API RP 581 - 3rd Ed.2016 - Add.2-2020 - Risk-Based Inspection Methodology
5-6 API RECOMMENDED PRACTICE 5812) Adjustment for Maintenance in Accordance with API STD 653, FAM – If the storage tank is maintained inaccordance with API STD 653, then FAM= 1; otherwise, FAM= 5 .3) Adjustment for Settlement, FSM – It is determined based on the following criteria:⎯ Recorded settlement exceeds API STD 653 criteria – FSM= 2F =⎯ Recorded settlement meets API STD 653 criteria – 1SM⎯ Settlement never evaluated – FSM= 1.5⎯ Concrete foundation, no settlement – FSM= 1Determination of the SCC Damage FactorsFollow calculating procedures outlined in Part 2, Section 5 through Section 14 for SCC of storage tankcourses, if applicable.Determination of the External Damage FactorsFollow calculating procedures outlined in Part 2, Section 15 through Section 18 for external damage ofstorage tank courses, if applicable.Determination of the Brittle Fracture Damage FactorsFollow calculating procedures outlined in Part 2, Section 21 for brittle fracture of storage tank courses, ifapplicable.Damage Factor Combination for Multiple Damage MechanismsFollow calculating procedures outlined in Part 2, Section 3.4.2 for combining DFs or multiple damagemechanisms of storage tank courses.Consequence of FailureThe COF is calculated in terms of affected area or in financial consequence. Consequences from flammableand explosive events, toxic releases, and nonflammable/nontoxic events are considered in both methodsbased on the process fluid and operating conditions. Financial consequences from component damage,product loss, financial impact, and environmental penalties are considered.The COF methodology is performed to aid in establishing a ranking of equipment items on the basis of risk.The consequence measures are intended to be used for establishing priorities for inspection programs.Methodologies for two levels of analysis are provided. A special COF methodology is provided for lowpressure and atmospheric storage tanks and is covered in this section.
RISK-BASED INSPECTION METHODOLOGY, PART 5—SPECIAL EQUIPMENT 5-7Consequence of Failure Methodology for Storage Tank CoursesThe COF associated with storage tanks is concerned primarily with the financial losses due to leakageand/or rupture of a storage tank course. Safety/area based consequences are addressed for the coursesfollowing the Level 1 or Level 2 consequence analysis methods provided in Part 3, Section 4.0 or Section5.0. Detailed procedures for calculating the financial COF for courses are provided in Section 2.5 throughSection 2.16.The procedure for determining COF of storage tank course components includes calculations for both areaand financial-based methods.Required Properties at Storage ConditionsFluid properties should be determined for the COF calculation. When calculating the safety COF area fortank courses, see Part 3, Section 5.1.2 Level 1 or 2 Consequence of Failure methodology. See Part 3,Section 5.1.2 for detailed description of required properties at storage conditions. The financial COF for fluidsother than those in Table 4.5 may be modeled if the stored as liquid data required in Table 4.5 and Part 3,Table 4.2 are provided by the user. Note that flammable COF would be calculated based on the equationconstants in Part 3, Table 4.8 and Part 3, Table 4.9 for the fluid closest matching the molecular weight (MW)and normal boiling point (NBP).Fluid properties at storage conditions are necessary to calculate the financial and area-based Level 1 andLevel 2 COF. Refer to the following paragraphs for a detailed description of the required properties atstorage conditions for tank course components:⎯ Level 1 COF methodology, see Part 3, Section 4.1.2⎯ Level 2 COF methodology see Part 3, Section 5.1.2Required Properties at Flashed ConditionsFluid properties are determined for a safety based COF for use in the Level 1 or 2 Consequence of Failuremethodology. See Part 3, Section 5.1.3 for detailed description of required properties at flashed conditions.Release Hole Size SelectionA discrete set of release events or release hole sizes are used for consequence analysis as outlined in Table4.4.Calculation of Release Hole SizesThe following procedure may be used to determine the release hole size and the associated generic failurefrequencies.a) STEP 2.1 – Determine the release hole size, dn, from Table 5.4 for storage tank courses.b) STEP 2.2 – Determine the generic failure frequency, gffn, for the dnrelease hole size and the totalgeneric failure frequency from Part 2, Table 3.1 or from Equation (5.4).gfftot4= ∑ gff(5.4)n=1nRelease Rate CalculationRelease rate calculations are provided for a leak in a storage tank course. The liquid head of the product isassumed to be constant over time, and the leak is to atmospheric pressure for a course leak.Storage Tank CourseThe discharge of a liquid through a sharp-edged orifice in a storage tank course with a liquid height abovethe orifices may be calculated using Equation (5.5).
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5-6 API RECOMMENDED PRACTICE 581
2) Adjustment for Maintenance in Accordance with API STD 653, F
AM – If the storage tank is maintained in
accordance with API STD 653, then F
AM
= 1; otherwise, F
AM
= 5 .
3) Adjustment for Settlement, F
SM – It is determined based on the following criteria:
⎯ Recorded settlement exceeds API STD 653 criteria – F
SM
= 2
F =
⎯ Recorded settlement meets API STD 653 criteria – 1
SM
⎯ Settlement never evaluated – F
SM
= 1.5
⎯ Concrete foundation, no settlement – F
SM
= 1
Determination of the SCC Damage Factors
Follow calculating procedures outlined in Part 2, Section 5 through Section 14 for SCC of storage tank
courses, if applicable.
Determination of the External Damage Factors
Follow calculating procedures outlined in Part 2, Section 15 through Section 18 for external damage of
storage tank courses, if applicable.
Determination of the Brittle Fracture Damage Factors
Follow calculating procedures outlined in Part 2, Section 21 for brittle fracture of storage tank courses, if
applicable.
Damage Factor Combination for Multiple Damage Mechanisms
Follow calculating procedures outlined in Part 2, Section 3.4.2 for combining DFs or multiple damage
mechanisms of storage tank courses.
Consequence of Failure
The COF is calculated in terms of affected area or in financial consequence. Consequences from flammable
and explosive events, toxic releases, and nonflammable/nontoxic events are considered in both methods
based on the process fluid and operating conditions. Financial consequences from component damage,
product loss, financial impact, and environmental penalties are considered.
The COF methodology is performed to aid in establishing a ranking of equipment items on the basis of risk.
The consequence measures are intended to be used for establishing priorities for inspection programs.
Methodologies for two levels of analysis are provided. A special COF methodology is provided for low
pressure and atmospheric storage tanks and is covered in this section.
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