An Introduction To Api 579 2006

An Introduction to API RP 579: Section 9, Assessment of Crack like Flaws

ROHIT RASTOGI REACTOR SAFETY DIVISION BHABHA ATOMIC RESEARCH CENTRE MUMBAI

INDIAN NUCLEAR SOCIETY LECTURES ON WELDING, NDE AND INTEGRITY ASSESSMENT September 18-22, 2006




[email protected]

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INDEX 1. Introduction to API RP 579

3

2. Overview of API 579, Section 9 (Assessment of crack like flaws) 2.1 General 2.2 Applicability And Limitations Of The Procedure

7 7 7

3. Flaw Characterization 3.1 Characterization Of Flaw Length 3.2 Characterization Of Flaw Depth

9 10 13

4. Level 1 Assessment

15

5. Level 2 Assessment

17

6. Level 3 Assessment

23

7. Remaining Life Assessment

25

8. Leak Before Break Analysis

27

9. Remediation

29

10. In-Service Monitoring

30

11. Documentation

30

12. Example Calculation

32

13. References

33

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1. Introduction to API RP 579 In classical engineering design, an applied stress is compared with the appropriate material resistance expressed in terms of a limit stress, such as the yield strength or fatigue endurance limit. As long as the material resistance exceeds the applied stress, integrity of the component is assured. It is implicitly assumed that the component is defect-free but design margins provide some protection against defects. Modern design and operation philosophies, however, take explicit account of the possible presence of defects in engineering components. Such defects may arise from fabrication, e.g., during casting, welding, or forming processes, or may develop during operation. They may extend during operation and eventually lead to failure, which in the ideal case occurs beyond the design life of the component. The analytical methods for safety evaluation of flaws are based on stress analysis, but they also require information on equipment operations, nondestructive examination (NDE), and material properties. Stress analysis may be performed using standard handbook or design code formulas or by means of finite element analysis (FEA). With modern computer technology, the use of FEA is quite common. Fitness for service assessment requires both knowledge of past operating conditions and a forecast of future operating conditions. Interaction with operations personnel is required to obtain these data. NDE is used to locate, size, and characterize flaws. The material properties should include information of material damage mechanisms and behavior in the service environment, especially on the effects of corrosion and temperature. The draft of API RP (American Petroleum Institute Recommended Practice) 579 [1] was started in 1994, and the first edition was published in January 2000. API 579 has been developed to provide guidance for conducting fitness for service (FFS) assessments of flaws commonly encountered in the refining and petrochemical industry, which occur in pressure vessels, piping, and tanks. However, the assessment procedures can also be applied to flaws encountered in other industries such as the pulp and paper industry, fossil fuel utility industry, and nuclear industry. The guidelines provided in API 579 can be used to make run-repairreplace decisions to ensure that pressurized equipment containing flaws that has been identified during an inspection can continue to be operated safely. API RP 579 is organized in modular fashion based on type of material damage or flaw to facilitate its use and updating. It incorporates a three-level assessment approach. The level of conservatism decreases with increasing level of assessment, but detail of analysis and data increase with increasing level of assessment. An inspector or a plant engineer can perform level 1 assessment. Level 2 assessment requires at least a plant engineer, whereas Level 3 assessment must be performed by an expert engineers or by a team of engineers that includes at least one expert engineer. Application of the higher levels of assessment is often limited by a lack of materials properties data and accurate operating data. A complete listing of the flaw and damage assessment procedures currently covered is shown in Table 1. The organization of each section of the API 579 code is shown in Table 2. Table 3 lists the appendices available in API 579.

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Table 1: Overview of flaw and damage assessment procedure

Section in API 579 3

Flaw or damage Overview mechanism Brittle fracture

4

General loss

5

Local metal loss

6

Pitting corrosion

7

Blisters laminations

8

Weld misalignment and shell distortions

9

Crack-like flaws

10

High temperature operation and creep Fire damage

11

metal

and

Assessment procedures are provided to evaluate the resistance to brittle fracture of in-service carbon and low alloy steel pressure vessels, piping, and storage tanks. Criteria are provided to evaluate normal operating, start-up, upset, and shutdown conditions Assessment procedures are provided to evaluate general corrosion. Thickness data used for the assessment can be either point thickness readings or detailed thickness profiles. A methodology is provided to guide the practitioner to the local metal loss assessment procedures based on the type and variability of thickness data recorded during an inspection Assessment techniques are provided to evaluate single and networks of Local Thin Areas (LTAs), and groove-like flaws in pressurized components. Detailed thickness profiles are required for the assessment. The assessment procedures can also be utilized to evaluate blisters Assessment procedures are provided to evaluate widely scattered pitting, localized pitting, pitting which occurs within a region of local metal loss, and a region of localized metal loss located within a region of widely scattered pitting. The assessment procedures can also be utilized to evaluate a network of closely spaced blisters. The assessment procedures utilize the methodology developed for local metal loss Assessment procedures are provided to evaluate either isolated, or networks of blisters and laminations. The assessment guidelines include provisions for blisters located at weld joints and structural discontinuities such as shell transitions, stiffening rings, and nozzles Assessment procedures are provided to evaluate stresses resulting from geometric discontinuities in shell type structures including weld misalignment and shell distortions (e.g. out-of-roundness, bulges, and dents) Assessment procedures are provided to evaluate crack-like flaws. Recommendations for evaluating crack growth including environmental concerns are also covered Assessment procedures are provided to determine the remaining life of a component operating in the creep regime. The remaining life procedures are limited to the initiation of a crack Assessment procedures are provided to evaluate equipment subject to fire damage. A methodology is provided to rank and screen components for evaluation based on the heat exposure experienced during the fire. The assessment procedures of the other sections of this publication are utilized to evaluate component damage

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Table 2: Organization Different Sections of API 579

Title

Overview

1

General

2

Applicability and limitations of the FFS assessment procedures Data requirements

The scope and overall requirements for an FFS assessment are provided The applicability and limitations for each FFS assessment procedure are clearly indicated; these limitations are stated in the front of each section for quick reference The data requirements required for the FFS assessment are clearly outlined; these data requirements include: • Original equipment design data • Maintenance and operational history • Required data/measurements for a FFS assessment • Recommendations for inspection technique and sizing requirements Detailed assessment rules are provided for three levels of assessment: Level 1, Level 2, and Level 3. Guidelines for performing a remaining life estimate are provided for the purpose of establishing an inspection interval in conjunction with the governing inspection code Guidelines are presented on methods to mitigate and/or control future damage. In many cases, changes can be made to the component or to the operating conditions to mitigate the progression of damage Guidelines for monitoring damage while the component is inservice are provided, these guidelines are useful if a future damage rate cannot be estimated easily or the estimated remaining life is short. In-service monitoring is one method whereby future damage or conditions leading to future damage can be assessed or confidence in the remaining life estimate can be increased. Guidelines for documentation for an assessment are provided; the general rule is – A practitioner should be able to repeat the analysis from the documentation without consulting an individual originally involved in the FFS assessment A comprehensive list of technical references used in the development of the FFS assessment procedures is provided; references to codes and standards are provided in this section Tables and figures including logic diagrams are used extensively in each section to clarify assessment rules and procedures A number of example problems are provided, which demonstrate the application of the FFS assessment procedures

3

4 5

Assessment techniques and acceptance criteria Remaining life evaluation

6

Remediation

7

In-service monitoring

8

Documentation

9

References

10

Tables and figures

11

Example problems

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Table 3: API 579 Appendices

Appendix Title

Overview

A

Thickness, MAWP and membrane stress equations for a FFS assessment

B

Stress analysis overview for a FFS Assessment Compendium of stress intensity factor Solutions

Equations for the thickness, MAWP, and membrane stress are given for most of the common pressurized components. These equations are provided to assist international practitioners who may not have access to the ASME code and who need to determine if the local design code is similar to the ASME code for which the FFS assessment procedures were primarily designed for. Recommendations for stress analysis techniques that can be used to perform an FFS assessment are provided including guidelines for finite element analysis A compendium of stress intensity factor solutions for common pressurized components (i.e. cylinders, spheres, nozzle, etc.) is given. These solutions are used for the assessment of crack like flaws. The solutions presented represent the latest technology and have been rederived using the finite element method in conjunction with weight functions A compendium of reference stress solutions for common pressurized components (i.e. cylinders, spheres, nozzle, etc.) is given. These solutions are used for the assessment of crack-like flaws Procedures to estimate the through-wall residual stress fields for different weld geometries are provided; this information is required for the assessment of crack like flaws Material properties required for all FFS assessments are provided including: • Strength parameters (yield and tensile stress) • Physical properties (i.e. Young’s Modulus, etc.) • Fracture toughness • Data for fatigue crack growth calculations • Fatigue curves (Initiation) • Material data for creep analysis including remaining life and creep crack growth An overview of the types of flaws and damage mechanisms that can occur is provided, concentrating on service-induced degradation mechanisms. This appendix only provides an abridged overview on damage mechanisms; API 571 is currently being developed to provide a definitive reference for damage mechanisms that can be used with API 579 and API 580 An overview of the studies used to validate the general and local metal loss, and the crack-like flaw assessment procedures are provided Definitions for common terms used throughout the sections and appendices of API 579 are given Forwarded to the API CRE FFS task group for resolution

C

D E F

Compendium of reference stress solutions Residual stresses in a FFS evaluation Material properties for a FFS assessment

G

Deterioration and failure modes

H

Validation

I

Glossary of terms and definitions Technical inquiries

J

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2. Overview of API 579, Section 9 (Assessment of crack like flaws) 2.1 General Section 9 gives assessment procedures to evaluate crack-like flaws. Guidelines for evaluating crack growth including environmental concerns are also covered in this section. Fitness-ForService (FFS) assessment procedures for evaluating crack-like flaws in components in this section are based on the Failure Assessment Diagram (FAD) method. This method has evolved as the most broadly accepted methodology for the analysis of components containing a crack-like flaw. Crack-like flaws are planar flaws that are predominantly characterized by a length and depth, with a sharp root radius. Crack-like flaws can be embedded or surface breaking. Examples of crack-like flaws include planar cracks, lack of fusion and lack of penetration in welds, sharp groove-like localized corrosion, and branch type cracks associated with environmental cracking. In some cases, it is conservative and advisable to treat volumetric flaws such as aligned porosity or inclusions, deep undercuts, root undercuts, and weld overlays as planar flaws, particularly when such volumetric flaws may contain micro cracks at the root. This is because the results from an NDE examination may not be sensitive enough to determine whether micro cracks have initiated from the flaw. It may be necessary to use the assessment procedures in Section 9 to compare the relative flaw tolerance of an existing component for screening purposes. In this type of analysis, a standard reference flaw must be postulated to undertake the fracture mechanics calculations. A standard reference surface flaw that can be used has a depth equal to 25% of the wall thickness and a length equal to six times this depth.

2.2 Applicability and Limitations Of The Procedure The assessment procedures of Section 9 can be used to evaluate pressurized components containing crack-like flaws. The pressurized components covered include pressure vessels, piping, and tanks designed to a recognized code or industry standard. The Level 1 and 2 assessment procedures in this section apply only if all of the following conditions are satisfied: •

The original design criteria were in accordance with the following codes o ASME B&PV Code, Section VIII, Division 1 o ASME B&PV Code, Section VIII, Division 2 o ASME B&PV Code, Section I o ASME B31.3 Piping Code o ASME B31.1 Piping Code o API 650 o API 620 The method can also be applied to pressure containing equipment constructed to other recognized codes and standards.

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•

The component is not operating in the creep range

•

Dynamic loading effects are not significant (e.g. earthquake, impact, water hammer, etc.).

•

The crack-like flaw is subject to loading conditions and/or an environment that will not result in crack growth. If a flaw is expected to grow in service, it should be evaluated using a Level 3 assessment and the remaining life should be evaluated using the procedures given in this section.

The following limiting conditions should be satisfied for a Level 1 Assessment. •

Limitations on component and crack-like flaw geometries: The limitations have been specified in terms of the radius and thickness of the spherical and cylindrical components. For cylindrical and spherical shell components, the crack-like flaw is oriented in the axial or circumferential direction (i.e. perpendicular to a principal stress direction) and is located away from major structural discontinuity. For a flat plate, the crack like flaw is oriented such that the maximum principal stress direction is perpendicular to the plane of the flaw. If the crack-like flaw is oriented such that it is not perpendicular to a principal stress plane, then the flaw may be characterized based on the recommended procedures.

•

Limitations on component loads: The loading on the component is from pressure that produces only a membrane stress field. Pressurized components subject to pressure that result in bending stresses (e.g. head-to cylinder junction, nozzle intersections, rectangular header boxes on aircooled heat exchangers) should be evaluated using a Level 2 or Level 3 Assessment. 7KH PHPEUDQH VWUHVVHV GXULQJ RSHUDWLRQ DUH ZLWKLQ WKH OLPLWV RI WKH RULJLQDO construction code and the component will not be subject to hydro test conditions. If a component being evaluated is to be subject to a future hydro test, the component’s metal temperature shall be at least above the nil ductility temperature. After the hydro test, the crack-like flaw shall be re-examined to ensure that the flaw has not grown. The weld joint geometry is either a Single-V or Double-V configuration.

•

The material meets the following limitations: The material is carbon steel with an allowable stress as per the original construction code that does not exceed 172 MPa (25 ksi).7KHVSHFLILHGPLQLPXP\LHOGVWUHQJWKIRU the base material is less than or equal to 276 MPa (40 ksi), the specified minimum tensile strength for the base material is less than or equal to 483 MPa (70 ksi), and the welds are made with electrode compatible with the base material. $SSHQGL[)RIWKH API 579 gives the methodology to estimate lower bound fracture toughness. The fracture toughness is greater than or equal to the lower bound KIC value obtained from Appendix F, computed using a reference temperature from Appendix F, This will be

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true for carbon steels where the toughness has not been degraded because of environmental damage (e.g., fire damage, over-heating, graphitization, etc.). A Level 3 Assessment should be performed when the Level 1 and 2 methods cannot be applied or produce overly conservative results. Conditions that typically require this assessment level include the following. •

Advanced stress analysis techniques are required to define the state of stress at the location of the flaw because of complicated geometry and/or loading conditions.

•

The flaw is determined or expected to be in an active sub-critical growth phase or has the potential to be active because of loading conditions (e.g. cyclic stresses) and/or environmental conditions, and a remaining life assessment or on-stream monitoring of the component is required.

•

High gradients in stress (either primary or secondary), material fracture toughness, or material yield and/or tensile strength exist in the component at the location of the flaw (e.g. mismatch between the weld and base metal).

3. Flaw Characterization Flaw characterization rules allow existing or postulated crack geometry to be modeled by a geometrically simpler one in order to make the actual crack geometry more amenable to fracture mechanics analysis. The nomenclature and idealized shapes used to evaluate cracklike flaws are shown in Table 4.

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Table 4: Flaw Characterization

Actual

Ideal Through-wall Flaw

Surface Flaw

Embedded Flaw

The rules used to characterize crack-like flaws are necessarily conservative and intended to lead to idealized crack geometries that are more severe than the actual crack geometry they represent. These characterization rules account for flaw shape, orientation and interaction.

3.1 Characterization of Flaw Length The flaw length is typically not difficult to determine for surface breaking flaws. If the flaw is oriented perpendicular to the plane of the maximum principal tensile stress in the component, then the flaw length to be used in calculations (c or 2c) is merely the measured length co or 2co. However, if the flaw is not oriented in a principal plane, then an equivalent flaw dimension with a Mode I orientation may be inferred by one of the following options. (a) Conservative option – The flaw dimension, c, to be used in the calculations can be set equal to the measured length, co, irrespective of orientation. In general, such an assumption leads to a conservative analysis. For fracture assessments, the plane of the flaw should be assumed to be normal to the maximum principal tensile stress. (b) Equivalent flaw length option – If the inclined crack propagates by brittle fracture or fatigue, it will tend to align itself perpendicular to the applied stress. That is, an initially mixed-mode crack will tend to become a mode I crack when it propagates, assuming the material is relatively homogeneous. If there are no planes of weakness in the material, the propagating crack will follow the path where the driving force is highest, which turns out to be approximately normal to the maximum principal stress in most cases. The preferred initial direction of propagation for a mixed-mode crack can be determined by evaluating the local energy release rate as a function of propagation direction [2]. The critical conditions for propagation can then be determined by comparing the maximum energy release rate to a critical value for the material. Finally, an equivalent mode I crack can be defined that will

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propagate given the same principal stresses as in the mixed-mode case. The recommended procedure for defining an equivalent Mode I flaw dimension is shown in figure 1.

Figure 1: Calculation of Equivalent flaw

STEP 1 – Project the flaw onto a principal plane. In the case of uni-axial loading, there is only one possible principal plane. However, when the loading is bi-axial (e.g., a pressurized component which is subject to a hoop stress and an axial stress), there is a choice of principal planes on which to project the flaw. In most cases, the flaw should be projected to the plane normal to the maximum principal tensile stress (the σ 1 plane), but there are instances where the σ 2 plane would be more appropriate (e.g., when the angle between the flaw and the principle plane 45o). STEP 2 – Compute the equivalent flaw length 1. For the plane of the flaw projected onto the plane normal to σ 1

2. For the plane of the flaw projected onto the plane normal to σ 2

In the above equations, the dimension c corresponds to the half flaw length to be used in calculations, co is the measured half length for the flaw oriented at an angle from the σ 1 plane, and B is the bi-axiality ratio, defined as: Lectures on Welding, NDE and Integrity Assessment

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The above equation is only valid when both σ 1 and σ 2 are positive. If σ 2 is compressive, B should be set to zero, and first equation in step 2 is used to compute the equivalent flaw length. If stress gradients occur in one or more directions, the sum of membrane and bending components should be summed for computing σ 1 and σ 2 . For uni-axial loading, B = 0 and first equation in step 2 reduces to

Figure 2 gives a comparison of the crack sizes characterized by methods (a) and (b) above.

Figure 2: Comparison of equivalent mode I crack length estimates from the energy release rate model and the simple projection method

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3.2 Characterization of Flaw Depth The part through-wall depth of a flaw can be considerably more difficult to estimate than the length. Either a default value or a value based on detailed measurements may be used for the flaw depth in the assessment. a) Flaw Depth by Default Values 1) Through-Wall Flaw – If no information is available about the depth of a flaw, a conservative assumption is that the flaw penetrates the wall (e.g., a = t for a surface flaw). In pressurized components, an actual through-wall flaw would most likely lead to leakage, and thus would not be acceptable in the long term. However, if it can be shown that a throughwall flaw of a given length would not lead to brittle fracture or plastic collapse, then the component should be acceptable for continued service with a part-through-wall flaw of that same length. Additional special considerations may be necessary for pressurized components containing a fluid where a leak can result in auto refrigeration of the material near the crack tip, or other dynamic effects. 2) Surface Flaw – Flaw depths less than the full wall may be assumed if justified by service experience with the type of cracking observed. However, if service experience is not available, then the assumed flaw depth should not be less than the following where length of the flaw is 2c and wall thickness is t.

a = min [t , c ] b) Flaw Depth from Actual Measurements If the flaw is normal to the surface, the depth dimension, a, is taken as the measured dimension, ao. However, if the flaw is not normal to the surface, see Figure 3, the following procedure may be used to compute the depth dimension, a.

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Figure 3: Flaw depth characterization

STEP 1 – Project the flaw onto a plane that is normal to the plate surface. STEP 2 – Measure the angle to the flaw, , as defined in figure 3, and determine W using the following equation

STEP 3 – Multiply ao by W to obtain the dimension a, which is used in calculations. Note that the dimension d for buried flaws may decrease when the flaw depth is determined using this approach. If the remaining ligament is small, it may be necessary to re-categorize the flaw depending on the remaining ligament size. An embedded flaw may be re-categorized as a surface flaw and a surface flaw may be re-categorized as a through-wall flaw. The code also gives guidelines for the categorization of branched and multiple flaws.

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4. Level 1 Assessment Level 1 Assessments are limited to crack-like flaws in pressurized cylinders, spheres or flat plates away from all structural discontinuities. The step-by-step procedure for FFS using this level is given next. STEP 1 – Determine the load cases and temperatures to be used in the assessment based on operating and design conditions. STEP 2 – Determine the length and depth of the crack-like flaw from inspection data. The flaw should be characterized using the procedures defined in the code. STEP 3 – Determine the case from the list below to be used in the assessment based on the component geometry and crack-like flaw orientation with respect to the weld joint. • • • • • • •

Flat Plate, Crack-Like Flaw Parallel to Joint (Figure 4 given here. Similar curves available for the remaining cases in the code) Cylinder, Longitudinal Joint, Crack-Like Flaw Parallel To Joint Cylinder, Longitudinal Joint, Crack-Like Perpendicular To Joint Cylinder, Circumferential Joint, Crack-Like Flaw Parallel To Joint Cylinder, Circumferential Joint, Crack-Like Flaw Perpendicular To Joint Sphere, Circumferential Joint, Crack-Like Flaw Parallel To Joint Sphere, Circumferential Joint, Crack-Like Flaw Perpendicular To Joint

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Applicability t = thickness t < 38mm Figure 4: Level 1 Assessment, Flat Plate Notes: 1. Definition of Screening Curves (solid line ¼-t flaw, dashed line 1-t flaw): A – Allowable flaw size in base metal. B – Allowable flaw size in weld metal that has been subject to PWHT. C – Allowable flaw size in weld metal that has not been subject to PWHT. 2. Crack dimension for a 1-t and ¼-t flaw are shown in figures on the right above 3. The maximum permitted flaw length from this curve is 2c = 203.2mm. 4. Tref = use 38oC (material specific can also be obtained from Section 3)

STEP 4 – Determine the screening curve from the case being analyzed. The following should be noted when selecting a screening curve. For each Figure in STEP 3, two sets of screening curves, ¼-t and 1-t crack depths are provided for three conditions. 1. Base metal 2. Weld that has been subject to PWHT 3. Weld metal that has not been subject to PWHT If the depth of the flaw can accurately be determined using qualified NDE procedures, then the ¼-t flaw curve can be used in the assessment; otherwise, the 1-t flaw curve should be used. If t ≤ 25.4 mm where t is the wall thickness of the component containing the flaw, then the ¼t flaw curves are directly applicable and the limiting crack depth is 0.25t. If t > 25.4 mm, then the ¼-t flaw curves are applicable when the absolute crack depth is less than or equal to 6.3 mm. The through-wall flaw or 1-t curves can be used for all wall thickness up to the screening curve limitation of 38 mm.

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If the location of the flaw is at or within a distance of two times the nominal plate thickness measured from the centerline of the weld, then the curves for welds should be utilized; otherwise, the curve for base metal may be used. Note that for flaws located at welds, the applicable assessment curve is based on heat treatment of the component. If there is question regarding the type and/or quality of PWHT, Curve C (i.e. no PWHT) should be used. STEP 5 – Determine the reference temperature. The assessment curves are based on a reference temperature, Tref = 38oC, and this value may be used in the assessment. Alternatively, a value of the reference temperature can be established using Section 3 of API 579. STEP 6 – Determine the maximum permissible crack-like flaw length. Enter the assessment Figure established in STEP 3 with the assessment temperature and reference temperature determined in Steps 1 and 5, respectively, to determine the maximum length of the flaw using the applicable screening curve. STEP 7 – Evaluate Results – if the permissible flaw size determined in STEP 6 is greater than or equal to the length of the crack-like flaw determined in STEP 2, then the component is acceptable for future operation. If the component does not meet the Level 1 Assessment requirements then a Level 2 or Level 3 Assessment can be done.

5. Level 2 Assessment The assessment procedure in Level 2 provides a better estimate of the structural integrity of a component than a Level 1 Assessment with a crack-like flaw. A: Assessment using partial safety factors In the assessment of crack-like flaws, partial safety factors are utilized along with the FAD acceptance criteria to account for variability of the input parameters in a deterministic fashion. Three separate partial safety factors are utilized 1. Factor for applied loading 2. Factor for material toughness 3. Factor for flaw dimensions The partial safety factors are applied to the stresses resulting from a stipulated loading condition, the fracture toughness and the flaw size parameters prior to the FAD analysis. The partial safety factors were developed based upon the results of a series of probabilistic analyses of components with crack-like flaws. In this procedure, Partial Safety Factors are applied to the independent variables (i.e. flaw size, material fracture toughness, and stress) to account for uncertainty. One such table is shown in Table 5.

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Table 5: Partial Safety Factors in API 579

RC is a cut-off value used to define the regions of brittle fracture and plastic collapse, when RC 5ky brittle fracture is predicted

The step-by-step procedure for assessment under primary loads is presented now. STEP 1 – Evaluate operating conditions and determine the pressure, temperature and loading combinations to be evaluated. STEP 2 – Determine the stress distributions at the location of the flaw based on the applied loads in STEP 1 and classify the resulting stresses into the following stress categories • • •

3ULPDU\VWUHVV 6HFRQGDU\VWUHVV 5HVLGXDOVWUHVV

Appendix E of API 579 contains a compendium of residual stress distributions for various weld geometries. These distributions are based on finite element analyses of weld residual stresses in a series of pipe girth welds, seam welds, and nozzle-to-head attachment welds performed under MPC sponsorship. Based on these results, a series of parametric residual stress distributions were developed and included in API 579 Appendix E. STEP 3 – Determine the material properties; yield strength, tensile strength and fracture toughness (Kmat) for the conditions being evaluated in STEP 1. The yield and tensile strength

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should be established using nominal values, and the toughness should be based on the mean value. Appendix F of API 579 contains information on material properties, including toughness. This appendix does not contain a database of toughness values, however. Rather, it provides correlations and estimation methods. For ferritic steels, there are lower-bound correlations of toughness to Charpy transition temperature. These correlations were adapted from Sections III and XI of the ASME boiler and pressure vessel code. For static loading in the absence of dissolved hydrogen, the lower-bound toughness correlation is as follows:

where Tref is the 20 J (15 ft-lb) transition temperature in the case of carbon steels. For probabilistic fracture analyses of steel structures, API 579 endorses the use of the fracture toughness Master Curve, as implemented in ASTM Standard E 1921-97. The Master Curve quantifies the temperature dependence of steels in the transition range, as well as the statistical distribution of toughness at a given temperature. The latter is characterized by a three-parameter Weibull distribution with two of the three parameters specified:

where F is the cumulative probability, B the specimen thickness (crack front length), and K0 is the Weibull mean toughness, which corresponds to the 63rd percentile value. The temperature dependence of the median (50th percentile) toughness is given by

where T0 is the index transition temperature material for the material of interest. It corresponds to the temperature at which the median toughness for a 25 mm (1 in.) thick specimen is 100 MPa√m (91 ksi√in). The median and Weibull mean are related as follows:

When fracture toughness testing is not feasible, T0 can be estimated from the 27J (20 ft-lb) transition temperature:

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STEP 4 – Determine the crack-like flaw dimensions from inspection data. The flaw should be categorized as per the guidelines given in the code. STEP 5 – Modify the primary stress, material fracture toughness, and flaw size using the Partial Safety Factors (PSF). 1. Primary Membrane and Bending Stress – Modify the primary membrane and bending stress components determined in STEP 2 (Pm and Pb , respectively) using the PSF for stress (see table 4). Pm = Pm .PSFS Pb = Pb .PSFS 2. Material Toughness – Modify the mean value of the material fracture toughness determined in STEP 3 (Kmat ) using the PSF for fracture toughness. K mat PSFk 3. Flaw Size – Modify the flaw depth determined in STEP 4 using the PSF for flaw size. If the factored flaw depth exceeds the wall thickness of the component, then the flaw should be re-categorized as a through-wall flaw. K mat =

a = a.PSFa However, if a given input value is known to be a conservative estimate (e.g. lower-bound toughness or upper-bound flaw size), a PSF of 1.0 may be applied to this value. STEP 6 – Compute the reference stress for primary stresses, σ refp , based on the factored primary stress distribution and factored flaw size from STEP 5 and the reference stress solutions in Appendix D of the API code. STEP 7 – Compute the Load Ratio or the abscissa of the FAD using the reference stress for primary loads from STEP 6 and the yield strength from STEP 3. The load ration is given by the following equation. P LPr = l Ply Where: Pl = is the generalized loading parameter, such as applied stress, bending moment, or pressure Ply = is the value of the generalized loading parameter evaluated for the component with a crack-like flaw at the yield stress. Alternatively, the load ratio can be written in terms of a reference stress.

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σ refp L = σy Where P  σ ref =  l  σ ys P   ly  Formulations for the ref for different geometries and loading condition have been provided in the Appendix D of API 579. STEP 8 – Compute the stress intensity attributed to the primary loads, K IP , using the factored primary stress distribution and factored flaw size from STEP 5, and the stress intensity factor solutions in Appendix C. If K IP < 0 then set K IP = 0. p r

SR , based on the STEP 9 – Compute the reference stress for secondary and residual stresses, σ ref secondary and residual stress distributions from STEP 2, the factored flaw size from STEP 5, and the reference stress solutions in Appendix D of API 579.

STEP 10 – Compute the stress intensity attributed to the secondary and residual stresses, K ISR , using the secondary and residual stress distributions from STEP 2, the factored flaw size from STEP 5, and the stress intensity factor solutions in Appendix C of the API 579 code. STEP 11 – Compute the plasticity interaction factor, in presence of secondary loads. A detailed procedure has been defined in the Section 9 of the code. STEP 12 – Determine toughness ratio or ordinate of the FAD assessment point where K IP is the applied stress intensity due to the primary stress distribution from STEP 8 and Kmat is factored fracture toughness from STEP 5. Kr =

K IP + ΦK ISR K mat

STEP 13 – Evaluate results. Section 9 of API 579 covers the assessment of cracks and other planar flaws. As is the case with other prominent procedures, such as R6 and BS 7910, the failure assessment diagram (FAD) methodology forms the basis of the flaw evaluation.

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Lecture number 19 a, Page 21 of 33

Figure 5: A level 2 FAD, showing the typical cut-off values Figure 5 illustrates the FAD concept. The toughness ratio, Kr, and the load ratio, Lrp , for the structure of interest are plotted on the diagram. The FAD curve represents the predicted failure locus. If the assessment point falls within the curve, it is considered acceptable. The FAD is based on the following equation.

(

K r = 1 − 0.14 ( LPr )

2

)(0.3 + 0.7 exp −0.65 (L )  ) for L ≤ L P 6 r

P r

P r (max)

The extent of the FAD on the Lrp axis is determined as follows = 1.00 for materials with yield point plateau (strain hardening exponent > 15), = 1.25 for carbon-Mn steels, = 1.80 for austenitic stainless steels σ = f for other materials where f is the flow stress and ys is the yield stress σ ys = 1 if the strain hardening characteristics of the material are unknown. B: Assessment without using partial safety factors In this procedure, conservative assumptions are made in determining the material fracture toughness and applied stress, and a lower-bound failure assessment diagram is utilized to compensate for uncertainty in the assessment procedure. In this procedure the partial safety factors used in STEP 5 previously, are not used. The flaw is acceptable if Kr is less than 0.7 and Lr is less than 0.8.

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Lecture number 19 a, Page 22 of 33

6. Level 3 Assessment The assessment procedure in Level 3 provides the best estimate of the structural integrity of a component with a crack-like flaw. In addition, this assessment level is required if sub-critical crack-growth is possible during future operation. Five methods are permitted in a Level 3 Assessment. Method A Assessment – The basis of this method is the Level 2 assessment procedure except that the FAD in figure 5 is utilized for the acceptance criteria with user specified Partial Safety Factors based on a risk assessment. Alternatively, a probabilistic analysis can be performed. Method B Assessment – The basis of this method is the Level 2 assessment procedure except that the FAD is constructed based on the actual material properties. This method is only suitable for base and weld material because it requires a specific material dependent stressstrain curve. The method should not be used for assessment of crack-like flaws in the HAZ. The procedure for the assessment is as follows: STEP 1 – Obtain engineering stress-strain data for the material containing the cracklike flaw at the assessment temperature. If a stress-strain curve for the actual material containing the flaw cannot be obtained, a stress-strain curve for a material with the same specification and similar stress-strain response can be used. The 0.2% offset yield strength, tensile strength, and modulus of elasticity should be determined together with sufficient data points to accurately define the stress-strain curve. It is recommended that the engineering stress-strain curve be accurately defined at the following ratios of σ 0.7, 0.8, 0.98, 1.0, 1.02, 1.1, 1.2 and intervals of 0.1 applied stress to yield stress: σ ys up to uts STEP 2 – Convert the engineering stress-strain curve obtained in STEP 1 to a true stress-strain curve. The true stress and strain can be computed from the engineering strain as shown below.

σ t = (1 + ε es )σ es ε t = ln (1 + ε es )

Where subscripts t = true, es = engineering

STEP 3 – Determine the material-specific FAD using the following equation: P 3  Eε L ( r ) σ ys ref K r (LPr ) =  P +  Lr σ ys 2 Eε ref 

K r (LPr ) = 1

   

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−1 2

for 0.0 < LPr ≤ LPr (max) for LPr = 0

Lecture number 19 a, Page 23 of 33

STEP 4 – Complete the assessment using the Level 2 assessment procedure except the material specific FAD generated above is utilized. Partial Safety Factors should be used in the assessment. Alternatively, a probabilistic analysis can be performed. Method C Assessment – The basis of this method is the Level 2 assessment procedure except that the FAD is constructed based on the actual loading conditions, component geometry and material properties. FAD is generated using the equation given below.

Kr =

J elastic J total

Partial Safety Factors should be used in the assessment. Alternatively, a probabilistic analysis can be performed. Method D Assessment – This method is a ductile tearing analysis where the fracture tearing resistance is defined as a function of the amount of stable ductile tearing. This method should only be used for materials that exhibit stable ductile tearing (e.g. ferritic steels on the upper shelf and austenitic stainless steels). Partial Safety Factors should be used in the assessment. Alternatively, a probabilistic analysis can be performed. Method E Assessment – The recognized assessment procedures listed below are subject to supplemental requirements that may include the use of Partial Safety Factors or a probabilistic analysis. • • • • • • •

BS PD6493 or BS 7910 Nuclear Electric R-6 SAQ/FoU Report 96/08 WES 2805 – 1997 DPFAD Methodology EPFM using the J-integral The J-integral-Tearing Modulus method

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Lecture number 19 a, Page 24 of 33

7. Remaining Life Assessment Sub-critical Crack Growth

Assessment procedures are given for evaluating sub-critical crack-growth in pressure containing components. In the refining and petrochemical industry, there are a wide variety of process environments and material degradation mechanisms that assist environmentally and service induced cracking. In API 579, in-service crack growth is categorized into four main types • • • •

Crack growth by fatigue Crack growth by stress corrosion cracking Crack growth by hydrogen assisted cracking Crack growth by corrosion fatigue

The methodology for crack growth evaluation used in API 579 is based on fracture mechanics. In this methodology, the growth of a pre-existing crack is controlled by a crack tip stress intensity factor. It is assumed that a crack growth law controls the growth of a crack. This law is available for each combination of material, environment, and crack tip stress intensity factor which can be measured or determined independently and applied to a component with a crack-like flaw. An important requirement for this methodology is that the material properties such as yield and flow stress, material toughness, and crack growth law including appropriate coefficients should be determined as closely as possible from conditions which represent the combination of material, equipment age, environment and loading conditions (applied stress intensity level) for the component being evaluated. A major difficulty with environmental cracking data is that crack growth rates can be highly sensitive to changes in the process environment. The models are fitted in carefully controlled conditions in a laboratory experiment. The composition and temperature of an actual process is subject to fluctuations, and the applicability of laboratory data is inappropriate in many cases. Another problem with predicting crack growth rates in structures is that the cracking often occurs as the result of an upset in operating conditions. For example, cracking that is detected after several years of service may have occurred over the space of several hours or days when atypical operating conditions were present. No cracking occurred before or after this upset. An average crack growth rate, obtained by dividing the crack size by the total time in service, would be meaningless in such instances. For cases involving fatigue, or environmentally assisted cracking, new cracks can initiate at other locations in the structure remote from the known cracks being analyzed. This occurs because corrosion, erosion, local cyclic or static stresses or local concentration of the environment is such that threshold values for crack extension are exceeded. Hence, when assessing the significance of known or postulated cracks for in-service crack extension and structural failure, the implications of exceeding such threshold values elsewhere in the structure must be considered. Evaluation And Analysis Procedures For Components With Growing Cracks –The analysis involves the use of a Level 3 assessment and the numerical integration of a crack growth law.

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Lecture number 19 a, Page 25 of 33

The method involves calculation of the limiting crack size for failure. For this the initial crack size is incremented for the loads and environmental conditions for the planned life of the component. The final flaw is checked for acceptability based on a Level 3 analysis. A stepby-step procedure is given below. STEP 1 – Perform a Level 3 assessment for the initial crack size. If the component is demonstrated to be acceptable per a Level 3 assessment, then an attempt to apply remedial measures to prevent further crack growth should be made. STEP 2 – If effective remedial measures are not possible and slow sub-critical crack growth is expected, then determine if a crack growth law exists for the material and service environment. If a crack growth law exists, then a crack growth analysis can be performed. Otherwise, a leak-before break analysis should be performed to determine if an acceptable upper bound crack size could be established. STEP 3 – Compute the stress at the flaw based on the future operating conditions. In these calculations, all relevant operating conditions including normal operation, start-up, upset, and shutdown should be considered. STEP 4 – Determine an increment in crack growth based on the previous flaw size (to initialize the process, the previous flaw size is the initial flaw size determined in STEP 1), stress, estimated stress intensity, and the crack growth law. For surface and embedded flaws, the increment of crack growth will have a component in the depth and length dimension. For embedded flaws, the increment of crack growth may also include a component to model the flaw location in the wall thickness direction. The increment of crack growth is established based on the applied stress intensity associated with the component of the crack and the crack growth law. For example, if a surface flaw is being evaluated, the crack depth is incremented based on the stress intensity factor at the deepest portion of the crack and the length is incremented based on the stress intensity factor at the surface. The flaw size to be used in STEP 5 is the previous flaw size plus the increment of crack growth. STEP 5 – Perform a Level 3 assessment for the current crack size. Demonstrate that for the current crack size, the applied stress intensity factor is less than the critical stress intensity factor for the applicable crack growth mechanism. If the assessment point for the current flaw size is outside of the FAD or the crack is re-categorized as a through-wall crack, then go to STEP 6; otherwise, go to STEP 4 and continue to grow the crack. STEP 6 – Determine the time or number of stress cycles for the current crack size (ao, co) to reach the limiting flaw size. The component is acceptable for continued operation provided: The time or number of cycles to reach the limiting flaw size, including an appropriate inservice margin, is more than the required operating period. 7KH FUDFN JURZWK LV PRQLWRUHG on-stream or during shutdowns, as applicable, by a validated technique. 7KHREVHUYHGFUDFN growth rate is below the value used in the remaining life prediction as determined by an onstream monitoring or inspections during shutdowns. 8SVHW FRQGLWLRQV LQ ORDGLQJ RU environmental severity are avoidable. ,IWKHGHSWKRIWKHOLPLWLQJIODZVL]HLVUH-categorized as a through-wall thickness crack, the conditions for an acceptable leak before break (LBB) criteria should be satisfied.

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Lecture number 19 a, Page 26 of 33

STEP 7 – At the next inspection, establish the actual crack growth rate, and re-evaluate the new flaw conditions per procedures of this section. Alternatively, repair or replace the component or apply effective mitigation measures.

8. Leak Before Break Analysis In certain cases, it may be possible to show that a flaw can grow through the wall of a component without causing a catastrophic failure. In such cases, a leak can be detected (taking into consideration the contained fluid and type of insulation) and remedial action could be initiated to avoid a component failure. This type of examination is called a LeakBefore-Break (LBB) analysis. The LBB methodology may be useful to determine an upper bound for a part-through flaw that is growing at an unknown rate; although the remaining life cannot be determined, detection of a leak can serve as an early warning. A leak-before-break analysis begins by re-categorizing the flaw as through-wall, and evaluating the new geometry. If the postulated through-wall flaw is acceptable, the existing flaw can be left in service as long as it does not grow through the wall. Limitations Of LBB – There are limitations of the leak-before-break methodology. This approach should not be applied to certain situations that are outlined below. a) The leak should be readily detectable. The LBB approach may not be appropriate if the affected area is covered by insulation, or if the cracking mechanism produces very tight cracks that do not produce leaks when they grow through the wall. The ability to detect a leak may also be influenced by the contained fluid (e.g. liquid or gas). b) The LBB methodology may not be suitable for flaws near stress concentrations or regions of high residual stress. The pitfalls of LBB in these situations are illustrated in Figure 6. When the stresses are higher on the surface than in the interior of the wall, the flaw may grow faster in the surface direction than in the depth direction. In some cases, the flaw can grow virtually around the entire circumference of the vessel before advancing in the depth direction. Therefore, LBB should not be applied to non-post weld heat treated cylindrical shell components with cracks in a circumferential weld joint (e.g. girth seams and head to-shell junctions) or shell-to-nozzle junctions with circumferential cracks unless it can be shown that the stress distribution will not promote accelerated crack growth at the surface.

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Lecture number 19 a, Page 27 of 33

Flaw at a stress concentration

Flaw subjected to high residual stresses

Flaw growth in predominantly length direction Figure 6: Leak before break, difficult cases

c) The LBB approach should not be applied when the crack growth rate could potentially be high. When a leak occurs, adequate time must be available to discover the leak and take the necessary action. This consideration is particularly important when the component is subject to pneumatic pressure. d) The possible adverse consequences of developing a leak must be considered, especially when the component contains hazardous materials, fluids operating below their boiling point, and fluids operating above their auto-ignition temperature. Pressurized components that contain gas at high pressure can experience pneumatic loading or other dynamic effects at the crack tip making LBB impractical. Pressurized components that contain light hydrocarbon Lectures on Welding, NDE and Integrity Assessment

Lecture number 19 a, Page 28 of 33

liquids, or other liquids with a low boiling point, can experience auto refrigeration that also make LBB impractical. LBB Procedure – The procedure for assuring that a leak before break criteria is satisfied is shown below. STEP 1 –Demonstrate that the largest initial flaw size left in the structure will not lead to fracture during the life of the component. STEP 2 –Determine the largest (critical) crack length of a full through-wall crack below which catastrophic rupture will not occur for all applicable load cases. STEP 3 – Compute the corresponding leak areas associated with the critical crack lengths determined in Steps 1 and 2. STEP 4 – Determine the leakage rate associated with the crack area computed in STEP 3, and demonstrate that the associated leaks are detectable with the selected leak detection system.

9. Remediation A FFS analysis provides the remaining life of a component containing a flaw so that operation can be assured until the next scheduled inspection. The remaining life of crack-like flaws can only be determined if information about the crack growth rate in the service environment is known. Typically, this information is not readily available or established for many of the process environments that occur in the refining and petrochemical industry. Therefore, a combination of analytical techniques, in-service monitoring, and remediation methods may be used to provide assurance that a component can be operated until the next scheduled inspection. Remediation measures for crack-like flaws generally fall into one of the categories shown below. One or a combination of these methods may be employed. Remediation Method 1 – Removal or repair of the crack. The crack may be removed by blend grinding. The resulting groove is then repaired using a technique to restore the full thickness of material and the weld repair is subject to PWHT in accordance with the in-service inspection code. Alternatively, repair of the groove is not required if the requirements of FFS assessment procedures in Section 5 of API 579 are satisfied. Remediation Method 2 – Use of a crack arresting detail or device. For components that are not a pressure boundary, the simplest form of this method is to drill holes at the end of an existing crack to effectively reduce the crack driving force. For pressurized components, a device can be added to the component to control unstable crack growth. Remediation Method 3 – Performing physical changes to the process stream. This method can be used to reduce the crack driving force (reduction in pressure) or to provide an increase in the material toughness at the condition associated with highest stress state. This may involve the introduction of a warm start-up and/or shutdown cycle into equipment operating procedures such that the temperature of the component is high enough to ensure adequate material toughness at load levels associated with the highest state of stress.

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Lecture number 19 a, Page 29 of 33

Remediation Method 4 – Application of solid barrier linings or coatings to keep the environment isolated from the base metal. In this method, the flaw is isolated from the process environment to minimize the potential for environmentally assisted sub-critical crack growth. Remediation Method 5 – Injection of water and/or chemicals on a continuous basis to modify the environment or the surface of the metal. In this method, the process environment is controlled to minimize the potential for environmentally assisted sub-critical crack growth. Remediation Method 6 – Application of weld overlay. In this method, weld overlay is applied to the component surface opposite to the surface containing the cracks to introduce a compressive residual stress field at the location of the crack. The compressive residual stress field should eliminate any future crack growth. This type of repair also increases the structural integrity of the component containing the flaw by the addition of extra wall thickness provided by the weld overlay. Remediation Method 7 – Use of leak monitoring and leak-sealing devices.

10. In-Service Monitoring In all cases where sub-critical in-service crack growth is permitted by the methods of API 579 section 9, in-service monitoring or monitoring at a shutdown inspection, as applicable, of the crack growth by NDE is required. The applicable NDE method will depend on the specific case. Before returning the component to service, the monitoring method should be validated to ensure that it could adequately detect the size of the flaw under service conditions. The NDE sensitivity and flaw sizing uncertainty associated with the in-service monitoring procedure should be taken into account when specifying a limiting maximum flaw size for continued operation.

11. Documentation A Fitness-For-Service analysis should be sufficiently documented such that the analysis can be repeated later. This information should be permanently stored with the equipment record files. The following information should be documented for a structural integrity assessment carried out according to the procedures of this section. a) Assessment Level – Any deviations or modifications used at a given level of analysis (Levels 1, 2 and 3). b) Loading Conditions – calculation, categorization and stress analysis tools used should be documented. c) Material Properties – The material specification of the component containing the flaw; yield stress, ultimate tensile stress, and fracture toughness at the temperature of interest (including whether the data was obtained by direct testing or indirect means and the source and validity of data); and a description of the process environment including its effect on material properties (Levels 1, 2 and 3).

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Lecture number 19 a, Page 30 of 33

d) Characterization Of Flaw – The flaw location, shape and size; NDE method used for flaw sizing and allowance for sizing errors; and whether re-characterization of the flaw was required (Levels 1, 2 and 3). e) Partial Safety Factors – A list of the Partial Safety Factors used in the analysis; in a Level 3 assessment, a technical summary should be provided if alternative factors are utilized in the assessment (Level 2 and 3). f) Reference Stress Solution – The source of the reference stress solutions (e.g. handbook solution or finite element analysis) used in the assessment including whether the local and/or global collapse was considered (Levels 2 and 3). g) Stress Intensity Factor Solution – The source of stress intensity factor solutions (e.g. handbook solution or finite-element analysis) used in the assessment (Levels 2 and 3). h) Failure Assessment Diagram – whether the Level 2 recommended curve, a material specific curve (including the source and validity of stress-strain data), or a curve derived from J-analysis is used in the assessment (Level 3). i) Flaw Growth – whether any allowance is made for crack extension by sub-critical crack growth mechanism (e.g. fatigue or stress corrosion cracking); the crack growth laws and associated constants utilized (from technical publication or laboratory measurements) should be summarized (Level 3). j) In-Service Margins – The results calculated for each loading condition of interest and for each category of analysis undertaken; assessment points should be displayed on the appropriate failure assessment diagram (Level 3). k) Sensitivity Analysis – A listing of the input parameters used to perform sensitivity studies (e.g. load, material properties, flaw size, etc.); the results of each individual study should be summarized (Level 3). All conservative assumptions used in the assessment procedures should be documented. In addition, all departures from the procedures in this section should be reported and separately justified. A separate statement should be made about the significance of potential failure mechanisms remote from the defective areas, if applicable. If an in-service monitoring system is instituted because of the potential for sub-critical crack growth or a leak detection system is installed as the result of a LBB assessment, then the following documentation should be kept with the equipment files: • • • • •

6SHFLILFDWLRQIRUWKHV\VWHP 3URFHGXUHVIRULQVWDOODWLRQRIWKHV\VWHP 6\VWHPYDOLGDWLRQDQGFDOLEUDWLRQ 3URFHGXUHVIRUUHFRUGLQJGDWD $OOGDWDUHDGLQJVZKLOHWKHFRPSRQHQWLVLQ-service readings

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Lecture number 19 a, Page 31 of 33

12. Example Calculation A sample case study is presented next. This problem is taken from Reference [3]. A plate of SA 516 Grade 70 steel has an edge crack with a depth (a) = 0.50 in. The plate thickness (B) = 1.25 in., and the plate width (W) = 5.00 in. It is in service at a temperature (T) = 100°F and is subject to a maximum axial load (F) = 240 kips. No material properties data are available. There are no bending loads and no cyclic loads or environmental exposure (non-growing crack). The problem is to determine if the flaw is acceptable and the maximum safe applied axial load by Level 2 assessment. From ASME Code Section II.D, the minimum yield strength of the plate material is 38 ksi. The minimum fracture toughness (KIc) is estimated using methods given in API RP 579. From Table 3.3 of API RP 579, Curve B applies to this steel. From Figure 3.3 of API RP 579 (ASME Code Section VIII, Division 1, Paragraph UCS-66), the reference temperature (Tref) = 40°F for this case. Then, from Paragraph F.4.4.1.c of API RP 579, an estimate of KIc is computed as follows:

Next, determine the reference stress ref, using Equation D.27 of API RP 579. The primary membrane stress (Pm) = 240/(1.25 x 5.00), and the primary bending stress (Pb) = 0. It follows that

From Equation D.28 of API RP 579,

D:WKXV

The stress intensity factor (KI) is

Where:

For a = 0.5, W = 5, and Pm = 240 kips,

Finally, determine Lr and Kr as follows:

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Lecture number 19 a, Page 32 of 33

When these results are plotted, the evaluation point falls above the Level 2-B FAD as shown by the (•) symbol in Figure 7. Thus, the crack-like flaw is NOT acceptable.

Example of Level 2 FAD

0.8 0.7

(1.12, 0.559)

Load = 171 kips

0.6

Kr

0.5 0.4 0.3 0.2 0.1 0 0

0.2

0.4

0.6

0.8

1

1.2

Lr

Figure 7: Example Level 2-B FAD

The acceptable load is calculated for a Lr value of 0.8. This comes out to be 171 kips (♦).

13. References 1. API. Recommended practice for fitness-for-service. API 579. Washington, DC: American Petroleum Institute, 2000. 2. T. L. Anderson, 1995, ‘‘Fracture Mechanics: Fundamentals and Applications,’’ 2nd edition, CRC Press, Boca Raton, FL. 3. Carl E. Jaske, “Process Equipment Fitness-for-Service Assessments Using API RP 579”, Process & Power Plant Reliability Conference, Clarion Technical Conferences, November 78, 2001. http://www.clarion.org

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