Condition Assessment of Supercritical Boilers-Challenges Ahead Dr. S.K.Nath Engineering Officer Central Power Research Institute Thermal Research Centre Koradi Nagpur-441111
Supercritical Technology in India Availability of coal both in quality and quantity Reduction in emission-environmental obligation Low Average efficiency of power plants in India in the range of 27% - 34%. Achieving the required economic growth Major Power Producer NTPC Ltd. has gone for first Supercritical Units in India followed by others. Estimated 25 Nos. of SC projects are under different phase of development while more than 35 projects are under proposal stage.
Rankine Cycle
Supercritical Rankine Cycle 374 0 C; 225 Kg/cm 2
Enhanced steam parameters require superior materials
Requirements of materials for high-temperature application 1. Adequate strength to resist deformation-high temp. and pr. 2. Adequate fatigue strength against vibratory stress 3. Sufficient ductility to accommodate cumulative plastic strain and notch strength against stress concentrations 4. Good resistance to service environment to withstand oxidation, corrosion and erosion 5. Structural ability to resist damaging metallurgical changes at operating conditions
Requirements of materials for high- temperature application (contd.) 6. Ease in fabrication (machining, forging, casting and welding) 7. Low coefficient of thermal expansion to resist thermal stresses 8. Good thermal conductivity to minimise thermal gradient 9. Low density to provide high strength-to-weight ratio – for last staging blading of large steam turbine 10. Availability of long-term test data to validate the design 11. Availability in the desired size and shape
Comparison of allowable stresses between conventional and advanced materials
Supercritical Boiler Material Specification Item Section (ASME) Water Wall Tubing SA213-T22 SA335 - P12 Header & Piping SA335 - P91 SA213-T12 Superheater SA213-T23 Tubing SA213-T91 SUPER 304H SA106-C Header & Piping SA335 - P12 SA335 - P91 SA210Gr.C Reheater SA213-T12 Tubing SA213-T23 SA213-T91 SUPER 304H Header & Piping SA106-C Economiser Tubing SA210-C Seperator Storage Tank SA302-C
Various damage mechanisms in supercritical boilers • Short term damage mechanisms: Erosion, Fireside corrosion, short- term overheating • Long term damage mechanism: Creep, Thermal Fatigue
Operational effects on supercritical boiler components • 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)
Manifestation • Mechanical Material loss Wall thinning Weld defect Crack Swelling Slagging, fouling Loss of material strength
Manifestation (contd.) • Metallurgical Creep life Structural integrity • Steam Starvation Sudden Rupture
Various damage mechanisms and suitable NDE methods Damage 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 Ultrasonic Time of Flight Diffraction (TOFD) inspection, and sizing potential drop technique Short Term overheating In-situ Metallography, Hardness Measurement Swelling Dimensional Measurement (OD)
What is Creep? -The time dependent, thermally assisted deformation of components under load (stress) is known as creep.
Structural Microstructure Action needed Expended Classificati features life on fraction Undamaged Ferrite & None 0.12 pearlite A Isolated None until next major 0.46 cavities scheduled maintenance outage B Oriented Replica test at specified 0.50 cavities interval preferably within 1.5 to 3 years C Linked cavities Limited service until repair 0.84 (micro cracks) and better to inspect within 6 months D Macro cracks Immediate repair 1.00
In-situ metallography (Replication)
Major Findings Bulging
Damaged microstructure (creep cavities)
Fatigue Start-ups, load changes Crack initiation – Stress Analysis, N f Crack propagation
CRACK PROPAGATION Initial Crack length – Assessed by a suitable NDE technique (e.g. Ultrasonic) Critical Crack length – Assessed based on the prevailing stress field and geometry of the job.
Crack propagation Paris Law:- da/dN = c k n c , n = material constants For k = M ( a) ac a -n/2 da = c n M n/2 dN ; ai M = parameter related flaw shape
Ultrasonic Time of Flight Diffraction (TOFD) Inspection
TOFD - How it works
COMPLEX WELD WITH DISSIMILAR THICKNESS
Specimen simulating complex geometry weld namely terminal weld between pipe and valve containing various defects S x 2 a Z t LW C 2 2 2 2 S T T H x 2 a B t BW C 2 2 2 2 S d d H x 2 a B t 1 C 2 2 2 2 S d h d h H x 2 a B t 2 C
Indian Boiler Regulation (IBR)
Statutory Perspective Objectives: 1. Safe Operation 2. Update Boiler memo
Statutory Perspective (Contd.) 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.
TABLE -1 Component Visua Ultrasonic Magnetic Liquid/ Replication Sampling Deposit Outside Fibroscopic Hardness Other l testing Particle Dye Analysis Diameter Inspection Inspection Penetrant And Inspection Thickness (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. Yes No No Yes No No No Yes Yes Yes Headers Attemperator Yes Yes No Yes Yes No No Yes Yes Yes Swell Header measur ement High Temp. Yes No No No No Yes No Yes No No Economiser tubes Low Temp. Yes No No No No Yes No Yes No No Economiser tubes Convection Yes No No No No Yes Yes Yes No Yes Superheater coils Primary Yes No No No No Yes No Yes No Yes Super heater coils Pre final Yes No No No No Yes No Yes No Yes Super heater coils
TABLE -1 (Contd.) Component Visua Ultrasonic Magnetic Liquid/ Replication Samplin Deposit Outside Fibroscopic Hardness Others l testing Particle Dye Analysis Diameter Inspection g Inspection Penetrant And Inspection Thickness Final Yes No No No No Yes No Yes No Yes Super heater coils Reheater coils Yes No No No No Yes No Yes No Yes High Temp. Yes Yes No Yes Yes No No Yes Yes Yes headers Final Yes Yes No Yes Yes No No Yes Yes Yes Swell Super heater measur header ement Reheater Yes Yes No Yes Yes No No Yes Yes Yes Swell header measur ement Main steam Yes No No No Yes No No Yes No Yes Piping Platen super Yes Yes No Yes Yes No No Yes Yes Yes Heater header Primary super Yes Yes No Yes Yes No No Yes Yes Yes heater header
TABLE -1 (Contd.) others Component Visual Ultrasonic Magnetic Liquid/ Replication Sampl Depo Outside Fibrosc Hardness testing Particle Dye sit Diamet opic ing Inspection Penetrant Anal er Inspecti Inspection ysis And on Thickn ess Economiser Yes No No Yes No No No Yes No No Header Auxiliaries Yes No No No No No No Yes No No Boiler Bank Yes No No No No No No Yes No No Tube Water Wall Yes No No No No Yes No Yes No No Furnace Water Yes No No No No Yes No Yes No No Wall
Table – 2. Component Visual Ultrasonic Magnetic Liquid/ Replicatio Deposit Outside Fibroscopic Hardness Other Samplin testing Particle Dye n Analysis Diameter Inspection g Inspection Penetrant And Inspection Thickness Drum(Steam) Yes No No Yes No No No Yes No No Water Drum Yes No No Yes No No No Yes No No Economiser Yes No No No No Yes No Yes No No Tubes Convection Yes No No No No Yes No Yes No No Super Heater coils Primary Super Yes No No No No Yes No Yes No No Non Heater coils destru ctive oxide thickn ess inspe ction Final Super Yes No No No No Yes No Yes No No Heater coils High Temp. Yes No No Yes Yes No No Yes Yes No Headers
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