-API-579.pdf

API 579

An Introduction to API RP 579:

Section 9

Assessment of Crack Like

Flaws

• 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

API 579

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 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

API 579

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

API 579

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

API 579

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

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

API 579

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

API 579

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

API 579

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

Data Requirement

• Original Equipment Design Data

• Maintenance and equipment

history

• Loads and stresses

• Material properties

• Flaw Characterization

• Recommendation for inspection

techniques

API 579

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

Flaw Characterization (Shape)

Through Wall Flaw

API 579

Flaw Characterization (length) when

flaw is not normal to principal stress

direction

• Conservative Option

– C_o (measured length), C (length used in calculations, normal to max. stresses) – Take C = C_o

• Equivalent flaw length

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

– Mixed mode to Mode I crack – Equivalent Mode I from energy

Flaw Characterization (Length)





0

1

,

2

,

API 579

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=a

_o

 

min ,

length=2c

Flaw Characterization (Depth)

 

o

API 579

API 579

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

API 579

Level 1 analysis

¼ t, flaw t flaw

A – flaw in base metal.

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

T_ref = use 38o_{C (material specific}

can also be obtained from Section 3)

API 579

Failure Assessment Diagram

' ref r ys

L







' _I r mat

K

K

K



 

r r

K



f L

Advantages of FAD

• Double criteria approach:

– Fracture

• LEFM • EPFM

– Collapse

• Elasto-Plastic Fracture Mechanics:

API 579

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

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

API 579

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

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   mat mat k

K

K

PSF



a



a PSF

.

_a

API 579

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

API 579

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

API 579

Reliability Index

The reliability index is given by





2

3

2

2

2

1

2

3

2

1









m

m

m

m

















R

R

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

Need for safety factors (PSF) on each component of load for consistent Reliability

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

API 579

Partial safety Factors

Ductile Brittle

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 ref p r y

L







API 579

Level 2 Analysis

12 – Determine toughness ratio

13 – Evaluate results on FAD

P SR I I r mat K K K K  F 

 









 





Level 2 Analysis

If Partial safety Factors are not used

0 0.7

0 0.2 0.4 Lr 0.6 0.8 1

API 579

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

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

API 579

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-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,

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 2 3 (max) for 0.0 2 1 for 0 P r ys ref P P P r r P r r r ys ref P P r r r L E K L L L L E K L L                   









1 ln 1 t es es t es         

API 579

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

elastic r total J K J 

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

API 579

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

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

API 579

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

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

API 579

Procedure for RLA

6 – Determine the time or number of stress cycles for the current crack size (a_o, c_o) 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

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

API 579

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

API 579

LBB Limitations

Flaw at a stress concentration

Flaw subjected to high residual stresses

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

API 579

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

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

API 579

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.

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’ = 100

o

_F

• Axial Load ‘F’ = 240 kips

• Yield stress ‘Sy’ = 38 ksi

• Toughness not known

API 579

Solution

• Kc, from Table 3.3 of API 579, Tref

API 579

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