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

3-106 API RECOMMENDED PRACTICE 581
mod 100 ppm
tox
lim
= = 2000 ppm
(3.190)
0.05
When the cloud dispersion analysis is performed, the consequence area would be based on that portion of the
cloud at grade level that exceeded 2000 ppm for a duration of 10 minutes or more.
5.9.7 Determination of Final Toxic Consequence Areas
The final toxic consequence areas are determined as a probability weighted average of the individual toxic
consequence areas calculated for each release hole size. The probability weighting utilizes the generic
frequencies of the release hole sizes selected per Section 4.2.
The equation for probability weighting of the personnel injury consequence areas is given by Equation (3.191).
CA
tox
f , inj
⎛
4
tox
⎜∑
gffn
⋅CAf , inj,
n ⎟
n=
1
= ⎜
⎟
⎜ gfftotal
⎟
⎜
⎝
⎞
⎟
⎠
(3.191)
In Equation (3.191), the total GFF is as calculated in STEP 2.2.
5.9.8 Calculation of Toxic Consequence Areas
tox
a) STEP 9.1—Determine the mole fraction of the release rate that contains a toxic component, molefrac .
b) STEP 9.2—Calculate the release duration, ld , using Equation (3.186).
tox
n
c) STEP 9.3—Determine the toxic impact criteria, tox
lim
, and the time durations associated with each. For
example, an AEGL-3 toxic criteria can be based on a 10-minute, 30-minute, or 1-hour duration.
d) STEP 9.4—Determine the modified toxic limit,
mod
tox
lim
, using Equation (3.189).
e) STEP 9.5—For each hole size and for each toxic criteria available for the fluid, use the vapor source rate
and source area determined in STEP 7.7, and perform a cloud dispersion analysis in accordance with
Section 5.7.4. The leak duration, ld , from STEP 9.2 is used as an input into this analysis. Note that the
tox
n
concentration averaging time used in the dispersion analysis should be equal to the time duration
applicable to the toxic criteria being evaluated.
f) STEP 9.6—From the cloud dispersion analysis, determine the grade level area or boundary of the cloud
that is at or above the modified toxic exposure criteria established in STEP 9.4. This area is the toxic cloud
area, cloud
CA .
f , n
g) STEP 9.7—For each hole size, determine the probability of toxic release, ptox
n
, using Equation (3.187)
and the results from STEP 8.10.
tox
h) STEP 9.8—For each hole size, calculate the personnel injury toxic consequence area, CA , using
f , inj,
n
Equation (3.188).