API RP 581 - 3rd Ed.2016 - Add.2-2020 - Risk-Based Inspection Methodology
2-56 API RECOMMENDED PRACTICE 5818.6 Determination of the DF8.6.1 OverviewA flow chart of the steps required to determine the DF for SSC is shown in Figure 8.1. The following sectionsprovide additional information and the calculation procedure.8.6.2 Inspection EffectivenessInspections are ranked according to their expected effectiveness at detecting SSC. Examples of inspectionactivities that are both intrusive (requires entry into the equipment) and nonintrusive (can be performedexternally) are provided in Annex 2.C, Table 2.C.9.6.If 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 3.4.3.8.6.3 Calculation of the DFThe following procedure may be used to determine the DF for SCC; see Figure 8.1.a) STEP 1—Determine the environmental severity (potential level of hydrogen flux) for cracking based onthe H 2 S content of the water and its pH using Table 8.2.b) STEP 2—Determine the susceptibility for cracking using Figure 8.1 and Table 8.3 based on theenvironmental severity from STEP 1, the maximum Brinnell hardness of weldments, and knowledge ofwhether the component was subject to PWHT. Note that a High susceptibility should be used if crackingis confirmed to be present.c) STEP 3—Based on the susceptibility in STEP 3, determine the Severity Index, S VI , from Table 8.4.d) STEP 4—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.e) STEP 5—Determine the number of inspections and the corresponding inspection effectiveness categoryusing Section 8.6.2 for past inspections performed during the in-service time. Combine the inspectionsto the highest effectiveness performed using Section 3.4.3.sscf) STEP 6—Determine the base DF for SCC, D fB , using Table 6.3 based on the number of inspectionsand the highest inspection effectiveness determined in STEP 5 and the Severity Index, S VI , fromSTEP 3.g) STEP 7—Calculate the escalation in the DF based on the time in service since the last inspection usingthe age from STEP 4 and Equation (2.25). 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.ssc11 .( fB ( ) )sscD f = min D ⋅ max[ age, 1. 0] , 5000(2.25)
RISK-BASED INSPECTION METHODOLOGY, PART 2—PROBABILITY OF FAILURE METHODOLOGY 2-578.7 Nomenclatureageis the component in-service time since the last cracking inspection or service start datesscD fis the DF for SSCsscD fB is the base value of the DF for SSCS VIis the Severity Index8.8 ReferencesSee References [4], [19] (pp. 541–559), [29], [30], [31], and [32] in Section 2.2.8.9 TablesTable 8.1—Data Required for Determination of the DF—SSCRequired DataSusceptibility (Low, Medium, High)Presence of water (Yes or No)H 2 S content of waterpH of waterPresence of cyanides (Yes or No)Max Brinnell hardnessAge (years)Inspection effectiveness categoryNumber of inspectionsCommentsThe susceptibility is determined by expert advice or using the procedures in thissection.Determine whether free water is present in the component. Consider not onlynormal operating conditions but also start-up, shutdown, process upsets, etc.Determine the H 2 S content of the water phase. If analytical results are notreadily available, it can be estimated using the approach of Petrie & Moore [30] .Determine the pH of the water phase. If analytical results are not readilyavailable, it should be estimated by a knowledgeable process engineer.Determine the presence of cyanide through sampling and/or field analysis.Consider primarily normal and upset operations but also start-up and shutdownconditions.Determine the maximum Brinnell hardness actually measured at the weldmentsof the steel components. Report readings actually taken as Brinnell, notconverted from finer techniques (e.g. Vickers, Knoop, etc.). If actual readingsare not available, use the maximum allowable hardness permitted by thefabrication specification.Use inspection history to determine the time since the last SCC inspection.The effectiveness category that has been performed on the component.The number of inspections in each effectiveness category that have beenperformed.
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RISK-BASED INSPECTION METHODOLOGY, PART 2—PROBABILITY OF FAILURE METHODOLOGY 2-57
8.7 Nomenclature
age
is the component in-service time since the last cracking inspection or service start date
ssc
D f
is the DF for SSC
ssc
D fB is the base value of the DF for SSC
S VI
is the Severity Index
8.8 References
See References [4], [19] (pp. 541–559), [29], [30], [31], and [32] in Section 2.2.
8.9 Tables
Table 8.1—Data Required for Determination of the DF—SSC
Required Data
Susceptibility (Low, Medium, High)
Presence of water (Yes or No)
H 2 S content of water
pH of water
Presence of cyanides (Yes or No)
Max Brinnell hardness
Age (years)
Inspection effectiveness category
Number of inspections
Comments
The susceptibility is determined by expert advice or using the procedures in this
section.
Determine whether free water is present in the component. Consider not only
normal operating conditions but also start-up, shutdown, process upsets, etc.
Determine the H 2 S content of the water phase. If analytical results are not
readily available, it can be estimated using the approach of Petrie & Moore [30] .
Determine the pH of the water phase. If analytical results are not readily
available, it should be estimated by a knowledgeable process engineer.
Determine the presence of cyanide through sampling and/or field analysis.
Consider primarily normal and upset operations but also start-up and shutdown
conditions.
Determine the maximum Brinnell hardness actually measured at the weldments
of the steel components. Report readings actually taken as Brinnell, not
converted from finer techniques (e.g. Vickers, Knoop, etc.). If actual readings
are not available, use the maximum allowable hardness permitted by the
fabrication specification.
Use inspection history to determine the time since the last SCC inspection.
The effectiveness category that has been performed on the component.
The number of inspections in each effectiveness category that have been
performed.
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