooling is

advised. The loop seal operation shall be checked in each shift physically.

Photo 5: It was seen that there were long metres of seal weld yet to be carried out. The seal

welding brought relief in ID fan capacity of CFBC boiler.



Photo 6: The photographs show the extent of pending seal welding work. There are many areas

of air infiltration zones.

Photo 7: There should be no dust at I pass roof top. The dust seen here is due to leakage from

SH outlet stubs from roof top. As per suggestion the POP was applied. Yet the POP shall be

finished with hessian cloth spread and then be painted with black bitumen paint.



Photo 8: There were inaccessible locations where moment brackets have been used for resting

beams. The sealing was carried out to reduce the ID load.

Photo 9: There was dust seen in the hanger tubes location on roof top of second pass. It was

advised to remove aluminum insulation cladding & use POP mastic finish. Generally POP /cement mortar finish gives a simple but crude finish with a better sealing.



Photo 10: The cyclone top is seen a long open line, wherein air was seen to be sucked inside

when tested with flame.

Photo 11: The cyclone top is seen with openings made for pouring refractory for roof casting

insitu. The patch plates are to be welded.



Photo 12: This is the location of wing wall SH seal box at front waterwall. The seal welding of

seal box is incomplete. The ash from furnace is seen to come out here.

Photo 13: When the ESP was inspected, it was seen that the doors were simply left at its place

without fasteners. Ash was seen around the door.



Photo 14: The duct crack developed in the secondary APH casing. This was suspected to be due

to flow induced vibration in secondary airpreheater.

Photo 15: Crack that propagated in the SAPH and PAPH interconnection gas duct. This crack is

very close to PAPH. If SAPH tube bundles are resonating under flow induced vibration, the

casing surrounding SAPH would fail in no time.



Photo 16: The SAPH to PAPH interconnecting gas duct is cracked on ESP side. The crack is

very close to PAPH.

Photo 17: During inspection it was seen that there were missed out seal welds, which had led to

direct air leakage from the SAPH & PAPH to flue path. Also there were welds missed out

between the structure and gas duct.



Photo 18: Tubes seen polished due to air leakage from PAPH to flue path due to lifting lug hole.

In all the four blocks, the lifting lug holes were not closed at all.

Photo 19: The polishing of tubes is due to large air leak from the Primary APH. This is because

the tube sheet to air duct was not closed as per drawing.



Photo 20: There was a long crack seen in the expansion joint below the secondary APH casing.

The joint needed repair / replacement. Note the expansion below was not welded to casing and

structure.

Photo 21: There were locations wherein proper fit up was not done and hence outside air was

coming in.



Photo 22: The expansion bellow was first welded to tube sheet and the gas duct was to be

welded to expansion joint. The weld was missing between the expansion joint and the air duct.

Photo 22: Improper closure of expansion joint with the tube sheet. The expansion bellow had to

be trimmed to complete the seal weld.



Photo 23: The expansion joint was not sealed to tube sheet at many locations.

Photo 24: During inspection it was seen that there were missed out seal welds, which had led to

direct air leakage from the SAPH & PAPH to flue path. Also there were welds missed out

between the structure and gas duct. All these details were well covered in the drawings. It was

the case wherein front line inspection was not carried out.



Photo 25: The Chinese drawings were in detail covering the various seal welds required for

performance. The above detail is the bellow seal required between side casing and the support

beams.

Photo 26: The expansion joint between the APH supporting beam and the casing below. Item no

16 was not found in place at some locations. This plate could prevent heat passing to the APH

support beams.



Photo 17: Generally it is seen that the ash settles over the baffles and scalloped plate, if erected

properly. The tubes here are not thinned down.

Photo 18: The protection shields on the SH tubes are seen in tact this time. During last

inspection many shields were upside down.

Photo 27: This drawing shows the air side sealing arrangement between the air duct and the

tubes sheet.

Photo 28: The following are the details from the drawing. Though the drawings were clear, the

project manager was helpless as he did not have a person who could read engineering drawings

and carry out proper inspection.

Documents

Diagnosis of a CFBC Boiler for Under Capacity Problem