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

RISK-BASED INSPECTION METHODOLOGY, PART 3, ANNEX 3.A—BASIS FOR CONSEQUENCE METHODOLOGY 3.A-23
3.A.4 Level 2 Consequence Methodology
3.A.4.1 General
The use of event trees and quantitative effects analysis forms the basis for the Level 2 consequence
methodology provided in Part 3, Section 5 with the details for calculating event tree probabilities and the
effects of pool fires, jet fires, flash fires, fireballs, VCEs, and BLEVEs are provided. Part 3 provides the
impact of most of these events with the closed-form equations.
3.A.4.2 Cloud Dispersion Analysis
Some events, such as VCEs and flash fires, require the use of sophisticated dispersion analysis software to
model how the flammable or toxic releases mix and disperse with air as they are released to the atmosphere.
There are several commercially available software packages that enable the user to perform dense gas
dispersion consequence modeling. Examples include, such as SLAB, DEGADIS and PHAST, some of which
are available in the public domain, while others are commercially available. A study contracted by the U.S.
Department of Energy provides a comparison of many different software packages, and recommendations
are provided to help select the appropriate package for a particular application.
In general, packages that perform dense gas dispersion modeling should be chosen as opposed to neutrally
buoyant models because hazardous releases typically will be materials with molecular weights heavier than
air. Even light hydrocarbons can be modeled accurately using dense gas modeling since the temperature of
the releases will result in releases with densities heavier than air.
Dispersion models will provide a cloud concentration profile. For flammables releases, the concentration
profile is used to assess which portions of the cloud are in the flammable range. For flash fires, the impact
area at grade is determined to be the area in the cloud that has flammable concentrations between the
released fluid’s LFL and UFL. For VCEs, a volumetric calculation is required since the total amount of
flammable volume and mass is required to assess the magnitude of the explosion.
3.A.5 Consequence Methodology For Atmospheric Storage Tanks
3.A.5.1 Overview
The consequence model for atmospheric storage tanks (ASTs) is based on a modification of the Level 1
consequence analysis. Only a financial consequence analysis is provided for the AST bottom.
3.A.5.2 Representative Fluid and Associated Properties
A representative fluid that most closely matches the fluid contained in the AST system being evaluated is
selected from the representative fluids shown in Part 3, Table 6.1. The required fluid properties for the
consequence analysis are also contained in this table.
In addition to selecting a fluid, a soil type must also be specified because the consequence model depends
on soil properties. Representative soil conditions and the associated soil properties required for the
consequence analysis is provided in Part 3, Table 6.2.