Condition Assessment of Condition Assessment of Supercritical Boilers-Supercritical Boilers-

Challenges AheadChallenges Ahead

Central Power Research Central Power Research InstituteInstitute

Thermal Research CentreThermal Research CentreKoradiKoradi

Nagpur-441111Nagpur-441111

Dr. S.K.NathEngineering

Officer

Supercritical Technology in IndiaSupercritical Technology in India

Availability of coal both in quality and quantityAvailability of coal both in quality and quantity Reduction in emission-environmental obligationReduction in emission-environmental obligation Low Average efficiency of power plants in India in Low Average efficiency of power plants in India in

the range of 27% - 34%.the range of 27% - 34%. Achieving the required economic growth Achieving the required economic growth Major Power Producer NTPC Ltd. has gone for Major Power Producer NTPC Ltd. has gone for

first Supercritical Units in India followed by first Supercritical Units in India followed by others.others.

Estimated 25 Nos. of SC projects are under Estimated 25 Nos. of SC projects are under different phase of development while more than different phase of development while more than 35 projects are under proposal stage.35 projects are under proposal stage.

Rankine CycleRankine Cycle

Supercritical Rankine Supercritical Rankine CycleCycle

3740 C; 225 Kg/cm2

Enhanced steam Enhanced steam parameters require parameters require superior materialssuperior materials

Requirements of Requirements of materials for high-materials for high-

temperature applicationtemperature application1.1. Adequate strength to resist Adequate strength to resist deformation-high temp. and pr.deformation-high temp. and pr.

2.2. Adequate fatigue strength against Adequate fatigue strength against vibratory stressvibratory stress

3.3. Sufficient ductility to accommodate Sufficient ductility to accommodate cumulative plastic strain and notch cumulative plastic strain and notch

strength against stress concentrationsstrength against stress concentrations

4.4. Good resistance to service environment Good resistance to service environment to withstand oxidation, corrosion and to withstand oxidation, corrosion and

erosionerosion

5.5. Structural ability to resist damaging Structural ability to resist damaging metallurgical changes at operating metallurgical changes at operating

conditionsconditions

Requirements of materials Requirements of materials for high-temperature for high-temperature application (contd.)application (contd.)

6. Ease in fabrication (machining, forging, 6. Ease in fabrication (machining, forging, casting and welding)casting and welding)

7. Low coefficient of thermal expansion to 7. Low coefficient of thermal expansion to resist thermal stressesresist thermal stresses

8. Good thermal conductivity to minimise 8. Good thermal conductivity to minimise thermal gradient thermal gradient

9. Low density to provide high strength-to-9. Low density to provide high strength-to-weight ratio – for last staging blading of large weight ratio – for last staging blading of large

steam turbinesteam turbine

10. Availability of long-term test data to 10. Availability of long-term test data to validate the designvalidate the design

11. Availability in the desired size and shape11. Availability in the desired size and shape

Comparison of allowable Comparison of allowable stresses between conventional stresses between conventional

and advanced materialsand advanced materials

Supercritical BoilerSupercritical BoilerItem Section

Material Specification (ASME)

Water Wall Tubing SA213-T22

Superheater

Header & PipingSA335 - P12

SA335 - P91

Tubing

SA213-T12

SA213-T23

SA213-T91

SUPER 304H

Reheater

Header & Piping

SA106-C

SA335 - P12

SA335 - P91

Tubing

SA210Gr.C

SA213-T12

SA213-T23

SA213-T91

SUPER 304H

EconomiserHeader & Piping SA106-C

Tubing SA210-C

Seperator Storage Tank SA302-C

Various damage Various damage mechanisms in supercritical mechanisms in supercritical

boilersboilers• Short term damage mechanisms:

Erosion, Fireside corrosion, short-

term overheating

• Long term damage mechanism:

Creep, Thermal Fatigue

Operational effects on Operational effects on supercritical boiler supercritical boiler

componentscomponents• High temperature effect (ageing)• High temperature corrosion (ash attack)• High velocity flue gas with particulate burden (erosion)• Thermal cycling• Steam side oxide scale growth• Maintenance repair (weld, foreign material entrapment)

ManifestationManifestation•MechanicalMaterial lossWall thinningWeld defectCrackSwellingSlagging, foulingLoss of material strength

Manifestation Manifestation (contd.)(contd.)

•Metallurgical

Creep lifeStructural integrity

•Steam Starvation

Sudden Rupture

Various damage Various damage mechanisms and suitable mechanisms and suitable

NDE methodsNDE methodsDamage Mechanism NDE Methods for detection

Erosion Visual Examination (VE), Ultrasonic Thickness Survey

Blockade in water circuit Fibroscopy

Welding defects Ultrasonic Test (UT), Magnetic Particle Test (MPT), Dye Penetrant Test (DPT), Radiographic Test (RT)

Creep In-situ Metallography, Hardness Measurement

Oxide Scale growth Ultrasonic Test (UT)

Thermal fatigue crack detection and sizing

Ultrasonic Time of Flight Diffraction (TOFD) inspection, potential drop technique

Short Term overheating In-situ Metallography, Hardness Measurement

Swelling Dimensional Measurement (OD)

What is Creep?What is Creep?

-The time dependent, -The time dependent, thermally assisted thermally assisted deformation of components deformation of components under load (stress) is known under load (stress) is known as creep.as creep.

Structural Classification

Microstructure features

Action needed Expended life fraction

Undamaged Ferrite & pearlite

None 0.12

A Isolated cavities

None until next major scheduled maintenance outage

0.46

B Oriented cavities

Replica test at specified interval preferably within 1.5 to 3 years

0.50

C Linked cavities (micro cracks)

Limited service until repair and better to inspect within 6 months

0.84

D Macro cracks Immediate repair 1.00

In-situ metallography (Replication)

Major FindingsMajor Findings

Bulging

Damaged microstructure (creep cavities)

FatigueFatigue

Start-ups, load changesCrack initiation – Stress Analysis, Nf

Crack propagation

CRACK CRACK PROPAGATIONPROPAGATION

Initial Crack lengthInitial Crack length – – Assessed by a suitable NDE Assessed by a suitable NDE technique (e.g. Ultrasonic)technique (e.g. Ultrasonic)

Critical Crack lengthCritical Crack length – – Assessed based on the Assessed based on the prevailing stress field and prevailing stress field and geometry of the job.geometry of the job.

Crack propagationCrack propagation

Paris Law:-Paris Law:-

da/dN = cda/dN = ckknn cc, n = , n = material material constants constantsFor k = MFor k = M((a) a)

aiaiacac a a-n/2-n/2da = cda = cnnMMn/2n/2dNdN; ;

M = parameter related flaw M = parameter related flaw shapeshape

Ultrasonic Time of Flight Ultrasonic Time of Flight Diffraction (TOFD) Diffraction (TOFD)

InspectionInspection

TOFD - How it worksTOFD - How it works

D-scan presentation of thro' wall condition

WT

Tx

Rx

Lateral Wave Backwall Reflection

Scan Motion

Wide Beam Coverage

Successive position related waveforms captured and processed on-line .....

..... for immediate graphical presentation in meaningfull

thro' wall format

RF Waveform

Time of Flight

Tx Signal

Amplitude Quantisation

Phase Reversal

Positional Encoding

PCS

Weld Length

Configuration

Diffracted Signals

COMPLEX WELD WITH DISSIMILAR COMPLEX WELD WITH DISSIMILAR THICKNESSTHICKNESS

Specimen simulating complex geometry weld Specimen simulating complex geometry weld namely terminal weld between pipe and valve namely terminal weld between pipe and valve

containing various defectscontaining various defects

C

ZaxStLW

2

C

BaxHTTStBW

2222 2

C

BaxHddSt

2222

12

C

BaxHhdhdSt

2222

22

Indian Boiler Indian Boiler Regulation (IBR)Regulation (IBR)

Statutory PerspectiveStatutory Perspective

Objectives:

1. Safe Operation

2. Update Boiler memo

Boiler Act

IBR- Rules & Regulations

Prescriptions

• Authority – Inspectorate of Boilers

• Jurisdiction – Within the State Territory

• Boilers > = 22.75 Litres

> = 1,00,000 Hours

> = 25 years old

Statutory Perspective (Contd.)

• Agency: Approved as per Act

• Methodology:

• Table 1 and Table 2

• NDT inspection of Drum, Headers, Pipes &Tubes by Visual, UT,DPT, Replication, OD & Thickness, Fiber optic inspection, Hardness, Oxide scale thickness measurement.

Statutory Perspective (Contd.)

Component Visual Ultrasonictesting

MagneticParticle

Inspection

Liquid/Dye

PenetrantInspection

Replication Sampling DepositAnalysis

OutsideDiameter

AndThickness

FibroscopicInspection

Hardness Other

(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12)

Drum(Steam) Yes Yes No Yes Yes No Yes Yes No Yes

Water Drum Yes Yes No Yes Yes No Yes Yes No Yes

Low Temp. Headers

Yes No No Yes No No No Yes Yes Yes

Attemperator Header

Yes Yes No Yes Yes No No Yes Yes Yes Swellmeasurement

High Temp.Economisertubes

Yes No No No No Yes No Yes No No

Low Temp.Economisertubes

Yes No No No No Yes No Yes No No

ConvectionSuperheatercoils

Yes No No No No Yes Yes Yes No Yes

PrimarySuper heatercoils

Yes No No No No Yes No Yes No Yes

Pre finalSuper heatercoils

Yes No No No No Yes No Yes No Yes

TABLE -1

Component Visual Ultrasonictesting

MagneticParticle

Inspection

Liquid/Dye

PenetrantInspection

Replication Sampling

DepositAnalysis

OutsideDiameter

AndThickness

FibroscopicInspection

Hardness Others

FinalSuper heatercoils

Yes No No No No Yes No Yes No Yes

Reheater coils Yes No No No No Yes No Yes No Yes

High Temp.headers

Yes Yes No Yes Yes No No Yes Yes Yes

FinalSuper heaterheader

Yes Yes No Yes Yes No No Yes Yes Yes Swell measurement

Reheaterheader

Yes Yes No Yes Yes No No Yes Yes Yes Swell measurement

Main steamPiping

Yes No No No Yes No No Yes No Yes

Platen superHeater header

Yes Yes No Yes Yes No No Yes Yes Yes

Primary super heater header

Yes Yes No Yes Yes No No Yes Yes Yes

TABLE -1 (Contd.)

Component Visual Ultrasonictesting

MagneticParticle

Inspection

Liquid/Dye

PenetrantInspection

Replication Sampling

Deposit

Analysis

OutsideDiamet

erAnd

Thickness

Fibroscopic

Inspection

Hardness others

EconomiserHeader

Yes No No Yes No No No Yes No No

Auxiliaries Yes No No No No No No Yes No No

Boiler BankTube

Yes No No No No No No Yes No No

Water Wall Yes No No No No Yes No Yes No No

Furnace Water Wall

Yes No No No No Yes No Yes No No

TABLE -1 (Contd.)

Table – 2.

Component Visual Ultrasonictesting

MagneticParticle

Inspection

Liquid/Dye

PenetrantInspection

Replication

Sampling

DepositAnalysis

OutsideDiameter

AndThickness

FibroscopicInspection

Hardness Other

Drum(Steam) Yes No No Yes No No No Yes No No

Water Drum Yes No No Yes No No No Yes No No

EconomiserTubes

Yes No No No No Yes No Yes No No

ConvectionSuper Heater coils

Yes No No No No Yes No Yes No No

Primary Super Heater coils

Yes No No No No Yes No Yes No No Non destructive oxide thickn

ess inspection

Final Super Heater coils

Yes No No No No Yes No Yes No No

High Temp. Headers

Yes No No Yes Yes No No Yes Yes No

Final SuperHeater Header

Yes No No Yes Yes No No Yes Yes No Swell measuremen

t

EconomiserHeader

Yes No No No No No No Yes No No

Auxiliaries Yes No No No No No No No Yes No

Boiler BankTube

Yes No No No No No No Yes No No

Water Wall Yes No No No No Yes No Yes No No

Main steamPiping

Yes No No No No Yes No Yes No No

TABLE -2 ( Contd.)

Statutory PerspectiveStatutory Perspective (Contd.) (Contd.)

OUTCOME

• Scientifically assured safe operation for a specified period.

Role of Indian Boiler Role of Indian Boiler Regulation (IBR) Regulation (IBR)

With age of currently available supercritical boilers in their infancy, the deployment of NDE will be limited to failure analysis and condition assessment of the component for quality control of repair programmes during any forced outage event and IBR relevance may be seen here.

Issues of Supercritical Issues of Supercritical Technology in IndiaTechnology in India

Critical issues to be resolved for faster Critical issues to be resolved for faster development :development :

Technological issues :-Technological issues :-

a) Water wall cracking a) Water wall cracking b) Negative flow characteristic b) Negative flow characteristic c) Slagging. c) Slagging. d) Designing as per Indian conditiond) Designing as per Indian conditione) Welding technology for new materiale) Welding technology for new material

Issues of Supercritical Issues of Supercritical Technology in IndiaTechnology in India

(contd.)(contd.) Operation & Maintenance Issues:Operation & Maintenance Issues: Availability of sufficient field data w.r.t. Availability of sufficient field data w.r.t.

material behavior, boiler tube leakage (BTL)material behavior, boiler tube leakage (BTL) Availability of critical spares for supercritical Availability of critical spares for supercritical

plants.plants. Availability of skilled manpowerAvailability of skilled manpower Large number of foreign suppliers of SC Large number of foreign suppliers of SC

Power Plants , but have comparatively long Power Plants , but have comparatively long delivery period . delivery period .

Availability of contractors for O&M activitiesAvailability of contractors for O&M activities

Reference:Reference:  1. 1. R.Viswanathan, “Damage Mechanisms and Life Assessment of

High Temperature Components”, ASM International, Metals Park, Chio 44073.

2. Arindam Sinha and Akhilesh Kumar Singh (2011), New Era in

Indian Thermal Power Sector-Evolvement of Supercritical Technology, Workshop on Remnant Life Assessment (RLA) Study of

Boilers organized by Central Power Research Institute on 4-5th March, 2011 at Raipur (CG)

3. Modern Power station practice, Boilers and Ancillary plant, BEI Volume – B.

4. Indian Boiler Regulation, Act 1923 & Regulations 1950, Akalank Publications, Delhi-110 006, 9th ed., July 2007.

5. S.K.Nath et al (2011), Non-Destructive Evaluation in Remnant Life Assessment (RLA) study of Boilers Workshop on Remnant Life Assessment (RLA) Study of Boilers organized by Central Power

Research Institute on 4-5th March, 2011 at Raipur (CG)

6. B.J.Robbins, D.M.Farrell, J. Stallings and S. Cardoso, The Monitoring of Circumferential Fatigue Cracking of Furnace Tubes in Supercritical Boilers, Rowan Technologies Ltd., Manchester, UK and

Electric Power Research Institute, Palo Alto, CA, USA.

7. Kulvir Singh (2006), Advances in Materials for Advanced Steam Cycle Power Plants, BHEL Journal