API RP 581 - 3rd Ed.2016 - Add.2-2020 - Risk-Based Inspection Methodology
2-108 API RECOMMENDED PRACTICE 58116.6.2 Inspection EffectivenessInspections are ranked according to their expected effectiveness at detecting the specific damagemechanism. Examples of inspection activities that are both intrusive (requires entry into the equipment) andnonintrusive (can be performed externally) are provided in Annex 2.C, Table 2.C.10.3.The number and category of the highest effective inspection will be used to determine the DF.16.6.3 Calculation of the DFThe following procedure may be used to determine the DF for CUI; see Figure 16.1.a) STEP 1—Determine the furnished thickness, t, and age, age, for the component from the installationdate.b) STEP 2— If the corrosion rate determined based on inspection history or assigned by a knowledgeablespecialist use the assigned value and skip to STEP 5.c) STEP 3—Determine the base corrosion rate, C rB , based on the driver and operating temperature usingTable 16.2.Corrosion rates can be higer than predicted in cyclic or intermittent service. Table 16.2 can be used tohelp estimate a more representative corrosion rate; with adjustments made for factors such as:e) The complete temperature range the equipment will see, including idle or out of servicef) Time spent at each temperature rangeg) Wet/dry cycling if going through the dew point rangeh) Potential for higher concentration of contaminantsi) Frequency of temperature cyclesd) STEP 4—Calculate the final corrosion rate using Equation (2.46).Cr = CrB ⋅FINS ⋅FCM ⋅FIC ⋅max⎡F EQ , F ⎤⎣ IF ⎦(2.46)The adjustment factors are determined as follows.1) Adjustment for insulation type; F INS , based on Table 16.3.2) Adjustment for Complexity, F CM —Established based on the following criteria.— If the complexity is Below Average, then F CM = 0.75.— If the complexity is Average, then F CM = 1.0.— If the complexity is Above Average, then F CM = 1.25.3) Adjustment for Insulation Condition, F IC —Established based on the following criteria.— If the insulation condition is Below Average, then F IC = 1.25.— If the insulation condition is Average, then F IC = 1.0.— If the insulation condition is Above Average, then F IC = 0.75.4) Adjustment for Equipment Design or Fabrication, F EQ —If equipment has a design that allows waterto pool and increase metal loss rates, such as piping supported directly on beams, vessel external
RISK-BASED INSPECTION METHODOLOGY, PART 2—PROBABILITY OF FAILURE METHODOLOGY 2-109stiffening rings or insulation supports, or other such configuration that does not allow water egressand/or does not allow for proper coating maintenance, then F EQ = 2; otherwise, F EQ = 1.5) Adjustment for Interface, F IF —If the piping has an interface where it enters either soil or water, thenF IF = 2; otherwise, F IF = 1.e) STEP 5—Determine the time in service, agetke, since the last known thickness, trde (see Section4.5.5). The trde is the starting thickness with respect to wall loss associated with external corrosion (seeSection 4.5.5). If no measured thickness is available, set trde = t and agetke = age. The measured wallloss from CUI, Le, may be used to calculate trde using Equation (2.47).trde= t−LeNOTEWhen using Equation (2.47), agetke is the time in service since Le was measured.(2.47)f) STEP 6—Determine the in-service time, agecoat, since the coating has been installed usingEquation (2.48).agecoat= Calculation Date − Coating Installation Date(2.48)g) STEP 7—Determine the expected coating age, Cage, based on coating type, quality of application andservice conditions. Cage should be 0 years for no coating or poorly applied coatings. Lower qualitycoatings will typically have a Cage of 5 years or less. High quality coatings or coatings in less harshexternal environments may have a Cage of 15 or more years. Cage may be adjusted based on anevaluation of the coating condition during a high-quality inspection.h) STEP 8—Determine the coating adjustment, Coatadj , using Equations (2.49) and (2.50).Ifagetke≥ agecoat:( )Coat = min C ,ageIfadj age coatagetke< agecoat:1. If the coating has failed at the time of inspection when agetke was established, thenCoat adj = 0(2.49)2. If the coating has not failed at the time of inspection when agetke was established, useEquation (2.36) to calculate Coatadj( ) ( )Coat = min C ,age −min C ,age −ageadj age coat age coat tke(2.50)i) STEP 9—Determine the in-service time, age, over which CUI may have occurred using Equation (2.51).age = age − Coattke adj(2.51)j) STEP 10—Determine the allowable stress, S, weld joint efficiency, E, and minimum required thickness,tmin, per the original construction code or API 579-1/ASME FFS-1 [10] . In cases where components areconstructed of uncommon shapes or where the component's minimum structural thickness, tc, maygovern, the user may use the tc in lieu of tmin where pressure does not govern the minimum requiredthickness criteria.k) STEP 11—Determine the Art parameter using Equation (2.52) based on the age and trde from STEP 5,Cr from STEP 3.ArtC⋅ ager= (2.52)trdel) STEP 12—Calculate the flow stress, FS CUIF , using E from STEP 10 and Equation (2.53).( + )CUIF YS TSFS == ⋅E ⋅ 11 .(2.53)2
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RISK-BASED INSPECTION METHODOLOGY, PART 2—PROBABILITY OF FAILURE METHODOLOGY 2-109
stiffening rings or insulation supports, or other such configuration that does not allow water egress
and/or does not allow for proper coating maintenance, then F EQ = 2; otherwise, F EQ = 1.
5) Adjustment for Interface, F IF —If the piping has an interface where it enters either soil or water, then
F IF = 2; otherwise, F IF = 1.
e) STEP 5—Determine the time in service, agetke, since the last known thickness, trde (see Section
4.5.5). The trde is the starting thickness with respect to wall loss associated with external corrosion (see
Section 4.5.5). If no measured thickness is available, set trde = t and agetke = age. The measured wall
loss from CUI, Le, may be used to calculate trde using Equation (2.47).
trde
= t−
Le
NOTE
When using Equation (2.47), agetke is the time in service since Le was measured.
(2.47)
f) STEP 6—Determine the in-service time, agecoat, since the coating has been installed using
Equation (2.48).
agecoat
= Calculation Date − Coating Installation Date
(2.48)
g) STEP 7—Determine the expected coating age, Cage, based on coating type, quality of application and
service conditions. Cage should be 0 years for no coating or poorly applied coatings. Lower quality
coatings will typically have a Cage of 5 years or less. High quality coatings or coatings in less harsh
external environments may have a Cage of 15 or more years. Cage may be adjusted based on an
evaluation of the coating condition during a high-quality inspection.
h) STEP 8—Determine the coating adjustment, Coatadj , using Equations (2.49) and (2.50).
If
agetke
≥ agecoat:
( )
Coat = min C ,age
If
adj age coat
agetke
< agecoat:
1. If the coating has failed at the time of inspection when agetke was established, then
Coat adj = 0
(2.49)
2. If the coating has not failed at the time of inspection when agetke was established, use
Equation (2.36) to calculate Coatadj
( ) ( )
Coat = min C ,age −min C ,age −age
adj age coat age coat tke
(2.50)
i) STEP 9—Determine the in-service time, age, over which CUI may have occurred using Equation (2.51).
age = age − Coat
tke adj
(2.51)
j) STEP 10—Determine the allowable stress, S, weld joint efficiency, E, and minimum required thickness,
tmin, per the original construction code or API 579-1/ASME FFS-1 [10] . In cases where components are
constructed of uncommon shapes or where the component's minimum structural thickness, tc, may
govern, the user may use the tc in lieu of tmin where pressure does not govern the minimum required
thickness criteria.
k) STEP 11—Determine the Art parameter using Equation (2.52) based on the age and trde from STEP 5,
Cr from STEP 3.
A
rt
C
⋅ age
r
= (2.52)
trde
l) STEP 12—Calculate the flow stress, FS CUIF , using E from STEP 10 and Equation (2.53).
( + )
CUIF YS TS
FS == ⋅E ⋅ 11 .
(2.53)
2
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