2. History and Background
API RP 571 Fitness-For-Service (January
2000)
reliable assessment of the structural integrity of
equipment for the refining and petrochemical
industry
to be used in conjunction with the API existing
codes for pressure vessels, piping and
aboveground storage tanks (API 510, API
570, API 653)
API & ASME
joint committee was formed in 2001
enhance the range to process, manufacturing
and power generation industries
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3. Introduction
API/ASME Construction Codes
The construction codes & standards do not provide rules
to evaluate equipment that degrades while in-service
and deficiencies due to degradation that may be found
during the service
Fitness-For-Service (FFS)
Quantitative engineering evaluations that are performed to
demonstrate the structural integrity of an in-service
component that may contain a flaw or damage
API 579-1/ASME FFS-1
This standard provides guidance for conducting FFS
assessments using methodologies specially prepared for
pressurized equipment
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4. Scope
The methods and procedures in this standard
are intended to supplement and augment the
requirements in API 510, API 570, API 653 and
other post construction codes that reference
FFS
The reference procedure in this standard can be used for
FFS assessments and/or re-rating of equipment designed
and constructed to the following codes;
ASME B&PV Code, Section VIII, Division 1
ASME B&PV Code, Section VIII, Division 2
ASME B&PV Code, Section I
ASME B31.1 Piping Code
ASME B31.3 Piping Code
API 650
API 620
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5. Scope
The assessment procedures in this standard may also
be applied to pressure containing equipment
constructed to other recognized codes &
standards, including international and internal
corporate standards
The FFS assessment procedures in this standard cover
both the present integrity of the component given a
current state of damage and the projected remaining
life
Analytical procedures, material properties including
environmental effects, NDE guidelines and
documentation requirements are included in this
standard
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6. Scope
The FFS assessment procedures in this
standard can be used to evaluate flaws
commonly encountered in pressure
vessel, piping and storage tanks
The procedures are not intended to provide
a definitive guideline for every possible
situation. However, flexibility is provided to
form an advanced assessment level to
handle uncommon situations
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7. Outcome
If the results of FFS assessment indicate that the
equipment is suitable for the current operating
conditions, then the equipment can continue to be
operated at these conditions provided
monitoring/inspection programmes are
established, otherwise the equipment is re-rated.
The re-rating of equipment is done by finding a
reduced Maximum Allowable Working Pressure
(MWAP) and/or coincident temperature for
pressurized components and reduced Maximum Fill
Height (MFH) for tank components
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8. Organization
The FFS assessment procedures in
this standard are organized by the aw
type and/or damage mechanism
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12. General FFS Assessment Procedure
If the damage mechanism cannot be identified, then a FFS
assessment should not be performed per API 579
Identification of damage mechanism is the key component in
the FFS assessment
Firm understanding of the damage mechanism is required to
evaluate the time-dependence of the damage
Time-dependence of damage is required to develop a
remaining life and inspection plan
API 579 provides guidance for conducting FFS assessments
using methods specifically prepared for equipment in the
refining and petrochemical industry; however, this
document is currently being used in other industries such as
the fossil utility, pulp & paper, food processing, and non-
commercial nuclear
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13. General FFS Assessment Procedure
General FFS assessment procedure used in API 579
for all flaw types is provided in Section 2 that includes
the following steps:
Step 1 - Flaw & damage mechanism identification
Step 2 - Applicability & limitations of FFS procedures
Step 3 - Data requirements
Step 4 - Assessment techniques & acceptance criteria
Step 5 - Remaining life evaluation
Step 6 - Remediation
Step 7 - In-service monitoring
Step 8 - Documentation
Some of the steps shown above may not be necessary
depending on the application and damage mechanism
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14. Assessment Levels
Level 1
The assessment procedures included in
this level are intended to provide
conservative screening criteria that can
be utilized with a minimum amount of
inspection or component information. A
Level 1 assessment may be performed
either by plant inspection or engineering
personnel.
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15. Assessment Levels
Level 2
The assessment procedures included in this level
are intended to provide a more detailed
evaluation that produces results that are more
precise than those from a Level 1 assessment.
In a Level 2 Assessment, inspection information
similar to that required for a Level 1 assessment
are needed; however, more detailed calculations
are used in the evaluation. Level 2 assessments
would typically be conducted by plant
engineers, or engineering specialists
experienced and knowledgeable in performing
FFS assessments.
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16. Assessment Levels
Level 3
The assessment procedures included in this level
are intended to provide the most detailed
evaluation which produces results that are more
precise than those from a Level 2 assessment.
In a Level 3 Assessment the most detailed
inspection and component information is
typically required, and the recommended
analysis is based on numerical techniques such
as the finite element method or experimental
techniques when appropriate. A Level 3
assessment is primarily intended for use by
engineering specialists experienced and
knowledgeable in performing FFS assessments.
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17. Acceptance Criteria
Allowable Stress
This acceptance criterion is based upon calculation of
stresses resulting from different loading
conditions, classification and superposition of stress
results, and comparison of the calculated stresses in
an assigned category or class to an allowable stress
value. The allowable stress value is typically
established as a fraction of yield, tensile or rupture
stress at room and the service temperature, and this
fraction can be associated with a design margin. This
acceptance criteria method is currently utilized in
most new construction design codes. In FFS
applications, this method has limited applicability
because of the difficulty in establishing suitable stress
classifications for components containing flaws.
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18. Acceptance Criteria
Remaining Strength Factor
(RSF)
Based on the concepts of
elastic plastic fracture
mechanics.
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19. Failure Assessment Diagram (FAD)
The FAD is used for the evaluation of crack like flaws in
components.
In a FFS analysis of crack-like flaws, the results from a stress
analysis, stress intensity factor and limit load solutions, the
material strength, and fracture toughness are combined to
calculate a toughness ratio, Kr , and load ratio, Lr . These two
quantities represent the coordinates of a point that is plotted
on a two dimensional FAD to determine acceptability. If the
assessment point is on or below the FAD curve, the component
is suitable for continued operation.
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21. Remaining Life Assessment
Once it has been established that the component
containing the flaw is acceptable at the current
time, the user should determine a remaining life for
the component. The remaining life in this Standard is
used to establish an appropriate inspection
interval, an in-service monitoring plan, or the need for
remediation. The remaining life is not intended to
provide a precise estimate of the actual time to
failure. Therefore, the remaining life can be estimated
based on the quality of available
information, assessment level, and appropriate
assumptions to provide an adequate safety factor for
operation until the next scheduled inspection.
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22. Part 10: Assessment of Components
Operating in the Creep Range
Provides assessment procedures for
pressurized components operating in the
creep range
Assessment procedures for determining a
remaining life are provided for components
with and without a crack-like flaw subject to
steady state and/or cyclic operating
conditions
The procedures in this Part can be used to
qualify a component for continued operation
or for re-rating
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23. PART 10: Level 1 Assessment –
Applicability and Limitations
Level 1 Assessment procedures apply only if
the following conditions are satisfied
Component has been constructed to a recognized
code or standard
Component has not been subject to fire damage
or another overheating event that has resulted in
a significant change in shape such as sagging or
bulging, or excessive metal loss from scaling
The material meets or exceeds the respective
minimum hardness and carbon content
limitations.
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25. PART 10: Level 1 Assessment –
Applicability and Limitations
The component does not contain:
An LTA or groove like flaw
Pitting Damage
Blister, HIC or SOHIC damage
Weld misalignment, out of roundness, or bulge that
exceed the original design code tolerances,
A dent or dent-gouge combination,
A crack-like flaw, or
Microstructural abnormality such as graphitization
or hydrogen attack.
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26. PART 10: Level 2 Assessment –
Applicability and Limitations
The Level 2 assessment procedures in this Part apply
only if all of the following conditions are satisfied:
Component has been constructed to a recognized code
or standard
A history of the operating conditions and
documentation of future operating conditions for the
component are available.
The component has been subject to less than or
equal to 50 cycles of operation including startup and
shutdown conditions, or less than that specified in
the original design.
The component does not contain any of the flaws
listed as in level 1 assessment requirements.
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27. PART 10: Level 3 Assessment –
Applicability and Limitations
A Level 3 Assessment should be performed when the
Level 1 and 2 methods cannot be applied due to
applicability and limitations of the procedure or when the
results obtained indicate that the component is not
suitable for continued service. Conditions that typically
require a Level 3 Assessment include the following.
Advanced stress analysis techniques are required to
define the state of stress because of complicated
geometry and/or loading conditions.
The component is subject to cyclic operation.
The component contains a flaw listed as in level 1
assessment requirements. A detailed assessment
procedure is provided for a crack-like flaw;
however, this procedure cannot be used to evaluate
crack-like flaws that are caused by stress
corrosion, oxide wedging, or similar environmental
phenomena.
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28. PART 10: Level 3 Assessment –
Applicability and Limitations
The Level 3 Assessment procedures, with the exception
of the procedure for the evaluation of dissimilar metal
welds, can be used to evaluate components that contain
the flaw types listed as in level 1 assessment
requirements. A separate procedure is provided to
evaluate components with crack-like flaws.
The assessment procedure provided for dissimilar metal
welds is only applicable to 2.25Cr – 1Mo to austenitic
stainless steel welds made with stainless steel or nickel-
based filler metals. An alternative assessment procedure for
this material and other materials that are not currently
covered may be used.
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29. Assessment Techniques and
Acceptance criteria
Level 1 Assessments are based on a comparison with specified
time-temperature-stress limits and a simplified creep damage
calculation for components subject to multiple operating
conditions (i.e. temperature and applied stress combinations). In
addition, a check on material properties in terms of hardness or
carbon content and a visual examination of the component is
made in order to evaluate the potential for creep damage based
on component distortion and material characteristics such as
discoloration or scaling.
Level 2 Assessments can be used for components operating in
the creep regime that satisfy the requirements for applicability.
The stress analysis for the assessment may be based on closed
form stress solutions, reference stress solutions, or solutions
obtained from finite element analysis.
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31. Assessment Techniques and
Acceptance criteria
Level 3 Assessments can be used to evaluate those
cases that do not meet the requirements of Level 1
or Level 2 assessments. A detailed stress analysis
is required to evaluate creep damage, creep-
fatigue damage, creep crack growth, and creep
buckling. In addition, a separate procedure is
provided to perform a creep-fatigue assessment of
a dissimilar-weld joint.
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32. Level -1 Assessment Procedure
The Level 1 assessment for a component subject to a single
design or operating condition in the creep range is provided
below.
STEP 1 – Determine the maximum operating
temperature, pressure, and service time the component is
exposed to. If the component contains a weld joint that is
loaded in the stress direction that governs the minimum
required wall thickness calculation, then 14ºC (25ºF) shall be
added to the maximum operating temperature to determine
the assessment temperature. Otherwise, the assessment
temperature is the maximum operating temperature. The
service time shall include past and future planned operation.
STEP 2 – Determine the nominal stress of the component for
the operating condition defined in STEP 1. The computed
nominal stress shall include the effects of service-induced wall
thinning.
STEP 3 – Determine the material of construction for the
component and find the figure with the screening and damage
curves to be used for the Level 1 assessment.
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33. Level -1 Assessment
STEP 4 – Determine the maximum permissible time for
operation based on the screening curve obtained from STEP
3, the nominal stress from STEP 2, and the assessment
temperature from STEP 1. If the time determined from the
screening curve exceeds the service time for the component
from STEP 1, then the component is acceptable per the
Level 1 Assessment procedure. Otherwise, go to STEP 5.
STEP 5 – Determine the creep damage rate, Rc and
associated creep damage Dc for the operating condition
defined in STEP 1 using the damage curve obtained from
STEP 3, the nominal stress from STEP 2, and the
assessment temperature from STEP 1. The creep damage
for this operating condition shall be computed using
Equation given below where the service exposure time is
determined from STEP 1.
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34. Level -1 Assessment
STEP 6 – If the total creep damage determined from STEP 5
satisfies Equation given below
then the component is acceptable per the Level 1
Assessment procedure. Otherwise, the component is not
acceptable and following requirements shall be followed.
Rerate, repair, replace, or retire the component.
Adjust the future operating conditions, the corrosion
allowance, or both; note that this does not apply if
based on the current operating time.
Conduct a Level 2 or a Level 3 Assessment.
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35. Level 2 Assessment
The creep damage based upon the results
of a stress analysis is computed as follows:
STEP 1 – Determine a load history based on
past operation and future planned
operation. The load histogram should
include all significant operating loads and
events that are applied to the component.
If there is cyclic operation, the load
histogram should be divided into operating
cycles as shown in Figure 1. Define K as the
total number of operating cycles.
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43. Level 3 Assessment
The Level 3 assessment procedures provide a means to evaluate
the remaining life of a component using advanced stress analysis
techniques. If the flaw is volumetric (i.e. LTA, pitting damage, weld
misalignment, out-of-roundness, bugle, dent, or dent-gouge
combination), then the stress analysis model used to evaluate the
remaining life must include the flaw so that that localized stresses
and strains are accounted for. These stress results are then
directly used in the assessment. If the component contains a
crack-like flaw, then the stress analysis used for remaining life can
be based on an un-cracked body analysis. The effects of the crack
are accounted for in the assessment procedure.
As in the case for the Level 2 assessment, the predominant failure
mode for components operating in the creep regime is creep
rupture. If the component is subject to cyclic operation, then the
effect of creep-fatigue interaction needs to be evaluated. Both of
these damage mechanisms involve a time-based failure mode;
therefore, a remaining life needs to be evaluated as part of the
assessment.
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44. Corrosion Assessment
Following three parts of API/ASME 579-1
address corrosion
Part 4 – Assessment of General Metal Loss.
Part 5 – Assessment of Local Metal Loss.
Part 6 – Assessment of Pitting Corrosion.
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45. Corrosion Assessment
Part 5 is usually less conservative than Part 4
because the former accounts for the finite extent of
the metal loss
The assessment in Part 4 assumes that the metal loss
is over the entire component.
The two assessments give similar answers when the
metal loss extends over long distances.
Both the Part 4 and Part 5 assessments use the RSF
concept to evaluate wall thinning.
Inspection data for local and general metal loss
assessments typically consists of wall thickness
readings in a grid pattern.
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46. Corrosion Assessment
The pitting corrosion assessment entails
computing an RSF that depends on the
diameter, depth, and spacing of pits.
In the Level 1 assessment, the RSF is
estimated by visually comparing pitting
charts with the observed pitting.
The Level 2 assessment requires
measurement of pit dimensions and
spacing and includes a series of
calculations to estimate the RSF.
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47. Part 4: General Metal Loss
Required Data/Measurements for a FFS
Assessment.
Thickness readings are required on the component
where the metal loss has occurred to evaluate the
general metal loss.
Two options for obtaining thickness data:
Point Thickness Readings: point thickness
readings can be used to characterize a metal loss
in a component if there are no significant
differences in the thickness reading values
obtained at thickness monitoring locations.
Thickness Profiles: thickness profiles should be
used to characterize metal loss in a component if
there is a significant variation in the thickness
readings.
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50. Part 5: Local Metal Loss
The type of flaws that are
characterized as local metal loss are
defined as follows
Local Thin Area (LTA) – local metal
loss on the surface of the
component; the length of a region of
metal loss is the same order of
magnitude as the width.
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51. Part 5: Local Metal Loss
Groove-Like Flaw – the following flaws are included in this
category; a sharp radius may be present at the base of a
groove-like flaw.
Groove – local elongated thin spot caused by directional
erosion or corrosion; the length of the metal loss is
significantly greater than the width.
Gouge – elongated local mechanical removal and/or
relocation of material from the surface of a
component, causing a reduction in wall thickness at the
defect; the length of a gouge is much greater than the
width and the material may have been cold worked in the
formation of the flaw. Gouges are typically caused by
mechanical damage, for example, denting and gouging of a
section of pipe by mechanical equipment during the
excavation of a pipeline. Gouges are frequently associated
with dents due to the nature of mechanical damage. If a
gouge is present, the assessment procedures of Part 12
shall be used.
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54. Part 6: Pitting
The assessment procedures is used to evaluate metal
loss from pitting corrosion
Pitting is defined as localized regions of metal loss
which can be characterized by a pit diameter on the
order of the plate thickness or less, and a pit depth
that is less than the plate thickness
Assessment procedures can be used to evaluate four
types of pitting
widely scattered pitting that occurs over a significant region of
the component
A local thin area (LTA) located in a region of widely scattered
pitting
localized regions of pitting, and
Localized Pitting confined within a region of a LTA.
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55. Part 6: Pitting
Pitting Charts
FFS by visually
comparing pit
chart to actual
damage plus
estimate of
maximum pit
depth
Pitting Chart – API 579
Grade 4 Pitting
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56. Part 6: Pitting
Pitting Charts
Pit charts provided for
a different pitting
damages measured
as a percentage of
the affected area in a
6 inch by 6 inch
RSF provided for each
pit density and four
w/t ratios
(0.2, 0.4, 0.6, 0.8)
Pitting Chart – API 579
Grade 4 Pitting
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57. Part 9: Crack-Like Flaws
Crack-like flaws are planar flaws which are
predominantly characterized by a length and depth, with
a sharp root radius, the types of crack-like flaws are
Surface breaking
Embedded
Through-wall
In some cases, it is conservative and advisable to treat
volumetric flaws such as aligned porosity or
inclusions, deep undercuts, root undercuts, and overlaps
as planar flaws, particularly when such volumetric flaws
may contain microcracks at the root
Grooves and gouges with a sharp root radius are
evaluated using Section 9, criteria for the root radius is
in Section 5
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58. Part 9: Crack-Like Flaws
The assessment procedures in Part 9 are based on a
fracture mechanics approach considering the entire
range of material behavior
Brittle fracture
Elastic/plastic fracture
Plastic collapse
Information required to perform an assessment is
provided in Part 9 and the following Appendices
Appendix C - Stress Intensity Factor Solutions
Appendix D - Reference Stress Solutions
Appendix E - Residual Stress Solutions
Appendix F - Material Properties
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