Flaw Size Acceptance Limits for a Stainless Steel Pressure Vessel

Transcription

1 Flaw Size Acceptance Limits for a Stainless Steel Pressure Vessel Consuelo Guzman-Leong Lucius Pitkin, Inc. Richland, Washington, U.S.A. Frederic A. Simonen, Ph.D. Lucius Pitkin, Inc. Richland, Washington, U.S.A. Stephen R. Gosselin, P.E. Lucius Pitkin, Inc. Richland, Washington, U.S.A ASME Pressure Vessels and Piping Conference July 14-18, 2013 Paris, France

2 Scope of Paper Develop flaw size acceptance thresholds for defects detected and characterized during inservice inspections of a stainless steel pressure vessel The approach used guidance from ASME Section XI Code Limits were based on elastic plastic fracture mechanics analyses that considered limit load and ductile tearing failure modes Fracture mechanics calculations employed the Failure Assessment Diagram Acceptable flaw sizes were calculated for several flaw aspect ratios and inspection intervals for axial shell welds Slide 2

3 Technical Basis ASME Section XI does not address vessel flaw size acceptance standards for stainless steel vessels For ferritic steel pressure vessels Section XI Table IWB presents allowable flaw size limits in terms of flaw depth, length and vessel thickness. These flaw size limits are based on linear elastic fracture mechanics calculations that assume a brittle fracture failure mode The 2-inch thick vessel was treated as a thick-walled stainless steel pipe Elastic plastic fracture mechanics methodologies in ASME Section XI Code Non-Mandatory Appendices C and H were applied Slide 3

4 Methodology Step 1: Calculate Stress Intensity Factor Step 2: Apply FAD Step 3: Calculate Fatigue Crack Growth Rates Obtain through-wall stress distributions from FEA calculations Apply Non-Mandatory Appendix A equations for stress intensity factors Apply Non-Mandatory Appendix H procedures Calculate Limit Load Apply Non-Mandatory Appendix C equations for stainless steel crack growth rates Slide 4

5 Assumptions For conservatism, all flaws were postulated to occur as surface breaking cracks, with semi-elliptical shapes, located at the inner or outer surface of the vessel surface having the highest level of tensile stress and/or fatigue crack growth rates A maximum allowable crack extension by ductile tearing of 6 mm was used Flaw lengths for growing fatigue crack depths were adjusted to maintain constant aspect ratios Consistent with ASME Section XI failure assessment methods, crack instability calculations were based on pressure induced primary membrane stresses Slide 5

6 Inputs For conservatism: fracture toughness properties for weld metals were used because data for weld toughness is consistently lower than for base metal toughness; yield and flow stress for base metal were used as these are lower than those for the weld metals The operating pressure and temperature were less than 400 psig and 150 F, with the primary membrane (hoop) stress < 16 ksi Conservative safety factors for Level A and B loads, were applied (pressure (2.7) and thermal (2.3) stresses) Selection of the aspect ratios (2, 3, 6, 10, 20, and 60) for flaws in axial welds was based on the ASME Section XI tables values for ferritic steel vessels Slide 6

7 Stress Intensity Factors: Step 1 Updated equations developed for ASME Section XI Appendix A were used to calculate stress intensity factors Calculations based on the stress intensity factor flaw model equations for a surface flaw in a flat plate of finite thickness at the deepest point crack depth a Stress profile through-the-weld thickness constants and stress intensity G coefficients No adjustment for crack tip plasticity was made, already accounted for in FAD Stress intensity factors as functions of crack depth and length were calculated for the thermal and pressure stresses from finite element analysis Slide 7

8 Failure Assessment Diagram (FAD): Step 2 ASME Code Section XI, Non-Mandatory Appendix H FAD exceedance checks determine acceptability for continued service Calculations were repeated for each initial crack depth and aspect ratio Flaw depths up to and including the (a/t) ratio of 0.75 Flaws allowed 6mm of crack extension by ductile tearing Allowable flaw sizes are identified as those that correspond to the acceptance region of the FAD curve and/or the 75 percent a/t cutoff Ductile tearing parameter, Vertical Axis K R FAD Limit Load Parameter, Horizontal Axis S r Cutoff Limit Load Sr-cutoff Slide 8

9 FAD: Step 2 (cont d) Ductile tearing parameter, Vertical Axis K R (K TOTAL /K R ) Total stress intensity factor included both pressure and thermal loads, K TOTAL J-integral resistance at ductile tearing of 6mm obtained from available test data, ->K R Negative K r values were treated as zero(s) when plotting against the FAD FAD Limit Load Parameter, Horizontal Axis S r SF P/P 0 Reference Limit Load Pressure (P 0 ) Cutoff Limit Load Sr-cutoff P L /P 0 Internal pressure at collapse limit load for axial flaw (P L ) Slide 9

10 Failure Assessment Diagram (FAD): Step 2 (cont d) Slide 10

11 Allowable Flaw Depths For flaw in axial weld No allowance for fatigue crack growth No consideration of inservice inspection interval Aspect Ratio Allowable Flaw Depth (in) Slide 11

12 Fatigue Crack Growth: Step 3 Water environment equations for fatigue crack growth rates in stainless steels developed for ASME Section XI Non-Mandatory Appendix C Surface breaking cracks were assumed to be located on the vessel surface having the highest level of tensile stress and/or fatigue crack growth rates The change in the stress intensity factor (cyclic stress range, KI (ksi in)) for applied loads at the weld location used the mean crack depth (a ) in cyclic flaw growth rate calculations Results are reported for fatigue crack growth corresponding to 10-year inspection intervals Slide 12

13 Results for Axial Weld Flaws These flaw depths (inches) ensure code margins against unstable crack growth per ASME Code Section XI fracture mechanics requirements, including fatigue crack growth between inservice inspections Aspect Ratio Inspection_Interval (years) Slide 13

14 ` Results (cont d) 2c = 3, a = 0.5 and t = 2 Acceptable Detected Crack Depths (a/t) 2c/a = 6 a/t = 0.25 Inspection Interval (yrs) Aspect Ratio (Flaw Length:Depth Ratio) Acceptable as long as inspection interval is <25 years Slide 14

15 Summary Flaw acceptance tables were established for a specific stainless steel vessel to address various flaw aspect ratios and inservice inspection intervals A special technical approach was needed because the current guidance provided by ASME Code Section XI is limited to ferritic steel vessels Evaluations were performed using the FAD methodology and procedures in Non-Mandatory Appendices A, C and H of ASME Code Section XI Insights gained from this work may be useful in developing a general ASME Section XI methodology for flaw acceptance tables and detailed fracture mechanics procedures Slide 15