In a thermal power plant, boiler overhauling is generally done yearly. The presentation highlights the procedure for the re-commissioning of the boiler after capital overhaul.

1. RECOMMISSIONING PROCEDURE
AFTER CAPITAL
OVERHAUL OF BOILER
By
Manohar Tatwawadi
+91 9372167165
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2. OBJECTIVES
Recommissioning of the boiler after capital overhaul aims at
following objectives.:-
1. Preparation of the equipments for reliable and trouble free
operation. This may involve.
A. Inspection, pre operational checking and preparation
of the equipment for satisfactory trial.
B. Trial run, initial operation and performance
monitoring and
C. Problem analysis and solution.
2. Compiling and creation of equipment performance data for
future reference and comparison.
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3. OBJECTIVES
3. Checking of all interlocks of the equipment and confirming the
operation of relays as per the protection provided and
tripping of the equipment accordingly.
4. Some of the equipment or subsystem of the boiler like
auxiliaries, coal handling plant, ash handling plant, service and
instrument compressors, electrical systems are overhauled by
different agencies. For successful commissioning of the steam
generator all these equipment should be made ready at
appropriate stage of boiler commissioning. This involves
effective coordination between the operation and
maintenance staff.
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4. ACTIVITIES
• Broadly, the activities during recommissioning of
a boiler can be divided into following different
stages.
 Planning and Preparation.
 Inspection and checks.
 Trial run of equipments.
 Trial run of boiler and setting parameters for
operation regimes.
 Performance Testing.
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5. Planning and Preparation
• Planning for recommissioning of the unit should start atleast 8
to 10 days prior to completion of the overhaul of the unit.
• Scope of work and clear demarkation should be there
regarding responsibilities of different section/ agencies i.e.
mechanical, electrical, Testing, I and C, operation and
maintenance etc.
• To facilitate smooth recommissioning, proper coordination
should be there among various sections/ agencies for trial
run, testing, activities like chemical cleaning, steam blowing
and other commissioning activities.
• For effective monitoring, a PERT chart or a bar chart of
commissioning activities should be prepared. This can be
done after ascertaining the planned dates of activities and
readiness of requisite inputs like power, water, fuel,
lubricants, service facilities and trained personnel.
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6. Inspection and
Recommissioning Checks
• The first important recommissioning activity at site is the
inspection and checking of equipment.
• Generally this activity is meant to ensure overhaul of the
equipment completion, compliance of critical dimensions,
tolerances, removal of unwanted material and access for
inspection.
• All the air and gas duct should be checked for completeness
of work, correct supporting, expansion provision wherever
required and proper flanged and welded connections.
• The complete system should be cleaned of all debris.
Proper application of insulation to minimise heat loss and
to ensure human safety should also be checked.
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7. Steam Drum
• The steam drum should be checked for correct fit
up of internals and cleanliness.
• All safety valve nozzles, water column nozzles,
vent nozzles, pressure gauge nozzles, down
comer nozzles etc. should be checked for any
obstruction.
• The drum should be free to expand in all the
required directions.
• The water level gauge and water level instrument
connection should be strictly as per drawing.
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8. Superheater, Reheater,
Desuperheater and Economiser
• Cleanliness is the one of the prime requirement
in this area.
• The assemblies should be checked for transverse
and longitudinal spacing. Any misalignment
should be corrected.
• Supports, expansion clearances, correct
installation of cooled and uncooled spacers gas
baffles etc. should be checked.
• The thermocouples for temperature
measurement should be checked for correct
location and installation.
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9. Waterwalls and Buckstays
• All the waterwalls and other areas subjected to
expansion movement should be checked up for
any tie up with structural steel. Adequate
expansion clearances and guiding must be
provided.
• Correct location of instrument tapping points
should be checked. For measurement of
verification of thermal expansion of boiler,
indicators should be mounted at the required
location.
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10. Waterwalls and Buckstays
• The buckstays system should be checked for
correct installation and insulation filling wherever
required. Corner connection and intersection
between vertical and horizontal buckstays, front
and rear pass buckstays should be checked for
proper arrangement.
• The bottom ash hoppers may be checked for any
possible spray or splashing of water on to the
waterwall tubes. Water seal plates should be
checked for expansion clearances and adequacy.
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11. Hangers and Support
• All hangers of the boiler, headers, ducts and
piping should be checked up for correct
location.
• Loading spring compression in case of spring
type hangers should be strictly as per design.
• Both the spring and constant load hangers
should have adequate expansion range.
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12. Safety valves
• The hydrostatic test plug to safety valves must
be removed prior to firing the unit.
• After hydrostatic testing all the valves should
be cleaned, assembled and made ready for
setting.
• The safety valve vent pipes should be properly
supported with adequate expansion around
the drip pan.
• Drains of adequate size must be provided.
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13. Rotating Equipment
• The rotating equipment including fans, airheaters and
pulverisers should be completely checked as per O & M
manuals.
• Gaps and clearances inside the mill, air heater and all
bearings must be checked and adjusted.
• Correct direction of rotation and proper alignment with
the drive should be ensured.
• The suction vanes, filters, strainers, guides must be
ensured to be clean and dust free.
• Lubricating system, oil with oil pumps must be ensured
to be working and all permits must be closed.
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14. Trial Run of the Equipment
• Normally before trial run of equipments, service
facilities like power, cooling water and lubricants
must be made available.
• Prior to trial run of the equipment, trial run of the
drive unit (electric motor) need to be taken in the
un-coupled condition. Motor’s protection and
interlocks should be tested.
• After successful trial run of drive, the equipment
be coupled and trial run in coupled condition be
taken.
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15. Trial Run of the Equipment
• Trial run of the rotating equipment must be
individually done.
• During trial run, equipment should be carefully
observed for any unusual noise, abnormal
vibrations or undue bearing temperature rise.
Any defect noticed should be immediately
attended.
• A record of trial run including vibrations bearing
temp, starting current, no load current etc should
be maintained for future reference.
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16. Safety Valve Setting Procedure
• The safety valve in the Boiler to be floated and set to operate
at the required pressure. This is done to ensure the
availability of the safety valves for pressure release in times of
necessity and also to check the adequacy of the venting
system.
• During safety valve setting, the Boiler steam pressure will be
raised to the maximum value of the setting.
• Before commencing floating, all the precommissioning checks
and servicing of the valves should have been completed.
• The boiler pressure is gradually raised at much lower rate to
facilitate measuring of waterwall and other pressure part
expansions.
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17. Safety Valve Setting Procedure
• After measuring the expansion at operating pressure, all the
safety valves are gagged except the one to be floated and
when the boiler pressure is around 10% lower than the set
value, the valve is warmed up and eased by hand popping.
• Then the pressure is gradually raised. When the safety valve
floats, the pressure is noted down using a calibrated gauge
and the firing is stopped.
• As the boiler pressure decays, the safety valve resetting
pressure is also noted.
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18. Safety Valve Setting Procedure
• Any adjustment required for setting the lifting
pressure, blow down is done as per the safety valve O
& M manual. Then the firing is restarted and pressure
raised. The procedure is repeated till the safety valve
lifting pressure and blow down are within acceptable
limits. The above procedure is repeated for all safety
valves. If floating is started from the valve set at
highest pressure and proceeded in the order of
decreasing set pressure, gagging of already floated
safety valves will not be normally required.
• Before declaring the completion of this activity, gags
from all the safety valves should be removed and
recorded.
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19. FILLING OF BOILER
– While filling the boiler with water, following care
should be taken.
• DM water only should be used to reduce
corrosion and deposition.
• The temperature of water fed should be within
allowable limit compared to the temperature of
metal to reduce thermal stresses. Temperature
difference within 50 0C is generally considered as
safe.
• Keep all the vents open while filling such that no
air pocket is formed.
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20. Hydraulic Testing of Boiler
Precautions to be taken during a Hydraulic Test.
• If the test is to be conducted above the operating
pressure, the safety valves are to be gagged.
• Only D.M. water treated with volatile chemicals such
as Hydrazine, Ammonia and proper P. H. value as per
the chemist should be used.
• Use water at ambient temperature but above 25 0C
• Check the supports and arrest all spring loaded
hangers to take up the additional load due to water.
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21. Hydraulic Test… Precautions
• Keep the boiler outlet valves tightly closed. Ensure
that water should not enter the Turbine.
• Arrest the hangers of pipe line.
• If the Boiler is to be kept idle after the hydraulic
testing, pressure parts are to be preserved by wet
method or nitrogen blanketing.
• Ensure that the safety valve gagging (or Hydraulic
test plug) is removed after completion of Hydraulic
test.
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22. TRIAL OPERATION OF BOILER
• After completing the Re-commissioning activities
satisfactorily, Boiler is ready for Trial Operation.
• At this time, all facilities and inputs for loading the unit and
putting into regular service should be ready.
• Same time, the Turbine, Generator and complete electrical
transmission system must be ready.
• Before trial run of the Boiler, general inspection of all
equipment is carried out. All rotating equipment is checked
for proper lubrication and smooth operation.
• Dampers and valves one checked for proper operation.
• The complete Instrumentation and control must be checked
for proper functioning.
• The boiler is then lighted up in accordance with operation and
maintenance instructions as per the light-up programme
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23. Performance Testing
• On successful completion of trial run of the boiler it is
declared ready for further full load operation.
• If at this stage there are some minor pending points or
problems which do not affect the safe operation of the
unit, the concerned agency may be asked to attend the
problem at the next available opportunity.
• When the unit is fully loaded and the performance is
found satisfactory, then performance testing of the unit
can be carried out to find out the improvement after
carrying out the capital overhaul.
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