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

3-74 API RECOMMENDED PRACTICE 581
n) Heat of combustion, HC
s
.
o) Toxic limits (e.g. IDLH, ERPG, AELG, probits, etc.).
5.1.3 Required Properties at Flashed Conditions
Analysis requires a fluid property package to isentropically flash (isenthalpic is acceptable) the stored fluid from
its normal operating conditions to atmospheric conditions. The effects of flashing on the fluid temperature as
well as the phase of the fluid at atmospheric conditions should also be evaluated. Liquid entrainment in the jet
release as well as rainout effects could be evaluated to get a more representative evaluation of the release
consequences. The isentropic flash calculation from storage conditions to atmospheric pressure, P
atm
,
simulates the release of the fluid from a leaking or ruptured storage container. The resulting flash temperature,
T , is determined along with the phase distribution and properties of each phase at these conditions. The
f
released mixture can either be a single-phase liquid, a single-phase vapor, or a two-phase mixture of both as
shown in Figure 5.1. The required fluid properties at the flashed conditions are listed below:
a) flashed phase (vapor, liquid, or two-phase);
T
f
b) flash temperature, ;
frac
fsh
c) flash fraction, ;
ρ l
d) density of the liquid, ;
ρ v
e) density of the vapor, ;
Cp
f) specific heat of the liquid, l
;
HC
g) heat of combustion of liquid, l
;
HC
h) heat of combustion of vapor, v
;
∆H
i) latent heat of vaporization of liquid, v
;
T
j) bubble point temperature of liquid,
b ;
T
k) dew point temperature of vapors,
d .
As shown in Figure 5.1, where a fluid is flashed to a single-phase liquid, a bubble point temperature calculation
is performed at atmospheric pressure to find the temperature, T
b
, at which vapor bubbles first appear. Similarly,
in the single-phase vapor case, a dew point calculation is performed at atmospheric pressure to find the
temperature, T
d
, at which liquid drops first start condensing.