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

RISK-BASED INSPECTION METHODOLOGY, PART 3—CONSEQUENCE OF FAILURE METHODOLOGY 3-73
5 COF—Level 2
5.1 Determine the Fluid Composition and Associated Properties
5.1.1 General
The Level 2 consequence analysis provides the equations and background information necessary to rigorously
calculate consequence areas for several flammable and toxic event outcomes. A summary of these events is
provided in Table 5.1.
The actual composition of the fluid, including mixtures, should be used in the analysis. Fluid property solvers
are available that allow the analyst to calculate fluid physical properties more accurately. The fluid solver
provides the ability to perform flash calculations to better determine the release phase of the fluid and to
account for two-phase releases. In many of the consequence calculations, physical properties of the released
fluid are required both at storage conditions and conditions after release to the atmosphere.
5.1.2 Required Properties at Storage Conditions
As shown in the flowchart of Figure 5.1, at the start of the consequence analysis, an isothermal flash is used
to determine the phase distribution and properties of the multi-component feed mixture at the storage
temperature, T
s
, and pressure, P
s
. The mass and mole fractions are determined along with the composition of
each phase. Thermodynamic properties such as entropy and enthalpy are calculated along with transport
properties such as thermal conductivity and viscosity. The required fluid properties at the storage conditions
are listed below.
a) Storage phase (vapor, liquid, critical, or two-phase).
b) Mass fraction liquid, frac
l
.
c) Mass fraction vapor, frac
v
.
d) MW.
e) Liquid density, ρ
l
.
f) Liquid viscosity, µ
l
.
g) Ideal gas specific heat ratio, k = Cp C .
v
h) Enthalpy of mixture.
i) Entropy of mixture (to perform flash calculations).
j) Critical pressure and temperature, T and P .
k) AIT.
l) Saturation pressure, Psat
s
, at storage temperature.
c
c
m) Flammability limits, LFL and upper flammability limit (UFL).