Text

Vermont Yankee July 2008 Evidentiary Hearing - Applicant Exhibit E4-08-VY, Flow-Accelerated Corrosion in Power Plants
	ML082530345
Person / Time
Site:	Vermont Yankee
Issue date:	08/12/2008
From:	Bouchacourt M, Chexal B, Dooley B, Horowitz J, Rosalyn Jones, Kastner W, Millett P, Nordmann F, Paul P, Remy F, Wood C; Electric Power Research Institute, Govt of France, Electricite de France, Siemens AG
To:	; NRC/SECY/RAS
	SECY RAS
References
	06-849-03-LR, 50-271-LR, Entergy-Applicant-E4-08-VY, NEC037447, RAS M-323, TR-106611-R1
	Download: ML082530345 (67)
	v • d • e

A*.G H -S27,-5 L7rF-7-**ý,

DOCKETED USNRC August 12, 2008 (11:00am)

OFFICE OF SECRETARY RULEMAKINGS AND ADJUDICATIONS STAFF

-AN D o. Official Exhibt N Flow-Accelerated...

mn Power Plants U.S. NUCLA R COMMI .SSION OFFERED by pcaItiLice ee Intervener NRC Other

,IIENTIR on OWitness/Panel sIEC

.-- .--- ENJ GO37 44&on TAC Ti"U-

,A*RjD REJECt1 WITHDRAWN 3 R epona C

FLOW-ACCELERATED CORROSION IN POWER PLANTS TR-106611-Rl by Bindi Chexal Jeffrey Horowitz Barry Dooley Peter Millett Chris Wood Robin Jones of EPRI Michel Bouchacourt Franqois Remy Francis Nordrnann Pierre Saint Paul of Electricitý de France Wolfgang Kastner of Siemens AG Power Generation

. .... .. . ... .. . -N E G 037447--

Table of Contents Nomenclature .............. .I... .. xxxix 1 Introduction to Flow-Accelerated Corrc)sion Corrosion ................................. . 1-2 Flow-Accelerated Corrosion .................... . 1-4 Historical and Technical Background of FAG..... 1-5 Laboratory Research .................. 1-5 Correlation Development ..................

Computer Programs To Model FAG ........... 1-6 Accident at the Surry Power Plant ............ 1t-7 Millstone Accident ........................

Pleasant Prairie Power Plant ................ 1-10 Fort Calhoun Accident ..................... 1-12 Current Global Industry Response ............ 1-12 References ........................... ...... 1-13 2 General Aspects of Corrosion Wall Thinning in Aqueous Systems ............. .2-1 Wall Thinning Caused by Chemical Effects .... .2-1 Wall Thinning Caused by Mechanical Effects ... .2-4 Combined Effect of Chemical and Mechanical Pr'ocesses. .2-8 General Aspects of FAC ........ .2-9 Conditions Required for FAC .............. .2-9 Description of the FAC Process .............. 2-10 Mechanisms of the FAC Process ............. 2-15 References ................................. 2-18 3 Key Parameters Influencing FAC Introduction . ... ........................... .3-1 Factors Influencing FAC ...................... I .3-1 Hydrodynamic Factors ................... .3-2 Environmental Factors .................... 4 I.

3-13 Effect of Materials ......................... 3-31 Problems in Modeling FAC .................... 3-36 xix

. . . . .. . ....- - N EC 037467- _

Table of Contents Semi-,Mechanistic Models for Predicting FAC ........ I *. 3-37 Semi-Mechanistic Models ............ t ,.3-37 C EG B Model ...... ........................ P .. 3-50 Investigation of the Semi-Mechanistic Models ....... .. 3-53 pH Effect ................................ .. 3-53 MassTransfer Effect ................... *. 3-62 Temperature Effect.... .. 3-78 Effect of Steel Composition .......... , .. 3-85 R e fe re nc e s . . . . . . . . . . . . . . . . . . . . . . . .3-97 4 Systems and Components Susceptible to Flow-Accelerated Corrosion Commonly Susceptible Systems .............. ................ 4-1 Fluid S tate ............................... ................ 4-1 Operating Tim e ........................... ........... ,:.....4-1 Size of Lines ............................. ............ 4-2

........................ I ................................ 4-3 Nuclear Power Plants Power System Flow Diagram .................. ...*........ .4-3 4-3 Piping Systems Typicaily Susceptible to FAC.. ............. 4-4 Fossil Power Plants ............ .... . ....... 4-5 Power System Flow Diagram .......

Piping Systems Typically Susceptible to FAC .

4-18 Differences Between Nuclear and Fossil Plants ........... 4-11 Plant Operating Conditions .................. ........... 4-13 W ater Chemistry ......... ................ ... ........ 4-14

..... :4-14 Damage to Other in-Line Components ............ ........... 4-16 Steam Turbines ........................

Moisture Separator Reheater ............. .. .......... 4-13 Feedwater Heaters ...... . ....... ............ 4-21 Heater Drains .. .......... ............... ............ 4-27 Steam Generators (Nuclear Plants) ............ ...............................

............ 4-27 4-27 Fossil Boilers ........................... .

Pumps ............................ . .............. 4-30 Other Notable Problem Areas ....... .... . ... .. 4-32 Systems Not Susceptible to FAC ................ ............................... 4-33 Plant Experiences with Flow-Accelerated Corrosion.. ............................. 4-34 XX NEC03746.--.

Table of Contents FAC in Steam Generators ...... 4-35 FAC Experience in Piping in the United States 4-43 FAC Experience in Canada ............... 4-55 FAC Experience (n Europe ................ 4-55 EPRI's Plant Experience Database ......... 4-73 References ................... .......................... 4-73 5 Water Chemistry Control Practices LW R Water Chemistry ................ ................ . .5-1 Optimization of Water Chemistry for Flow-Accelerated Corrosion,. .5-6 PWR Secondary System Chemistry ........................... *5-6 Balance of Plant Considerations .......................... .5-7 U.S. Utility Practices ............ . ................. . .5-8 EDF Water Treatments ............ 5-19 VGB Water Chemistry Guidelines for PWRs ........... >.... 5-22 Miscellaneous. PWR Chemistry Considerations ....... 5-27 Boiling Water Reactors ................ ............. 5-30 BWR Chemistry Control for Flow-Accelerated Corrosion (FAC) 5-34 VGB Guidelines for German Boiling Water Reactors. ....... .5-37 Fossil Units ...................... II II 4 * ......... 5-41 United States ........ J

......... 5-41 EDF .................... . . I B I h 5-55 VG B ........... ,... . . . . . 4 P 4 4 *

....... 5-56 References.. ................ ......... 5-61 6 Plant Practices to Detect and Control FAC Introduction .............. ....... .6-1 Nuclear Plants .................... I I I I 6 U

6-1 NUMARC Guidelines ............ 1 g 4 4 1 P 4 q

.6-1 NSAC-202L .' ....... .. ... . q q 4 q I 4

6-3 FAC Control Strategies ....... p I 4 F 4

.6-4 Fossil Plants.. ........... ...... .6-5 Typical Utility Tasks to Control FAC

6-7 Programmatic Aspects of Inspection Programs.

6-9 Inspection Planning ............. 6-10 Initial Inspection . ........... 6-10 I

Xxi

MECO37'169- -- --

Table of Contents Subsequent Inspections ........

6-11 The Inspection Process.......... *6-12 Ultrasonic Methods ............. .6-12 Radiographic Methods ...........

6-17 Optical Inspection Methods .......

6-22 New NDE Methods .............. .6-23 Interpretation of Inspection Data..

6-29 Actions To Reduce or Eliminate FAC .6-39 Water Chemistry Considerations,., .6-40

. 6-40 Selection of Pipe Material......

Modification of Operating Conditions .6-45 Local Design Changes ...........

6-46 Qualification of Thinned Components D I i q

I b I q *6-47 Piping Design Codes ......... L

4 q 4 4

.6-47 Piping Design Considerations ..... I d I b I I

6-48 Local Wall Thinning ........... 0 p I b I I I d .6-49 References.................. .6-52 7 Applications of FAC Prediction Software Keller's Model . ..... . ............................. 7-4 7-4 Models for Computer Applications ..................

Software from EDF ...................... ........... 7-4 The Siemens/KWU WATHEC and DASY Programs. . ........... 713 The EPRI CHEC Programs .................... .......... 7-19 Software Description .................... ... ........ 7-36 References ........................ ........... 7-44 8 Self-Assessment of Flow-Accelerated Corrosion Programs N uclear Utilities ............................ .................

....... 8-1 Enforcement of FAC Programs ............. .. . ............

. . . . . . . . .. 8-2 EPRI Recommendations ................. ...

... ... ... 8-2 Fossil Utilities .............................. ............................. 8-5 Enforcement of FAC Programs ............. .............................. 8-5 Process Plants ............................. .............................. 8-5 R eferences ................. .............. .............................. 8-6 xxii

- - - NECO3747O~-----~--..

Table of ContIents 9 Conclusion General Aspects ...................................

9-1 Key Param eters ....................................

9-2 Systems and Components Susceptible to FAC .................. .9-2 Plant Practices to Detect and Cbntrol FAC.................. .9-3 4

Applications of FAC Prediction Software................... .9-4 Sum mary ...............................................

9-4 A Scientific Basis of Corrosion in Aqueous Systems General Aspects of Aqueous Corrosion ............. .. ......... A-1 The Corrosion Cell ............... .. .. ....... o..I A-1 Thermodynamics of Aqueous Corrosion ......... A-4 Polarization ....... I .............. ....

. . ...... A-7 Aqueous Corrosion in Alkaline Solutions ............ ......... A-12 Chemically Stable Species and Oxide Composition ......... A-12 Behavior of Magnetite, Crystals ....... . A-19 The Phenomena of Aqueous Corrosion ........... ......... A-20 Models of General Aqueous Corrosion ............ ......... A-21 Corrosion Kinetics .......... ........... . ......... A-23 R eferences ..................................... . ......... A-28 B Rate Aspects of Wall Thinning Effect of Velocity on the Rate of Wall Thinning ......... .......... B-3 Stagnant W ater . ...... ý..................... .......... B-3 Low Velocity ........................... ........... 1B-4

.......................... 6 -4 C ritical Velocity ...............................

High Velocity ................................ . .......... B-4 R eferences ..................... ....... ........ B-6 C Illustrations of FAC-Induced Wall Thinning D An Introduction to Fluid Flow and Mass Transfer Introduction To Fluid Flow ............................ .D-1 Interaction of a Liquid with the Wall of a Flowing System..... .D-1 Hydraulic Diameter .............................. ,D-3 Roughness Effect on Turbulent Flows in Pipes............ 0-6 Dimensionless Approach for Roughness .................... ID-9 Xxiii

... -NEC037471---------

T'hie of Contents Introduction to Mass Transfer .......... ......... D-1O Reynolds Number............... ......... D -11 Schmidt Number ................... I......... D-12 Sherwood Number .................. ......... D-14 Pressure Drop in Components ............ ......... 13-18 Introduction to Two-Phase Flow .......... ......... D-19 Steam Quality and Void Fraction ....... . 4........ D-21 Flow Regimes ...................... ......... D-22 Correlating Void Fraction. ........ ,. D-23

.... D-23 Chexal-Leflouche Void Model .........

Void Fraction Behavioral Characteristics. ......... D-24 References ............................ ......... D-30 Glossary . _G-1 Ind ex . ..... ................... .. . 1-1 Xxiv

-- ~ -- ~.-----.---NEC037472-----

List of Figures 1- Chexal's Four Quadrant Known-Unknown Diagram.... p. xvi 1-1 Loss of Availability Due to Corrosion of U.S. Nuctear Plants.... p. 1-2 1-2 Sample of Surface Damaged by Flow-Accelerated Corrosion.... p. 1-4 1-3 Surry Unit 2 Condensate Failure.... p. 1-7 1-4 Millstone Unit 3 Heat Drain Failure.... p. 1-10 1-5 Failed Pipe from the Feedwater System at the Pleasant Prairie Fossil Power Plant.... p, 1- 11 1-6 Failed Sweep Elbow from the Extraction Steam System of Fort Calhoun Station.... p. 1-12 2-1 FAC in a PWR Drain Line.... p. 2-3 2-2 Interactions of Single or Two-Phase Flows with a Solid Surface....

p. 2-4 2-3 Liquid Impact Erosion Observed in Copper Alloy Condenser Tubes after 36,000 Hours Operating in a PWR.... p. 2-6 2-4 Material Loss Caused by Erosion.... p. 2-7 2-5 Typical FAC Superficial Damage Produced in a Very Strongly Reducing Environment (High Purity Feedwater with Cation Conductivity of - 0.1 pS/cm, Oxygen < 1 ppb, Hydrazine of - 25 ppb, and an ORP of Approximately -350 mV).... p. 2-10 2-6 Scalloped Appearance of Single-Phase FAC (450 Impingement Test at 356 'F (180'C) and 184 ft/s (56 m/s) with a pH of 9.0 (at 77°F

,(26"C)) with Ammonia Water Treatment) as Observed Under a Scanning Microscope.... p. 2-11 2-7 Tiger Striped Appearance of Two-Phase FAC.... p. 2-12 2-8 Microstructure of Carbon Steeo in the Plane Parallel to the Roiling Direction.... p. 2-13 2-9 Scanning Electron Microscope Observation Showing the Selective Attack on the Pearlite Phase on a Carbon Steel Surface.... p. 2-13 2-10 Scanning Electron Microscope Observation of a Magnetite Cross Section.... p. 2-14 2-11 Schematic Representation of the FAC Process.... p. 2-15 xXv

.. .. .. . .. .. . .. ....,. . .. . . .. . . .. . . .N C 0 3 '1 . 3 .. ... . . . . .. . . . . ... . . . . . . . . . . . ... .. . ....

List of Figures 2-12 Schematic Representation of Oxide Formed on Iron-Based Feedwater Surfaces During Operation with Deoxygenated AVT under reducing conditions (ORP < 0 mV).... p. 2-18 3-1 Effect of Flow Rate on Corrosion in Circulating 5720F (300'C) Water of Low Alloyed Steel.... p. 3-3 3-2 Schematical Representation of the Change in the Mechanism of FAC as a Function of the Liquid Velocity.... p. 3-4 3-3 Effect of Roughness on Mass Transfer.... p. 3-7 3-4 Effect of the Dimensionless Roughness on Mass Transfer.... p, 3-9 3-5 Processes that May Occur when Steel is Exposed to Contaminated, Flowing Water.... p. 3-14 3-6 Influence of Copper Ions in Solution on the Rate of FAC.... p. 3-18 3-7 Influence of Copper Ions in Solution on the Rate of FAC.... p. 3-19 3-8 Flow and Temperature Dependence of Single-Phase FAC with Ammonia at a pH of 9.04 at 770 F (25CC) (Data Taken Downstream of Orifice).... p. 3-25 3-9 Temperature Dependence of Single-Phase FAC Under Neutral Conditions, pH of 7 at 770 F (250C) (Data Taken on Geometry; Plate.

Specimens).... p. 3-26 3-10 Temperature Dependence of Single-Phase FAC for High and Low Velocity Specimens with Ammonia at a pH of 5.6 at 77 0 F (25CC) (Data Taken on Inner Surface of Annulus).... p. 3-27 3-11 Temperature Dependence of Single-Phase FAC, with Ammonia at a pH of 9.0.... p. 3-28 3-12 Temperature Dependence of Two-Phase FAG.... p. 3-29 3-13 Temperature Dependence of Two-Phase FAG with a Steam Quality of 65% and a Velocity of 185 ft/s (56 m/s).,... p. 3-30 3-14 Temperature Dependence of Two-Phase FAC.... p. 3-30 3-15 Material Dependence of FAC.... p. 3-32 3-16 Material Dependence of FAG.... p. 3-33 3-17 Material Dependence of FAC for Geometry: Plant Specimens....

p. 3-34 3-18 Material Composition Effect on the Magnetite Porosity at 435°F (225'C) with pH of 9.0 Using Ammonia.... p. 3-35 3-19 Schematic Representation of the Berge Model.... p. 3-39 3-20 Schematic Representation of the FAC Process.... p. 3-40 xxvi ,

NEC037474----

List of Figures 3-21 Schematic Representation of the MIT Model.... p. 3-43 3-22 Plot of the Reaction Rate Constant K*... p, 3-46 3-23 Scanning Electron Microscope Observations of the Cross Sections of the Oxide Surface on 2 Steel Specimens Exposed to Single-Phase Flow at 355 0 F (1800C) with pH of 9.0 at 770 F (25CC) using Ammonia.... p. 3-47 3-24 Effect of the Oxide Thickness of the FAC Process.... p. 3-48 3-25 Effect of the Reaction Rate Constant on the FAC Process_... p. 3-49 3-26 Effect of pH on the Rate of FAC for Geometry: Plant Specimens and a Pipe Specimen,... p. 3-54 3-27 Effect of pH on the Rate of FAC.... p. 3-55 3-28 Scanning Electron Microscope Observations of Ferrite Grains Exposed to Two-Phase Flow in the Five Cail Babcock Loop at 3500 F (1750C), with Impinging Flow at 185 ft/s (60 m/s), Steam Quality of 64% at 1700x.... p. 3-56 3-29 Scanning Electron Microscope Observations of Pearlite Grains Exposed to Two-Phase Flow in the Five Cail Babcock Loop at 350'F (175°C), with Impinging Flow at 185 ft/s (60 m/s), Steam Quality of 64% at 480x.... p. 3-57 3-30 Equilibrium Iron Solubility Dependence of Single-Phase FAC Obtained in the EDF-Ciroco Loop at 355°F (1 80C).... p. 3-59 3-31 Equilibrium Iron Solubility Dependence of Two-Phase FAC Obtained in the EDF Five-Call Babcock Loop at -350°F (-I 75°C) .... p. 3-60 3-32 Equilibrium Iron Solubility Dependence of Single-Phase FAC Obtained in the EDF-Ciroco Loop.... p. 3-61 3-33 Dependence of Single-Phase FAC Rates on Mass Transfer Coefficient at 3000 F (1490C) with a pH of 9.05.... p. 3-64 3-34 Mass Transfer Coefficients as a Function of Temperature .... p. 3-65 3-35 Mass Transfer Coefficient Dependence of FAC Rates in Single-Phase Flow at 3557F (180'C) and a pH of 9.0 at 77°F (25CC) Using Ammonia.... p. 3-67 3-36 Mass Transfer Coefficient Dependence of FAC Rates in Single-Phase Flow at 355 0 F (180'C) and Using Several pHs.... p. 3-68 3-37 Appearance of Test Surface Before the Test (as Received Conditions Machined and Polished).... p. 3-70 xxvii

-- -. -----. NEC037475 - -.

List of Figures 3-38 Appearance of Test Surface After Testing Carried Out at 3550 F (1800C) with a pH of 9.0 Using Ammonia on a 0.315 inch (8 mm) 10 Tube with a Velocity of 12 ft/s (4 m/s) and a Wear Rate of 11 mils/yr (0.03 micron/hr).... p. 3-71 3-39 Appearance of a Test Surface After Testing Carried Out at 3553 F (1800C) with a pH of 9.0 Using Ammonia on a 0.315 inch (8 mm) tD Tube with a Velocity of 50 ft/s (16 mis) and a Wear Rate of 121 mils/yr (0.35 micron/hr).... p. 3-72 3-40 Appearance of a Test Surface After Testing Carried Out at 3550 F (180'C) with a pH of 9.0 Using Ammonia on a 0.157 inch (4 mm) 10 Tube with a Velocity of 105 ft/s (34 m/s) and a Wear Rate of 862 mils/yr (2.5 micron/hr).... p. 3-73 3-41 Roughness Effect on FAC Rates.... p. 3-74 3-42 Steam Quality Dependence of Two-Phase FAC Rates at 345°F (1750C) with pH of 9.0 Using Ammonia.... p. 3-76 3-43 Scanning Electron Microscope Views of Oxide Cross Sections After Tests at the EDF Ciroco Loop at 3550 F (180 0 C) with a pH of 9.0 at 77 0 F (25°C) Using Ammonia.... p. 3-77 3-44 Mass Transfer Coefficient Dependence of the FAC Rates in Single-Phase Flow.... p. 3-79 3-45 Temperature Dependence of FAC Rates in Single-Phase Flow....

p. 3-80 3-46 Scanning Electron Microscope Views of the Surface Appearances After Testing in the EDF Ciroco Loop (Test Duration was Approximately 48 to 60 Hours).... p. 3-81 3-47 Scanning Electron Microscope Views of the Surface Appearances After Testing in the EDF Ciroco Loop (Test Duration was Approximately 48 to 60 Hours).... p. 3-82 3-48 Scanning Electron Microscope Views of the Surface Oxide Obtained from Plant Samples.... p. 3-84 3-49 Scanning Electron Microscope Views Showing the Steel Composition Effect on the Porosity of the Magnetite Layer at 435°F (225CC) with a pH of 9.0 Using Ammonia.... p. 3-86 3-50 Profile Analysis Using Glow Discharge Spectrometry on a Steel Specimen Containing 0.025% Chromium, Single-Phase, at 435DF (2250C) with a pH of 9.0 Using Ammonia.... p. 3-87 4xviLi

List of Figures 3-51 Profile Analysis Using Glow Discharge Spectrometry on a Steel Specimen Containing 1.54% Chromium, Single-Phase, at 4350 F (2256C) with a pH of 9.0 Using Ammonia,... p. 3-88 3-52 FAC Rate Dependence on the Steel Composition for Both Single and Two-Phase Flows.... p, 3-89 3-53 Scanning Electron Microscope Examination of Carbon Steel, Steam Generator J Tube with a Chromium Concentration of 0.0166%,....

p. 3-91 3-54 Scanning Electron Microscope Examination of Carbon Steel, Steam Generator J-tube with a Chromium Concentration of 0.135%....

p. 3-92 3-55 Main Improvement Carried Out in the FAC Model to Take Account of the Chromium Effect.... p. 3-94 3-56 Time and Chromium Concentration Dependency of FAC for PWR J-tubes at 22100 and 9 m/s.... p. 3-95 3-57 Temperature Dependence of FAC Rates in Single-Phase Flow Obtained in the EDF Ciroco Loop for Several'Steels with a pH of 9.0 at 77TF (250C) Using Ammonia.... p. 3-96 4-1 Typical Nuclear Power Plant Heat Balance Diagram.... p. 4-3 4-2 Typical Drum Boiler Fossil Plant Cycle Showing Locations of Impurity Ingress, Corrosion and Deposition.... p. 4-7 4-3 Typical Once-Through Boiler Fossil Plant Cycle Showing Locations of Impurity Ingress, Corrosion and Deposition.... p. 4-7 4-4 Failed Elbow from the Feedwater System at the Navajo Fossil Power Plant.,, p. 4-8 4-5 Mollier-Diagram Showing Flow-Accelerated Corrosion Zones of Saturated Steam Turbines [4,7].... p. 4-15 4-6 Wet Steam Velocities Calculated for the Initial Design of the MSR Used for EDF's 900 MWe PWR Units.... p. 4-17 4-7 Damage Caused by Secondary Flow in the MSR of a 900 MWe EDF PWR Unit.... p, 4-18 4-8 Damage Caused by the Jet from a Leak in a Reheater Tube in the MSR of a 900 MWe PWR EDF Unit (Steam at 51 8°F (270C) and 798 psi (5.5 MPa), Water at 507°F (264°C) and 725 psi (5 MPa))....

p. 4-19 1 4-9 MSR Tube Protection Resulting from a Change in the Water Treatment from Ammonia to Morpholine in a 900 MWe EDF PWR Unit.... p. 4-20 xxix

. .. ...... .. .. . .. ... ... .. .... . .... .. ... ...._ N EC0 3 7 427-7 . . . . . .. .. .. .... .. . . .. . .. . . . . . ... .. .

List of Figures 4-10 Damage in a Low Pressure Feedwater Heater of a 900 MWe PWR EDF Unit.... p. 4-22 4-11 Damage at the Feedwater Heater Inlet of a 900 MWe EDF PWR Unit.... p. 4-23 4-12 Theoretical versus Measured Extraction Line Quality.... p. 4-24 4-13 Damage Caused by Bypass Flow Around a Baffle in a Low Pressure Feedwater Heater of a 900 MWe EDF PWR Unit.... p. 4-25 4-14 Changes in the Design of the Feedwater Heaters of some of EDF's 900 MWe PWR Units.... p. 4-26 4-15 Cross Section Through Economizer Inlet Header and Tubes Showing Flow-Accelerated Corrosion in the Tubes.... p. 4-29 4-16 Flow Accelerated Corrosion and Burst in Economizer Inlet Header Tubes.... p. 4-30 4-17 Material Dependence of Single-Phase FAG Rates Obtained in the Creil Loop of EDF.... p. 4-31 4-18 FAC Damage to J-tubes in the Fessenheim Power Plant (a 900 MWe EDF PWR unit).... p. 4-36 4-19 FAC Damage to the Feed Rings at the Diablo Canyon Plant..... p. 4-38 4-20 Local FAC Damage on the Feed Ring at the Impact Area of Jets from the J-tubes at the Fessenheim Power Plant (a 900 MWe EDF PWR unit):.... p. 4-39 4-21 Typical Aspect of Damaged Flow Holes (Nominal Flow Hole Diameter of 19 mm).... p. 4-40 4-22 Steam Generator Tube Eggcrate Support Damage at San Onofre Nuclear Generating Station.... p. 4-42 4-23 Failed Elbow from the Feedwater System at the Navajo Fossil Power Plant.... p. 4-45 4-24 Surry Unit 2 Condensate Failure.... p. 4-46 4-25 Location of the Rupture in Surry Unit 2.... p. 4-47 4-26 Failed Pipe Downstream of a High Pressure Turbine Nozzle in Unit 2 of the Arkansas Nuclear One Nuclear Station.... p. 4-49 4-27 Millstone Unit 3 Heater Drain Failure.... p. 4-50 4-28 Millstone Unit 2 Failure in the Rleheater Drain System .... p, 4-51 4-29 Millstone Unit 2 Failure in the Heater Drain Bypass System.... p. 4-52 4-30 Failed Pipe from the Feedwater System at the Pleasant Prairie Fossil Power Plant.... p. 4-53 xxx

.... ....... . . .. .... .... .. . .. . .N EC0 37,4...7 8 .... .. . .. . . . . . . . .. .. . . .... ... . . . . .. .. . .

List of Figures 4-31 Failed Sweep Elbow from the Extraction Steam System of Fort Calhoun Station.... p. 4-54 4-32 Damagein the Steam Generator of the Saint Laurent Gas Cooled Reactor Power Plant.... p. 4-59 4-33 Damage in the Discharge Pipe of a Feedwater Pump at Bouchain Unit 1 (a 250 MWe Fossil Fired Power Plant).... p. 4-60 4-34 Damaged Section Between the High Pressure Turbine and the Moisture. Separator inlet at the Belgium Tihange Nuclear Power Plant.... p. 4-61 4-35 Damaged Section Between the High Pressure Turbine and the Moisture Separator Inlet at the EDF Bugey Nuclear Power Plant....

p, 4-62 4-36 Damage in the Moisture Separator of a CP1 (a 900 MWe PWR Power Plant).... p. 4-63 4-37 Arrangement of the High Pressure Feedwater Heaters and High Velocity Separators in the French 1,300 MWe Nuclear Plants....

p. 4-64 4-38 Details of the Leak in the Heater Drain System at Bugey (a 900 MWe PWR):.,.. p, 4-65 4-39 Upstream View of the Feedwater Failure at Loviisa (a 445 MWe Finnish PWR).... p. 4-67 4-40 Downstream View of the Feedwater Failure at Loviisa (a 445 MWe Finnish PWR).... p. 4-6B 4-41 Rupture of Extraction Line from a Russian Designed VVER.... p. 4-70 4-42 Characteristic FAC Wear Patterns of the Component Shown in Figure 4-41 .... p. 4-70 4-43 Feedwater Locations Identified as Sensitive to FAC.... p. 4-71 5-1 PWR Secondary Chemistry Optimization Diagram.... p. 5,3 5-2 PWR Primary Chemistry Optimization Diagram.... p. 5-4 5-3 Boiling Water Reactor Water Chemistry Optimization Diagram....

p. 5-4 5-4. Implementation of Advanced Water Chemistries in U.S. PWRs....

p. 5-11 5-5 Comparison of Amine Feedwater Concentrations Required for pH Control ..... p.5-13 5-6 Comparison of Condensate Polisher Loading for Various Alternate Amines..... p. 5-14 Xxxi

List of Figures 5-7 Distribution of Amines in Use in U.S. PWRs.... p. 5-15 5-8 Evolution of Materials Required for Various Components During the Construction of the EDF's PWRs.... p. 5-22 5-9 Sampling Points at PWR Required by VGB Water Chemistry Guidelines [5.15].... p. 5-24 5-10 Effects of Normal Water Chemistry, Noble Metals Chemical Addition, and Hydrogen Water Chemistry on FAC.... p. 5-35 5-11 Relevant Sampling Points at German BWRs.... p. 5-38 5-12 Schematic Representation of Oxide Formed on Iron-Based Feedwater Surfaces During Operation with OT.... p' 5-45 5-13 Changes in OxidLzing Reducing Potential (ORP) and Feedwater Iron' Levels (Fe) at the Economizer Inlet when Hydrazine (N2 H4 ) is Gradually Reduced on a 600 MW Drum Unit with an All-Ferrous Feedwater System.... p. 5-47 5-14 Change in Feedwater Iron and Oxygen Levels at the Economizer Inlet Upon Elimination of Oxygen Scavenger (Carbohydrazide).... p. 5-48 5-15 Change in Boiler Pressure Drop Increase when Hydrazine is Eliminated and when Oxygen is Injected.... p, 5-49 6-1 Fossil Plant FAC Program Road Map.... p. 6-6 6-2 Thickness Measurements Using the Pulse-Echo Method.... p. 6-13 6-3 Panametrics' 36DL+ Hand Held UT Instrument...: p. 6-14 6-4 Sample Component Gridded for UT Inspection.... p. 6-16 6-5 Sample Radiograph Showing FAC Degradation in a Pipe Downstream of a Socket Welded Elbow.... p. 6-18, 6-6 Radiographic Exposure Using the Tangential Beam Approach....

p. 6-19 6-7 Radiographic Exposure Using the Perpendicular and the Tangential Beam Approaches.... p. 6-20 6-8 Sketch of the Autogrid UT System.... p, 6-25 6-9 A Conceptual Sketch Showing Application of the Digital Radiographic System on Piping.... p. 6-26 6-10 Schematic of Magnetostrictive System.... p. 6-28 6-11 Band Method .... p. 6-31 6-12 Area Method .... p. 6-33 6-13 Moving Blanket Method.... p. 6-35 6-14 Histogram of Wear - Showing No Wear.... p. 6-37 xxxii

.... . NEC0374 --..

List of tigures 6-15 Histogram of Wear - Showing Considerable Wear.... p. 6-38 6-16 Histogram of Wear - Showing Some Wear.... p. 6-39 6-17 Before and After Weld Overlay.... p. 6-44 6-18 High Velocity Separator.... p. 6-46 6-19 Wall Thinning Logic Diagram.... p. 6-49 6-20 Types Damaged Amenable for Local Thinning Analysis.... p. 6-51 7-1 Plot of Wail Thinning Number, W, versus the Sherwood Number, Shtr... p.7-6 7-2 Effect of the Upstream Element on the Geometric Factor of the Downstream Element.... p. 7-8 7-3 Computed Wall Thinning of a Piping System with WATHEC Residual Life Expectancy for a Component.... p. 7-16 7-4 Assessment of Wall Thicknesses of Components by 3-D Displays with DASY Trending of Wall Thinning out of Examinations from 2 Outages.... p. 7-17 7-5 Rough and Detailed Weak Point Analysis with WATHEC & DASY....

p. 7-18

7-6 Chexal-Horowitz FAC Model, Impact of Chromium.... p. 7-22 7-7 Chexal-Horowitz FAC Model, Impact of Molybdenum.... p-.7-23 7-8 Chexal-Horowitz FAC Model, Impact of Liquid Velocity.... p. 7-24 7-9 Chexal-Horowitz FAC Model, Impact of Pipe Diameter.... p. 7-25 7-10 Chexal-Horowitz FAC Model, Impact of Oxygen Level .... p. 7-25 7-11 Chexal-Horowitz FAC Model, Impact of Change in pH .... p. 7-26 7-12 Chexal-Horowitz FAC Model, Impact of Using Ammonia or Alternate Amines at a pH of 9 at 770F (250C).... p. 7-27 7-13 Chexal-Horowitz FAC Model, Impact of Fitting Geometry.... p. 7-28 7-14 Chexal-Horowitz FAC Model, Impact of Steam Quality.... p. 7 7-15 ChexaL-Horowitz FAC Model, Impact of Hydrazine Concentration....

p. 7-30 7-16 Chexal-Horowitz FAC Model, Comparison Against Laboratory Data.... p. '7-32 7-17 Chexai-Horowitz FAC Model, Comparison Against Plant Data....

p. 7-33 7-18 Mtss Transfer Entrance Effect-.., p. 7-35 A-1 Schematic of Corrosion Reactions of Iron in Acid Solutions...; p. A-2 xxxi~i

.. .... ... ........... . . .. .. . .. .. . .... . .. .. . . . ...N EC 0 3 "7 4 O .1

List of Figures A-2 The Pourbaix Diagram for Iron in Water at 77°F (25°C).... p. A-5 A-3 Schematic of the Helmholtz Double Layer Showing the Inner Helmholtz Plane and the Outer Helmholtz Plane with their Associated Ions.... p. A-9 A-4 Soluble Ferrous and Ferric Ion Concentrations in Equilibrium with Magnetite Calculated Using Sweeton & Baes [A.13] and Tremaine &

LeBlanc EA.14].... p. A-15 A-5 Concentration of Ferrous and Ferric Ions Relative to the Total Concentration of Iron in a Saturated Solution at 3920 F (2000C)....

p. A-16 A-6 pH Diagram for the Iron-Water System at 392°F (2000C),.... p. A-17 A-7 Schematic Representation of the Magnetite Crystal.... p, A-18 A-8 Models of Carbon Steel Corrosion in High Temperature Water....

p, A-21 A-9 Parabolic Rate Constants for the Corrosion of Mild Steel Corrosion in Pure Water and Steam.... p. A-25 A-10 Schematic of the Difference Between a Protective Corrosion Process and Flow-Accelerated Corrosion.... p. A-27 B-1 Change in the Corrosion and Erosion Mechanisms as a Function of Liquid Velocity.... p. B-2 B-2 Kinetics of Erosion and/or Corrosion Processes.... p. 8-3 C-1 A 10" (275 mm OD) 90 degree elbow from an MSR gravity drain line exhibiting a scalloped type damage on the extrados.... p. C-1 C-2 A 10Q1 (275 mm OD) 90 degree elbow from an MSR gravity drain line exhibiting a scalloped type damage on the extrados.... p. C-2 C-3 A concentric 6" X 14" (170 mm X 355 mm OD) reducer from a heater drain line exhibiting a scalloped surface and general wall thinning....

p. C-2 G-4 Fabricated 24" (610 mm OD) tee from a low pressure extraction steam line exhibiting a surface damage morphology characterized by sharp edged excavations which are preferentially oriented toward the incoming flow.... p. C-3 C-5 A concentric 6" X 14" (170 mm X 355 mm OD) reducer from a heater drain line exhibiting a scalloped surface and wall thinning downstream of a backing ring.... p. C-3 C-6 A section of 6r (170 mm OD) pipe taken from a horizontal portion of an MSR gravity drain line.... p. C-4 Xxxiv Xx NCO742. -

List of Figurets C-7 A 12" (325 mm OD) 90-degree elbow from a high pressure extraction steam line exhibiting areas of both smooth excavation and a textured oxide layer.... p. C-4 C-8 A 12" (325 mm OD) 90-degree elbow exhibiting longitudinal and circumferential grid line markings used for UT wall thickness measurements.... p, C-5 C-9 A 12" (325 mm OD) 90-degree elbow from a high pressure extraction steam line exhibiting areas of smooth excavation and tiger striping....

p. C-5 C-10 A 12" (325 mm OD) 90-degree elbow from a high pressure extraction steam line exhibiting areas of smooth excavation and a textured oxide layer.... p. C-6 C-1 1 An 8" (220 mm OD) tee from a heater drain line exhibiting a both a coarse and fine scalloped surface pattern and wall thinning in the branch (outlet) leg.... p. C-6 0-12 A 12" (325 mm OD) 90-degree elbow from a high pressure extraction steam line exhibiting area of smooth excavation with a glossy black magnetite surface finish.... p. C-7 C-13 A 24" (610 mm OD) 90-degree elbow from a high pressure extraction steam line exhibiting smooth excavations and tiger striping at small bore branch connections on the extrados of the upstream end....

p. 0-7 C-14 A 24" (610 mm OD) 90-degree elbow from a high pressure extraction steam line exhibiting well defined tiger striping on the intrados of the upstream end.... p. C-8 C-15 A 24" (610 mm OD) 90-degree elbow from a high pressure extraction steam line exhibiting tiger striping damage at small bore branch connections on the median at the upstream end.... p. C-8 C-16 A 24" (610 mm OD) 90-degree elbow from a high pressure extraction steam line exhibiting smooth excavations and tiger striping along the median and extrados.... P. C-9 C-17 A 24" (610 mm OD) 90-degree elbow from a high pressure extraction steam line exhibiting markings for UT wall thickness measurements.... p. C-9 C-18 A 24" (610 mm OD) 90-degree elbow from a high pressure extraction steam line exhibiting smooth excavations and tiger striping along the median and extrados.... p. C-10 xxxv 14EC03748L

List (if Figures C-19 A 10" (275 mm 00) 90-degree elbow from a high pressure extraction line exhibiting large areas of smooth excavations along the extrados ..... p. C-10 C-20 A 10" (275 mm 00) 90-degree elbow from an MSR drain line exhibiting irregularly shaped oxide patterns.... p. C-11 C-21 A 1.5" (48 mm OD) 90-degree elbow from a steam system drain line which discharges to the main condenser.... p. C-11 C-22 A 10" (275 mm OD) 90-degree elbow exhibiting a glossy magnetite finish with a scalloped area on the upstream end..... p. C-12 D-1 Moody Diagram of Darcy Friction Factor..., p. D-4 D-2 The Velocity Distribution in Turbulent Flow.... p. D-6 D-3 Details of the Three Regimes for Turbulent Flow.... p. D-7 D-4 Relative Roughness of New Commercial Piping.... p. D-8 D-5 Simplified Moody Diagram.... p. D-9 D-6 Development of the Diffusion (Mass Transfer) and Hydrodynamic Boundary Layers.... p. D-13 D-7 Comparison of the Thickness of the Hydrodynamic and Diffusion Boundary Layers.... p, 0-14 D-8 Flow Regimes in Convective Boiling.... p. D-20 D-9 Homogeneous Void Fraction.... p. D-21 D-10 Generalized from Regime Map for Horizontal Two-Phase Flow,....

p. 0-22 D-1 I First Quadrant Void Fraction Characteristics.... p. D-25 D-12 First Quadrant Void Fraction Characteristics .... p. D-26 D-13 First Quadrant Void Fraction Characteristics .... p. D-27 D-14 Third Quadrant Void Fraction Characteristic.... p. D-28 D-15 Third Quadrant Void Fraction Characteristic.... p. D-29 D-16 Third Quadrant Void Fraction Characteristic.... p. D-30 xxxvi NE C03 7404__.___..

List of Tables 3-1 Published Geometric Enhancement Factor Values for Piping Components with Single Phase Flow as Used in Various FAC Models....p. 3-11 4-1 Most Important Fossil Plant Areas Experiencing FAC (Results from 1997 Survey of 63 Utilities [4.2]) .... p. 4-9 4-2 Serious Failures in Fossil Plants Since 1982 [4.3]) .... p. 4-9 4-3 FAC Resistant Materials and Applications for Steam Water Cycle Components [4.7]...-. p, 4-16 4-4 Alloy Content of the Materials Shown in Figure 4-17....p. 4-31 4-5 History of U. S, Major FAC Experience....p. 4-44 4-6 History of EDF FAC Experience .... p. 4-57 5-1 Recirculating Steam Generator Power Operation (> 5% Reactor Power) Blowdown Sample/Feedwater Sample....p. 5-10 5-2 Comparison of Amines....p. 5-17 5-3 EDF Feedwater Chemistry Specifications ....p. 5-21 5-4 Requirements for the Feedwater of Steam Generators at German PWRs Under Continuous Operation .... p. 5-25 5-5 Requirements for the Steam Generator Blowdown of German PWRs in Continuous Operation....p. 5-26 )

5-6 Requirements for the Water of Steam Generators of German PWRs in Continuous Operation when Conditioned with Sodium Phosphates.

.... p. 5-27 5-7 BWR Chemistry Guidelines - Reactor Feedwater/Condensate -

Power Operation ....p. 5-31 5-8 Median Values of Iron and Copper Concentrations for Table 5-7 ....p. 5-32 5-9 Requirements for the Reactor Feedwater and Reactor Water at German BWRs Under Continuous Operation .... p. 5-39 5-10 Estimated Percentage of Units Around the World Operating with Either Oxygenated or Deoxygenated Feedwater Treatment .... p. 5-42 5-11 Economizer Inlet-OT Normal Target Values (N) and Action Levels 1, 2 and 3 for Once-Through Units ....p. 5-44 5-12 Chemistry Limits for Fossil Plant Feedwater Treatments.... p. 5-51 Xxxvii

.... . ... ... NEC037405..

List of Tables 5-13 Drum Boiler Water Treatments....p. 5-53

.5-14 Est)mated Percentage of Drum Units Around the World Operating with the Three Main Boiler Water Treatments.... p. 5-54 5-15 Fossil Water Chemistry Specifications....p. 5-55 5-16 Fuly Demineralized Feedwater, Continuous Operation ....p. 5-57 5-17 Boiler Water of Demineralized Feedwater, Continuous Operation .... p. 5-58 5-18 Fully Demineralized Feedwater, Continuous Operation. ...p. 5-59 5-19 Boiler Water of Demineralized Feedwater, Continuous Operation ....p. 5-60 5-20 Steam, Continuous Operation....p, 5-61 6-1 Single Outage Inspection Data ....p. 6-36 6-2 Multiple Outage Inispection Data....p. 6-36 6-3 FAG Resistance of Commonly Used Replacement Alloys.... p. 6-40 6-4 Post-Weld Heat Treatment Requirements for Chrome-Moly....p. 6-42 7-1 Keller's Geometry Factors [7. 1].,p,' 7-3 8-1 FAC Program Self-Assessment Questionnaire ....p. 8-3 B-1 Summary of Damage Mechanisms (from Poulson [B.21) ....p. B-5 D-1 Heat-Mass Transfer Analogy [D.16]....p. D-15 KXxviii NEC037486..-.-

Nomenclature cc Void fraction Charge transfer coefficient for hydrogen evolution 6* Oxide thickness a,.) Mass Lransfer boundary layer thickness 1lHydrodynamic boundary layer thickness 11 Absolute viscosity 0 Oxide porosity 0 Thermal diffusivity A Thermal conductiti&.*

}.t. Dynamic viscosity LtL Liquid dynamic viscosity

1) Kinematic viscosity

'UL Liquid kinematic viscosity p Density PL Liquid density o Specific heat t: Shear stress e Turbulent eddy diffusivity a Constant A Cross sectional area xxxix NEGO37487------

~ouzeaclati're A Component enhancement factor in EDF formulation A Empirical coistant in Equation 3-42 6 Constant = 0 1, 2, or 3 depending on the degree oc hydrolysis of the ferrous ion b Constant B Empirical constant in Equation 3-42 B pH dependent coefficient c Concentration fBlasius fi'iction factor cfr

Blasius frictim factor - rough cf, Blasius friction factor - smooth.

C Weight of material removed by corrosion CR? Corrosion rate Ceq Ferrous ion concentration in equilibrium C'"eq Ferrous ion concentration in equilibrium at standard hydrogen parlial pr'essure CO Ferious ion concentration at the metal/oxide interface CS Ferrous ion concentration at the surface C". Ferrous ion concentration in the bulk Cr% Weight. percent of chromium CuOP Weight percent of copper d Pipe diameter xl

. NEC03748..-

Nomenclature dl. Hydraulic diameter D Diffusion coefficient of iron in water Do Outside diameter DR Magnetite dissolution rate e Roughness element height e4 Dimensionless roughness height E Longitudinal joint efficiency E EMF of the corrosion cell at equilibrium EME of the corrosion cell at standard conditions f ' Praction of the oxidized metal converted to magnetite at the mnetalJoxide interface f UtDa!y friction factor F Faraday's constant FI,2,3,4,5,67,8 Factors in the Chexal-Horowitz model and the Kastner Model

( Gibbs F.'ree Energy h Material composition h Heat transfer coefficiern ia Anodic current y Cathodic current J Mass flux k Mass tr'ansfer coefficient xli

. NEG037499-.-.

Nomenclature ki Loss Coefficient K Reaction rate constant K* Reaction rate constant K1,23,s4 Equilibrium constants Kc Keller geometry factor KD Vapor-liquid distribution coefficient (also known as the vapor-liquid partition coefficient)

Kp Parabolic rate constant Ko ConsUtant in Equation A-35 K., Constant in the Keller equation Kx Constant in the Keller equation L Pipe length Mo% Weight percent molybdenum Ap Pressure drop P Intemal pressure pK Base disassociation constant.

q Heat flux Q Total mass flow rate Q Activation energy for a diffusion process R Universal gas Constant Re Reynolds number Re,, Reynolds nuniber of the'liquid x..i

Nomenclature Re,, Reynolds number of the orifice S Allowable streSIs Sc Schmidt number Sh Sherwood number She Standard hydrogen electrode Shfd Sherwood number calculated by the Berger & Hau correlation for a straight tube Shba 0 Maximum Sherwood number Shl, Sherwood number - rough Shx Local Sherwood number Size Maximum grid spacing t Time talooe Local allowable thickness tinit Initial thickness

't. Maximum thickness tmeas Measured thickness t1nift Minimum thickness t1"OMJ Nominal thickness tfprd Predicted thickness T Temperature 71,),)p Thickness required to withstand internal pressure V Molecular volume lijil

.... ..... . . NEG03749-1 .--.

Nomenclature V Average velocity VI, Liquid velocity W Wall thinning number x Distance x Steam quality Xcrit Critical defect suie y Distance from the wall y Coefficient in Equation 6-2 z Number of transferred electrons xHiv NEC037492 -.----- -

Introduction to Flow-Accelerated Corrosion Corrosion is the degradation of a material by means of chemical reactions with the environment. Any time a metal's energy is raised above the ground state, it is no longer passive, and will tend to return to a state of lower energy. That is to say, the act of reiining ore to create usable metal ensures that the metal is subject to corrosion. There are different types of corrosion that can occur in a variety of situations. Some forms of corrosion are very common; for examrple, the rusting of steel in moist environmenls. This book deals with a less familiar form of corrosion known as flow-accelelrated conrosion (FAC). This form of corrosion has plagued nuclear and fossil power plants for many years. Although FAC can occur in many different metals, it has been of most. concern in the carbon steel portiou of the.high temperature piping and equipment found in power plants. FAC results in thinning of piping, vessels, and-equipment [rom the inside oat, therefore it cannot be detected except by special means.

i-1

........ - --. NEC037493--------

Flow-Accelerated Corrosion Flow-Accelerated Corrosion Flow-acceleraied con osjon1 is a process whereby the normally protectlve oxide layer on carbon or low-alloy steel dissolves into a stream of flowing water or a water-steam mtxlure. The oxide layer becomes thinner arid less protective, and the corrosion rate increases. Evenually a steady state is reachr4 where the corrosion and dissolution 'ates are equal and stable. corrosion rates are maintained. In some areas, the oxide layer may be so thin as to expose ana apparently bare metaI surface.

More commonly however, the corroded surface exhibits a black color typical of magnetite.

"1o the naked eye, the damaged surface has Ctvariable appearance. The appearance is often different tar single-phase and two-phase conditions. In single-phase flow, often under a small degree of magnification a scalloped,wavy or orange-peel appearance is observed. In two-phase conditions, a condition called tiger striping is often observed (see Figure .1-2). Further examples of damage are presented in Appendix C.

Figure 1-2. Sample of Surface Damaged by Flow-Accelerated Corrosion (Courtesy of Altran Corporation)

., 11tht Unite(d States, flow-accelerated coIToS]iof is commonly but incorrectly known as Crosion-Cor,'rsioc For measons unit, will be explaind later, the "crosion-corrosion pro-cess'" is in reality a pure corrosion process that does not have an erosion cor-ponent.

'riTe term "(lomw-assismad c oJl" ling also been usedl to desciibe (his prmIcess.

1-4 NECO37496.-----

Introduction to Flow-Accelerated Corrosion Chapter Damage caused by flow-accelerated corrosion can be characterized as a general reduction of wall thickness rather than a local attack, such as pitting or cracking.

Although FAC occurs over a wide area within a given fitting, it is localized in the sense that it frequently occurs over a limited area of a piping fitting due to local high areas of turbulence. In this context, "localized" may mean within several feet

(- I meter) of the fitting or region of turbulence. However, if one fitting is found to be thinned, then most likely Ihere will be others that have also lost material.

A thinned component will typically fail due to overstress from operating pressure, or abrupt changes in conditions such as water hammer, stant-up loading, etc. Large fittings may rupture suddenly rather than provide warning of their degraded condition by first leaking.

FAC occurs under both single and two-phase flow conditions. Because water is necessary in order to remove the oxide layer, FAC does not occur in lines transporting dry or superheated steam, Two-phase FAC has been recognized as a world-wide problem since about 1970.

Since the mid-1980s, single-phase FAC has been acknowledged as a major problem in the balance-of-plant and secondary piping of U.S. and foreign nuclear and fossil plants.

Historical and Technical Background of FAC Since the 1970s, there have been many studies of the mechanisms of flow-accelerated corrosion. This research was carried out principally in France, Germany, and the United Kingdom. The efforts were a combination of laboratory research and attempts to correlate the laboratory results with plant experience.

Laboratory Research The laboratory work concentrated on developing an understanding of the mechanism of flow- accelerated corrosion. This effort enabled the researchers to describe the corrosion process. In short, the process was found to be a dissolution of the normally protective oxide layer from the metal surface, leading to local thinning of the oxide and a consequent increase in corrosion rates resulting from rapid diffusion through the oxide film. This research identified the fundamental nature of the process and the governing factors such as: fluid temperature, mass transfer (related to the fluid bulk velocity), alloy composition, oxidizing/reducing potential (ORP, related to the dissolved oxygen and reducing agent), fluid pH 1-5

NEC03749I7_.______-

Historical and Technical Background of FAC level, component geometry and upstream influences, and steam quality. The corrosion process can be viewed as the mass transfer limited dissolution of the oxide into ihe flowing stream. As such, the important variables are the solubility and porosity of the oxide, the rate of mass transfer to the stream, and the free stream concentration of soluble iron.

Correlation Development Early attempts were made to reduce the various laboratory results to a form that would be usable by poxwer plant engineers. The appropriate physical and chemical parameters were mathematically fitted to the FAC rate. Such correlations could then be used to predict the rate of flow-accelerated corrosion as a function of plant conditions. The early attempts at correlating the laboratory data and plant.

experience were not completely successful, but'they did suggest ways for better predictions.

Flow-accelerated corrosion is unusual compared to other corrosion processes because of its greater degree of predictability. For most corrosion processes, the rate of wastage cannot be predicted to within an order of magnitude. Early work to correlate flow-accelerated corrosion with system design and operational parameters showed that FAC was reasonably predictable.

During the past several years, successful correlations have been developed by at number of organizations. To be successful, a correlation must predict the rate of corrosion actually occurring in plant systems as wellas in laboratory experiments.

Recently, these con'elations have been incorporated in computer software. Most of this software has been designed for use on personal computers.

Computer Programs To Model FAG As part of the re-spoase to a 1986 FAC-induced failure at the Surry Power Plant, EPRI developed and introduced the CHEC (Chexal-Horowitz Eiosion-Corrosion)Icomputer program (1.2] in 1987. This was the first implementation of a flow-accelerated corrosion predictive algorithm on a personal computer.

Subsequently, EPRI prepared a family of codes with expanded capabilities to calculate the rate of two-phase FAC and to manage the data produced by an, inspection program. This family of codes has now been combined into a single computer code called CHECWORKSIM WChexal-Norowitz Engineering-

-CorrosionWORKStation) [.1.3]. In addition, programs to address both single and L At that time, the term erosion-corrosion was used instead of FAC.

1-6

~.--~---~ ~NEC037490 -

Introduction to Flow-Accelerated Corrosion Chapter two-phase FAAC were, doveloped by ElectricitC dto France (LP.F) (the 3BRIIt-CI(".LRO() CoLdC) and by Sicmens/KWU" in Geimany (the WATHEC"M and DASYIM cod). These codes and their correlationrs are described in later sections.

Accident at the Surry Power Plant Although power plants throughotu the world have been experiencing FAC problems for decades, ti1e rtpture Of aEM elbow in the condensace smslem at the Surry Nuclear Power Phlnt in 1986 initiated the present L.,S. interest in this problem.

The, Surry Nuclear Power Plant, located, in Gravel Neck, Vigin ia, n.issof two Westinghouse 822 MWe pressurized water reactors (PWRs). On December 9, 10Y6, an 18-inch elbow in the condensate system ol" Unit 2 ruIptured during a plant transient (Figure 1-3), Four workers were killed and four othier workers were severely scalded. Even though the plant was safely shut down, the United States.

Nuctae' Regulatory Commission (NRC) became concerned because it was apparent t1hat s*Af ety-re*tated syslems can be damaged by failures in non-safety-related pipes. Post-accide nt inspections of the Uhnit I and Unit 2 piping revealed widespread degradation due 1o FAC, As a result, 190 componetS)t were replaced because of pipe wall thinnirng at the two units [1.41.

Figure 1-3. Surry Unit 2 Condensate Failure (Courtesy of Viwginia Power) 1-7 NEC03-7499.

Historical and Technical Background of FAC The Surry accident highlighted the possible consequences of sudden failures that can be caused by FAC. High-energy line breaks are of concern both from safety and economic standpoints. Clearly, a rupture in a high-energy line is a major safety concern. Less obviously, a rupture can damage or actuate safety systems in the area. Finally, failures can result in expensive repairs and purchase of replacement power to offset lost production.

Global Response to the Surry Accident The unexpected nature and the severity of the Surry accident prompted a quick response from both the U.S. utility industry and the regulatory community. The Surry accident also had implications for fbreign nuclear utilities.

The U.S. Industry Response The initial effort was led by EPRI and by the Virginia Power Company. EPRI sent a letter to all utility chief executive officers of nuclear and fossil power plants providing quick guidance on where and how to look, when to look, and what to do. Virginia Power Company, in parallel, presented detailed briefings on the accident in several locations of the country. The ininiediate concern was that, similar failure potential may exist at other nuclear and fossil power plants. ElRI issued a report titled, "Single-Phase Erosion-Corrosion of Carbon Steel Piping,"

in February 1987 [1.5]: EPRI also initiated the development of CHEC which was released in July 1987.

In response to the Surry accident, the Nuclear Management and Resources Council (NUMARCy-an umbrella organization1 that coordinated the nuclear power industry's activities on major issues--formed a working group in April 1987 to address FAC in the nuclear industry. The working group included personnel friom utilities, EPRI, and the Institute of Nuclear Power Operations (INPO). Their goal was to formulate an industry approach to Ilow-accelerated corrosion that would help to prevent further serious failures. The group concluded that large-scale inspection efforts would neither be practical nor necessarily solve the problem. They focused on single-phase FAC because it was felt at the time that degradation of two-phase lines was already adequately addressed by existing utility inspection programs.

The working group recommended a unified industry approach to the issue. This approach included susceptibilly analysis combined with a limited number of inspections of components most likely to be affected by FAC. Briefly, the 1.In 1993, NUMARC and several other industry orgaribzations were combined into the Nuclear Energy Institute (NEI).

i-8 NEG037500-_-O -

Factors Influencing FAC For the "completely rough turbulent flow" generally found in power plant piping, the roughness height is greater than the laminar boundary layer and is of the same magnitude or greater than the diffusion boundary layer. This indicates that for power plant piping, the Sherwood number will be nearly proportional to the velocity. Thus, FAC will be nearly proportional to velocity.

Effect of Flow Geometry Both the size and the shape of a component directly influence the velocity and the local mass transfer rate, As one would expect, components with geometries that promote increased velocity and turbulence tend to experience more severe FAC.

FAC is less frequently observed in straight lengths of pipes free from hydrodynamic disturbances. Still, FAC can and does occur in straight piping,.

especially when the bulk fluid velocity is high. FAC is more often encountered at points of hydrodynamic disturbance, mainly inside and just downstream of fittings in steam and water systems. These include elbows, pipe bends, reducers, tees, at pipe entries, downstream of flow control orifices, valves, etc, A more detailed discussion of damage locations in both single and two-phase flows can be found in the literature [3.9, 3. 111.

A geometric enhancement factor is generally used to represent the effec of increasing turbulence on FAC. The first recognized geometry factors were those from Keller [3.1911. These factors were determined from the experience of FAC in turbines (high wet-steam velocity impingements). When compared to plant data they were nyt found to be representative of single-phase FAC [3.20].

EPRI has performed extensive studies to correlate different piping geometries with FAC. The resulting, empirically developed, geometry factors [3.21] provide more accurate predictions than were previously available. Some geometiy factors are more detailed in that they consider the effect of the upstream configuration on the rate of FAC in the downstream piping (3.22]. In addition, these factors account for FAC, upstream of certain components (e.g. expanders).

Table 3-I presents a review of geomnetric factors found in the literature and used to predict FAC [3.19, 3,22, 3.23, 3.24]. The Chexal-Horowitz geometiy factors in Table 3-1 are refinements over thoe previously published earlier [3.22].

3-10

.. .NE Q 0317536 .......... .... . ... ... .... . . .. . . . . . .....

Introduction to Flow-Accelerated Corrosion Chapter Table 3.1. Published Geometric Enhancement Factor Values for Piping Components with Single Phase Flow as Used in Various FAC Models Geometric Factors for FAC F Chexal-Fitting Keller Horowitz Remy Woolsey Kastner

[3.19] rin3.22] * [3.23] [3.24] [3.25]

Srefinements Straight to 1.0 1.0 1o0 1'0 Pipei 900 Elbow 5.75 to 13 3.7 2.1 1.7 6.0 to 11 Reducer (large end) 2.5 3.2 (small end) 1.8 Pipe Entry 4.0 25 3.5 to 6.24 Expander (large end) 3.0 3.6 4 (small end) 2.8 Pipe Expansion - 2.02 Orifice 4.0 to 6.0 5,0 2.9 3.0 to 4,o2 Tee: Flow (run) 3.74 1 5.0 5.7 2.0 to Combination (branch) 5.0 2.52 Tee: Flow (run) 18,75 5.0 5.7 Separation (branch)j 4.0 1.All the geometry factors are based on comparison with straight pipe.

2. The reference flow is based on the downstream pipe.

Effect oa Steam Quality When the stream flowing past a metal surface is steam and water (two-phase flow),

the system pressure (or temperature) and the amount of steam as a mass fraction (the quality) are important. These variables help determine the distribution of voids within the flow at a given cross-section. The ratio of area occupied by vapor to total pipe area at a given cross-section is called the void fraction. The mass 3-Il

.. ..... . . . .. . .. ....... . .. .. . _... N EC0 3.7 5 3 L_.. . . . * . . .. . . . ... . . . ... . ..

Introduction to Flow-Accelerated Corrosion Chapter In practice, oxygen-dosed neutral water chemistry and oxygenated treatment for fossil plants use additions of at least 30 ppb of oxygen to the feedwater. These types of water chemistries are used mainly by fossil-fueled plants since oxygeni is not tolerated in the boilers of nuclear units [3.291, (See Chapter 5.)

Metallic impurities. Metaflic impurities have a very ininor impact on the FAC rate. However, copper, nickel, molybdenum, and lead ions in solution can affect the rate of FAC, even at a feedwater concentration as low as I ppb 13.34, 3,35).

The influence of copper appears to be the result of the electrodeposition of Cu 2 +.

This is caused by the negative surface potential formed on surfaces experiencing FAC [3,351. When this occurs, metallic copper precipitates into the pores of the oxide. As a consequence, oxide porosity and FAC are reduced. The presence"of copper ions can be the result of corrosion of copper alloys used in condenser tubes, low pressure heater tubes, etc.

Copper's effects can be important where a high concentration of ammonia is used for the all-volatile treatment (AVT) of the feedwater. Figures 3-6 and 3-7 illustrate data obtained from the CIROCO Loop at EDF using thin layer activation (TLA>

corrosion monitoring t . Figure 3-6 shows the inhibition of corrosion when copper ions are present in the solution. They result from the ammoniated corrosion of copper alloys located in the low pressure part of the feedwater ciicuit (pH 9.6) and the transport of these ions into the test section located in the high pressure part of the loop. Figure 3-7 shows the increased FAC when the copper alloy tubes are eremoved and the full feedwater flow passes around a copper lube section using a bypass.

It appears that metallic copper is deposited on the magnetite layer during the high pH operation. When the pH is reduced, the corrosion rate remains low for a period of time apparently due to blockage of the magnetite surface. This result lasted for approximately forty hours until the corrosion rate returned approximately to the original value.

1.The thickrneýss of the activawd spot isdirectly proportional to the comWt rate Thus, the corrosion rate, Vc, is elual to the slope of a count rate versus time plot.

3-17 NEC037543......

Factors Influencing FAC pH25 0c: 9.0 pH2.-c: 9.6 pH25'C; g.0 3550 (NH4OH) (NH 4OH) (NH 40H)

I FAC Inhibition I 3500

ý.i.V¢ 0.02 mnrl0 t000v h* vc =0.45 ram/100 000 h (40 hi delay) 340 Vet 8.BImrnVDO 000 h 34100

. .. . .oxygen:

4.. -~~ .. . . I-. .5 . . g/kg

.4 P l P- PPP M PPJ 560 600 650 700 rime (h)

Figure 3-6. Influence of Copper Ions In Solution on the Rate of FAC (From Bouchacourt[3,36,)

3-IS NECQO375A44A --..

Introduction to Flow-Accelerated Corrosion Chapter 1920 pH: 9.6 Ammonia Oxygen = - 1.tg/kg

-;1910 ( -

VC=t3I.:.

AVc=1.3 mm/lOOO000h i Copper Free Water K "

7 F Inhibition Without Copper With Copper 1880 Bypass Bypass 300 320 340 360 380 400. 420 440 Time (h)

Figure 3-7. Influence of Copper Ions in Solution on the Rate of PAC (From Bouchacourt [3. 19])

An electrowdeposition process could explain the presence of other elements such as nickel and molybdenum in the surface oxide layer. It is difficult to rule out the possibility that observed concentrations in the oxide layer are derived solely from the steel [3.351. Stainless steel is an identifiable source of both nickel and molybdenum in power plant systems.

Deposition of elements such as copper and nickel wight eventually lead to complete inhibition of the FAC phenomenon. This depends on the FAC rate and both the concentration and the deposition rates of the metallic ions.

Other impurities. No detailed systematic studies of other water impurities on fAC have been published, probably because most of the laboratory work has been conducted on feedwater with low conductivity. Nevertheless, the presence of low levels of acid forming anions such as CE-or SO42, has been shown to have no influence on FAC 13.18). At higher concentrations, if these acidic species are concentrated by'an evaporation prrocess, they would be expected to decrease the 3-19

-- NE-Q0375A45 -. ~ -- 1,-- --.-.

Factors Influencing FAC pH at the both oxide/water and metal/oxide interface and to promote steel oxidation or the magnetile dissolution process. Two particular circumstances in which one may see increased FAC are:

The presence of CO 2 caused by air inleakage through condensers or by a thermal decomposition of carbonates or other organic impurities [3.18, 3,331.

The presence of organic acids such as acetic acid and formic acid generated by decomposition o17 organic impurities, chemical reagents such as morpho line, ion exchange resins and water treatment additives (3.33].

According to Woolsey [3.241, it is not known whether there are any specific chemical reactions between anion and iron species that would alter the FAC behavior.

Hydrazine Hydraiine is a reducing agent added to the feedwater/condensate system in a power plant. It is a scavewging agent (removes oxygen), and also maintains a reducing environment in the steam generators (nuclear plants) and in the feed train, Hydrazine is unique in the chemical species in that it is reactive and unstable. It reacts with oxygen forming water and nitrogen. Most of the bydrazine which does not react with oxygen thermally decomposes to form ammonia.

Recent inforination indicates that in the 0-150 ppb range of hydrazine level, the IAC rate increases with increasing hydrazine level as the oxidizing reducing potential (ORP) becomes more reducing. The decrease in potential in this range leads to greater dissolution of the surface magnetite (Fe30 4) and thus to an increase in the rate of FAC, Above the 150 ppb hydrazine level, the potential is lowered significantly enough that it leads to slower kinetics. Thus, any further increase in the bydrazine level leads to a decrease in the FAC rate. Therefore, a plot of FAC rate versus hydrazine level is a bell-shaped curve with a peak at 150 ppb, Theoretical considerations show that the FAC rate should be proportional to the concentration of hydrazine to the 1/6 power. Recent plant and laboratory information shows that this does not continue above 150 ppb of hydrazinc.

Hydrazine is commonly added to the feedwater of the PWR secondary circuit to keep feedwater oxygen leveIs lower than 5 ppb. Hydrazine is used to maintain a reducing environment in the feed train and the steam generator as a scavenger of 3-20 NEC037546- .

Introduction to Flow-Accelerated Corrosion Chapter Unmonitored FAC damage can be just as dangerous in cogeneration, process plants, and fossil plants as it is in nuclear plaints.

Differences Between Nuclear and Fossil Plants

'rhere are significant design and operating differences between ouclear and fossil plants that affect FAC. These are discussed below.

Plant Operating Conditions Nuclear plants operate with the throttle steamrn-steam entering the high pressure turbine--saturated, or in the case of plants wilh once-throagh steam generators, slightly super'heated. In contrast, the throttle steam in modern fossil units is highly superheated and may also be sopercritical (above the critical pressure of water).

To understand what this means, it is necessary to compare the power cycles typically ased. In both nuclear and fossil stations, condensate leaves the condenser, passes through a number of feedwater heaters, and entelN. the heal.

source-boiler, steam generator, or nuclear core. The heat source adds energy to the feedwater, creating steam. The steam leaving the heat source eoslers the high pressure turbine, Some of the steam flowing through this turbine is removed and used to heat the feedwater. This is known as "extraction steam." The bulk of the steam exils the high pressure turbine. 'The condition of the steam at the exit of the high pressure turbine and the downstream equipment are the most important differences between nuclear and fossil steam cycles.

The steam exiting the high pressure turbine in nuclear plants contains significant moisture, This moisture lust be removed from the steam before it can enter the lower pressure turbines. Nuclear plants have a moisture separator to remove this moisture and sometimes have a steam heated reheater to superheat the steam. This steam then passes through the low pressure turbine and on to the condenser.

4-11

- NEC03-7639-.......

Differences Between Nuclear and Fossil Plants In contrast, the stean leaving the high pressure turbine in a fossil plant is still superheated or only slightly wet. Moisture separators, steam reheaters, and their associated drainage systems are not needed between .the high pressure turbine and the lower pressure turbines&. Most extraction piping in fossil plants carries superheated steam and is thus not susceptible to FAC. Therefore, a fossil unit often does not have the wet steam conditions that contribute to damage in certain nuclear power plant piping systems. However, during start-up and at reduced power levels, steam qualities are lower and some steam systems may be susceptible to FAC. There also may be steam lines directly from the steam drums that have saturated conditions and possibly have some moistut-e.

The condensate, feedwater, and heater drain systems of fossil and process plants are susceptible to FAC damage. In fact, in fossil plants the areas nonnally damaged by :AC are between the condensate pump discharge and the boiler entrance and- in the heater drains (see the section entitled "Damage to Other In-Line Components" later in this chapter for further information on the type of damage encountered at the entrance to the boiler). Additionally, fossil plants that are used in the peaking mode can also experience PAC-caused damage in the two-phase systems.

Fossil plants can operate as either "base-load" units or ".peaking" units. Base-load units operate continuously at high power levels. In this service, operational transients are limited to start-up, shutdown, and equipment malfunctions. Peaking units are started and stopped, or operate at reduced power levels, as needed to meet periods of heavy load demand. Peaking units experience more frequent operational transients than base-load plants.

Peak-load plants are called on to closely follow system load demands and maintain condenser vacounu while in siandby mode. By maintaining vacuum, these plants can be on-line in a shorter time than if vacuum had to be re-established. To re-establish a vacuum, the boiler must continuously generate a small amount of steam to operate the air ejectors. While operating in the standby mode, most of the steam-filled portions of a plant operate under conditions that are 1.There is some confusion in nomenctature between nuclear units and fossil units relative to the term "reheaters." In fossil units reheating refers to the practice of routing stc*am from the high pressure turbine back through the reheating section of the boiler where thle temperature of the steam is raised. The steam then is returned to an intermediate pressure turbine.

in nuclear units the reheater is a heat exchanger which heals tIhe steam exiting the in1is5 tre separator with diveited main or extraction steam. In the nuclear case, the reheater drains have been an area of FAC problems. No analogous area exists in a fossil unit.

4-12

.. . -. . NECO37640 .

Introduction to Flow-Accelerated Corrosion Chapter far from normal. There can be high-velocity, two-phase flow in portions of the system thai. carry superheated steam when at normal power. Peaking units can also operate for substantial periods at reduced power. This also can lead to two-phase flow in lines that carry superheated steam at full power. These modes of operation complicate FAC susceplibilil.y analyses and inspection planriing.

Water Chemistry Modern fossil plants normally use one of several different water treatments f4.41.

Drum boilers typically use all-volatile treatment (AVT), oxygenated treatment (OT), phosphate treatment (PT), congruent phosphate treatment (CPT), or equilibrium phosphate treatment (EPT). Once-through boilers usually use either AVT or 0T (see Chapter 5 for further information). Ammonia is the feedwater treatment of choice because it does not break down under the high temperatures experienced in boilers. More complicated-amines, such as morpholine, would thermally decompose at boiler temperatures.

As at nuclear plants, hydrazine is often used to produce a reducing feedwater environment, particularly for mixed metalurgy systems, and to remove oxygen from the feedwater. Any hydrazine that enlers the boiler is thermally decomposed to form ammonia, hydrogen and nitrogen.

Damage to Other In-Line Components FAC damage is not restricted to piping and piping components. Any component in the stream is subject to the same corrosion mechanism. The same cause produces die same effects. In general, the most vulnerable areas are where one or more of the following conditions exist:

0 The flow has a high velocity.

There is impingement on a surface, T

There is a large pressure difference that induces internal flows.

There is a flow with high quality (low moisture) that tends to have a lower concentration of pH control ammonia or amine and consequent decrease in pH.

4-13 NEC03764.1 .

Cr

Damage to Other In-Line Components In evaluating corrosion within equipment, it is necessary to consider the con sequences of the damage as well as to know the limit of allowable damage. The engineer should be aware that:

If there is thinning of internal elements, both the direct and indirect consequences of ibis thinning should be considered.

If the wear occurs on a pressure boundary, the engineer must be able to determine if there is sufficient remaining material to withstand the applied load, at least until the component can be repaired or replaced.

To help illustrate the information presented above, some typical experiences wilh FAC of in-line components will now be desciibed, The items below apply to both nuclear and fossil plants. Note that. a detailed presentation. of plant-specific FAC events is provided later in this chapter.

Steam Turbines With regard to damage in steam turbines, two phenomena, flow accelerated corrosion and droplet impingement erosion are particularly noteworthy. This section will briefly discuss wet steam turbines in nuclear power plants and conventional steam turbines in fossil fueled power plants.

The well known bladings damage of the f nal stages in conventional steam turbines turned out to be comparatively limited in saturated steam turbines in nuclear power plants (4.5]. Experience shows that enlarging the linear blade sizes and increasing the peripheral velocities essentially has positive effects for wet steam turbines. This is connected with the more favorable steam flow of the final stages with the selection of higher condenser pressures dictated by environmental considerations and with the partial load characteristic of the water-separator and superheater.

On the other hand, flow accelerated corrosion has caused considerable wear in unalloyed or low-alloyed steels in wet steam flow exposed parts such as housings, blade series, and shaft seals in the first NPP turbines' early design [4.6].

As a result of steam expansion in the turbine, water is separated out in the wet steam region, which causes the steam flow rate to decrease steadily. Figure 4-5 shows the expansion curve for a saturated steam turbine with external monisture separation and steam reheat contrasted with that of a notL.reheat turbine; the endangered areas in which flow-accelerated corrosion tend to occur are depicted.

As can be seen, the expansion of reheated steam takes place in a region less 4-14 NEC0376.42

E*priences with Flow-Accelerated Corrosion Plan ix Fort Calhoun Station On April 21, 1997, with the Foit Calhoun Station (owned by We, OmIaiha Public Power District) retctor operating a a noitmiTnai 100% power, the third elbow downstream froom the turbine in the extraction steam line ruptured (Figure 4--31).

Fort Calhoun Station is 478 MWe PWR. The elbow was cons1ructed of I 2-in1ch bent pipe with a radius to diameter ratio of 5. The fish.-nouth' rupttjre Was approximately 54 inches long and 18 inches wide. This el]bow was localed behind several non-safety related Motor Control Centetrs (MCCs). The event was further complicated by the activation of the sprinkler system within the turbine bitlding.

Art additional personnel safety issue restItted [rom damnage to Some asbestos insulation., resulting in the need to restrict access to the turbine building due co con tatmiinraoUn.

Figure 4-31. Failed Sweep Elbowlror the EXtraction Steam.System of Fort Calhoun Station ""

(Courtesy of Omaha Pubtic Power Distric, of he pipe rupture iridi cates that it resulted from exces sive pilpe walt Iusessneil.

thinning caused by Flow Accelerated Corrosion (FC'j. "Th pipe thinming occurred over a relatively long period of time, antd sigrfiicant hinning should bare been detected well before, th* event occurred. Th13e site had not been inspected, 4-54

. .. NEC037 62---------------------- ..........-

Introduction to Flow-Accelerated Corrosion Chapter Fort Calhoui Station has an FAC control program, however, there was oot a detailed, step by tep metlodology for tile process of selecting inspection sites.

Such a inethodology would define the s uscept ibil ily evaluation pro*e~ss and identify sihuations that would require expansion of the selected inspection kiciitionls.

The rupiure klcation was in a system that had been categoized as "suseTptibleP to FAC and had been incorporated into the EPRI CHECWORKS analytical model, but had not been selected for inspection under the FAC control program. Other sites in this line were inspecewd, including several shovter radius elbows, a tee, and a ,*educer, These coxponeils had showh expected rates of Wear.

The. section of piping immediately upstream of the rupture site was replaced in 1985 to a*ddress FAC( wear of the piping. This indicates Ibat the rupMtur, site would have been an appropriate candidate for inspection.

FAC Experience in Canada SInaddit ion to problems in BOP sys*tems, CANDU plants hav-e experielnced FAC in (he primary system. CAND.)U plants are of a pressure tube design with heavy wawr as a coIIant and moderator Other than the pressure tubes, and the steam generat*or tubes, the system is constructed out of carbon steel. At the outlet of each pressure tube, there are several fittings and pipes leading up to the outlet header.

These lilting have experieuced FAC in seveý-al CANDU plants. This experience is ULnusual for severil reasons, namely: the einperature is very high 590F.(310C),

the velocities arc very high (33-.59 r'eet per second (1 0- f8 m/s)), and the 0luid is heavy water.

To xNsuro against excessive future thinning, the plant operatovrs are conducting increased iuspections and investigating water chemistry r'emedies [-4.22].

FAC Experience in Europe In addition to the experience cyf the U..S. power plants, there have been significant problmIi with FA(G elsewhere, throuaglhout the world.

4-55

....... N E C037683 ...................

Plant Expernences witL Fltow-Acclenited CorrosiOrl

.-. '.;':ii::::

"':::i:::::.:

Figure 4-41. Rupture of Extraction Line from a Russian Designed VVER (Courtas'yof Nuclear Research Instilute)

Figure 4-42. Characteristic FAC Wear Pattemrs of the Component Shown in Figure 441 (Courtesy of Nuclear Research Inslituto) 4--)0 NEQ03769.8 .~

Introduction to Flow-Accelerated Corrosion Chapter Table 7-1. Keller's Geometry Factors [7.13 FLOW PATTERN REFERENCE VELOCITY Kc At Pipes 1.00 Primary low I *At Blades !0 Primary Flow EVelocity of Initial Flow Stagnation At Plates (Upstream of Stagnation 1.00 Points Obstacle)

.. . 0.75 S -'In Pipe Junctions 0.60 RID 0,35 0.52 VR/D= 15 In 0.30 Secondary ......... Elbows Flow R/D= 2,5 Flow Velocity 0.23 Stagnation Points Behind Pipe Joints 0.15 StagnaCt Behind sharp edged 0.16 Points Due to entrances Vortex Forma- I Flow Velocity 0.16 tion IT"

At and behind t ibarriers N tnaIn straight pipes Flow Velocity 0,04 No Stagna-tion Points In loose horizontal Velocity Calculated from 0.08 lurbine seals Pressure Drop Ln turbine gland I. Velocity calculaled from 0.08 seals pressure drop Complicated Flow Through At andabove tur- 0,30 Turbines bine blades and at Average circumferential drainage collecting blade 'velocity

__rings 7-3 NEC0_3 7825 - - -,--

Models for Computer Applications III0,000,000 I . .. - . .... . . . .

Test Loop Data Plant Data 1,00,000

,_ Iio~oooW ¥ i " ,'" I, .. "* ...

100 + . .. . . ..

i09 1000 ioooo ioouoo oooo00 1,o000,000 Sherwood Number, Sb Figure 7-1. Plot of Wall Thinning Number, W, versus the Sherwood Number, Shr (From Bouchacourt[7.31)

Tha CIROCO loop test results were obtained on straight tubes (OD -0.31 inches (8 mm)) with Reynolds numbers between 3x 10' and 2x 10J,Ammonia or morpholine water treatments (cold pH between 8.8 and 9.6), and a temperature range of 300' to 480 0F (-1.50 0 C to 250 0 C) were used. The plant. measurements were niade on straight, corroded pipes- 'rhe thinning rates were large enough to have caused obvious wear in the straight portion of die pipes. The temperature was between 355' and 4350 F (-1 800 C and 225TC), and the cold pH was 8.8.

In addition to validating the assumption of equal Sherwood number and wall thinning rate number, this figure shows two interesting resu]ls-The thinning is directly proportional to time, as evidenced by comparing results of laboratory lests of 200 hour0.00231 days 0.0556 hours 3.306878e-4 weeks 7.61e-5 months duration with plant results after 60,X)0 hours0 days 0 hours 0 weeks 0 months of operation.

As a direct consequence of the above conclusion, there is no incubation period present in the wear mechanism, i.e. the wear rate is always independent o1 time.

7-6 NEC0.37828.

Introduction to Flow-Accelerated Corrosion Chapter When several mass transfer correlations were examined, the Reynolds number exponent was between 0.6 and 0.8. This difference corresponds to an uncertainty of +/-20% in predicting mass transfer which conatibutes to the overall wall thinning rate uncertainty of +/-50%.

The discussion above refers only to straight pipes. But FAC damage appears first in pipe components. In components, there are no general correlations to deduce the mass transfer from hydraulic conditions. To obtain data in components, the same correlation used for a straight tube is used with an enhancement factor, A.

The enhancement factor depends on the type of component.. More generally, the relationship belween the Sherwvood number and the wall thickness rate number can be written as:

W = A - 51r (eq. 7-5) where:

A is the geometry factor.

The value of the enhancement factor was obtained from thickness measurements recorded at plants operated by EDF and other utilities. The chromium content for each data point was estimated from samplings of 200 fittings at EIF plants, and was approxi mated by the operators at non-EDF plants. Generally, no chromium data exists for the inspected components.

It is clear that the behavior of each component is somewhat difterent. Parrtof this discrepancy is caused by uncertainties in the experience feedback analysis such as unknown initial thicknesses and chromium concentrations, uncertainties in wall thickness measurements, actual water chemistry experienced (mainly in the oldest data), and the lime of full power operation. To take into account the range of possible predictions, three parameters are defined: the mean, the range, and the maximum. The mean value is considered the most probable and is used in BRT-CICERO to predict the wear rate of other components or for the same component under different conditions. The range is given by the standard deviation of the dispersion. The range is used for the analysis of inspections to conclude if the rmeasured value agrees with the previous value, The maximum value of Agives the most conservative result for evaluating the mechanical behavior of the compolents.

7-7

-b EC037829 * . -~

Models for Computer Applications EDF maintains a database of measured enhancement factors. Included in this database are data for straight pipes, elbows, tees, pipe downstream of regulating valves, orifices, and diffusing sections. The maximum value of A is less than ten and generally is between three and seven.

In addition to the empirical methods of determining A, theoretical methods can be used to estimate A by taking into account the specific geometry of the component.

For elbows, the value of A depends on the turning angle and the curvature of the elbow. The wear rate in a tee depends on the velocity ratio of the main to the branch and on the angle of the lateral pipe to the main run. The geometry factor also considers the effect of the upstream component on the downstream component as presented in Figure 7-2.

It0s \

0.4 - 1

At A+ B x A(Upstream EIement) o0.3 .

U .4, m0.2 0.1 0l .. I i iL . .i .

0 2 4 6 8 .10 12 14 Number of Diameters Beween TWo Fittings, Figure 7-2. Effect of the Upstream Element on the Geometric Factor ofthe Downstream Element (From Bouchacourt(7,31)

BRT-CICERO software was designed and developed by the Engineering and Construction Department of the Power Engineering Division of EDF. It is a complete methodology to deal with flow-accelerated corrosion. It is designed to:

0 Be comprehensive

Be conservative

Optimize the design margin available

Be both an inspection optimization and a design tool 7-8 NEC037830..

Introduction to Flow-Accelerated Corrosion Chapter BRT-CICERO software is composed of two parts. The first part, "Plant Piping Inventory," characterizes the plant systemrs and their available margins. The second part perfo rms analyses and is designed for use by plant operators. It also uses the data logger files that are produced when performing non-destructive evaluation (NDE) thickness measurements-Plant Piping Inventory The system characterization is performed by engineering staff on a mainframe computer using computer aided drafting (CAD) and stress analysis software. The EDF procedure consists of the following three steps:

J. All systems in the plant are classified as being susceptible, or not susceptible to FAC. By considering all of the systems in the plant and eliminating only the ones that are not susceptible, the engineer minimizes the chances of omitting a susceptible line. Systems are excluded from the evaluation for reasons of material, lack of operating time, or for being a small bore line (2 inches (50 mm) or less). The resulting, laige-bore susceptible lines are modeled from the available isometric drawings on a CAD system. All available mechanical data such as design conditions, steel grade, code of construction, etc., are included in the database.

2. The next step is to calculate the available structural margin for each component. The first part of this process is to determine whether the pressure stress (the hoop stress) is the governing load. If not, the minimum allowable thickness will be greater than the hoop staress allowable thickness. .The design thick-ness is then compared with an estimated initial thickness obtained by considering the design documentation, the nominal thickness and manufacturing process used. The initial design margin is the difference between the estimated initial thickness and the design thickness. If the initial design margin is too small, additional analyses can be used to compare the local stress level in the thinned area. If the area where the stress level is high does not coincide with the worn area, the thickness loss does not affect the design stress level. For small values of thinning (less than 10% of the nominal thickness), generic calculations are performed for standard components such as elbows, reducers, etc., in order to take advantage of this additional margin from the beginning of operation. Details of this process are explained subsequenty. The result of the second step is the identification of the margin available for fEAC damage for each component.

3. The third step is to perform the first FAC calculation for each plant using plant specific thermal-hydraulic data and the normal water tieatment. The goal of this calculation is-to identify groups of components which have 7-9

_ N.EC037031_ _----

Introduction to Flow-Accelerated Corrosion Chapter General Plant Diagnosis During the Tough analysis, a plant system list is established that contains information regarding operating conditions of all safety related plant systems. The systems in the list are evaluated by WATHEC to indicate whether they can be disregarded from further analysis. If system operating conditions indicate that a system may be subject to FAC, a detailed analysis is initiated, again using WATHEC.

Detailed Plant Diagnosis The detailed analysis is thus limited to those lines that are susceptible to FAC and may therefore jeopardize safety. The detailed analysis requires input data on system operating conditions, system design criteria, and the geometric arrangement of piping elements. The program computes the minimum life expectancy for piping components by considering local stress conditions. Based on these values, inspection deadlines are determined. If an evaluation indicates a high risk of piping failure, an NDE examination is scheduled for the next outage.

NDE Wall Thickness Measurement The PC program DASY handles the storage, administration, evaluation and documentation of wall thickness measurements on individual piping elements.

Since the progrnus WATHEC & DASY have compatible data formats, NDE results are made available to WATHEC and can be used to "calibrate" the predicted susceptibility for all components considered. Additionally, this data allows the elimination of inaccuracies included with input parameters, e.g. true matedal composition (content of Cr, Cu, Mo of piping elements) or no exact original thickness measuremenis of components.

Actions to Prevent Flow-Accelerated Corrosion Damage If significant degradation by flow-accelerated corrosion is detected, an assessment is perlbrmed to avoid a safety risk. Assessments may call for periodical checks on piping component wall thickness (monitoring) using WATHEC inspection deadline recommendations, Other options are component repair or replacement or changes in operating conditions. The effectiveness of options proposed can be chocked with WxFrHEC before implementation.

The EPRI CHEC Programs The CHEC computer program [7.181 was the first of the CHEC series to use the Chexal-Horowitz flow-accelerated corrosion model developed by EPRI in 1987 in response to the Surry accident. The model is empirical and a "best fit" of all data 7-19 NECO7OA41. ..

Models for Computer Applications available [7.19, 7.201. The model was modified in stages and incorporated in CHECMATE [7.211 and CHECWORKS [7.22J (Chexal-Horowitz Engineering coirosion XYQr~~tation), the most current in the CHEC series of computer programs. The latest version of the Chexal-Horowitz flow-accelerated corrosion model is provided in CHECWORKS.

Chexal-Harowitz FAC Model Provided below is a description of the model used to predict the rate of single-phase and two-phase flow accelerated corrosion which depends on a large number of interrelated factors. These factors can be divided into three groups: (1) water chemistry variables-pH, dissolved oxygen, hydrazine concentration, and the p1-control amine; (2) hydrodynamic variables&---fluid velocity, pipe diameter, temperature, steam quality, and the geometry of'the Jlow path; and (3) material variables---percentage of chromium, molybdenun and copper in the steel. The model was developed by correlating:

All pertinent British, French and German laboratory data

Assembled U.S. plant data

EPRk-sponsored tests in support of model development.

The geneial formulation of the Chexal-l.orowitz model is as follows:

CR = F,(7) -F2(AC) F3(MT) -F 4 (02 ) .FpSqIH) FC(G)* FT(()

'FS,(H) (eq. 7-7) where:

CR is the FAC rate, F'(1) is the factor for temperature effect, F2(AC) is a'e factor for alloy content effect, F3(MI) is the factor for mass transfer effect, F4(02) is the factor for oxygen effect, Fs(ptt) is the factor for pH effect at temperature, F6r(G) is the factor for geometry effect, F/(a) is the factor for void fraction, and Fr(gw is the factor for hydrazine concentration.

7-20 NEG037.842 . .. .... .. _ . .. . . . . . .

Introduction to Flow-Accelerated Corrosion Chapter Since the interrelationship between the parameters F, through F7 was not initially apparent, the formulation was developed empirically. In doing so, the following principles were upheld:

All of the above parameters were incorporated into the model.

All of the collected data were used in the model development.

The model did not presuppose a form for the con-elation.

o Although the model is empirical, steps were taken to ensure that each pat of the model made mechanistic "sense" using EPRI in-house corrosion experience.

Using these principles, an iterative procedure was used until an optimum model was obtained. This model included all of the experimental trends, and con-elated well with the bulk of the laboratory data.

The model wa further refined by comparing its predictions with actual wear data obtained from nuclear power plants and with additional laboratory data. The use of these additional data (particularly tW take into account various geometrical mass transfer enhancement factors) further improved the model. t is worth noting that the FAC rate goes to zero if any of these factors becomes zero. This is the situation when stainless steel is used, where F2 (AQ approaches zero for high amounts of chromium in the alloy. Each of these factors is discussed below.

Temperature Factor.Fluid temperature influences several variables. The variation of FAC rate with temperature is a bell shaped curve with the maximum around 300'F (150'). The FAC rate is controlled by oxide dissolution kinetics at low temperatures and by mass transfer limitations at high lemperatures. The reason for this behavior is believed to be due to the competing behavior of three separate mechanisms in the temperatu re range of interest (about 200-500"F (-I 00-250'C)):

1. The solubility of the oxide layer decreases with increasing temperature above 300°7 (1500 C) and the flow-accelerated corrosion phenomena is mass transfer controlled.

2. The kinetics of the dissolution rate increases with increasing temperature below 300'F (I50'C) andi the flow-accelerated corrosion phenonmena is partially kinetics controlled.

3. The hot pH of an aqueous solution of a pH control agent decreases with temperature in the temperature range of interest.

7-21

.. NE Q3E3743 .

Models for Computer Applications These three competing effects may explain the shape of the temperature dependency curve.

Alloy Factor.The alloy factor used was a modified form of the Ducreux [7-23]

correlation. This correlation relates the flow accelerated cornosion rate with the presence of three alloy elements: chromium, copper, and molybdenum. The substantial decrease in the rate of FAC with even small amounts of chromium is due to the increase of stability of the oxide layer, Chromium tends to reduce drastically the solubility of iron oxides in pure water and thus its presence greatly reduces the FAC rate. The dependence of the predicted FAC rate on the amount of chromium and molybdenum are presented in Figures 7-6 and 7-7.

Geometry Factor.The EPRI geometry factors are more'detailed in that they consider the effect of the upstream component on FAC in the downstream piping.

In addition, these factors account for FAC upstream of certain components (e.g, expanders). They have been refined over time with additional data and are used in the CHECWORKS code.

QheaNlub Flow M**nsfd Conodoan Modt

.1 -iS - Cr =0.03%

0.. - -- -- Cr=0.10%

.... Cr = 0.20%

,. -*-* lCr=0,50%

rise Mo = CU = 0.03%

UA - V = 20 ft/see 040 0,0 Oxygen to I p OPpH:=7 atTr*F 0.40 9W GNW 1001 110 MO M 30 M5 400 450 Figure 7-6. Chexal-Horowitz FAC Model, Impact of Chromium 7-22 NECO3784t..... ... ..

-.. .-.-... .44. .-

Introduction to Ftow-Accelerated Corrosion Chapter Chexal-Harowiz Flow Accelerated Corrosion Model

,0.140 -

0.140 - - MoG= 03%

- - - Ma = 0.10%

3 0,120

... Mo Ma M!--

2OJ0%

0,--%%

So0.0o SO.IW Cr = Cu . 003%

D= 4' M0,020 C

V 20 N/ac

.100 Oxypn - I ppb Ao pH 7 at rr 202 elbow ISO 200 250 300 350 400 450 Temperature (*F)

Figure 7-7. ChexalI-Horowitz FAC Model, Impact of Molybdenum Mass Thansfer Factor The mass transfer coefficient is one of the key factors that affects both single-phase and two-phase flotw-aecelerated corrosion rate. The.

value of the mass transfer coefficient, k, varies with the local hydrodynamic conditions. Its dependence is expressed in dinmensionless form using the corresponding Sherwood number.

F3(MT) k where:

k is Sh-DldH, dr+ is the hydraulic diameter, and D is the diffusion coefficient for iron in solution, and the Sherwood number is determined by:

,Sh = a. Reb . Scc (eq. 7-8)

Where:

Re is the Reynolds number (Re = VdH/v),

Sc is Ihe Schmidt number (Sc = v/D),

7-23 NE0037845-........

Models for Computer Applications V is the liquid velocity, v is the kinematic viscosity, and a, b, c: are experimentally determined constants.

For two-phase flow the Reynolds number, Re, is based on the velocity in the liquid layer given by,

( 7-9) where:

Q is the total mass flow rate, A is the pipe flow area.

X is the steam quality, PL is abe liquid density, and a is the steam void fraction.

The dependence of the predicted FAC rate on the liquid velocity and the pipe diametr are presented in Figures 7-8 and 7-9.

I 0.180 0.180 0.140 nall-Hor*wtz Raw Aceleated Crmoslon Model a30 WSWo t20 ftt l0oft/sec 5fsocm 0.120 Cr =Mo = Cu = 0.03%

O.M4 Oxygen I ppb pH =7 at 77"F W olbow 100 ISO 200 RMO* SS 3* 0 400 459 Temperature (*P)

Figure 7-8. Chexal-Horowitz FAC Model, Impact of Liquid Velocity 7-24 NEC037846

Introduction to Flow-Accelerated Corrosion Chapter Chexal-Horowitz Flow Accelerated Corrosion Modal

.0 o160 13.14D S0,120- .'..... 16" S..241$

0C Cr = Mo = Cu = 0.03%

V= 20 fWfsec Oxygen = 1 ppb 0.

pH = Tat 77'F 900 gfbgw c/ 0.000 -

1O0 160 200 250 300 350 400 430 Temperature (F)

Figure 7-9. Chexal-Horowltz FAC Model, Impact of Pipe Diameter Oxygen Factor.It has been observed widely in flow-accelerated corrosion that the rate of corrosion varies, inversely with the amount of dissolved oxygen present.

Data from various sources were corrlated and used to develop the oxygen factor used in the Chexal-Horowitz model. The dependence of the predicted FAC rate oi the dissolved oxygen is presented in Figure 7-10.

Chexal-Horowutz Flow Accelerated Corrosion Model

-0 ppb "i0.140 --

00120 - --- 10 ppb 30 ppb 09100 ..... 50ppb 0.0800 Cr v Mo = Cu = 0.03%

0 - 4" V 20 ftfsec 0.040 pH = 7 at 7-7F

,- 0,020

. 901 elbow u*0.000-100 150 200 250 300 350 400 450 Temperature 16F)

Figure 7-10, Chexal-Horowitz FAC Model, Impact of Oxygen Level 7-25 NEC037047 ..

Models for Computer Applications pH Factor.The rate of metal loss is strongly dependent upon the solubility of ferrous ions at the metal surface. One of the main parameters controlling the solubility of iron is the operating temperature pH in the aqueous phase at the oxide-solution interface. The value of pH at operating temperature is calculated by the solution of several non-linear simultaneous equations involving mass balance, charge balance, dissociation constants for water, base dissociation constants and partitioning coefficients of the relevant alkalizing agents and anions. The pH factor used in the model is based on the pH at the operating temperature; This means that the flow-accelerated corrosion rate is a function of amine type, amine concentration and temperature. The dependence of the predicted FAC rate on the condensate pH and the pH control amine are presented in Figures 7-ti and 7-12.

Chexel4*orowltz Flow Accelerated Corrosion Model ri 0*,00 8.9 at 774 F S.h..... 9.1 at 771F S010 ... 9.4a1 77'F Or Mo = Cu = 0.03%

Dg V0.020 I4"p

.C - Hydrazine = 20 ppb 0.10 ... .A mine = amm nia 100 10 200 250 Sa0 SW 400 450 Temperature (OF)

Figure 7-11. ChexaI-Horowltz FAC Model, Impact of Change In pH 7-26 NE,03784_8... -

i Introduction to Flow-Acceletated Corrosion Chapter Chemal-Horowitz Flow Accelerated Corrosion Model c,0.400 O.0310 0o.05 Ammonia

-aaETA

-,-*-- Morpholdsa C

U.26 Cr = Moz Cu = 0.03%

V= 20ftec

.Oxygen =1 b C6 **. *0100 Hydrezine PH =- 9at = 20 ppb n77F 0.665 90W elbow 0.000 I II 100 160 200 250 300 3W0 400 40 Temperature (OF)

Figure 7-12. Chexal-Horowitz FAC Model, Impact of Using Ammonia or Alternate Amines at a pH of 9 at 77 6 F (25bC)

Geonmtry Factor.The geometry factor accounts for the increased mass transfer that takes place in fittings due to increased flow turbulence (e.g. from flow direction changes as in an elbow) versus the mass transfer that occurs in a straight pipe. At the time the development of the Chexal-Horowitz model was started, the only widely recognized geometry factors were those of Keller [7.1 ]. These values, which were developed through pressure drop considerations and designed to be applied to two-phase flow, were compared to plant data and were found not to be representative of single-phase flow-accelerated corrosion. In view of the lack of other published infornnaton, plant data were used to establish the geometric factors.

Additionally, NEI International Research & Development Company, Ltd. in England was asked to employ the method of Poulson [7.24] to investigate single-phase and two-phase geometry factors. Briefly, Poulson's method consists of modeling the flow-accelerated corrosion of steel in water with the corrosion of scaled copper components in an acid ferrous chloride solution. The comparable two-phase steam water simulation is done using an air-acid mixture. The use of this method dramatically inureases the corrosion rate and allows rapid, cost-effective testing of a variety of geometries. In this method, the rate of corrosion is 7-27 NEC037849 -........-.-.- ---.-.

Models for Computer Applications controlled by the reduction of ferric ions which is the cathodic reaction, and copper dissolving as a monovalen. copper chloride comples. This method ha.s been tested in single-phase conditions and as expected:

4 The'corrosion is proportional Co the ferric ion concentration and is zero with no ferric ions present.

, Corrosion rate profiles are similar to known.mass transfer profiles.

Peak, plateaus and enhancement factors are the correct function of Reynolds number.

Actual corrosion rates are close to those predicted from existing mass transfer correlations.

Another innovation in this area was the definition of a component categoty to cover the straight pipe immediately downstream of a fitting. Separate geomeiry faclors were developed for each situation.

In 1994, hundreds of records of plant inspections were evaluated to refine and improve ihe geometry flictors. These improvements have been incorporated in C14ECWORKS version i.OC and later versions of the code.

The dependence of the predicted FAC rate on the fitting geometry is presented in Figure 7-13.

3 Chexal-Horow#tz Flow Acveleratod Corrosion Model

"- 0,2, - 181 return

- a - 90 elbtow X: --.--... 450 elbo 0.200 --- pipe S0,00 - ..

Cr - Mo = cu =0.03%

0 4" V 20 ftyssec 0-060, -. *";"""""4 *ONN- oxygen pH =7 at=I7"7*F ppb 1CD 1.

" 200 250 300 3N0 400 450 Tempetature (TF1 Figure 7-13. Chexal-Horowitz FAC Model, Impact of Fitting Geometry 7-28 NE C037050 __

Introduction to Flow-Accelerated Corrosion Chapter Void Factor.Two-phase flow adds to the complexities of single-phase flow,-

accelerated corrosion. To represent the two-phase phenomena, al. least one more correlating variable must be added, This variable was chosen to be the void fraction, oa. The void fraction is defined as the ratio of the area occupied by ihe vapor to the total area of the channel. It should be noted that void fraction and quality (the ratio of steam flow rate to the total flow rate) are not equivalent because in general the steam and water phases are moving with different velocities. Also, qualily is a mass-based parameler, while the void fraction is an area based quantity.

The void fraction for a component containing a two-phase flow environment is calculated using a void fraction correlation developed by Chexal et at. [7.25]. The key variables needed for deteinining the void factor are pressure, orientation, total mass flow r*ates, quality and pipe diameter. When the void fraction is zero, i.e. if the flowing fluid is single-phase liquid, F7(a) = I and the model becomes a single.-

phase flow-accelerated corrosion rate predictor. When the void firaction is one, i.e. there is no liquid present, F7(a) = 0, The dependence of the predicted FAC rate on the steam quality is presented in Figure 7-14.

ChexaI-Hagowitz Flow Adeelerated Corrosion Mq(el 0.300

". 10% Quality

-- - 5% Quality

-'o.. . 0% Ouality t* *..""*D Cr =4"=Mo Cu= 0.03%

,,.*.-*V -*20 ftse¢ Oxygen = 1 ppb PH = 7 at 7"F A 0.050 - -90" elbow 100 I50 200 250 300 350 400 450 Temperature ('F)

Figure 7-14. Chexal-Horowltz FAC Model, Impact of Steam Quality 7-29 NEC037R81 .. .

Models for Comrputer Applications Ilydrazinefactor. Recent work has indicated 'a strong relationship between the oxidizing reducing potential (ORP) and the rate of FAC. The ORP is related to the amount of dissolved oxygen and the concentration of a reducing agent such as hydrazine. The oxygen dependency is included as the P'4 factor previously discussed. This relationship appears to be most signilicant in fossil plants that operate at a pH about 9 with and without hydrazine [7.26, 7.27). In this operatin g regime, the rate of FAC appears to be greatly reduced when the hydrazine is eliminated.

To fully account for the influence of ORP on the rate of FAC a new factor has been developed, This factor, F8 has been designed to account for the presence of hydrazine. The dependence of the predicted FAC rate on the hydrazine level is presented in Figure 7-15. The hydrazine factor has been developed to cover the entire range of hydrazine concentrations from 0 ppb (typical of some fossil plants) to about 500 ppb (typical of sorme Japanese PWRs). This factor has been added to version 1.OF of CHL-CWORKS.

The FAC rate dependence on hydrazine can be seen in Figure 7-15, below.

Chexal-Hovowitz Flow Accelerswtd Corrosion Model oe ---- - O150ppb 0.o~o - ,.-. ...... 20 *o*

== o.* ..... Oppbt

" I).040 0Cr= 4'mo = Cu = 0.03%

V = 20 Wsec W00-0 "' ' oygen = tppb

_ - ,* Am~ne = ammonia 0.010 M 9.g0elbow lea 1SO 200 250 amm 360 0 450 Temperature (OF)

Figure 7-15. Chexal-Horowltz FAC Model, Impact of Hydrazine Concentration 7-30

... NEG037.52 - ---..

Introduction to Flow-Accelerated Corrosion Chapter Model Performance Against Laboratory Data The predictive model was validated by comparing it against all of the available laboratory data from EDF in France and CEGB in England. Ii should be noted that laboratory data tend to be more accurate than plant data because:

The initial thickness of the sample is well characterized, and the thickness measurements are typically made with thin layer activation. This is a veiy precise way of measuring the wall thickness The chemistry and flow conditions are well characterized and accurately measured.

The EDF data were taken at the Ciroco Loop at the EDF facility in Les Renardi6res, France. The bulk of the data were taken itn 0.315 inch (8 mm) inside diameter carbon steel tubes. The CEGB data were taken at the CEGB Loop in Leatherhead, The bulk of the data were taken in 0.354 inch (9 mm) inside diameter carbon steel tubes.

7-31 NEG037853

Models for Computer Applications Figure 7-16 shows the performance of the Chexal-Horowitz correlation against single-phase laboratory data. As can be seen, the agreement is quite good.

0.80 0-60 C

0.40 2

0.20 0.00 M 0.00 0.20 0.40 0.60 0.80 Measured Corrosion Rate (mm/yr)

Figure 7-16. Chexal-Horowitz FAC Model, Comparison Against Laboratory Data Comparison with Plant Data The purpose of the predictive algorithm is to predict actual plant behavior. To validate the model, data from twenty nuclear plants were used. As mentioned before, plant data are inherently less accurate than laboratory data because of uncertainties in piping operating conditions, and also because of the lack of baseline thickness measurements of the piping components.

In most cases, exiperiience has found that discrepancies between model predictions and plant data results from uncertainties in actual operation of the system and plant, and actual condition of the as-built piping. These uncertainties include:

The original thickness and thickness profile of the piping components, 7-32 NEG037854

Introduction to Flow-Accelerated Corrosion Chapter

Trace amounts of alloying elements that are present in the piping, but have not been included in the predictions.

" Inaccuracies in the NDE inspection data.

, Actual steam quality of two-phase systems.

Mistaking other corrosion related damage for FAC (e.g. cavitation or general corrosion during shutdown periods).

Uncertainties in the actual number of hours or fraction of time that a system or tiain operates.

Uncertainties in the plant chemistry history.

Unknown internal discontinuities within the piping such as counterbore, backing rings, and mismatches with regards to piping fit-tip.

However, in spite of these uncertainties, application of the code at operating power plants has repeatedly demonstrated the reliability of the model to identify problem areas needing to be inspected.

Figure 7- 17 shows the performance of the Chcxai-lorowitz correlation against single and two-phase plant data. As can be seen, the agreement is still quite good.

Chexal-Horowltz Flow-Accelerated Corrosion Model 500 400 300 200 100 0

0 100 200 300 400 500 Measured Wear (mil)

Figure 7-17. Chexal-Horowitz FAC Model, Comparison Against Plant Data 7-33 NECO37055

ML082530345

Text

Navigation menu

Search