Session03 PET-04.Maintenanne Technology1

7/29/2019 Session03 PET-04.Maintenanne Technology1

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Power Engineering and Training Services Co.,Inc.

2011 Climate Change Action Clean Coal Technology International

Cooperation Project CCT Transfer Project Dispatch Technology

Interaction (USC Coal-fired Power Plant Operation Technology)

Power plant O&M techniques

(Maintenance techniques)



1

Copyright© Power Engineering and Training Services Co.,Inc. All rights reserved.

Table of contents

Sheet No.

1. Damage part of boiler equipment by aging degradation 3 ～ 5

【Reference】 Standard of maintenance 7 ～ 11

2. Total inspection of boiler 13 ～ 19

3. Remaining life assessment for boiler pressure-resisting part 21 ～ 24



1. Damage part of boiler equipment by agingdegradation



3


◎ Boiler holds high-temperature and high-pressure fluids and conducts heat exchange with extremely high-temperaturecombustion gas. Thus, it is employed under very tough condition.

1. Proportion of damage by facilitiesPressure-resisting parts such as furnace walls, SH, RH, ECO, piping, and so on. ‥‥ 67%

2. Damage factorsBoiler is affected by working temperature, types of load stresses, atmosphere, and others, so its damage factors aremulti-faceted. In most cases, it is influenced at the same time by a compound of more than two factors.

Thermal fatigue, corrosion fatigue and creep damage ‥‥ 83%

3. Measures to improve reliability Prevent fatigue of pressure-resisting part, corrosion fatigue, and creep damage.

As regards makers' basic design for boiler, boiler is designed and manufactured to be able to operatefor 100,000 hours.

Propensity of damages to boiler facilities

炉壁, 31%

SH,RH,ECO, 20%配管, 16%

ファン, 10%

弁, 5%

非耐圧部, 4%

その他, 14%Furnace

walls31%

SH, RH, ECO20%

Piping16%

Fans

10%

Others 14%

Non-pressure-resisting part 4%

Valves5%

types of boilerfacilities

pressure-resisting parts

熱疲労・腐食疲労,68%

クリープ, 15%

磨耗, 5%

腐食, 5%

その他, 7%thers 7%

Corrosion 5%

Wear 5%

Creep 15%

Thermal fatigue/Corrosion fatigue

68%

Proportion of causes bydamage locations

★Propensity of damages to boiler facilities

Boiler holds high-temperature and high-pressure fluids and conducts heat exchange withextremely high-temperature combustion gas. Thus, it is employed under very tough condition.

Boiler consists of main body, pipes, instrument and control equipment, combustion equipment andauxiliary machines. The main body consists of pressure-resisting parts where high-pressure feedwater and steam flow from SH to RH and Non-pressure-resisting parts such as casing, buck stayand duct.

While boiler equipment generally function and deliver specified performance under theenvironmental stress, the damage and deterioration become advanced under the control of timefactor. The figure of deterioration damage ratio occurring at each boiler equipment shows that67% of total damage is pressure-resisting such as furnace wall, SH, RH, ECO and piping. Then,factors of equipment damage are exhibited in complicated aspects under the influence such asoperating temperature, type of load stress and environment, and it is likely that deterioration anddamage become advanced by more than two factors at the same time. So heat fatigue, corrosionfatigue and creep account for 83% of the total factor at boiler pressure-resisting parts.

As regards makers' basic design for boiler and turbine, boiler and turbine are designed andmanufactured to be able to operate for 100,000 hours, however generating facilities recentlyoperates over 100,000 hours. For this reason, it is needed to prevent fatigue, corrosion fatigue andcreep damage at pressure-resisting part for improving reliability of the equipment.



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Root causes of damage

◎ Design of boiler pressure-resisting part: According to makers' design, pressure-resisting part

has a strength to withstand creep rupture for over 100,000 hours.・ As a mater of fact, the strength of boiler pressure-resisting part is weakened due to various deteriorations

such as excessive rise of load stress as well as initiation and propagation of cracking which result from

swelling deformation and decrease of effective thickness. As a consequence, the strength drop leads to

fracture in boiler pressure-resisting parts (e.g. evaporator tubes, pressure vessels, etc.) and brings about

such failures like rupture and leakage.

Boiler damage factors

Excessive rise of load stress due todecrease of effective thickness

Swelling

Deformation

Generation and propagation of cracking

Generation and propagation of cracking

Fracture due tostatic stress

Corrosionfatiguefracture

Fatigue fracture

Creep fracture

Rupture

Leakage

Overheating

(Tube plugging, Flow imbalance and Deposition

of scale in tubes)

High-temperature corrsion

(Coal ash and Vanadium attack)

Corrosion of tube inner surface

Wear(Ash erosion and high-speed gas flow)

Fatigue

(Thermal fatigue and mechanical fatigue)

Corrosion fatigue

(Fatigue under corrosion of tube inner surface due to

boiler water)

Deterioration of materials

(Weakening strength, Degradation of material

properties and Material flaws )

★Root causes of damage

As noted before, as regards makers‘ basic design for boiler and turbine, boiler and turbine aredesigned and manufactured to be able to operate for 100,000 hours. This is the design forpressure-resisting part of boiler and it has a strength to withstand creep rupture for over 100,000hours.

(reference)

Creep rupture strength … Metal material becomes soft and deformable when it gets hot.While the metal is subjected to constant load at high temperature, it keeps ondeforming. This is called creep phenomenon. Creep limit stress is a stress under whichstrain or deformation volume reaches a certain value at a certain temperature after aconstant time (for example 100,000 hours). Creep rapture strength is a stress underwhich rupture occurs in that time.

<explanation of figure>

Phenomenon causing boiler damage includes deterioration, overheating, corrosion, wear,fatigue and corrosion fatigue of material. Through this phenomenon, the strength ofboiler pressure-resisting part is weakened due to various deteriorations such asexcessive rise of load stress as well as initiation and propagation of cracking whichresult from swelling deformation and decrease of effective thickness. As a consequence,the strength drop leads to fracture in boiler pressure-resisting parts (e.g.evaporator tubes, pressure vessels, etc.) and brings about such failures like ruptureand leakage.



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Leakage in boiler pressure-resisting part

◎ The significant thing in deciding boiler life/renewal time is occurrence of the leakage atpressure-resisting part of boiler that is the most important part.

Cause of leak Description

Wear

(Erosion)

It results in decreasing thickness of tube outer surface due to the collision and the like of combustion gas containing

fine particles and others.

Creep[Overheat]

It is a tendency to deform with time as a resul t of exposure to certain levels of high temperature and load stress. Under

such influences, deformation and voids (hollow holes) ensue and proceed due to slip i n crystal grain boundary of

metals. As a consequence, this gives rise to cracking.

Fatigue Slip lines are created in crystal grains due to cyclic loading of repeat stress, and thus slip bands are formed. Initiation

and propagation of cracking along slip bands eventually leads to leakage.

Corrosion The strength of metal materials is weakened in a variety of forms due to a combination of mechanical, electrical and

chemical factors.

Classification of factors causing leakage in boiler pressure-resisting part

Wear (Erosion) Creep (Overheat) Corrosionatigue

・Make effective diagnosis during daily inspection to ensure an early detection of problematic parts. Since these parts

can be replaced with new ones, the equipment life is likely to be prolonged.

★Leaks in boiler pressure-resisting part

The significant thing in deciding boiler life/renewal time is occurrence of theleakage at pressure-resisting part of boiler that is the most important part. Factorsof leakage in pressure-resisting part are basically divided into 4 categories, whichare (1) wear(erosion), (2) creep, (3) fatigue and (4) corrosion. All the factors makethe material keep on deteriorating due to long operation and finally leads to theleakage.

According to statistics of boiler manufacturer in Japan “Past record of leakage atpressure-resisting part of boiler (monthly number of cases)”, average numbers ofleakage at pressure-resisting part of boiler for ten years from 1997 to 2006 are about19 cases/year for utility boiler, about 30 cases/year for industrial boiler. Theleakages in the industrial boilers tend to occur more. However, since there are muchmore operating industrial boilers, the proportion of leakage shows 1 boiler / 8 boilerfor the utility boiler and 1 boiler / 25 boiler for the industrial boiler. That meansthat proportion of leakage in the utility boiler is 3 times much more than that in the

industrial boiler. However, the cases of leakage in the utility boiler recently tendto decrease.



【Reference】 Standard of maintenance



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Periodic inspection items (boiler inspection intervals and

details)①

The standard periodic inspection items created by the Nuclear and IndustrialSafety Agency of the Ministry of Economy, Trade and Industry are as follows:

*1 The “ Supplemental procedures” are the procedures required for prolonging the periodic

licensee inspection.

*2 For “periodic inspections” that are to be performed every other time the periodic

licensee inspection is performed, perform them every time the periodic licensee

inspection is performed if the periodic inspection interval is extended to four years.

Inspection item Periodic inspection Remarks*1

*2

1. B oi ler s

(1) Drums

Including flush

tanks for

startup

bypasses

a. Remove as many steam

separators as necessary to

perform a visual inspection of

the interior of drums and a

liquid penetrant test (PT

inspection) of weld lines on

the interior. However, if welds

on the interior of nozzles

have been smooth-polished,

removal of steam separators

may be performed every other

periodic licensee inspection

time.

• In addition to periodic licensee

inspections, it is desirable to conduct

the following special detailed

inspections at intervals of eight years or

60,000 to 80,000 hours, starting with the

initial inspections conducted after the

first 80,000 hours of operation.

• Choose a sample of welded section s on

the exterior of nozzles, longitudinal

jo in ts, and cir cum ferent ial jo in ts and

perform magnet parti cle tests (MT

inspection) of these sections.

• Remove as many interior units attached

by welding as necessary to perform an

MT inspection of welds on the interior of nozzles.

The standard periodic inspection items created by the Nuclear and Industrial SafetyAgency of the Ministry of Economy, Trade and Industry areshown in the table.



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8



details)②



*2

(2) Water drums Same as above, except that theexpression “steam separators”should be replaced by “ interior equipment.”

(3) Pipe headers(A) Furnaces

Economizers

a. Perform an exterior inspection of pipe headersand metal suspensions for pipe headers.

b. Choose a sample of weldedsections of pi pe header nozzles that have not beenflexible-processed andwhose edges have not beenrounded and perfo rm a PTinspection of these sections.

c. Choose a sample of two or more pipe headers from allpipe headers to perform aninterior inspection wheninspecting pipe headersevery other periodic licenseeinspection time.

• Pipe headers that have heat-retainingmaterials attached to them do not need tobe removed when performing an exterior inspectio n of pip e headers and metalsuspensions for pipe headers.

• Af ter a to tal of 80,000 ho urs of operat ion,it is desirable to cho ose a sample of welded sections of pipe header nozzlesand metal supports and p erform a PTinspection of these sections.

• Af ter a to tal of 80,000 ho urs of operat ion,

it is desirable to conduct the followingspecial detailed inspection in addition toperiodic licensee inspections. Choose asample of welded sections on theexterior of longitudinal andcircumferential header joints and performan MT inspectio n of these sections.

The standard periodic inspection items created by the Nuclear andIndustrial Safety Agency of the Ministry of Economy, Trade and Industryare shown in the table.



9

9



details)③



*2

(B) Superheaters

Reheaters

a. Perform an exterior inspection of headers

and metal suspensions for headers.

b. Choose a sample of welded sections of pipe

header nozzles that have not been flexible-

processed and who se edges have not been

rounded, and perform a PT inspection of

these sections.

Same as above *3

Pipe headers or large-diameter pipes

Furnace evaporation pipe headers

Superheater pipe headers or main steampipes

Reheater pipe headers or high-temperaturereheat steam pipes

*3 Make a remaining service life assessment concerning creeps for pipe headers or large-diameter pipes designed for temperatures of 450°C or higher.

Choose sections that are likely to be exposed to the severest conditions for assessment.

The standard periodic inspection items created by the Nuclear and

Industrial Safety Agency of the Ministry of Economy, Trade and Industryare shown in the table.



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10



details)④



*2

(4) Piping(A)Steam

generatingtubing

[For boilers other than oil-fired, gas-fired, or black-liquid-fired boilers]

a. Perform an exterior inspection of pipes inside furnaces.

b. Construct scaffoldings up to theheight of furnace burners every other periodic licensee inspection time toperform visual inspections using lifts,inspection robots, or other similar means.

c. Choose a sample of sections of non-erosion-proof pipes that need to be

steam-cut and measure their thickness.

• Aft er a t otal o f 80,000hours of operation, it isdesirable to choose asample of weldedsections of metal pipeattachments, as needed,and perform a PTinspection of thesesections.

• It is desirable to choose asample of pipes and tocheck their interior conditions at regular intervals.

• App ropr iate measu resneed to be taken toprevent erosion.

• PT inspections need tobe performed for selected samples of welded sections of fintubes.

• For refuse incinerationwaste-heat boi lers,appropriate measuresneed to be taken toprevent corrosion.

• For refuse incinerationwaste-heat boi lers, thethickness of exposedpipes needs to bemeasured up to theheight of burners insideburners.

The standard periodic inspection items created by the Nuclear andIndustrial Safety Agency of the Ministry of Economy, Trade and Industry

are shown in the table.



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Periodic inspection items (boiler inspection intervals and details)⑤


*2

(B) SuperheatersReheatersEconomizers

[For boilers other than oil-fired and gas-fired boilers]

a. Perform a visual inspection of pipesof su perheaters, reheaters, andeconomizers.

b. For non-erosion-proof boilers,perform a hand inspection of pipes of superheaters, reheaters, andeconomizers.

c. For non-erosion-proof boilers,choose a sample of sections of pi pesof su perheaters, reheaters, andeconomizers, and measure their thickness.

d. Choose a sample of sections of dissimilar metal joints that are notwelded with Inconel welding rods,and perform a PT inspection of th esesections.

• It is desirable to choosea sample of sections of pipes of superheatersand reheaters at regular intervals and measuretheir thickness.

• Aft er a t ota l o f 80,000hours of operation, it isdesirable to choose asample of weldedsections of metal pipeattachments, as needed,and perform a PTinspection of thesesections.

• App ropr iate measu resshould be taken toprevent erosion.

• For boilers for which noSUS-scale pr eventivemeasures have beentaken, it is necessary tochoose a sample of sections and check theaccumulation of scalesin these sections.

The standard periodic inspection items created by the Nuclear andIndustrial Safety Agency of the Ministry of Economy, Trade and Industry

are shown in the table.



2．Total inspection of boiler



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Total inspection of boiler body forthermal power station

◎ Process of total inspection

Exemplification of the cases offaults/failures in similar units

Extension from the facts/figures of faults/failures in own boiler

Confirmation of the repaired parts ofpast faults/failures and their soundness

SecondarySH tubes

TertiarySH tubes

FourthSH tubes

SecondaryRH tubes

Firing

Gas ductEvaporator

tube

PrimaryRH tube

PrimarySH tube

ECO tubeCO tube

＊Sort out faults/failuresexperienced by similar unitsin the past

＊Track down the facts ofequipment damagesdiscovered in previousperiodic and intermediateinspections from the verybeginning

＊As regards the previouslyrepaired parts, make surethat they are in sound stateby means of visualinspection and NDT on thebasis of designed targets.

Misuimi power station is the latest large coal-fired generating unit in Chugoku Electric PowerCo.,Inc., which is regarded as very important power plant. So it conducts continuous operation atrated output(1,000MW) and high reliability of their equipment is required. When the total operating

time of boiler exceeds 60,000 hours, leakage accident of furnaceevaporator tube occurred inMisumi power station in March 2006. Total inspection of boiler body was conducted duringrestoration work for this trouble and intermediate inspection of generating facility in September toNovember 2006 as a countermeasure to prevent reoccurrence of similar trouble in the future. Thistotal inspection will be introduced.

◎ Process of total inspection

Exemplification of the cases of faults/failures in similar units

Extension from the facts/figures of faults /failures in own boiler

Confirmation of the repaired parts of past faults/failures and their soundness

＊ Sort out faults/failures experienced by similar units in the past anddraw map of damage by summarizing lists of faults/failures afteranalyzing the contents of faults/failures (structure and factor) andconsidering needs of horizontal development to apply to your plant.

＊ Track down the facts of equipment damages discovered in previousperiodic and intermediate inspections from the very beginning andconduct inspections by using a check list after confirming theinstallation status.

＊ As regards the parts improved in the past periodic inspections, makesure their safeness by means of visual inspection and NDT on the

basis of designed targets.



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Map of faults/failures in boiler body(Overall)

Locations Causes of damage

①

Elements

・Peripheries of sootblower

(Erosion)

・Tube intersections,

baffles

(Contact thinning)

・Lower bent part

(High-temp. corrosion)

・Supports

(Contact/thermal fatigue)

②

Evaporator

tubes

・Evaporator tubes around

deslagger

(Ash erosion)

・Upper stages of burner～

Around deslagger(Sulfidation corrosion)

③

Furnace

bottom

・Furnace bottom tubes

(Falling clinker)

・Orifices in tubes

(Scale deposits)

Secondary

RH tubes

TertiarySH tubes

FourthSH

tubes SecondaryRH tubes

Firing

Gas ductEvaporator

tube

PrimaryRH tube

PrimarySH tube

ECO tubeCO tube

◎ When drawing a map of damage, make a study of cases of faults/failures in similar units, look into their details(mechanisms and possible causes), thoroughly think about the need for horizontal development, and then draw amap which caters to the components of own boiler.

◎Drawing map of faults/failures in boiler body (Overall)When drawing a map of damage, make a study of cases of faults/failures in similarunits, look into their details (mechanisms and possible causes), thoroughly thinkabout the need for horizontal development, and then draw a map which caters to the

components of own boiler.

Structural soundness of equipment is checked after deciding concrete locations andcontents of inspection in faults/failures of each equipment by using this.

Locations Causes of damage

①Elements

・Peripheries of soot blower(Erosion)

・Tube intersections, baffles(Contact thinning)

・Lower bent part(High-temp. corrosion)

・Supports(Contact/thermal fatigue)

②Evaporator tubes

・Evaporator tubes around deslagger(Ash erosion)

・Upper stages of burner～Arounddeslagger(Sulfidation corrosion)

③Furnace bottom

・Furnace bottom tubes(Falling clinker)

・Orifices in tubes(Scale deposits)



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Map of damage to boiler body (Furnace walls)

(Sulfidation corrosion)

Corrosion

SecondarySH tube

TertiarySH tube

FourthSH tube

Secondary

RH tube

Firing

Gas ductEvaporator

tube

PrimaryRH tube

PrimarySH tube

ECO tubeCO tube

In the range fromthe upper part ofburner to deslaggerlevel, low NOxoperation ispracticed andreducingatmosphere issecured. Thus,sulfidation corrosionproceeds.

★ Features of damage: In view of avatar, tubesurface corrodes and get thinned due to severesulfidation corrosion.

(See the photo below)

★ Action/solution: For the boiler where sulfidationcorrosion becomes apparent, conduct claddingwelding repair, panel replacement, and so onduring periodic inspection.

◎Damage of deslagger level range on furnace walls

It is promoted to reduce amount of generated NOx in the range from the upper part ofburner to deslagger level in many coal-fired boiler. Strong reducing atmosphere issecured near burner zone due to this low NOx operation, and terrible wastage bysulfidation corrosion is found around water wall tube in the center of boiler sidewall.



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Map of damage to boiler body (Furnacewalls)

indicates the injection extent of deslagger.Thinning damage occurs due to deslagger steam.

写真

（Thinning of deslaggersection)

Location ofthinning damage

SecondarySH tube

Tertiary SH tube Fourth SH tube

Secondary

RH tube

Firing

◎ Deslagger is a short retractable removaldevice (same as soot blower) used to

prevent drop of thermal efficiency of furnacewalls stemming from the deposits of slag,dust and ash contained in combustion gas.

★ Action/solution: After removing the scale in the location of

thinning damage which is produced by blast, conduct visual

examination and measure wall thickness to clarify the extent of

damage.

◎Damage of evaporator tubes by deslagger on furnace wall

Deslagger is a short retractable removal device (same as soot blower) used to preventdrop of thermal efficiency of furnace walls stemming from the deposits of slag, dustand ash contained in combustion gas.

Therefore, deslagger is used for mainly cleaning the wall where the soot blowers areinstalled.



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Map of damage to boiler body (Furnacebottom)

Striking on furnace bottom tubes

Big and hard clinker grows and falls down on furnacebottom. As a result, furnace bottom tubes are damaged.

Plugging of orifice holes in tubes

When orifice holes are blocked due to deposits of scale,evaporator tubes may get overheated.

（ Diagram of orifice in

tube)

Orifice hole

Orifice in tube

(Damage to furnace

bottom)

9mm

5mm

SecondarySH tube

Tertiary SH tube Fourth SH tube

Secondary

RH tube

Firing

◎ Furnace bottom is a place at comparatively low temperaturewhere is not prone to be affected by heating damage.However, in the events of coal-fueled unit, feedwatertreatment, and so on, damage tends to occur.

◎ Furnace bottom is placed in the uppermost stream part ofevaporator tubes of which boiler is constructed. For thisreason, if furnace bottom is damaged (primary damage),evaporator tubes in the downstream part will besubsequently damaged (secondary damage).

◎Damage of furnace bottom

Furnace bottom is the part which has less probability of damage in boiler bodyequipment by heat due to the low temperature. However, it will be damaged in case that

coal is used as the fuel or some type of water treatment is applied and so on. Sincethe furnace bottom is located at the inlet of evaporator tube constituting the boiler,when the furnace bottom is damaged as a primary damage, it causes the secondary damagein the evaporator downstream.

・Striking on furnace bottom tubes ： Clinker generated by burning adheres to thefurnace wall of boilers using coal with high slugging property. If the hard clinkergrows bigger and falls down, striking signature appears on the surface of furnacebottom tubes. A hole can be created depending on the size and hardness of fallingclinker and steam leakage may occur.

★ Treatment of striking signature : Check if the thickness of struck part satisfiesthe minimum requirement by measuring it and so on, and replace short tube of thestruck damaged part or conduct weld overlay.

★ Measures to prevent clinker dropping : ① Install soot blowers at the upper part offurnace where clinker is easily created to prevent the growth. ② In case that coalwith high slugging property is used, reduce the mix ratio of the coal to control theslugging.

・Plugging of orifice holes in tubes : Adhesion of scale inside evaporator isprogressed by boiler water treatment. If the scale removes and drops off, it isaccumulated at orifice regulating flow inside tubes and blocks a hole of the orifice.When the flow inside the tube stops by the block, the evaporator tubes lyingdownstream of the orifice are overheated and damaged by creep.

★ measures to prevent blocking : Conduct visual inspection of orifice inside tubesaccording to the plan of cutting the tubes.



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Map of damage to boiler body (Frontelements)

Contact thinningof tube

Contact thinningof tube

Cracking/burning due to thermal

fatigue of supports and baffles

Erosion byatomizingmedium

SecondarySH tubes

TertiarySH tubes

FourthSH tubes

×

×

×

×

×

×

×

◎ SH tubes are prone to buckle and deform during operating condition. To overcome this problem, metalsupports and spacers are used to avoid disarray of tube rows. Meanwhile, care is also taken to easethermal expansion.

◎ When fuel options and boiler types are different, SH configuration also differs. In coal-fired units, it issignificant to take account of such problems as slagging due to molten ash deposits, fretting due to ash.Therefore, it is essential to carefully select appropriate tube wall temperature, gas temperature, gas flowrate, tube pitch, etc.

◎ Damage of front elements of boilerThe condition of pressure and temperature of SH steam have gotten higher through thedemand in high efficiency and the progress in material development. With the higherpressure and higher temperature, heat transfer area of water wall have decreased due

to a decrease in latent heat generated by evaporation. On the other hand, heattransfer of SH have been increased by an increase in heat absorption of SH. Therefore,the end part at SH outlet is located in the area around furnace outlet to receiveenough radiant heat.

SH tubes are prone to buckle and deform during operating condition. To overcome thisproblem, metal supports and spacers are used to avoid disarray of tube rows. Meanwhile,care is also taken to ease thermal expansion. When fuel options and boiler types aredifferent, SH configuration also differs. In coal-fired units, it is significant totake account of such problems as slugging due to molten ash deposits, fretting due toash. Therefore, it is essential to carefully select appropriate tube wall temperature,gas temperature, gas flow rate, tube pitch, etc.

Damage of front elements are as follows.

★ Erosion by atomizing medium of soot blower

★ Thinning of tube by contact with support tube such as cooling spacer tube

★Cracking/burning due to thermal fatigue of supports(spacer) and baffles

ETC.



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Map of damage to boiler body (Backelements)

Ash erosion

Contact

thinning of tube

Erosion byatomizingmedium

SecondarySH tube

TertiarySH tube

FourthSH tube

Secondary

RH tube

Firing

Gas ductEvaporator

tube

PrimaryRH tube

PrimarySH tube

ECO tubeCO tube◎ In SH, RH, and the like, air column

vibrates due to Karman vortex takingplace in the slipstream of tube nest.

◎ Insertion of anti-vibration baffles in thelocations as needed to prevent noise

and vibration.◎ In horizontal type SH/RH which spanis relatively longer, tube deflectionwill become bigger if both ends aresupported. Because of this, it isneeded to support more than threepoints, so support tubes (e.g. SHtubes, water tubes, etc.) are erectedvertically to reinforce support.

◎ Damage of boiler back elements

In SH, RH, and the like which is installed in the gas duct downstream of the furnace,air column vibrates due to Karman vortex taking place in the slipstream of tube nest.Insertion of anti-vibration baffles in the locations as needed to prevent noise andvibration. In horizontal type SH/RH which span is relatively longer, tube deflectionwill become bigger if both ends are supported. Because of this, it is needed tosupport more than three points, so support tubes (e.g. SH tubes, water tubes, etc.)are erected vertically to reinforce support.

It is necessary to appropriately select temperature of tube wall, gas temperature, gasvelocity, tube pitch and so on in boiler back elements in the same way as frontelements

Damage of back element are as follows.

★ Erosion by atomizing medium of soot blower

★ Thinning of tube by contact with support tube

★ Erosion by collision of fly ash included in combustion gas

ETC.



3．Remaining life assessment for boilerpressure-resisting part



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Purpose of remaining l ife assessment

In recent years, there has been a rapid increase in aging thermal power facilities. For example,

over 60% of turbine facilities have a total operating hours that exceeds 1000,000 hours.

Meanwhile, due to circumstances such as changes in the demand for electric power and thediversification of fuels, there is an increasing difficulty for operations such as DSS (daily

startup and shutdown) and WSS (weekly startup and shutdown).

Therefore, the maintenance/improvement of proof strength and extension of life through

planned preventive maintenance has become even more important for aging boiler and turbine

facilities.

The primary measures are remaining life assessment and proof strength

improvement techniques.

【Remaining life diagnosis】・It is a potent procedure that involves the planned replacement of parts to extend the life of an entire plant,

through a quantitative understanding of the life of that target equipment.

・It is helpful for the reduction of equipment maintenance cost to understand the damage to the boiler that has

been operated for a long period, and then to decide the optimal timing for repair/replacement.

【Proof strength improvement techniques】・Make efforts to change the structure by means of improvement of material quality of target equipment

or application of reinforcing materials, etc., and to enhance the equipment proof strength. As a result,

life can hopefully be prolonged.

In recent years, there has been a rapid increase in aging thermal power facilities. Forexample, over 60% of turbine facilities have a total operating hours that exceeds1000,000 hours. Meanwhile, due to circumstances such as changes in the demand for

electric power and the diversification of fuels, there is an increasing difficulty foroperations such as DSS (daily startup and shutdown) and WSS (weekly startup andshutdown).

However, an investment on thermal power plant is increasing through a shift to nuclearpower generation for CO2 emission reduction. Therefore, proper investment onmaintenance for existing plants is required and it is important to maintain and improve thestrength and extend the remaining life by planed preventive maintenance. The mainmeasures are remaining life assessment and strength improvement technology.

The purpose of remaining life assessment is to extend the life of an entire plant through aquantitative understanding of the life of that target equipment and the plannedreplacement of parts. It is helpful for the reduction of equipment maintenance cost tounderstand the damage to the boiler that has been operated for a long period, and then to

decide the optimal timing for repair/replacement.

Proof strength improvement techniques make it possible to prolong the life throughchanging the structure by means of improvement of material quality of target equipmentor application of reinforcing materials, etc., and enhancing the equipment proof strength.



22


What is creep damage?

If stationary stress is applied at high temperature, deformations will occur over time.

Subsequently, carbides coagulate and grow larger inside parts/materials, voids

(holes) develop in crystal grain boundaries, and then rupturing will occur as thesevoids (holes) grow larger and join together.

Initial damage state

→Voids are not detected.

Appearance of voids

→Appearance and growthof voids in grain boundaries

Appearance of microscopic

cracks→Appearance and growth

of microscopic cracks due

to the merging of voids

Growth of cracks

→Appearance of macroscopic cracks due to

the growth of microscopic

cracks

・ Creep void: Those where bigger holes are formed when microscopic defects originally

existing inside metal crystals concentrate in crystal grain boundaries as a result of

diffusion phenomena.

Creep damage process

Crystal grain

boundaryCreep void Microscopic crack Macroscopic crack

While a constant stress is applied to metal material under high temperature, if the

stress is smaller than the elastic limit, strain is accumulated over time and it will be

deformed. This deformation is called creep damage. This figure shows process of

creep damage occurring in grain boundaries.

At the initial damage stage, creep voids are not detected. The creep voids are those

where bigger holes are formed when microscopic defects originally existing inside

metal crystals concentrate in crystal grain boundaries as a result of diffusion

phenomena. At the voids appearance stage, voids appear, grow and connect in grain

boundary. At microscopic cracks appearance stage, microscopic cracks appear and

grow through connection of voids. At the cracks growth stage, macroscopic cracks

appear due to the growth of microscopic cracks and finally fracture.



23


Non-destructive test methods (Void observation

method①)

As one of the non-destructive test methods, parameter A method is described below.

・Number of grain boundaries intersected by the reference line: N0

・ Number of grain boundaries with voids intersected by thereference line: Nc

Definition of Parameter A A ＝

The master curve that shows the relationship

between the value for Parameter A and creepdamage is created in advance, and creep damage

is assessed based on the value for Parameter A

from the assessed area.

Ａ

Ｄ

Master curveP ar am e t e r A

Creep damage

Void

Nc

N0

【Parameter A method】・ Use the number of creep voids per unit of grain boundaryas a parameter for creep damage assessment.

Assess creep damage based on the value for Parameter A from the assessed area by making use

of the master curve that is created in advance to show the relationship between the value for

Parameter A and creep damage.

Reference

line

As one of the non-destructive test methods, parameter A method is described below.

Parameter A method is to use the number of creep voids per unit of grain boundary as aparameter for creep damage assessment.

Creep voids appears on the crystal grain boundaries with growth of creep damage. Whenany reference line is drawn on the metal structure in assessmentarea, Parameter A isdefined as a ratio of number of grain boundaries intersected by the reference line tonumber of grain boundaries with voids. The method is to conduct remaining lifeassessment in the assessment area by the master curve that shows the relationshipbetween Parameter A and creep damage. Parameter A is calculated by observing replicafilm on which structure in the assessment area is transcribed by optical microscope orscanning electron microscope.



24


Non-destructive test methods (Void observation

method②)

As one of non-destructive test methods, void area ratio method is described below.

Major axis (a)

Minor axis (b)

Void area A (①+②)

Observation field area (a×b)

・Observation field method＝a×b

②

①ａ

ｂ

・Void area＝Major axis (a)×Minor axis

(b)

【Void area ratio method】・Use the area of creep voids per unit area as an indicatorfor creep damage assessment.

Void area ratio ＝

Procedures for void area ratio method

A master curve created in advance that shows the relationship between the void area

ratio and creep damage is used to assess creep damage based on the void area ratio of

the assessed area.

As one of non-destructive test methods, void area ratio method is described below.

Void area ratio method is to use the area of creep voids per unit area as an indicator forcreep damage assessment.

Void area ratio is defined as a ratio of observation field method to void area. The methodis to conduct life assessment in the assessment area by master curve that showsrelationship with void area ratio and creep damage. The ratio of observation field to voidarea is calculated as indicated and it is judged that the larger creep damage occurs whenthe ratio is larger.

Documents

Session03 PET-04.Maintenanne Technology1