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

3-80 API RECOMMENDED PRACTICE 581
determine P bg ,
. The evaporation rate in this case is primarily limited by how fast the newly generated vapor can
be carried away from the interface by diffusion or convection.
The vapor rate generated off of the surface of a non-boiling pool,
(3.105) as provided by Shaw and Briscoe [10] .
erate
nb
, can be estimated using Equation
erate = C
⎛P
⋅
⋅ MW ⎞
u ⋅r
⎝ ⎠
bg , 0.78 1.89
n 15 ⎜ ⎟ w pn ,
RTs
(3.105)
The size of the non-boiling pool reaches a steady state when the evaporation rate, erate , is equal to the
bn ,
pool release rate,
pool
W
n
, as discussed in Section 5.7.3.
5.7.5 Cloud Dispersion Modeling
The ability to perform cloud dispersion analysis is a key component to performing the Level 2 consequence
analyses. Modeling a release depends on the source term conditions, the atmospheric conditions, the release
surroundings, and the hazard being evaluated. Employment of many commercially available models, including
SLAB, account for these important factors and will produce the desired data for the Level 2 assessments [12] .
Annex 3.A provides background on performing these studies and provides some guidance on available
software. Additional guidance is provided by Hanna and Drivas [13] .
The dispersion analysis is needed to determine several things. For flammable releases, such as flash fires,
this will typically entail determination of the portion of the cloud area (area footprint, ft 2 , at grade) where the air
to fuel mixture is between the LFL and the upper flammability limit (UFL). For VCEs, the amount of flammable
mass in the cloud is required. In this case, the amount of flammable material (lb) is required and therefore the
cloud dispersion model must be able to predict the volumetric portion within the cloud that is above the LFL of
the mixture.
For toxic releases, the cloud dispersion model must be able to calculate the concentration (ppm or vol%) of
the toxic component of the release throughout the cloud. The portion of the cloud in terms of plant area that
has a higher concentration than the relevant toxic impact criteria is determined. The toxic criteria may be based
on a probit value, IDLH, ERPG, AEGL, LC-50, or other acceptable value.
5.7.6 Cloud Dispersion Calculation
a) STEP 7.1—For each release hole size, calculate the adjusted release rate, rate
n
, using Equation (3.13)
where the theoretical release rate, W
n
, is from STEP 3.2. Note that the release reduction factor, fact
di
,
determined in STEP 6.4 accounts for any detection and isolation systems that are present.
b) STEP 7.2—For each release hole size, calculate the leak duration, ld
n
, of the release using Equation
(3.15), based on the available mass, mass , from STEP 4.6 and the adjusted release rate, rate
avail,
n
n
,
from STEP 7.1. Note that the leak duration cannot exceed the maximum duration, ld , determined in
max,
n
STEP 6.5.
c) STEP 7.3—Determine the rainout mass fraction from the released fluid using Equation (3.98) or (3.99),
based on the flash fraction calculated in STEP 1.3.
d) STEP 7.4—For each hole size selected in STEP 2.1, calculate the release rate of liquid that settles to the
ground for the pool calculations, W , using Equation (3.100).
pool
n