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

RISK-BASED INSPECTION METHODOLOGY, PART 3—CONSEQUENCE OF FAILURE METHODOLOGY 3-9
4 COF—Level 1
4.1 Determine the Representative Fluid and Associated Properties
4.1.1 Representative Fluids
In the Level 1 consequence analysis, a representative fluid that most closely matches the fluid contained in
the pressurized system being evaluated is selected from the representative fluids shown in Table 4.1. Because
very few refinery and chemical plant streams are pure materials, the selection of a representative fluid almost
always involves making some assumptions. Annex 3.A provides guidance on selecting a representative fluid
when an obvious match in Table 4.1 cannot be found or when the fluid is a mixture with or without toxic
components.
4.1.2 Fluid Properties
The required fluid properties estimated for each of the representative fluids as provided in Table 4.2 are
dependent on the stored phase of the fluid below.
a) Stored liquid:
1) NBP;
2) density, ρ
l
;
3) AIT.
b) Stored vapor or gas:
1) NBP;
2) MW;
3) ideal gas specific heat capacity ratio, k;
4) constant pressure specific heat, C p ;
5) AIT.
The properties of fluids (or individual components of mixtures) typically can be found in standard chemical
reference books. The NBP is used in determining the phase of the material following the release to atmosphere,
and either the MW or density is used in determining the release rate, depending on whether a liquid or gas,
respectively, is released.
4.1.3 Choice of Representative Fluids for Acids and Caustic Fluids
The appropriate choice of reference fluid for acids and caustics is Acid/Caustic. Acid/Caustic should be chosen
whenever the fluid is nonflammable and nontoxic but would still present a hazard to personnel that may come
in contact with the release. Acid/Caustic is modeled as a liquid spray; see Section 4.10.3.