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

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.

CONTENTS 1.GENERAL CHARACTERISATION OF IN-SERVICE DAMAGES AND FAILURES OF BASIC EQUIPMENT OF HEAT POWER PLANTS 2.FRACTURE MECHANICS APPROACHES 3.ENGINEERING APPLICATIONS FOR SERVICEABILITY ASSESSMENTS OF POWER ENGINEERING PIPELINES

1. GENERAL CHARACTERISATION OF IN-SERVICE DAMAGES AND FAILURES OF BASIC EQUIPMENT OF HEAT POWER PLANTS 1.Water-wall tubes of high-pressure steam boilers 2.Super heater tubes of sub- and supercritical pressure boilers 3.Water economizers 4.Non-heated boiler’s elements 5.Feeding pipelines of supercritical pressure power generating units

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

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

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

2. 2. SURFACE FATIGUE CRACK NUCLEATION AS RESULT OF CORROSION DEFORMATION INTERACTIONS EXPERIMENTAL BACKGROUND   s 1 2 pH=3,0 I c (  ) 12Kh1МF  max I, nА, МPа Fig. 2.5 Fig. 2.6 N I IIIII (2.3)

EXPERIMENTAL PROCEDURE P Fig Testing equipment. a) b) c)

IIIIII Fig Correlation between parameter  s and level of corrosion fatigue damaging of cyclic deformed surface (08Kh18N12T steel; pH=6.5).

ASSESSMENT OF SURFACE CORROSION FATIGUE CRACK NUCLEATION s  s   neMM z   Fig (2.4)

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

EXPERIMENTAL METHODOLOGY Fig 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 a) c) b)

Fig 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. F pH E PC a) b)

ELECTROCHEMICAL CONDITIONS IN THE CORROSION CRACKS Fig Dependencies of pH(x) and E(x). Fig Distribution of pH values in the corrosion crack cavity for cracks of different length (40Kh13steel - reactor water of boron regulation; pH=8.0) x / a pH(x) 8 - a = 3.40mm -a = 5.80mm -a= 13.8mm (2.7) (2.8)

METHOD FOR FORECASTING OF THE THRESHOLD STRESS INTENSITY FACTOR UNDER STRESS CORROSION CRACKING AND CORROSION FATIGUE 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;  is a frequency of cyclic loading; T 0 and N 0 are respectively time and number of cycles loading, that correspond to the beginning of hydrogen formation in the crack tip; T b and N b are the base of tests in hours and in cycles of loading, respectively. Subscripts s and c specify static or cyclic loading conditions, respectively. (2.9) (2.10)

Fig 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; 5 - corrosive environment, load ratio R= METHOD FOR DETERMINING OF BASIC CHARACTERISTICS OF CORROSION CRACK GROWTH RESISTANCE (2.11)

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

3.2. DETERMINING OF PERIOD FOR SURFACE CORROSION FATIGUE CRACK NUCLEATION (3.1) pH 12Kh1MF steel08Kh18N12T steel а, mm рН=3, рН=6, рН=9, Table 3.1

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

Table 3.2. Operating aqueous environments and their chemical composition Number of environment Environment Chemical composition 1 Boron regulation 1 %-solutionH 3 BO 3 +KOH (pH8) 2 Boron regulation with chloride admixtures 1 %-solutionH 3 BO 3 +KOH (pH8) + +5 mg/kg Cl – (10,5mg/kg KCl) 3 Boron regulation with nitride admixtures 1 %-solutionH 3 BO 3 +KOH (pH8)+ +10mg/kg  3 NO (16,3mg/kg KNO 3 ) 4 Ammoniac Distilled water+NH 3 (pH9) 5 Hydrazine-ammoniac (I) H 2 O+NH 3 (pH9)+100  g/kg N 2 H 4 6 Hydrazine-ammoniac (II) H 2 O+NH 3 (pH9)+100 mg/kg N 2 H 4 7 Ammoniac with chloride admixtures H 2 O+NH 3 (pH9)+ +10mg/kg Cl – (16,5mg/kg NaCl) 8 Ammoniac with admixtures of hydrochloric acid H 2 O+NH 3 (pH9)+ + 10mg/kg Cl – (HCl);pH3,95 9 Ammoniac with admixtures acetic acid H 2 O+NH 3 (pH9)+ +10 –5 mole/l CH 3 2 COOH;pH5,9

Table 3.3. Coefficients in Paris equation for tested conditions Steel Number of environment C n R ,61 0, ,95 0, Kh18N12Т ,48 0, ,13 0, ,15 0, ,79 0, ,26 0, ,50 0, Kh1МF ,39 0,8051

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  l,mm Shape of crack Steel Number of environment 500 cycles 1000 cycles 2000 cycles 3000 cycles 5000 cycles 1 7,2 7,1 7,0 6,9 6,8 2 7,1 7,0 6,9 6,8 08Kh18N12Т 3 7,9 7,6 7,3 7,1 7,0 4 7,1 7,0 6,8 6,7 6,6 5 7,0 6,9 6,7 6,6 6 7,2 7,0 6,9 6,8 6,6 7 7,1 7,0 6,9 6,8 8 7,2 7,1 6,9 6,8 6,7 201  al 12Kh1МF 9 6,2 5,9 5,8 1 7,8 7,6 7,5 7,4 7,2 2 7,6 7,5 7,4 7,3 7,2 08Kh18N12Т 3 8,6 8,2 7,8 7,7 7,4 4 7,7 7,5 7,3 7,2 7,1 5 7,5 7,4 7,2 7,1 7,0 6 7,8 7,6 7,4 7,2 7,1 7 7,6 7,5 7,4 7,4 7,2 8 7,8 7,6 7,4 7,3 7,1 31  al 12Kh1МF 9 6,6 6,4 6,3

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 a)a) l *, mm  10 3, hours c)c) l *, mm  10 3, hours b)b)  10 3, hours l *, mm d)d)  10 3, hours l *, mm

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 %).

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

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.

Fig 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.

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

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.