boiler comm 24

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XXIV. BOILER COMMISSIONING

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Contents

1. INTRODUCTION

2. PLANNING AND PREPARATIONS

3. INSPECTION AND PRECOMMISSIONING CHECKS

4. TRIAL RUN OF THE EQUIPMENTS

5. PRECOMMISSIONING ACTIVITIES

6. TRIAL OPERATION OF BOILER

7. PERFORMANCE TESTING

8. TRAINING OF OPERATING AND MAINTENANCE STAFF

FIG. XXIV-1 SCHEME OF CHEMICAL CLEANING (INFORMATIVE)

FIG. XXIV-2 STEAM BLOWING - PUFFING METHOD FOR

REHEAT BOILERS

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XXIV. BOILER COMMISSIONING

1. INTRODUCTIONThe present trend in utility and industrial market is to go in for large capacity, high pressure

steam generators. These steam generators commonly known as boilers have a number of

auxiliaries and sophisticated controls. Their size and complexity render them unsuitable for

shop assembly and testing in most cases. Hence all the components of the steam generator

have to be assembled and tested only at site. The operation of checking the equipments and

tuning them before putting into commercial service is commonly termed as Commissioning.

Generally commissioning a steam generator aims at accomplishing the following objectives:

i) Preparation of the equipments for reliable and trouble-free operation.

This may involve:

a) Inspection, pre-operational checking and preparation of the epuipments including

specialised cleaning.

b) Trial run, initial operation and performance monitoring.

c) Problem analysis and solution.

ii) Fulfilling contractual obligations like proving performance guarantee.

iii) Compiling and creation of equipment performance data for future reference and

comparison.

iv) Training of operation and Maintenance personnel.

Frequently some of the equipments or subsystems of the boiler, like auxiliaries, coal handling

plant, ash handling plant, service equipments and electrical systems are engineered by different

agencies. For successful commissioning of the steam generator all these equipments should

be made ready at appropriate stages of boiler commissioning This involves effectiveco-ordination, and is an aspect of project management in commissioning. While details of

this aspect of commissioning are not being covered in this chapter, mention will be made of

the -same at the relevant sections for the sake of completeness.

Broadly, the activities during commissioning of a boiler can be divided into seven stages

from preparation to performance testing of the unit:

i) Planning and preparations.

ii) Pre-commissioning inspection and checks.

iii) Trial run of equipments.

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iv) Pre-commissioning activities Including specialised cleaning.

v) Trial operation of boiler and setting parameters for operation regimes.

vi) Performance testing.

vii) Training of operation and Maintenance personnel.

2. PLANNING AND PREPARATIONS

Planning for commissioning the unit should start even during the early stage of contract. The

contract specification should be clear in scope of work and demarkation of responsibilities of

executing agencies.

To facilitate smooth commissioning, agreements should be reached on procedures for trial

runs, testing, activities like chemical cleaning, steam blowing and other commissioning

activities even in the contract stage. For effective utilisation of resources at site as also forefficient monitoring a PERT chartor 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. For more

realistic scheduling this is best done after a site inspection by an experienced commissioning

engineer.

The commissioning team, before setting up office at site will study the contract specification,

design calculations, anticipated, and guaranteed performance figures, various boiler schematics

and operation and maintenance instructions. The lead commissioning engineer will do the

planning for getting ready all commissioning documents like equipment check lists, trial run

instructions report format etc. He will also establish the site office and organise other siterequirements like manpower, instruments etc.

3. INSPECTION AND PRECOMMISSIONING CHECKS

The first important commissioning activity at site is the inspection and checking of equipments.

Generally this activity is meant to ensure erection completion, compliance of critical

dimensions, tolerances, removal of unwanted material and access facilities for maintenance

and inspection.

All the air and gas paths including ducts should be checked for completeness of work, correctsupporting, expansion provision where 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 up.

3.1 Steam Drums

The steam drum should be checked for correct fit-up of internals and cleanliness, All safety

valve nozzles, water column nozzles, vent nozzle, pressure gauge nozzles, down corner 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 connections should be

strictly installed as per drawings.

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3. 2 Superheater, Reheater, Desuperheater and Economiser

Cleanliness is one of the prime requirements in this area. The assemblies should be checked

for transverse and longitudinal spacings. Any misalignment should be corrected . Supports,

expansion clearances, correct installation of cooled and uncooled spacers, vibration snubbers,

gas baffles, etc., should be checked. The permanent and temporary thermocouples are to be

checked for correct location and installation. These should be identified and tagged. Heatcheck should be done on each thermocouple to ensure continuity and response.

3.3 Waterwalls and Buckstays

All the waterwalls and other areas subjected to expansion movements should be checked up

for any tie up with structural steel. Adequate expansion clearances and guiding must be

provided. Correct location of openings like access doors, observation ports and instrument

tapping points should be checked. For measurement and verification of unrestrained thermal

expansion, indicators should be mounted at the required locations.

The buckstay system should be checked for correct installation and insulation filling wherever

envisaged. Corner connections and intersections between vertical and horizontal stays and

front and rear pass buckstays should be checked for proper arrangement. Generally the surface

guides also form part of the buckstay system. These should be checked for correctness of

location, sturdiness and marked up for indicating expansion movements.

The bottom ash hoppers may be checked for any possible spray or splashing of water on to the

waterwall tubes. Water seals also should be checked for expansion clearances and adequacy.

3.4 Hangers and Supports

All hangers of the boiler, headers, ducts and piping should be checked up for correct location

and loading. Spring compression in the case of spring type hangers, should be strictly as per

design. Both the spring and constant load hangers should have adequate expansion range. All

spring stops and locking devices should be brought to operating positions or removed and

accounted.

3.5 Safety Valves

Correct location of the safety valves should be ensured. The hydrostatic test plug to safety

valves must be removed prior to firing the unit. After hydrostatic testing all the valves shouldbe cleaned, assembled and made ready for setting.

The safety valve vent pipes should be properly supported with adequate expansion clearances

around the drip pan. Drains of adequate size must be provided as indicated in the 0 & M

instruction.

3.6 Tangential Wind Boxes

The wind box in general should be checked for proper installation as per drawing. The fuel

piping should have adequate and proper supports. The shouldnt load the coal nozzles. Thesecondary air dampers must be free to open and close with adequate opening for cooling air at

fully closed position. Tilt should be operable through full range. The linkage connection for

tilting and wind box dampers should be checked witty respect to drawing. While tilting, all

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the nozzles in the same corner and at each elevation in different corners should tilt in unison

within the allowed variation. The internal clearances between wind box frame end nozzles

and burner panel should be as per design.The oil piping around the wind box should be

properly routed and supported. Flexible hoses must be connected without any twist or bending

and must allow for cubical expansion of the wind box and free travel of retract mechanism.

The location of the diffuser in the oil compartment and the tip position with respect to diffuseris a critical dimension. This should be strictly within the tolerance specified.All the oil and

atomising air lines must be steam blown and the oil pipingflushed sufficiently long enough

to ensure cleanliness.

The area around the wind boxes must be clean. Adequate clearance and handling provision

for removing oil guns and coal nozzles must be ensured.The ignitors and ignition system

including control cabinets should be critically examined for proper installation.

3.7 Rotating Equipments

The rotating equipments including fans, airheaters and pulverisers should be completely

checked as per the 0 & M manuals and manufacturers recommendations. Gaps and clearance

inside the mills airheaters and all bearings must be checked and adjusted. Correct direction

of rotation and proper allingnment with the drive also should be ensured. The correct

functioning of all the protective interlocks must be checked and recorded.

3.8 Instrument Tappings

All the instrument tappings and impulse lines must be checked for proper installation with

adequate facilities for cleaning and maintenance.

3.9 F.S.S.S.

Checking the F.S.S.S. system involves elaborate work of checking the field equipments, wiring

and simulation including testing various protections. As the F.S.S.S. is intended for ensuring

safe operation of the boiler checking should be meticulous and by passing of interlocks and

protections should be avoided.

4. TRIAL RUN OF THE EQUIPMENTS

All the rotating equipments must be individuality trian run. 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, the drive unit (normally an electric motor) will be trial run

in the uncoupled condition. To start with pre-commissioning checks on the motor should be

completed and protection and interlocks tested. After successful completion of trial run of the

drive, the equipment will be coupled and trial run at no load, for about 8 hours. During the

trial run, equipments will be Carefully observed for any unusual noise, abnormal vibrations

or undue bearing temperature rise. Any defects noticed should be immediately attended and

the trial run repeated till satisfactory results are obtained.A record of the trial run including

the vibration readings, bearing temperature rise, starting current, no load current, time of

acceleration, deceleration, defects noticed and rectifications carried out should be maintainedfor future reference.

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5. PRECOMMISSIONING ACTIVITIES

It is a general practice to carry out the following tests and activities before commencing

trial operation of the boiler:

i) Air and gas tightness tests.

ii) Chemical cleaning of the boiler.

iii) Thermal flow test.

iv) Steam blowing.

v) Safety valve setting.

vi) Clean air flow test of pulverisers.

vii) Testing of protection and interlocks.

viii) Calibration of instruments.

5.1 Air and Gas Tightness Tests

The air tightness and gas tightness tests are conducted to detect the source of leakages in the

system. This should be carried out before applying the insulation. Prior to carrying out this

test all the pre-commissioning checks of components in the system including dampers should

have been completed and the access doors, observation ports, and other openings closed andsecured properly.

For conducting gas tightness test following procedure is generally used - Run F.D. fan and

open all the dampers in air and flue gas path excluding I.D. fan discharge dampers. (I.D. fan

should not be run and the I.D. fan running interlock for F.D. fan running should be bypassed).

Pressurise the flue gas path to at least 50 mm of water column. The furnace pressure should

not exceed 100 mm of W.C. Light up a smoke bomb in the furnace and one in the second pass

through suitable openings, the opening should be closed. Check manhole doors, peephole

doors, duct joints, fan flanges, hopper joints etc., for leakage. The location of leakage points

may be marked and recorded for rectification. Repeat the test after rectification to ensure that

the system is tight in all respects.

Similar procedure is adopted for conducting air tightness test. This is done by running F.D.

fans and keeping all the dampers in air path open except for the dampers in the wind box. The

system is pressurised to 100 mm of water column. A smoke bomb is introduced and lighted

up in the suction of each fan.

5.2 Chemical Cleaning of the Boiler

The internal surfaces of a boiler in contact with water or steam must be kept clean for efficient

heat transfer, maintaining steam purity and to avoid pressure part failures. Most steam generator

manufacturers recommend chemical cleaning of the unit for this purposes. Cleaning processadopted can be generally classified under alkali-boil out and acid cleaning.

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Alkali boil out is carried out to remove materials such as lubricants, oil and rust.

Acid cleaning of boilers removes scales and deposits formed on the heat transfer surfaces in

contact with water. This also removes mill-scales and corrosion products found in the steam

generator.

As a standard practice, the steam generating surface (i.e. water walls) only are chemicallycleaned. For low pressure industrial boilers alkali boil out alone may be adequate, while for

boilers with high operating pressure alkali boil out is followed by acid cleaning.

Chemical cleaning of the boiler should be preceded by cleaning of the pre-boiler system. This

is accomplished by hot alkaline mass flushing to remove loose debris and silica bearing

deposits.

5.2.1 Alkali Boil Out

As already stated the alkali boil out is carried out to remove water and alkali soluble andsaponifiable compounds from water side surfaces of the unit. Sand, loose Mill scale, and

corrosion products formed on the tube surfaces during erection and following hydraulic test

are also removed by blow-down during the boil out process.

Generally the alkaline chemicals used for boil out is a combination of one or more of the

following chemicals :

i) Caustic soda NaOH

ii) Soda ash Na2

CO3

iii) Tri sodium phosphate Na2PO

412H

2O

iv) Sodium sulphite Na2SO

2

v) Sodium nitrate Na2NO

3

Of the above soda ash and tri sodium phosphate are the most commonly used chemicals.

Sodium sulphite is used to reduce oxygen corrosion and sodium nitrate is added to prevent

the possibility of Caustic embrittlement. In addition to those chemicals a detergent may also

be added.The boil out process may be preceded by hot and cold water flushing of the System to remove

loose debris and sand. As this may be the first time the steam generator is fired, the firing

system, the draft system and the instrumentation must be ready. To facilitate checking up free

and uniform expansion of the unit while raising pressure, the firing rate should be maintained

at minimum level. For modern high pressure boilers with welded water walls and wall

superheaters, any refractory drying will also be carried out during this process itself.

Preparatory to boil out, the boiler should be filled with water to the normal operating level in

steam drum. The chemicals may be added through the normal chemical feeding system in the

drum or introduced into the steam drum through an open manhole.

Steam pressure is then raised slowly by firing the ignitors and warm up guns. To ensure

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adequate circulation of water and good mixing of chemicals in natural circulation boilers, the

pressure is raised to about 1/5 the normal operating pressure or 25 kg/Cm2 (G) whichever is

lower. Low pressure units may be boiled out at pressures above 7 kg/cm2 (G). The pressure is

maintained at this level till satisfactory results are obtained. Every 2 to 4 hrs. low point drains

in the ring headerare openedfor blow down for about 2 minutes after putting out the fire.

During the process alkalinity, silica level, pH and oil content are monitored at frequent intervals.Samples may be taken from the continuous blow-down line and ring header drain. The process

is terminated when the oil content reduces to satisfactory level (generally less than 1 P.P.M.).

At the completion of the process the boiler will be boxed up for slow cooling, drained and

flushed free of residues with hot water followed up by cold water flushing.

5.2.2 Acid Cleaning

For all High pressure units the alkali boil out will be followed up by acid cleaning. There are

two methods in vogue for acid cleaning :

i) Soaking method.

ii) Circulation method,

In the soaking method, the internal surfaces of the boiler are kept soaked in inhibited acid

solution for 4 to 6 hours after firing the unit to raise temperature. In the circulation method the

inhibited acid solution is maintained under circulation for 4 to 5 hours by means of acid

circulation pumps and a temporary circulating system of dissolving tank and piping. Required

water temperature is maintained by addition steam from external source in the dissolving

tank. In both methods inhibited acid is used for preoperational cleaning.

Fig. XXIV-1 gives a scheme of circulation system adopted for circulation method which is

commonly practised for acid cleaning. In this method use system is first filled with

dernineralised water and temperature raised to the required value by adding steam to the

dissolving tank. The flow required is maintained under circulation. Inhibitor is first added to

the circulating water followed by acid. Ammonium bi-fluride is sometimes added to increase

the reaction rate and to dissolve silica. The solution is kept under circulation by operating the

pumps. Samples of circulating water are taken every 15 minutes from different locations and

analysed for acid concentration (percentage), pH value, silica, and iron contents.

The process is terminated, when the iron concentration stabilises. Normally this will happen

in about 4 hours. At this stage the system is drained under positive nitrogen pressure, and

rinsed with demineralised water. To ensure thorough iron removal this rinsing may be followed

by similar cleaning with citric acid.

The system is then drained under nitrogen capping and rinsed till no traces of acid are detected

in the sample.

After rinsing neutralisation may be done with an alkaline chemical like soda ash. Finally in

the passivation stage the chemicals (normally hydrazine and ammonia) are added to the hotcirculating water and circulation is maintained for a pre-determined period after which whole

system is drained hot. The complete system is well ventilated by opening all the vents and

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manholes. Throughout the acid cleaning process the superheaters are filled with treated D.M.

water and pressurised. Once the surfaces are sufficiently cool the internal surfaces of the

system should be inspected for cleanliness and passivation coating.

Chemical cleaning of the unit removes some weld slag also. Hence after cleaning, the

boiler must be checked for weld sweating or leakages. This is best done by hydraulic

testing of the unit.

The passivation coating created immediately after acid cleaning will be lessadherent. To

achieve permanent base protection layer, second stage passivation is carried out at a

temperature of about 250oC. This is achieved by firing the boiler and raising the pressure to

about 40 kg /cm2 with adequate content of hydrazine as the prime chemical.

5.3 Thermal Flow Test

Thermal flow test is conducted to detect passage obstruction in waterwall, economiser and

superheaters. The chocked tube is identified with the help of thermal flow meter using theprinciple that the rate of cooling of a heated tube depends upon the fluid flow rate inside the

tube. The thermal flow meter used in the test Measures the time taken for a heated tube to fall

in temperature from 10oC above the ambient to 3oC above ambient. This timing for the different

tubes of the circuit is compared for detecting the chocked tubes.

For conducting the test, a flow circuit should be established and flow maintained so as to

create the required velocity in the tubes to be tested. For obtaining reasonably accurate results

a minimum velocity required is 0.1 m/sec. Normally the acid cleaning circuit and pumps used

will be sufficient for the purpose. While conducting test in the waterwall, the downcomers

are to be blanked for obtaining satisfactory velocity in the waterwall tubes.

It is advantageous to carry out the test in the waterwall and economiser immediately before

the acid cleaning and in the superheater system after blowing.

When testing superheaters only volatile chemicals should be used for treating the demineralised

water. It is essential to take necessary precaution to avoid water going to the turbine side.If

main steam line is filled up with water, the hangers should be locked.

5.4 Steam Blowing

Steam blowing operation is intended to remove mill-scales, welding slag, debris or otherforeign material left over in the superheaters, reheaters and steam piping. The basis behind

steam blowing is to create a momentum equal to or preferably greater than that during normal

operation. This will blow out all the loose debris from the system. The two methods in practice

for steam blowing are puffing method and continuous blowing. In the puffing method a thermal

shock is created which helps in dislodging the scales from the internal surfaces.

In the continuous blowing method constant purge steam rate is maintained by firing the unit

for few minutes, whereas in puffing the fire is quenched and pressure is allowed to decay

during blowing. As the pulling method is Most commonly employed, only this method is

described here. Refer Fig. XXIV-2 for scheme generally adopted for steam blowing. Theboiler is filled up to the normal level with treated D.M. water. Pressure is raised by firing to

about 40 atmospheres, and firing discontinued. The outlet valves in the steam lines are opened,

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venting out the steam to atmosphere. The high differential pressure between boiler and

atmosphere creates a high Mass flow of steam at velocities much higher than normal. When

the pressure drops down to about 25 atmospheres the valves are closed and the firing restarted

to raise the pressure again for the next blow. The cycle is repeated till the lines are adequately

clean. As every blow induces a thermal shock to the boiler components, the pressure drop

during each blow should be limited to about 40oC drop in saturation temperature.

The effectiveness of steam blowing is evaluated by a target plate placed in the stream near the

outlet to atmosphere. As the main object of blowing is to prevent damage to turbine blades

from foreign materials carried by steam the target plate also is to be made from similar Material

as that of turbine blade. Blowing should be continued till clean targets are obtained.

5.5 Safety Valve Setting

At the end of steam blowing, the safety valves in the boiler will 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 sturdiness and adequacy of the ventingsystem.

During safety valve setting the boiler steam pressure will be raised to the Maximum value for

the first time. In Most cases the safety codes require this to be done in the presence of boiler

inspector.

Before commencing floating all the pre-commissioning checks and servicing of the valves

should have been completed. The boiler pressure is gradually raised at much lower rate than

normally recommended in start up curve to facilitate measuring waterwall and other pressure

part expansions. The waterwall expansions would be proportional to saturation temperature

rise corresponding to boiler pressure. Free expansion and expansion clearance at different

locations should be carefully checked. The expansion data is collected for different pressures

including the full operating pressure.

After measuring the expansion at operating pressure, all the safety valves (except the one to

be floated) are gagged. 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

stopped. As the boiler pressure decays, the safety valve reseating pressure is also noted. Any

adjustment required for setting the lifting pressure or blow down is done as per the safety

valve 0 & 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 setat 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

operation gags from all the safety valves should be removed and accounted.

5.6 Clean Air Flow Testing of Pulverisers

Pulveriser clean air flow test is conducted for the following purposes:

a) To verify correct orifice installation in pulveriser fuel pipes.

b) To check that coal pipes and riffle distributors are free from foreign

Materials.

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c) To check if the exhauster (of suction type mills) has sufficient head to assure

100 per cent air flow at 100 per cent mill loading.

d) To calibrate the venturi or piccolo or jam tubes for mill air flow measure

ment (in pressurised mills).

Before commencing the test, all the pre-commissioning checks on the pulveriser shouldhave been completed and the primary air system commissioned.

For pressurised mills the test is conducted as follows:

i) F.D. and W. fans are started and normal furnace draft maintained.

ii) The primary air fans are run and the required hot primary air header to furnace

differential and mill air flow is maintained.

iii)Air flow through the pulverised coal pipes is measured by a standard pitot tube and

adjusted by operating hot air and cold air dampers to obtain approximately standardmill air flow.

iv) The velocity profile in each of the coal pipes is accurately measured by using a

standard pitot tube.

v) The mill Bowl differential and the air flow measuring instrument differentials are

also measured.

All these data are recorded in a test format.

From the above readings the air flow through each pipe is calculated and the variation in

percentage from the average is computed. This should be within + 5%. If the variation is

higher, then the coal pipes should be checked for any foreign materials or incorrect orifice.

After correcting the problem the test is re-run until desired results are obtained.

From the data collected the calibration curve for the flow measuring instrument, is also

computed and furnished to the instrumentation supplier for setting the mill air flow control

system.

Completion of clean air flow test marks the readiness of mills for operation on coal.

5.7 Testing of Protections and Interlocks

Protection and interlocks do the very important function Of safeguarding the equipments

from damages due to malfunctioning or mal-operation. All the interlocks and protections

provided for the individual equipments should necessarily be checked, and made functional

before trial run of equipments.

The protection interlocks are normally supplied by electrical and instrument contractor to the

design of project engineering consultants. Boiler commissioning engineer prepares the full

list of approved final interlocks and protections envisaged for different equipments and systems.

A detailed sequence of testing (simulation as well as actual testing wherever possible)

envisaging all conditions is also prepared. Testing is done jointly with the concerned agencies.Observations and adjustment done are recorded. Defects if any should be rectified. The testing

should be repeated till all the protections and interlocks are checked and ensured.

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5.8 Calibration of Instruments

All the instruments both at local and in unit control board should be calibrated before putting

them into service. Pressure gauges should be calibrated by dead weight gauges. Draft gauges

should be checked with local manometers. Temperature indicators may be compared with

thermometers and thermocouples should be read by potentiometers. Flue gas oxygen readIng

should be verified with Orsat analysers.Normally test pockets are provided at different locations for the purposes. At critical locations

like Economiser water inlet, Superheater outlet, Reheater outlet and Desuperheater inlet and

outlet, Calibrated. Thermocouples should be fixed in the test pockets for periodic reading and

comparison with UCB indication till the reliability of the UCB indication is established. Air

heater gas inlet and outlet ducts should be fitted with grids for multipoint measurement of

temperature and oxygen content in flue gas.

6. TRIAL OPERATION OF BOILER

Satisfactory completion of precommissioning activities is a milestone in commissioning. This

indicates the readiness of boiler for Trial operation. At this time all facilities and inputs for

loading the unit and putting into regular service should be ready. For utility units the turbine

generator and the complete electrical transmission system must be ready- In the case of

industrial boilers the process equipments should be ready. Before trial run of the boiler a

general inspection of all equipments is carried out. All the rotating equipments are checked

for proper lubrication and smooth operation. Dampers, and valves are rechecked for proper

operation. The complete instrumentation and control must be checked for proper functioning.

The automatic controls should be checked and made ready for commissioning. Adequate

number of operating and maintenance staff and good communication between different areas

and the control room must be ensured. The boiler is then lighted up in accordance with operationand maintenance instructions.

The rate of pressure rise, furnace outlet temperature and any other limitation given in the 0 &

M instructions should be strictly adhered to. The unit is then loaded and the load raised

according to starting diagrams given in the operating instructions. During the first few start

ups it may be worthwhile to raise the load at a slower rate, in order to acquaint the operators

with the behaviour of various systems. The milling system can be tried one after the other.

After checking for smooth working of the mills without undue vibration, noise, pounding or

temperature rise, the unit load can be increased progressively. All the automatic controls

including the furnace draft, drum level, total airflow, steam temperature and milling system

automatics are commissioned and put into operation progressively. Combustion regimeadjustments can be taken up at this stage for optimising milling system performance and

setting up secondary air damper controls.

The main activity in optimising milling system performance is to set classifier to obtain the

required fineness. For large utility boilers the recommended pulverised fuel fineness is 70 per

cent passing through U.S. 200 Mesh. However in some cases adjustments may be required at

site due to varied burning characteristics, slagging tendency or abrassiveness of coal being

fired. Any adjustment should aim at obtaining minimum carbon loss and least wear of milling

system components. To start with the Mill classifier vanes May be positioned in the middle of

their operating range and Mill loaded to its full design capacity for the coal being fired. The

Mill air flow and Mill outlet temperature must be maintained as per design recommendations.

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After the Mills have stabilised, pulverised coal sampling is conducted as given in the operating

instructions. Simultaneously bottom and fly ash sample should be collected. The furnace

must be visually checked for proper combustion and flame stability. If fineness vary

significantly from recommended values classifier is adjusted suitably till optimum conditions,

i.e., good combustion, acceptable carbon loss and least wear of milling system components

are obtained. After the trials fineness must be recorded and intimated to all concerned.

Optimising the milling system may be followed by setting the secondary air dampers. Secondary

air dampers serve to proportion the combustion air between fuel and auxiliary air. In tangential

firing system, more fuel air tends to shift the coal ignition point away from the tip and vice

versa. For reliable operation the coal ignition point must be adjusted for obtaining stable

ignition of coal at all feeder speeds and a healthy scanner, pick up. This is achieved by stroking

the fuel air dampers in tune with feeder speed between a minimum and a maximum opening.

The maximum and minimum limits of fuel air damper opening and the recommended wind

box pressure should be checked before commissioning the secondary air control system on

automatics.

Before taking the unit upto full load, the boiler should be maintained at different loads to

check the performance of individual equipments as well as the systems. At different loads the

superheater metal temperature distribution should be measured and recorded. Water and steam

temperatures, air flows, oxygen content in flue gas entering and leaving air heaters should be

periodically checked locally with reliable instruments andcross checkedwith controlroom

indication.

While raising load, the different regimes and parameters should be maintained as

envisaged in the design of equipments and their performance watched. Any problems in

this regard should be intimated to the designer for analysis and solution. This feed back is

essential for the designer to know the equipment performance and for improving the

equipment.

When the unit is fully loaded and the performance is satisfactory the boiler is

declared ready for trial run. During the trial run the boiler is operated according to the load

requirements agreed between the owner and the supplier in accordance with the contract

terms. Normally during trial run the boiler is operated at varying loads for sometime and at

full load for a specific period.

On successful completion of trial run the boiler is declare rteady for commercial

operation and taken over by the operation group. Even at this stage there can be someminor pending points or problems which deo not affect the safe operation of the unit . the

concerned agencies may agree to attend to these problems at the next oppurtunity even

after the boiler is taken over.

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7. PERFORMANCE TESTING

The test is normally carried out within 3 months or as agreed upon after declaring the

boiler for commercial operation. Generally when more than one unit is supplied under the

same contract, the testing will be done on one unit only. The conditions for the test are

normally agreed at the contract stage itself. For details of conducting performance test, the

relevant chapter may be referred.

8. TRAINING OF OPERATING AND MAINTENANCE STAFF

Training of operating and maintenance personnel is best done during commissioning of

the boiler.

A well-planned programme result in correct operating and maintenance procedures being

followed.This will reflect in high availability of the unit.The training programme should

be devised to acquaint the operators and engineers in

1. Basic theory.

2. Constructional features.

3. Equipment operation - Regular and emergency situation.

4. Adjustment and maintenance procedures.

Basic document for use in training is the operating and maintenance instructions of the

equipment. The relevant sections should be completely read and understood by all operating

and maintenance personnel. The information may be supplemented by lectures from designers,

commissioning engineers and experienced operating personnel. Visits to running stations

and sites during erection of the equipment will greatly help in the understanding of the

equipments.

The operation and maintenance personnel should be identified well in advance so that they

can participate in the commissioning of the equipments. They should be associated with

commissioning engineers right from initial stages. After completion of trial runs the equipment

can be operated by the operating staff under the guidance of commissioning engineers.

This mode of training normally results in better understanding of the equipments and their

behaviour by the operating staff much before the units are ready for commercial operation.

This procedure is also advantageous in that the operating and maintenance personnel willknow of all the problems faced with the equipments during commissioning and their solutions.

Completion of trial run or the performance test marks the successful commissioning of the

unit.

After successful commissioning and handing over, a document highlighting the various

activities, problems faced, record of data collected and protocols signed should be prepared

and sent to all the agencies for their records.


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FIG. XXIV-1 SCHEME OF CHEMICAL CLEANING(INFORMATIVE)

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FIG. XXIV-2 STEAM BLOWING - PUFFING METHOD

FOR REHEAT BOILERS

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