API RP 581 - 3rd Ed.2016 - Add.2-2020 - Risk-Based Inspection Methodology
2-78 API RECOMMENDED PRACTICE 581If multiple inspections of a lower effectiveness have been conducted during the designated time period, theycan be equated to an equivalent higher effectiveness inspection in accordance with Section 3.4.3.12.6.3 Calculation of the DFThe following procedure may be used to determine the DF for ClSCC; see Figure 12.1.a) STEP 1—Determine the susceptibility for cracking using Figure 12.1 and Table 12.2. If cracking isconfirmed to be present, set the susceptibility to High and skip to STEP 4. If cracking is present and wasnot been removed, a FFS evaluation is recommended to ensure that the component is suitable forcontinued service.b) STEP 2— Determine the susceptibility using Table 12.3.c) STEP 3—Modify the base susceptibility in STEP 2 with the susceptibility modifier from Table 12.4.d) STEP 4—Determine the Severity Index, S VI , from Table 12.5 using susceptibility from STEP 1 (ifcracking is present) or STEP 3.e) STEP 5—Determine the time in service, age, since the last Level A, B, or C inspection was performedwith no cracking detected or cracking was repaired. Cracking detected but not repaired should beevaluated and future inspection recommendations based upon FFS evaluation.f) STEP 6—Determine the number of inspections and the corresponding inspection effectiveness categoryusing Section 12.6.2 for past inspections performed during the in-service time. Combine the inspectionsto the highest effectiveness performed using Section 3.4.3.g) STEP 7—Determine the base DF for ClSCC,ClSCCD fB, using Table 6.3 based on the number ofinspections and the highest inspection effectiveness determined in STEP 6 and the Severity Index, S VI ,from STEP 4.h) STEP 8—Calculate the escalation in the DF based on the time in service since the last inspection usingthe age from STEP 5 and Equation (2.29). In this equation, it is assumed that the probability for crackingwill increase with time since the last inspection as a result of increased exposure to upset conditionsand other non-normal conditions.ClSCC11 .( fB ( ) )ClSCCD f = min D ⋅ max[ age, 1. 0] , 5000 (2.29)12.7 Nomenclatureageis the component in-service time since the last cracking inspection or service start dateClSCCD fis the DF for ClSCCClSCCD fBis the base value of the DF for ClSCCS VIis the Severity Index
RISK-BASED INSPECTION METHODOLOGY, PART 2—PROBABILITY OF FAILURE METHODOLOGY 2-7912.8 ReferencesSee References [10], [58], [59], [60], [61], [62], [63], and [64] in Part 1, Section 2.2.12.9 TablesTable 12.1—Data Required for Determination of the DF—ClSCCRequired DataSusceptibility (Low, Medium, High)Cl − concentration of process water (ppm)Operating temperature, °C (°F)pH of process waterAge (years)Inspection effectiveness categoryNumber of inspectionsCommentsThe susceptibility is determined by expert advice or using theprocedures in this section.Determine the bulk Cl − concentration of the water phase. Ifunknown, the default value for ppm is >1000. Consider Cl −content of any water present in system (i.e. hydrotest, boilerfeed, steam). Also, consider the possibility of concentration ofCl − by evaporation or upset conditions.Determine the highest operating temperature expected duringoperation (consider normal and non-normal operatingconditions).Determine pH of the process water. High pH solutions withhigh chlorides generally are not as susceptible to cracking aslow pH solution with chlorides.Use inspection history to determine the time since the last SCCinspection.The effectiveness category that has been performed on thecomponent.The number of inspections in each effectiveness category thathave been performed.
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2-78 API RECOMMENDED PRACTICE 581
If multiple inspections of a lower effectiveness have been conducted during the designated time period, they
can be equated to an equivalent higher effectiveness inspection in accordance with Section 3.4.3.
12.6.3 Calculation of the DF
The following procedure may be used to determine the DF for ClSCC; see Figure 12.1.
a) STEP 1—Determine the susceptibility for cracking using Figure 12.1 and Table 12.2. If cracking is
confirmed to be present, set the susceptibility to High and skip to STEP 4. If cracking is present and was
not been removed, a FFS evaluation is recommended to ensure that the component is suitable for
continued service.
b) STEP 2— Determine the susceptibility using Table 12.3.
c) STEP 3—Modify the base susceptibility in STEP 2 with the susceptibility modifier from Table 12.4.
d) STEP 4—Determine the Severity Index, S VI , from Table 12.5 using susceptibility from STEP 1 (if
cracking is present) or STEP 3.
e) STEP 5—Determine the time in service, age, since the last Level A, B, or C inspection was performed
with no cracking detected or cracking was repaired. Cracking detected but not repaired should be
evaluated and future inspection recommendations based upon FFS evaluation.
f) STEP 6—Determine the number of inspections and the corresponding inspection effectiveness category
using Section 12.6.2 for past inspections performed during the in-service time. Combine the inspections
to the highest effectiveness performed using Section 3.4.3.
g) STEP 7—Determine the base DF for ClSCC,
ClSCC
D fB
, using Table 6.3 based on the number of
inspections and the highest inspection effectiveness determined in STEP 6 and the Severity Index, S VI ,
from STEP 4.
h) STEP 8—Calculate the escalation in the DF based on the time in service since the last inspection using
the age from STEP 5 and Equation (2.29). In this equation, it is assumed that the probability for cracking
will increase with time since the last inspection as a result of increased exposure to upset conditions
and other non-normal conditions.
ClSCC
11 .
( fB ( ) )
ClSCC
D f = min D ⋅ max[ age, 1. 0] , 5000 (2.29)
12.7 Nomenclature
age
is the component in-service time since the last cracking inspection or service start date
ClSCC
D f
is the DF for ClSCC
ClSCC
D fB
is the base value of the DF for ClSCC
S VI
is the Severity Index
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