API RP 581 - 3rd Ed.2016 - Add.2-2020 - Risk-Based Inspection Methodology

RISK-BASED INSPECTION METHODOLOGY, PART 5—SPECIAL EQUIPMENT 5-7
Consequence of Failure Methodology for Storage Tank Courses
The COF associated with storage tanks is concerned primarily with the financial losses due to leakage
and/or rupture of a storage tank course. Safety/area based consequences are addressed for the courses
following the Level 1 or Level 2 consequence analysis methods provided in Part 3, Section 4.0 or Section
5.0. Detailed procedures for calculating the financial COF for courses are provided in Section 2.5 through
Section 2.16.
The procedure for determining COF of storage tank course components includes calculations for both area
and financial-based methods.
Required Properties at Storage Conditions
Fluid properties should be determined for the COF calculation. When calculating the safety COF area for
tank 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 fluids
other 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 equation
constants 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 and
Level 2 COF. Refer to the following paragraphs for a detailed description of the required properties at
storage conditions for tank course components:
⎯ Level 1 COF methodology, see Part 3, Section 4.1.2
⎯ Level 2 COF methodology see Part 3, Section 5.1.2
Required Properties at Flashed Conditions
Fluid properties are determined for a safety based COF for use in the Level 1 or 2 Consequence of Failure
methodology. See Part 3, Section 5.1.3 for detailed description of required properties at flashed conditions.
Release Hole Size Selection
A discrete set of release events or release hole sizes are used for consequence analysis as outlined in Table
4.4.
Calculation of Release Hole Sizes
The following procedure may be used to determine the release hole size and the associated generic failure
frequencies.
a) STEP 2.1 – Determine the release hole size, d
n
, from Table 5.4 for storage tank courses.
b) STEP 2.2 – Determine the generic failure frequency, gff
n
, for the d
n
release hole size and the total
generic failure frequency from Part 2, Table 3.1 or from Equation (5.4).
gff
tot
4
= ∑ gff
(5.4)
n=
1
n
Release Rate Calculation
Release rate calculations are provided for a leak in a storage tank course. The liquid head of the product is
assumed to be constant over time, and the leak is to atmospheric pressure for a course leak.
Storage Tank Course
The discharge of a liquid through a sharp-edged orifice in a storage tank course with a liquid height above
the orifices may be calculated using Equation (5.5).