API 579 An Introduction to API RP 579: Section 9

Assessment of Crack Like Flaws

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• Classical engineering design – applied stress : material resistance

– component is defect-free

• Possible presence of defects – casting, welding, forming, develop during operation

• Fitness for Service (FFS) procedure – Determining the residual life of damaged plant

– Ensuring safe operation beyond design life

– Down-rating damaged plant below design

– Demonstrating tolerance to defects within a safety case

– Extending inspection intervals

– Reducing duration of outage and shutdown

Introduction

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Codes • API: American Petroleum Industry

• API Codes and Standards for: – design, fabrication, inspection and testing of new pressure

vessels, piping systems and storage tanks • do not address the fact that equipment degrades while in-

service

• deficiencies due to degradation or from original fabrication may be found during subsequent inspections.

• Can be applied to other industries

• API Codes – API 510: Pressure vessel inspection code

– API 570: Piping inspection code

– API 653: Tank inspection code

– API 580: Risk based inspection

• API 579

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API 579

• to ensure safety:plant personnel, public

• to provide sound FFS assessment procedures

• to ensure consistent remaining life predictions

• to enhance long-term economic viability

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API 579

• API's Recommended Practice 579 for FFS

• API 579 can be used to make run-repair-replace decisions

• The 1,000-page document is organized into modules

• Each section is based on a type of flaw or damage, such as crack-like flaws

• The document is primarily aimed at the petrochemical industry

• types of damage listed seen in petrochemical applications – they are present in other industries

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Overview of Damage Assessment Procedures

Section

1 Introduction and Scope

2 Outline of Overall Methodology

3 Brittle Fracture

4 General Metal Loss

5 Local Metal Loss

6 Pitting Corrosion

7 Blisters and Laminations

8 Weld Misalignments and Shell Distortions

9 Crack Like Flaws

10 High Temp. Operation and Creep

11 Fire Damage

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Methodology for All Damage Types

1 Flaw and damage mechanism identification

2 Applicability and limitations of the FFS assessment procedures

3 Data requirements

4 Assessment techniques and acceptance criteria

5 Remaining life evaluation

6 Remediation

7 In-service monitoring

8 Documentation

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Assessment Levels • Three levels of assessment for each flaw and

damage type – Level 1 to 3

• Assessment level – Conservatism

– Amount of information required

– Skill of the assessor

– Complexity of analysis

• Level 1 – NDE inspector

• Level 2 – Plant Engineer

• Level 3 – FFS Expert

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API 579 Section 9 - ASSESSMENT OF CRACK-LIKE FLAWS

• FFS for crack like flaws

• Based on Failure Assessment Diagram (FAD) method

• Crack like flaws observed from inspection: – planar flaws

– Length, depth, sharp root radius

– Conservative to treat volumetric flaws as cracks • Micro-cracks at root

• Relative flaw tolerance at design stage – Risk to fracture

– a/t = 25%, length = 6a

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Applicability and Limitations of the Procedure

• Level 1 and 2

– Original Design Criteria

– Operating temperature less than Creep range

– Dynamic Loading effects not significant

– No in-service crack growth

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Applicability and Limitations of the Procedure : Level 1

• Geometries – Flat plate, cylinder or sphere

– R/t > 5

– t < 38 mm

– Away from major structural discontinuity

• Loads – Only membrane stress field, within design limits

• Material – C-Steel with specified max. tensile prop. And

min. fracture properties

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Data Requirement

• Original Equipment Design Data

• Maintenance and equipment history

• Loads and stresses

• Material properties

• Flaw Characterization

• Recommendation for inspection techniques

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Flaw Characterization

• Simple geometry, amenable for fracture mechanics analysis

• Objective is to get a crack of conservative size in plane to maximum principal stress direction

• Cracks from inspection: – irregular in shape

– arbitrarily oriented

– multiple cracks

– branched cracks

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Flaw Characterization (Shape)

Through Wall Flaw

Surface Flaw

Embedded Flaw

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Flaw Characterization (length) when flaw is not normal to principal stress

direction

• Conservative Option

– Co (measured length), C (length used in calculations, normal to max. stresses)

– Take C = Co

• Equivalent flaw length

– Inclined cracks -> align itself perpendicular to the applied stress

– Mixed mode to Mode I crack

– Equivalent Mode I from energy considerations

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Flaw Characterization (Length)

0 1 2, ,c c f

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Flaw Characterization (depth)

• Depth difficult to measure

• A. Flaw depth by default values

– Through wall flaw, a = t,

– Surface flaw,

• B. Flaw depth from actual measurements

– Normal flaw, a=ao

min , length=2ca t c

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Flaw Characterization (Depth)

oa a W

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Flaw Characterization (Branch Crack)

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Flaw Characterization (Multiple Cracks)

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Level 1 Analysis • STEP 1 – Determine the load cases and temperatures:

operating and design conditions.

• STEP 2 – Determine the length and depth of the crack: characterize

• STEP 3 – Determine the case from the list below

o Flat Plate, Crack-Like Flaw Parallel To Joint

o Cylinder, Longitudinal Joint, Crack-Like Flaw Parallel To Joint

o Cylinder, Longitudinal Joint, Crack-Like Perpendicular To Joint

o Cylinder, Circumferential Joint, Crack-Like Flaw Parallel To Joint

o Cylinder, Circumferential Joint, Crack-Like Flaw Perpendicular To Joint

o Sphere, Circumferential Joint, Crack-Like Flaw Parallel To Joint

o Sphere, Circumferential Joint, Crack-Like Flaw Perpendicular To Joint

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Level 1 analysis

¼ t, flaw

t flaw

A – flaw in base metal.

B –flaw in weld metal that has been subject to PWHT.

C –flaw in weld metal that has not been subject to PWHT

Tref = use 38oC (material specific

can also be obtained from

Section 3)

At Tref +33o Cv = 68J, l.e. >.89mm

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Failure Assessment Diagram

' ref

r

ys

L

' Ir

mat

KK

K

r rK f L

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Advantages of FAD

• Double criteria approach:

– Fracture

• LEFM

• EPFM

– Collapse

• Elasto-Plastic Fracture Mechanics:

– J-Integral calculation not required

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Level 2 Analysis

• If the component does not meet the Level 1 Assessment requirements then a Level 2 or Level 3 Assessment can be done.

• Method A: Using partial safety factors

– Factor for applied loading

– Factor for material toughness

– Factor for flaw dimensions

– Based on probabilistic methods

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Level 2 Analysis 1– Evaluate operating conditions and determine the pressure, temperature and loading combinations to be evaluated.

2–Stress distributions at the location of the flaw. Classify Primary stress

Secondary stress

Residual stress

Appendix E of API 579 contains a compendium of residual stress distributions for various weld geometries

3 – Determine the material properties

yield strength

tensile strength

fracture toughness

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Level 2 Analysis • Appendix F of API 579 contains information

on material properties, including toughness

• Appendix does not contain a database of toughness values

• It provides correlations and estimation methods

• For ferritic steels, there are lower-bound correlations of toughness to Charpy transition temperature – From Sections III and XI of the ASME boiler and

pressure vessel code

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Level 2 Analysis API 579 endorses the use of the fracture toughness Master Curve, as implemented in ASTM Standard E 1921-97

4 – Determine the crack dimensions: characterize

5 – Modify the primary stress, material fracture toughness, and flaw size using the Partial Safety Factors ( PSF )

.

.

m m S

b b S

P P PSF

P P PSF

matmat

k

KK

PSF . aa a PSF

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Need for Partial safety Factors (PSF)

Consider a Design

R = L1 + L2 + L3

Let the factor of safety be 1.5

Thus:

R/(L1+L2+L3) = 1.5

1.5 to account for scatter in R, L

Probability of failure P(R < [L1+L2+L3])

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Estimating the Probability of failure

Let all the variables R, L1, L2, L3 follow a normal distribution.

Coeff. Of Var (/ m)

R 0.1

L1 0.1

L2 0.2

L3 0.3

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Reliability Index

The reliability index is given by

2

3

2

2

2

1

2

321

mmmm

R

R

Now we will try to estimate probability

of failure for different load combinations

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m1 m2 m3 Sm Pf

200 0 0 200 2.8x10-3

0 200 0 200 2.3x10-3

0 0 200 200 6.8x10-2

mR = 300 Global Factor of safety = 1.5 i.e. mR/Sm = 1.5

Need for safety factors (PSF) on each component of

load for consistent Reliability

R/f = f1.L1 + f2.L2 + f3.L3

PSF ensures guaranteed lower bound reliability

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Partial safety Factors Ductile Brittle

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Level 2 Analysis 6 – Compute the reference stress for primary stresses

–reference stress solutions: Appendix D

7 – Compute the Load Ratio

8 – Compute the stress intensity attributed to the primary loads

9 – Compute the reference stress for secondary and residual stresses (used for F)

10 – Compute the stress intensity attributed to the secondary and residual stresses

11 – Compute the plasticity interaction factor, F in presence of secondary loads

p

refp

r

y

L

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Level 2 Analysis 12 – Determine toughness ratio

13 – Evaluate results on FAD

P SR

I Ir

mat

K KK

K

F

2 6

(max)1 0.14 0.3 0.7exp 0.65 for P P P P

r r r r rK L L L L

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Level 2 Analysis

If Partial safety Factors are not used

0

0.7

0 0.2 0.4 0.6 0.8 1Lr

Kr

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Residual Stress Profiles

• Listed in Appendix E of API 579 Section 9

• Residual stress distributions are provided for the following weld joint configurations – Full Penetration Welds in Piping and Pressure Vessel

Cylindrical Shells

– Full Penetration Welds in Spheres and Pressure Vessel Heads

– Full Penetration Welds in Storage Tanks

– Full Penetration and Fillet Welds at Corner Joints

– Fillet Welds at Tee Joints

– Repair Welds

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Residual stress profiles

• Based on upper bound values of the extensive numerical analyses and a literature survey of published results

• Residual stress distributions are provided for both the as-welded and PWHT conditions

• Distinction is not made concerning the material of construction – Weld joint geometry – Single V-Type – Double V-Type – Fillet welds – Repair welds

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Data required • The material specification • The material specified minimum yield strength • The wall thickness of the component • The heat input used to make the weld • The type of weld (i.e. girth or circumferential

joint, longitudinal seam, repair weld, or attachment weld)

• The weld joint configuration (i.e. single V-groove, double V-groove, corner joint, fillet weld, or repair weld)

• Procedures aimed at reducing the residual stress level – hydrotest to 150% of the maximum allowable

working pressure (MAWP)per the ASME Code, Section VIII,

– post weld heat treatment per the original construction code

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Level 3 Analysis Method A Assessment –Level 2 the FAD with user specified Partial Safety Factors based on a risk assessment

Method B Assessment – FAD is constructed based on the actual material properties

1 23

(max) for 0.02

1 for 0

P

r ysrefP P P

r r r rP

r ys ref

P P

r r r

LEK L L L

L E

K L L

1

ln 1

Where subscripts t = true, es = engineering

t es es

t es

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Level 3 Analysis Method C Assessment –FAD is constructed

based on the actual loading conditions, component geometry and material properties

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

elasticr

total

JK

J

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Level 3 Analysis

• 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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Remaining Life Assessment (RLA)

• Sub-critical Crack Growth

– Crack growth by fatigue

– Crack growth by stress corrosion cracking

– Crack growth by hydrogen assisted cracking

– Crack growth by corrosion fatigue

• Growth of a pre-existing crack is controlled by a crack tip stress intensity factor

• Laws for crack growth rates for these mechanisms have been provided in Appendix F

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Difficulties in RLA

• Crack growth rates can be highly sensitive to changes in the process environment

– Models are fitted in carefully controlled conditions in a laboratory experiment

• Cracking often occurs as the result of an upset in operating conditions

– Average crack growth rate would be meaningless in such instances

• New cracks can initiate at other locations in the structure

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Procedure for RLA 1 – Perform a Level 3 assessment for the initial crack size

If the component is acceptable apply remedial measures to prevent further crack growth

2 – If effective remedial measures are not possible and slow sub-critical crack growth is expected

If a crack growth law exists for the material and service environment: a crack growth analysis can be performed else, a leak-before break analysis should be performed

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Procedure for RLA

3 – Compute the stress at the flaw based on the future operating conditions

4 – Determine an increment in crack growth

5 – Perform a Level 3 assessment for the current crack size If the assessment point 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

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Procedure for RLA 6 – Determine the time or number of stress cycles for the current crack size (ao, co) to reach the limiting flaw

size Acceptable if time to reach the limiting flaw size,with FOS, is more than the required operating period

If the depth of the limiting flaw size is re-categorized as a through-wall thickness crack, the conditions for an acceptable leak before break (LBB) criteria should be

satisfied

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

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LBB Procedure

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

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Leak Before Break

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LBB Procedure Limitations

The leak should be readily detectable

Insulation

Tight crack

Contained fluid

The LBB methodology may not be suitable for flaws near stress concentrations or regions of high residual stress

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LBB Limitations

Flaw at a stress concentration

Flaw subjected to high

residual stresses

Flaw growth in

predominantly length

direction

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LBB Limitations

Crack growth rate high

Adequate time must be available to discover the

leak and take the necessary action

Possible adverse consequences of developing a leak

hazardous materials

fluids operating below their boiling point

fluids operating above their auto-ignition temperature

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LBB Procedure 1 –Demonstrate that the largest initial flaw size left in

the structure will not lead to fracture during the life of the component.

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.

3 – Compute the corresponding leak areas associated with the critical crack lengths

4 – Determine the leakage rate associated with the crack area computed above, and demonstrate that the associated leaks are detectable with the selected leak detection system

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Remediation • Method 1 – Removal or repair of the crack. The crack

may be removed by blend grinding

• Method 2 – Use of a crack arresting detail or device

• Method 3 – Performing physical changes to the process stream

• Method 4 – Application of solid barrier linings or coatings to keep the environment isolated from the base metal

• Method 5 – Injection of water and/or chemicals on a continuous basis to modify the environment or the surface of the metal

• Method 6 – Application of weld overlay

• Method 7 – Use of leak monitoring and leak-sealing devices

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In-service monitoring In all cases where sub-critical in-service crack growth is permitted

– in-service monitoring or

–monitoring at a shutdown inspection

of the crack growth by NDE is required. The applicable NDE method will depend

on the specific case.

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Example Calculation

• A plate of SA 516 Grade 70 steel

• Edge crack, depth ‘a’ = 0.5 inch

• Width of plate ‘W’ = 5 inch

• Thickness ‘B’ = 1.25 inch

• Service temp.’T’ = 100o F

• Axial Load ‘F’ = 240 kips

• Yield stress ‘Sy’ = 38 ksi

• Toughness not known

• Safe ? Using a Level 2 analysis

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Solution

• Kc, from Table 3.3 of API 579, Tref = 40o F

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Solution

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FAD

Example of Level 2 FAD

0

0.2

0.4

0.6

0.8

0 0.2 0.4 0.6 0.8 1 1.2

Lr

Kr

(1.12, 0.559)Load = 171 kips

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Thank You