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

RISK-BASED INSPECTION METHODOLOGY, PART 3—CONSEQUENCE OF FAILURE METHODOLOGY 3-11
— autoignition temperature, AIT [K (°R)], can be estimated from Table 4.2.
2) For a stored vapor:
— molecular weight, MW [kg/kg-mol (lb/lb-mol)], can be estimated from Table 4.2;
— ideal gas specific heat ratio, k, can be estimated using Equation (3.1) and the C
P
values as
determined using Table 4.2;
— autoignition temperature, AIT [K (°R)], can be estimated from Table 4.2.
d) STEP 1.4—Determine the steady state phase of the fluid after release to the atmosphere, using Table 4.3,
and the phase of the fluid stored in the equipment as determined in STEP 1.2.
4.2 Release Hole Size Selection
4.2.1 General
A discrete set of release events or release hole sizes are used since it would be impractical to perform the
consequence analysis for a continuous spectrum of release hole sizes. Limiting the number of release hole
sizes allows for an analysis that is manageable, yet still reflects the range of possible outcomes.
The release hole sizes shown in Table 4.4 are based on the component type and geometry as described in
Annex 3.A. In addition, the release hole sizes are limited to a maximum diameter of 16 in. This diameter
represents a practical maximum value for a release calculation because catastrophic failures of components
generally do not involve disintegration of the equipment item.
4.2.2 Calculation of Release Hole Sizes
The following steps are repeated for each release hole size; typically four release hole sizes are evaluated.
a) STEP 2.1—Based on the component type and Table 4.4, determine the release hole size diameters, d
n
.
If D< d , d = D.
n
n
b) STEP 2.2—Determine the generic failure frequency, gff n , for the n th release hole size from Part 2, Table
3.1, and the total generic failure frequency from this table or from Equation (3.2).
gff
total
4
= ∑ gff
(3.2)
n=
1
n
4.3 Release Rate Calculation
4.3.1 Overview
Release rates depend upon the physical properties of the material, the initial phase, the process operating
conditions, and the assigned release hole sizes. The correct release rate equation must be chosen, based on
the phase of the material when it is inside the equipment item and its discharge regime (sonic or subsonic), as
the material is released.
The initial phase of the hazardous material is the phase of the stored fluid prior to coming into contact with the
atmosphere (i.e. flashing and aerosolization is not included at this point). For two-phase systems (condensers,
phase separators, evaporators, reboilers, etc.), some judgment as to the handling of the model needs to be
taken into account. In most cases, choosing liquid as the initial state inside the equipment is more conservative