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

3-26 API RECOMMENDED PRACTICE 581
4.9.5 Determination of the Toxic Release Rate and Mass
The toxic release rate or mass to be used in the toxic consequence analysis is determined based on the mass
tox
fraction of the toxic component, mfrac , that is present in the release fluid.
tox
tox
raten
mfrac Wn
= ⋅ (3.61)
tox
tox
massn
mfrac massn
tox
For pure toxic fluids ( 1.0)
= ⋅ (3.62)
mfrac = , the toxic release rate, rate , is equal to the release rate, W
n
, as calculated
tox
in Section 4.3 and the toxic release mass, massn
is equal to the release mass, mass
n
, as calculated in Section
4.7. For mixtures, the toxic release rate and release mass are modified based on the percentage of the toxic
component in the mixture and the storage phase (liquid or vapor) of the mixture.
Note that the magnitude reduction factor, fact
di
, to account for detection and isolation systems is not applied
to toxic releases as it is to flammable releases (see Section 4.6).
4.9.6 Estimation of Toxic Consequence Area for HF Acid and H2S
a) The background for the development of the toxic consequence equations for HF acid and H2S is provided
in Annex 3.A. For determination of the toxic consequence areas, the assumption was made that the release
phase would always be a gas or vapor.
b) The toxic consequence areas for continuous releases of HF or H2S as a function of the release rate may
be calculated using Equation (3.63).
tox
n
CA
10 tox
( c log 10
⎡C 4 B rate ⎤
−
⋅ ⋅ n + d
⎣ ⎦ )
,
= C8⋅
tox CONT
inj n
tox
( ⎡ ⎤ )
tox−CONT
CAinj, n
= C8⋅ ⎣C4B ⋅raten
⎦^ c ⋅10 ^ d
(3.63)
Note that for continuous releases, the toxic release rate, tox
rate
, is used as the input to Equation (3.63).
n
The constants, c and d , to be used in Equation (3.63) are provided in Table 4.11 as a function of release
duration. Interpolation between curves using the actual duration (defined in Section 4.9.10) is acceptable.
c) The toxic consequence areas for instantaneous releases of HF or H2S as a function of the release rate
may be calculated using Equation (3.64).
CA
10 tox
( c log 10 ⎡C 4 B mass ⎤
− ⋅ ⋅ n + d
⎣ ⎦ )
,
= C8⋅
tox INST
inj n
tox
( ⎡ ⎤ )
tox−INST
CAinj, n
= C8⋅ ⎣C4B ⋅massn
⎦^ c ⋅10 ^ d
(3.64)
Note that for instantaneous releases, the toxic release mass,
mass
tox
n
, is used as the input to
Equation (3.64). The constants, c and d to be used in Equation (3.64) are provided in Table 4.11.