UNIVERSST&TSBIBUOTHEK HANNOVER

Transcription

1 The Spalling of Steam-Grown Oxide From Superheater and Reheater Tube Steels FP-686 Technical Planning Study Final Report, February 1978 Prepared by CENTRAL ELECTRICITY RESEARCH LABORATORIES Kelvin Avenue, Leatherhead, Surrey, KT22 7SE, England Principal Investigators J. Armitt R. Holmes M. I. Manning D. B. Meadowcroft E. Metcalfe UNIVERSST&TSBIBUOTHEK HANNOVER TECHNISCHE INFORMATIONSBIBLIOTHEK Prepared for Electric Power Research Institute 3412 Hillview Avenue Palo Alto, California EPRI Project Manager R. I. Jaffee Fossil Fuel and Advanced Systems Division UB/TIB Hannover

2 TABLE OF CONTENTS Section page 1 INTRODUCTION SCOPE OF STUDY HISTORY OF PROBLEMS ARISING FROM OXIDE SPALLING U.K. Experience of Oxide Spalling Problem Japanese Experience of Oxide Spalling Problem Russian Experience of Oxide Spalling Problem Summary of Main Conclusions from Plant Experience Outside the United States THE U.K. COLLABORATIVE RESEARCH PROGRAMME ON OXIDE SPALLING The Central Electricity Research Laboratories of the CEGB The Regional Scientific Services Laboratories of the CEGB The South of Scotland Electricity Board Co-operating Organizations LABORATORY AND PLANT OBSERVATIONS OF OXIDE GROWTH RATES MORPHOLOGIES AND SPALLING SOURCES OF OXIDATION AND SPALLING DATA PRINCIPAL RESULTS Earlier Oxidation Studies Results from the U.K. Collaborative Programme SUMMARY OF RESULTS Scale Morphologies Oxidation Rates Spalling Observations PHYSICAL PROPERTIES OF STEAM GROWN OXIDE LAYERS INTRODUCTION FRACTURE STRESSES AND STRAINS 3-1

3 - A Section Page 3.3 YOUNG'S MODULUS FRACTURE SURFACE ENERGIES THERMAL EXPANSION COEFFICIENTS DISCUSSION Fracture Stresses and Strains Young's Modulus Measurements Applicability of Data THE STRAIN TOLERANCE OF DUPLEX OXIDE LAYERS SOURCES OF OXIDE STRAINS IN OPERATIONAL PLANT Cooling Strains Heat Flux Strains Thermal Shock Strains Oxide Transformation Strains Flexural Strains Enhanced Strains at Tube-tube Welds OPTIONS FOR STRESS RELIEF Non-catastrophic Strain Accommodation Fracture in Tension - Through-scale Cracking 4-13 Regular through-scale cracking 4-14 Interphase slip at metal oxide interface 4-17 Delamination associated with through-scale cracking Fracture in Compression and Spalling Strain Energy Criterion 4-18 Comparing model predictions with plant experience and determination of y' 4-19 Extension of the strain energy model to ferritic steels and tensile stresses NUMERICAL ASSESSMENT OF DEBRIS RELEASE QUANTITIES HYDRODYNAMIC STUDIES ON TRANSPORT AND BLOCKAGE FORMATION OF OXIDE SCALES EXPERIMENTAL OBSERVATIONS Separate Flake Behaviour Aggregation of Flakes and Behaviour of Aggregates BLOCKAGE FORMATION PROBABILITY 6-16 VI

4 Page CRITERIA FOR BLOCKAGE CLEARANCE Effect of Flakes on Flow Velocity in Tube Stresses in a Plug of Flakes in a Straight Tube Strength and Stability of Arches of Flakes Overall Clearance Criteria Breakage of Flakes 6-29 CONCLUSIONS 6-30 APPENDIX TO SECTION NUMERICAL ASSESSMENT OF BLOCKAGE PROBABILITIES 7-1 EROSION BY OXIDE PARTICLES 8-1 PRACTICAL SOLUTIONS FOR THE OXIDE SPALLING PROBLEM 9-1 CHEMICAL REMOVAL OF OXIDE 9-2 CONTROL OF THE TIME OF RELEASE OF THE OXIDE Control of the Oxygen Potential of the Steam The Possible Effects of Thermal Shock 9-4 PREVENTION OF DAMAGE BY RELEASED DEBRIS Off-load Inspection Technigues On-load Technigues Design Modifications 9-7 MATERIALS SOLUTIONS 9-8 SURFACE TREATMENT SOLUTIONS 9-11 FUTURE WORK REQUIRED lo-l Vll

5 FIGURES Figure Page 1-1 Spalled Scale Distribution in Two Reheater Platens Weight of Scale Debris in Reheater Tube Bends Experimental Results for Type 316 Steels Oxidized in Steam Oxidation Rates of Type 316 Tube Materials in Steam The Effect of Tensile Strains Applied to Oxidized Specimens on Their Subsequent Oxidation Behaviour Comparison of Experimental and Calculated Values of Normalized Crack Spacing with Total Oxide Strain (Total Strain for Fracture) Versus Reciprocal Flaw Size Spalling Propensity as a Function of Inner Layer Oxide Thickness on AISI 316, 321 and Esshete Probability of Spalling as a Function of Operating Hours for AISI 316, AISI 321 and Esshete Metal Loss Rates for Parabolic Oxidation of Ferritic Steels True Linear Thermal Expansion Coefficients of Steam Grown Oxides Calculated Cooling Strain for Oxide Grown on a Iron Calculated Cooling Strain for Oxide Grown on lcrmo Steel Calculated Cooling Strain for Oxide Grown on 2^CrlMo Steel Calculated Cooling Strain for Oxide Grown on 9CrlMo Steel Calculated Cooling Strain for Oxide Grown on Type 316 Steel Temperature Distribution in Tube Wall and Oxide for Calculation of Heat Flux Strains Temperature Distribution in Tube Wall and Oxide for Calculation of Thermal Shock Strains Critical Fracture Surface Energy Density, y, as a Function of Elastic Modulus, E, for Various Materials Model Considered for Development of Regular Through-Scale Cracking Limiting Crack Separation when Maximum Force Sustained by Oxide Island is Controlled by Metal/Oxide Interfacial Slip Calculated Stored Energy Function hee e for Oxide Grown on Type 316 Steel at Different Temperatures Observed Fraction of Spalling as a Function of Stored Energy Function, heeiei, obtained from Metallographic Data IX

6 Figure 4-13 Oxide Failure Modes as a Function of Thickness and Impressed Strain Method of Calculating the Debris Release due to a Temperature Transient AT Flake Behaviour Rig with Conventional 'U' Tube Superheater Platen Model Used for Flake Trapping on Tube Wall Initial Lift Velocities for Various Flat Square Flakes at Rest in the Bottom of a 'U' Tube Fluid Velocities at which Flakes Fall Fluid Velocities at which Flakes Rise Idealized Wedge of Flakes Forces Acting on a Wedge of Flakes in a Vertical Tube Behaviour of Accumulated Flakes Behaviour of Accumulated Flakes Effect of Blockage on Flow Rate in 13mm Internal Diameter Superheater Tubes as a Function of Flake Quantity Stress in Blockage as a Function of Flake Quantity Model for Stress Distribution in Flake Arches Pressure Sustained by Flat Flakes Simply Supported Across a Span for Various Thicknesses Decision Tree for Assessing Blockage Risks Simple Model for Indentation Fracture Erosion Simplified Steam Flow Diagram Showing the Turbine By-pass System. 9- x

7 TABLES Table Page 2-1 Thicknesses of scales obtained in a Japanese experiment on the effects of surface finish on austenitic steels; superheated steam at 680 C for 200h. {2-11} Typical weight gain figures for austenitic steels after exposure to steam at 600 C and ^ 600 p.s.i Results of inspection of tubing in Cockenzie Trial after 8800h Oxide thicknesses of ferritic steels from the Bankside Corrosion Trials and other data. {2-21} List of ferritic steel applications in CEGB superheaters and reheaters. {2-22} Examination of 12Cr tubing from No.9 boiler Aberthaw 'B' (10,000 hours Unit running time) Fracture stress and strain measurements by 3-point bend tests. The results are given with 90% confidence limits Comparison of fracture stress and fracture strain results for iron oxides Results of Young's moduli measurements by acoustic and 3-point bend techniques Comparison of Young's moduli measurements for iron oxides Some additional physical properties of Fe-0., and Fe O Strain ranges in the oxide for various alloys upon cooling from 540 C for Fe.O. and Fe 0 outer layers Maximum thermal shock strains for different cooling rates Typical physical properties of oxide flakes Lift velocities for magnetite flakes. 6-7 xi