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PROBLEMS OF LIFETIME ASSESSMENT OF WATER-STEAM CIRCUIT ELEMENTS OF POWER UNITS

1st Hungarian - Ukrainian Joint Conference “Safety, Reliability and Risk of Engineering Plants and Components” Bay Zoltan Institute for Logistics and Production Systems Miskolc, HUNGARY, 11- 12 April 2006. PROBLEMS OF LIFETIME ASSESSMENT OF WATER-STEAM CIRCUIT ELEMENTS OF POWER UNITS.

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PROBLEMS OF LIFETIME ASSESSMENT OF WATER-STEAM CIRCUIT ELEMENTS OF POWER UNITS

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  1. 1st Hungarian - Ukrainian Joint Conference “Safety, Reliability and Risk of Engineering Plants and Components” Bay Zoltan Institute for Logistics and Production Systems Miskolc, HUNGARY, 11- 12 April 2006 PROBLEMS OF LIFETIME ASSESSMENT OF WATER-STEAM CIRCUIT ELEMENTS OF POWER UNITS I. M. DMYTRAKH and V. V. PANASYUK Karpenko Physico-Mechanical Institute, National Academy of Sciences of Ukraine, Lviv, UKRAINE

  2. ABSTRACT The analysis and synthesis of the modern scientific and engineering approaches for life assessment of the structural elements of basic heat and mechanical equipment for heat power plants are presented. Basic concepts and methods for strength and durability assessment of materials and structural elements are stated grounding on the fracture mechanics approaches. The examples of calculations of residual life of the basic structural elements are given with take into account of the actual data of metal properties and operating conditions of equipment.

  3. CONTENTS • GENERAL CHARACTERISATION OF IN-SERVICE DAMAGES AND FAILURES OF BASIC EQUIPMENT OF HEAT POWER PLANTS • FRACTURE MECHANICS APPROACHES • ENGINEERING APPLICATIONS FOR SERVICEABILITY ASSESSMENTS OF POWER ENGINEERING PIPELINES

  4. 1. GENERAL CHARACTERISATION OF IN-SERVICE DAMAGES AND FAILURES OF BASIC EQUIPMENT OF HEAT POWER PLANTS • Water-wall tubes of high-pressure steam boilers • Super heater tubes of sub- and supercritical pressure boilers • Water economizers • Non-heated boiler’s elements • Feeding pipelines of supercritical pressure power generating units

  5. 1. WATER-WALL TUBES OF HIGH-PRESSURE STEAM BOILERS 2. SUPERHEATER TUBES OF SUB- AND SUPERCRITICAL PRESSURE BOILERS 3. WATER ECONOMIZERS

  6. 4. NON-HEATED BOILER’S ELEMENTS 5. FEEDING PIPELINES OF SUPERCRITICAL PRESSURE POWER GENERATING UNITS

  7. 2. FRACTURE MECHANICS APPROACHES Surface film breakdown MATERIAL CORROSION FRACTURE Corrosion pits development Pit-crack transition ENVIRONMENT STRESS STATE Crack growth to critical size Fig. 2.1. Factors that define corrosion fracture Catastrophic fracture Fig. 2.2. Stages of corrosion fracture

  8. 2.1. ASSESSMENT OF CORROSION PITS DEVELOPMENT 1 0,1 i, mA/cm 2 0,01 0,001 1 2 3 4 5 6 7 1,2 8 m 9 c, m 10 1,03 15 , 20 25 Fig. 2.4 Dg % 0,49 28 Fig. 2.3 STAGE I (2.1) STAGE IІ (2.2)

  9. 2. 2. SURFACE FATIGUE CRACK NUCLEATION AS RESULT OF CORROSION DEFORMATION INTERACTIONS 400 12Kh1МF , nА pH=3,0 s I 300 max 2.2.1. EXPERIMENTAL BACKGROUND 200 ) Ds 2 ( 100 c I 1 s s 0 0 0 100 200 300 s , МPа II III I N Fig. 2.5 Fig. 2.6 (2.3)

  10. 2.2.2. EXPERIMENTAL PROCEDURE 12 16 13 1 5 9 1 4 11 8 P 10 3 1 2 4 6 7 5 b) 17 a) Fig. 2. 7. Testing equipment. c)

  11. , MPa s s 3 10 2 10 1 10 2 3 4 N, cycles 10 10 10 I II III Fig. 2.8. Correlation between parameter s and level of corrosion fatigue damaging of cyclic deformed surface (08Kh18N12T steel; pH=6.5).

  12. 2.2.4. ASSESSMENT OF SURFACE CORROSION FATIGUE CRACK NUCLEATION + Û + z M M ne s ³ s ³ s s s s Fig. 2.13 (2.4)

  13. 2.3. CORROSION FATIGUE CRACK GROWTH 2.3.1. MODEL OF THE CORROSION CRACK (2.5) Where da/dN - corrosion fatigue crack growth rate; Pj(s) - parameters, that characterise stress-strain state of materials and are function of the external applied load ; An(t)- parameters, that determine in time physicochemical processes that occur between deformed material and environment; Bm(St)- parameters, that characterise the material surface state S which is created during fracture processes; Ciare constants that characterise given system “material - environment”; i, j, m, n = 1, 2, 3… P P (2.6) Where pHtandEt- hydrogen exponent of environment and electrode potential in the crack tip; KI- stress intensity factor. Fig. 2.9. Model presentation of a material prefracture zone at corrosion crack

  14. 2.3.2. EXPERIMENTAL METHODOLOGY b) 6 1 2 c) 4 3 a) 5 Fig. 2. 10. Technique for electrochemical measurements in corrosion crack: a) - Scheme of the minielectrodes installation: 1- specimen; 2- crack; 3- crack propagation front; 4- crack propagation plane; 5- mini electrodes; 6 – driver. b) - Specimen geometry. c) - Minielectrodes: 1 – teflon tube; 2-antimony indicator; 3-ions conductor; 4-isolator

  15. PC 13 14 11 E 15 pH 12 16 F 9 5 10 4 8 2 1 3 6 7 b) a) Fig. 2.11. General view (a) and principal scheme of testing system (b): 1- specimen; 2- corrosion cell; 3- heater; 4-temperature gauge; 5- temperature control unit; 6- load mechanism; 7- load registration; 8-minielectrodes; 9- mini electrodes motion mechanism; 10- step motor; 11-operating unit; 12- registration unit; 13-PC; 14-keyboard; 15- monitor; 16- printer.

  16. 2.3.3. ELECTROCHEMICAL CONDITIONS IN THE CORROSION CRACKS (2.7) (2.8) pH(x) - a = 3.40mm 8 -a = 5.80mm 7 -a= 13.8mm 6 5 4 3 2 1 0 0.2 0.4 0.6 0.8 x / a Fig. 2.13. Distribution of pH values in the corrosion crack cavity for cracks of different length (40Kh13steel - reactor water of boron regulation; pH=8.0). Fig. 2.12. Dependencies of pH(x) and E(x).

  17. 2.3.4. METHOD FOR FORECASTING OF THE THRESHOLD STRESS INTENSITY FACTOR UNDER STRESS CORROSION CRACKING AND CORROSION FATIGUE (2.9) (2.10) Where A and m are the constants “material-environment” system; a and b are a thermodynamic constants, that define an electrochemical conditions of electrolytic hydrogen forming from corrosion environment; w is a frequency of cyclic loading; T0and N0are respectively time and number of cycles loading, that correspond to the beginning of hydrogen formation in the crack tip; Tband Nbare the base of tests in hours and in cycles of loading, respectively. Subscripts s and c specify static or cyclic loading conditions, respectively.

  18. 2.3.5. METHOD FOR DETERMINING OF BASIC CHARACTERISTICS OF CORROSION CRACK GROWTH RESISTANCE Fig. 2.14. Comparison the cyclic crack growth resistance diagrams for pressure vessels metal those have been built according to ASME data (curves 1 and 2) and Bamford’s data (curves 3 and 4) and also on the base of proposed method (curve 5). Note that different curves represent different test conditions: 1 - dry air; 2 - humid air; 3 - corrosive environment, load ratio R<0.5; 4 - corrosive environment, load ratio R>0.5; 5 - corrosive environment, load ratio R=0.7. (2.11)

  19. 3. ENGINEERING APPLICATIONS FOR SERVICEABILITY ASSESSMENTS OF POWER ENGINEERING PIPELINES d S Fig 3.1. Typical distribution and view of crack-like defects in the pipeline wall.

  20. 3.1. SUBJECT OF STUDIES a) b) Fig 3.2. Element of pipe (a) and schematic cutting plan (b).

  21. pH 12Kh1MF steel 08Kh18N12T steel 3.2. DETERMINING OF PERIOD FOR SURFACE CORROSION FATIGUE CRACK NUCLEATION а, mm а, mm 1 5 10 1 5 10 рН=3,0 32565 45285 52272 217541 255481 274469 (3.1) рН=6,5 49777 69925 81093 134573 160021 173304 Table 3.1 рН=9,0 34464 46270 54051 184605 213375 227405

  22. 5 6 10 10 12Kh1МF 5 08Kh18N12Т 5 4 5 4 3 4 pH3,0 3 pH3,0 2 pH6,5 2 1 , cycles pH6,5 , cycles 3 1 pH9,0 5 pH9,0 2 4 3 1 2 1 а =1 mm 1 а =1 mm cal 1 cal N N 2 5 2 а =2 mm 2 а =2 mm 1 4 3 3 а =5 mm 3 а =5 mm 2 5 4 4 а =10 mm 4 а =10 mm 3 5 а =20 mm 5 а =20 mm 1 4 5 10 10 4 5 5 6 N , cycles 10 10 10 10 N , cycles ex ex Fig. 3.3. Experimental and predicted according to formula (7.1) values of number cycles of loading N for corrosion fatigue surface crack of different length.

  23. 3.3. ASSESSMENT OF ADMISSIBLE CORROSION FATIGUE CRACK DEPTH The assessment of admissible crack depth in pipelines walls has been done on the base of corrosion fatigue crack growth rate, i.e: (3.2) where is the maximum crack growth rate that may be admitted in the wall of pipelines during planned time of exploitation

  24. Table 3.2. Operating aqueous environments and their chemical composition Number of Environment Chemical composition environment 1 Boron regulation 1 % - solution H BO + KOH ( pH 8) 3 3 Boron regulation with 1 % - solution H BO + KOH ( pH 8) + 3 3 2 – chloride admixtures + 5 mg/kg Cl (10,5 mg/kg KCl ) 1 % - solution H BO + KOH ( pH 8)+ Boron regulation with 3 3 3 - nitride admixtures +10 mg/kg (16,3 mg/kg KNO ) NO 3 3 4 Ammoniac Distilled water+ NH ( pH 9) 3 m 5 Hydrazine - ammoniac ( I ) H O + NH ( pH 9)+100 g/kg N H 2 3 2 4 6 Hydrazine - ammoniac ( II ) H O + NH ( pH 9)+100 mg/kg N H 2 3 2 4 Ammoniac with chloride H O + NH ( pH 9)+ 2 3 7 – admixtures +10 mg/kg Cl (16,5 mg/kg NaCl ) Ammoniac with admixtures H O + NH ( pH 9)+ 2 3 8 – of hydrochloric acid + 10 mg/kg Cl ( HCl ); pH 3,95 Ammoniac with admixtures H O + NH ( pH 9)+ 2 3 9 – 5 ace tic acid +10 mole/l CH CH COOH ; pH 5,9 3 2

  25. -6 10 4 dl/dN, m/cycle 1 dl/dN, m/cycle 5 2 -6 10 6 3 -7 10 7 8 -7 9 10 -8 10 -8 10 08Kh18N12T 12Kh1MF -9 -9 10 10 10 100 10 100 1/2 D × K , МPа (m) 1/2 D × K , МPа (m) I I Fig 3.4. Corrosion fatigue crack growth rate diagrams of steels 08Kh18N12Т and 12Kh1МF in operating environments of different composition. The numbers of points correspond to numbers of environments in Table 3.2.

  26. Table 3.3. Coefficients in Paris equation for tested conditions Number of 2 Steel C n R environment . - 17 1 2 10 7,61 0,8162 . - 19 08 Kh 18 N 12Т 2 6 10 8,95 0,9339 . - 12 3 4 10 3,48 0,8306 . - 16 4 2 10 7,13 0,9181 . - 16 5 3 10 7,15 0,9289 . - 14 6 1 10 5,79 0,9199 12 Kh 1М F . - 21 7 7 10 10,26 0,9 528 . - 14 8 2 10 5,50 0,8978 . - 31 9 3 10 21,39 0,8051

  27. Table 3.4. Admissible crack depth in the wall of pipelines versus number cycles of loading of the heat plant power units Admissible depth of crack , mm l * Shape of Number of Steel 500 1000 2000 3000 5000 crack environment cycles cycle s cycles cycles cycles 1 7,2 7,1 7,0 6,9 6,8 08 Kh 18 N 12Т 2 7,1 7,0 6,9 6,8 6,8 3 7,9 7,6 7,3 7,1 7,0 4 7,1 7,0 6,8 6,7 6,6 = l a 1 20 5 7,0 6,9 6,7 6,7 6,6 6 7,2 7,0 6,9 6,8 6,6 12 Kh 1М F 7 7,1 7,0 6,9 6,9 6,8 8 7,2 7,1 6,9 6,8 6,7 9 6,2 6,2 5,9 5,8 5,8 1 7,8 7,6 7,5 7,4 7,2 08 Kh 18 N 12Т 2 7,6 7,5 7,4 7,3 7,2 3 8,6 8,2 7,8 7,7 7,4 4 7,7 7,5 7,3 7,2 7,1 = l a 1 3 5 7,5 7,4 7,2 7,1 7,0 6 7,8 7,6 7,4 7,2 7,1 12 Kh 1М F 7 7,6 7,5 7,4 7, 4 7,2 8 7,8 7,6 7,4 7,3 7,1 9 6,6 6,6 6,4 6,3 6,3

  28. 9 9 , mm , mm a) b) * l * l 8 8 7 7 1 2 3 1 2 3 6 6 0 100 200 300 400 0 100 200 300 400 3 3 t × t × 10 , hours 10 , hours 8 8 , mm , mm 4 5 6 d) c) 7 8 9 * * l l 7 7 6 6 4 5 6 7 8 9 5 5 0 100 200 300 400 0 100 200 300 400 3 3 t × 10 , hours t × 10 , hours Fig 3.5. Assessment of admissible crack-like defect depth on operation time of pipe-line made from steel 08Kh18N12Т (a, b) and steel 12Kh1МF (c, d): a, c – a/b=1/10; b, d – a/b=2/3. Numbers of points correspond to numbers of environments in Table 4.1. 0

  29. 3.4. ASSESSMENT OF METAL PROPERTIES DEGRADATION UNDER LONG TERM EXPLOITATION Table 3.5. Statistic data on exploitation regimes of power plant units. Table 3.6. Chemical composition of steel 16HS (in weight %).

  30. Table 3.7. Corrosion fatigue crack growth resistance of feeding pipelines metal (16HSsteel).

  31. Fig 3.6. Comparison of the corrosion fatigue crack growth resistance diagrams of new metal (1) and used pipe-line metals from Vyhlehirska Power Plant (2) and Ladyghynska Power Plant (3) for operating environments of different composition: (a) - environment of nominal composition; (b) - with organic additions.

  32. Fig. 3.7. Dependencies of admissible rack-like defects depth on the planned service life for new metal (1) and used pipeline metals from Vyhlehirska Power Plant (2) and Ladyghynska Power Plant (3). Environment: (a) and (c) - nominal composition; (b) and (d) - with organic additions. Type of crack-like defect: (a) and (b) - furrow-type; (c) and (d) - ulcer-type.

  33. EXPERT SYSTEM FOR ASSESSMENT OF RELIABILITY AND DURABILITY OF STRUCTURAL COMPONENTS OF HEAT POWER PLANTS

  34. CONCLUTIONS 1. Analysis of the characteristic types of in-service damages and failures of basic equipment of heat power plants (water-wall tubes of high pressure steam-boilers, super heater tubes of sub- and supercritical pressure boilers, water economizers, non-heated boiler’s elements and feeding pipelines of supercritical pressure power generating units) has showed on predominantly cracks nucleation and growth processes as result of as result of long time of exploitation or as result of different reflections of operating regimes of equipment. 2. Fracture mechanics approaches are preferable as basic concept for expert assessments of technical state and reliability of heat-and-power engineering equipment. 3. Service life extension of such equipment is to be carried out on the base of diagnostics of its actual state and residual life of the basic structural elements should be evaluated with takes into account of the actual data of metal properties and operating conditions of equipment.

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