Difficulties Faced

7/27/2019 Difficulties Faced

1/6

PROJECT : JSL DUBRI UNIT-1

PROBLEM : UNIT-1 STABLE OPERATION COULD NOT BE

ACHIEVED DUE TO DIFFICULTIES FACED IN

THE FOLLOWING AREAS.

- AIR FLOW CONTROL COULD NOT BE PUT ON AUTO - WIND BOXTO FURNACE DP WAS LOW

- DIFFICULTY IN PARALLELING OF PA FANS- DEAERATOR LEVEL CONTROL COULD NOT BE PUT ON AUTO- MILL OUTLET TEMPERATURE AND FLOW CONTROL-

SUPERHEATER AND REHEATER TEMPERATURE CONTROL

AIR FLOW CONTROL

MILL AIR FLOWS:

It was observed that the unit was running at 115 MW with 5%oxygen level instead of 3.5 %. Total secondary air flowmaintained was much less. Due to low secondary air flow, the

wind box DP maintained was also lower. Also the O2 calibrationwas to be done with standard gas.

Mill air flows were suspected to be high, as the hot air

dampers were almost in full open condition. Hence all the mill

air flows were to be checked for correctness of measurement.


2/6

Recalibration of air flow transmitters of all Mills :

While recalibration of flow transmitters, it was observed that the

mill air flows were far exceeding the transmitter range. It was

verified with actual manometer measurement, which indicated 100mm WCL against the range of 80 mmWCL. All the

transmitters were recalibrated to 100 mmWCL and thecorresponding corrections were incorporated in MAX control. The

air flow curve was also extrapolated to a range of 0 to 100 mm WCL.

After completing this exercise, the mill air flows maintainedwere observed to be far in excess and was at 70 TPH. Any reduction

in mill air flow was also drastically reducing mill outlet temperature.

Higher fuel flow in feeders suspected, which are to be attended by

site one by one.

SECONDARY AIRFLOWS :

Due to higher flow of primary air going through the mills,

the secondary air flow requirement was lower. In addition, it was

observed that the blade pitch positions indicated at control roomfor both FD FANS were around 5-10 % only. On physical inspection, it

was observed that the blade pitch positions had slipped from original

setting.

Both FD FANs blade pitches were attended without stopping the unitor load reduction, one at a time. After rectification, the blade

pitch positions have improved and air flow control could be put onauto. Presently both fans are running at 30% blade pitch on auto.

02 analyzer was calibrated with standard oxygen cylinder and air

flow control was put on auto.


3/6

WIND BOX TO FURNACE DP LOW

The lower wind box DP was due to the less flow of secondaryair because of more primary air flow through mills. Also discrepancieswere observed in the physical position of SADC dampers. The

same were corrected and recalibrated. After recalibration of mill airflow, oxygen analyser and correction of SADC dampers, windbox to furnace differential pressure was put on auto and it was

maintaining at 100 mmWCL.

PARALLEL OPERATION OF PA FANS : Step 1 :

PA fans were taken out of service, one at a time and blade pitch &

control slide travel were checked by adjusting the stoppers.

Step 2 :

Stroking time of servomotor of blade pitch was checked for each

fan and was found to be equal ( 17 seconds each).

Step 3 :

The stroking time of both the fans was adjusted to 26 seconds

each with blade pitch power cylinder.

Step 4 :

The approximate paralleling point identified from fan curve was at 30


4/6

% of blade opening ( around 200 Tons of PA flow ) for the running

flow conditions and system resistance. The second fan was started

and the load on first fan was reduced to maintain PA header pressureless than the trough point on the curve (fan curve). The second fan

blade pitch was increased to 30 %.

As the running fan blade pitch position was slightly decreased, both

the PA fans got paralleled. This operation was taken up with twomills in service and third mill on no load, to maintain total primary

air flow of 200 TPH. After paralleling, PA header pressure was put

on auto and tuned to the requirement. While tuning, the speed ofresponse of blade pitch matters and shall be equal. This was taken

care by steps 1 & 2. Once paralleled, the operating point was

taken further to the right on the curve by increasing the air flowfor the same header pressure.

During the first paralleling operation, as the blade pitch

paralleling point and responses need to be studied, it was donemanually and then put on auto operation. It was paralleled at 585-610

mmWCL of hot air header pressure.

After paralleling, it is imperative that the operating point shallbe taken further right on the curve to avoid de - paralleling andbefore putting PA header pressure on auto operation

Points to be noted:

In the initial trial, there can be a reduction in mill air flows

followed by increase after paralleling, which may create fluctuationsin furnace draft. Hence auto loops such as furnace draft, mill


5/6

air flow and temperature controls shall be commissioned to makeparalleling of PA FANS much simpler.

Suggestions :

To eliminate this problem, anti stall ring installation can beconsidered.

Leakage air in air heaters Vs PA header pressure curve by

BHEL/Ranipet will help in identifying clearly the operating point ofboth PA fans.

DEAERATOR LEVEL CONTROL:

In the indirect method, the controller sees and assigns higher order

of priority to hot well level. Whenever the deaerator level falls, the

hot well make up will be increased and as the level builds up more in

hot well, more flow will be allowed to Feed Storage Tank ( FST )

of deaerator. Since the capacity of FST is large in relation to hot

well, this will reduce the emergencies and result in a stablecontrol. Deaerator level control was put on auto and tuned.

SUPER HEATER AND REHEATER TEMPERATURE CONTROL

The logic blocks built in super heater and reheater level controls had

certain deficiencies. For example, the DESH outlet temperatureset point (variable) as decided by the output of the PID of


6/6

final temperature controller was not functional because of missinglinks between the feed forward blocks provided from fuel control,

steam flow control, drum pressure. A similar problem was also

noticed in reheater temperature controls.

The above problems were resolved by suitably inserting the missinglinks of the logic blocks and temperature controls put on auto and

tuned.

After the above correction and changes, the boiler follow up mode wastuned and put on auto for a short duration.

The machine was taken to rated load and 72 hours of trial operationwas completed.

Documents

Difficulties Faced